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DESIGN AND DEVELOPMENT OF A PERVAPORATION MEMBRANE
SEPARATION MODULE
Weihua Xu
A thesis submitted in conformity with the requirements for the Degree o f Master of Applied Science
Graduate Department of Mechanical and Industrial Engineering University of Toronto
@Copyright by Weihua Xu 2001
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DESIGN AND DEVELOPMENT OF -- . -- *- A PERVAPORA-TION-MEmRANE-SPARATION MODULE
A thesis subrnitted in confodty with the requirements for the Degree of Master of Applied Science
Graduate Department of Mechanical and Industrial Engineering University of Toronto
2001
ABSTRACT
This thesis contributes to the design and development of a new pervaporation membrane
separation module based on axiomatic design theory and Design for X methodology. The overall
design pmcess consists of needs generation, task analysis, knowledge preparation, design
methodology selection, conceptual design, detailed design, working drawings, manufacturing
arrangement, prototyping and design evaluation. The conceptual design was executed on four
levels to form the basic configurations and features under the guidance of axiomatic design
theory. Thereafter, the detailed design was perfonned based on Design for X methodology. The
design incorporates the customer requirements, cost, manufactunng, performance, structure,
maintenance, reliability, schedule and human factor considerations, etc. Numerous innovative
design features have been incorporated in the module and have been made to obtain the best
combination of various design concepts which have been validated by prototyping.
Recommendations for the further improvement of the module are addressed.
ACKNOWLEDGMENTS
1 would like to thank Professor Ron D. Venter for providing me with the opportunity to
work in this project. His inspiration and guidance have been essential to my educational and
career oriented pursui ts.
My sincere thanks are expressed to Ian McGregor and Darren Lawless of Fielding
Chernical Technologies Inc., for their creative thinking, program management, critical design
review and financial investment.
My sincere gratitude is also due to Materials and Manufacturing Ontario and Michael D.
Burgoyne for the financial support via the interact program.
Working with Mark Nye was not on1 y educational, but refreshing. Mark's help with the
manufacturing of the prototype and jig was much appnciated.
Thanks to Brenda Fung, Administrator of the MIE Graduate Studies Office, for her
consistent support over the past few years.
To al1 the friends and coworkers 1 met while studying at the University of Toronto, thank
you for your friendship and support.
Finally, 1 would like to thank for my family, Yan, rny wife for sharing wealth and woe
with me. Her love and support dunng the last two years have been beyond compare; to my
parents for their encouragement; to Jeanne and Matthew, my lovely kids for the great joy they
provide.
iii
TABLE OF CONTENTS
1.2 General S tatement and Research Objectives .. . ..... .. .. . . .... . . .. ... . ... . . .. . . .. .. . . . . . .. . . . . . . . . . . . . 3
1.3 Overview of the Thesis ............. . ................................................. . . . . . ......... ............. 4
CHAITER 2 MEMBRANE P E R V A P O R A T I O N m m m m m m a m ~ m m m m * m m m ~ m m m m m m m m * m m m m m m m m m m m m m m a m m m m a m m a m m m m m m m a 7
2.1 Description of Pervaporation Separation ....... .. .......... . ...... ...... . .. . . . . . . . . . . . . ........ . . 7
2.2 Pervaporation Membranes .. .... .. . .. . ... . . . ... .. .. . .. .. . .... . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Separation Mechanism of Pervaporation Membranes ....................................................... 8
2.4 Specifications of Pervaporation Membranes ............................................................. 9
2.4.1 Membrane Selectivi ty .... ... ..... ... ... .. . .. .... .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4.2 PermeslteFlux ....... ,,, ............. .. ............................................ . ................................... 10
2.5 Configuration of Pervaporation Rocess .... ..... ... ...... . ... .... . . . .. .. .. . . . .. . . . . . . . . . . . . . 10
2.6 Various Pervaporation Membrane Separation Modules .. . . . .. .. ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 3
2.6.1 Plate-and-Frarne Modules ......... ,.. .... ... .. . .... ....... . . . . ......... . . . . . . . . . . . . . 13
2.6.2 Hollow-FiberModules .... ...... . ................... . .... . . . . . . ................. . ........ . .............. 14
iv
........................................................................................... 2.6.3 Spiral-Wound Modules 15 ......-a= . - .... . . a-: . &
2.6.4 Tubular Module ...... ........... ........................................................... ............... 17
.............................................................................................................. 2.6.5 FCT Module 18
..................................................................... 2.6.6 Cornparison of Pervaporation Modules 19
CHAPTER 3 OVERVIEW OF DESIGN THEORIES o m o m o o o o o o o o o o m o m m o o a o o o ~ o o ~ o o m o o o o o m o o m o o m o o o 21
...................................................................................................................... 3.1 Introduction 21
3.2 Overview of Design Methodologies ............................................................................. 2 2
3.3 Axiomatic Design Theory .................................................................... .. ...................... 24
....................................................................................................... Design Axioms 2 5
.................................................................................................... Design Equation 2 5
.............................................................................................. Uncoupled Design 2 6
Decoupled Design ............................................................................................. 2 7
...................................................................................................... Coupled Design 2 8
Constrains ............................................................................................................... 28
......................................................................... Task Decomposition and Hierarc h y 2 8
................................................................................................................ Corollaries 3 0
................................................................................................................... Theorems 31
3.4 Design for X .................................................................................................................... 3 2
3.4.1 Definition of DFX .................................................................................................... 32
3.4.2 Principles of DFX ...................................................................................................... 33
3.4.2. L Ptocedures of DFX .............................................................................................. 34
................................................................................................... 3.4.2.2 Tools of DFX 3 5
............................................................................................... 3.4.3 Benefi ts from DFX 3 6
CHA.Pl'ER 4 CONCEFIVAL DESIGN AND ANALYSIS oooomoooomooooooomomooooooooooooooooaooaomoooomo 37
4.1 Introduction ...................................................................................................................... 37 -._:_.. : ... :- :..... =. . IL . ..i..-.-_.. .. ., . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 EHect of the Pervaporation Rocess Conditions ....................................................... 3 8
4.3 Conceptual Design Pmcess of the Pervaporation Module ............................................... 41
4.3.1 Lever O-Task Definition and hplementation .......................... .. .......................... 4 5
4.3.2 Level 1-General Functions and Physical Solutions .................................................. 45
4.3.3 Level 2-Major Functions and Physical Solutions ..................................................... 47
4.3.3.1 Conceptual Design of the Perrneate Generator ............................................... 4 7
4.3.3.2 Conceptual Design of the Penneate Removing .................................................. 5 3
4.3.4 Level3-DetailedConceptualDesign ...................................................................... 54
4.3.4.1 Conceptual Design of Maximizing the Membrane Area .................................... 55
4.3.4.2 Conceptual Design of the Disk Membrane Support .......................................... 5 8
4.3.4.3 Conceptual Design of the Sealing ...................................................................... 6 2
4.3.4.4 Conceptual Design of Minimizing the Flow Restriction .................................... 64
4.4 Summary ..................................................................... ................... .................... 6 8
CHAPI'ER 5 DETAILED DESIGN AND ANALYSIS ....................................................... 70
Introduction .................................................................................................................... 70
Design of the Permeate Flow Velocities .......................................................................... 71
............................................................................ Design of the Separate Disk Etements 7 4
Design of the Channel Configuration .............................................................................. 76
Design of the Built-in Membrane Spacers ................................................................. 7 8
Design of the Segmented Disk Joints .......................................................................... 8 2
Design of the Membrane Disk Hubs ................................................................................ 87
Design of the Central Removal Tube .............................................................................. 91
Design of the Membrane Disk Rein forcements ............................................................... 95
................................................................... 5.10 Plastic Material Selection ........................ 9 8 ........ --......- -. . . =
510.1 Specifications of the Plastic Components ............................................................... 98
5.10.2 GeneralDescriptionofPolymers ........................................................................ 9 9
............................................................................ 5.10.3 Cornparison of Typical Pol p e r s 99
....................................................................... 5.1 1 Design for Assembly and Disassembly 102
5.12 RototypingofthePervaporationModule .................................................................. 104
...................................................................................................................... 5.13 Sumrnary 107
CHAITER 6 ASSEMBLY JIG DESIGN ....... ..,..... ........ .................................................. 108
.................................................................................................................... Introduction 108
...................................................................................... FRs of the Membrane Disk Jig 108
Disk Holding Assembl y Design ................................................................................... 110
................................................................................................ Roller Assembly Design 1 1 1
......................................................................................................... Design of the Base 1 1 1
CHAPTER 7 CONCLUSIONS ............................................................................................ 112
....................................................................................................... 7.1 Sumrnary of Results 112
................................................................................ 7.2 Recommendation for Future Work 114
REFERENCES ....................................................................................................................... 127
vii
-x*- -L . L .
LIST OF TABLES - . .
Table 2.1 Cornparison of Pervaporation Modules .................................................................... 20
..... ............................................................................. Table 3.1 Overview of Design for X .. 3 3
...................................................................... Table 3.2 What Does a DFX Tool Accomplish? 36
Table 5.1 Cornparison of Typical Thennoplasts ..................................................................... LOO
Table 5.2 Cornparison of Typical Thermosets ..................................................................... 101
Table 5.3 Cornparison between the Real Module and its Prototype ....................................... 106
.
F i p 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Figure 2.1 1
Figure 2.12
Figure 2.13
Figure 2.14
Figure 3.1
Figure 3.2
Figure 3.3
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
LIST OF FIGURES
................................................... Pervaporation Membrane Separation Schematics 7
................................................................................ Vacuum Driven Pervaporation 11
Temperature Gradient Driven Pervaporation .......................................................... 11
....................................................................................... Carrier Gas Pervaporation Il
Pewaporation with a Condensable C h e r .............................................................. 12
............................. Pervaporation with a Two-phase Permeate and Partial Recycle 12
................................. Pewaporation with Fractional Condensation of the Pemeate 12
A Plate-and-Frame Membrane Module .................................................................. 14
...................................................................... Monsanto Prism Closed-end Module 15
............................................................................. A Capillary (spaghetti) Module 15
Spiral-wound Membrane Module .......................................................................... 16
Four4eaf. Spiral-wound Module ........................................................................ 17
A Tubular Module ............................................................................................. 17
....................................................................... FCT Pervaporation Module Layout 19
........................................................................... Block Diagram of Design M e s s 21
Design Mapping Process Illustration .................................................................... 25
....................... Hierarchical Structure. Task Decomposition and Zig-zag Mapping 29
Hierarchy of Functional Requirements .................................................................... 43
Hierarchy of Design Parameters ............................................................................. 44
.................................................. Cornparison between Glue and Mechanicd Seals 5 6
Illustration of Disk Structure Instability ................................................................ 59
Possible Disk Frarnes ............................................................................................... 61
............................................ Penneate Gas Fiowing Mec hanism in a Mesh S pacer -65
ix
Figure 4.7 - d ..........
Figure 4.8
Figure 5.1
Figure 5.2
Figure 5.3
Figure 5.4
Figure 5.5
Figure 5.6
Figure 5.7
Figure 5.8
Figure 5.9
Figure 5.10
Figure 5.11
Figure 5.12
Figure 5.13
Figure 5.14
Figure 5.15
Figure 5.16
Figure 5.17
Figure 6.1
Figure 6.2
Permeate Pressure Pattern between Channels ......................................................... 66 . . ....
Built-in Mesh Spacer Concept ................................................................................ 67
Membrane Disk Element ..................................................................................... 7 5
................. Comparison of the Polar Parabola Channels and the Straight Channels 77
Membrane Disk with Spiral Channels ..................................................................... 78
Dot Pattern of the Membrane Spacer ................................................................... 8 1
.................................................... V Shape Grove Pattern of the Membrane Spacer 81
Circle Shape Grove Pattern of the Membrane Spacer ............................................. 81
....................................................................................... Joints with Bolts and Nuts 84
........................................................................................................ Spacer Joints 8 4
Aluminum Strap Joining .......................................................................................... 86
................................................................. Configuration of Membrane Disk Hub 8 8
...................................................................... Membrane Disk Hub Configuration 89
............................................... Central Structure of the Membrane Separator Unit 94
................................................................... Reinforcement Clip Configuration 9 6
......................................................................... Reinforcement Rig Configuration 96
Reinforcement Clip Installation ......................................................................... 97
Collar Torching Mechmisnt ................................................................................ 103
......................................... Assembly and Disassembly of a Reinforcement Clip 104
Configuration of Membrane Disk Jig ............................................................... 109
Positional Structure of the Disk Inner Penphery ................................................... 1 IO
Figure App-A- 1 Two Membrane Cell Assembly (Plastic Disk) ........................................... 1 1 6
.............................. Figure App-Ad Two Membrane Ce11 Assembly (Steel Disk. Prototype) 117
Figure App-B-1 Central Collar .............................................................................................. 119
Fig~re App-B-2 -...... _Lm-&_- .. a._
Figure App-B-3
Figure App-B-4
Figure App-C- l
Figure App-C-2
Figure App-C-3
Figure App-C-4
Figure App-C-5
Figure App-C-6
Sepmte Disk Element ................................................................................. 1 2 0 - . .................................................................................................... Pin Washer 121
Reinforcement Clip .................................................................................... 122
Pervaporation Module Corn ponen ts ............................................................ 124
Pervaporation Module Assembl y ................................................................ 124
Raw Membrane Steel Disk Plate .................................................................. 125
Membrane Separation Cell ........................................................................... 125
Jig in Action (Top View) ............................................................................. 126
Jig in Action (Side View) ............................................................................ 126
. .
-A-- a L-. . . & . . . . . .
LIST OF APPENDIXES
APPENDIXES A Selected Assembly Dniwings .................................................................... 115
APPENDIXES B Selected Component Drawings .................................................................. 118
APPENDIXES C Selected Pictures ......... , ........ .,., ............... ... .......................................... 123
"-- NOMENCLATURE AND ACRONYMS
-
Roman LRtters
Element of the design matrix
Active surface area of the membrane
Concentration of the solvent component which has lower volatility
Concentration of the solvent component which has higher volatility
Diarneter of the disk hub
Inner Diameter of the center tube
Diameter of the membrane support disk
Membrane flux rate
Number of the channels in each membrane disk
Number of the disk membrane cells in a pervaporation module
S t e m pressure at the temperature of 100 OC
Permeate pressure in the pervaporation module at the temperature of LOO OC
Volumetnc permeate flow
Total cross section area of the channels on the membrane disk
Thickness of the steel disk
Maximum temperature in the pervaporation module
Volume per pound steam at the pressure of Po
Volume per pound steam at the pressure of Pi
Velocity of the permeate flow in the disk channels
Velocity of the permeate flow in the center tube
Width of the outer periphery seals
Width of the disk channels
xiii
Greek Letters
Acronyms
CE
DFX
DP
FCT
FR
P A
PVA
~embrane selectivity
Concurrence Engineering
Design for X
Design Parameter
Fielding Chemical Technology Inc.
Functional Requirement
Isopropyl Alcohol
Pervaporation
xiv
CHAPTER 1 INTRODUCTION
1.1 Background and Motivation
Pervaporation is a membrane separation process used to separate mixture of dissolved
solvents. In recent years there has k e n increased interest in the use of pervaporation membrane
separation techniques for the selective separation of organic liquid mixtures, because of its high
separation efficiency and flux rates coupled with potential savings in energy costs.
Systematic studies of membrane phenornena can be traced to the eighteenth century
philosopher scientists. Early investigators experimented with any type of diaphragm available to
them, such as the bladders of pigs, cattle or fish and sausage casings made of animal gut. In later
work collodion membranes were preferred. By the early 1930s micro-pomus collodion
membranes were commerciall y available. During the next 20 years this earl y micro-filtration
membrane technolog y was expanded to other pol yrners.
Membranes found their first significant applications in the filtration of drinking water
sarnples at the end of World War II. By 1960. thetefore, the elements of modem membrane
science had been developed. But membranes were used in only a few laboratones and small
industrial applications. Membranes suffered from four problems that prohibited their widespread
use: they were too unreliable, too slow, too unselective, and too expensive. Partial solutions to
each of these problems have been developed during the last 30 years. and as a result there is a
surge of interest in membrane-based separation techniques.
The 20-year period from 1960 to L980 produceci a tremendous change in the status of
membrane technology. Using the techniques such as interfacial polymerization or multi-layer
composite casting and coating, it is now possible to make membranes as thin as 0.1 ,un or less.
In 1980s GFT, a small German engineering company, introduced the first commercial - A-- -- - -
pervaporation systems for dehydratioi of aÏcohol.
Membrane process techniques can be classified into membrane formulation and
membrane pervaporation separation modules. The former is to develop and produce various
membranes and to exploit the separation mechanisrn of pervaporation membranes. The latter
focuses on the design and development of various membrane separators such that the potential of
pervaporation membranes is maximized with minimum cost, weight and size.
With the developrnent of new pol yrner matenals and membranes, poor membrane
performance is no longer a problem. In this situation, a good pervaporation separator plays an
important role to maximize the membrane performance. Although tkre are a number of different
pervaporation separators available in today's market, most of them suffer from some comrnon
problems, such as high cost, which inhibit their widespread use.
U.S., Europe and Japan are the major countries active within pervaporation membrane
industry. Fielding Chemical Technologies Inc. (FCT) is a medium size company located in
Mississauga, Ontario, Canada. Their mission is to be a world leader in chemical separation
technologies and to provide the leadership in environmental practices so that companies can
safely use and Ruse chemicals. As the largest chemical recycling company in Canada, FCT is a
Canadian pioncer in the industriat application of membrane sepamtion technologies. FCT not
only systematically explored the pervaporation membrane formulation, membrane separation
mechanisms and membrane applications, but also developed the fiat generation of FCT
pervaporation membrane module for the deh ydration of PAlwater mixture. This membrane
module has been used by FCT for the testing of various membranes.
