Catalytic Partial Oxidation of Propylene to Acrolein: The ...
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Transcript of Production of Acrolein
IN THE NAME OF ALMIGHTY ALLAH,
WHO IS THE MOST BENEFICENT AND THE
MOST MERCIFUL
Production of Acrolein by partial
oxidation of Propylene
Project Advisors
Madam Saira Bano
Sir Abdul Rehman
Project Members
Sweeba Zafar 2008-CPE-14
Aleem Naeem 2008- CPE-82
Muhammad Naeem 2008- CPE-38
Muddasar Safdar 2008- CPE-02
DEPARTMENT OF CHEMICAL AND POLYMER ENGINEERING
UNIVERSITY OF ENGINEERING & TECHNOLOGY
LAHORE
Production of Acrolein by partial
oxidation of Propylene
This project 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: _______________
Sign: _______________
Name: _______________
External Examiner: Sign: ________________
Name: ________________
DEPARTMENT OF CHEMICAL AND POLYMER ENGINEERING
UNIVERSITY OF ENGINEERING AND TECHNOLOGY
LAHORE
i
All praises to Almighty
Allah, Whose uniqueness,
oneness & wholeness is
beyond any comparison. All
respects are for His Holy
Prophet, Muhammad (peace
be upon him) who enabled
us to recognize our Creator.
ii
Dedicated to
Our loving Parents, their
resolute patience and guidance
to bring us to this position.
iii
Abstract
This report presents the final year project design of a chemical plant producing
3500 kg/day of Acrolein by partial oxidation of propylene using mixed catalyst.
The mixed catalyst is the bismuth molybdate-based catalyst having an average
particle size of 3.5mm.We selected this catalyst because it is highly active and
selective than other catalysts used for the production of Acrolein. We selected the
capacity on the basis of demand and supply of Acrolein worldwide and with
respect to Pakistan. The process that we selected for the production of Acrolein is
an optimum one because of low cost of propylene. Also propylene is easily
available and the yield of Acrolein obtained is maximum by this process than any
other process. After selecting the capacity and process for production of Acrolein
we did material and energy balance of whole plant and determined the flow rates
and fractions of components across each equipment being used in the plant and
also the heat load for each unit. We designed the four major units of the plant that
are heat exchanger, reactor, absorber and distillation column. Also we did the
mechanical design of reactor. After that we applied control scheme to heat-
exchanger, PFR and distillation column. We did the HAZOP analysis of absorber.
We studied the environmental impacts of Acrolein and the also the steps of
minimizing these impacts. Finally, we determined the cost of all designed
equipments.
iv
Acknowledgement
All praise to ALMIGHTY ALLAH, who provided us with the strength to
accomplish this main project. All respects are for His HOLY PROPHET (PBUH),
whose teachings are true source of knowledge & guidance for whole mankind.
Before anybody else we thank our Parents who have always been a source of
moral support, driving force behind whatever we do. We are indebted to our
project advisors Madam Saira Bano and Sir Abdul Rehman for their worthy
discussions, encouragement, technical discussions, inspiring guidance, remarkable
suggestions, keen interest, constructive criticism & friendly discussions which
enabled us to complete this report. They spared a lot of precious time in advising
& helping us in writing this report.
We are sincerely grateful to Dr. Mahmood Ahmad & Dr. Shaukat Rasool for their
profound gratitude and superb guidance in connection with the project.
Authors
v
Preface
It is a design project and purpose is to present the production of Acrolein by
partial oxidation of propylene using mixed catalyst.
Chapter 1 provides basic knowledge of Acrolein, methods of manufacturing,
physical and chemical properties, applications and other uses of Acrolein.
Chapter 2 deals with capacity selection and different processes for the
manufacturing of Acrolein and the selection of optimum one.
Chapter 3 deals with process description.
Chapter 4 consists of material and energy balance calculations across all
equipments in the plant.
Chapter 5 includes detailed design of shell and tube heat exchanger, reactor,
absorber and distillation column. It also consists of basic knowledge of these
equipments and the specification sheets of all these equipments are also given.
Chapter 6 includes mechanical design of reactor.
Chapter 7 Instrumentation and control for the process is being discussed in this
chapter.
Chapter 8 deals with hazard and operability analysis. Why and how HAZOP
analysis is done.
Chapter 9 includes environmental impacts of Acrolein and what steps are under
taken to minimize these impacts.
