production of cyclohexane from benzene.doc

Production of Cyclohexane from Benzene

Session 2005-2009

Project Advisor

Prof. Dr. Shahid Naveed

Authors:

Zaeema Tahir 2005/FC-CPE-10

Sidra-tul-Muntaha 2005/FC-CPE-18

Ahmad Waqas 2005/FC-CPE-16

Usman Hameed 2005/FC-CPE-03

DEPARTMENT OF CHEMICAL ENGINEERING

U.E.T - LAHORE -PAKISTAN

This report is submitted to department of Chemical

Engineering, University of Engineering & Technology

Lahore- Pakistan for the partial fulfillment of the

requirements for the

Bachelor’s Degree

In

CHEMICAL ENGINEERING

Internal Examiner: Sign:_____________

Name:______________

External Examiner Sign:_______________

Name:_______________

DEPARTMENT OF CHEMICAL ENGINEERING

UNIVERISITY OF ENGINEERING AND TECHNOLOGY

LAHORE-PAKISTAN

DEDICATED TO

Our

Beloved Parents,

Respected Teachers

And

Sincere Friends

ACKNOWLEDGEMENT

We express gratitude and praise to ALMIGHTY

ALLAH, the creator of universe, who is beneficent and

merciful, guided us in difficult and congeal circumstance,

who endowed us with the will to undertake this design

project. Great respect our Holy Prophet Hazrat Muhammad

(PBUH), who taught us to learn till lap of grave.

At this point, the end of a purposeful learning

period, our emotions are very strong, feelings are deep, and

we are still remembering the time when our dreams came

true and we came at U.E.T, a dynamic institution with

professionals loving and professional making setup.

The time which was spend over here, the practical

and conceptual knowledge which we gained made this

golden time, of course, a milestone in our professional

career with the name of department of chemical engineering

a long list of polite cooperative and affectionate professional

teachers came across our mind. For whom we confess our

negligence of vocabulary to say thanks for their assistance.

We pay special homage to our respective

teachers; Dr. Saleemi (Chairman of Department) and our

lenient and cooperative project advisor Prof. Dr.Shahid

Naveed, who really paid their special attention in the

completion of our project.

TABLE OF CONTENTSPREFACE.......................................................................................................................................1

CHAPTER 1......................................................................................................................................

Introduction................................................................................................................................4

CHAPTER 2......................................................................................................................................

Process selection and description of flow sheet....................................................................10

CHAPTER 3......................................................................................................................................

Material balance & Energy balance.......................................................................................22

CHAPTER 4......................................................................................................................................

Design of Equipments..............................................................................................................40

Reactor design ..................................................................................................................41

Vapor/liquid separator design............................................................................................58

Stabilization column design..........................................................................................................72

Heat Exchanger design..................................................................................................................89

CHAPTER 5......................................................................................................................................

Mechanical Design of Heat Exchanger................................................................................108

CHAPTER 6......................................................................................................................................

Instrumentation and Process control..................................................................................112

Control scheme of Outer-recirculation Cooler................................................................119

CHAPTER 7......................................................................................................................................

HAZOP Study ........................................................................................................................120

HAZOP Study of Gas/Liquid Separator.............................................................................................126

CHAPTER 8......................................................................................................................................

Environmental impacts of Cyclohexane Plant.....................................................................129

CHAPTER 9.....................................................................................................................................

Material of Construction.......................................................................................................136

CHAPTER 10....................................................................................................................................

Cost Estimation .....................................................................................................................140

REFERENCES..........................................................................................................................143

APPENDIX.................................................................................................................................145


PREFACE

This project is submitted to the Department of Chemical Engineering, University Of

Engineering And Technology Lahore, Pakistan, for the fulfillment of the Bachelors

Degree.

This research report is concerned about the activity of designing a plant for manufacture

of Cyclohexane. The study of said subject offers a way to make Pakistan self supported in

cyclohexane, as all consumer society of it imports this chemical from Saudi Arabia,

China, UAE and Malaysia. The report describes the most economical way to produce

cyclohexane in Pakistan keeping in view all the resources of country.

Cyclohexane is the major pre-cursor for the production of Nylon. Automotive

applications of nylon have been growing strongly where there has been a drive to replace

metals with plastics to reduce the weight of motor. Hence it will a cost effective solution

in this sector also.

The survey of demand of cyclohexane in Pakistan has been made with the help of Lahore

Chamber Of Commerce and Trade. Hence the production capacity of our plant is based

on the present needs of it.

The basic structure of report is given below.

Chapter # 1 is the introduction of cyclohexane that covers the areas of demand of it in

present days and as well as in future. A brief view of natural resources and physical

properties has been given. The properties of benzene and hydrogen have been given

where ever their need is. Important Industrial applications and discovery of this chemical

has also been given.

Chapter # 2 is Process selection and description of flow-sheet. This chapter gives the

concise listing of commercial processes used for synthesis of cyclohexane. These include

liquid and vapor phase processes, while the one by one description of each process has

1


been avoided but major strengths of each category has been stated to the level as is the

demand of the work. Contrary to this, process selection is based on the economic analysis

of different methods of production. In the end a detailed description of flow-sheet and

process has been given. The flow-sheet has been taken from Encyclopedia of design and

process for chemical engineers.

Chapter #3 is Material and energy balance of the plant. The lengthy calculations have

been given in tabular form.

Chapter # 4 is the Design of Equipments in which operating conditions, process

conditions and Design of equipments has been given in details. The conventional design

of each equipment has been preferred on the complex designing procedures. Authors

have made efforts to their level best to make all the equipments economical and easy to

handle for plant personnels.

Chapter # 5 is the Instrumentation and process control. This is the vast field of

research and makes the job complex. Report states the control of temperature in outer

recirculation cooler. The complete PID has been given. Above it automatic control is the

recommended throught out the plant that results in saving labor expenses along with

improved ease and efficiency of operations.

Chapter # 6,7,8.9 and 10 covers Mechanical design, Hazop study, Environmental

impacts of cyclohexane, Material of construction and Cost estimation respectively.

The contents of these chapters give an over view of the respected subject. There is no

such detail demanded in this report.

Plant has not its own power set-up rather it is recommended to purchase on commercial

level.

Although nomographs, simplified equations, and shortcut methods are included, every

effort has been made to indicate the theoretical background and assumptions for these

relations.SI units are emphasized but older fps and cgs systems have not been completely

2


removed. Conversion factors have been given where ever is needed. The property data

has been collected from various books and simulation software Hysys at the process

conditions.

Finally, as is customary, the errors that remain are our.

The Authors

Zaeema TahirSidra-tul-muntaha

Usman HameedWaqas Ahmed

3


Chapter # 1

INTRODUCTION

Cyclohexane is a cycloalkane. Cycloalkanes are types of alkanes which have one or more

rings of carbon atoms in the chemical structure of their molecules. Alkanes are types of

organic hydrocarbon compounds which have only single chemical bonds in their

chemical structure. Cycloalkanes consist of only carbon (C) and hydrogen (H) atoms and

are saturated.

Cyclohexane has following synonyms. Benzenehexahydride,Ciclohexano,

Hexahidrobenceno, Hexahydrobenzene, Hexamethylene, Hexametileno, Hexanaphthene,

Naphthene.

Nylon growth, which is the main driver in the cyclohexane market, has stagnated in many

applications to below GDP levels although there is still some growth in nylon plastics for

automotive and other resin applications. One of the better performing markets for nylon

is engineering thermoplastics. These materials have tough physical properties such as

high tensile strength, excellent abrasion, chemical and heat resistance, which allow them

to replace metals. Automotive applications have been growing strongly where there has

been a drive to replace metals with plastics to reduce the weight of motor vehicles.

FUTURE DEMAND

Future global demand growth for cyclohexane is put at

around 2-3%/year. SRI Consulting estimates global demand

for cyclohexane was just over 5m tonnes in Figure1.1 2005.

With an average growth rate of 3%/year, demand should reach

6m tonnes by 2010.

In Europe, future demand growth is about 2%/year with the

4

http://www.petr0.com/Cycloalkane/encyclopedia.htm





main growth in engineering plastics and some industrial filament uses.

Any new investment in cyclohexane is likely to be focussed in the Middle East and Asia

where demand growth is strongest. Aromatics Thailand started up a 150,000 tonnes/year

plant in May 2006 although production could be increased to180,000 tonnes/year

depending on feedstock availability.

NATURAL RESOURCES

Cyclohexane occurs naturally in crude oil and can be released from petroleum

fractions whenever they are refined, stored or used. Another major release is from motor

gases. Naturally is also released from volcanos. It is produed in large quantities for nylon

production and is released in water from plants. However, cyclohexane is resistant to

biodegradation, it degrades only in the presence of other petroleum fractions.

Volatization from water should be the fate process for aquatic life. While bio-

concentration in aquatic organism and adsorbtion to sediments is estimated to occur to a

moderate extent.

PROPERTIES

Cyclohexane, C6H12, formula weight 84 is a colorless, water-insoluble, non-

corrosive liquid. It is an excellent solvent for cellulose ethers, resins, fats, waxes, oils,

bitumen and crude rubber. The main use is as an intermediate in the manufacture of

nylon.

Table 1.1

Cyclohexane

Solvent Properties

5


CAS 110-82-7

Physical Properties

Molecular weight 84.16

Boiling point 80.72°C

Vapor pressure 77.5 Torr at 20°C

Freezing point 6.54°C

Refractive index 1.4262 at 20°C

Density 0.7785 g/mL (6.497 lb/gal) at 20°C

0.7739 g/mL (6.457 lb/gal) at 25°C

Dielectric constant 2.02 at 20°C

Dipole moment 0 D at 20°C

Polarity index (P') 0.2

Viscosity 1.0 cP at 20°C

Surface tension 24.98 dyn/cm at 20°C

Solubility in water 0.006% at 25°C

Solubility of water in cyclohexane 0.01% at 20°C

Storage Store in an area designed for

flammable storage, or in an approved

6


metal cabinet, away from direct

sunlight, heat and sources of ignition.

Flash point -4°F (-20°C) by closed cup

Lower explosive limit 1.3%

Upper explosive limit 8.0%

HISTORY

In 1867 Marcellin Berthelot reduced benzene with hydroiodic acid at elevated

temperatures. He incorrectly identified the reaction product as n-hexane not only because

of the convenient match in boiling point (69°C) but also because he did not believe

benzene was a cyclic molecule (like his contemporary August Kekule) but rather some

sort of association of acetylene. In 1870 one of his sceptics Adolf von Baeyer repeated

the reaction and pronounced the same reaction product hexahydrobenzene and in 1890

Vladimir Markovnikov believed he was able to distill the same compound from Caucasus

petroleum calling his concoction hexanaphtene

In 1894 Baeyer synthesized cyclohexane starting with a Dieckmann condensation of

pimelic acid followed by multiple reductions:

7

http://en.wikipedia.org/wiki/Pimelic_acid

http://en.wikipedia.org/wiki/Dieckmann_condensation

http://en.wikipedia.org/wiki/Petroleum

http://en.wikipedia.org/wiki/Vladimir_Markovnikov

http://en.wikipedia.org/wiki/Adolf_von_Baeyer

http://en.wikipedia.org/wiki/Acetylene

http://en.wikipedia.org/wiki/August_Kekule

http://en.wikipedia.org/wiki/Boiling_point

http://en.wikipedia.org/wiki/N-hexane

http://en.wikipedia.org/wiki/Hydroiodic_acid

http://en.wikipedia.org/wiki/Benzene

http://en.wikipedia.org/wiki/Organic_reduction

http://en.wikipedia.org/wiki/Marcellin_Berthelot


and in the same year E. Haworth and W.H. Perkin Jr. (1860 - 1929) did the same in a

Wurtz reaction of 1,6-dibromohexane.

Surprisingly their cyclohexanes boiled higher by 10°C than either hexahydrobenzene or

hexanaphtene but this riddle was solved in 1895 by Markovnikov, N.M. Kishner and

Nikolay Zelinsky when they re-diagnosed hexahydrobenzene and hexanaphtene as

methylcyclopentane, the result of an unexpected rearrangement reaction.

INDUSTRIAL APPLICATIONS

1-Commercially most of cyclohexane produced is converted into cyclohexanone.

Cyclohexanone is the organic compound with the formula 5CO. The molecule consists of

six-carbon cyclic molecule with a ketone functional group. This colorless oil has an

odour reminiscent of pear drop sweets as well as acetone.

2-Cyclohexanol (or "KA oil") is the organic compound and is formed by

catalytic oxidation. KA oil is then used as a raw material for adipic acid. Adipic acid is

the organic compound with the formula 4(CO2H)2. From the industrial perspective, it is

the most important dicarboxylic acid: About 2.5 billion kilograms of this white

crystalline powder are produced annually, mainly as a precursor for the production of

nylon.

3-Cyclohexane is also an important organic solvent.

8

http://www.petr0.com/Adipic_acid/encyclopedia.htm



http://www.petr0.com/Cyclohexanol/encyclopedia.htm

http://www.petr0.com/Cyclohexanone/encyclopedia.htm



http://en.wikipedia.org/wiki/Rearrangement_reaction

http://en.wikipedia.org/wiki/Alkyl_cycloalkane

http://en.wikipedia.org/wiki/Nikolay_Zelinsky

http://en.wikipedia.org/w/index.php?title=N.M._Kishner&action=edit&redlink=1

http://en.wikipedia.org/wiki/Wurtz_reaction

http://en.wikipedia.org/wiki/File:CyclohexaneBayerSynth.png

http://en.wikipedia.org/wiki/File:CyclohexaneSynthesisPerkin.png

http://en.wikipedia.org/wiki/File:CyclohexaneBerthelot.png


Also it is used in Electroplating , Electroplating - Vapor Degreasing Solvents, Laboratory

Chemicals, Solvents – Extraction, Machinery Mfg and Repair , Rubber Manufacture,

Solvents - Rubber Manufacture, Wood Stains and Varnishes

STRUCTURE

Cycloalkanes (also called naphthenes , especially if from petroleum sources) are types

of alkanes which have one or more rings of carbon atoms in the chemical structure of

their molecules. Alkanes are types of organic hydrocarbon compounds which have only

single chemical bonds in their chemical structure. Cycloalkanes consist of only carbon

(C) and hydrogen (H) atoms and are saturated because there are no multiple C-C bonds

to hydrogenate (add more hydrogen to). A general chemical formula for cycloalkanes

would be CnH2(n+1-g) where n = number of C atoms and g = number of rings in the

molecule. Cycloalkanes with a single ring are named analogously to their normal

alkane counterpart of the same carbon count: cyclopropane, cyclobutane, cyclopentane,

cyclohexane, etc. The larger cycloalkanes, with greater than 20 carbon atoms are

typically called cycloparaffins.

Cycloalkanes are classified into small, common, medium, and large cycloalkanes,

where cyclopropane and cyclobutane are the small ones, cyclopentane, cyclohexane,

cycloheptane are the common ones, cyclooctane through cyclotridecane are the medium

ones, and the rest are the larger ones.