FCT's experience shows that a pervaporation module plays an important role in
excavating the performance of a pervaporation membrane. The existing FCT pervaporation
module has been validated by a large number of tests over the past few years. Some innovative & - - --a& - - 4-
design concepts have been successful~y proven. However, some weaknesses has been identified
in the application, such as limited production capacity, over weight, structure defects and sealing
problem. etc.
Over the past one year, there have been a numkr of exploratory meetings between FCT
and the creative design team from the University of Toronto. The discussions focused on
launching a project to design and develop a new generation of pervaporation membrane
separation modules to boost market. This new module is specially required to have higher
production capacity to overcome this limitation of the earlier modules. Moreover, the new
module should build on and extend the approved design concepts as much as possible.
University of Toronto undertwk the project and has king collaborated with FCT on the design
and development of the new module, under the support of MM0 interact program. This thesis
details the entire design process performed for the project requirements descri bed above.
General Statement and Research Objectives
Engineering design is the key technical ingredient in the product realization process, and
the means by which new products are conceived, developed and brought to market. Design is
also a creative process which is an essential feature of the human king's ability to survive and
prosper. Based on the first generation of FCT module, a new concept module is proposed in this
thesis for the chemical separation business. This new module should inherit the proven design
concepts in the baseline module as much as possible. Besides, the defects identified in earlier
prototype modules should be avoided. Such a challenge could not be completed by regular
design approaches based on detailed drawing in isolation of the actual pervaporation process.
First, a thorough understanding of chemical pervaporation separation techniques was required.
3
Second1 y, a comprehensive design and anal y sis process was undertaken, including needs L .- - - -
generationi ta& &aiy&, knowledg& preparation, design methodology selection, conceptual
design, detailed design, working drawings, manufactunng arrangement, prototyping and design
evaluation. Finally, the design is a creative process based on the designer's experience, with the
help of certain design theones and methodologies.
Since the last decade, a relatively new field of research which has emerged to promote
the understanding of design and referred to and known design theory and methodology. On the
one hand, engineering product design is seeking the guidance of design theory. On the other
hand, there is a need to evaluate and verify various design methodologies developed recently.
Axiomatic design theory developed ai MIT by Suh, Nakazawa, Bell, Gossard et. al. is
one of the most systematic design theones. The axiomatic design approach defines design as the
creation of synthesized solutions that satisfy perceived needs through the mapping between
functional requirements and design parameters. Axiomatic design theory offers the means by
which good and bad design is distinguished and optimum design solutions are selected.
Design for X is a concurrent design concept by which the product realization prwess is
organized. The design pmcess based on Design for X uses, wherever possible, information and
knowledge about al1 the issues in a product life.
This thesis contributes to the bRdging between the latest design methodotogies and r
practical design example. The Axiomatic Design Theory in conjunciion with Design for X has
been selected to guide the design and development of a novel pervaporation module.
Overview of the Thesis
This thesis consists of seven Chapters and two Appendixes. In Chapter 1, the background
and general statement are presented.
4
The understanding of the pervaporation separation technique is essentid for the * - -- - - - - .d -L - - - . -. - -- - - - -
development of a pervapor&on module. In Chapter 2, various pervaporation membranes are
introduced and the membrane separation mechanism is explained. Then current pervaporation
process is described and different pervaporation modules are compared to understand the
advantages and disadvantages of different design concepts. The earlier versions of the FCT
module are presented; these are particularly important as the new design is based on the
established working success of the design.
Chapter 3 is the overview of the design theoties and methodologies. Over the past
decade, axiomatic theory has been developed by rnany researches. There are numerous
publications dedicated to this theory and axiomatic design has become one of the most
systematic of the emerging design theones. Thus it is possible to systematically introduce the
axiomatic theory in this thesis. Unfortunately, Design for X (DFX) is a methodology still under
development; there is no unique design method suitable for al1 of the design goals, Le. "Xs". It is
very therefore difficult to demonstrate the overall methodology conveniently. However. the
author will summaxize the process of DFX such that it can be followed for the subsequent
detailed design.
The conceptual design of the pervaporation module is addressed in Chapter 4. The
cvrtceptuat design witt start from the identification of the effect of pervaporatim process
conditions, since module design is to ma te a process condition which can maximize the
membrane potential. Under the guidance of axiomatic design theory, the conceptual design is
perfomed on four levels. The main configuration and features of the module are detennined
during the conceptual design phase.
The detailed design process follows the conceptual design. Chapter 5 describes the details
of the components and assemblys individually. DFX is employed to set the design goals. The
engineering drawings are generated in the detailed design phase as well. Finally, the prototyping ----..a L -- - -- - A & - - - - - - -- -
process is presented to evaluate the design concepts.
In Chapter 6, a membrane disk jig, as important equipment to facifitate the assernbly of
the membrane disks is designed and detailed.
Conclusions and recommendations for future work are addressed in Chapter 7. In the
Appendix A and B, the selected assembly drawings and component drawings are enclosed for
reference.
CHAPTER 2 MEMBRANE PERVAPORATION
2.1 Description of Pervaporation Separation
Pervaporation is a membrane process used to separate mixture of dissolved solvents. The
process is shown schematically in Figure 2.1. A liquid mixture contacts one side of a none-
porous permselective membrane; the permeate is removed as a vapor from the other side.
Transportation through the membrane is induced by the difference in partial pressure between
the liquid feed solution and vapor permeate. The permeate undergoes a phase change, from
liquid to vapor, during its transport through the membrane barrier.
Feed Mix ture
Pe rmea te Gas
Figure 2.1 Pervaporation Membrane Separation Schematics
2.2 Pervaporation Membranes - - .
The famil y of pervaporation membranes pnmaril y consists of symmetrical membranes,
asymmetric membranes and composite membranes. Cerarnic and metal membranes and liquid
membranes are also the member of this family, but they are not popular in industries.
Syrnrnetrical membranes are unifomly isotropic throughout. The membrane can be
porous or dense, but the permeability of the membrane material does not change from point to
point within the membrane.
Typical asymmetic membranes have a relatively dense, thin surface layer supported on an
open, often micropomus substrate which has the s m e material as the surface layer. The surface
layer generally performs the pervaporation separation and is the principal bamer for the flow
through the membrane. The open support layer provides mechanical strength.
Composite membranes comprise a porous support layer with a thin dense coated layer on
top of it. This top layer is made of a different matenal from the suppon layer and this support
layer is often applied on a fabric non-woven. Composite membranes are one of the most popular
pervaporation membranes used in the chemical industries.
2.3 Separation Mecbanism of Pervaporation Membranes
Pervaporation separation mechanisms have been studied by a large number of researches
for a long time. There are different opinions on how selectivity and transport could be explained
in a pervaporation membrane system. Binning et al. [32] suggested that the selectivity took place
in a boundary layer between the liquid zone and the gas zone in the membrane. Michaels et al.
[33] interpreted the selectivity as a result of sieving by the polyrner crystals. Schrodt et al. 1341
suggested that hydrogen bondings between polymer and solvent components plays an important
d e . Long [35] considered that the diffusion and concentration gradients in the different solvent
8
-- - -
components were the goveming factors. Matsuura et al. 136, 37) regarded the pervaporation --- -- .*-- -- -
mechanism as a combination of reverse osmosis (RO) separation, followed by evaporation and
vapor transport through the capillary pores on the surface layer of a RO membrane. Yoshikawa
et al. [38,39,40,41, 421 explained that specific and selective separation of substances through
membranes may be realized by differences in strength of h ydrogen bonding interaction w hic h
leads to selective separation through the membranes.
Although pervaporation transport mechanism can be explained in different ways. most
people agree that the transport of the permeate through a pore-free permselective film involves
three successive steps as follows:
a) Upstrearn partitioning of the feed-components between the flowing liquid mixture
and the swollen upstrearn layer of the membrane.
b) Diffusion of the penetrants through the unevenl y-swo!len pemselective bamer,
c) Permeate desorption, which takes place at the downstream surface of the film.
In this thesis, the project present focuses on the pervaporation membrane separation
devices, instead of pervaporation membranes. Therefore, the three separation processes above
are good enough to guide the design and development of the pervaporation membrane
separation module.
2.4 Specifications of Pervaporation Membranes
The behavior of a pervaporation membrane used to separate a given binary A-B liquid
mixture is characterized by two parameten: Selectivity and Flux.
2.4.1 Membrane Se1ectiMty -->L-- ----- --- - - .- - - - - - . - - -
Membrane selectivity is defined as follows:
Where a - Membrane selectivity
Ci - Concentration of component which has higher volatility
Ci - Concentration of component which has lower volatility
2.4.2 Permeate Flux
Penneate flux (J) through the membrane is defined as the amount of permeate per
effective membrane surface area and tirne unit, specified in Kglh mZ. In most commerical
applications, penneate flux is the most important specification of membranes. A higher flux
improves the pmcess economics.
2.5 Configuration of Pervaporation Process
Maintaining a vapor pressure gradient across the membrane produces transport through
pervaporation membranes. The vapor pressure gradient used to produce a flow across a
pervaporation membrane can be generated in a number of ways. Figure 2.2-2.7 illustrates several
configurations of the pervaporation membrane separation processes. The first two are introduced
below only for simplicity.
Figure 2.2 shows a vacuum driven pervaporation. Vapor penneate is sucked out by using
a vacuum pump, maintaining a pressure gradient across the membrane. The use of a vacuum
pump speeds up the permeate transportation. Figure 2.3 illustrates a temperature gradient dnven
pervaporation without any vacuum pumping. The vapor permeate is driven out by condensation. -----. - - - e- & 8 - --- " -- --
This system is simple, but the permeate transport speed is lidted by the condensation efficiency.
In FCT, the standard configuration combined the vacuum dnven and temperature
gradient driven approaches. which takes the advantages of both systems. The heater improves the
membrane separation efficiency. The transportation of the vapor permeate is speeded up by
vacuum pumping.
Liquid
Figure 2.2 Vacuum Driven Pervaporation
Feed -
Liquid
Vapor
/////////////////L
Condenser
- R e t e n t a t e
- Permea te
F e e d C
H e a t e r
Figure 2.3 Temperature Gradient Driven Pervaporation
- R e t e n t a t e 7 / / , - / / / / / / / / / / / / / ' .
Permeo te
Liquid
Feed - - R e t e n t a t e
H e a t e r C o n d e n s e r
- liquid Vapor
Vapor
P e r m e a t e liquid
Nonecondensable c a r r i e gas
Figure 2.4 Carrier Gas Pervaporation
11
Liquid
Feed -{y- R e t e n t a t e
E v a p o r a t o r
?////////////////i Condenser
Vapo r
Permea t e Decan te r Iiquid
Imiscibie Liquid c a r r i e r
Figure 2.5 Pervaporation with a Condensable Carrier
Liquid
Feed 1-- Reten ta te
Vapor I I I
Permea t e Decan te r Liquid
Figure 2.6 Pervaporation with a Two-phase Pemeate and Paitial Recycle
Feed - R e t e n t a t e
I Vapor
Condenser I .t T l (y Condenser 1
at T 2 < T 1
First Second f r a c t i o n f r a c t i o n
Figure 2.7 Pervaporation with Fractional Condensation of the Permeate
2.6 Various Pervaporation Membrane Separation Modules - ..a - - -
The application of membrane techniques on industrial purposes requires the development
of high quality membrane modules. The earliest design were based on simple filtration
technology and consisted of flat sheets of membrane held in a type of filter press. These are
called plate-and-frame modules. Systerns containing a number of membrane tubes were
developed at about the same time. Later, with the advent of spiral-wound and hollow-fiber
modules, the application of membrane techniques was quickly extended to wider industrial arers.
Spiral-wound and hollow-fiber modules have dominated commercial market for r quite long
period. However, Recently, new generation of plate-and-frame pervaporation modules is
developed for some special applications. Refined design and new manufacturing techniques
make the plate-and-frame configuration more cornpetitive than it used to be. In this section, the
major pervapmtion module configurations are introduced.
2.6.1 Plate-and-Frame Modules
Plate-and-frame modules were among the earliest types of membrane systems and the
design has its ongins in the conventional filter-press and used stainless steel. Membrane feed
spacers and product spacers are layered together between two end plates, as illustrated in Figure
2.8. Many plate-and-frarne units developed for small-scale applications are expensive, cornpared
with their alternatives. The leaks caused by the many seals are another serious problem. Another
disadvantage is that the plate-and-frarne design gives the lowest surface arealunit-volume ratio.
Currently, the maximum area of membrane is 200 ft2 per ft3 of module.
Feed
S ~ a c e r
y \ Membrane \ support
Sp'cer p!ate
Figure28 APlate-and-Frame MembraneModule
2.6.2 Hollow-Fiber Modules
Hollow-fiber membrane modules are formed in iwo basic geometry. The first is the
closed-end design illustrated in Figure 2.9 and used, for example, by Monsanto in their gas
separation systems or Dupont in their reverse osmosis fiber systems. In this module, a loop of
fiber or a closed boundle is contained in a pressure vessel. The system is pressurized from the
shell side and penneates pass through the fiber wall and exits via the open fiber ends. This design
is easy to make and allows very large fiber membrane areas to be contained in an economical
system. Because the fiber wall must support a considerable hydrostatic pressure, these fiben
usually have a small diameter, on the order of 100 p n ID and 150-200 pm OD.
IFeed -
, Hollow Fiber Membranes
Permea -
Figure 2.9 Monsanto Prism Closed-end Module
The second type of hollow-fiber module is the flow through system illustrated in Figure
2.10. The fibers in this type of unit are open at both ends. In this system, the feed fluid can be
circulated on the inside of the fibers. The fibers often have larger diameters than the very fine
fibers used in closed loop systems. These so-called spaghetti fibers are used in ultrafiltration
pervaporation and in some low to medium pressure gas applications, with the feed king
circulated through the lumen of the fibers. Feed pressures are usually limited to less than 150
psig in this type of application.
1 Hol low Fiber Membranes
/ P e r n e a t e
Figure 2.10 A Capillary (spaghetti) Module
2.63 Spiral- Wound Modules
The design shown in Figure 2.11 is the first and simplest design. consisting of a
membrane envelope wound around a perforated central collection tube. The wound module is
--- -
placed inside a iubular pressure vesse1 and feed gas is circulated axially down the module across - - --- 4 --- - - - --
the membrane envelope, where it spiral~ t-owids the center and exits via the collection tube. The
major problem for this module is the large pressure loss, especially with high flux membranes.
To minimize the pressure loss encountered by the perrneate gas travelling towards the
central pipe, another spinl-wound module with multi-leaf warping mund the central collection
pipe is illustmted in Figure 2.12. Spiral-wound modules are the more widely used configuration
throughout the industry.
Figure 2.1 1 Spiral-wound Membrane Module
\ Men
env b r a n e etope
Apoce r
Membrane envetope
Figure 2.12 Four-leaf. Spiral-wound Module
2.6.4 Tubular Module
A tubular module is shown in Figure 2.13. Tubular modules are no used in only a few
ultrafiltrations where the benefit of resistance to membrane fouling because of good fluid
hydrodynarnics overcomes the problem of their high capital cost.
Feed
Hollow P e r n e a t e
thiri walled . porous
Permea t e
R e t e n t a t e
Figure 2.13 A Tubular Module
2.63 FCT Module a- & - - * - -
Over the past few yean, Fielding Chernical Technologies Inc. (Fa) has been
significantly investing in the research and development of membrane pervaporation separation
technologies. Their mission is to become a Canadian leader in chernical separation technologies.
Various membrane formulations, membrane manu fac turi ng and pervaporation separation
processes have been investigated in FCT. A PAN UF composite pervaporition membrane is
recognized as a preferable pervaporation membrane, especially for their special business,
currently focusing in the mixture of IPA and water. The composite membrane is made in FCT
and consists of PAN UF membrane (polyacrylntrile ultrafiltration membrane) and polyvinyl
alcohol active layer. Polyester unwoven is used as the backing material for the PAN UF
membrane.
A pervaporation membrane module i s designed to suit FCT's composite membrane.
Figure 2.14 shows the layout of the module. The module comprises a center, axial penneate
removal tube having a wall with at least one inlet opening. At least one separator element is
mounted on the penneate removal tube adjacent to the inlet opening. The separate element
includes a permeate transport plate having a transverse opning for the passage of the permeate
removal tube through the pemate transport plate. The penneate transport plate also defines
fiuid passages disposed radialIy relative to the permeate removal tube and communicating with
the inlet opening. A pervaporation membrane envelops the permeate transport plate. Annular
sealing rings are located concentrically about the permeate removal tube in engagement with the
pervaporation membrane, so that fluid has to pass through the pervaporation membrane to enter
the permeate removal tube inlet opening. Two plastic mesh spacers are layered on both sides of
the permeate removal tube and between membrane and the plate.
The separator module is located in a solution tank for containing fluid to be separated. ---..A - ---
The separator module has a central axial penneate nmoval tube extending from the tank.
Vacuum is applied to the penneate removal tube to extract vapor permeate from the module.
Figure 2.14 FCT Pervaporation Module Layout
Cornparison of Pewaporation Modules
There are many different pervaporation modules available in the market. It is necessary to
undentand the differences between them. A pervaporation module design for a particular
membrane separation is a tra&-off of a number of factors, such as cost, application environment
conditions and membrane performance. etc. The actual sales price of a membrane module varies
widel y with the application di fference. Generall y. high-pressure systems are more ex pensive
than low-pressure or vacuum ones. The second major factor determining module selection is
resistance to fouling. Membrane fouling is a particularly important problem in liquid separations
such as reverse osmosis and ultrafiltration. In gas separation application fouling is more easily
19
controlled. The third factor i s the ease with which various membrane materials can be fabncated -----A- -1- "A- L -A- - -
into a particular module design. ~ n y membrane can be fitted into a plate-and-frame module, for
example, but relatively few membrane matenals can be fabncated into fine fibers or capillary
fibers. Finally, it is necessary to consider the suitability of the module for high-pressure
operation and the relative magnitude of pressure drops on the feed and permeate sides.
