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GAS PERMEATION PROPERTIES AND CHARACTERIZATION OF
MATRIMID BASED CARBON TUBULAR MEMBRANE
NORAZLIANIE BINTI SAZALI
A thesis submitted in
fulfillment of the required for the award of the
Degree of Master of Mechanical Engineering
Faculty of Mechanical and Manufacturing Engineering
Universiti Tun Hussein Onn Malaysia
MAC 2015
v
ABSTRACT
Carbon membranes offer high potential in gas separation industry due to its highly selective. Therefore, this study aims to investigate the effect of carbonization parameter such as polymer composition, carbonization temperature, and carbonization environment on the gas separation properties. Polyimide (Matrimid 5218) was used as a precursor for carbon tubular membrane to produce high quality of carbon membrane via carbonization process. The polymer solution containing 5wt%, 10wt%, 13wt%, 15wt%, and 18wt% of Matrimid are prepared based carbon tubular membrane. The polymer solution was coated 3 times on the surface of tubular ceramic tubes by using dip-coating method. Dip-coating technique offer high potential in fabricating defect free carbon membrane. The polymer tubular membrane was then carbonized under nitrogen or argon atmosphere at temperature of 600, 750, and 850 oC with heating rate of 2 oC/min. Matrimid-based carbon tubular membranes were fabricated and characterized in terms of its structural morphology, chemical structure, thermal stability, and gas permeation properties by using scanning electron microscopy (SEM), Fourier transform infrared (FTIR), thermal gravimetric analysis (TGA), and pure gas permeation system, respectively. From the results, it is found that the best polymer composition was 15wt% while the best carbonization temperature for the preparation of carbon membrane-based Matrimid 5218 was at 850 oC. The highest CO2/CH4 and CO2/N2 selectivity of 83.30 and 75.73, was obtained for carbon membrane prepared under nitrogen environment. Meanwhile, for carbon membrane prepared under argon environment, the highest CO2/CH4 and CO2/N2 selectivity of 87.30 and 79.69, respectively, was achieved. The result indicated that the performance of the carbon tubular membrane can be controlled by varying the carbonization environment.
vi
ABSTRAK
Membran karbon menawarkan potensi yang tinggi dalam industri pemisahan gas kerana sifatnya mempunyai kepemilihan yang tinggi. Oleh itu, objektif kajian ini ialah untuk mengkaji kesan parameter pengkarbonan seperti komposisi polimer, suhu pengkarbonan dan persekitaran pengkarbonan terhadap sifat pemisahan gas. Polimida (Matrimid 5218) telah digunakan sebagai bahan utama bagi tiub membran karbon untuk menghasilkan membran karbon yang berkualiti tinggi melalui proses pengkarbonan. Larutan polimer yang mengandungi 5wt%, 10wt%, 13wt%, 15wt%, dan 18wt% Matrimid disediakan berdasarkan tiub membran karbon. Larutan polimer telah disalut 3 kali pada permukaan tiub seramik dengan menggunakan kaedah penyalutan celup. Kaedah penyalutan celup menawarkan potensi yang tinggi dalam menyediakan membran karbon yang sempurna. Polimer membran karbon kemudiannya dikarbonkan di bawah persekitaran gas nitrogen atau gas argon pada suhu 600, 750, dan 850 oC dengan kadar pemanasan 2 oC / min. Tiub membran karbon berasaskan Matrimid kemudian dicirikan dari segi struktur morfologi, struktur kimia, kestabilan terma, dan sifat-sifat penelapan gas dengan menggunakan mikroskopi imbasan elektron (SEM), spektroskopi inframerah (FTIR), analisis gravimetri terma (TGA), dan sistem penelapan gas tulen. Daripada keputusan kajian, didapati bahawa komposisi polimer yang terbaik adalah 15wt% manakala suhu pengkarbonan yang terbaik untuk penyediaan karbon membran berasaskan Matrimid 5218 adalah pada 850 oC. Nilai tertinggi kememilihan untuk CO2/CH4 dan CO2/N2 ialah 83.30 dan 75.73, yang telah diperolehi daripada membran karbon yang disediakan di dalam persekitaran gas nitrogen. Sementara itu, bagi membran karbon yang disediakan di dalam persekitaran gas argon, nilai tertinggi kepemilihan yang untuk CO2/CH4 dan CO2/N2 ialah 87.30 dan 79.69. Keputusan yang diperolehi menunjukkan bahawa prestasi tiub membran karbon boleh dikawal dengan mengubah persekitaran pengkarbonan.