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http://www.petr0.com/out/Cyclohexane/encyclopedia.htm

http://www.petr0.com/out/Cyclopentane/encyclopedia.htm

http://www.petr0.com/out/Cyclobutane/encyclopedia.htm

http://www.petr0.com/out/Cyclopropane/encyclopedia.htm

http://www.petr0.com/out/Alkane/encyclopedia.htm

http://www.petr0.com/out/Chemical_formula/encyclopedia.htm

http://www.petr0.com/out/Hydrogenation/encyclopedia.htm

http://www.petr0.com/out/Hydrogen/encyclopedia.htm

http://www.petr0.com/out/Chemical_bond/encyclopedia.htm

http://www.petr0.com/out/Chemical_compound/encyclopedia.htm

http://www.petr0.com/out/Hydrocarbon/encyclopedia.htm

http://www.petr0.com/out/Organic_compound/encyclopedia.htm

http://www.petr0.com/out/Molecule/encyclopedia.htm

http://www.petr0.com/out/Chemical_structure/encyclopedia.htm

http://www.petr0.com/out/Atom/encyclopedia.htm

http://www.petr0.com/out/Carbon/encyclopedia.htm

http://www.petr0.com/out/Alkane/encyclopedia.htm

http://www.petr0.com/out/Petroleum/encyclopedia.htm


Chapter # 2

PROCESS SELECTION &

DESCRIPTION OF FLOW

SHEET

Commercially cyclohexane is synthesized by various processes. Each process has its

own merits and demerits. Categorizing various processes we can differentiate

among them on following characteristics;

1) OPERATING CONDITIONS

There exist two types of processes one is called liquid phase process and other

is called vapor phase process of cyclohexane manufacture. The phase to be

handled dictates the operating conditions of process. In liquid phase processes the

operating temperature is comparatively low. Hence is less costly process.

Vapor phase processes yield an undesirable low output per unit volume of

reactor zone. This is not only due to low density of treated products but also due

to difficulties encountered in cooling of said reactor zone. It is necessary to use

bulky apparatus comprising critical and costly cooling coils.

2) CATALYST TYPE

Liquid phase or vapor phase

Type of metal used

3) TEMPERATURE CONTROL

10


The method used to offset the rise in temperature due to exothermicity of the

reaction. In a fixed bed, this problem can be solved in two ways.

I. By installing several adiabatic reactors in series, and lowering the

temperature between each reactor, by direct quench, or by cooling in heat

exchangers; however, this solution requires considerable equipment.

II. By using a Latitude reactor with1 circulation of a heat transfer fluid on the

shell side; however, the need to fill each catalyst tube uniformly to

guarantee uniform pressure drops, flow rates and unit conversions, as

well as the necessarily large no. of these tubes, makes this solution costly

in terms of capital expenditure and problematic in operation.

If the catalyst is in suspension, the heat can be removed by the circulation of the

medium outside the reactor, through a heat exchanger. Various liquid and vapor

phase processes are tabulated below with their prominent characteristics.

LIQUID PHASE PROCESSES

TABLE 2.1

Process Name Operating cond. Catalyst Heat Removal

UPO (Universal oil Temp: 200 - 300°C Fixed bed of of

Pt

Quenching shots from

products) Hydrar Press: 3xl06Pa abs pt based catalyst cooled reactor effluent

process

Houdry Process Temp: 160 - 235°C Pt-based catalyst Three reactors in

Press: several atms in fixed beds. series; 1st treats bulk of

feed and recycle, ,

11


2nd treats effluent from

1st, remainder feed and

recycles. Adiabatic

operation.

Sinclair/engelhard Temp; 250°C Noble metal Heat is removed in

process fixed bed. Situ by means of a tube

bundle with the

production of steam

IFP (Institut Temp: 200 - 240°C Raney 'Nickel in Outer- recirculation

Francais du Petrole) Press: 35 atm Suspension Heat removal.

VAPOR PHASE PROCESSES

Table 2.2

Process Name Operating cond. Catalyst Heat Removal

Bexane DSM:

Nederlandse

Temp. 370°C

Pressure 3xl06pa

abs

Pt-based catalyst By a coolant

Hytoray ProcessTemp. 370°C

Pressure 3xl06pa abs

Pt-based

catalyst

By a coolant

12


SELECTED PROCESS FOR CYCLOHEXANE

MANUFACTURE

For this design report, IFP liquid phase process is selected. IFP process is a

mixed phase process; i.e; it is a hybrid of liquid phase and vapor phase process.

This process enjoys the benefits of both process and makes it economical.

Majorly it converts benzene in liquid phase at low temperature after that it

eliminates the inherited drawback of liquid phase process of low purity by

converting rest of the benzene in vapor phase hence also relaxes the need of costly

reactor.

The main features of this process are given below;

1. It is a liquid phase process that is a stable system with respect to control point of

view.

2. Better heat removal system i.e., by outer-recirculation cooler, so an isothermal

reaction is achieved.

3. Pressure is high which give higher yields at a particular temperature.

4. Lower temperatures can be selected in liquid phase which give higher equilibrium

constant values as the process is exothermic.

PROCESS DETAILS

(I) BASIC CHEMISTRY

The hydrogenation of benzene proceeds according to:

C6H6 +3H2 C6H12

One mole of benzene reacts with three moles of hydrogen to produce one mole of

cyclohexane. The reaction is highly exothermic, liberating 91500 btu/lb-mol of benzene

converted at 300 oF.

13


(II) REACTION KINETICS

The kinetics are first order in hydrogen partial pressure, zero order of benzene, and

independent of the pressure of cyclohexane.

PROCESS DESCRIPTION & PROCESS FLOW DIAGRAM

Fresh benzene from storage tank at 25oC and 1 atm, make-up hydrogen, and

recycle hydrogen are heated to reaction temperature, (benzene in heat exchanger and

hydrogen is heated by compressing adiabatically) and fed to the slurry reactor. Slurry

phase reactor is an isothermal reactor in which benzene in liquid form and hydrogen in

gas phase is introduced and reaction takes place on Raney nickel catalyst. The conversion

in this reactor is 95%. Slurry phase reactor is provided with an outer-recirculation heat

exchange/cooler which removes the heat of reaction and low pressure (70 psi) steam in

generated. Temperatures in the reactor are held below 204oC to prevent thermal

cracking, side reactions and an unfavorable equilibrium constant that would limit benzene

conversion.

Next to the slurry phase reactor, a catalytic fixed bed pot reactor is provided

which makes-up the conversion almost to 100%. In this reactor the reaction takes place

in vapor phase .Effluent from the fixed bed reactor is condensed and cooled to 160°C and

then this Gas liquid mixture is flashed to 10 atm in a gas liquid flash separator. Excess

hydrogen is recycled to slurry phase reactor and liquid from separator is fed to the

stabilizer column to remove dissolved hydrogen. Liquid product from bottom of

stabilization column at 182oC is cooled in product cooler and send for final storage. The

overheads of low pressure flash are 95% hydrogen which is used as fuel gas or mixed

with sales gas.

14


15


HYDROGENATION CATALYSTS

1. FOR LIQUID PHASE

Nickel and noble metals (rhodium, ruthenium and Platinum) are catalysts for benzene

hydrogenation, commonly and for this project Raney Nickel in suspension is used as a

catalyst for liquid phase hydrogenation. Nickel catalysts require generally high

temperatures and pressures.

Raney Nickel is powdered alloy of Nickel with aluminum, activated with caustic soda

solution. Normal percentage of aluminum in the alloy is 10--15%.

Raney Nickel is classified as W1 W2, W3, W5, W6, W7 and W8 due to the activity

difference mainly imparted from the method of preparation. Most active grade is W 6 but

minimum allowed temperature is < 100°C. We select W2 grade because it can be easily

stored under solvent contained sealed container. Nickel catalysts are especially

susceptible to sulfur poisoning. Sulfur compounds in feed are kept below Ippm. Carbon

monoxide is also mentioned as a catalyst poison for Nickel and concentration in the

feeds should be kept below 20 ppm. Catalysts must have high degree of hydrogenation

activity because benzene conversion must be nearly complete to meet product purity.

Activity increases with hydrogen adsorption on the surface.

SLURRY CATALYST SYSTEM

Particle size = 150 °A

Density of cat. = 8:9 g/cc.

Conc.in solution = 0.07%.

2. VAPOR PHASE CATALYST

16


Instead of Raney Nickel, Nickel oxide (NiO) supported on alumina (Al2 03) is used for

vapor phase hydrogenation in) fixed bed pot reactor.

The Characteristics of system used are given below:

Diameter of pellet, Dp =60µm

Specific surface, Sg =278m2/g

Specific Volume, Vg =0.44cm3/g

Density of catalyst, pg =2.63g/cm3

Density of pellet, pp =2.24g/cm3

αµ =29oA

REACTION CONDITIONS SELECTION

TEMPERATURE SELECTION

Because it is an exothermic reaction, the equilibrium constant decreases as

the temperature is increased. Conversely at very low temperatures, the reaction rate

is impractical. There are two limits for high temperature selection.

• At 260oC, thermal cracking of benzene begins.

• At 248oC, isomerization of cyclohexane to methyl cyclopentane begins.

So upper temperature range is 248.88oC

In the following, a table T 2.3 is produced which shows the variations in equilibrium

constant values versus temperature.

Table 2.3

17


TEMPERATURE

(OC)EQUILIBRIUM CONSTANT,K.

93 2.29 XlO10

149 2 . 6 x 1 0 6

204 2.18X103

260 7.10

315 7.03 x 10-2.

We selected 204oC at which value of K is appreciable. For pure feed, the yield at

this temperature and system pressure is almost 100%.

PRESSURE SELECTION

High pressure i.e., 35 atmosphere" is chosen due to following reasons.

(i) At 204°C, the vapor pressure of benzene is very high, so to get

a liquid phase reaction, high pressure must be specified.

(ii) The expression for equilibrium constant for this reaction is

K = [C6H12] [E/π]3

[C6H6][H2]3

Where π = pressure in atmospheres absolute.

The expression shows clearly that higher Pressure favours higher

C6 H12 yield.

18


(iii) The stoichiometric equation for reaction is

C6H6 + 3H2 C6H12

According to Le' chattier principle, high pressure will favour more benzene

inversion.

SELECTION OF HYDROGEN TO BENZENE RATIO

A table is given below which shows the impact of H2/Bz ratio on reaction

conversion at 204 °C.

Table 2.4

Temperature

(OC)

H2/Bz

(Mol/Mol)

% Excess

Hydrogen

Benzene

Concentration

204 3 0 11700 ppm.

204 3.03 1.0 5350 ppm.

204 3.15 5.0 205 ppm.

204 3.75 25.0 6 ppm.

204 6.00 100.0 1ppm.

204 α α 0.5 ppm.

Our choosen conversion is 99.998% equivalent to 5-

10 ppm equilibrium benzene so 25% excess benzene

is used.

19


ASSUMPTIONS AND THEIR JUSTIFICATION

1. All the sulfur in benzene feed is converted to H2S.

S + H2 —> H2S

The H2S in ppm is discarded in purge stream from liquid/gas separator. Although

for purge, concentration of CO is cared about, low ppm H2S is assumed to be

blown - off.

2. Pressure effects on solubility is neglected because total

condensed cyclohexane flashed from separator is recycled back via over-head

condenser.

3. Steady state equimolar flow of cyclohexane (vapor and liquid) is assumed in

stabilizer because both streams are fed when they are saturated.

4. For some heat exchangers, average transfer coefficients are used which are

justified for preliminary design.

RECOMMENDED DESIGN CAPACITY

Data taken from Lahore Chamber Of Commerce for the import of cyclohexane in

Pakistan is in the range of batch operation to make it in continuous operation range the

minimum capacity is 40 tons/day so we have selected it. Continuous processes are less

expensive and product cost per unit of time is less than batch operations.

The final purity of product is 99.98% that is suitable to market.

GENERAL DISCUSSION ON DESIGN

When one sees the design results, two prominent features are

highlighted.

1. As the capacity selected is claimed for a pilot plant, the design dimensions of all

equipments support the claim, i.e., no commercial scale dimensions are encountered.

2. The dimensions are consistent i.e., design methods/strategies work well and no

unevenness is found.

20


These features not only confirm the design strategies, but also justify the

assumptions made in the design. Secondly, although it is a pilot plant, there is no need

for special fabrication i.e., all the heat exchangers and rotary machinery is readily

available/fabricated by vendors/fabricators.

21


Chapter # 3

MATERIAL BALANCE &

ENERGY BALANCE

MATERIAL BALANCE

Basis

40 tons (19.84 Kg mole/ hr or 1668.56 kg / hr) per day of cyclohexane

Bz : H2

1 : 3.75 (in mol fraction )

REACTION

C6H6 + 3H2 C6H12

From Encyclopedia

Product composition: (wt. basis)

C.H. =

0.9988

M.C.P. = 0.00022

Benzene = 10 ppm

Impurities (CH4 + C2H6 etc) = 0.00122


Total = 1.00

Benzene Feed Composition (Wt. basis)

Benzene = 0.9978

C.H. = 0.00016

M.C.P. = 0.00012

Impurities = 0.00057

Sulfur = 0.5 ppm

Total = 1.00

Hydrogen Feed Composition

(Wt. basis) (Mol basis)

H2 = 0.9111 0.98798

C02 = 0.0002 0.00001

CO = 0.00013 0.00001

CH4 = 0.08853 0.012

TOTAL = 1.00 1.00

23


BALANCE ACROSS REACTOR (R-O1)

R-O1

Components In (Kg/hr) Out (Kg/hr)

Benzene 1548.80 78

Cyclohexane 0.3 1583.6

M.C.P. 0195 0.4

Impurities 1.00. 1.7

Sulfur Trace. Trace

Hydrogen 150 36

Carbon dioxide 0.06 0.06

Carbonmonoxide 0.04 0.04

Methane 25 25

Total 1725 1725

Temp (°C) 204.4 204.4

Press (atm) 35 34.625

24


BALANCE ACROSS REACTOR (R-O2)

R-O2


Benzene 78 0.02

Cyclohexane 1583.6 1667

M.C.P. 0.4 0.4

Impurities 1.7 1.7

Sulfur Trace Trace

Hydrogen 36 30

Carbon dioxide 0.06 0.06

Carbonmonoxide 0.04 0.04

Methane 25 25

Total 1725 1725

Temp (°C) 204.4 273

Press (atm) 34.625 33.6

25


BALANCE ACROSS FLASH DRUM (V-O1)

V-O1


Liquid Purge Recycle

Benzene 1.7 0.02 - -

Cyclohexane 1666.545 1666.5 - -

M.C.P. 0.4 0.4 - -

Impurities 1.7 1.7 - -

Sulfur Trace - - -

Hydrogen 30 0.498 16 13.25

Carbon dioxide 0.06 6-10x6.6 0.03 0.025

Carbonmonoxide 0.04 6-10x4.2 0.02 0.0167

Methane 26.0 3-10x3 13.14 11.5

Total 1725 1669 30 25

26


BALANCE ACROSS STABILIZATION COLUMN (V-O2)

V-O2


Bottoms Overheads

Benzene 0.02 5.18X10-3 0.01482

Cyclohexane 1666.5 1666.5 0

M.C.P. 0.4 3.6x10-4 0.3996

Hydrogen 0.996 0.0258 0.9702

Carbon dioxide 6-10x6.6 0 6-10x6.6

Carbonmonoxide 6-10x4.2 0 6-10x4.2

Methane 3-10x3 0 3-10x3

Total 1669 1666.53 1.3876

OVERALL MATERIAL BALANCE27


ENERGY BALANCE

28


LATENT HEAT OF VAPORATION :-

Watson Equation;

Lv = Lvib [ (Tc –T)/(Tc –Tb) ]0.38

Where;

T = Temperature (OF)

Tc = Critical temperature for cyclohexane

= 996 R.