Chapter 10 includes cost estimation of all the designed equipments.
vi
Table of Contents
Page #
Chapter # 1
Introduction of Acrolein --------------------1
1.1 Acrolein -------------------------------------------------------1
1.2 History and Origin --------------------------------------------1
1.3 Methods of manufacturing------------------------------------1
1.4 Properties of Acrolein ----------------------------------------2
1.4.1 Physical properties of Acrolein--------------------------2
1.4.2 Chemical properties of Acrolein-------------------------3
1.5 Uses and applications of Acrolein----------------------------3
Chapter # 2
Process and Capacity selection ----------------6
2.1 Process Selection-------------------------------------------------6
2.1.1Vapor phase condensation----------------------------------6
2.1.2 Vapor phase oxidation--------------------------------------6
2.1.3 Partial oxidation of propylene------------------------------6
2.2 Capacity Selection-------------------------------------------------7
Chapter # 3
Process Description-----------------------------11
3.1 Process Description -----------------------------------------------11
vii
Chapter # 4
Material and Energy Balance -----------------14
4.1 Material Balance --------------------------------------------------14
4.1.1 Material Balance across reactor------------------------------14
4.1.2 Material Balance across quench cooler---------------------15
4.1.3 Material Balance across absorption column----------------16
4.1.4 Material Balance across water distillation column---------17
4.1.5 Material Balance across propylene distillation column----18
4.1.6 Material Balance across acrolein distillation column------19
4.2 Energy Balance-----------------------------------------------------19
4.2.1 Energy Balance across mixing point-------------------------19
4.2.2 Energy Balance across preheater-----------------------------20
4.2.3 Energy balance across reactor--------------------------------21
4.2.4 Energy balance across quench cooler------------------------22
4.2.5 Energy Balance across absorption column------------------23
4.2.6 Energy Balance across water distillation column-----------24
4.2.7 Energy Balance across propylene distillation column------25
4.2.8 Energy Balance across acrolein distillation column--------26
Chapter # 5
Designing of Equipments ------------------------27
5.1 Design of Shell and Tube Heat Exchanger ---------------------27
5.1.1Heat Exchanger--------------------------------------------------27
5.1.2 Main Categories of Heat Exchangers------------------------27
5.1.3 Heat exchangers are used--------------------------------------27
5.1.4 Selection of Heat Exchanger----------------------------------28
5.1.5Shell and Tube Heat Exchanger-------------------------------29
5.1.6 Types of Shell and Tube Heat Exchanger-------------------29
viii
5.1.7 Design Calculations--------------------------------------------30
5.1.8 Specification Sheet of heat exchanger-----------------------41
5.2 Design of Reactor--------------------------------------------------42
5.2.1 Selection of Reactor Type-------------------------------------42
5.2.2 Design Calculations--------------------------------------------44
5.2.3 Specification Sheet of reactor--------------------------------54
5.3Design of Absorber-------------------------------------------------55
5.3.1 Packed Columns------------------------------------------------55
5.3.2 Choice of plates or packing-----------------------------------55
5.3.3 Types of packing-----------------------------------------------57
5.3.4 Column Internals-----------------------------------------------60
5.3.5 Packing support----------------------------------------------61
5.3.6 Liquid distributors--------------------------------------------62
5.3.7 Liquid redistributors--------------------------------------------65
5.3.8 Hold-down plates-----------------------------------------------66
5.3.9 Liquid hold-up--------------------------------------------------67
5.3.10Wetting rate-----------------------------------------------------68
5.3.11Column Auxiliaries--------------------------------------------68
5.3.12 Design Calculations-------------------------------------------70
5.3.13 Specification Sheet of absorber------------------------------83
5.4 Design of Distillation Column ----------------------------------84
5.4.1Distillation-------------------------------------------------------84
5.4.2 Types of Distillation Columns-------------------------------85
5.4.3 Choice between plate and packed columns----------------85
5.4.4 Plate Contractors-----------------------------------------------86
5.4.5 Selection of Tray----------------------------------------------86
5.4.6 Factors affecting Distillation Column operation----------87
5.4.7 Design Calculations-------------------------------------------89
5.4.8 Specification Sheet --------------------------------------------103
ix
Chapter # 6
Mechanical design of Reactor------------------104
6.1 Mechanical Design-------------------------------------------------104
Chapter # 7
Instrumentation and Control ------------------106