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TABLE OF CONTENTS
TITLE i
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLE x
LIST OF FIGURES xi
LIST OF SYMBOLS AND ABBREVIATIONS xii
LIST OF APPENDICES xv
CHAPTER 1 INTRODUCTION 1
1.1 Background of the study 1
1.2 Problem Statement 5
1.3 Objectives 6
1.4 Scope of the Study 7
1.5 Thesis Outlines 7
CHAPTER 2 LITERATURE REVIEW 9
2.1 Carbon Membrane 9
2.1.1 Separation Mechanism in Carbon
Membranes
9
viii
2.1.2 Configuration of Carbon Membranes 14
2.1.3 Type of Carbon Membrane 15
2.1.4 Advantages of Carbon Membrane 16
2.1.5 Disadvantages of Carbon Membrane 17
2.1.6 Application of Carbon Membrane 19
2.2 Steps Involved in Carbon Membrane
Preparation
22
2.2.1 Polymeric Precursor 23
2.2.2 Polymeric membrane Preparation 26
2.2.3 Stabilization Process 28
2.2.4 Carbonization Process 29
2.3 Forthcoming Guidelines in Carbon
Membrane Study
33
CHAPTER 3 METHODOLOGY 37
3.1 Materials 37
3.2 Experimental procedures 37
3.3 Polymer-based Tubular Membrane
Preparation
40
3.3.1 Dope Formulation 40
3.3.2 Dip-coating Method 42
3.4 Carbon-based Tubular Membrane
Preparation
43
3.5 Characterization Methods 46
3.5.1 Thermogravimetric Analysis (TGA) 46
3.5.2 Fourier Transform Infrared
Spectroscopy (FTIR)
46
3.5.3 Scanning Electron Microscopy (SEM) 47
3.6 Membrane Module Construction 47
ix
3.7 Pure Gas permeation Measurements 49
CHAPTER 4 RESULT AND DISCUSSION 52
4.1 Introduction 52
4.2 Effect of polymer composition (5 wt%, 10
wt%, 13 wt%, 15 wt% and 18 wt%)
52
4.2.1 TGA Results 53
4.2.2 SEM Results 55
4.2.3 FTIR Results 58
4.2.4 Gas Permeation Properties 61
4.3 Effect of Carbonization Temperature
(600 oC, 750 oC, 850 oC)
64
4.3.1 SEM Results 65
4.3.2 FTIR Results 67
4.3.3 Gas Permeation properties 68
4.4 Effect of Carbonization Environment 70
4.4.1 SEM Results 72
4.4.2 FTIR Results 73
4.4.3 Gas Permeation Properties 73
4.5 Comparison with literature 76
CHAPTER 5 CONCLUSION 79
5.1 Introduction 79
5.2 Conclusion 79
5.3 Recommendations 81
PUBLICATIONS 82
x
REFERENCES 84
APPENDICES 95
xi
LIST OF TABLES
Table 1.1 Type of membrane process 2
Table 2.1 Comparison of gas separation performance of supported
carbon membranes reported by previous researchers
21
Table 2.2 Glass transition temperature of polyimides 26
Table 2.3 Stabilization and carbonization process parameter 32
Table 3.1 Chemical and physical properties of NMP and Matrimid 38
Table 3.2 Polymer Solution Formulation 40
Table 3.3 Gas molecules characteristic 50
Table 4.1 Gas Permeation Properties of the Matrimid–based
Carbon Tubular Membrane prepared at different
polymer composition
61
Table 4.2 Gas Permeation Properties of the Matrimid–based
Carbon Tubular Membrane prepared at different
carbonization temperature
69
Table 4.3 Comparison with previous literatures 78
xii
LIST OF FIGURES
Figure 2.1 Typical molecular sieving transport mechanism 11
Figure 2.2 Idealized arrangement of a pore in a carbon material 16
Figure 2.3 Schematic diagram of the carbon membrane preparation 23
Figure 3.1 Flow chart of works 39
Figure 3.2 Illustration of dope solution preparation equipment 41
Figure 3.3 Dip-coating process 43
Figure 3.4 (a) Furnace operation 44
Figure 3.4 (b) Complete furnace operation 44
Figure 3.5 Heat treatment profile 45
Figure 3.6 Tubular membrane module 48
Figure 3.7 Gas permeation apparatus 51
Figure 4.1 Weight loss of Matrimid-based membrane versus
temperature as measured by TGA
54
Figure 4.2 SEM images of cross-sections of the carbon membrane
prepared from (a) 5 wt% concentration, (b) 10 wt%
concentration, (c) 13 wt% concentration, (d) 15 wt%
concentration, (e) 18 wt% concentration
57
Figure 4.3 FTIR analysis of polymeric precursor membrane
prepared at different polymer composition
59
Figure 4.4 FTIR analysis of carbon membranes prepared at
different polymer composition
60
Figure 4.5 Comparison between CO2 permeance readings with CH4
and N2 permeance readings for Nitrogen environment.
63
Figure 4.6 Comparison between CO2/CH4 selectivity readings
CO2/N2 selectivity readings for Nitrogen environment.
63
xiii
Figure 4.7 SEM images of the cross-sections for Matrimid/NMP-
based (a) Polymeric membrane, (b) CM-600, (c) CM-
750 and (d) CM-850
66
Figure 4.8 FTIR analysis of Matrimid-based carbon membranes
prepared at different carbonization temperature
67
Figure 4.9 SEM images of the cross section of Matrimid-based
carbon membranes prepared under (a) Nitrogen gas
environment, (b) Argon gas environment
72
Figure 4.10 FTIR analysis of Matrimid-based carbon membranes
prepared under Nitrogen and Argon gases
73
Figure 4.11 Comparison between Nitrogen gas with Argon gas
permeance readings
75
Figure 4.12 Comparison between Nitrogen gas with Argon gas
selectivity readings
75
xiv
LIST OF SYMBOLS AND ABBREVIATIONS
CO2 - Carbon dioxide
H2 - Hidrogen
O2 - Oxygen
N2 - Nitrogen
CH4 - Methane
SEM - Scanning electron microscopy
FTIR - Fourier transform infrared spectroscopy
TGA - Thermogravimetric analysis
TADS - Thermal analysis data station
H2S - Hydrogen sulfide
H2O - Water
KBr - Potassium bromide
PI - Polyimide
PPO - Polyphenylene oxide
PEI - Polyetherimide
PAN - Polyacrylonitrile
PFA - Polyfurfuryl alcohol
PVDC-AC - Polyvinylidene chloride-acrylate terpolymer
NMP - N-methyl-2-pyrrolidone
GPU - Gas permeation units
BTDA - 3, 3, 4, 4-benzophenone tetracarboxylic dianhydride
xv
DAPI - 5(6)-amino-1-(4’-aminophenyl)-1,3-trimethylindane
P - Permeability
dpP/dt - Pressure increase rate of permeate
Ji - Permeate flux
D - Diffusion coefficient
dCi/dx - Concentration gradient
Tg - Glass Transition Temperature
A - Membrane surface area
(P/l) - Pressure normalized flux
Q - Volumetric flow rate
i - Component i
j - Component j