Tb = Boiling point, for cyclohexane

= 636.36 R

HEAT OF REACITON :-

C6H6 + 3H2 C6H12

(1) (g) (g)

[Sum of products Heat of formation] – [Sum of products Heat of formation] =Heat of

reaction

[- 29430] - [11720 + 0] = -74135.32 btu/lb-mol

SPECIFIC HEAT OF CYCLOHEXANE VAPORS:-

From537 R to 960 R

C0p = (1.8) (-7.701 +125.675xl0-3 T- 41.58x10-6 T-2) dt ÷ (1.8) dt C°p =

37.15 Btu/lb mol. °F

C°p = 154.43 kJ/ kg-mol. K

29


Critical pressure = 588 psia

Critical temperature = 996 R

Reduced Pressure,Pr = 0.87

Reduced temperature,Tr = 0.96.

Cp - C°p = 9.6 x 10-6

Specific Heat,Cp = 37.15 Btu/lb mol. °F

Specific Heat,Cp =155.5 kJ/ kg-mol.K

SPECIFIC HEAT OF HYDROGEN:-

Cpo = (6.52+0.78xl0-3T+0.l2xl05 T-2)dt ÷ dt

= [(6.52 T +0.78x10-/23T2 -0.12x105 /T ) ] ÷ [960-537]

Cp° = (1532.2 + 76.16 + 17.754)/235

= 6.92 Btu / lb-mol-oF

=28.96 kJ/ kg-mol.K

SPECIFIC HEAT OF LIQUID BENZENE:-

a, Cp at 77 °F = 0.45 Btu / lb-mol-oF

b, Cp at 400 °F= 0.6 Btu / lb-mol-oF

c, Cp = (0.6-0.45)/(400-77)

30


= 4.644xl0-4 Btu / lb-mol-oF

Specific heat, Cp = (a + ct)dt ÷ dt

Specific heat, Cp = [ 0.45dt + 4.644/2x10-4 Tdt] ÷[400-77]

= 43.74 Btu/lb mol °F

= 183.09 kJ/ kg-mol. K

SPECIFIC HEAT OF LIQUID CYCLOHEXANE:-

Average Temperature =

= 434K

Reduced Temp.,Tr = 0.784

Accentric factor ,ω = 0.214

Cp°, vapor heat capacity = -7.701 + 125.675 x 10-3 (434)

- 41.584 x 10-6 (434)2

= -7.701 + 54.543-0.02

= 46.824 Btu/lb mol. °F

= 195 KJ/ kg-mol.K

Using Sternling and Brown relation:-

31


(Cp l - Cpo )/2 = (0.5 + 2.2 ω)[3.67 + 11.64(1-Tr)4 + 0.634(1-Tr)-1]

Where;

R = 2 Btu/ lb mol - ° F

(Cp l - Cpo )/2 = (0.971) [3.67 + 0.0253 + 2.935]

(Cp l - Cpo )/2 = 6.44

CpL = 59.7 Btu/ lb- mol °F

= 248.17 KJ/ kg-mol. K

32


ENERGY BALANCE AROUND REACTORS:-

ΔHR,77F + ΔH PRODUCTS,500F - ΔHREACTANTS,400F (A)

1. Δ Hr,77

= 74135.32 Btu/lb mol (C.H.) °F x 45.157 moles/hr = 337728.65 Btu/hr.

2. ΔHPRODUCT FROM 400 TO 500 °F

ΔHp = mCpΔT

= 45.157x37.15 Btu/lb mol - °F (500-77) +36.21(500-77) (6.93)

= 709617 + 106145.632 = 815762.632 Btu/hr.

33


3. ΔH reactants from 77 to 400 °F

ΔHR =mCpΔT

= 45.45 moles/hr x 43.74 Btu/lb mol - °F x (400 - 77) +

166.26 x 6.91 x (400-77)

= 1013052.4 Btu/hr.

Inserting in (A):

= -3347728.65 + 815762.632-1013052.4

= - 3.5 xlO6 Btu/hr.

So, = 3.5 x 106 Btu/hr or 5.9 x 104 Btu/min.

5.9 x 10 Btu/min. has to be removed by outer circulation.

FIXED BED REACTOR OUT-LET TEMPERATURE:-

Conversion = 98 % to 100%

Moles converted= 45.45 (0.02)

= 0.909 lb moles/hr.

Heat generated at 77 °F = 67389 Btu/hr.

Inlet temperature = 500 °F

Assume adiabatic operation:

34


= 45.45 (-7.701+125.675x10-3 T)dt + 33.383(6.52+ 0.78x10-3T)dt

37438.33 = [-7.701(T2-533) + (T22 – 5002)] (45.45) + [6.52(T2 – 500) +

(T22– 5002)](33.38)

37438.33 = [-350T2 + 186555.57+2.856T22 -

811348l]

+ [217.66T2 - 116011.3 + 0.013 7/ -

3698.66]

37438.33 = -132.34 T2 + 2.87 T22 - 744502.5

Hence;

2.87 T22 - 1 3 2 . 3 4 T 2 - 781940.82 = 37438.33

On solving the above quadratic equation, we get temperature in oF

T2 = 522.55 °F

35


ENERGY BALANCE OF HEAT EXCHANGERS

ENERGY BALANCE OF OUTER RECIRCULATION COOLER:-

Item NO. E-01

PARAMETERS STREA

M

1

STREAM

2

Fluid Entering Benzene Water

Flow-rate (kg/hr) 26877.3 7978.7

Inlet Temperature 0C 248.88 150.5

Outlet Temperature 0C 204.44 243.3

Change in temperature 0C 44.44 93.3

Heat Capacity (J/kg K) 2590.36 4169.7

Inlet Enthalpy kJ/kg 579 520

Oulet Enthalpy kJ/kg 191.9 907.4

36


Duty of exchanger

(MJ/hr)

3094 3094

Inlet enthalpy = outlet Enthalpy

579+520=191.9+907

1099kJ/kg=1099KJ/kg

ENERGY BALANCE OF CONDENSER FOR CYCLOHEXANE

VAPORS:-

Item No. E-02

37

PARAMETERS STREAM

1

STREAM

2

Fluid Entering Cyclohexane

+ Gas

Water

Flow-rate (kg/hr) 1725 2478.5

Inlet Temperature 0C 272.5 26.7

Outlet Temperature 0C 62 149

Change in temp. 0C 202 122.3

Heat Capacity (j/kgK) 3.6x103 4.19x103

Inlet Enthalpy kJ/kg 891 7.123

Oulet Enthalpy kJ/kg 378.563 519.56

Duty of exchanger

(MJ/hr)

1266 1266


Inlet Enthalpy

= Outlet Enthalpy

891+7.123 = 519.56+378.563

898.123kJ/kg = 898.123 kJ/Kg

ENERGY BALANCE OF OVERHEAD CONDENSER:-

Item No. E-03

38

PARAMETERS STREAM

1

STREAM

2

Fluid Entering cyclohexane Water


Inlet Temperature 0C 125 55.24

Outlet Temperature 0C 125 65.6

Heat Capacity (J/kg K) 3.0x103 4.19x103

Inlet EnthalpykJ/kg 515 126.7

Outlet Enthalpy kJ/kg 474 167.6

Duty of exchanger (MJ/hr) 600 600


Inlet Enthalpy = Outlet Enthalpy

503+9.23 = 419.56+84.03

512.23kJ/kg = 512.59 kJ/Kg

ENERGY BALANCE OF PRODUCT COOLER:-

Item No. E-05

39


Inlet

Enthalpy= Outlet Enthalpy

275.42=275.42(kJ/kg)

Chapter # 4

40

PARAMETERS STREAM

1

STREAM

2

Fluid Entering cyclohexane Water


Inlet Temperature 0C 184 25

Outlet Temperature 0C 30 43

Heat Capacity (J/kg K) 3.0x103 4.19x103

Inlet Enthalpy kJ/kg 233.52 41.9

Outlet Enthalpy kJ/kg 200 75.42

Duty of exchanger (MJ/hr) 723.85 723.85


DESIGN OF EQUIPMENTS

41


REACTOR DESIGN

WHAT IS A REACTOR?

a. A container to which reactants are fed and products removed, that

provides for the control of reaction conditions.

b. A device that encloses the reaction space, and which houses the catalyst

and reacting media & is designed to provide residence times for reactants

so that chemical reaction occur among them under proper reaction

conditions.

REACTION

• Main reaction

ΔH = - 214 KJ/mole

• Highly exothermic

• Favored by low T & high ppH2

• Side reactions

• Isomerization

• Impacts final product quality

• Ring opening

42


• Favored by high T

COMMON TYPES OF MULTIPHASE CATALYTIC

REACTORS

1. Fixed-bed Reactors

a. Packed beds of pellet or monoliths

b. Multi-tubular reactors with cooling

c. Slow-moving pellet beds

d. Three-phase trickle bed reactors

2. Fluid-bed and Slurry Reactors

a. Stationary gas-phase

b. Gas-phase

c. Liquid-phase

i. Slurry

ii. Bubble Column

iii. Ebulating bed

SELECTION OF REACTOR TYPE

Slurry reactors are commonly used in situations where it is necessary to

contact a liquid reactant or a solution containing the reactant with a solid

catalyst. To facilitate mass transfer and effective catalyst utilization, the

catalyst is usually suspended in powdered or in granular form. This type of

reactor has been used where one of the reactants is normally a gas at the

reaction conditions and the second reactant is a liquid, e.g., in the

hydrogenation of various oils. The reactant gas is bubbled through the

liquid, dissolves, and then diffuses to the catalyst surface. Obviously mass

43


transfer limitations can be quite significant in those instances where three

phases (the solid catalyst and the liquid and gaseous reactants) are present

and necessary to proceed rapidly from reactants to products.

Satterfield has discussed several advantages of slurry reactors relative to other modes of

operation. They include the following.

1. Well-agitated slurry may be kept at a uniform temperature throughout,

eliminating "hot" spots that have adverse effects on catalyst selectivity.

2. The high heat capacity associated with the large mass of liquid facilitates

control of the reactor and provides a safety factor for exothermic reactions

that might lead to thermal explosions or other "runaway" events.

3. Since liquid phase heat transfer coefficients are large, heat recovery is

practical with these systems.

4. The small particles used in slurry reactors may make it possible to obtain

much higher rates of reaction per unit weight of catalyst than would be

achieved with the larger pellets that would be required in trickle bed

reactors. This situation occurs when the trickle bed pellets are characterized

by low effectiveness factors.

5. Continuous regeneration of the catalyst can be obtained by continuously

removing a fraction of the slurry from which the catalyst is then separated,

regenerated and returned to the reactor.

6. Since fine catalyst particles are desired, the costs associated with the

pelleting process are avoided, and it becomes possible to use catalysts that

are difficult or impossible to pelletize.

A major deterrent to the adoption of continuous slurry reactors is the fact

that published data are often inadequate for design purposes. Solubilization

and mass transfer processes may influence observed conversion rates and

44


these factors may introduce design uncertainties. One also has the problems

of developing mechanical designs that will not plug up, and of selecting

carrier liquids in which the reactants are soluble yet which remain stable at

elevated temperatures in contact with reactants, products, and the catalyst.

A further disadvantage of the slurry reactor is that the ratio of liquid to

catalyst is much greater than in a trickle bed reactor. Hence, the relative

rates of undesirable homogeneous liquid phase reactions will be greater in

the slurry reactor, with a potential adverse effect on the process selectivity.

TYPES

Slurry reactors may take on several physical forms: they may be simple

stirred autoclaves; they may be simple vessels fitted with an external pump

to recirculate the liquid and suspended solids through an external heat

exchanger; or they may resemble a bubble-tray rectifying column with

various stages placed above one another in a single shell. Since a single

slurry reactor has a residence time distribution approximating a CSTR, the

last mode of construction gives an easy means of obtaining stagewise

behavior and more efficient utilization of the reactor volume.

WHY BUBBLE SLURRY COLUMN REACTOR

• They have excellent heat and mass transfer characteristics, meaning high

heat and mass transfer coefficients.

• Little maintenance and low operating costs are required due to lack of

moving parts and compactness.

• Wide range of possible operating pressures(5-150bar)

• Absorption of reaction heat is obtain so that isothermal conditions are

approached

• Low pressure drop across reactor

• Little floor space is requried

45


• High wetting of external catalyst surface to delay catalyst fouling

• Solids can be handled without significant errosion or plugging problems

• The durability of the catalyst or other packing material is high. Moreover,

online catalyst addition and withdrawal ability

SLURRY BUBBLE COLUMN REACTOR

INTRODUCTION

A bubble column reactor is basically a cylindrical vessel with a gas distributor at the

bottom. The gas is sparged in the form of bubbles into either a liquid phase or a liquid–

solid suspension. These reactors are generally referred to as slurry bubble column

reactors when a solid phase exists.

46


Bubble columns are intensively utilized as multiphase contactors and reactors in

chemical, petrochemical, biochemical and metallurgical industries. They are used

especially in chemical processes involving reactions such as oxidation, chlorination,

alkylation, polymerization and hydrogenation, in the manufacture of synthetic fuels by

gas conversion processes and in biochemical processes such as fermentation and

biological wastewater treatment. Some very well known chemical applications are the

famous Fischer–Tropsch process which is the indirect coal liquefaction process to

produce transportation fuels, methanol synthesis, and manufacture of other synthetic fuels

which are environmentally much more advantageous over petroleum-derived fuels.

47


REACTOR SKETCH & MATERIAL AND ENERGY

BALANCE

48


DESIGN CALCULATIONS

STEPS AFFECTING THE GLOBAL RATE

1. Mass transfer of gas from bubble to bubble/liquid interface.

kg = mass transfer coefficient for gas diffusion

ag = gas bubble-liquid interfacial area per unit volume of

bubble free slurry

Cg = concentration of hydrogen in gas

Cig = H2 concentration at benzene-hydrogen

bubble interface (at gas side)

2. Mass transfer from the stagnant liquid film of bubble to bulk of

liquid.

kl = mass transfer coefficient for gas absorption

ag = gas bubble-liquid interfacial area per unit

volume of bubble free slurry

Cil = H2 concentration at benzene-hydrogen

bubble interface (at liquid side)

Cl = bulk concentration of H2 in solution

3. Mixing & diffusion in bulk liquid.

49


The rise of bubbles through liquid is sufficient to achieve uniform conditions in

bulk liquid. Hence the resistance of step 3 can be neglected.

4. Mass transfer of dissolved gas from the bulk liquid to the outer

surface of solid catalyst.

kc = mass transfer coefficient for particles

ac = external surface area of paticles

Cl = bulk concentration of H2 in solution

Cs = concentration of H2at the external surface area of catalyst

pellet

5. Reaction on the catalyst and diffusion of products to liquid phase.

k = specific reaction rate constant

ac = external surface area of paticles

Cs = concentration of H2at the external surface area of catalyst

pellet

CATALYST SYSTEM CHARACTERISTICS

Particle size = 150 °A (spherical)

Catalyst density = 8.9 g/cc

Cone, in solution = 0.07%

50


mcat = Conc. in solution x liq. density

= (0.0007) (0.51) = 3.57 xlO-4 g/cc

= 160.45 cm2/cc

BUBBLE DIAMETER CALCULATIONS

Kumar and Kuloor Correlation

Db = bubble diameter, cm

𝜐 = kinematic viscosity, cm2/sec = 1.9 x 10-3

Q = Vol. flow rate of gas, cc/sec = 23419.34

g = gravitational constant, cm/sec2 = 980

Db3 = 1.247

Db = 1.076 cm = 10.76 mm

ag = 4𝜋r2 = 3.637 cm2

BUBBLE FREQUENCY CALCULATION

Q = Vol. flow rate of gas = 23419.34 cc/sec

D = Orifice diameter = 8mm = 0.8 cm

51


θ = Surface tension.