The ciifference and sirni larity between different pervaporation modules are summarized in
Table 2.1.
Table 2.1 Cornparison of Pervaporation Modules
Manufacturing Cosî ($lmz)
Packing Density
Resistance to fouling
Parasitic pressure dmps
Suitable for high pressure
operation?
Limitecl to specific types of
membrane?
Hollow Fine
Fibers
5-20
high
Very poor
High
Yes
yes
CapUlary
Fibers
20- 100
modera te
Moderate
No
Y=
Plate-and-
Frame
100-300
low
8 d
moderate
Yes
no
Spiral-Wound
30-100
moderate
moderate
moderate
No
No
Tu bular
50-200
low
ver^ good
low
Can be done
with difficulty
no
-7 *
CHAPTER 3 OVERVIEW OF DESIGN THEORIES . - 2- - - + - - L A - - L - L -
3.1 Introduction
"Design" is taken to mean al1 the process of conception, invention, visualization,
calculation, marshalling, refinement, and specification of details which determines the form of
engineering products. Design generally begins with a need. This need may be met already by
existing designs, such as in the pervaporation module. In such a case the designer hopes to meet
the need better.
A design process may Vary with different applications, designer's preferences and
experiences. According to Michael French [4], r normal design process is shown as a block
diagram in Figure 3.1.
Analysis of Pmblem h 1 Statement of Roblem 1
1 Conceptual Design 1
1 Selected Schemes 1
I Mailing I
Figure 3.1 Block Diagram of Design Process
DMng the 1s t decades, design methoàologies were studied by a number of researchers - .-
and significant progresses had been made. ~ l t h o u ~ h design methodology was widely accepted at
the university level, design methods were seldom applied in the industry. Two nasons for this
paradox could be identified:
i) Designers were unable to introduce methods because of crowded schedules and
cost pressures. and
ii) Industrial engineers were often skeptical toward methodology. claiming the
methodology were too abstract and theoretical for their practical design work.
Recently, the author got a chance to practice modem design theories and methodologies
in the design and development of a pervaporation membrane separation module for the chernical
industry. Decisions were made to select the appropnate design methodology. A review of design
theories and methodologies, therefore, is perfonned in this Chapter.
3.2 Overview of Design Methodologies
Dnving by continuous enhancement of design process to create better design solutions.
there are two major developments in prescnptive design research. The first stream emphasized
on the methodologies of design deployment while the second stream evaluated the design
solution by design pnnciples.
For the first stream, the method of robust design (G. Taguchi, 1986-1989) suggests that a
go& design is the one that provides a robust solution for the functional objectives. It can be
accomplished through a design process constituted from three phases: the conceptual (system)
design. parameter design, and tolerance design phase. For the second Stream, Hubka (1980), Phal
and Beitz (1988) first broaden the design pmblem through abstraction of requirement
22
specifications by transfomiing quantitative into qualitative data, thus fomlating the problem in -a-= -,L- L - - - -- - -- - - - - - - - - -- . "solution neutral terms" - the problem is specified independent of solution, so that different
possible solutions can be evolved with bias towards a particular solution.
Pahl and Beitz have addressed the many issues of conceptual design in general. However,
they have not investigated: (a) relationships between solution principles (aspects such as
coupling and non-coupling), (b) relationships between different domains of design. suc h as
product design and process design, (c) means to mathematically express and analyze the above
relationships, (d) means to eliminate or reduce dependence on weight factors for evaluation of
competing designs, which tends to make the evaluation subjective.
Hubka, Eder and Andreasen [28, 291 viewed a design problem as formulation of a
technical system (TS) which has inputIoutput relationships witli humans (Hu) and active
environment (Aenv). They considend a technical process (TS) converting input operand state
(0d') into output operand state (0d2) through effects from Hu. Ts and Aenv. Although the
spbolic mode1 of design process were different. closer study revealed that the underlying
procedure of conceptual design was almost similar to the one put forward by Pahl and Beitz. The
procedure again involved abstraction from design speci fication, establishment of organ stmc tures
(term used for function carriers) in the form of morphological matnx, combination of function
carriers to form concept alternatives (tenn used for concept voriants), nrd finally ranking of the
concept based on evaluation criteria using point scores.
An important milestone in design theory is the introduction of axiomatic theory
developed by Nam P. Suh 116). Suh assumed that a design problem could be stated in terms of
functional requirements (FRs) and that the design solution was defined in tenns of design
parameters @Ps). He addressed that a design was a mapping process from functional domain to
physical domain. Suh proposed that a design concept could be evaluated by the use of the
independent cnteria and minimum information cnteria. Suh's axiomatic theory contains two = --e------ -
axioms, seven coroll&es and seven theo-, which will be introduced in the section 3.3 in
detail.
Design for X (DFX) is a design methodology or concept receiving widespread attention
h m mearchers and engineers. DFX refen to methods and twls for design specific objectives
('X's), such as manufacturing, assembly, performance, cost, automation, etc. When a design
process takes multi-objectives into consideration, concurrent engineering (CE) concept is
implemented. An important contributor to the DFX method is Boothroyd 1181 who studied
aspects of product redesign to make them more amenable to assemble. His work is very practical
and useful, as it provides direct suggestions for modifications in a product's design, for easier
assembly. DFX method is to be introduced in Chapter 3.4.
Axiomatic Design Theory
Axiomatic design theory was introduced by Suh, Bell. Nakazawa, Gossard et. al.. Dunng
the past fifteen years, axiornatic design method considea design as a mapping procedure from
one domain to another.
In the process of pmduct design, a designer first defines a set of functional requirements
to fulfill the customer's needs. Then the development of a physical embodiment in physical
domain is begun. The axiomatic design defines design as the creation of synthesized solutions
that satisfy perceived needs through the mapping between functional requirements (FRs) and
design parameters (DPs). FRs are defined in the functional domain in order to satisfy the original
needs given in the customer domain, while DPs are created in the physical domain to satisfy the
FRs. In the case of process design, the pmcess variables (PSs) are designed in the process
domain to satisfy the defined DPs. The mapping process is shown in Figure 3.2.
24
Figure 3.2 Design Mapping Process Illustration
33.1 Design Axiom
There are two design axioms that govern good design as given below :
Axiom 1 The independence Axiom
Maintain the independence of FRs.
Axiom 2 The in formation Axiom
Minimize the information content of the design.
Axiom 1 states that the mapping pmcess between the FRs and the DPs must be such that
a perturbation in a paticular DP must affect only its referent FR so as to create good design.
Axiom 2 states the criterion for selecting the optimum design solution from among those
that satisfy Axiom 1.
333 Design Equation
The mapping process may be characterized mathematicaily as follows:
(FR)=[Al{DP}
W hete {FR} is the functional requirement vector,
25
{FR) = [FR, F& .. . FR, )I -a----*- - - -
{ DP ) is the design parameter vector:
{DP} = [DP, DP2 ... DP, 1-1 [A] is the design rnatrix:
a11 ail * * * Qin
a,, a,, ... a,
3.33 Uocoupled Design
A design is called uncouple design if the design matrix [A] is diagonal. The condition for
uncouple design is
a k k f 0
ai, = O when i # j
That is
a,, O O O
O a , O O . O
O O O a ,
This matnx equation can be wntten as
FRIWI~DPI
FR2=22DP2
A uncouple design satisfies Independence Axiom 1, because the independence of FRs is --a- : -.%%,- & .-- - -+ . -- - - - - - - - - -
assured when e&h DP is changed. Each FR can be changed independently without affecting any
other FRs by varying other DPs.
33.4 Decoupled Design
A design is called decoupled design if the design matrix is triangular.
The condition for decoupled design is
a k k f 0
aij = O when i > j or
aij = O when i < j
That is
This matnx equation can be written as
F R I = ~ I iDPr
FR2=a 1 2DPi+a2rDPz
FR*, DPl +adDP2+. . . . . . +a,,DP,
In this case the independence of the FRs can be assured if we adjust the DPs in a
particular order. For example, if DPi is determined, the FR2 is only affected by DP2.
33.5 Coupled Design LA-- -- -J - - -
The converse of an uncoupled design is the coupled design whose design matrix consists
of mostly nonzero elements. The relationship between FRs and DPs is
F R l a l iDPi+ a ,2DP2+. . . . . .+ a ,.DPn
FR2= a DP + u =DP2+. . . . . . + a 2nDP.
A Change in FRs cannot be accomplished by simply changing DP,, since this will also
affect FR2 and FRi. Such r design clearly violates Axiom 1.
3.3.6 Constrains
Constrains in the context of axiomatic design represent the limits on an acceptable
solution. A constrain does not have to be independent of other constrains and FRs; Another
feature is that they do not normal1 y have tolerances associated wi th them.
33.7 Task Decomposition and Hierarchy
Task decomposition has been recognized as an important issue in a design process by
many researchers. However, Suh was the first to relate the issue of task decomposition with
hierarchical decomposition.
In Suh's axiomatic design theory, FRs and DPs can be identified by the decomposition of
design process, which can be implemented at different levels called hierarchy. FRs at the ith
28
-- - "
level can not be decomposed into the next hierarchy without completing the mapping process ---x -- . L
bèiwe& thè funciion domain and t h e pfiysical-domGn at the ith kvel of the FR
Decomposition requires mapping between one level of hierarchy of a domain, to a
different level of hierarchy of another domain, calied Zig-zagging.
A hierarchical tree structure, decomposition and zigzag mapping is illustrated in Figure
Suh's research on the relationship between task decomposition and hierarchy of FRs and
DPs nsulted in two very important conclusions as given below:
1) a good designer can identify the most important FRs at each level of the functional tree
by eliminating secondary factors from consideration.
2) Less-able designers often try to consider al1 the F R s of every level simultaneousl y, rather
than making use of the hierarchical nature of FRs and DPs. Consequently, the design
pmcess become too complex to manage.
Figure 3.3 Hierarchical Structure, Task Decomposition and Zigzag Mapping
-- 7
33.8 Corollaries -a----= -
' ~ l i h o u ~ h axiornatic theory consists of two axioms, many corollaries can be derived as a
direct consequence of the original axioms. There are seven corollary concluded in Suh's
publications. These corollaries may be more useful in making specific design decisions, since
they cm be applied to actual situations more readily than the original axioms. They rnay be
called design rules which any designer should comply with at every signal moment dunng the
entire design processes.
Comllary 1 (Decoupling of Coupled Design)
Decouple or separate parts or aspects of a solution if FRs are coupled or become
interdependent in the designs proposed.
Corollary 2 (Minimization of FRs)
Minimize the number of FRs and constrains.
Corollary 3 (Integration of Ph ysical Parts)
Integrate design features in a design physical part if FRs can be independently
satisfied in the proposed solution.
Corollary 4 (Use of S tandardization)
Use Standardized or interchangeable parts if the use of these parts is consistent
with the FRs and constnins.
Corollary 5 (Use of Symmetry)
Use syrnmetrical andor arrangements if they are consistent with FRs and
constrains.
Corollary 6 (Largest Tolerance)
Specify the largest allowable tolerance in stating FRs.
Corollary 7 (Uncoupled Design with Less Information)
30
Seek an uncoupled design that requires less information than coupled designs in
---FA..- L=.- - satisfying-a set of FRs.
Suh concluded four theorems addressing to the consequences of the number of DPs king
less, more, or equal to the number of FRs. The implication of these theorems is that al1 attempts
should be made to match the number of DPs and FRs.
1) Theorem 1 (Coupling Due to Insufficient Number of DPs)
When the number of DPs is less than the number of FRs, either a coupled design
results or the FRs cannot be satisfïed.
2) Theorem 2 (Decoupling of Coupled Design)
When a design is coupled due to the greater number of FRs than DPs (i.e., mx) , it
may be decoupled by the addition of new DPs, so as to make the number of FRs and
DPs equal to each other, if a subset of the design matrix containing n x n elements
constitutes a triangular.
3) Theorem 3 (Redundant Design)
When there are more DPs than FRs, the design is either a redundant design or a
coupted destgn.
4) Theorem 4 (Ideal Design)
In an ideal design, the number of DPs is equal to the number of FRs.
5) Theorem 5 (Need for New Design)
When a given set of FRs is changed by the addition of a new FR, or substitution of
one of the FRs with a new one, or by selection of a completely different set of FRs,
the design solution given by the new set of FRs. Consequently, a new design solution
must besough,
6) Theorem 6 (Path Independence of Uncoupled Design)
The information content of an uncoupled design is independent of the sequence by
which the DPs are changed to satisfy the given set of FRs.
7) Theorem 7 (Path Dependence of Coupled and Uncoupled Design)
The information contents of a coupled and decoupled design depends on the sequence
by which the DPs me changed and on the specific paths of the changes of these DPs.
3.4 Design for X
Another design methodology is called Design for X (DFX). In this section, DFX concept
is presented; this method is used for the detailed design of the pervaporation module.
3.4.1 Definition of DFX
DFX is a philosophy and a methodology that focus on a limited number of vital elements
at a time. This allows nvailable resources to be put into the best use. Application of a certain
DFX approach means adapting the product development process in order to improve the product
with a certain focus and target. The definition of DIX can be given as making decisions in
product development related to "X'. In DFX methodology, product design can be structured
such that "X" is explicitly emphasized. For exarnple, design for assembiy @FA) focuses on the
business process of "Assembly" which is part of the life cycle "Production", meaning the design
of the pmduct for the ease of the assembl y.
Since the publication of "Engineering Design: A systematic approach" by Matousek in
1957, the DFX family has been expanded significantly. Currently, the X stands for any aim and
32
-* appücation of a pmduct. The different DFXs shown in Table 3.1 are presented in various
. . - --- . - - - - - - p&katipns :
Table 3.1 Overview of Design for X
Design for Assembly
Design for Manufacturing
Design for Safety
Design for Cost
Design for Quality
Design for Reliability
Design for Profits
Design for Dimension Control
Design for Automation
Design for Inspectability
Design for Human Factors
Design for Materials
Design for Marketability
Design for EMC
Design for Environment
Design for Ease of Rec ycling
Design for Maintainability
Design for Modularity
Design for Life Cycle
Design for Service
Principles of DFX
It is difficult to precisely define the approach and procedure of DFX becruse of the
Design for Distn bution
Design for Production
Design for Flexibility
Design for Efficiençy
variety of element "X" in DFX. Different "X" results in different method and procedure. Even
though different application uses the same target 'X', the method and procedure may differ.
However, to demonstrate the generd concept of DFX, the author summmizes the DFX
mechanisrn here by using procedures and tools.
Design for Risk
Design for Technical Merit
3.4.2.1 Procedures of DFX
--- -- - .- Eor of tht p&ct design cases,the DFX application consists of four steps of design
and analysis summarized below:
Stepl : Clari fy product design specification
The first step of DFX is to make sure that the right design requirements are derived from
the customer/rnarket needs. At this stage, the "X" requirements are of paramount importance and
thenfore should be specified explicitly in the analysis matrix. This creates the appropriate "mind
set" for the designers right from the start and gives new dimensions to creative thinking. The
author experienced great freedom in creativity in this step.
Step 2: Select technical solutions
The second step of DFX is to establish technical solutions that meet product design
specifications. First, when the Step 1 is completed, many sub-functions are fomed. The
functional integration may be dealt with later. Next, it is beneficial to distinguish common sub-
functions required by al1 customers from optional sub-functions specific to customers. This helps
establishing product variants later. Finally, several technical solutions may surface and selections
have to be made.
Step 3: Generate concepts
The objective of this step is to establish product concepts based on "X" to achieve sub-
functions selected at the previous step. At this stage, a number of design concepts may be
generated. A selection is necessary to narrow down the number. There are different methods for
this selection. For exarnple, The Pugh's selection matrix is used by some of the researchers for
this purpose. h this method, comparing satisfactory achievement of the manufacturing goals
-- - - - - - - , d e s A u u t the selection of technical soiutions,
Step 4: Evaluate concepts
The design concepts generating at the proceeding step are evaluated using certain
evaluation criteria which is dependent on the specified "X' and varies with different " X . For
example, the minimum part-count cnteria is applied to Bwthroyd-Dewhurst Design for
Assembly @FA). In Design for Modularity, Olesen and Gunnar Erixon uses the so-called
universal virtues-Cost, Time, Quality, Efficienc y, Flexibi lity, Risk and Environment. Such an
evaluation is important to assess the pmposed changes and to compare with the earlier situation.
During a development prwess there are many cross-roads to pass and choices have to be made.
3.4.2.2 Tools of DFX
DFX tools are usually the computer software packages which con offer the designer the
analysis means when using DFX methodology in the product design. It is reported that some of
the DFX tools are successful. such as DFA tool based on Bwthroyd-Dewhurst methods. The
main DFX functionality accomplished by DFX tools and their human usen are summarized in
Table 3.2. DFX twls usually provide the first four functions and the second five functions are
carried out mainly by human usea although a few research system can achieve them to some
extent.
3 Measure performance. I I
1
2
Gather and present facts about products and processes.
Clarify and analyze relationships between products and processes.
1 5 1 Diagnose why an area is strong or weak.
4
6 Rovide redesign advice on how a design can be improved. I I 1
Highlight strengths and weaknesses and compare alternatives.