7.1 Instrumentation and Process Control---------------------------106
7.2 Process instrument-----------------------------------------------107
7.3 Control------------------------------------------------------------107
7.3.1Temperature measurement and control----------------------107
7.3.2Pressure measurement and control---------------------------107
7.3.3 Flow measurement and control------------------------------108
7.4 Control scheme of distillation column--------------------------108
7.5 Heat exchanger control-------------------------------------------111
7.6 Control Scheme of PFR------------------------------------------111
Chapter # 8
HAZOP Study ------------------------------------ 114
8.1 Introduction ---------------------------------------------------------114
8.2 Background ---------------------------------------------------------114
8.3 Types of HAZOP---------------------------------------------------115
8.4 HAZOP guide words and meanings------------------------------116
8.5 HAZOP study of an absorber--------------------------------------116
x
Chapter # 9
Environmental Impact analysis of acrolein -118
9.1Hazards Identification-----------------------------------------------118
9.1.1Potential Acute Health Effects---------------------------------118
9.1.2 Potential Chronic Health Effects------------------------------118
9.2Fire and Explosion Data---------------------------------------------119
9.3Accidental Release Measures---------------------------------------119
9.4 Handling and Storage------------------------------------------------120
9.5Exposure Controls/Personal Protection----------------------------120
9.6First Aid Measures----------------------------------------------------121
Chapter # 10
Cost Estimation -----------------------------------123
10.1 Cost Indexes---------------------------------------------------------123
10.2 Cost Estimation of designed equipments-------------------------124
APPENDICES-------------------------------------129
REFERENCES -----------------------------------155
1
CHAPTER NO: 1
INTRODUCTION OF ACROLEIN
1.1 Acrolein
Acrolein is the basic compound in the series of unsaturated aldehydes. Its
chemical formula is C3H4O and chemical name is 2-propanol. Acrolein is
colorless and highly volatile liquid and soluble in many organic liquids.
1.2 History and origin
Acrolein is highly toxic and flammable material with extreme lachrymatory
properties. Degussa has produced Acrolein commercially since 1938.The process
was based on vapors phase condensation of acetaldehyde and formaldehyde. By
following the Degussa method of acrolein production the first plant to
manufacture acrolein first started in 1942. In 1945 shell started the production of
acrolein by pyrolysis of diallyl ether, a byproduct of synthesis of allyl alcohol by
saponification of allyl chloride. In 1959 shell began producing acrolein by partial
oxidation of propylene.
Acrolein, low mole weight aldehyde containing a C=C solid bond, is a clear to
yellow, flammable, poisonous liquid with a disagreeable odor; boiling at 52.7 0C;
soluble in water, alcohol, and ether; causing tears. Commercial acrolein is
produced by gas-phase oxidation of propylene in the presence of bismuth or
molybdenum oxide. It is also produced as a by-product during the production of
acrylic acid or acrylonitrile.
1.3 Methods of Manufacturing
It was produced commercially starting in 1938 by the vapor-phase
condensation of acetaldehyde & formaldehyde. In 1959, the direct oxidation
of propylene in presence of a catalyst became the preferred commercial
2
process, & variations of this process are the only methods currently used
commercially. The acetaldehyde-formaldehyde route was last used in the
USA in 1970
Manufactured: By oxidation method I-e (A) by oxidation of acetaldehyde;
(B) by oxidation of propylene in liquid phase; (C) by oxidation of propylene
in vapor phase; (D) by oxidation of allyl alcohol;
By heating glycerol with magnesium sulfate.
Prepared industrially by passing glycerol vapors over magnesium sulfate
heated to 330-340 0C.
1.4 properties of acrolein
1.4.1 Physical properties of acrolein
Molecular weight 56.06 kg/kg mole
Odor Extreme sharp, pungent and disagreeable
Color Colorless or yellowish
Boiling point 52.50C at 760 mmHg
Melting point -880C
Density 0.8389 g/cm3 at 20
0C, 0.8621 g/cm
3 at 0
0C
Heat capacity 2139 kJ/kg.K (17 to 440C, liquid)
1200 kJ/kg.K (3000C, vapor)
Standard heat of
formation
-74.483 kJ/mol
Heat of combustion -29098 kJ/kg
Heat of vaporization 542.191 kJ/kg
Heat of
polymerization
-80.4 kJ/mol
PH 6 in 10% solution in water at 250C
Surface tension 0.024N/m at 200C
3
Vapor density 1.94 (Air =1)
Viscosity 0.35 cp at 200C
1.4.2 Chemical properties
CH2=CH-CHO the carbonyl group in the conjugate with the C=C bond is present
in molecule of acrolein because of its two functional group; acrolein is highly
reactive, easily polymerized compound. Its reactive centre can be reacted
selectively and simultaneously. The reaction of acrolein can be understood as
typical of olefin activated for nucleofilic attack by influence of electron attracting
carbonyl group or as a reaction of aldehyde that is unsaturated.