Estimate the surface tension, θ, using the generalized corresponding state correlation of

Brock and Bird and the Miller relationship. The correlation and the relationship are as

follows:

where K is defined as follows:

Where

θ = surface tension in dynes/cm

Pc = critical pressure, bar

Tc = critical temperature, K

Tb = normal boiling point, K

Tb = 80.1 °C = 353.1 °K

Tbr = 0.63 Tr = 0.85

Pc = 48.3 atm, Tc = 562.1 °K, K = 0.6366

θ = 6.857 dynes/cm

f = 686102 Bubbles/sec.

FINDING OVERALL RATE EQUATION

Solving all diffusion & reaction equations simultaneously gives final equation of the form

52


Where

Under the assumptions:

Gas is pure so Cg = Cig

Catalyst is highly active so k is very large

Equilibrium exist at bubble-liquid interface, Cig and Cil are related by Henry’s

Law Cig = H Cil

As bubbles are small and in large cone (Large ag), while ac is low and poor agitation so

final expression for ko is

And overall reaction rate is

LIQUID FILM DIFFUSION COEFFICIENT CALCULATION

Mass transfer correlations from Bulk liquid to catalyst particle

Where,

D = diffusivity of hydrogen = 3.8 x 10-5

53


µl = viscosity of benzene = 0.001 poise

ρl = density of benzene = 0.51 g/cc

Hence

kc = 0.0305 cm/sec

OVERALL RATE CALCULATION

H = 1765.4

pp H2 = 35 atm

ac = 160.45 cm2 /cc

= 8.931 x 10-4 gmol/cc

rv = 2.476 x 10-6 mol of benzene/cc-sec

REACTOR VOLUME CALCULATION

V = volume of slurry cm3

Xe – Xi = 0.95

rv = 2.476 x 10-6 mol of benzene/cc-sec

F = 5.74 gmol/sec

54


V = 2202431.43 cm3 =2.20 m3

Reactor volume is obtained by 25 % increment

Vreactor = 2.75 m3

Vessel is cylindrical and in slurry bubble column reactors height to diameter ratio is from

3 to 6. Let height to diameter ratio be 4:1 so

Diameter of vessel = 0.96 m

Height of vessel = 3.83 m

RESIDENSE TIME CALCULATION

Volume of vessel = 2.75 m3

Volumetric flowrate = 0.0234 m3/sec

Γ = 1.958 min

SPARGER SELECTION

Porous plate distributor is selected because

• Low price

• Easy manufacturing

• Variety of specfications

55


PRESSURE DROP CALCULATIONS

ΔP across sparger = 0.1-0.3 atm

• Let ΔP be 0.2 atm

ΔP due to liquid head = 0.175 atm

Total ΔP = 0.375 atm

56


SPECIFICATION SHEET

Identification

Item Reactor

Item No. R-101

No. required 1

Position Vertical

Function Production of cyclohexane from benzene hydrogenation

Operation Continuous

Type Catalytic

Slurry Bubble Column Reactor

Chemical Reaction

C6H6 + 3H2 C6H12 ∆H = -214 KJ/mol

Catalyst Raney Nickel

Shape : Spherical

Size : 150 oA

Material Contained Benzene

Quality of material Slightly Corrosive

Working Volume 2.20 m3

57


Design Volume 2.75 m3

Residense time 1.958 min

Temperature

(process

temperature)

2047.4 oC

Working Pressure 35 atm

Diameter of Vessel 0.96 m

Height of Vessel 3.83 m

Height to Dia Ratio 4:1

Sparger Type Porous plate

Pressure Drop 0.375 atm

58


VAPOR-LIQUID SEPARATOR

DESIGN

A vapor-liquid separator drum is a vertical vessel into which a liquid and vapor mixture

(or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the

bottom of the vessel, and is withdrawn. The vapor travels upward at a design velocity

which minimizes the entrainment of any liquid droplets in the vapor as it exits the top of

the vessel.

OPTIONS AVAILABLE

HORIZONTAL VAPOR LIQUID SEPARATOR

VERTICAL VAPOR LIQUID SEPARATOR

59


The size a vapor-liquid separator drum (or knock-out pot, or flash drum, or compressor

suction drum) should be dictated by the anticipated flow rate of vapor and liquid from the

drum. The following sizing methodology is based on the assumption that those flow rates

are known.

Use a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4, and size the

vessel to provide about 5 minutes of liquid inventory between the normal liquid level and

the bottom of the vessel (with the normal liquid level being at about the vessel's half-full

60


level).

SELECTION CRITERIA FOR VAPOR LIQUID SEPARATORS

The configuration of a vapor/liquid separator depends on a number of factors. Before

making a vessel design one has to decide on the configuration of the vessel with respect

to among others:

Orientation

Type of feed inlet

Type of internals

Type of heads

Orientation of the Vessel

The selection of the orientation of a gas-liquid separator depends on several factors. Both

vertical and horizontal vessels have their advantages. Depending on the application one

has to decide on the best choice between the alternatives.

Advantages of a vertical vessel are:

a smaller plot area is required (critical on offshore platforms)

it is easier to remove solids

liquid removal efficiency does not vary with liquid level because the area in the

vessel available for the vapor flow remains constant

generally the vessel volume is smaller

Advantages of a horizontal vessel are:

61


it is easier to accommodate large liquid slugs;

less head room is required;

the downward liquid velocity is lower, resulting in improved de-gassing and foam

breakdown;

additional to vapor / liquid separation also a liquid / liquid separation can be

achieved (e.g. by installing a boot).

The preferred orientation for a number of typical vapor / liquid separation applications are:

Application Preferred orientation

Reactor Effluent Separator (V/L) Vertical

Reactor Effluent Separator (V/L/L) Horizontal

Reflux Accumulator Horizontal

Compressor KO Drum Vertical

Fuel Gas KO Drum Vertical

Flare KO Drum Horizontal

Condensate Flash Drum Vertical

Steam Disengaging Drum Horizontal

62


Feed Inlet

Inlet Nozzle

The feed nozzle size and the type of feed inlet device (if any) have an impact on the

vapor / liquid separation that can be achieved. The feed nozzle is normally sized to limit

the momentum of the feed. The limitation depends on whether or not a feed inlet device

is installed.

Inlet device

Various inlet devices are available to improve the vapor / liquid separation. Among

others the following inlet devices may be installed:

a deflector baffle

a slotted tee distributor

a half-open pipe

a 90 ° elbow

a tangential inlet with annular ring

a schoepentoeter

For vertical drums, preferably a deflector baffle or a half open pipe shall be selected. In

case of a slug flow regime in the inlet piping, or if a high liquid separation efficiency is

required, a tangential inlet nozzle with annular ring can be used. However, in case a high

liquid removal efficiency is required, the application of a wire mesh demister is preferred.

For horizontal drums normally a 90° elbow or a slotted diverter is installed. In some

cases a submerged inlet pipe is installed, but this shall not be done in the case of a two-

phase feed.

Normally the selected inlet device for a horizontal drum shall be:

63


a 90° elbow or a slotted diverter in case of an all liquid or vapor-liquid feed

a submerged pipe when the feed is a subcooled liquid and the mixing of liquid

and blanket gas is to be minimized

two 90° elbow inlets in case of high vapor loads

Internals

After passing through the feed inlet, the vapor stream will still contain liquid in the form

of droplets. The maximum size of these entrained droplets depends on the vapor upflow

velocity. A separation device can reduce this entrainment significantly. Wire mesh

demisters are the most commonly used as separation device. They are used for two

reasons:

To minimize entrainment

Of the drum services having such a requirement, suction drums for reciprocating

compressors are the most notable examples

To reduce the size of a vessel

The allowable vapor velocity in a drum can be increased significantly by using a wire

mesh demister. So, when sizing is governed by vapor-liquid separation criteria, this will

result in a smaller diameter of the vessel

Major disadvantages of wire mesh demisters are:

They are not suitable for fouling services

Their liquid removal decreases significantly at reduced throughput

Although the size of the vessel often can be reduced by applying a wire mesh demister,

there are also many services where there is normally no demister installed. Reflux

accumulators, for example, seldom have mist eliminators.

64


There are several other types of mist eliminators such as vanes, cyclones, and fiber beds.

They are used when conditions are not favorable for wire mesh screens. Selection criteria

for these types of internals are the required efficiency, capacity, turndown ratio,

maximum allowable pressure drop and fouling resistance. These types however will not

be further addressed in this design guide.

Vessel Head

Most vessels have 2:1 elliptical heads, welded to the shell of the vessel. However, in

some cases other types of heads are used. The major alternatives are:

Flat heads

In case of small vertical vessels (diameter less than approximately 30”) often a flanged

top head is used, which also serves to provide access to the vessel. Depending on the

pressure rating, this type of head can either be flat or elliptical, and shall be selected in

consultation with the mechanical engineer

Hemispherical heads

A hemispherical head should be considered for an extremely large, high-pressure vessel

A dished head should be considered in the case of a large diameter, low-pressure

vessel

65


INLET STREAM

C.H= 1666.545 kg/hr

M.C.P= 0.367 kg/hr

Benzene= 0.0167 kg/hr

Impurities= traces

S= traces

H2=150-120= 30 kg/hr+ XH2R

CO2= 0.0327 kg/hr+ X CO2R

CO= 0.02 kg/hr+ X CO R

CH4=14.5 kg/hr+ X CH4R

INPUTS

Operating pressure : P=10 atm

66


Vapour mass flow rate: WV = 56.05 kg/hr

Vapor density = 1.23 kg/hr

Liquid mass flow rate : WL = 1669 kg/hr

Liquid density : = 39.6 kg/m3

VAPORS

H2= 30 kg/hr

CO2= 0.0327 kg/hr

CO= 0.02 kg/hr

CH4=26 kg/hr

LIQUID

C.H= 1666.545 kg/hr

M.C.P= 0.367 kg/hr

Benzene= 0.0167 kg/hr

Impurities= traces

S= traces

Kg mole of Gases

H2= 15 kg mole

CO2= 1.363×10-3 kg mole

67


CO= 1.42857×10-3 kg mole

CH4=1.625 kg mole

VOLUME OF GASES

n= total moles=16.627 kg mole

= 16.627×0.082×335/10

kgmole×atm×m3×k / atm× kg mole×k

= m3

V= 45.676 m3/ hr

V=0.76 m3/ min

Density of vapours

Mass = 56.1 kg/ hr

= 0.935 kg/min

ρv = 0.935/0.76

68


= 1.23 kg/ m3

Density of liquid

n= total moles=19.84 kg mole

Specific gravity = 0.313

Density of liquid = 31.3 kg/m3

STEPS

Vv=A× Uv

Uv = kv {(ℓL - ℓv)/ ℓv}1/2

kv= 0.0107 m/s with a mist eliminator

A=πD2/4

LLA=ts× VL

3≥ ts ≤5

L=LL+1.5D+1.5ft

CALCULATIONS

First we find velocity of gase

Uv = kv {(ℓL - ℓv)/ ℓv}1/2

= 0.0579m/s

Now we find area69


Vv=A× Uv

A= Vv/ Uv

0.76 m3 1 min sec

min 60 sec 0.05798 m

= 0.218 m2

= 2.346 ft2

DIAMETER

D= 1.72 ft

= 1.75 ft

LENGTH OF LIQUID ENTRAINED

LLA=ts× VL˘

ts= 4 min

We assume 5 percent of entrainment of liquid in vapors

VL˘= VL× 5 %

70


= 0.908× 5 %

= 0.0454 m3 / min

LLA=ts× VL˘

LL=ts× VL˘/ A

= 0.0454 ×4 / 0.218 m2

m3 / min×min×1/ m2

=0.633027 m

= 2.73 ft

= 2.75 ft

L= LL+1.5D+1.5 ft

= 6.875 ft

Minimum length should be 8.5 ft

According to “vertical and horizontal vap liq separator design”

So length is 8.5 ft

L/D= 8.5/1.75

= 4.85

L/D < 5 for vertical separator

71


S pecific ation s heetItem Vapor liquid separator

Num ber of Item 1

Item C ode V-1204

Operating Tem perature 62◦C

Operating P ress ure 10atm

height 8.5 ft

D iam eter 1.75 ft

Vortex breaker R adia l vane vortex breaker

Materia l of cons truction C arbon s teel

72


STABILIZATION COLUMN

DESIGN

Stabilization column is the last mass transfer Operation in the production of cyclohexane.

Its Objective is to increase the purity of cyclohexane to the level as is demanded in

market and also to stabilize cyclohexane for safe storage, hence is the name stabilization

column.

It removes absorbed light gases( H2 , CH4 , CO , CO2 ) from cyclohexane by raising its

temperature in column and hence stripping gases in counter current contact of gases and

liquid.

A typical design of stabilization column is the cold feed

stabilizer with out reflux.

The stabilizer is a conventional distillation column with

reboiler but no overhead condenser. The lack of overhead

condenser means there is no liquid reflux from the overhead

stream. Therefore feed is provided on the top of column and

must provide all the cold liquor for the tower.

Fig.4.1 Cyclohexane stabilizer column

This type of design can be used when operating pressures are

high; typically stabilizers operate in the range of 700kpa -1400kpa. High pressure

eliminates the need of cold reflux stream. Also as the pressure of system is high, the

flashing of feed is avoided. At the bottom a product cooler is install whose temperature

varies between 90-200oC depending upon operating pressure.

73


Contrary to this there is another design in which the requirement of cold feed may be

relaxed as the need of cold stream is fulfilled with the help of refluxing a part of overhead

vapors. But it is costly due larger number of auxiliary equipments required.

I selected Cold feed without reflux stabilizer column .Saturated liquid feed at 10 atm is

entering at top of the packed column via liquid distributor. As the liquid flows down the

column making a film on packing an intimate contact of liquid and gas phase, which is

coming from bottom reboiler, takes place hence gases desorb from cyclohexane because

temperature of gas phase is higher than liquid: a favorable condition for gases to leave

liquid phase. At the bottom reboiler boils a portion of bottom product and sends back to

column to increase the purity of product to 99.98%. Saturated steam at 10.5 atm is

entering in reboiler’s coils. Overhead products contains majorly hydrogen and trace

amount of other products. This gas is used as a fuel gas and is stored after cooling in

overhead cooler.

MATERIAL BALANCE:

Components

Feed Bottoms Overheads

Mol

fraction

xf

Kg-

mol/hr

Mol

fractio

n

xb

Kg-

mol/hr

Mol

fraction

xd

Kg-

mol/hr

C6H6 0.9733 19.841 0.9998 19.841 0 0

H2 0.0245 0.498 5.0x10-4

9.9x10-

3 0.995 0.4851

74


1.0x10-5

2.0x10-

4

1.07x10-7

2.1x10-

6 2.6x10-5 1.9x10-4

CO 1.5x10-7

3.0x10-

6 0 0 3.0x10-7 3.0x10-6

CO2 1.5x10-7

3.0x10-

6 0 0 3.0x10-7 3.0x10-6

CH4 1.8x10-4

3.7x10-

3 0 0 3.65x10-4 3.7x10-3

MCP 2.15x10-3 0.043

2.19x10-7

4.3x10-

6 2.7x10-5 0.0429

Total 1.0 20.385 1.0 19.845 1.0 0.540

DESIGNING OF COLUMN

The general design of stabilization column include following steps:

1) Selection of tray or packed column

2) Selection of packing

3) Calculation of Diameter and Area of column

4) Calculation of pressure drop

5) Calculation of Number of transfer units

75


6) Estimation of height of transfer units

7) Height of Column

8) Calculation of Liquid Hold Up

Step # 1: SELECTION OF COLUMN :-

Application of stripping in practical process requires the generation of large contact area

between liquid and gas phase. This is usually done with three basic techniques.