9 Allow iteration to take place. 1 1
7
8
3.43 Benefits from DFX
Predict what-if effects.
Canyoutimprovements.
DFX methodology has ken successfully used in many cases. The benefits can be
grouped into three categories.
The first category is directly related to the cornpetitiveness measure, including improved
quality, compressed cycle time. redwed life-c ycle cosis, increased flexibi lity and productivity.
more satisfied customers, safe workplace, etc.
The second category of benefits includes improved and rationalized decisions in
designing produc ts, process and resources.
The third is its far-reaching effect on operational efficiency in product development, such
as better communication and closer cooperation, concurrence and transparency and better job
hang-over.
36
CHAPTER 4 CONCEPTUAL DESIGN AND ANALYSIS
4.1 Introduction
The development of a pervaporation membrane module is a prerequisite for the technique
realization of a membrane prevaporation process. A pervaporation membrane module actually is
a mechanical apparatus in which a working environment is created such that the membrane
pervaporation performance is maximized. This apparatus is also designed such that the
pervaporation separation process can be operated continuousl y.
A conceptual design usually is the first phase of a product design. Generally. conceptual
design may be performed within every design stage, from the initial design concept to the final
detailed design. However, in this thesis, the conceptual design is on1 y used to determine the basic
configuration of the pervaporation module. This process ensures that the entire design process
will be in the right direction. It is in the conceptual design phase that the design particularly
requires the guidance and evaluation of design theories. Multiple modem design theories have
been compared and their suitability for the conceptual design of the pervaporation module has
been investigated as well. The author especially prefen the use of the axiomatic design theory in
the conceptual design phase. This preference is based on the simple fact that the axiomatic theory
can meet the requirements of conceptual design. In addition, in the conceptual design phase.
axiomatic design theory has better operability than it does in the detailed design phase.
The conceptual design of a pervaporation module starts from the identification of the
effect of the pervaporation process conditions. Then a pervaporation module should be designed
such that the negative effects on the membrane performance are suppressed, whereas the positive
effects are enhanced. Comprehensive experimental researches and theoretical analysis, regarâing
to the pervaporation conditions, have been accomplished by numerous researches. Continuous
expioratim air thea effccts is beynia the-scupc-of this thesis; This thesis witt focus on the
mechanical design of the pervaporation module, not on the membrane discussion. However, the
effect of the pervaporation process conditions will be summarized as the starting of the
conceptual design process.
In this Chopter, the effect of the pervaporation process conditions is described. Then the
conceptual design process of the pervaporation module i s presented. Based on Suh's axiomatic
theory, four levels of the hierarchy are generated for the decomposition of the functional
requirements and for the mapping from the functional domain to the physical domain. Then the
independence of the functional requirements is examined by analyzing the design equations.
Finally, the conceptual design is summarized.
Effixt of the Pervaporation Process Conditions
The capacity of a pervaporation module pnmarily depends on the membrane
characteristics which are direct1 y affected by the pervaporation process conditions. In this
section, the major effects of the pervaporation process conditions are summarized as follows:
a) Feed concentration
A change in feed concentration directly affects the sorption behavior at the liquid-
membrane interface and the diffusion of the component in the membrane. Sorption and diffusion
of the feed components in membrane characterize the membrane separation properties.
Therefom, the permeation characteristics are obviously dependent on the feed concentration.
b) Feed Pressure
If the membrane is more than 20 p n and the feed pnssure does not exceed 10 bars,
pervaporation- tmsportis not signifie-antly-affeted by feeepressure (upstream pressure) since
the pmcess is mainly govemed by the diffusion of the penetrants through the membrane.
However. results from experiments also showed that for thinner films the upstream pressure
sometime contributed to the pervaporation transport. Some experiments also concluded that for
higher permeate pressure, the feed pressure influences the pervaporation characteristics when the
pressure approaches the saturation pressure of the permeate side.
Usually, the thickness of industrial membranes is less than 20 pn; the feed pressure is
less than 10 bars. Thus the effect of feed pressure is ignored in pervaporation membrane
sepmtion industries. In this project, the author also assumes the effect of feed pressure is
negligi ble. There is no feed pressure applied in the pervaporation module.
c) Permeate Pressure
Permeate pressure strongly influences the pervaporation characteristics. The maximum
driving force can be obtained at zero pressure in permeate chamber. Increases of pemeate
pressure leads to decreases of the pervaporation flux rate. The selectivity of the membrane,
however, may increase or decrease when the permeate pressure is nised.
d) Porosity of the membrane support
The membrane characteristics are significantly effected by the local pressure at the
surface of the active layer of a membrane. For a composite membrane with backing polymer
support, the polymer support layer influences the membrane separation characteristics through its
pomsity, A pressure drop over the porous support will degrade the activity gradients of the
components in the active membrane. In a pervaporation module design. the membrane structure,
39
- such as the backing polymer material of a composite membrane, is predetermined. Investigating
--A- - - the-effect of porosity of a membrane support c m provick some refermce to the reduction of flow
restriction in the pervaporation module to be addressed next.
e) Fiuidrestrictioninpenneateremovalpassageways
The local pressure at the down Stream surface of a PAN UF membrane is determined by
the flow restriction over both support polymer and permeate removal passageway, if the
permeate flow and vacuum pump setting arc fixed. Once the composite membrane is selected,
the flow restriction over the polymer support is determined. The membrane user has no control
on the pol ymer support. Hence the restriction of passageway becomes the only parameters which
can be adjusted by the pervaporation module design. That is to Say, the passageways on permeate
side must be large and smwth enough to handle specific volume without significant pressure
loss.
f ) Temperature
Pervaporation characteristics in tenns of flux are dependent on the temperature since
solubility and diffusivity of the feed mixture component in a polymer membrane generally
depend on the operating temperature. However, the temperature range is limited by several
factors, such as sealing matenals. In FCT, 60-80 O C is the temperature range used by the
membrane separation devices. Provision should be made to increase the temperature to
approximately 100 O C such that the membrane potential can be used to its maximum limit.
The temperature also has effect on the selectivity of the membrane. In most cases, a smal l
decrease of selectivity is found at increasing temperature.
g) Concentration Polarization
- ~cerifl.at1on pokïzation tatCés pke when mass transfer occurs from a more
concentrated boundary solution to a less concentrated bulk. In pervaporation. a binary liquid
mixture is permeating through a semi-permeable membrane with different pemeation rates. This
nsults in an increase of the less penneable component in the boundary layer near the membrane
surface, and a concentration of the less permeable component in this region will develop. For al1
separation membranes, including MF, üF and RO membrane, concentration polarization can
result in the decrease of flux.
4.3 Conceptual Design Process of the Pervaporation Module
Once the effect of the pervaporation process conditions is understood, the conceptual
design can be started. The main configuration of the pervaporation module are generated in the
conceptual design phase. Conceptual design is considered to be the most important stage in
product design. A conceptual design should refiect the designer's phi losoph y; the fundamental
features should be embodied at a system level by a conceptual design. A good conceptual design
must be a trade-off of various design requirements. Any design defects in conceptual design will
be a catastrophic to the final product; a system design fai lure cannot be full y compensated by an y
future design work. Therefore, more attention has to be paid to the concepturl design than any
other design phases in this thesis.
Traditionally, conceptual design is perfomed based on the experience and personal
attributes of the designer. This approach is no longer appropriate for the new generation of the
pervaporation modules. It would be very significant to design a product under the guidance of
design theones. As mentioned before, Suh's axiomatic design theory i s one of the most
systematic design methodologies. The author is especially amenable to the application of
- aiiomatic theoiy to the conceptual dësign.
Before starting the conceptual design of the pervaporation module, one thing we need to
think about is how far and how much details the conceptual design should go. The answer differs
from project to project. We decided to stop the conceptual design process as soon as the main
sub-assemblies are fomulated. leaving the remaining processes to the detailed design phase.
Based on this design cnterion, as shown in Figure 4.1 and Figure 4.2, four levels of the design
hierarchy are constructed in both the functional domain and the physical domain, respectively.
The design concept of the pervaporation module is generated by the mapping and Zig-Zagging
between the two domains. The design independence is evaluated by using Suh's axiomatic
theory to optimize the design solutions.
The design constraint is defined as follows: the new module should buiid on and extend
the design concepts pmven on the eaclier version qfthe FCT module as much as possible.
43.1 Lever O-Task Definition and Implementation - - --- - - -
Level O of the FRs hierarchy defines the general task and implementation of a
pervaporation separation module. Market analysis shows that in the next decades, the
pervaporation separation systems with low cost and high performance will have very g d
commercial prospects, especially in small to medium site chemical Company. As one of the
important subsystems, a new pervaporation module has been identified as important for this
highly profitable market. In addition, it is also urgent for FCT to have a pervaporation separation
system for their chemical recycle business. Therefore, the FR at level O is to separation binary
dissolved solvents (the FR) by using a pervaporation membrane separation module (the DP).
43.2 Level 1-General Functions and Physical Solutions
Definition of FRs
There are two FRs defined on this level for the pervaporation module that has k e n
defined on level O. Fint, a penneate generator is required for the chemical pervaporation
separation of binary dissolved solvents. Second, the vapor pemeate generated by pervaporation
should be removed continuously such that the pervaporation pmcess can take place continuously.
These two FRs may be written as:
FRi = Perrneate Generation
FR2 = Penneate Removing
The functional hierarchy of the pervaporation separation device is shown on level 1 in
Figure 4.1.
7
a Choice of DPs ------ L --- - L - A - - -
It is clear that one way of generating permeate (the FRi) is to use a pervaporation
membrane separator/cell. There are a variety of configurations available for a membrane cell. As
mentioned before, the new pervaporation module must be a generic product of the earlier FCT
module on which the pervaporation performance of ''the disk membrane cell" has been validated.
Furthemore, the structure suitability for the FCT environment has dso been demonstrated.
Although there are some weaknesses on the earlier disk membrane cell, a disk type of membrane
ceIl is selected as the physical implementation of generating permeate with minimum
engineering risk. The basic structure of a "disk membrane cell" is a disk chamber enveloped by
the pervaporation membranes. The feed mixture solvent and the permeate vapor are separated by
the membrane.
To conceive a physical solution for the pemeote removing (the FR2), a fluid transport
mechanism is required. The author intentionally lumps this permeate transport mechanism into a
single DP2 in ternis of permeate removal structures, since the details will be discussed at the
subsequent levels.
Hence for this design solution, the DPs may be written as:
DPi = Disk Membrane Cells
DPz = Permeate Removal Structures
The physical solution hierarchy of the pervaporation separation device is shown on the
first level in Figure 4.2.
a Design Equation and Analysis
-- - Having defined FRs ami DPs, the next task calls for the anal ysis of the proposed design
----A- --- --- A - -
solution to see whether it violates the l f i s o f nature and fie design axioms. FRi is satisfied by
DPi and DPl only, whereas FR2 is satisfied by DPl and DP2 since membrane cell is part of the
permeate removal structures. The independence is assured if we adjust the DPs in a particular
order. For example, the membrane cell design can be done first, then the remaining permeate
removal structure becomes the only effect on the permeate removing. Thus Axiom 1 is satisfied.
The design is decoupled. This relation establishes a solid base for the future design processes.
The design equation is shown as followr:
4.33 Level2-Major Funetions and Physical Solutions
Decomposition requires mapping between the first level of hierarchy of the physical
domain, to the second level of hierarchy of the functional domain, called Zig-zagging. In this
section, the mapping between FRs and DPs on the second level of hierarchy is perfomed; and
the major configurations can be defined. The mapping will be executed on the two design
parameters, the "disk membrane cell" and the "penneate removal structure".
4.3.3.1 Conceptual Design of the Permeate Generator
Definition of FRs
In pervaporation separation technique, it is essential to select an appropriate
pervaporation membrane (the FRi); the pervaporation efficiency is mainly detemined by the
membrane pervaporation characteristics. In addition, a reliable membrane is the basis of reliable
pervaporation module.
47
. .-
The production capacity of a pervaporation membrane is proportional to the active -:- - - - - 2 - - - - u r - - -- - - L & & . - -
membrane area if the flux rate of the membrane is cktermined. It is very important to make use
of every inch of the membrane material, maximizing the effective membrane area (the FR2).
Pervaporation membranes are flexible sheet materials. However, the membrane materials
are weak, easy to be darnaged. Therefore, membranes should be reinforced by using certain
membrane supports (the FR3).
After the diffusion of the penetrants thmugh the uneven-swollen perm-selective barrier,
the pemeate desorption is taking place at the downstream surface of the membrane fimi, where
the pemeate will be stored for a short pend, then is sucked out by a vacuum pump via the
penneate removal passageways. Therefore, a permeate storage between the membrane and its
support is required (the m). Sealing is another important FR in a pervaporation module. Seals are required on al1 of
the joints. It is very challenging. but essential for a pervaporation module to have good sealing.
On the disk membrane cell, the seals are used to join the membrane and its support together
along the inner and outer peripherys (the FR5).
Moreover, concentration polarization may degrade the capability of membrane
separation. The minimization of the concentration polarization is considered to be another
importent functional requirement f the FRs).
The functional hierarchy of the permeate generator consists of six sub FRs as given
FRi = Use high performance membrane
FR2 = Maximize the active membrane area
FR3 = Support membrane
= Create pemeate storage
FRs = Seals dong the inner and outer peripherys - - -. --
= Minimize the concenkation polarization
Choice of DPs
Then ore various industrial pervaporation membranes available in the market. FCT has
king involved in the application research of high performance pervaporation membranes for
many years. The objective is to meet the requirement of their chemical recycling business and
the potential chemical separation market. Comprehensive investigation shows that a FCT in-
house PAN üF pervaporation composite membrane (FCT membrane later) has satisfactory
pervaporation separation performance (the Dei). The FCT membrane consists of a PAN UF
membrane (polyacrylnitrile ultrafiltration membrane), a polyvinyl alchohol activc layer and a
polyester unwoven fabric. The polyester unwoven fabric is used as the backing material for the
PAN UF membrane. Mechanical property tests show that the PAN UF membrane is a semi-
flexible material. Folding is not allowed for this type of membrane since it may result in the
cracking of the PAN UF membrane layer. Any tiny damage on the membrane layer may result in
the leakage of the feed mixture to the permeate chamber. Therefore, plate-and-fnme design is
considered to be a suitable membrane support mechanism for the FCT membrane, since plate-
and-frame module provides the membrane with the lewt stress.
The membrane is finnly seated on the membrane support when the vacuum is applied in
the permeate chamber. To isolate the feed mixture from the permeate, the inner and outer
periphenes of the membrane must be completely sealed; the inner periphery of the disk
membrane ce11 is also used to interface with other support components. The smaller the seals and
the support components, the larger the active membrane area is. Therefore, to satisfy the FR2, it
is necessary to minirnize the size of the membrane-contacted components, including the seals .+__ .i. _ & . - - - - . -
and support components (the DP2).
There are various approaches to support the membranes, such as frame structures or solid
disks. The earlier FCT module uses the steel disk plates because disk type supports are light.
Therefore, membrane support disks (the DP3) are also selected to support the membrane in the
new module. The main reason for this selection is that the new module should use the proven
design concepts as much as possible. The detailed structure of a membrane support disk will be
addressed in the further decomposition of the membrane supporting section.
The mapping between the permeate storage (the FR,+) to its physical solution leds to the
use of membrane spacen. A spacer between the membrane and the disk support (the DPS) can
create a chamber for storing the permeate. This chamber, as a passageway, can also functionally
cornmunicate with the main penneate removal structure.
In the earlier FCT module, glue was used to seal the outer periphery of the disk. There
are several problems associated with the glue seal. First, it is not easy to control the width of the
glue seals. In the 11 inches disk, the m a of the glue seals accounted for approximately 17% of
the total disk area, which causes significant loss of the active membrane area. Next, the
membrane layer on the outer periphery may be gradually peeled off under the flush of the solvent
flow. Furthermott, the worlcing temperature of the cunmt glue is limited to arwnd 60-70 O C . It
is expected to use the new module at a temperature range of 60-80 O C , with a potential of
increase up to 100 OC, since higher temperature may likely improve the membrane
pervaporation. Moreover, the membrane ce11 wi th glue seals cannot be rec yc led. There fore, new
sealing methods, so-called composite seal (the DPS) is proposed. The basic concept is to use the
protection straps and some solid sealing materials to seal the disk peripherys, such as aluminum
strap, mbber membranes or adhesive tapes. This design concept focuses on resolving the sealing
problems existed on the earlier module. The details will be described in the next level of the
Concentration polarization effect can be minimized by generating an environment of flow
flush (the DP5). How to create this environment will be addressed in the next level of the
hierarchy as well.
Therefore, the DPs for the pervaporation separation may be written as:
DPl = PAN UF pervaporation composite membrane
DPÎ = Minimize the size of the membrane periphery components
DP3 = Disk membrane support
DP4 = Spacer between membrane and disk
DP5 = Composite seals
DP6 = Flush flow environment
a Design Equation and Analysis
The proposed design solution can be analyzed by exarnining the design equation as
follows, to see whether it violates the laws of nature and the design axioms.
The design matrix has zero value in most of the elements, indicating that the
independence has k e n successfully implemented in most of the FRs. Nevertheless, it is also
51
-,-- -
noticed that the design matrix is almost triangular, with exception of the none zero element as3. --.- --- -- - - - - 2 %
This couple&iationshipcan be clearly illustrated in the following equations:
FR3 = a33DP3+~3flP4+a.&P5
F& = a@P4
FRs = a53DP3+as5DP5
Where the FF4 can be satisfied by the DP4, and DP4 only. However, both DP3 and DP5
affect the FR3 and FR5.