α - Selectivity
γ - Gamma
Δp - Pressure difference
(SPPO) - Sulfonated poly(phenylene oxide)
6FDA:BPDA-DAM
- 6FDA dianhydride and DAM diamine monomers
Å - Kinetic Diameter
AgNO3 - Silver nitrate
Ar - Argon
ASCM - Adsorption-selective carbon membrane
ATR - Attenuated total reflectance
C=C - Alkene
C=O - Carbonyl group
C5H9NO - N-Methyl-2-pyrrolidone, N-Methyl-4-pyrrolidone, 4-Piperidinone, 2-Piperidinone
C-H - Alkane
xvi
CM - Carbon Membrane
CMSM - Carbon molecular sieve membrane
C-N - Carbon–nitrogen bond
C-N-C - Alkane
CO - Carbon monoxide
ID - Internal Diameter
KBr - Potassium bromide
Kr - Krypton
N-H - Hydrogend Bond
OD - Outer Diameter
PBI - Poly(benzimidazole
PEG - polyethylene glycol
PES - Polyethersulfone
PMDA - Pyromellitic dianhydride
SPAEK - sulfonated poly(aryl ether ketone)
TiO2 - Titanium Dioxide
ZrO2 - Zirconium Dioxide
xvii
xviii
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Sample of Calculation 94 B Gas Permeation Results 97
CHAPTER 1
INTRODUCTION
1.1 Background of the study
There are lots of membrane definitions proposed in the literature [1]. A
selective barrier between two phases is a general definition of membrane and
can be classified into biological and synthetic membranes[2-3]. The synthetic
membranes can be classified into organic (polymer or liquid) and inorganic
membranes while biological membranes is a separating membrane that acts as
a selectively permeable barrier within living things. Membrane process is a
process that uses membrane to achieve particular separation[4]. Membrane
processes can also be categorized according to the driving force as shown in
Table 1.1. As among those processes; membrane for gas separation process has
the highest growth percentages from past decades. The continuity in
development of membrane for gas separation process has been research by
Baker, R.W.[5].
Membrane might be restricted into two categories depending to its
structure or morphologies (symmetric and asymmetric). The determinations of
the total membrane are from the thickness performance of the symmetric
2
membranes. The thicknesses of symmetric membranes are around 10 up to 200
µm. The membrane thicknesses need to be decrease in order to increase the
permeation ratio. In contrast, an asymmetric membrane involve of very solid
top layer or skin with a thickness of 0.1 to 0.5 µm supported by a porous sub
layer with a thickness about 50 up to 150 µm[6].
Table 1.1: Type of membrane process
Driving Force Type of Membrane process
Pressure Gas separation, Reverse Osmosis, Ultrafiltration,
Nanofiltration and Microfiltration.
Concentration Dialysis, Controlled release, Donan dialysis
Partial pressure Vapor Separation, Pervaporation
Electrical Potential Membrane Electrolysis, Electrodialysis, Energy
conversion
Membrane technologies are used in a number of industrial processes
such as the purification of H2; the production of O2 enriched air, separation of
CO2 and H2O from natural gas, and recovery of vapors from vent gases.
Among these separation processes, separation of CO2 from natural gas is the
most emerging application because its emission is strongly associated with the
industrial development and with the energy production from fossil sources
which, and up to now, it cannot be substituted by any other sources except by
nuclear energy[7]. Due to the economic risks involved in treating larger
streams, initial acceptance was slow and limited to smaller streams. This is
because of the general downturn in the oil and gas industry in the 1980s[8].
Membrane separation compromises an encouraging alternate towards
conservative separation technologies for example the separation of CO2 by
amine absorption from light gases [8-9].
The separation of natural gas by thin barriers termed as membranes is a
3
productive and promptly expanding, and it has been verified to be
economically and technically excellent to additional appearing tools[2].
Recently, the membrane gas separation technology has prominently
progressive and nowadays can be marked as a competing industrial gas
separation technique[10]. The two furthermost significant parameters which
can be determined by permeation measurement are the permeance and
selectivity. Permeance is a measure of the amount of permeates that permits
throughout a certain membrane area in a specified time and pressure difference,
while the selectivity is a measure of the membrane quality. In this study, single
gas permeation measurement was used in order to estimate the gas permeation
properties of the carbon tubular membranes.
Nowadays, membranes processes are very important in the industry, for
example in medical applications or separation of petrochemicals and waste
water treatment. Conventional separation methods used for purification of
chemical products, extraction, crystallization, and notably distillation are
energy and cost intensive. The advantages contributed by membrane gas
separation are its simplicity in operation that can be fitted easily onto the power
plant without requiring complicated integration and no need to add chemical or
to regenerate an absorbent or adsorbent[11]. To date, extensive schemes on
polymeric membrane remain broadly applied in dissimilar separation methods
besides require conquered the membrane market in the worldwide[12].
Polymide membrane can gives better separation plus satisfactory permeation
flux. Yet, in the harsh environment, polymeric membranes are not right to be
used for instance those prone towards erosion are high temperatures operation.
As a result, inorganic membranes have rapidly received global attention in
being considered as one of the potential candidates to replace available
polymeric membranes.