1- Breaking up gas into small bubble into continuous liquid stream

(Tray Column)

2- Dividing the liquid streams into numerous thin films that flow through continuous

Gas phase ( Packed Column)

3- Dispersing the liquid as multitude of discrete droplets within continuous gas

phase( Spray Contactor)

Tray Column Packed Column

It is used for non corrosive,

non-foaming and clean

liquids.

These are preferred for

corrosive liquids.

Tray columns are for large

installations

They are efficient in small

installations.

It is used for low to medium

liquid flow-rates

For high liquid to gas ratios,

packed columns are installed.

They are preferred when

internal cooling is required

For low pressure drop application

they work best.

76


between stages

I have selected packed column because cyclohexane is slightly corrosive

and as it is very small scale plant. Also packed columns are less expensive than

plate columns for small column diameter (<0.6 m).

Above this the fabrication of trays in small diameter column is a difficult job.

The main components of packed columns are given bellow:

1. Shell

2. Packings

3. Packing support

4. Liquid distributor

Figure 4.2: Packed column with

its internals

77


Step # 2: SELECTION OF PACKING TYPE :-

The distributor and packing are the heart of the performance of this

equipment. Their proper selection entails an understanding

of packing operational characteristics and the effects on performance.

The broad classes of packings for vapor-liquid contacting are either

random or structured . The former are small, hollow structures with large

surface per unit volume that are loaded at random into the vessel.

Structured packings may be layers of large rings or grids, but are most

commonly made of expanded metal or woven wire screen that are stacked

in layers or as spiral windings.

GENERAL CRITERIA FOR SELECTION OF PACKING :

1. It should provide large contact area between liquid and gas streams.

2. It should have high flooding limits.

3. It should have high wetting characteristics.

4. There should be less pressure drop in it .

5. It should have open structure so that packing may not plug.

6. It should have good liquid distribution characteristic.

7. It should be mechanically robust.

8. It should be economical and easily available.

Most commonly used packings include:

78


Figure 4.3 Examples of some packing

Literature survey reveals that structured packings have the merits of low

pressure drop, good mass transfer characteristics, high capacity and hence

lower diameter of column but they have the demerits of high cost and

relatively less availability. Also structured packings are employed

particularly in vacuum services where pressure drops must be kept low .

79


On the other hand comparative analysis of different random packings

supports Pall rings with respect to availability, cost and pressure drop

features.

Pall rings have open structure and high flooding and loading limits. Good

liquid / gas distribution and high mass transfer efficiency. Metal rings are

easily wettable. Mechanical Strength of Metal Pall Ring Packing is high.

Other than this it is mechanically robust and can withstand high

temperature of our process hence I selected metal pall rings.

PACKING SIZE:-

In general the largest size of packing that is suitable to the size of packing

should be used; up to 50mm. Smaller sizes are more expensive than larger

ones. Above 50mm lower cost per cubic meter does not compensate for

lower mass transfer efficiency. Uses of too large size packings in small

diameter column make liquid distribution poor .

R e c o m m e n d e d s i z e s a r e g i v e n b e l o w

Column diameter Packing sizes

<0.3m <25mm

0.3-0.9 25-38mm

>0.9 50-75mm

The design data of different rings is given in appendix, Figure 4 .1.

Step # 3: COLUMN DIAMETER CALCULATIONS :- 80


In high pressure system the capital cost of column is very important. So, it

is generally recommended to reduce the diameter of column and hence

reduce the cost of equipment.

The other choice is to increase the diameter of column and decrease the

height of column, which is not a suitable rule for high pressure systems as

the increase in diameter of column has a very little effect on reduction of

height of tower.

In calculation of appropriate diameter for column the steps followed are

given below:

a) Find percentage flooding

With the help of calculated F L V , flow parameter, read values of K 4 & K 4/

from Figure 4.3 given in appendix.

The formulae to calculate flow parameter and percentage flooding are

given below with:

FL v = L/V(√ρv / ρ l )

Where;

L = Mass flow-rate of liquid in kg/sec

= 0.46kg/sec

V = Mass flow-rate of vapor in kg/sec

= 0.0143 kg/sec

ρ l = Density of liquid, kg/m 3

= 778kg/m 3

ρv = Density of vapor, kg/m 3

81


=1.5kg/m 3

FL V = 2.3

Percentage flooding = (K 4/K4/)0 .5x 100

Assume pressure drop per unit height of packing to be 21mm/m height of

packing

K4 = 0.13

K4/ = 0.23

% flooding = 75%

Percentage flooding is in satisfactory range.

b) Find Vapor Mass velocity kg/m 2-sec

The formula to be used is given below;

ρv = Density of liquid,kg/m 3= 778kg/m 3

ρ l =Density of vapor ,kg/m 3=1.5kg/m 3

ν l =Viscosity of liquid ,Nm/s 2

Fp = Packing factor m - 1

= 160 (taken from Richardson and coulson Vol. 6)

G = 0.367 kg/m 2-sec

82


c) Required Area= V/G m 2

V = 0.0142 kg/sec

G = 0.367 kg/m 2-sec

Area ,A= 0.039 m 2

Diameter, D = √(4xA)/3.14

D= 0.223m

Actual area comes out to be;

A= π/4(D 2)

A= 0.039 m 2

Step # 4: CALCULATION OF PRESSURE DROP IN

PACKING :-

Pressure drop per unit height of packing differ from packing to other. It

also depends upon size of packing. Smaller sizes have larger pressure drop

than bigger one.

By graphical Method

The X and Y co-ordinates are given below:

X- Coordinate;

Gx /Gv√ (ρv /ρx-ρv)

Y-Coordinate;

Gv2 Fpµx

0 . 1/g c(ρx-ρv) ρv

83


Pressure drop per unit height of packing has been calculated with the help

of graph given in appendix Figure 4.2.

∆P = Pressure drop in inches of water /ft of the packing height

G = Gas superficial mass velocity lb/s-ft 2 tower cross section

=0.07414 lb/s-ft 2

L = liquid superficial mass velocity lb/s-ft 2 tower cross section

=2.419lb/s-ft 2

ρg =Gas density ,lb/ft 2

g c = 32.14 lbm-ft/lb f-sec2

Fp=48.48

X=1.596

Y=0.0002

∆P = 0.2424 in H 2O/ft of packing

=20mm H 2O/m of packing

Assumed value was 21mm H2O / m Height of packing calculated value is

close to assumed value so it is acceptable.

Step # 5: CALCULATION OF NUMBER OF TRANSFER

UNITS :-

Number of transfer units has been calculated with the help of Kremser’s

Equation given below;

84


(S N + 1 – S) / (S N + 1 – 1) = Mol fraction of solute gas stripped

This equation is applicable because the equilibrium data of hydrogen

desorption from cyclohexane is straight line. Equilibrium constant,K, has

been measured by using graph given in appendix Table 4.4

Whereas nomenclature used is given below :

N= No Of Transfer Unit

S= Stripping Factor

= KV/L

L= Liquid flow-rate,kg-mol/sec

L= 5.66x10 - 3kg-mol/sec

V= Vapor flow-rate,kg-mol/sec

V= 1.5x10 - 4kg-mol/sec

K= 76

S= 2.024

N= 8

The number of transfer units for required separation is 8.

Step #6: HEIGHT OF TRANSFER UNITS :-

Height of transfer units is the height of packing that is required to change

the mol fractions of components equivalent to one theoretical plate in tray

85


column. It is some time called the efficiency calculation of packed

column.

It is calculated by using the concept of Height equivalent to theoretical

plate, HETP, given by peters.

HETP is a strong function of packing material and its size. Pall rings has

economical HETP. As the size of packing is reduced HETP reduces

because mass transfer efficiency increases, hence we get the benefit of

reduced cost. On the other hand in small size packings pressure drop per

unit height of packing is greater than larger one’s. According to Walas for

a given type of packing material the ratio of HETP to pressure drop

remains constant for all sizes. Hence it is not recommended to decrease

size of packing to have small height of column.

HETP/HOG = ln[mG/L]

[ mG/L-1]

Where

HO G =Vapor phase height of transfer unit

As rule of thumb in when D<0.5m [ 7 ]

HETP=D

HETP=0.233m

HO G = 0.54m

Step #7: HEIGHT OF COLUMN :-

86


Z =N x HO G

Z = Height of Packing in meters

Z=8x0.54

Z =4.32m

Z t= Z +Hd +Hb

Height for the disengagement region, H d =0.289m

Height for the Bottom, H b =0.4365m

Z t = Total height of the column

= 5.04m

Step # 8: LIQUID HOLD UP CALCULATIONS :-

HLw = 0.0004(Lm/Dp)0.6

Dp =equivalent spherical packing diameter (inches)

Lm =liquid rate ( lb/s-ft2 )

Hlw=0.050m3/m3 of packing

87


PROCESS SPECIFICATION SHEET FOR STABILIZATION COLUMN

88


89

1. Item Number V-02 Type:Packed

2 Service Cyclohexane Stabilizer

3. NO OF EQUIPMENTS 1

4. OPERATION CONTINUOUS

PROCESS CONDITIONS

5. Liquid Handled Cyclohexane

6. Liquid Flow-rate

Kg-mol/hr20.385

7. Liquid Quality Slightly corrosive

8. Liquid Viscosity cP 0.12

9. Vapor Handled Cyclohexane

10. Vapor Quality Slightly corrosive

11. Vapor mol.wt 84

12. Temperature K 453

13. Pressure atm 10

OPERATIONAL CONDITIONS

14. Nature of operation Stripping

15. Feed nozzle location Top/Sat. liquid

16. Temperature

K442

17. Percentage Flooding 75%

COLUMN INTERNALS

18. Packing Size

mm25

19. Nature of packing Pall rings

20. Material of Packing Metallic, Carbon Steel

21. Liquid Distributor Spray nozzles


HEAT EXCHANGER DESIGN

HEAT EXCHANGER

Heat exchanger is a device that is used to transfer heat between two fluids at different

temperature.

DIFFERENT TYPES OF HEAT EXCHANGERS

The principle types of heat Exchanger used in Chemical and allied industries are as follows:

1. Double Pipe heat Exchanger

2. Shell and Tube Heat Exchanger

3. Plate and Frame Heat Exchanger

4. Plate and Fin Type Heat Exchanger

5. Spiral Type Heat Exchanger

6. A Cooled: Cooler and Condenser

SELECTION CRITERIA

Selection process includes a No. of factors all of these are related to the heat transfer

application.

1. Thermal Requirements

2. Material Compatibility

3. Operational Maintains

4. Environmental, Health & Safety Consideration

5. Availability

6. Cost

90


SHELL & TUBE HEAT EXCHANGER

Basic Construction of Shell & Tube Heat Exchangers

Shell and tube heat exchangers represent the most widely used vehicle for the transfer of

heat in industrial process applications. They are frequently selected for such duties as:

• Process liquid or gas cooling

• Process or refrigerant vapor or steam condensing

• Process liquid, steam or refrigerant evaporation

• Process heat removal and preheating of feed water

• Thermal energy conservation efforts, heat recovery

• Compressor, turbine and engine cooling, oil and jacket water

• Hydraulic and lube oil cooling

• Many other industrial applications

Shell and tube heat exchangers have the ability to transfer large amounts of heat in

relatively low cost, servicable designs. They can provide large amounts of effective tube

surface while minimizing the requirements of floor space, liquid volume and weight.

Shell and tube exchangers are available in a wide range of sizes. They have been used in

industry for over 150 years, so the thermal technologies and manufacturing methods are

well defined and applied by modern competitive manufacturers. Tube surfaces from

standard to exotic metals with plain or enhanced surface characteristics are widely

available. They can help provide the least costly mechanical design for the flows, liquids

and temperatures involved.

Although there exist a wide variety of designs and materials available, there are

components common to all designs. Tubes are mechanically attached to tube sheets,

which are contained inside a shell with ports for inlet and outlet fluid or gas. They are

designed to prevent liquid flowing inside the tubes to mix with the fluid outside the tubes.

Tube sheets can be fixed to the shell or allowed to expand and contract with thermal

stresses by have one tube sheet float inside the shell or by using an expansion bellows in

the shell. This design can also allow pulling the entire tube bundle assembly from the

91


shell to clean the shell circuit of the exchanger.

Fluid Stream Allocations

There are a number of practical guidelines which can lead to the optimum design of a

given heat exchanger. Remembering that the primary duty is to perform its thermal duty

with the lowest cost yet provide excellent in service reliability, the selection of fluid

stream allocations should be of primary concern to the designer. There are many trade-

offs in fluid allocation in heat transfer coefficients, available pressure drop, fouling

tendencies and operating pressure.

1. The higher pressure fluid normally flows through the tube side. With their

small diameter and nominal wall thicknesses, they are easily able to accept

high pressures and avoids more expensive, larger diameter components to be

designed for high pressure. If it is necessary to put the higher pressure stream

in the shell, it should be placed in a smaller diameter and longer shell.

2. Place corrosive fluids in the tubes, other items being equal. Corrosion is

resisted by using special alloys and it is much less expensive than using

special alloy shell materials. Other tube side materials can be clad with

corrosion resistant materials or epoxy coated.

3. Flow the higher fouling fluids through the tubes. Tubes are easier to clean

using common mechanical methods.

4. Because of the wide variety of designs and configurations available for the

shell circuits, such as tube pitch, baffle use and spacing, multiple nozzles, it is

best to place fluids requiring low pressure drops in the shell circuit.

5. The fluid with the lower heat transfer coefficient normally goes in the shell

circuit.This allows the use of low-fin tubing to offset the low transfer rate by

providing increased available surface.

Tubes

Tubing that is generally used in TEMA sizes is made from low carbon steel,

copper,Admiralty, Copper-Nickel, stainless steel, Hastalloy, Inconel, titanium and a few

92


others. It is common to use tubing from 5/8 to 1-1/2 in these designs. Tubes are either

generally drawn and seamless or welded. High quality ERW (electro-resistancewelded)

tubes exhibit superior grain structure at the weld. Extruded tube with low fins and interior

rifling is specified for certain applications. Surface enhancements are used to increase the

available metal surface or aid in fluid turbulence, thereby increasing the effective heat

transfer rate. Finned tubing is recommended when the shell side fluid has a substantially

lower heat transfer coefficient than the tube side fluid. Finned tubing has an outside

diameter in the finned area slightly under the unfinned, or landing area for the tube

sheets. This is to allow assembly by sliding the tubes through the baffles and tube

supports while minimizing fluid bypass. U-tube designs are specified when the thermal

difference of the fluids and flows would result in excessive thermal expansion of the

tubes. U-tube bundles do not have as much tube surface as straight tube bundles, due to

the bending radius, and the curved ends cannot be easily cleaned. Additionally, interior

tubes are difficult to replace, many times requiring the removal of outer layers, or simply

plugging the tube. Because of the ease in manufacturing and service, it is common to use

a removable tube bundle design when specifying U-tubes.