This coupled design is caused by the closed relationship between the membrane, the
membrane support and the seal. The second level of the hierarchy shows that the membrane and
its support are physically connected together by the sealing method. Thus the membrane support
design has to be taken into account in the design of the sealing method.
According to Suh's axiomatic theory, a coupled design is not a good design. However,
the author's experience in the practice of various product designs shows that design decoupling
is very difficult in most of the design practices; actually, completely uncoupled designs have
never been seen by the author in large industrial projects. This does not mean that the axiomatic
theory is incorrect. On contrary, the independence axiom is one of most successful design twls
useû by the author. A main convibution of the independence axiom, in my view, is io define a
design target. If Suh's independence axiom is kept in mind, an acceptable design with the least
problerns can be created, even though it might not be uncoupled or decoupled.
The coupled design on level 2 is the best that we can achieve so f a . From the system
design's point of view, this slightly coupled design is not only acceptable, but also necessary
sometimes. This is called "strategic couple concept" herein, which means to use some coupled
designs by strategy in the selected situations? so as to achieve optimal design solutions on a
m-.-
system level. The application of the "strategic couple concept" can often result in some -u----LL & - --- = z- * --- -
innovative design s o u o G , such as "one Stone, two bi&. TheUstrategic couple concept" will
be fûrther demonstrated on the third level of the decomposition by some practical examples.
433.2 ConceptuPl Design of the Permeate Retnoving
Definition of FRs
In the pervaporation separation processes, the last separation action is the permeate
desorption taking place at the downstrearn surface of the film. After this desorption process, the
permeate completes a phase change from liquid to vapor. To maintain a low pressure on the
downstrram surface of the membrane, the permeate has to be continuously removed. Usually a
vacuum pump is used to take the pemeate vapor out via fluid transport passageways. If there is a
large pressure loss over the passageway, the pressure on the downside surface of the membrane
will be raised up, leading to the decrease of flux rate. Hence, minimization of flow restriction
(the FRi) is an important sub-FR under the permeate removing FR.
Petmeate removal structure is one of the major contributors to the module weight. Thus
the weight reduction (the FRi) is defined as another sub-FR under the pemeate removing FR.
FR = Low flutd rtstriction
FR2 = Minimize the weight
a DPs for the Pemeate Removing
To maintain a minimum flow restriction, the passageway design should take into account
the flowing of vapor permeate. The design of a low restriction passage is partially involved in the
design of the membrane cell, since the permeate flow is generated in, then gets out of the
--* - -
membrane cell. There are various appnxiches for the reduction of flow restriction. In level2, for L . 3 - - --- . -- - -
simplicity, the physical solution of F& is lumped into the "low flow restriction passage" (the
DPI). The detailed implementation is to be presented in the fourth level of the hierarchy. The
weight reduction can be implemented by using light material, such as plastics or aluminum, to
build the parts.
DPi= Low flow restriction passage
DP2= Light material
Design Equation and Analysis
The weight of a pemeate removing structures is also determincd by the component size.
Low flow restriction usually requires large fluid passageway which increases the size of the
related components. To demonstrate the relationship between FRs and DPs, a design equation is
given below:
This equation shows that this design is decoupled. If the fluid transport structure is
detennined first, the independence can be satisfied.
4.3.4 Level3-Detsiled Conceptual Design
Although the decomposition of level 2 defines the major functional structure of the
pervaporation module, it is still too rough to demonstnte the concrete configuration. Funher
decompositions iire required. In level 3, axiomatic design theory will be continuously employed
to ensure optimal or near-optimal design solutions. The decomposition and mapping processes
.
are perfonned on the four sub-structures only, i.e. the smail membranecontacted components, L.. - ---. - . -
the àisk membrane support, the composite s&l and the low flow restriction.
4 A 4 J Conceptuaï Design of Maximizing the Membrane Arta
a Defini tion of FRs
As mentioned before, the minimization of the membrane periphery components can
maximize the active membrane area. There are inner and outer periphery seals on the membrane
cell. Both of them contribute to the loss of membrane area. For the outer periphery, the disk
diameter has ken pre-detemiined as 24 inches. The width of the seals becomes the only
parameter which can be minimized (the FR1). The inner periphery is a two degrees of fmedom
geometry, called inner hub later on. The size of the inner hub, including the diameter and width,
will affect the membrane area. Therefore, an inner hub with minimum diameter and width is
defined as the FR2, that is:
FRi = Minimize the loss of membrane area on outer periphery
FR2 = Minimize the loss of membrane area on inner periphery
a Choice of DPs
The DPl is deterrnined by designing the outer periphery seals. Generalty. there are two
types of appropriate seals for the membrane-disk configuration, Le. glue and mechanical seals.
Glue seal is an economical sealing method widely used in various areas. To ensure a reliable
sealing, the glue band must be wide enough. In the earlier FCT module, for instance, the glue
band occupies approximately 17% of the total membrane area on the outer periphery. Besides, if
the injection of glue is out of control, the width of glue seal will not be circumferentially unifonn
due to the liquid state of glue. See Figure 4.3-a.
Alum Clapm 7 Membrane 7 Glue 7 Membrane 7
Glue
A - A
Membrane Pad
B- a
Figure 4.3 Cornparison between Glue and Mechanical Seals
Mechanical seal is an alternative of glue seal. Compared with glue seal. it is easier for a
mechanical seal to achieve good sealing by using less contact surface. In mechanical seals the
membrane and disk surfaces a-e seamlessl y contacted each other by using a narrower mechanical
rim, rather than by sticking two surfaces together on a wider band. A narrower mechanical band
usually provides better sealing than a wider one. because a narrow band has less sensitivity to the
roughness of surface. Therefore. mechanical seals (the DPi) provide the possibility of using less
membrane area. For a 24 inches disk, the mechanical seal will use on1 y appmximately 4% of the
membrane m a , compared with at least 8% with the glue seal.
In practice, there are various mechanical sealing methods available. In the pervaporation
module, as shown in Figure 4.3-b, an aluminum wrap is proposed as the ernbodirnent of this
design concept, generating a wrapping force to bring the membrane fimly against the disk. A
56
T -
rubber strap or adhesive tape may be put . =% .. ---L-d - L * - - - L - - -
sealing. The wrap seal can not only satisfy
between the membrane and the disk to improve the
the I k o f ''mhimum loss of membrane". but also is
an optimal solution for the FR of "composite seal". In addition, it will be presented in the
subsequent chapters that the wrap design is also beneficial to the joining of the membrane disk.
After completing the mapping between the FRi and DPi, now we are going to discuss the
physical solution of the FR2. To satisfy the FR2, the inner periphery should be as small as
possible; this, however, conflicts with other functionai requirements; for example. the inner
periphery is the interface between the disk membrane ceIl and the central permeate removal tube
(to be described later). The permeate flow passes the inner periphery, accessing to the central
penneate removal tube. Sufficient passage size is required for the reduction of fiow restriction. In
addition, the disk cells are connected to the central removal structures at the inner periphery of
the disk cells. Hence the inner hub must be wide enough to provide sufficient strength and good
structure stability. In such a highly coupled location, it is very important to balance the different
requirements. That is to Say, when siring the inner periphery component (the DP2), we should
take into account the minimum allowed permeate flow passageway and the installation strength.
The design parameters may be written as:
DP,=Mechanical seal on outer periphery
DP2=Sizeâ inner periphery component
O Design Equation and Anal ysis
The design equation below shows that the design concept results in an uncoupled design.
This uncoupling relationship provides the designer with great freedom to work on the inner and
the periphery seal separately.
4.3.4.2 ConceptuPl Design of the Dlsk Membrane Support
Definition of FRs
A component used to support a membrane should have four functional requirements.
The first functional requirement is the "light" (the FRi). As the analysis described above,
the membrane support is a disk plate. Each module comprises a stack of disk plates. The number
of the disk plate depends on the productivity. Usually there are 20 to 40 disks per module. Thus
the disk is one of the major contnbuton to the total weight of the module. When the disk
diarneter is increased to 24 inches, the weight specification becomes extremely cntical. A heavy
module may cause a lot of maintenance and assembly difficulties. Actually, the reduction of disk
weight is one of the main thmsts to launch this new design project.
To reduce the weight, one of the approaches is to use thin disks. However, the reduction
of the disk thickness may have negative impact on the structure stiffness. The increased disk size
will futther degrade the stiffhess of the disks. Therefore. ensunng a good stiffness without
sacrificing the weight specification is defined as the second functional requirement (the FRI).
Many disks were warped in the earlier FCT 1 1 inches module assembly, as shown in
Figure 4.4. There are many reasons for this problern, such as the design defect, the material
deformation, the manufacturing tolerance and the improper assembl y processes, etc.; the rmt
cause is the use of the thin disks which are required by the weight reduction. This phenomenon is
called as insufficient structure stability or robustness. An unstable structure is harmful for many
design specifications, including product performance, structure integrity and appearance for
marketing. In addition, the warped disks locally block the path of the air bubbles which are used
for preventing the concentration polorization. Hence, it is important to maintain uniform spacing
=-- - -
bctween the disks, In other word, the design should ensure that the membrane cells have good p.---.- --AL-A-- -. -. - -. -- > -
structure stability (the FR3).
Figure 4.4 Illustration of Dis k Structure Instabi lity
The membrane disk is a critical part. In the eadier FCT module the disk is made of steel.
A series of spiral channels are cut on the disk plate by using laser. Hence the cost of material and
manufacturing is significant, especially when the product is put into a large scale production.
Hence low cost (the F&) is defined as an essential requirement in the new module. The four FRs
are listed as follows:
FR] = Light Weight
FR2 = Sufficient Stiffness and Strength
FR3 = Good Structure Stability
F& = Low Cost
Choice of DPs
There are many ways to reduce the weight of membrane support disks. Light material is
considered to be the best solution for weight reduction here, because the disk diameter has k e n
predetennined and the thickness also links to other requirements, such as stiffhess and flow
7 - -
restriction. Thenfore, aluminum or polymer materials should be selected as the material of --Le- a-= - - - -- .. - - -
membrane disks (the D P ~ ).
It is very difficult to make a large and thin disk plate very stiff and strong. Obviously, it is
not smart to impmve the disk stiffness by simply increasing the thickness. Our design concept is
to thin the disks as much as possible on the condition that: (1) the disk structure c m endure the
assembly impacts; (2) the disk thickness pmvides sufficient flow passage for the vapor pemeate.
Then take care of the stiffness and s t~ngth issues on the module assembly. This philosophy
nquires a fine design of the disk structure. As we know from the earlier FCT module, there are a
series of pemeate channels on the disk plate. The solid materials between the channels function
as frames. The shape of the frames determines the stiffness and strength of the disk assembly.
Generally speaking, there are thne frames available, as shown in Figure 4.5. Configuration "a"
has very bad strength and stiffness due to its n m w e r mot and wider tip. Configuration "b" goes
to the opposite way with very good stress pattern because of its wider mot and narrower tip, thus
provides g w d strength and stiffness. However, it is impossible to implement the configuration
"b" in a membrane support disk because there are no sufficient supporting materials at the outer
periphery of the disk. The configuration "c" with unifonn frame width provides a reasonable
stress pattern. From the stress point of view, this structure is satisfactory. In the detailed design,
it c m be seen that configuration "c" can be implemented in a disk plate by using polar parabola
curves. Therefore, it i s conctuded that the channel curve should ensure uniform frarne width on
the disk (DP2).
Figure 4.5 Possible Disk Frames
It is not feasible to think about the stability issue without refemng to the structure
stiffncss issue. The bad stability pmblem on the earlier FCT module is mainly caused by the thin
disk plates which are free of supporting at the outer periphery. Here reinforced disks have ken
proposed to address both the stiffness and stability issue. The design concept is to put several
reinforcements (the DS) along the periphery of the disk to rnechanicaliy constrain the disk outer
rim. The disk assembly is significantly reinforced by using this methods on system level without
violûting the weigh specification. This reinforced disk design will be further discussed in Chapter
5 in detail.
How to reduce the cost of the disk is an important issue. As mentioned above, plastics is
an ideat material for the membrane disk. However, plastic disks require injection motdmg
technique. The cost of an injection-molded part depends on the volume of production and the
component size. It is estimated that the new module only has a production quantity of
approximately 50 pilot units per year. Hence the injection mold will be one of the major
contributors to the cost. An injection mold for a 24 inches solid disk may be pnced at amund
MSK-30K. Therefore, our idea is to assemble the plastic disks by using smaller separate disk
elements made by a smaller injection mold. This concept will not only reduce the mold cost, but
61
a- - also reduce the cost by significantly increasing the quantity of injection mold parts. For example,
- 2---i_rl-2 . . - - _ l _ -._ -
if a disk is seprated into 16 small the d production for the injection mold parts
will be 16 times more than the solid disk does. Therefore, the segment disk is defined as a major
approach of cost reduction.
Based on the descriptions above, the rnapping for the membrane support leads to the
following list:
DPi = Plastic sheet plate
DP2 = Uniform width frame
Df3 = Disk reinforced at rim
DPI = Segmented disk
Design Equation and Analysis
The design equation below is used to illustrate the relationships between the FRs and DPs
defined above. The design matrix is triangular. Therefore if a special sequence is employed, the
independence between FRs can be assured. The design does not violate the axiomatic design
theory .
4.3.4.3 Conceptual Design of the M i n g
On level 2, a composite seal is proposed for the outer periphery sealing. In this section,
further decomposition and mapping are performed to define the detailed design solution.
Definition of FRs --- . - - -- - - LA - - - - - --
AS mentioned bef&e, the working temperature of the glue seal used for the ed ie r FCT
module is 60-70 OC. Actually in FCT the pervaporation temperature is set at approxirnately 64
OC. To improve the flow through the membrane, a higher temperature may be applied. The new
module design should take into account the potential increase in temperature. Thus the first
functional requirement is that the composite seal should be able to work well at a maximum
temperature of LOO O C (the FRi).
The second functional requirement is defined as replaceable seals (the FR2). Membranes
usually are very thin polymer sheets which are easily damaged in the field. In the earlier FCT
module, the whole disk membrane cell is thrown away once the membrane is broken since the
glue is not removable from the disk. For the new module, provision should be made to recycle
the membrane supports. Replaceable seals offer the possibility to recycle the disk plate. Thus the
list of FRs is written as follows:
FRI = Work at a Maximum Temperature of LOO O C
FR2 = Replaceable Seals
a Choice of DPs
To meet the new temperature requirement, The rubber stmp andor the 3M adhesive tape
(the DPi) are the candidates. Both are capable of working at the required temperature. The basic
concept is to put one of the sealing materials between the membrane and its support disk.
Another one is then used to wrap around the disk periphery. The nluminum wrap, which is
originally proposed for minimizing loss of active membrane area, can be used to clamp the
mbber strap or the adhesive tape. The service persons can remove the seal material by simply
taking the aluminum wrap and mbber seal off, if they want to replace the damaged membranes.
63
z - = m --
Therefore the combination of rubber, adhesive tape and aluminum wrap (the DP2) becomes the - - L - - L - -LL e - . - - - -
physicai solution of FRr. The list of DPs is given as follows:
DP, = Rubber strap and 3M adhesive tape
DP2 = Aluminum Strap
Design Equation and Analysis
The design equation as given below indicates that the design matrix is triangular.
Therefore if a special sequence is e iployed, the independence between FRs ciin be assured.
43Am4 Conceptual Design of Minimidng the Flow Restriction
In level 2, a low flow restriction passage is defined as the approach of reducing the flow
restriction in the module. The analysis at level 2 is too coarse to show the embodiment of the
functional requirement. In this section, further decomposition and mapping processes are
performed. The design concept will be detailed.
Definition of FRs
How to reduce the flow restriction is a traditional topic which has been studied by a
number of researchen for a long p e n d The Rasons causing flow restriction differ from case to
case. A number of approaches have been developed to reduce the fluid transport restriction.
Detailed description is beyond the scope of this thesis. The author is trying to avoid the detailed
fluid dynarnic analysis. By picking up the most significant factors, the author intended to find
most effective, but economical approaches to minirnize the fluid restriction.
In the membrane pervaporation separation, usually the nuid medium is vapor or stem. ---.-=---A- 2 - - d s = -- L-% - - - -
The restriction of vagr transport mainly depends on the vapor speed and flow path. The
pervaporation module design should avoid generating supersonic flow in the pemeate gas
transport passageway. The flow restriction will dramatically increase when the gas speed reaches
supersonic point. In addition, for subsonic flow, the flow restriction consists of steady flow
restriction and turbulent flow restriction. The steady flow restriction is increased with the
increased length of flow path and flow speed. Turbulent flow restriction is usually caused by the
eddies of flow. The pipe tuming, especially sharp tuming is the major contributor to the
generation of eddies. Therefore, the low pemeate gas speed (the FRi) and the minimum number
of sharp tuming (the FR2) are the essential functional requirement for the low flow restriction.
Figure 4.6 Pemeate Gas Flowhg Mechmism in a Mesh Spacer
The spacer used in al1 of the pervaporation modules is one of the major contnbutors to
the pressure loss in the module. Take the earlier FCT module as an example. Figure 4.6 shows
the penneate gas flowing mechanism in the membrane mesh spacer. It is clear that the plastic
mesh frames block the passages towards the channels. The fluid from the membrane downstrearn
surface cannot pass the mesh free of restriction, reaching the spiral channels on the disk plate.