Polymeric membranes as Matrimid 5218 are widely used in the
membrane separation process due to their process ability, flexibility and of
4
course low cost[13]. Superior membrane stability and durability, high
permeation flux and selectivity and low production cost are always the most
important criteria when developing a membrane for a specific separation[14].
In membrane processes, the permeability and selectivity are the most basic
properties of a membrane that need to be considered. An important concern in
membrane gas separations is to produce a membrane that is economically
feasible while maintaining a high permeability and selectivity with good
mechanical and thermal stability[15]. However, some limitations of thermal
and mechanical stability need to be considered of the existing commercial
polymers. It has been reported that the permeability’s obtained with carbon
membranes are much higher than those typically found with polymeric
membranes, and these selectivity’s are achieved without sacrificing
productivity. Furthermore, carbon membranes show higher thermal and
chemical stability. These features make the development of this type of
membrane attractive for industrial gas separation systems[16].
Previous researcher stated that when the driving force (pressure ratio)
was lower, the selectivity results was higher and the separation process said to
be more appropriated; in fact, the operating costs for the separation system also
lower[4]. When the permeability was higher, the cost of the system was
lowered and the membrane needed was also small. In order to surpass
Robeson’s upper bound, various researchers have been conducted. Previous
membrane researchers mention that membranes which have the potential to
exceed such upper bound were inorganic membranes. Therefore,
ultramicroporous (0.3–0.5 nm) membranes such as carbon membranes have
shown their promising performance[17]. Carbon membranes also lack of being
damaged in thermal cycles and easily handle at high temperatures when work
with supported ceramic membrane. High permeation flux and high selectivity
are essential requirements for a successful membrane[18].
According to the literature, carbon membranes have continuously
5
earned world attention and considered as one of future nominee to replace any
others membrane such as polymeric membranes. There are a lot of studies of
carbon membranes such as membrane synthesis, characterization, membrane
fabrication, and theoretical modeling have been done to get a superior
membrane performance[19]. As a result, the interest in carbon membrane
application in various separation developments has been increased.
1.2 Problem Statement
Polymeric membranes seem to have several disadvantages that limit
their industrial applications, since the performance of polymeric membranes
would deteriorate with time when they are used in harsh environment[20]. The
inadequacies of polymeric membranes in separation process have encouraged
the researchers to study carbon membrane. This is because of their mechanical
strength, high thermal and chemical resistance and higher pore volume with
high separation factors, as compared to polymeric membrane. In early 1980s,
carbon membranes produced from the carbonization of polymeric materials
have proved to be very effective for gas separation by Koresh and Soffer[21] as
well as Kapoor and Yang[22]. The pore dimensions of the prepared carbon
membrane can be fined by controlling the carbonization conditions. Carbon
membrane has been identified as a solution for the challenges faced by the
current membrane technology[23]. It offers a good combination of
permeability-selectivity trade-off and the separation maintained in the
environment that was prohibited by polymeric membranes previously.
Carbon membranes are very brittle and fragile. Without any support, it
will not easily operate in the system. The most common substrate used by the
previous researchers are α-Al 2O3[24], TiO2-ZrO2 macroporous tubes[25], and
porous graphite[26]. When a supporting substrate is used for the development
6
of carbon membrane, the substrate must be chemically and physically stable
and possess a diffusion resistance, which is lower than that of the carbon
membrane. Tubular substrates are mechanically stronger against a compressing
pressure than flat substrates. In this study, a ceramic tube is used as a supporter
to overcome the brittleness of the carbon membranes. It is due to high thermal
resistance; which necessary to sustain high carbonization temperature and
chemically stable against various solvent [27].
Different process condition will result in different structure of carbon
membrane and gas permeation performance. There are many types of precursor
that have used by previous researchers to develop carbon membrane with better
selectivity and permeability. In fact, it is well known that the membrane
performance appears to be a trade-off between a selectivity and permeability,
for example, a highly selective membrane tends to have low permeability[28].
Therefore, in order to prepare a good performance of carbon membrane,
several process parameters was manipulated in this study. It is involving the
manipulation of polymer composition, carbonization temperature, and
carbonization environment.
1.3 Objectives
Based on the research background and the problem statements above-
mentioned, the objectives of this study are:
1. To fabricate and characterize Matrimid-based carbon tubular membrane
in terms of its thermal stability and structural morphology properties.
7
2. To study the effect of process parameter (polymer composition,
carbonization temperature and carbonization environment) on the gas
separation performance (CO2/CH4 and CO2/N2 separation).
1.4 Scope of the Study
In turn to achieve the above mentioned objectives, the subsequent
scopes of works have been drawn:
1. The polymer solution formulation (5wt%, 10wt%, 13wt%, 15wt%, and
18wt %) is fabricated for polymer tubular membrane.
2. The polymer tubular membrane is fabricated by using 3 times dip-
coating process.
3. The thermal and structural properties of the polymer tubular
membranes is characterize using thermal gravimetric analysis (TGA),
scanning electron microscopy (SEM), and Fourier transform infrared
(FTIR) .
4. The carbon tubular membrane is prepared by heat treatment process.
5. The thermal and structural properties of the carbon tubular membranes
is characterized using thermal gravimetric analysis (TGA), scanning
electron microscopy (SEM), and Fourier transform infrared (FTIR) .
6. The effect of polymer composition (5wt%, 10wt%, 13wt%, 15wt%, and
18wt %), carbonization temperature (600 oC, 750°C, and 850 oC) and
carbonization environment (Nitrogen or Argon gas) is studied on the
gas separation performance.