The tendency of acrolein to polymerize is very great; the acrolein can only be
stored in the presence of considerable amounts of stabilizers. In spite of the
presence of stabilizer, small amounts of polymerization catalysts which are able to
initial radical, anionic or cationic propagating polymerization are sufficient to
cause highly polymerization reaction.
1.5 Uses and applications of acrolein
Some of direct and indirect uses of acrolein are
Manufacturing of Acrylic Acid
The largest single use for acrolein is as an isolated intermediate in the
manufacturing of acrylic acid, most of which is converted to its lower alkyl esters.
Preparation of Polyester Resin
Acrolein is used in the preparation of polyester resin, polyurethane, propylene
glycol, acrylic acid, acrylonitrile and glycerol.
Production of Methionione
Acrolein is basic raw material for the production of essential amino acid
methionine because of lack of methionine in many nutrient protein compounds
4
with the average biological demand, it is necessary to add methionine to the
natural food materials for boilers to improve their biological efficiency which is a
protein supplement used in animal feed.
Manufacturing of Glycerol
The chemical reduction of acrolein via alkyl alcohol is the technical process for
the manufacturing of synthetic glycerol.
Microbiological Activity of Acrolein
In biological systems one may expect rapid reactions with any reactive N-H, S-H,
O-H or C-H bond which would lead to molecular modification. In the subsurface
injection of waste waters the addition of 6-10 ppm acrolein controls the growth of
microbes in the food lines thereby preventing plugging and corrosion.
The microbiological activity is further utilized in protecting the liquid fuel against
microorganism. About <500 is in jet fuels or distillate feed tank bottoms. The
dialkyl acetyls of acrolein are also effective in such cases; as a biocide in oil wells
and liquid petrochemical fuels. The growth of algae, aquatic weeds and mollusks
in recirculation process water is controlled by acrolein.
Slime Formation
Slime formation is a serious problem in paper manufacturer: acrolein at 0.4 to 0.6
ppm is effective slimicide in this application.
Acrolein as Tissue Fixative
Acrolein has received quite a bit of attention as a tissue fixative. This property of
acrolein has been utilized for preservation of red blood cells. Acrolein may be
used to cross link invertase at PH 7 to give a water insoluble product which
possesses constant activity for inversion of sucrose for the period of 12 weeks.
Acrolein is sometimes used as a fixative in preparation of biological specimens
for electron microscopy.
5
Immobilization of Enzymes
Conversion of acrolein into polymers or copolymers processing pendant aldehyde
groups provides polymers which have been utilized for Immobilization of
enzymes.
Other uses
Acrolein has been used to make modified food starch.
In the cross-linking of protein collagen in leather tanning.
In the manufacture of colloidal forms of metals.
In the production of perfumes.
6
CHAPTER NO: 2
PROCESS AND CAPACITY SELECTION
2.1 Process Selection
Acrolein can be produced by different methods.
2.1.1 Vapor phase condensation
Acrolein was first produced commercially in the 1930s through vapor phase
condensation of formaldehyde and acetaldehyde.
2.1.2 Vapor phase oxidation
Acrolein was then produced in 1940s by vapor phase oxidation of propylene using
cuprous oxide catalyst; however, this method was not used at first due to the poor
performance of cuprous oxide catalysts.
2.1.3 Partial oxidation of propylene
Acrolein is being produced by partial oxidation of propylene using mixed catalyst
now a days from 1960s and to produce acrolein by this method using bismuth-
molybdate based catalyst is important one. This is most favored and most
economical method. By the critical study of the processes, catalytic oxidation of
propylene has proved to be the most attractive in terms of raw material and high
yield of acrolein than any other process. This process is attractive because of the
availability of highly active and selective catalysts and the relatively low cost of
propylene.
The process that we have chosen for the production of acrolein is by the “Partial
oxidation of propylene”.
7
2.2 Capacity Selection
Market trends/Demands
Acrolein as a chemical product is rarely sold in large amounts on open market.