The vapor only uses the gap between the spacer and membrane as the passageway. However, the --a- .=- --A - -- - - - - - - 5 - - & - -- -
gap is very small, especially when the vacuum is applied to the permeate chamber. The small gap
results in very large fiow restriction. The pressure pattern between the two channels is shown in
Figure 4.7-a. Therefore, a spacer with minimum flow restriction (the FR3 is the third functional
requirement. Based on the analysis above, the following three FRs are defined:
FRI = Low Permeate Gas Speed
F R 2 = Minimize Turbulent Flow
FR3 = Low Restriction Membrane Spacer
Pressure P r e s s u r e
(a) b!esh Spacer Design (b) Builci-in Sparer Design
Figure 4.7 Permeate Pressure Pattern between Channels
Choice of DPs
To assure the permeate gas speed is subsonic, the maximum pemeate Row rate has to be
understood. The fluid speed can be computed based on the rate of permeate flow and the cross
section m a of the fluid passage. In the subsequent section, the calculation will be presented to
demonstrate the estimation of the permeate gas speed. Here we only identified the sized passage
(the DPl) as the physical solution of FRl.
As we know, turbulent flow usually appears at the location where there are shaip tuming --- . - -.. - - -
of f l Ïw paths Gd &&natic change area. The passageway structure is incorporated with
other parts. It is very difficuit to design a passageway without any flow tumings. Therefore, it is
required to keep the number of sharp tumings to a minimum level and try the best to avoid the
dramatic change of flow area. This design concept is defined as the DP2.
The mesh spacers used for the earlier FCT modules are the plastic mesh sheets. In the
new module, a membrane disk plate with built-in mesh spacer (the DP3) is proposed to avoid the
high flow restriction in the spacer. Figure 4.8 shows partial built-in spacer. Injection molded
membrane disk enables the built-in spacer concept. This design concept connects two di fferent
functional requirements together by sharing the same manufacturing technique.
Thus the mapping results are as follows:
DPi = Sized Fiuid Transport Passageway
DP: = Minimum Flow Tuming Design
DP3 = Built-in Membrane Spacer
Figure 4.8 Built-in Mesh Spacer Concept
Design Equation and Analysis
-
Suh's axiomatic theory is u r d to evafuate the design quality. As shown in the design -L--:--.: - - . -
equation below. The design matrix is triangul&. Thenf& if a special sequence is employed, the
independence between FRs c m be assured
4.4 Summary
The objective of conceptual design is to generate a product requirement document to
ensure an optimal system design. In this chapter, the conceptual design of the pervaporation
module starts from identifying the effect of the membrane separation conditions. Then Suh's
axiomatic design methodology is used to support the conceptual design processes: including
Definition of FRs, Choice of DPs and Design Equation and Analysis. The mappings between the
functional domains and the physical domains are executed on four Ievels of the hierarchy. The
evaluation of independence is performed on the individual branch of the hierarchy tree. The
evaluation cnteria are to ensure that the conceptual design meets the minimum requirement of
independence, i.e. decoupled design.
The conceptual design described in this chapter results in the following design features:
1) The module shouid be a plate-and -&une configuration,
2) The module should consist of a stack of disk membrane cells and a central tube,
3) The disk membrane cells should be mechanically reinforced at rim by using certain
reinforcements,
4) The outer periphery of the membrane should be sealed by using composite seol, including
an aluminum wrap and replaceable mbber or adhesive tape,
5 ) The inner periphery of the membrane should be sealed by replaceable rubbers wraped by . .<-L - . -
the interface components betweG the disk membrihe cells and the central tube,
6) The channel shape should provide uniform width àisk frames,
7) The membrane disk should be constnicted using a senes of separate plastic pieces made
by injection rnolding technique,
8) Buiit-in mesh spacers should be incorporated on both surfaces of the membrane disks,
9) Al1 of the components should be sized to balance the multiple functional requirements,
including structure strength, fluid dynamics and pervaporation separate conditions, etc.
Note that the conceptual design can only create the major design features, far away from
the final engineering design. A detailed design is required to define every single design feature,
which will be presented in Chapter 5.
CHAPTER 5 DETAILED DESIGN AND ANALYSIS -=-aLA- - - L - -
5.1 Introduction
In Chapter 4 axiomatic design theory is used to optirnize the relationship between the
functions and the implementations of a product design. The conceptual design of the
pervaporation module is executed. Conceptual design is the most important design process which
generates the major configuration of a product and provides a design direction for the future
design processes. However, product design is still far away from king finalized in the
conceptual design phase, since many details have not been considered. Hence a detailed design is
required to refine the design concept. Theoretically, detailed design can also be perfomed under
the guidance of axiomatic design theory. The author, however, prefea to use the axiomatic
design theory in the conceptual design only. The practical design experience shows that if we
completel y rel y on the axiomatic theory, the design anal ysis processes may become very difficul t
since the independence of FRs may not be assured. In addition, the axiomatic design
methodology does not lend itself to the design job under a specified design goal, such as
improving quality, reducing cost, compressing cycle times, increasing flexibility, raising
productivity and efficiency, and improving the social image, etc. Therefore, it would be nice to
perform the detailed design of a multiple disciplinary product, such as the pervaporation module.
by using the objective orientated design methodologies.
In this Chapter Design for X methodology will be used for the detailed design of the
pervaporation module. The X in DFX represents one of the objectives of the product design.
There may be multiple 'Xs" required in a pmduct. Accoràing to the DFX methodology, the
detailed design of the pervaporation module will be performed based on the design configuration
-- - -
proposed in Chapter 4. The detailed structures of the pervaporation module will be created based . . ---- - Z L &- - . a*&=- L -2 k.. - - A
on the selected- '%" i n this Chapter.
5.2 Design of the Permeate Flow Velocities
A product is designed to meet certain requirements. A satisfied performance is the basic
design requirement of any products. The pervaporation module is a mechanical product used for
chernical pervaporation separation. The basic requirement for a pervaporation module is its
pervaporation separation capacity which is affected by some process conditions, including
membrane pervaporation performance (flux and selectivity), penneate pressure. temperature and
effective membrane area, as explained in the previous chapten. Here the penneate pressure is
selected as a major performance-related specifications which the module can contributed to.
Detailed design is perfonned to minimize the flow restriction along the permeate transport
passageway.
The velocity of vapor permeate is one of the most important contributors to the flow
restriction. The components of the pervaporation module should be sized such that the permeate
vapor velocity is minimized. Therefore, Design for Velocit y should be used through the module
design.
In the disk membrane cell, the highest flow speed is Iocated at the root of the membrane
disk. Before estimating the velocity in the disk channels. there are certain simplifying
assumptions as follows:
1) Separation liquid: mixture of IPA and water.
2) The permeate gas is pure water stem.
According to water s t em property chart 1311, the nlationship between pressure,
kmkrature and volume for gr pound s6am is as follows:
Pressure: PO = 3,6273 psia
Temperature: T - - 100 O C
Volume of per pound steam: vo O - 99.33 ft3/lb
The permeate pressure: PI - - 7.6 Torr
- O O. 147 psia
The permeate gas behaves as an ideal gas obeying the relationship: p,vl=povo
Al1 of the permeate gas flows through the channels in plane; None flows radially through
the mesh underlying the membrane.
The velocity of the penneate flows in the channels is estimated based on the following
predetermined parameters:
Temperature: T - - PAN WF Pervaporation Composite Membrane Flux: J - - Diameter of the Membrane Support Disk: D - - Diameter of the Disk HublSeat: d - - Inner Diarneter of the Central Tube: diirbc - - Width of the Outer Periphery Seals: x - -
Width of the Channels on the Disk: w - -
Thickness of the Steal Disk: t - - Number of the Channels on the Disk: n - - Number of the Disk Membrane Cells: N - -
72
-A=--& 5 L-
~ i 6 t . calculate active surface -&a of the membrane. Each disk has two active surfaces.
The total cross section area of the channels:
The active surface area of the membrane:
The volume per pound permeate gas under the conditions of working temperature and
pressure:
in3 llb
The volumetric permeate fiux rate:
The volumetnc permeate flow rate:
The velocity of the permeate gas through the disk channels:
The velocity of the permeate gas through the central removal tube:
The estimation of the velocity of pemeate flow shows that the maximum velocity is at
the mot of the spiral channels where the channel passages communicate with the central tube.
Because there is no the sound speed in vapor permeate medium available, the evaluation of
permeate velocity is accomplished by refemng to the critical Mach number (M=l) of air at sea
level standard condition, i.e. approximately 340 d s . Here the maximum permeate velocity is
109 d s . The velocity in the central tube is 97 mls when there are 40 disk membrane cells
installed. This big margin is necessary for the steady flow, because in practice the flow
restriction caused by the turbulent flow should be taken into consideration. For example. the
pressure drop taking place at the conjunction of the disk channels and the central tube is
pnmarily caused by the turbulent flow. Additionally, the pervaporation module design should
allow for further enhancement of the production capacity by simply adding extra more disks on
the central tube.
Ovenll, the passage-related components are sized b y Design for Veloci t y.
5.3 Design of the Separate Disk EIements
Most of the pervaporation devices in the market are too expensive to many customen,
especially small to medium size companies. Thus low cost pervaporation modules are highly
desirable. Design for Cost concept focuses on the cost reduction issue. The three step's design
procedure described in Chapter 3 is used to address this issue.
Specifications - -L -
As one of the most important design requirements in a pervaporation module design, cost
reduction can be dealt with by reducing the manufacturing cost. As described in Chapter 4,
injection-molded membrane disk is the best solution of weight reduction. However, the injection
mold of a 24 inches disk may cost as high as $25K to $30K. Injection-molded disks may account
for 3040% of the total manufocturing cost in a pervaporation module. provided that the annual
quantity of the module production is not large enough, such as less than 50 units. Therefore, our
design tries to find a trade off between cost and weight.
Technique Solutions
The size of an injection-molded part is directly associated to the mould pnce. A
membrane disk can be a solid disk or a segmented one assembled by several separate pieces. One
of the rnethods for cost ~duction is to use segmented disks to assemble a disk. Figure 5.1 shows
the layout of a segmented element made by injection-molding approaches. There is a channel cut
in the middle of the segmented piece. The two side brims will fom another two channels due to
the neighborhood bnms. The channel configuration will be discussed in the next section.
Figure 5.1 Membrane Disk Element
75
- - L" - Ëvhate Concept
Why is a segmented disk cheaper than a solid one? There are two reasons which
contribute to the cost reduction. The first is the reduced mold size; a solid disk needs larger
mold. leading to higher mold cost; whereas a segmented element can be made by c heaper mold
due to its reduced dimension. The second reason is the increased production volume. For
example, if the disk is divided into 16 duplicate pieces, the total quantity of the injection-mold
parts will be 16 times of the solid disks. This increase in production volume is dramatically
beneficial to the reduction of manufacturing cost.
Therefore, it is concluded that the application of Design for Cost concept on the module
design results in the use of a segmented membrane disk.
Design of the Chamel Configuration
As mentioned before, the membrane disk is a thin polymer disk made by injection
molding technique due to the weight and the cost considerations. However, structure strength
and stiffness are another major concem for a sheet part. To transport the vapor permeate, there
will be a number of the permeate channels incorporated on the disk plate, which will degrade the
stiffness and strength of the disk structure. The curve of the channels has significant effect on the
stiffness and strength of the disk. In Chapter 5, it is proposed by the conceptual design that the
channel curves should be design such as to provide uniform width disk frarnes. Thus this section
focuses on the determination of the ph ysical implementation of uni f o m width frames.
(331
Figure 5.2 Cornparison of the Polar Parabola Channels and the Straight Channels
Figure 5.2-a shows a disk with polar parabola channels incorporated. Figure 5.2-b
illustrates a disk with straight channel configuration. The important feature of polar parabola
curves is that they can provide radially uniform spans ôetween the two curves, except at the mot
area of the disk where the frame width is slightly smaller. Nevertheless. the straight channel
schematics are quite different; the span between the channels is dramatically reduced with
reduced radian. In order to shorten the circumferential fluid path at the outer periphery, more
channels are required, leading to very namw frames at the rwt of the disk. Therefore, i t is
concluded that polar parabola channels are ôetter than straight channels in the implementation of
uni form width frames.
The polar parabola curves are generated by the equation: Y=~o'; where Y is the radian of
the spiral çuwe to the origin; 0 denotes the angle between the radian and x-mis; k is a constant.
It is noticeable that the parameter Y is the function of 0 only; there is no relationship between Y
and R which is the diameter of the disk. This indicates that no matter what the diameter is, the
disk has the sarne shape and features. This advantage will provide the potential for the future
expansion of the disk diameter by simply scaling the module.
After defining the channel configuration, the disk assembly shown in Figure 5.3 can be
demonstrated. This disk assembly is constnicted by 16 duplicate pieces of separate elements. The
77
spiral channels form the envelope of the elements. At the outer periphery of each element, there ---- =---a- -- L- - -2 - --
are a notch and stick-out ear on each side. ~ a t i n g the notch with the stick-out ear of the
neighborhood elements can enhance the stiffness of the disk assembly.
In this design, there is a small hole at the inner periphery of each element which is used
to position the element when it is installed on the inner hub described later.
Figure 5.3 Membrane Disk with Spiral Channels
5.5 Design of the Built-in Membrane Spacers
T h m functiond requirernents are defined for e membrane spacer:
- to support the membrane,
- to generate a space as pemeate storage
- to direct the permeate gas towards the flow channels with minimum flow
restriction.
Generally, the second requirement is satisfied by simply increasing the thickness of the
spacer. Hence the detailed design here fwuses on the membrane support and flow passages.
-- -- -
There -- =: .JZ --A %. -
fiber giasses,
are various membrane spacers available for the pervaporation modules, such as
filter papers, porous si& m e d pads and polymer meshes, etc. As mentioned in
Chapter 4, the k s t solution for our new module is to use so-called built-in mesh spacers molded
on the surfaces of the plastic disk, see Figure 4.8 for the design concept. However, the
conceptual design only defines the general concept of using a built-in spacer, rather than the
detailed spacer pattern. In fact, the spacer mesh pattern directly associates with the flow
restriction and membrane supporting. As a result, the mesh pattern design is the key topic of this
section.
When vacuum is applied on the downside surface of the membrane, part of the membrane
material will firmly set on the membrane supports. The rest of membrane does not have any
support, providing a lot of stress to the membrane at the brim of the individual supports. The
periodical pressure pulse of pumping may speed up the breakage of the membrane along the
edge of the membrane supports. To avoid this problem, the design of the membrane support
should rninimize the size of non-supported membrane, increasing the density of supports. On
contrary, Design for Flow Restriction requires the increase of the perrneate flow passage as much
as possible. This requirement leads to increase of the size of non-supported membrane. This
conflicts with the previous requirement. Figure 5.4,5.5 and 5.6 show three possible built-in mesh
specen. Anal ysis is being performed to disclose the appropriate design solution.
Figure 5.4 shows a dot membrane support which features a number of small short stick-
outs on the disk surfaces. There are no mechanical linkages between the stick-outs except the
disk pad. This design provides the best flow condition because of its free passages in four
directions. The flow trajectories are shown in the arrow lines in Figure 5.4.
Figure 5.5 and 5.6 are the gmve type of membrane supports. A series of groves are cut on
the disk surfaces. The fluid is directed to the main channels via the groves. From the membrane
79
. - . . .
supporting point of view, these two grove membrane supports are quite similar. However, --- - - -- : - - - - A - - - - - - - . % . - - --&-- -
considering the Design for Flow Restriction, the des& shown in Figure 5.5 provides better flow
trajectories than Figure 5.6 because of its shorter flow path and less fiow tuming.
Compared with grove and dot membrane supports, we can find the former has more
supporting on the membrane, whereas the dot design is a little bit better in reducing flow
restriction. Therefore, it is concluded that the V shape grove design is the best solution.
Figure 5.4 Dot Pattem of the Membrane Spacer
Figure 5.5 V Shape Grove Pattem of the Membrane Spacer
Fiow Trajet tor ies
Figure 5.6 Circle Shape Orove Pattem of the Membrane Spacer
Design of the Segmented Disk Joints
As described in the previous sections, the membrane disk consists of 16 pieces of the
small plastic elements, as shown in Figure 5.3. To make them work, it is necessary to join the
separate elements together by putting some binding mechanism on both of the inner and outer
peripherys. The inner periphery structure is associated with the installation of the disk membrane
cells and the central removal tube, so it will be addressed in other sections. This section only
discusses the design solution of the outer periphery joints. Although having been designed in the
conceptual design, the outer periphery seals will be mentioned again in this section to support the
analysis. Design for lntegration @Fi) concept is the fundamental design guidance to meet multi
functional requirements. The four step of DFX is presented below.
Clarify Product Design Specification
Design specifications are reexamined for the product assortment. There are numerous
approaches for binding the disks. No matter what kind of the method is used, the binding
mechanism should be at least Reliable, Light, Small, Simple and Economical.
It has been defined in the conceptual design that the separate disk pieces will be made of
plastics. The design of joints plays a critical role in the reliability of a plastic product. Joints
usually are the weakest components in a plastic product. Therefore, joining of the membrane disk
should have minimum stress on the membrane disk.
Light and small are the basic specifications. For a plastic disk, the weight of joints is very
important since the weight at the outer periphery will increase the defornation of the membrane
disk. putting more stress on the working pieces. Besides, the space between the two disks is
--- -- -
limited. The reduction of joint size allows the use of more membrane disks. improving the c ------A---- P. - -
productivity . A design feature may link to the other requirement. For example, the joining of disk
elements is directly associated with the sealing and protection of the membranes. It would be
smart to design the joints such that the sealing and protection of the membrane are enhanced. In
addition, the disk reinforcements proposed in Chapter 4 will definitely affect the joint design. It
is mon than a joint design that does not degrade the associated functions. Our goal is to enhance
the associated functions by a proper joining. The DFI concept is used for this purpose.
a Select Technique Solution
Technique solutions for component joining Vary with the application conditions.