8
1.5 Thesis Outlines
Chapter 1 of this thesis is about the background of the membrane and
current development in membrane gas for gas separation. Then it includes the
problem caused in carbon membranes process and objective concerning about
the investigation of carbon membranes on effect of process parameter (polymer
composition, carbonization environment and carbonization temperature) on the
gas permeation performance (CO2/CH4 and CO2/N2 separation) and to fabricate
and characterize polymer-based carbon tubular membrane in terms of its
thermal stability and structural morphology properties. Chapter 2 presents
literature review that will focus on previous studies related to the carbon
membranes. The formation of carbon membranes are also discussed here.
From this chapter, the author will get more knowledge on the results of the
previous researches and can expect the result for the project.
Chapter 3 is the overview of the preparation of the polymeric
membranes and carbon membranes. Carbon membrane based supported tubular
have been carbonized inside furnace and the performance was evaluated in
terms of selectivity and permeation rate. Polymeric and carbon membranes
have been characterized by using SEM, TGA, and FTIR. Chapter 4 focuses on
the outcomes of the research and discussion. The gas result needs to be
calculating using specific formulation to get their selectivity and permeance.
The result was analyzed to understand the better selectivity and permeance by
each gas measured. Chapter 5 focuses on the conclusions of the project and
recommendations for future work.
9
CHAPTER 2
LITERATURE REVIEW
2.1 Carbon Membrane
2.1.1 Separation Mechanism in Carbon Membranes
Membrane-based separations processes are becoming increasingly
important in various industrial processes, for instance gas separation process.
Tremendous efforts have been put forward to develop new membrane materials
with excellent permeability and selectivity. Currently, carbon membranes have
been identified as very promising candidates for gas separations, granted by
their attractive characteristics especially in terms of their separation properties.
In fact, numerous numbers of reports have proven and indicated that carbon
membranes can be served as an ideal candidate for gas separation purpose as a
result of their excellent permeability and selectivity [29-30]. Moreover, carbon
membranes also have attracted significant attention as they can offer great
performance without increasing energy and processing cost as the cooling step
can be eliminated[30]. Dislocation in the orientation of aromatic micro
domains in like glasslike matrix gives rise to free volume and ultra micro
10
porosity. The microspores are usually considered to be nearly slit-shaped and
the pore mouth dimensions are similar to the diameters of small molecules[31].
Different mechanisms for gas transport through membranes are depends
on the properties of both permeate and the membrane. Separation mechanism
of gas separation process can be classified into solution diffusion, Knudsen
diffusion, and the molecular sieve effect. These models have been found to be
suitable only for material systems and finite number of gases. The solution
diffusion mechanism is based on the principle that the mixture component with
higher solubility and higher diffusion rate permeates preferentially through the
membrane, which independent on the component sizes. Moreover, solution-
diffusion membranes feature free volume sites which cannot be occupied by
polymer chains due to restricted motion and packing density of the polymer
chains[32].
The components are transported through the membrane by successive
movement between the transient free volume gaps close to the feed side to
those close to the permeate side due to thermal motion of segments of the
polymer chains. Previous researcher stated that various mechanisms can be
elaborated in the transportation of gases through a porous membrane[33]. It
will offer a schematic diagram of the mechanisms for the permeation of gases
through porous in addition to dense membranes[34]. As advances in
technology and separation requirements and applications develop beyond
difficulties, new membrane materials are required to meet the promising output
and effectiveness of the separation. The ultimate morphology of the membrane
achieved will be diverging extremely, varying on the properties of the materials
besides the process conditions applied.
Carbon membranes require the pore size in the range of pore diameter
more than 30 Å in order to achieve a very high selectivity[35]. The carbon
membranes contain constriction in the carbon matrix that approaches the
11
molecular dimensions of the absorbing species[36]. Molecules that are three-
dimensionally smaller than the size of the slit width or plannar shape with
small cross-sections are selectively permeated through the molecular sieve [37-
38]. Hence, carbon membranes are able to separate the gas molecules with
similar size efficiently. Figure 2.1 shows the typical molecular sieving
transport mechanism. According to this mechanism, the separation is caused by
passage of smaller molecules of gas mixture through the pores while larger
molecules are obstructed. It exhibits high selectivity and permeability
components of gas mixture[20].
Figure 2.1: Typical molecular sieving transport mechanism[20]
The concept of membrane for gas separation can be found in the early
1970 which were formed from cellulose esters and polysulfone [39]. Inorganic
membranes are usually designed from metals, ceramics or pyrolyzed
carbon[40]. They have compressed micro porous carbon particles into stainless
steel plugs and measured the diffusion coefficients for different gases like
helium (He), hydrogen (H2), nitrogen (N2), krypton (Kr) and argon (Ar)[41].
However, the remarkably growing interest in developing carbon membranes
12
had started earlier in 1980 when Koresh J. E. and Soffer A.[21] had
successfully prepared crack-free carbon molecular sieve membrane from
carbonization of cellulose hollow fiber membrane. Since then, numbers of
research and publications have been done related to the carbon membranes
[42]. On the other hand, carbon membranes offer much permeability and
selectivity than polymeric membranes, but they are expensive and difficult for
large-scale manufacture[39]. Therefore, it is highly desirable to provide an
alternative cost-effective membrane in a position above the trade-off curves
between permeability and selectivity. Carbon membranes have existed refined
for variation of industrialized operation involving gas separation[43]. For gas
separation, the selectivity and permeability of the membrane material defines
the competence of the gas separation process.
A membrane can be categorized generally into two classes which are
porous and nonporous, based on density and selectivity. A porous membrane is
a rigid, extremely voided structure with about delivered inter-connected pores.