Whilst there are producers that sell certain amounts of it, the chemical is
immediately used in the production of other chemicals due to its instability and
safety hazards.
In the case of this project, we will be designing a plant that will produce Acrolein,
which will be piped out directly to the neighboring plant that uses Acrolein to
produce other chemicals. This allows a small scale plant to be designed whilst
avoiding the problem of transporting Acrolein.
Whilst our plant will be producing Acrolein, the price and market of chemical is
fully dependent on products it is used to create and as such market analysis must
be extended to these chemicals. There are six main products that are produced
using Acrolein. These are polyurethane, methonine , Polyester resins, acrylonitrile
and acrylic acid. In the section of the report we will analyze the market for these
products alongside the Acrolein product.
It is possible to collect the information on the global market for the chemicals in
this report but finding exact figures and market percentages is difficult due to
commercial selling of such information. We have tried to obtain as many figures
as possible but they are mostly based on US imports. Whilst this does not show
the global market but it is a reasonable indicator of global market.
Acrolein is not a staple import/export product and due to its overall lack of value
unless further processed, the market is centre around countries and areas with
facilities that process the chemical further.
This can be seen when trying to source prices for Acrolein alone. The majority of
the manufacturers selling Acrolein are doing so from mainland China. Our
product buying websites, the manufacturers are usually nearly all Chinese based.
Looking at the change in market share and Acrolein exports, being imported into
the USA, over the past year, this viewpoint is only reinforced.
8
Figure 2.1. Dominant exporters of acrolein in the world with respect
to number of shipments
Figure 2.2. Dominant exporters of acrolein in the world with respect
to market share changes
9
This data in the tables clearly indicates the Chinese dominance of the Acrolein
export business. Few other countries even more close to affecting the market
share, with hundreds of countries having no noticeable effect at all.
It also shows the same scale of Acrolein import/export market. While other
chemicals having high market share changes, they are only in the single figures in
the most cases. Due to the small scale of Acrolein market however, the market
share changes are far higher as small individual shipment have far greater affect.
This leads to market share changes such as China gaining 27% more market share
from the year before while the Germany loses over 1/3rd
of the market share.
General Acrolein involved market
The current market for Acrolein and its subsequent products has a downward
outlook in the short-term. Asian markets prices dropping has a knock on effect
throughout the global market as potential buyers demand lower prices in the
European and USA markets. Profits are likely to be lower than normal in the
fourth quarter due to this.
The long term outlook for the market is mixed. Prices will rise again due to the
cost of raw materials and increased demand. This should in turn buoy profits
again. However, the dependence on the propene, and thus Acrolein for the
products previously mentioned may soon be threatened due to the rise of new
technologies.
Capacity in
Kg(Demand)
Capacity in Kg (Supply) Years
2523981 1913681 2006-2007
2945678 2283406 2007-2008
3515630 2697086 2008-2009
3940560 3080172 2009-2010
4512567 3673672 2010-2011
10
In the Scenario of Pakistan industry, there is no special attention towards the
generation of acrolein. The Desired chemical is totally exported from different
countries e.g. China, Germany, Malaysia, Iran etc.
So by keeping in view the importance of the above described chemical, special
attention is given to the manufacturing of the acrolein by the Engineers of
University of Engineering and Technology, Lahore.
The suggested pilot plant has the capacity of 3500 kg/day with the annual amount
1277500 kg with the increasing demand and importance of chemical with the
passage of time.
Selected Capacity: 3500kg /day
0500000
100000015000002000000250000030000003500000400000045000005000000
Am
ou
nt
in K
g
Years
Comparison of Demand Vs Supply
Supply
Demand
11
CHAPTER NO: 3
PROCESS DESCRIPTION
3.1 Process Description
Propylene (Stream 2), steam (Stream 4) and compressed air (Stream 6) are mixed
and heated to 250°C. The resultant stream (Stream 8) is sent to a catalytic packed
bed reactor where propylene and oxygen react to form acrolein. The reactor
effluent is quickly quenched to 50°C with deionized water (Stream 10) to avoid
further homogeneous oxidation reactions. Stream 12 is then sent to an absorber,
T-101, where it is scrubbed with water and acrolein is recovered in the bottoms
(Stream 15). The off gas, Stream 14, is sent to an incinerator for combustion.