Tube sheets

Tubesheets are usually made from a round flat piece of metal with holes drilled for the

tube ends in a precise location and pattern relative to one another. Tube sheet materials

range as tube materials. Tubes are attached to the tube sheet by pneumatic or hydraulic

pressure or by roller expansion. Tube holes can be drilled and reamed and can be

machined with one or more grooves. This greatly increases the strength of the tube joint.

93


The tubesheet is in contact with both fluids and so must have corrosion resistance

allowances and have metalurgical and electrochemical properties appropriate for the

fluids and velocities. Low carbon steel tube sheets can include a layer of a higher alloy

metal bonded to the surface to provide more effective corrosion resistance without the

expense of using the solid alloy.

The tube hole pattern or pitch varies the distance from one tube to the other and angle of

the tubes relative to each other and to the direction of flow. This allows themanipulation

of fluid velocities and pressure drop, and provides the maximum amount of turbulance

and tube surface contact for effective heat transfer. Where the tube and tube sheet

materials are joinable, weldable metals, the tube joint can be further strengthened by

applying a seal weld or strength weld to the joint. A strength weld has a tube slightly

reccessed inside the tube hole or slightly extended beyond the tube sheet. The weld adds

metal to the resulting lip. A seal weld is specified to help prevent the shell and tube

liquids from intermixing. In this treatment, the tube is flush with the tube sheet surface.

The weld does not add metal, but rather fuses the two materials. In cases where it is

critical to avoid fluid intermixing, a double tube sheet can be provided. In this design, the

outer tube sheet is outside the shell circuit, virtually eliminating the chance of fluid

intermixing. The inner tube sheet is vented to atmosphere so any fluid leak is easily

detected.

Shell Assembly

94


The shell is constructed either from pipe up to 24 or rolled and welded plate metal. For

reasons of economy, low carbon steel is in common use, but other materials suitable for

extreme temperature or corrosion resistance are often specified. Using commonly

available shell pipe to 24 in diameter results in reduced cost and ease of manufacturing,

partly because they are generally more perfectly round than rolled and welded shells.

Roundness and consistent shell ID is neccessary to minimize the space between the baffle

outside edge and the shell as excessive space allows fluid bypass and reduced

performance. Roundness can be increased by expanding the shell around a mandrell or

double rolling after welding the longitudnal seam. In extreme cases the shell can be cast

and then bored to the correct ID.

In applications where the fluid velocitiy for the nozzle diameter is high, an impingement

plate is specified to distribute the fluid evenly to the tubes and prevent fluid induced

erosion, cavitation and vibration. An impingement plate can be installed inside the shell,

which prevents installing a full tube bundle, resulting in less available surface. It can

alternately be installed in a domed area above the shell. The domed area can either be

reducing coupling or a fabricated dome. This style allows a full tube count and therefore

maximizes the utilization of shell space.

End Channels and Bonnets

End channels or bonnets are typically fabricated or cast and control the flow of the tube

95


side fluid in the tube circuit. They are attached to the tube sheets by bolting with a gasket

between the two metal surfaces. In some cases, effective sealing can be obtained by

installing an O-ring in a machined groove in the tube sheet. The head may have pass ribs

that dictate if the tube fluid makes one or more passes through the tube bundle sections.

Front and rear head pass ribs and gaskets are matched to provide effective fluid velocities

by forcing the flow through various numbers of tubes at a time. Generally, passes are

designed to provide roughly equal tube-number access and to assure even fluid velocity

and pressure drop throughout the bundle. Even fluid velocities also affect the film

coefficients and heat transfer rate so that accurate prediction of performance can be

readily made. Designs for up to six tube passes are common. Pass ribs for cast heads are

intregrally cast and then machined flat. Pass ribs for fabricated heads are welded into

place. The tube sheets and tube layout in multi-pass heat exchangers must have provision

for the pass ribs. This requires either removing tubes to allow a low cost straight pass rib,

or machining the pass rib with curves around the tubes, which is more costly to

manufacture. Where a full bundle tube count is required to satisfy the thermal

requirements, this machined pass rib approach may prevent having to consider the next

larger shell diameter.

Cast head materials are typically used in smaller diameters to around 14 and are made

96


from iron, ductile iron, steel, bronze or stainless steel. They typically have pipe thread

connections. Cast heads and tube side piping must be removed to service tubes.

Fabricated heads can be made in a wide variety of configurations. They can have metal

cover designs that allow servicing the tubes without disturbing the shell or tube piping.

Heads can have axially or tangentially oriented nozzles, which are typically ANSI

flanges.

Baffles

Baffles serve two important functions. They support the tubes during assembly and

operation and help prevent vibration from flow induced eddies and direct the shell side

fluid back and forth across the tube bundle to provide effective velocity and heat transfer

rates. The diameter of the baffle must be slightly less than the shell inside diameter to

allow assembly, but must be close enough to avoid the substantial performance penalty

caused by fluid bypass around the baffles. Shell roundness is important to acheive

effective sealing against excessive bypass. Baffles can be made from a variety of

materials compatible with the shell side fluid. They can be punched or machined. Some

baffles are made by a punch which provides a lip around the tube hole to provide more

surface against the tube and eliminate tube wall cutting from the baffle edge. The tube

holes must be precise enough to allow easy assembly and field tube replacement, yet

minimize the chance of fluid flowing between the tube wall and baffle hole, resulting in

reduced thermal performance and increased potential for tube wall cutting from vibration.

Baffles do not extend edge to edge, but have a cut that allows shell side fluid to flow to

the next baffled chamber. For most liquid applications, the cuts areas represent 20-25%

of the shell diameter. For gases, where a lower pressure drop is desirable, baffle cuts of

40-45% is common. Baffles must overlap at least one tube row in order to provide

adequate tube support. They are spaced throughout the tube bundle somewhat evenly to

provide even fluid velocity and pressure drop at each baffled tube section.

97


Single-segmental baffles force the fluid or gas across the entire tube count, where is

changes direction as dictated by the baffle cut and spacing. This can result in excessive

pressure loss in high velocity gases. In order to affect heat transfer, yet reduce the

pressure drop, double-segmental baffles can be used. This approach retains the structural

effectiveness of the tube bundle, yet allows the gas to flow between alternating sections

of tube in a straighter overall direction, thereby reducing the effect of numerous changes

of direction. This approach takes full advantage of the available tube surface but a

reduction in performance can be expected due to a reduced heat transfer rate. Because

pressure drop varies with velocity, cutting the velocity in half by using double-segmental

baffles results in roughly 1/4 of the pressure drop as seen in a single-segmental baffle

space over the same tube surface.

STANDARD DESIGN STEPS

a. Define the duty; Heat Transfer Rate and Temperature 98


b. Collection of Fluid Physical Properties

c. Assume the value of Heat Transfer Coefficient

d. Calculate the Mean Temperature Difference

e. Calculate the Area Required

f. Decide the Heat Exchanger Layout

g. Calculate the Pressure

STREAM CONDITIONS

HOT FLUID

Inlet Temperature T1 =184 C

Outlet Temperature T2 =30 C

Mass flow rate mh =1669 kg/hr

COLD FLUID

Inlet Temperature t1 =25 C

Outlet Temperature t2 =43 C

Mass flow rate mc =7969 kg/hr

PHYSICAL PROPERTIES

HOT FLUID

Specific heat Cp =2.186 kJ/kg K

Thermal conductivity k =0.1041 W/m K

Density 𝝆 =695.2 kg/m3

99


Viscosity μ =1.88 kg/m sec

COLD FLUID

Specific Heat Cp =4.17 kJ/kg K

Thermal conductivity k = 0.590W/m K

Density 𝝆 = 995kg/m3

Viscosity μ =2.37 kg/m sec

HEAT LOAD

Using Hot Fluid

Q = m Cp ∆T

=5.62 ×103 kJ/hr

LOG MEAN TEMPERATURE DIFFERENCE

T1 T2

Hot Fluid 184 C 30 C

t2 t1

Cold Fluid 43 C 25 C

ΔT1 = T2-t1 ΔT2=T1-t2

LMTD = (ΔT1- ΔT2)/(ln ΔT1/ ΔT2)

LMTD = 41.72 K

100


Ft Factor

R= (T1-T2)/(t2-t1)

= 8.55

S=(t2-t1)/(T1-t1)

=0.113

Ft=0.976

ΔTm=0.967×41.72= 38.684°K

ASSUMED OVERALL COEFFICIENT Ud

Ud =100 W/m2K

APPROXIMATE AREA

A=Q/Ud× ΔTm

=40.35m2

TUBE SPECIFICATIONS

Outside Dia of Tube (OD) = 0.019m

Inner Diameter of Tube (ID) = 0.016m

Tube Pitch Pt = 0.024m

Thickness = 16 BWG

Area of a Single Tube a = 0.2911m2

No of Tubes A/a=139

TUBE SIDE CALCULATIONS 101


Mass Flow rate (mc) = 7969 kg/hr

Total Area of Tubes (at) = 0.0125 m2

Mass Velocity Gt = (mc/at ) = 63752 kg/hr.m2

Calculation of Reynold’s Number

Reynold’s Number (NRe) = DGt / μ

= ( 0.016× 63752)/2.37

= 5425.70

TUBE SIDE HEAT TRANSFER

hi = [4200(1.35+0.02t)]/ID0.2

= 4877.57W/m2K

=4107.43W/m2K

SHELL SIDE CALCULATIONS

Internal Diameter of Shell (ID) = 0.387 m

Tube Clearance (C′) = 0.0048m

Baffle Spacing (B) = 0.2035 m

FLOW AREA

102


= (0.387×0.004×0.205 )/0.024

= 0.011 m2

MASS VELOCITY

Mass velocity

= 1669/0.011

= 151727kg/hr.m2

SHELL SIDE EQUIVALENT DIAMETER

Shell Side Equivalent Diameter

(De )= 0.0139 m

Reynold’s Number

= (0.0139×151727)/1.88

= 24434.5

Jh = 90

Prandtl Number

Pr = Cpμ/k

103


= (2.186×1.88 )/0.104

=1.3

OUTSIDE HEAT TRANSFER COEFFICIENT

= [90×0.104×(1.3)1/3]/0.0139

= 876.23W/m2 K

CLEAN OVERALL COEFFICIENT

= 742W/m2K

ACTUAL AREA Ac

=40.70m2

CORRECTED Ud

= (156×103)/(40.70×38.684)

= 99.15 W/m2K

DIRT FACTOR

104


Dirt factor

=0.0087 W/m2K

SHELL SIDE PRESSURE DROP CALCULATION

Friction factor (f) = 0.0051

Eq., Diameter Of Shell (De) = 0.0139m

Specific Gravity (S) =0.79

Number of crosses (N+1) = L/B=23.98

Gs = 42.27kg/sec .m2

SHELL SIDE PRESSURE DROP

= 741.98N/m2

TUBE SIDE PRESSURE DROP CALCULATION

Friction factor (f) = 0.0025

Inside Diameter (I.D) = 0.016m

Specific Gravity (S) =1

Length L = 4.88m

n=139

105


Gt = 177kg/sec.m2

TUBE SIDE PRESSURE DROP

=239.45 N/m2

106


SPECFICATION SHEET

I Item S Shell & Tube Heat Exchanger

F Function S Stabilizer Product Cooler

P Position Horizontal

N No. of Unit N One

1

N No. of Shell Passes 1 1

No. of Tube Passes 2 2

Heat Transfer Area 440.7m2

Diameter of Shell 0 0.387m

107


P Pitch (Triangular Pitch) 0 0.025m

No. of Tubes 1 139

T Type of Tube Used 1 16BWG

T Tube Length 4 4.88m

I ID & OD of tube 0 0.0157m, 0.01908m

hi 4 4877.57W/m2K

ho 8 876.23W/m2 K

P Pressure drop on Shell Side 7 41.98N/m2

108


Chapter # 5

MECHANICAL DESIGN

MECHANICAL DESIGN OF SHELL & TUBE HEAT

EXCHANGER

SHELL SIDE

SHELL THICKNESS

ts=Shell thickness =?

P=Design Pressure=1.51N/mm2

Ds=Inner Diameter of Shell=387mm

Permissible Strength for Carbon Steel f=95N/mm2

Joint factor=J = 85%

So ts =3.65

109


Allowable Corrosion allowance C =4mm

So the Shell Thickness

ts =3.65+4

=7.65mm

NOZZLE DESIGN FOR SHELL SIDE

Material used Carbon Steel

For Shell Fluid Inlet Nozzle

Mass Flow Rate of Shell Side fluid =1669kg/hr

Density of the Shell side Fluid=695.2kg/m3

Velocity of Shell Side Fluid= 218m/hr

A =m/𝝆v= 0.011m2

Dn = 118mm

Outer Nozzle also has the same Diameter

NOZZLE THICKNESS

C = Corrosion allowance

=5.11mm

110


HEAD THICKNESS

P = Design Pressure = 4 N/mm2

Rc =Outer Radius of Shell = 200mm

Rk = Kunckle Radius of Shell = 0.06Rc =12mm

W= 1.7706

th = 7.270mm

BAFFLE DIAMETER (Using 25% Cut Baffle)

Db = Ds -4.8

=382.2mm

TUBE SIDE

Material Used Carbon Steel

No. of Passes = 2

No. of Tubes = 139

Outside Diameter = 0.019m

Inside Diameter = 0.016m

Wall Thickness of Tube= 0.165m

111


Length of Tube= L=4.88m

Tube Pitch Pt =0.025m

Working Pressure =0.6 N/mm2

Design Pressure =0.66 N/mm2

NOZZLE DESIGN FOR TUBE SIDE

Material Used Carbon Steel

Mass Flow Rate of Tube Side Fluid = 7969kg/hr

Density of Cold Fluid =995 kg/m3

Velocity of Tube Side Fluid = v =640.72m/hr

= 0.0125m2

Dn = 126.18mm

NOZZLE THICKNESS

tn = 0.51

Nozzle thickness with Corrosion Allowance

tn = 0.51+4

112


= 4.51mm

Chapter # 6

INSTRUMENTATION &

PROCESS CONTROL

INSTRUMENTS

Instruments are provided to monitor the key process variable during plant operation. They

may be incorporated in automatic control loops or used for manual monitoring of the

process operation. They may also be a part of an automatic computer data logging

system .Instruments monitoring critical process variable will be fitted with automatic alarm

to alert the operator to critical and hazardous situation.

It is desirable that the process variable to be monitored be measured directly, often,

however, this is impractical and some dependent variable that is easier to measure is

monitored in its place. For example, in the control of distillation columns the continuous on

line, analysis of the overhead product is desirable but difficult and expensive to achieve

reliably, so temperature is often monitored as an indication of composition. The temperature

instrument may form part of a control loop controlling, say , reflux flow ; with the

composition of the overhead checked frequently by sampling and laboratory.

113


INSTRUMENTATION & CONTROL OBJECTIVE

The primary objective of the designer when specifying instrumentation and control schemes

are:

Safer Plant Operation

a. To keep the process variable within known safe operation limits

b. To detect dangerous situation as they develop and to provide alarms and

automatic shut down system.

c. To provide inter locks and alarms to prevent dangerous operating

procedures.

Production Rate

To achieve t6he design product output

Product Quality

To maintain the product composition within the specified quality

standards

Cost

To operate at the lowest production cost, commensurate with the other

objectives.