The separation of materials by porous membrane is generally a function of the
permeate character and membrane properties, for example the molecular size
of the membrane polymer, pore-size, and pore-size distribution [33]. Basically,
only those molecules that have different greatly in size might be divided
efficiently by micro porous membranes. A porous membrane is appropriate
exact in its configuration and function to the common filter. Porous membranes
for gas separation perform excessive levels of density nonetheless obtain little
selectivity values. Micro porous membranes are characterized by the average
pore diameter d, the membrane porosity, and asymmetry of the membrane.
They demonstrate unique permeability properties for carbon dioxide together
with high selectivity towards CH4, CO2 and N2 [20].
Carbon membranes have an amorphous porous structure that was
created by the evolution of volatile gases during the carbonization of the
polymeric precursors. The porous structure in carbon membrane contains pores
13
or apertures that approach the molecular dimension of the diffusing gas
molecules[44]. Normally, the pore system of carbon membrane is non-
homogeneous because they consist approximately wide opening among
uncommon conditions. The pores differ in size, figure also degree of the
connectivity reliant by the morphology of the organic precursor besides the
chemistry of its carbonization[45].
The structure of inorganic membranes can be divided into two main
categories based on its configuration which are porous inorganic membranes
and dense (non-porous) inorganic membranes [10, 20]. Nonporous or dense
membranes require prohibitive selectivity properties nonetheless the transport
ratio of the gases upon the medium are frequently low. Even permeates of
related sizes could be divided if their solubility in the membrane different
suggestively said to be an essential property of a nonporous dense membrane.
With excellent permeability, selectivity, good chemical and mechanical
properties, carbon membrane can be well-known through the world. According
to Muraoka, D.[46], asymmetric membranes are the most favoured membranes
structure for industrial applications due to their high gas permeance. For most
cases, asymmetric membrane is generally attempted for many gas separation
membranes to improve the permeability of the designed membrane[47]. In
membrane fabricating process, asymmetric membranes are usually prepared
via phase inversion techniques which are dry-wet spinning and wet
spinning[48].
The carbon matrix is assumed to be impervious, and permeation
through carbon membranes is attributed entirely to the pore system[21]. The
technique of activation or partial burn off, which is used to enlarge the pore
system in carbon, together with annealing at high temperatures in an inert
atmosphere which brings about pore closure, has been employed recently to
prove that the permeability of gases through the carbon membranes proceeds
14
through a pore system of molecular dimensions. The pore system consists of
relatively wide openings with narrow constructions[49].
The openings contribute the major part of the pore volume and
responsible for the adsorption capacity, while the constrictions are responsible
for the stereo selectivity of pore penetration by host molecules and for kinetics
of penetration[21]. Hence, the diffusivity of gases in carbon membrane change
abruptly depending on size and shape of molecules because the carbon
membrane has ultra micropores with critical pore mouth dimensions and pores
size similar to the dimension of gas molecules[50]. The pore size in the carbon
molecular sieve depends upon a balance between removal of O2 containing
groups on the surface around the pore mouth (pore enlargement) and
crystallite rearrangement enhancing sintering (pore shrinkage).
2.1.2 Configuration of Carbon Membranes
Configurations of carbon membrane can be divided into two major
categories: unsupported and supported carbon membranes[51]. Unsupported
membranes have three different configurations: flat (film), hollow fiber and
capillary while supported membranes consisted of two configurations: flat and
tubular. The unsupported carbon membranes are more brittle than the
supported carbon membranes. On the other hand, the supported carbon
membranes require several times the cycle of polymer deposition-carbonization
steps in order to produce an almost crack-free membranes[51]. The choice of a
membrane configuration depends on many factors, consisting of nature of the
polymer, the straightforwardness of formation of a assumed configuration and
structure, the reproducibility of a given structure, the membranes performance
characteristics and structural strength, the nature of the separation, the extent of
use and the separation economics.
15
In order to produce a high quality carbon membrane, some optimum
conditions need to be measured. If the carbon membranes are having low
qualities, the carbonization process might not satisfy the selectivity of the
particular membranes. Hence, polymeric membranes or precursors for
fabricating carbon membranes must be prepared in defect free form in order to
reduce the problems occur in the manufacturing process of the carbon
membranes[52]. Due to the brittleness of carbon membranes, a quite careful
handling is required for carbon membranes[53]. It makes them difficult to be
fabricated especially for larger surface area of membrane. However, this
problem can be overcome by introducing supported carbon membranes either
tube or flat sheet as mention by Kelly et al.[25].
2.1.3 Type of Carbon Membrane
Carbon membrane can be divided into two groups based on their
effective pore size and the separation mechanism towards gas separation[54].
They are adsorption-selective carbon membrane (ASCM) and carbon
molecular sieve membrane (CMSM). A minor modification in the effective
microspore size of the carbon membranes might seriously modify its
permeation rate and consequently its separation[28]. Thus, two different groups
of carbon membrane can be distinguished. Figure 2.2 shows the idealization
arrangement of a pore in a carbon material[55].
Due to Fuertes, A.B.[56], carbon membrane has been mainly focused
on the preparation of CMSM. Among the precursor used for making the
CMSM are phenolic resins [50, 57], polyimide [31, 44, 58], phenol
formaldehyde resin [59-60], and polyetherimide [61-62]. It is believed that the
pore size of CMSM is in the range of 3-5 Å [56]. CMSM is capable to separate
16
the mixtures of permanent gases (<4) such as O2/N2, CO2/N2, CO2/CH4 [63]
based on molecular sieving mechanisms. The separation properties of CMSM
charge abruptly depending on the kinetic diameter and the shape of penetrating
gases[64].