Generally, there are four types of joints available for the connection of sheet parts, including
welding, fasteners, holding and clamping.
a Generate Concept
Plastic welding is a special and new technique for joining plastics by melting the plastic
material at the bond area. There are a lot of methods for melting the plastics, such as soldering
gun, hot iron, hot gas, part spinning, ultrasonic wetd and vibration.
Fasteners are the rnost popular and reliable binding approaches. Figure 5.7 shows two
possible design concepts. The separate disk elements are mechanically jointed together by using
bolts and nuis. Holes are required to position the bolts and also to enhance the binding
mechanism.
Another binding mechanism is shown in Figure 5.8. Two stiff frames are holding the
separate disk pieces. There is no hole required on the separate disk pieces. The holding forces
appliod by the frames are generated by bolts attached on the frame, rather than on the âisk - .
elements.
Another possible solution for the disk connection is to use a metal strap warped around
the outer periphery of the disk. By selecting an appropriate thickness and width, the metal strap
can provide sufficient binding force to firmly clamp the separate pieces together without the
assistance of any other cornponents. Refer to Figure 4.3 for the details.
Figure 5.7 Joints with Bolts and Nuts
S p a c e d J o i n t
,---Membrane Disk
F o r c e - - F o r c e
Figure 5.8 Spacer Joints
. P T
Evaluate Concept -- -- --
AS described above, there are many different approaches for the joining of the separate
disk pieces. Each design concept will be evaluated to determine the best solution for the
pervaporation module.
Plastic welding is nlatively a new technique. Cumntly, this technique is mainly used in
some special area, such as aerospace industry. In commercial industry, plastic welding is still not
very popular. Plastic weld has highest operating cost and equipment cost. The welding matenal
and temperature a- hard to control by a regular operator. Special manpower and equiprnent are
required.
Drilling holes may degrade the strength of a component due to concentration of stress.
especially for a plastic part. When bolting load is applied at the points where they are located, the
concentration of stress can lead to failure due to creep, stress relaxation, crazing and etc. The
bolts and nuts olso add may extract weight to the outer periphery. Most importantly, the holes on
the disk pieces will interfere with the outer periphery sealing which is another important
functional requirement. Therefon, from Design for Integration point of view, the bolt and nut
concept is not a favorable solution.
The frame concept has been used for some of the existing plate-and-frame pervaporation
modules. The frrst advantage of this design is the good smicttm stability. Besicles, this
configuration integrates with the membrane outer periphery sealing very well. Mechanical
sealing can be effective because of the load applied to the membrane through the stiff frames.
However, this design also has several disadvantages. Fintly, the frame is a complex 3 dimension
component which requires higher manufactunng cost and techniques. Secondly, in order to
ensure a sufficient stiffness, the fi=ame dimension must be large enough, leading to increased
module weight. Another problem is the surface deformation dong the periphery of frame. The
m- -
mechanical membrane seding relies on perfect surface contacting. Bad flatness on the frame -- -
contact suifaces cm result in sealing failure.
Figure 5.9 Aluminum Strap Joining
Aluminum strap joint is very different from al1 of the above-mentioned design concepts.
An aluminum sûap is used to wrap the disk pieces around the disk periphery. as shown in Figure
5.9. Experiment results show that 25 gauge X 0.5 inches width is an appropriate size for
providing sufficient binding strength, without sacrificing the flexibility specification. As
mentioned in the conceptual design. Sufficient flexi bi li ty is necessary for compensating the
manufacturing tolerance and deformation. The ductile aluminum material enables flattening of
the surface wnnkles caused by the mismatch between the straight aluminum and the curvature of
the disk outer periphery. Monover, compared with bolting and holding methods, aluminum wrap
design utilizes one single part. Furthemore, it is also advantageous because of less weight and
minimum space volume.
The use of aluminum strap directly associates with the outer periphery seals as well as the
reinforcement of the disk cell. As we mentioned in the conceptual design, a composite seal needs
a rnechanism to clamp the nibber or adhesive tape and to protect the outer periphery of the
membrane from peeling. An aluminum strap is an ideal solution for these functional
requinments.
.-- .
To avoid àamaging the membrane, it is not allowed to directly clip the reinforcements on ----A- -.
the memb-e material wihout any pktectiork. Fr& this point of view, the aluminum strap
provides a protection cushion between the membrane and the clips.
Design for Assembly @FA) is part of the Design for Integration (DFI). DFA concept
requires us to take the assembly issues into consideration. The aluminum strap design has k e n
selected as the best solution for the joining of the separate disks. However, the incorporation of
the aluminum strap into the membrane disk assembly is still challenging. The answer is to use a
special jig to secure the separate disk pieces and to roll fold the aluminum strap. The jig design
will be described in Chapter 6 in details.
Design of the Membrane Disk Hubs
So far, significant progress has been made in the design of disk membrane cells, since
most of the structures for a membrane cell have been generated, including the membrane disk
plate, the built-in spacer, the outer periphery joint and composite seal. However, the design of a
membrane ce11 has not been completed. because the configuration of the inner periphery has not
k e n discussed yet. The inner periphery structure is called disk hub in which multi functions are
integrated. The design of disk hub will be presented in this section. Fint, the multiple functions
are defined; then the design concepts are presented. Finally, the design concept is analyzed to
ensure an optimal solution.
a Specifications
The multiple functions of a disk hub includes:
a) to join the separate disk pieces on the inner periphery,
b) to direct the permeate flow,
c) to pmvide an installation interface for the membrane cells.
87
There are a number of design concepts available for the disk hub. Analysis has k e n
perfonned during the design process to demonstrate the advantages and disadvantages of the
possible design schematics. Because of the limited space in this thesis, we are not going to show
al1 design concepts and anal ysis. Only the final solution is presented here.
Figure 5.10 shows partial cross section of the disk hub. There are two membrane disk
cells shown in the drawing. The picture on the left shows the assembly of the rwt of a disk hub.
The right side shows the installation relationship.
a - rubber gasket, b - pin washer, c - segmented membrane disk,
d - membrrine, e - flac wrisher, f - pin on the pin wcistier
g - central tube, h - spacet
Figure 5.10 Configuration of Membrane Disk Hub
As mentioned in the conceptual design, removable seals are preferred in the new module
to improve the reliability and easy of maintenance. Therefore, mechanical seals are selected for
the inner periphery of the disk. A mechanical seal means the use of soft sealing materials and
mechanical clamping mechanism. It is obvious that two annular washers (b and e) are required to
88
rn *- -
bridge the permeate --. A- 2 -. .
membrane. - ~ i t h o u t
channels, providing a flat solid cushion for the downside surface of the
the washers, the membrane can be sucked into the permeate channels,
leading to the failure of sealing. The perimeate gas can pass the spiral channels under the
bridging washer, then get into the central removal tube. On the feed side of the membrane, a
rubber gasket (a) is put between the membrane (d) and the spacer (h). When the two spaces (h)
are forced together, the depressed gaskets can provide a good isolation between the feed chamber
and the permeate chamber.
As shown in Figure 5.1 1, the bridging washer design has been integrated with the joining
of the separate disk elements. One of the washers has 16 pins stick-out on the washer flat
surface. Another one is just a regular flat washer. The use of the pin washer is for positioning the
separate disk elements circumferentialiy. If there is no positioning mechanism, some of the
permeate spiral channels may be blocked, since the separate disk elernents are movable.
89
Therefore, the pin washer is a critical design f e a t u ~ in the hub design. The pins should be sized ~-sL-A*-.- - - A - - - P L - -
in accordance with the available space at the rootof thedisk, since a senes of holes are required
to mate the pins on the washer. It is noticed that the available space at the inner periphery of disk
is associated with the inner diameter. A larger inner diameter provides more space for the
positioning holeslpins than a smaller one does. However, to maximize the active membrane area
and to minimize the module weight, a smaller inner diameter is expected on the disk. Therefore,
the size of disk hub will be the result of a trade-off between the multiply specifications. In this
module, the inner and outer diameters of the washers are sized to 2 94 inches DIA and 4 inches
DIA, respectively. The pin is 118 inches DIA x 0.04 inches LONG.
The use of pin washer is also significantly beneficial to the permeate flow. As shown in
Figure 5.9, the hub configuration enables the permeate flow to pass the thmat of channels,
directly accessing to the central removal tube, without any axial tuming. This design provides a
short and sttaight flow path, thus has less flow restriction.
Design for Manufactunng @FM) concept should not be ignored when we try to meet the
requirements of Design for Howing (DFF) and Design for Performance (DFP). Actually. during
the critical design review (CDR), the pin washer is the part most questioned by the reviewers.
The major concem was that the manufacturing cost of pin washer would be very high if the
- - regtrlar cold mactnning w m emptoyed. This htgh cost can be expected due to a couple of
reasons. The 16 pins, for instance, must be positioned accurately, which requires the use of
precise milling machines. Besides, the assembly of the 16 pins will be time consuming. And
there was comment that the pin washer was too expensive for its worth.
On the component level, the comments to the manufacturing cost are very reasonable.
However, if the pin washers are made by injection molàing methods, the cost and accuracy
problem will be minirnized. It is correct that the pin washer may cost more at the early stage,
- - - .
because the machined metal pin washer is used to venfy the design concepts dunng prototyping ------A -a-- - ."&-- d - - - - - - - - . -- - - -
stage. However, as soon as the desi& is put into production, the injection-molded pin washer
will bring the cost down to an acceptable level. This conclusion has been verified by the
prototype of the new module. The design concept of pin washer has been also validated.
5.8 Design of the Central Removal Tube
The design of a single membrane separator is presented in the previous sections.
Integration of the individual separators for the construction of a pervaporation module assembly
is illustrated in this section. In addition, removing the pemeate gas generated by each
pervaporation separator is incorporated to the separator installation mec hanism.
As described in section 5.7. the inner periphery seals are two rubber gaskets. These
gaskets are finnly pressed by an axial force via a spacer. In the earlier FCT module, the
membrane separators are installed on a central removal tube. A spacer ring is used to provide an
appropriate spacing between two separators. A stack of the membrane separaton and spacers sit
on a stop, then are fastened against the stop by using an nut at the opposite end of the central
removal tube. Analysis indicates that this installation concept suffers from some problems. The
first one is difficulty in maintenance. It is known that the membrane material on a composite
membrane may be damaged in field operations. The maintenance action must be carried out in
the field to replace the failed membrane separaton or the membranes only. In order to take the
failed separators off, other hedthy separaton have to be disassembled as well, if the failed
separators are buned in the other separators. For a 24 inches disk, this maintenance procedure
will be time consuming. Besides, the disassembly process may degrade the seals of healthy
separators. Moreover, as shown in Figure 2.14, the left end of the module assembly is quite long,
which will waste the space when the module is instdled in a solvent tank. That is to Say, the - - --- --- - -- A -
module is not compact.
Design for Unit concept has been widely used in industrial design, especially in
aerospace and automotive industries. It is a state-of-art design concept focusing on the
improvement of maintenance and manufacturing quality. The basic concept is to structure a
product according to its major function module. A machine may consist of several independent
sub units/modules which are fastened together. This configuration allows for the making of the
module separately. If one of the modules faiis, replacement can be performed on the failed
module only without disassembling the other units. Therefore, unit design concept can
significantly enhance the product quality and reduce the maintenance tirne.
Figure 5.1 1 illustrates the design concept of the central removrl tube and the installation
of disk membrane cells. A collar is working together with another collar to hold a membrane ceIl
assembly. The two collm are structurally identical. A female thread is made on the left side,
which lits the male thread on the right end of another collar. The function of spacer and central
removal tube are incorporated in one collar. The collar flanges implements the function of the
spacers described in section 5.7. The membrane cell is hold and sealed at the inner ring when one
collar is tightly screwed ont0 another. The central hole on the collar functions as a fluid transport
passage. The spirat chmels on the membrane ceil can send the permeate gas to the central
removal tube via 10 radial holes on the collar. As shown in Figure 5.12, the assembly consisting
of five components, i.e. membrane disk cell, two rubber gaskets and two collars. is called a
membrane separator. Each membrane separator is a unit. Installing multiple membrane
separators can create a membrane module.
The benefits of this design concept are given below:
1) Moreflexibiliiy.
n i e provision is made for the customer to change the production capacity of a
pervaporation module wfihout any new design jobs. By simply changing the number of
the membrane separator units, the production capacity is adjustable to some extent. In the
earlier FCT module, however, the central removal tube is varied with the change of the
number of the membrane separators.
Cost Saving
Because of the provision rnentioned above, the customers do not need to order new
components for different production runs. The pervaporation module suppliers, including
the manufactures and dealers, do not need any special design, testing and manufactunng
for the low volume, special orders. One modular can be suited for different purposes.
This will significantly reduce the product cost, resulting in better margins for both
customers and suppliers.
Reduced delivery time and Low Inventory
Standard separator modules are available at any time for any special orden. It is not
necessary for the customers to store too many different components for emergencies.
Convenient maintenance
Provision is made to break the centrai tube at any locations without disassembling other
membrane sepanitor units. Thus any damages in a membrane sepanitor unit can be fixed
by replacing the failed unit only. This feature will significantly improve the maintenance
efficiency and quality.
The module becomes compact, since the ends of the module is much shorter than the
earlier FCT module.
5.9 Design of the Membrane Disk Reinforcements - - --
As mentioned before, there are numerous disk membrane cells warped in the earlier FCT
11 inches module, which is unacceptable. This problem would be much more serious in the 24
inches module if there is no design changes, since the disk stability is getting worse with
increased diameter. However, disk warping is inevitable in reality. Solution. therefore, should be
found to overcome this problem.
In this section, Design for Stability is addressed at the system level, instead of at the
component level. This design concept allows the use of thinner membrane disk. The thickness of
the disk is detemined based on the minimum acceptable flow passage, i.e cross section area of
the spiral channels, then to deal with the stability issues on the final module assembly.
The design shown in Figure 5.13 is the embodiment of system stability concept. A
reinforcement clip ("a") with two slots is fitted to the outer rims of the two neighborhood disks.
When clamping forces are applied on both ends of the clip. The two disks will be mechanically
linked together with a fixed spacing equal to the distance between the two slots. At least three
reinforcement clips are required on each disk along the periphery, but provision is made to allow
more, depending on the actual requirement. The clips are usually put at the place where the disk
gets warped most seriously. A clip arrangement is shown in Figure 5.15.
The disk piates can provide mutual supporting when every membrane disk is
mechanically connected to its two neighborhood disks thmugh the clips. This supporting
mechanism significantly improves the stability of the disk group.
For further enhancement of the robustness of the module. a reinforcement rig is installed
on the collar shell. Four clamps are welded at the tip of the rig, as shown in Figure 5.14. This
design assures that the first membrane disk get very strong structure support, thus the first disk
becomes a solid base of the second one, just like what the reinforcement rig does for the first A--- --A- - L. - --- .=
disk. This ninforcemmt will be transfekd one by one to al1 of the membrane disks.
Overall, the clip mechanism described above is an ideal solution for the disk
reinforcement with minimum module weigh and lowest cost.
Figure 5.13 Reinforcement Clip Configuration
Figure 5.14 Rein forcement Rig Configuration
Figure 5.15 Reinforcement Clip Installation
- -- -
5.10 Plastic Material Selection -. -- - -A --- .- A- - - -- -
Material selection is directly or indirectly related to many issues, such as product life
cycle, cost, manufacturability and weight. In the pervaporation module, the material selection is
the detemination of the plastics used for the membrane support disks and the central collars.
Plastics are an important group of raw materials for a wide array of manufacturing
operations. Applications include large chemical storage tanks and industrial piping system
handling high 1 y corrosive chemicals. When properl y designed and applied, plastic products
possess the advantages of light, sturdiness, econorny, and comsion resistance.
There are numerous plastics available for industrial purposes. What kind of plastics is
suited for the pervaporation module? To answer this question, fint, the specifications of plastics
used for the pervaporation module must be clarified. Secondly, a basic understanding of plastic
properties is needed. Finally, design should be made based on the comparison of various plastics.
5.10.1 Specifications of the Plastic Components
Based on the application of the pervaporation module, the specifications of the plastic
disks and collars are defined as follows:
a) excellent comsion resistance to the mixture of water-organics, such as IPA and
ethanol,
b) medium mechanical properities, such as tensile strength, compressive strength,
fiexunl strength,
c) operating temperature of 60-100 O C ,
d) cost of raw material and processing.
5.10.2 General Description of Polymers
Plastics an in reality polymers. There are t h e general categories of polymers:
thermoplastic polymers comrnonly called thermoplasts, thennosetting polyrners called
thermosets, and elastomers, more commonly called rubber. Themoplasts are longthain linear
molecules that can easily formed by heat and pressure at temperatures above a critical
temperature. Rather than a long-chah molecule, thermosets consist of a three-dimensional
network of atoms. Thennosets are polymers that assume a permanent shape or set when heated,
although some will set at room temperature. Elastomers are polymetric materials whose
dimensions can be changed deastically by applying a relatively modest force. but which retum to
their original values when the force is released. Obviously, the possibility of using elastomers as
the disk and collar material should be fintly mled out.
In general. thermoplastic materials tend to be tougher and less brittle than thermoset
polymers so they can be used without the need for incorporating fullers. Compared to
thermoplasts, thermoset polymers are more brittle, stronger, harder, and genenlly more
temperature resistant, better creep resistant. By virtue of their basic polymer structure,
thennoplastics have been less dimensionally and thermally stable than thermosetting polymers.
Their disadvantages lie in the facts that most are more difficult to process and more expensive.
Therefore, thennosets have offered a performance advantage, although the lower processing
costs for thennoplastics have given the latter a cost advantage.