These are not separate objectives and must be considered together. The

order in which they are listed is not meant to imply the precedence of any

objective over another, other than that of putting safety first. Product

quality, production rate and the cost of production will be dependent on

sales requirements. For example, it may be a better strategy to produce a

better quality product at a higher cost.

In a typical chemical processing plant these objective are achieved by

combustion of automatic control, manual monitoring and laboratory

analysis.

114


COMPONENTS OF A CONTROL SYSTEM

Process

Any operation or series of operations that produces a desired final result is a process. In this

distillation the process is the cracking of Naphtha.

Measuring Means

Of all the parts of the control system the measuring elements is perhaps the most important.

If measurement is not made properly the remainder of the system cannot operate

satisfactorily. The available is dozen to represent the desired condition in the process.

Analysis of measurements Variable to be measured

1. Pressure measurements

2. Temperature measurements

3. Flow Rate measurements

4. Level measurements

Variable to be Recorded

Indicated Temperature, Composition, Pressure etc

Controller

The controller is the mechanism that responds to any error indicated by the error detecting

mechanism. The output of the controller is some predetermined function of the error.

In the controller there is also and error detecting mechanism which compares the measured

variable with desired value of the measured variable, the difference being the error.

Final Control element

115


The final control element receives the signal from the controller and by some predetermined

relationship changes the energy input to the process.

CLASSIFICATION OF CONTROLLER

In general the process controllers can be classified as:

a) Pneumatic controllers

b) Electronic controllers

c) Hydraulic controllers

In the ethylene manufacturing from naphtha the controller and the final control

element may be pneumatically operated due to the following reasons:

i) The pneumatic controller is vary rugged and almost free of maintenance. The

maintenance men have not had sufficient training and background in electronics,

so basically pneumatic equipment is simple.

ii) The pneumatic controller appears to be safer in a potentially explosive

atmosphere which is often present in the petro-chemical industry.

iii) Transmission distances are short. Pneumatic and electronic transmission system

are generally equal upto about 250 to 300 feet. Above this distance, electronic

systems begin to offer savings.

MODES OF CONTROL

The various type of control are called "modes" and they determine the type of

response obtained. In other words these describe the action of the controller that is the

relationship of output signal to the input or error signal. It must be noted that it is error

that actuates the controller. The four basic modes of control are:

116


i) On-off Control

ii) Integral Control

iii) Proportional Control

iv) Rate or Derivative Control

In industry purely integral, proportional or derivative modes seldom occur alone

in the control system.

The On-off controller in the controller with very high gain. In this case the error

signal at once off the valve or any other parameter upon which it sits or completely sets

the system.

ALARMS AND SAFETY TRIPS AND INTERLOCKS

Alarms are used to alert operators of serious, and potentially hazardous,

deviations in process conditions. Key instruments are fitted with switches and relays to

operate audible and visual alarms on the control panels.

The basic components of an automatic trip systems are:

i) A sensor to monitor the control variable and provide an output signal when a

preset valve is exceeded (the instrument).

ii) A link to transfer the signal to the actuator usually consisting of a system of

pneumatic or electric relays.

iii) An actuator to carry out the required action; close or open a valve, switch off a

motor. -

117


A safety trip can be incorporated in control loop; as shown in figure . In this

system the high-temperature alarm operates a solenoid valve, releasing the air on the

pneumatic activator closing the valve on high temperature.

INTERLOCKS

Where it is necessary to follow the fixed sequence of operations for example, during a

plant start-up and shut-down, or in batch operations-inter-locks are included to prevent

operators departed from the required sequence. They may be incorporated in the control

system design, as pneumatic and electric relays or may be mechanical interlocks.

DIFFERENT TYPES OF CONTROLLERS

Flow Controllers

These are used to control feed rate into a process unit. Orifice plates are by far the

most type of flow rate sensor. Normally, orifice plates are designed to give pressure drops

in the range of 20 to 200inch of water. Venture tubes and turbine meters are also used.

Temperature Controller

Thermocouples are the most commonly used temperature sensing devices. The two

dissimilar wires produce a millivolt emf that varies with the "hot-junction" temperature.

Iron constrictant thermocouples are commonly used over the 0 to 1300°F temperature

range.

Pressure Controller

Bourdon tubes, bellows, and diaphragms are used to sense pressure and differential

pressure. For example, in a mechanical system the process pressure force is balanced by

the movement of a spring. The spring position can be related to process pressure.

Level Controller

Liquid levels are detected in a variety of ways. The three most common are:

Following the position of a float, that is lighter them the fluid.

Measuring the apparent weight of a heavy cylinder as it buoyed up more or less

by the liquid (these are called displacement meters).118


Measuring the difference in static pressure between two fixed elevations, one in

the vapour above the liquid and the other under the liquid surface. The differential

pressure between the two level taps is directly related to the liquid level in the

vessel.

Transmitter

The transmitter is the interface between the process and its control system. The job of the

transmitter, is to convert the sensor signal (millivolts, mechanical movement, pressure

differential, etc.) into a control signal 3 to 15 psig air-pressure signal, 1 to 5 or 10 to 50

milliampere electrical signal, etc.

Control Valves

The interface with the process at the other end of the control loop is made by the final

control element is an automatic control valve which throttles the flow of a stem that open

or closes an orifice opening as the stem is raised or lowered. The stem is attached to a

diaphragm that is driven by changing air-pressure above the diaphragm. The force of the

air pressure is opposed by a spring.

119


CONTROL OF HEAT EXCHANGER

1. The Normal Way

The normal method for controlling a heat exchanger is to measure exit

temperature of the process fluid and adjusts input of heating or cooling medium to hold

the desired temperature.

2. Cascade Control

The control objective is to keep the exit temperature of stream 2 at a distance

value. The secondary loop is used to compensate for the changes in the flow rate of

stream 1.

In chemical process, flow rate control loops are almost always cascade with other control

loops.

CONTROL SCHEME

The figure shows a cascade control in which the hot fluid temperature is constantly

measured by the temperature transmitter. The temperature is matched with the set point

when the temperature changes, the deviations are communicated to the secondary controller

which is then adjusted the flow rate of the cold fluid.

120


Chapter # 7

HAZOP STUDY

INTRODUCTION

A HAZOP survey is one of the most common and widely accepted methods of systematic

qualitative hazard analysis. It is used for both new and existing facilities and can be

applied to a whole plant, a production unit, or a piece of equipment. It uses as its database

the usual sort of plant and process information and relies on the judgment of engineering

and safety experts in the areas with which they are most familiar. The end result is,

therefore reliable in terms of engineering and operational expectations, but it is not

quantitative and may not consider the consequences of complex sequences of human

errors.

121


The objectives of a HAZOP study can be summarized as follows:

1) To identify areas of the design that may possess a significant hazard potential.

2) To identify and study features of the design that influence the probability of a

hazardous incident occurring.

3) To familiarize the study team with the design information available.

4) To ensure that a systematic study is made of the areas of significant hazard

potential.

5) To identify pertinent design information not currently available to the team.

6) To provide a mechanism for feedback to the client of the study team's detailed

comments.

BASIC PRINCIPLES OF HAZOP STUDY

The basic concept of the hazard and operability study is to take a full description of

process and to question every part of it to discover what deviation from the intention of

the design can occure and what can be their causes and consequences.

The seven guide words recommended in the chemical industries association

(CIA) booklet are used. In addition to these words, the following words are also used

with precise meaning.

Intention

Deviation

Causes

Consequences

122


Hazards

In HAZOP study, each segment (pipeline, piece of equipment, instrument

etc) is carefully examined and all possible deviations from normal operating conditions

are specified. Some of guide words works recommended by CIA are given in the

following Table.

Table: HAZOP Guide Words and Meanings

Guide Words Meaning

No

Less

More

Part of

As well as

Reverse

Other than

Negation of design intent

Quantitative decrease

Quantitative increase

Qualitative decrease

Qualitative Increase

Logical opposite of the intent

Complete substitution

These guide words are applied to flow, temperature, pressure, liquid level,

composition and any other variables affecting the process. The consequences of these

deviations on the process are then assessed and the measures needed to detect and correct

deviations are established.

A HAZOP study is conducted in the following steps:

1) Specify the purpose, objective, and scope of the study. The purpose may be

the analysis of a yet to be built plant or a review of the risk of an existing unit.

Given the purpose and the circumstances of the study, the objectives listed

above can he made more specific. The scope of the study is the boundaries of

the physical unit, and also the range of events and variables considered. For

example, at one time HAZOP's were mainly focused on fire and explosion

123


endpoints, while now the scope usually includes toxic release, offensive odor,

and environmental end-points. The initial establishment of purpose,

objectives, and scope is very important and should be precisely set down so

that it will be clear, now and in the future, what was and was not included in

the study. These decisions need to be made by an appropriate level of

responsible management.

2) Select the HAZOP study team. The team leader should be skilled in HAZOP

and in interpersonal techniques to facilitate successful group interaction. As

many other experts should be included in the team to cover all aspects of

design, operation, process chemistry, and safety. The team leader should

instruct the team in the HAZOP procedure and should emphasize that the end

objective of a HAZOP survey is hazard identification; solutions to problems

are a separate effort.

3) Collect data. Theodore16 has listed the following materials that are usually

needed:

Process description

Process flow sheets

Data on the chemical, physical and toxicological properties of all raw

materials, intermediates, and products.

Piping and instrument diagrams (P&IDs)

Equipment, piping, and instrument specifications

Process control logic diagrams

Layout drawings

Operating procedures

Maintenance procedures

Emergency response procedures

Safety and training manuals

124


4) Conduct the study. Using the information collected, the unit is divided into

study "nodes" and the sequence diagrammed in Figure, is followed for each

node.

Nodes are points in the process where process parameters (pressure,

temperature, composition, etc.) have known and intended values. These values

change between nodes as a result of the operation of various pieces of

equipment' such as distillation columns, heat exchanges, or pumps. Various

forms and work sheets have been developed to help organize the node process

parameters and control logic information.

When the nodes are identified and the parameters are identified, each node is

studied by applying the specialized guide words to each parameter. These

guide words and their meanings are key elements of the HAZOP procedure.

Repeated cycling through this process, which considers how and why each

parameter might vary from the intended and the consequence, is the substance

of the HAZOP study.

5) Write the report. As much detail about events and their consequence as is

uncovered by the study should be recorded. Obviously, if the HAZOP

identifies a not improbable sequence of events that would result in a disaster,

appropriate follow-up action is needed. Thus, although risk reduction action is

not a part of the HAZOP, the HAZOP may trigger the need for such action.

125


The HAZOP studies are time consuming and expensive. Just getting the P & ID's

up to date on an older plant may be a major engineering effort. Still, for processes with

significant risk, they are cost effective when balanced against the potential loss of life,

property, business, and even the future of the enterprise that may result from a major

release.

126


HAZOP STUDY OF GAS/LIQUID SEPARATOR

Deviation from

Operating

conditions

Events cause

deviation

Consequence of

Deviation

Notes

Level

127


Less

1. Inlet flow

stops

1. Lowering of

pressure

Check for any

valve failure

2. temp, rises 2. Separator dries

out

Check condenser

temp rise

3. Pressure

lowers

3. Feed to

Stabilizer

interrupted

See the vent

valve

4. Vent on

Condenser is

uncontrolled

4. Purge

composition

changes

Check the purge

composition

Leakage

detection

5. Reflux is

interrupted

5. Feed to

Stabilizer

interrupted

Check for any

valve failure

More

1. Temp, drops 1. Pressure

increases

Consider for

column

2. Pressure

increased

2. Separator over-

loaded

shut down

3. Feed

vaporized

plugged

3. More

vaporizing steam

Check reflux

quantity

128


4. High temp of

reflux condenser

See reflex

condenser

No Same as less

Temperature

Less

1. Subcooling in

condenser

1. Less flashing Check flow from

condenser

2. Pressure

drops.

2. Purge / recycle

composition

change

Vent/purge

composition

analysis

3. Reflux temp,

drops

3. Level builds-up Check flow from

condenser

More Opposite to less

No Same as less

Pressure

Less

1. Purge valve

openned/uncontr

olled

1. Composition of

purge / recycle

changed

Check for gas

composition

2. Temp, drops

from condenser

2. Quanity to feed

vaporizer changes

Follow the

process

conditions

129


3. Temp, drops

from refluxed

condenser

3. Composition of

purge / recycle

changed

Consider the

vaporizer &

compressor shut

down

More Opposite to less

No Same as less

Chapter # 8

130


ENVIRONMENTAL IMPACTS

OF CYCLOHEXANE PLANT

Chemicals can be released to the environment as a result of their manufacture,

processing, and use. EPA has developed information summaries on selected chemicals to

describe how you might be exposed to these chemicals, how exposure to them might

affect you and the environment, what happens to them in the environment, who regulates

them, and whom to contact for additional information. EPA is committed to reducing

environmental releases of chemicals through source reduction and other practices that

reduce creation of pollutants.

HOW MIGHT I BE EXPOSED TO CYCLOHEXANE?

Cyclohexane is a colorless, flammable liquid. It occurs naturally in petroleum crude

oil, in volcanic gases, and in cigarette smoke. It is produced in large amounts (an

estimated 338 million gallons in 1992) by four companies in the United States. US

demand for cyclohexane is likely to increase at a rate of 2% to 2.5% per year. The largest

users of cyclohexane are chemical companies that make adipic acid and caprolactam,

chemicals used to make nylon. Chemical companies also use cyclohexane to make

benzene, cyclohexanone, and nitrocyclohexane. Cyclohexane can be added to lacquers

and resins, paint and varnish removers, and fungicides. It is also used as a fuel for camp

stoves.

Exposure to cyclohexane can occur in the workplace or in the environment following

releases to air, water, land, or groundwater. Exposure can also occur when people use

products that contain cyclohexane or when they smoke cigarettes. Cyclohexane enters

the body when breathed in with contaminated air or when consumed with contaminated

131


food or water. It can also be absorbed through skin contact. Cyclohexane is not likely to

remain in the body due to its breakdown and removal in exhaled air and in urine.

WHAT HAPPENS TO CYCLOHEXANE IN THE ENVIRONMENT?

Cyclohexane evaporates when exposed to air. It dissolves when mixed with water.

Most direct releases of cyclohexane to the environment are to air. Cyclohexane also

evaporates from water and soil exposed to air. Once in air, it is expected to break down

to other chemicals. Because it is a liquid that does not bind well to soil, cyclohexane that

makes its way into the ground can move through the ground and enter groundwater.

Plants and animals living in environments contaminated with cyclohexane can store small

amounts of the chemical.

We can, in brief asses the impacts of cyclohexane producing plant in the following

major areas of ecology and sociology.

HUMAN HEALTH

Cyclohexane is not a highly toxic chemical. For 600 to 700 ppm exposure, no chronic

effects have been observed. The recommended threshold limit for cyclohexane is 300

ppm by volume. Exposure time is also important. Usually several days are needed, for

both human and animals, to cause any problem at these ppms.

A. Pharmacokinetics

1. Absorption - Cyclohexane is absorbed following inhalation and nominally by the

skin. Massive applications of the chemical to the skin of rabbits have produced

microscopic changes in the liver and kidneys. Systemic toxicity observed in animals

exposed orally to cyclohexane indicates that gastrointestinal absorption of the chemical

also occurs. In workers exposed to atmospheric cyclohexane, 22.8% of the total

respiratory intake was absorbed, and a "significant amount" of the absorbed cyclohexane

was either retained or metabolized.