Figure 2.2: Idealized arrangement of a pore in a carbon material[55]
2.1.4 Advantages of Carbon Membrane
Recently, Ismail, A.F. and David, L.I.B.[16] have thoroughly reviewed
the advantages of carbon membrane and its potential application for used in gas
separation. Carbon membrane has been proved to demonstrate higher
selectivity than polymeric membrane especially in the separation of similar gas
molecules such as O2/N2, CO2/CH4 and CO2/N2 [21, 65]. Interestingly, this
higher selectivity was achieved without sacrificed its permeability or
productivity[44]. Most of the time, carbon membranes able to provide excellent
shape selectivity and high chemical stability which are required in gas
separation processes [7, 66].
17
Another interesting feature of carbon membrane is its ability to separate
a gas mixture at any desired temperatures, up to temperature where carbon
membrane begins to deteriorate. For non-oxidizing gases, this temperature may
be high as about 1000 oC [20]. Koresh et al. [21] have continuously tested their
carbon membrane at the temperature at 400 oC during one month and they
found that no deterioration was discernible. In another study, they interestingly
found that carbon membrane also can work at sub ambient temperature which
leads to tremendous increases in selectivity with little or no loss in permeability
especially in separating O2 from Ar to produce pure Ar[67].
Carbon membrane also offers the advantage of operation in
environments that was prohibitive to polymeric membrane. It can perform well
in the presence of organic vapor or solvent and non-oxidizing acids or bases
environments[21]. Furthermore, it is much more resistant towards radiation,
chemicals and microbiological attack. CMSM is usable for a prolonged period
under an atmosphere with contains low levels of oxidants in air
atmosphere[68]. Thus, this time independent factor makes the operating life of
carbon membrane much longer than polymeric membranes.
The membranes become more attractive a useful if they are able to
change and control their pore sizes to effect many gas pair separations. In
fabrication of carbon membrane, the same material can be used to develop
carbon membrane with different permeation properties for different gas
mixtures [50, 69].This was done by simple thermo chemical treatment to meet
different separation requirements and objectives. This attracted more research
in carbon membrane in order to tailor its separation ability to different
application.
18
2.1.5 Disadvantages of Carbon Membrane
Carbon membrane is very brittle and fragile [11, 19, 42]. In the cases of
unsupported carbon membrane for example hollow fiber, careful handling is
required for practical uses. Although the brittleness can be reduced by using
supported carbon membrane, but the preparation of effective support requires
the sequence of polymer deposition-carbonization designate recurring a few
times in turn to achieve an approximately crack-free membrane [20, 25]. Due
to the brittleness of carbon membranes, a quite careful handling is required for
carbon membranes[53]. It makes them difficult to be fabricated especially for
larger surface area of membrane. However, this problem can be overcome by
introducing supported carbon membranes either tube or flat[7].
Carbon membranes surfaces exhibit a strong affinity for O2 even at
room temperature. They are readily chemisorbed O2 on the exposure to air and
the resulting oxygen-containing complex surfaces. This complex surface acts
as a primary site for water sorption[44]. Then, the water molecules attract
additional water molecules through hydrogen bonding, leading to the formation
of clusters. The clusters grow and coalesce, cause the pores filling. This has
negative effect on the properties of CMSM that separate gases based on its
narrow micro porosity[28]. The micro pores were gradually plugged with water
at room temperature and as a result, the permeability to non-polar gases was
decreased. Thus, the modification of the surface properties of CMSM is a key
technology for selective gas separation[68].
Like polymeric membranes, carbon membrane also has the problem of
being vulnerable to fouling due to impurities in various hydrocarbon
compounds. Certain hydrocarbon impurities cause fouling to the carbon
membrane which in turn reduces the gas selectivity[35]. The impurities are n-
hexane and toluene vapor. Even small amount of such hydrocarbons can
significantly impair the performance of the membrane. Therefore, carbon
19
membrane also requires a pre-purifier for removing traces of strongly
adsorbing vapors and impurities by various filtration, separation or extraction
techniques[70]. However, the advantages of carbon membrane are apparently
much more than its disadvantages. This unique characteristic of carbon
membranes has accounts for the wide application of this new technology in
recent industry[19].
2.1.6 Application of Carbon Membrane
Membrane material becomes more attractive if it can offer the balance
in terms of permeability and selectivity for gas mixture separations. It can be
expected that the application of carbon membrane is more important in the new
membrane technology development. Most research on carbon membrane is
tested aimed to gas separation application. Concentrated researches are
specified aimed to the separation of CO2/CH4 and CO2/N2. Separation of these
gas mixture is very difficult task because of it close kinetic diameter. Carbon
membrane offers high selectivity for these gas pairs because of the size
sensibility and precise discrimination in the porous structure of carbon
membrane. The separation factor of CO2/CH4 and CO2/N2 that is greater than
10 is usually achieved with carbon membrane.
Power plants mingled through coal gasification processes required a
hydrogen separation phase around 300- 500 oC. At those hot environments, just
a membrane driven separation process could segregate the H2 from additional
carbon compounds for example CO2 and carbon monoxide (CO). Yet still
organic polymer membranes cants resists excessive temperatures and start to
decompose or react with some components. Inorganic carbon membranes are
chemically inert and thermally secure equal to beyond than 500 oC is much
appropriate for this purpose [50, 63].
20
Separation of CO2 in flue gas emissions from power plants that burn
fossil fuels constitutes gives a potential way to recover this gas and avoid its
emission to the atmosphere. That could be a possible application of carbon
membrane in a near future as suggested by the high separation selectivity
CO2/N2 achieved with these materials[71]. Table 2.1 shows the comparison of
gas separation performance of supported carbon membranes reported by
previous researchers.