5.103 Cornparison of Typical Polymers
Table 5.1 and 5.2 summarize the properties of selected thermoplasts and thermosets. The
data come from different publications, subject to the possible updating, since the materiai
pmperties Vary with the manufacturing and processing.
99
Analysis shows that ethylene chlorotrifluoroethylene (ELTFE) is the best polymer for - 2 - 1 - /- . _AC--.- - ___ _ - * --- - -
pervaporation module -if th'emoplast s are prefekd. When t hermosets are used, rein forced
Polyester (Halogenated Polyester Type) is recommended.
Note the new module with plastic components has not yet being put into production.
Further discussion with the component suppliers are still needed for the determination of module
Table 5.1 Cornparison of Typical Thermoplasts
Thermoplasts Corrosion
Resistance
Max Operating Mechanical
ProperÜes
Cost
(PVC Type 1)
Good Low
Normal Good Low
Fi berg lass-mored PVC
Chlorinated Polyvinyl Chloride
(CPVC)
Polypropylene (PP)
Good Good Medium
Very Good Normal
Good Good Low
Polyethylene (PE)
Ethylene
Chlorotnfluoroethylene
@L"TFE)
Vinylidene Fluoride (PVDF)
NIA
Excellent
Normal NIA
Excellent Medium
Good Good Medium
Table 5.2 Cornparison of Typical T'hermosets
Reinforced Polyester
(Isophthalic Polyester Type)
Rein forced Polyester
(Bisphenol Polyester Type)
Reinforced Polyester
(Halogenated Polyester Type)
Reinforced Vinyl Ester
Reinforced Epoxy
Reinforced Phenol-
Fonnaldehyde
Reinforced Furan Resin
Reinforced Phenolic
Corrosion
Resistance
Normal
Good
Good
Good
Normal
Normal
Very good
NIA
Normal
Mur Operating
Normal 1
Mechanical
Normal &+ Normal
1
80 Normal
149
Medium
Good
149
~ e d i um
Medium
NIA
Good
High
High
Very
High
Medium
*-- ..--
5.11 Design for Assembly and Disassembly --- ---- - 1 A - - -
The ability to assemble or disassemble module components is very important
measurement of product design quality. In the new pervaporation module, the assembly and
disassembly have been considered in every component as follows:
a) Binding of outer disk periphery
The assembly of the segmented disk is the biggest topic in the new module
design. How to clamp the aluminum stmp to the outer periphery of a flexible thin disk is
quite challenging. The answer is to use a jig. The jig design will be presented in the
Chapter 6 in detail.
b) Positioning of inner and outer disk periphery
The inner and outer periphery is unable to fit perfectly due to the manufactunng
emrs. Design should ensure that there is no double constrain occurred at the inner and
outer penpherys. Sufficient clearances have been designed for the pin holes on the disk
elements as well as the notches at the tip of the disk elements. This clearance can
compensate the manufacturing tolerances and component deformation.
c) Assembly and disassembly of collars
The collars have to be firmly connected each other so as to hold the membrane
disk cells and to assure the inner ring sealing. Therefore, torching the collars has to use
certain tools. Four radial holes have been designed on collar flange as shown in Figure
5.16. With the help of a rod inserted into the holes, a mechanist can easily torch the
collars. The reason why we use the holes, instead of regular hex structure, is to simplify
102
the component structure at the prototyping stage, reducing the manufacturing cost. Once P m " . - .----= -.--- ._ *_ _ .> . - - . -. .--- = - - ..- - - - - .
the collar goes to injection molded part, hex structure may be better.
,-, Torch ing Rod
Figure 5.16 Collar Torching Mechanism
d) Assembly and disassembly of reinforcement clips
The reinforcement clips should be designed such that they can be easily installed
to the disks and taken from the disks. To assure a diable clipping, tow opposite forces
have to be applied to the ends of the clip. The deformation of the dots will clamp the disk
rim firmly. Aluminum is an ideal material for the clips because of its ductile property.
In order to disassemble of the clips, three holes are put on the clip. As shown in
Figure 5.17, the slots can be stretched by using ceriain tools, with the help of two short
bars fiüing into the holes.
Figure 5.17 Assembly and Disassembly of a Reinforcement Clip
e) Assembly and disassembly of reinforcement ng
As shown in Figure 5.14, the reinforcement ng consists of two identical half-rigs
which can be bolted together on the flange of a central collar. This structure will facilitate
the assembl y and disassembl y significantl y.
5.12 Prototyping of the Pervaporation Module
Prototyping is a process of new product development widely used in industry. The
general objective of prototyping is to reduce the risk of product development by validating the
key design concepts on prototypes, prior to producing commercial quality of the matenal. The
new pervaporation module described above includes a number of innovate design concepts.
Some expensive manufactunng processes, such as injection molding, are required. Therefore, it
is definitely necessary to go through the prototyping process. The objective of pervaporation
module prototyping includes:
104
a) validate the segmented membrane disk concept, - - - - - - - -
b ) validate the composite seal conceptual, then determine the best arrangement of
sealing matenals,
c) validate the binding mechanism of aluminum clamp.
d) validate the clip reinforcement mechanism,
e) examine the necessity of reinforcement rig,
f) validate the central collar design concept,
g) expose the hidden design defects ahead of time.
In the new pervaporation module. the highest investment is in the injection molded parts.
According to the above-listed objectives, it is unnecessary to validate the key design concepts by
using injection-molded parts. Regular machining paris can be used as the alternatives to check
the design concepts. Based on this idea, a pervaporation module prototype is developed by using
alternative matenals and manufacturing techniques. The prototype is mainly used to examine the
structure design of the module, instead of its chemical separation performance. However.
provision is made for certain selected chemical separation tests. The following table 5.3
compares the difference between the real module and its prototype.
Table 5.3 Cornparison between the Real Module and its Prototype
Membrane
Septrate disk
elements
Penneate channels
Membrane spacers
Rein forcement clips
Bridging washers
Central Collars
Aluminum strap
Outer seals
Inner seals
Design in the Real Module
PAN UF pervaporation composite
membranes
Injection molded plastic parts
Injection molded spiral channel
Build-in spacers on plastic disk
elements
Aluminum clips
Injection molded plastic w a s h e ~
Injection molded or aluminum collars
30 GA x 314 inches aluminum foi1
3M adhzsive tape
0.062 inches THK rubber gaskets
(60 Duro EPDM)
Design in the Prototype
Papen
Laser cut flat Steel sheet parts
Laser cut spiral channel
White polyester mesh sheets
(MSH03880 1200-38WH
SAATI HITECH 80 inches
PW200)
Steel clips
Machining steel washers
Aluminum collars
30 GA x 518 inches aluminum
foi l
3M adhesive tape
0.062 inches THK nibber
gaskets
(60 Duro EPDM)
=--- - .
5.13 Summary --- *-LLP - . - -- - -
This Chapter covers the detailed design processes based on the design concepts proposed
in Chapter 4. The thrust of the detailed design is the individual design goals. Therefore, Design
for X guides the detailed design processes. The design features are summarized as follows:
the membrane support plate is a segmented disk which consists of 16 duplicate
elements,
the outer periphery of the membrane disk is bound by using an aluminum wrap in
conjunction with a composite seals,
the inner periphery of the membrane disk is connected by using a pin washer and
is fixed by two collars.
the collars are designed such that the membrane ce11 spacing. membrane ceIl
holding and penneate transpoitation are integrated in one pan,
Built-in membrane spacers with V shape flow path pattern are molded on the
membrane disk plates,
Separate reinforcement clips and rigs are used for strengthen the membrane disk
and maintain unifonn spacing between the disks at the outer periphery of the
disks without sacrificing weight specification.
Breakable central tube is used to facilitate maintenance.
See the assembly and component drawings in Appendix A and B for the details.
. -
CHAPTER 6 ASSEMBLY JIG DESIGN -----a - - 2- - - -
6.1 Introduction
Jigs can be integral to a successful manufacturing operation. The application of jigs play
a critical role in quality assurance and production efficiency. Jig design and manufacturing are
part of the product development. The financial investment in jigs usually has a significant effect
on the total cost of product development. Jigs can k designed such that the operational assembly
process becomes semi-automated, leading to high production quality, but this will nomally
require increased investment. Nevertheless, jigs of modest cost can be developed to ensure
satisfactory production, en hanced quali ty and reasonable production e fficienc y.
To facilitate the assembly of the segmented disks, it was necessary to design and produce
the appropriate jig; the jig design, which is an essential part of the module development process,
is presented in this Chapter. This jig consists of three sub-assemblies: the disk holding assembly,
the roller assembly and the base assembly.
6.2 FRs of the Membrane Disk Jig
The membrane disk developed in Chapters 4 and 5 is essentiany a thin polymer disk that
consists of 16 duplicate pieces [segments] bound together by a folded aluminum clamp wrapped
dong the periphery of the disk. Each individual piece will, in the final design, be proâuced by
using injection molding techniques. In order to assemble such a flexible component, the jig to be
employed must satisfy the following functional requirements:
a) Position and constrain the individual disk elements,
b) Enhancethestifiessofthediskassemblytoallowfortherollclampingoperation - .
of the folded aluminum strip,
c) Provide for the actual roll folding of the aluminum strip, and
d) Be simple to use and be of Iow cost
Based on these funciional requirements, a jig has k e n designed as shown in Figure 6.1.
The descriptions and explanations are presented in the subsequent sections.
Figure 6.1 Configuration of Membrane Disk Jig
- --= --
6.3 Disk Holàing Assembly Design ------A 2 - A - - - - -
Two solid aluminum disks (23 '/4 inches àiarneter x 95 inches thick) are used to constrain
the 24 inches segmented disk. As shown in Figures 6.1 and 6.2, these two identical solid disks,
'a', secure the membrane disk, 'b'. The two square flats, 'c' and 7, are used simply to fasten the
àisk plates, 'a', in position by using a central bolt, 'e'. The collar, 'w', is used as the central tube
for extraction of the permeate. The collar is also used to radially position each of the separate
disk elements. The threaded nut, 'v', can be screwed to the collar such that the membrane disks
are fastened at the inner ring. In order to ensure that the bolt passes the center of the membrane
disk, a concentric spacer, 7. is used to position the inner diarneter of the collar with the bolt
center. B y threading the concentric spacer up and down, the vertical position of the collar can be
adjusted to suit the position of the membrane disk assembly. Once al1 components are positioned,
the segmented disk will be firmly constrained in the holding plates. A 318 inches namw band
proinides at the periphery of the holding plates which are some 314 inches diameter smaller than
the membrane disk. By using this methoâ of assembly, the flexible membrane disk is sufficiently
secured to withstand the clamping load generated dunng the wrapping of an aluminum strip ai
the outer periphery of the disk.
The author intended to use the simplest approaches to implement the functiond
requirements (a) and (b). All items used in this design can be purchased or simply machined. No
item requires any expensive manufactunng process.
Figure 6.2 Positional Structure of the Disk Inner Periphery
110
6.4 Rolier Assembly Design - L -
The roller assembly is illustrated on the right side of Figure 6.1. Two rollers. 'h' and 'k',
roll fonn the aluminum st ip to warp around the periphery of the membrane disk. The handle, 'p',
drives the top roller, 'h', with the membrane disk rotating about the center, supported by the
bearing, 'g'. The rotating membrane disk drives the bottom roller, 'k'. If the gap between the two
rollers is appropriate, the aluminum strip will firmly wrap around the separate disk elements. The
gap is adjustable by changing the thickness of the spacer positioned between the suppon
structure of the rollea. This type of adjustment provides for a stable roller gap which has a
significant effect on the reliability of the composite sealing.
In order to match the height of the membrane disk with the rollers. spacer nuts are
threaded and positioned on the boit, 'e'. The height of membrane disk can be incrementally
adjusted by the positioning of these nuts. The use of such double nuts allows for inexpensive
manufactunng, incremental adjustment of the membrane for positioning and easy of locking.
Incremental adjustment of disk height is qu i red to compensate for the fixed positioning of the
rol lets.
The double roller mechanism is the physical implement of the functional requirement (c).
6.5 Design of the Base
The base of the assembly is designed to position the disk holding assembly and roller
assembly. The support bearing, 'g', is welded to a channel to increase the contact surface of the
cenier shaft, 'et. Slots are milled on the channel flange to allow for the positional adjustment of
the roller assembly. Provision has k e n made for the base to accommodate different membrane
disk diameters. The use of structural matenal, C channel, again minimizes the manufacturing
cost. Four holes are used for fastening the entire jig assembly to any ground base. since the
operational force required in the rolling of the alurninum wrap can be significant. 111
CHAPTER 7 CONCLUSIONS
7.1 Summary of Results
Engineering design is a comprehensive project management process. A successful
engineering design includes at least the following processes: needs generation, task analysis,
know ledge preparation, methodology selection, conceptual design, detai led design, working
àrawings, manufacturing arrangement, prototyping and design evaluation.
The engineering design of the pervaporation module presented in this thesis has involved
the entire procedure refereed to above. A new pervaporation membrane separation module has
been developed; the new module incorporates many of the successful design concepts of the
earlier FCT module. In addition, numemus innovative design concepts have been satisfactorily
implemented in the new module, according to the revised design specifications.
Suh's axiomatic theory and Design for X methodology have been successfully employed
to guide the overall design process. The former is primaily used to select the optimal design
solutions during the conceptual design phase and allow for the creation of viable design
solutions. The latter focuses on varioiis targets, from concurrent engineering, to the creation of
the detailed design concepts. It is concluded that although the designer's experience is essentid
in creating novel concepts, modern design theories and methodologies also contnbute an
important dimension to creative thinking.
One of the contributions of this thesis is the practical incorporation of two design
methodologies within a real project solution; the impiementation and the bndging between these
design theones and a practical design example requiring resolution.
-7 - ,
Compared with the earlier FCT module, the new module builds on and entends the plate- - &----- -- A n L - - -.- - - - - - - - - - -
and-frame style and the spiral permeate channels. The specific design contributions for the
pervaporation module are as follows:
The design enables the membrane disk diameter to be doubled with a significant increase
in surface area to enhance the pervaporation process; the diameter was increased from 11
inches to 24 inches in the working prototype.
The weight concem was resolved by segrnenting the disk into smaller elements which
can be easily injection-molded. The assembly also incorporates the use of the central
collars through which the permeate can be fed. The collars would also to be injection-
molded eliminating al1 machining costs. Furthemore. gnd patterns molded into the
surfaces of the segmented disks could furiher enhance the flow between the disk and the
membrane.
The jig to assemble the segrnented disks and the membrane to allow for the roll folding of
an aluminum wrap along the periphery (up to 24 inches diameter) has been designed and
demonstrated to function satisfactorily. This aluminum wrapper can be further enhanced
by the use of adhesive treatment of the aluminum to create a composite seal (roll folding
plus adhesive), thereby improving the operation at higher temperature, the reliability and
rnaintainabili ty of the pervaporation modu te.
The unifonn spacing between pervaporation modules disks assembled into a system has
been accomplished by the design of simple positional reinforcement clips which clip ont0
the periphery of the adjacent disks to maintain a defined distance between the disks.
The pin washer assembly facilitates the permeate flow path and represents an important
design feature.
- 6) The design concept is modular and allows for the assembly of multiple disks attached
. - -. - + - . - --. - - - - . L L - A
together by central collars. It &ows for the easy removal and replacement of individual
units within a system to provide for an efficient maintenance service.
The functional requirements and design goals have k e n satisfactorily implemented by
the new design concepts that have been validated by experimentation and prototyping.
7.2 Recommendation for Future Work
The following recommendations are based on the outcome of the current prototype
development :
1) To assemble a number of the pervaporation membrane disks to evaluate the overall
performance using the 24 inc hes âiarneter module.
2) The current approach is based on a segmented disk that provides the advantage of
teduced injection molding costs in the short-terni developrnent stage, i.e. the reduced cost
of the tool, die and molds for smaller components. Eventually, the cost of a decreased
number of disk segments should be considered, leading, possibly, to a unit disk.
APPENDIXES A Selected Assembly Drawings
APPENDIXES B Selected Component Drawings
Figure App-B- 1 Central Col lar
uirm / P M MI ABS PLASTICS
2) DE-BWP
I Figure App-B-2 Separate Disk Element
W H 1rnCRrJiCfS uwlcss o1IvRmr swciriro
UNIVERSITY OF TORONTO l f l L C
DISK ELEMENT
@ K r
DICR O
, O 135 , 035 , 1 5
, 5
, 5 .O
Ki I W C E S
ORAWiNG
SiZE
A
mm sl RnU CH(C*IO Br
.rPPUîMO Br
c R yv riutsu
UP IO O125 025 1 15 5 15 40 M - ./- 1 MG.
uu-oo-w Mlt
MlC
OIIC
S W E I M E 1
/ - O O M 0010 0020 DOJO 0 M D 0060 oim 0 125 INIILU ~cw.-
D * l i 6Sü iD
Figure App-B-3 Pin Washer
APPENDIXES C Selected Pictures
Figun App-C-1 Pewaporation Module Components (Including Aluminum Süap, Reinfomment Clips,
Collar, Plain Washer, Pin Washer, Membrane Disk Element)
Figure App-C-2 Pervaporation Module Assembly (Including Two Membrane Disk Units Reinforced by Thne Clips on the Periphery)
124
Figure A@-3 Raw Membrane Steel Disk Plate (Including Spiral Channels, Uniform Frames and F m Outer Penphery)
Figue AppC4 Membrane Sepmtion Cell (Including Membrane, Aluminum Clamp and Center Collar)
125
Figure App-C-5 Jig in Action (Top View) (Including Roll Folding Illustration, Disk Outer Periphery and Jig Rollers)
Figure App-Cd Jig in Action (Side View) (Including Height Adjustments, Rotating Shafl, Disk Holding and Centenng)
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