132


2. Distribution - Following inhalation exposure of Wistar rats to concentrations

of cyclohexane ranging from 300-2000 ppm, perirenal fat concentrations of the chemical

were 23- to 38-fold greater than brain concentrations after one week of exposure and 50-

to 80-fold greater than brain concentrations, after two weeks. No information was found

regarding distribution to other organs.

3. Metabolism - Cyclohexane is metabolized via the hepatic, vascular, and renal

systems. Microsomal hydroxylases oxidize cyclohexane to cyclohexanol in the presence

of NADPH and oxygen. Other metabolites identified in mammalian systems include

trans-cyclohexane-1,2,-diol, cyclohexanone, and adipic acid.

B. Acute Effects

Cyclohexane has low acute toxicity, producing eye irritation in humans and

neurological symptoms , other organ effects, and death in animals at very high doses.

1. Humans - According to one source, cyclohexane is detectable by odor and is

irritating to the eyes at 300 ppm; another source suggested 25 ppm as the odor threshold

(ACGIH 1991). Undiluted cyclohexane is also irritating to the skin. No other

information was found in the secondary sources searched for the acute toxicity of

cyclohexane to humans.

2. Animals - The oral LD50 for cyclohexane in rats ranges from 8.0 to 39 mL/kg

(both greater than 5 g/kg), depending upon the age of the animals. The oral LD50 for

mice is 1.3 g/kg; the minimum lethal oral dose in rabbits is 5.5-6.0 g/kg; and the dermal

LD50 in rabbits is >180 g/kg. Within 1 to 1.5 hours, lethal doses to animals produced

severe diarrhea, vascular damage and collapse, hepatocellular degeneration and toxic

glomerulonephritis. Exposure of rabbits to 3330 ppm (duration not given) produced no

effect; 18,500 ppm for 8 hours was non-lethal; and 26,600 ppm for 1 hour was lethal.

Application of 1.55 g/day of cyclohexane to the skin for 2 days produced minimal

irritation.

C. Subchronic/Chronic Effects133


Cyclohexane administered subchronically is of low toxicity, producing

neurological effects, ocular, gastrointestinal, and respiratory effects in animals at very

high, lethal concentrations.

1. Humans - No information was found for the subchronic/chronic toxicity of

cyclohexane in humans in the secondary sources searched.

2. Animals - No effects were observed in rabbits exposed to 434 ppm

cyclohexane for fifty 6-hour periods or in rhesus monkeys exposed to 1234 ppm under

identical exposure conditions. Concentrations of ò7445 ppm, 6 to 8 hours/day for 2 to 26

weeks were lethal to rabbits, producing neurological effects as well as closure of the eyes,

conjunctival infection, salivation, labored respiration, cyanosis and diarrhea prior to

death. Rats exposed by inhalation to 1500 or 2500 ppm cyclohexane for 9-10 hours/day,

5 days/week for 7, 14, or 30 weeks exhibited no adverse effects.

D. Carcinogenicity

1. Humans - No information was found in the secondary sources searched

regarding the carcinogenicity of cyclohexane in humans.

2. Animals - No information was found in the secondary sources searched regarding

the carcinogenicity of cyclohexane in animals.

E. Genotoxicity

Cyclohexane was negative for viral enhanced cell transformation in Syrian hamster

embryo (SA7/SHE) cells and for histidine reverse gene mutation in Salmonella

typhimurium.

F. Developmental/Reproductive Toxicity

1. Humans - No information was found in the secondary sources searched

regarding the developmental/reproductive toxicity of cyclohexane in humans.

134


2. Animals - No information was found in the secondary sources searched

regarding the developmental/reproductive toxicity of cyclohexane in animals.

G. Neurotoxicity

The central nervous system is a major target organ for the toxicity of cyclohexane.

High concentrations of the chemical produce various effects, ranging from trembling to

death.

1. Humans - At high concentrations, cyclohexane is a central nervous system

depressant and may cause dizziness and unconsciousness.

2. Animals - Mice exposed to 50 mg/L (14,500 ppm) for 2 hours exhibited minimal

narcotic effects. Exposure to 18,000 ppm produced trembling within 6 minutes,

disturbed equilibrium within 15 minutes, and completes recumbency within 30 minutes.

Cyclohexane caused an excitation of the vestibulo-oculomotor reflex (threshold blood

level, 1.1 mmole/L). Concentrations of ò7445 ppm, 6 to 8 hours/day for 2 to 26 weeks

were lethal to rabbits, producing convulsions, tremors, narcosis, and paresis of the legs.

LAND

In the plant erection and installation stage, extensive damages to the land may

take place. Various concrete and metal dumping and digging etc. are major costs.

In plant operation, chemical leakages and certain solids dumping may impart

effects on land, along with the social activities impacts of plant people.

WATER POLLUTION

Water is extensively used for cooling and for other purposes in the plant, so water level

of locality and surface concentrations of various salts may disturb.

135


TLm values for fish range from 32 to 57.7 mg/L, indicating that the chemical is

moderately toxic to aquatic organisms in acute tests. Cyclohexane is expected to be of

low toxicity to terrestrial organisms and has a smog-forming potential.

A. Toxicity to Aquatic Organisms

TLm values for fish (24-96 hr) are 43-32 mg/L (Pimephales promelas, fathead

minnow), 43-34 mg/L (Lepomis macrochirus, bluegill). Mussel larvae (Mytilus edulis)

exposed to 1 to 100 ppm (mg/L) cyclohexane exhibited a 10-20% increase in growth rate.

The threshold concentration of cyclohexane in the cell multiplication inhibition assay,

measured in the protozoa Uronema parduczi Chatton-Lwoff, was >50 mg/L.

B. Toxicity to Terrestrial Organisms

Based on the low toxicity of cyclohexane to laboratory animals, the toxicity of the

chemical to terrestrial animals is expected to be low.

C. Abiotic Effects

Limited information indicates cyclohexane may have potential to contribute to the

formation of photochemical smog. The ozone-forming potential for cyclohexane has

been measured as 2 on a scale of 5. Ozone-forming potential is an indicator of the smog-

forming potential of a chemical.

AIR POLLUTION

Purge from the plant can be burned in boiler furnace but blow-downs and

fugitive emissions (form valve etc.) may pollute air, similarly high temperatures involved

may also warm the atmosphere.

CULTURAL ACTIVITIES

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Extensive cultural activities are involved in erection and production phases of the

project. Also a lot of economical activity is involved. These can affect cultural, social and

morale of the people involved.

ECONOMICS & BUSINESS

A huge amount of finances are spent as fixed and working capital of the project.

New markets will be explored and captured. Existing financial activity (imports etc.) will

be affected. Also the living status of the people involved will be changed.

POPULATION THICKENING EFFECTS

If the plant is to be installed near to a thickly populated area, the necessities like

accommodation, transportation, schooling etc. may get sever.

137


Chapter # 9

MATERIAL OF

CONSTRUCTION

Any engineering design, particularly for a chemical process plant, is only useful when it

can be translated into reality by using available materials of construction combined with

the appropriate techniques of fabrication can play a vital role in the success or failure of a

new chemical plant.

IMPORTANT MATERIAL AVAILABLE

Material of construction may be divided into two general classifications of metals

and non-metals. Pure metals and metallic alloys are included under the first classification.

1) Iron and Steel

Although many materials have greater corrosion resistance than iron and steel cost

aspects favor the use of iron and steel. As a result they are often used as a material of

construction when it is known that some corrosion will occur. If this is done the

presence of iron salts and discoloration in the product can be expected and periodic

replacement of the equipment should be anticipated. In general, cost of iron and

carbon steel exhibit about the some corrosion resistance. They are not suitable for use

with dilute acids, but can be used with many strong acids; since a protective coating

composed of corrosion products forms on the metal surface.

2) Stainless Steel

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There are more than 100 different types of stainless steels. The main reason for the

existence of stainless steels is their resistance to corrosion. Chromium is the main

alloying element, and the steel should contain at least 11%. Chromium is a reactive

element but it and its alloys passivate and exhibit excellent resistance to many

environments. A large number of steels are available. So stainless steel contains

chromium, nickel, iron, and also containing small amount of other essential properties.

They have excellent corrosion resistance and heat-resistance properties.

3) Nickel and its Alloy

Nickel exhibits high corrosion resistance to most alkalies.' Nickel-clad steel is used

extensively for equipment in the production of caustic soda and alkalies. The strength

and hardness of nickel is almost as great as carbon steel. In general, oxidizing

conditions promote the corrosion of nickel, and reducing conditions retard it. Monel,

an alloy of nickel containing 67% nickel and 30% copper is often used in food

industries. This alloy is stronger than nickel and has better corrosion resistance

properties than either copper or nickel.

4) Copper

It has been the traditional metal in breweries for centuries, but with the advent of new

alkaline cleaner, some corrosion problems have occurred. Copper and copper base

alloys are used in the formation of heat exchanger tubing, piping, fittings, etc.

Although the corrosion rates are comparatively high. In the range from room

temperature upto 100°C, the corrosion rate of copper is comparatively small.

However, the corrosion rate of 100°C is about five times that which takes place at

room temperature.

5) Aluminium

The lightness and relative ease of fabrication of aluminum and its alloys are factors

favoring the use of these materials. Aluminium resists attack by acids because a

surface film of inert hydrated aluminium oxide is formed. This film adheres to the

139


surface and offers good protection unless materials which can remove the oxide, such

as hydrogen acids or alkalies are present.

6) Lead

Pure lead has low creep fatigue resistance, but. its physical properties can be

improved by the addition of small amounts of silver, copper, antimony or tellurium.

Lead-clad equipment is in common use in many chemical plants. Lead shows good

resistance to sulfuric acid and phosphoric acid but it is susceptible to attack by acetic

acid and nitric acid.

7) Hastelloy

The beneficial effects of nickel, chromium, and molybdenum arc combined in

Hastelloy C to give an expensive but highly corrosion-resistant material. A typical

analysis of this shows 56% nickel, 17 molybdenum, 16% chromium, 5% iron and 4%

tungsten with manganese, silicon, carbon, phosphorus, and sulfur making up the balance.

Hastelloy C is used where structural strength and good corrosion resistance are necessary

under conditions of high temperature. The material can be machined and is easily

fabricated.

8) Coatings

Breweries are large consumer of quality coatings, not only for tankage but also for

structural steel, flooring and other working areas. The coating used range from high

heat silicones for stacks to special super resistant grouts for floor pavers.

8) Floor Materials

Considerable giazod tile is used in breweries and special expoxies with good

adhesion to very smooth surfaces have employed to coat glazed ceramic tile in order

to prevent crazing (cracking). Bacterial contamination deep in the pores of the

concrete is a common occurrence. If floors are not properly sealed, corrosion of

140


concrete rebars and structural steel can result, with eventual cracking and spalling of

concrete.

10) Plastics

For corrosion control point of view, plastics materials are very useful, therefore

they have found application in breweries, water treatment tanks, acid storage, roofing,

and gutters are application for plastics that are common to most industrial activity.

Fiberglass and polyvinyl chloride are among the plastics that have been employed. Small

polypropylene tanks for yeast culture and other specialty service have some record of use.

EQUIPMENT MATERIAL OF

CONSTRUCTION

Storage tank for benzene Carbon steel

Storage tank for cyclohexane Carbon steel

Slurry hydrogenation reactor 316L Stainless Steel

Fixed Bed Cyclohexane Reactor 316L Stainlees Steel

Gas/Liquid Separator Carbon Steel

Stabalization Column Carbon Steel

141


Chapter # 10

COST ESTIMATION

An acceptable plant design must present a process that is capable of operating under

conditions which will yield a profit.

It is essential that chemical engineer be aware of the many different types of cost

involved in manufacturing processes. Capital must be allocated for direct plant expenses;

such as those for raw materials, labor, and equipment. Besides direct expenses, many

other indirect expenses are incurred, and these must be included if a complete analysis of

the total cost is to be obtained. Some examples of these indirect expenses are

administrative salaries, product distribution costs and cost for interplant communication.

ESTIMATION OF EQUIPMENT COST

STORAGE TANK

TK-1 = 3.1 x 106 rupees

TK-2 = 3.54 x 106 rupees

PUMPS

P-01 = 3.54 X 105 rupees

P-02 = 2.88 x 105 rupees

P-03 = 6.64xl04 rupees

142


COMPRESSORS

C-01 = 5.7.6x106 rupees

HEAT EXCHANGERS

E-01 = 1.45 xlO5 rupees

E-02 = 7.27xl05 rupees

E-03 = 5.8x105 rupees



E-06 = 9.25 xlO5 rupees

VESSELS

R-01 = 3.76xlO5 rupees

R-02 = 9.5xl04 rupees

V-01 = 3.3 x 105 rupees

V-02 = l.lxlO5 rupees

STABALIZER (V-03)

Shell cost = 3.54xlO5 rupees

Packing cost = 1.94 x 104rupees

Total cost = 3.73xlO5 rupees

143


Total purchased equipment cost = Rs. 2.56xl07 rupees

ESTIMATION OF TOTAL CAPITAL INVESTMENT

Direct Cost (Rs)

Installation costs = 6.4 x 105 rupees

Instrumentation & control, installed 4.61x105 rupees

Piping, installed = 1.15x105 rupees

Electrical, installed = 6.4 xlO4rupees

Building, process & auxiliary = 1.28 x 106rupees

Service facilities & yard improvement = 1.8x105 rupees

Land = 1.53 x 106 rupees

Total direct cost = 8.68 x 106rupees

Indirect Cost

Engineering & supervision = 1.514x106 rupees

Construction & contractor's fee= 1.56x106 rupees

Contingency = 1.33 x 107 rupees

Total indirect costs = 4.41 x 107 rupees

Total fixed capital investment = 1.31x107 rupees

Working capital = 3.3x106 rupees

Total capital investment = 1.64x107 rupees144


REFERENCES

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3) John B. Butt, “Reaction kinetics and reactor design”

4) Kuppan, T., “Heat Exchanger Design Hand Book, Marcel Dekker Inc., New York, 2000.

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1972.

6) William L. Luyben, Chemical Reactor Design and Control”

7) Peters, M.S. and Timmerhaus, K.D., “Plant Design and Economics for Chemical

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145


13) Peacock, D.G., “Co ulson & Richardson’s Chemical Engineering”, 3rd ed, vol,

Butterworth Heinenann, 1994.

14) Sinnot, R.K., “Coulson and Richardson’s Chemical Engineering”, 2nd ed, vol 6,

Butterword Heinemann, 1993.

15) Kern, D.Q., “Process Heat Transfer”, McGraw Hill Inc., 2000.

16) McCabe, W.L, “Unit Operations of Chemical Engineering”, 5th Ed, McGraw Hill, Inc,

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17) Perry, R.H and D.W. Green (eds): Perry’s Chemical Engineering Handbook, 7th edition,

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Professional Publishers,1972.

21) Kohl,L. and Richard Nelson, B., Gas Purifications, vol.5; Gulf

Professional Publishers,1997.

22) Walas.S M.,Chemical Process Equipment Selection and Design,Reed

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23) Richardson.J.F & Coluson,J.M., Chemical Engineering,

vol.6;McGraw Hill, New York, 1997.

24) Ernest,L.E., Applied Process Design For Chemical And

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Petrochemical Plants,edit.3;vol.2; Gulf Professional Publishers.

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APPENDIX

Figure 4.1

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Figure 4.2

148


Figure 4.3

149


Figure 4.4

150

Documents

production of cyclohexane from benzene.doc