Carbon membrane additionally could be applied for the separation of
alkenes with alkanes and olefins with paraffin’s gas mixtures. The preparation
of supported polyimide carbon membrane reported that virtuous separation
performance for the separation of ethylene/ethane and propylene/propane
mixtures can be achieved[68]. Meanwhile, Zhao et al. [72] analyzed
olefins/paraffin’s separation by using asymmetric hollow fiber polyimides
carbon membranes. Recently, Fuertes et al. [56] prepared phenolic resin based
carbon membrane for olefins/paraffin’s and n-butane/iso-butane separations. It
is conceivable to use carbon membrane to distinct isomers of hydrocarbons
between normal and branched polymers [21].
Table 2.1: Comparison of gas separation performance of supported carbon membranes reported by previous researchers.
Polymer/Precursor Configuration CO2/CH4 CO2/N2 N2/CH4 O2/N2 H2/N2 References
Poly(benzimidazole) (PBI)
blends with Matrimid
Flat-sheet 131.65 - 4.53 8.70 257.14 [73]
6FDA-bisAPAF Flat sheet 45.0 - - - - [74]
PMDA–ODA polyimidea Flat-sheet - 12.3 - 10.4 76.3 [15]
sulfonated poly(phenylene
oxide) (SPPO)
Hollow-fiber 197 58 - 12.1 400 [75]
Matrimid 5218 Hollow Fiber - 21.83 - 5.5 146.5 [76]
Matrimid-M Hollow Fiber 34.5 33.2 - 6.5 - [77]
PEI/PVP Hollow Fiber 69.00 34.50 - - - [78]
Zeolite Tubular 97.3 51.6 - - - [24]
Phenolic resin Film - - - 5.0 - [57]
Sulfonated poly(aryl ether
ketone), SPAEK
Film 67 - - - 220 [31]
21
22
2.2 Steps Involved in Carbon Membrane Preparation
Carbon membranes are typically prepared by the carbonization of
aromatic polymers[75]. Carbonization is the most important step in the
fabrication of carbon membrane and can be recognized as the heart of the
carbon membrane process. There are few important steps that need to be
regarded in order to enhance the performance of carbon membrane such as
precursor selection, membrane preparation as well as carbonization
conditions[42]. As advances in technology and separation requirements and
applications develop further challenging, new membrane materials are required
to fulfil separation productivity and efficiency. Carbon membranes are
fabricated to overcome the incapability of polymeric membranes. Figure 2.3
shows the schematic diagram of the carbon membrane preparation.
Selection of altered precursors might give dissimilar results of carbon
membranes. Therefore, the select of the polymeric precursor is the most
imperative aspect. For polymeric precursor selection, several factors should be
considered. Ideally, polymeric precursor with good permeability-selectivity,
high thermal, mechanical and chemical stability is favourable to produce high
performance ofcarbon membrane. Polyimide-based carbon membranes showed
that the maximum encouraging performances[79]. Membrane defect free
carbon additionally is well derived from polyimide membranes that can endure
high temperatures and without treatment softens and breaks down suddenly and
quickly. These materials can produce carbon membrane with great carbon yield
and maintain their structural form after heat treatment at high temperature
because of unusually durable and surprising heat and chemical resistance[18].
23
Figure 2.3: Schematic diagram of the carbon membrane preparation.
2.2.1 Polymeric Precursor
Polyimides are often employed as carbon precursors to prepare in
various morphologies. Polyimides have been developed as thermo resistant
polymers, and have been used in different fields, especially in the field of
electronics[65]. Due to their practical and promising applications, there have
been commercially available polyimide films with different molecular
structures, as a consequence, with different performances in applications. Also
different polyimides have been studied by the synthesis in the laboratories.
Polyimides are said to be greatest stable classes of polymers. The temperatures
can be rising up to 300 °C and higher, and determinations frequently
Precursor Selection
Polymeric Membrane
Stabilization
Carbonization
Heat Treatment
Carbon Membrane
24
decompose before reaching their melting point. Normally, they cannot examine
a melting phase transition and lose their shape. Polyimides are also very
decent precursor’s selection for carbon membranes[34].
In this research, polyimides have been used is Matrimid 5218. Matrimid
5218 not only can be carbon precursors, it also produces high carbon yields and
the procedures for carbonization and graphitization being simple. Moreover,
Matrimid 5218 gives high viscosity solutions at low polymer or solvent ratios.
Fully imidized polyimides with high glass transition temperatures (Tg), such as
Matrimid 5218, are suitable polymer precursors because their structure
gradually changes during carbonization; this is a key characteristic for
obtaining defect-free Carbon Molecular Sieve Membranes (CMSM)[80]. The
glass transition temperature, Tg of polyimides are shown in Table 2.2. The
purpose of using Matrimid 5218 as the main polymeric precursor because it is
readily soluble and exhibits an excellent combination of selectivity and
permeability for many significant gas pairs that is superior to those of most
other commercial polymers[81]. Due to the interesting of Matrimid 5218 as a
polymer for gas-separation membranes, its provide good solubility of in
common organic solvents enables bulk chemical modifications to be performed
as long as the employed reagents do not induce chain degradation or cross
linking reactions with the more sensitive benzophenone and imide carbonyl
structural segments present.
The heat treatment of Matrimid membrane up to 850 oC has formed
carbon membrane comprising both ultramicropores and larger micropores. The
ultramicropores are supposed to be commonly responsible for molecular
sieving mechanism while the micropores offer insignificant resistance to
diffusion but provide high capacity absorption site for penetrates[34]. The
characterization of the obtained Matrimid polymer based carbon tubular
membrane is also a key issue because it is not always straightforward to
analyze real samples of supported carbon membranes. Some studies have
83
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