Stream 15 is then distilled in T-102 to separate acrolein and propylene from water
and acrylic acid. The bottoms (Stream16) consisting of wastewater and acrylic
acid are sent to waste treatment. The distillate (Stream 17) is sent to T-103 where
propylene is separated from acrolein and the remaining water in the system. The
distillate from T-103 contains 98.4% propylene. The bottoms (Stream 19) are then
sent to T-104 where acrolein is separated from water. Stream 21 is sent to waste
treatment, and the distillate (Stream 20) consists of 98% pure acrolein.
12
Figure 3.1. Process flow diagram
13
Table 3.1. Equipment Description
Equipment No. Equipment Name Equipment No. Equipment Name
C-101 Feed air
compressor
P-103A/B Reflux pump
E-101 Reactor preheater P-104A/B Reflux pump
E-102 Condenser R-101 Packed bed
reactor
E-103 Reboiler T-101 Acrolein absorber
E-104 Condenser T-102 Water distillation
tower
E-105 Reboiler T-103 Propylene
distillation tower
E-106 Condenser T-104 Acrolein
distillation tower
E-107 Reboiler V-101 Reflux vessel
P-101A/B Water pump V-102 Reflux vessel
P-102A/B Reflux pump V-103 Reflux vessel
14
CHAPTER NO: 4
MATERIAL AND ENERGY BALANCE
4.1Material Balance
Our plant has capcity of 3500 kg/day.
From capacity selection data,we have to produce acrolein based on above
mentioned capacity so here is materail balace acording to our capacity.
4.1.1Material balance across Reactor
Stream No. /Name 8 9
Mass Flow Rate (kg/hr) 2730 2730
15
4.1.2Material balance across Quench cooler
Stream No./Name 9 10/11 12
Mass Flow Rate(kg/hr) 2730 40527 43257
16
4.1.3 Material balance across Absorption column
Stream No./Name 12 13 14 15
Mass Flow Rate(kg/hr) 43257 1800 1725.9 43332
17
4.1.4 Material balance across Water distillation column
Stream No./Name 15 16 17
Mass Flow Rate (kg/hr) 43332 43086 246.61
18
4.1.5 Material balance across Propylene distillation column
Stream No./Name 17 18 19
Mass Flow Rate (kg/hr) 246.61 17.91 228.7
19
4.1.6 Material balance across Acrolein distillation column
Stream No./Name 19 20 21
Mass Flow Rate (kg/hr) 228.7 147.57 81.21
4.2 Energy Balance
Reference Conditions:
Temperature = 298.15K
Pressure = 101.325kN/m2
4.2.1 Energy balance across the Mixing Point
20
Q = n CpT2
T1 dT
Stream
No./Name
1 2 3 4 5 6
Temperature
(k)
477.15 470.15 432.15 417.15 298.15 384.15
Heat load
(KJ/hr)
-24111.6 -29491.3 203333.8
4.2.2 Energy balance across the Preheater
dTCnQiPi
2
1
T
T
Stream No. /Name 7
8
Temperature (k) 413.15
523.15
Heat load (kJ/hr) 432427.1
21
4.2.3 Energy balance across the Reactor
Q reactor= ∆H reactants+∆H reaction+∆H products
Stream No./ Name 8
9
Temperature (k) 523.15
600.15
Heat load ( kJ/hr) 867393.3
1889203.93
∆H reaction = ∆H reaction 1 + ∆H reaction 2 + ∆H reaction 3+ ∆H reaction 4
∆H reaction = -1273275.84 + -140565.6 +-91359.52+-1103420.9
∆H reaction=-2608621.903 kJ/hr
Q reactor =867393.3+ (-2608621.903) +1889203.93
Q reactor=147975.327 kJ/hr
22
4.2.4 Energy balance across the Quench Cooler
dTCnQiPi
2
1
T
T
Stream No./Name 9 10/11
12
Temperature(k) 600.15 298.15
310.15
Heat load (kJ/hr) 1889203.93 0
957166.1
23
4.2.5 Energy balance across the Absorber
dTCnQiPi
2
1
T
T
Stream No. /Name 12 13 14 15
Temperature(k) 310.15 298.15
299.15 310.15
Heat load(kJ/hr) 957166.1 0 1914.5 991745.6
24
4.2.6 Energy balance across the Water Distillation Tower
dTCnQiPi
2
1
T
T
Stream No./Name 15 16
17
Temperature(k) 310.15 373.15
302.15
Heat load (kJ/hr) 991745.6 4480530.449
2768.57
Q condenser = 308.2kJ/hr
Q reboiler = -97272kJ/hr
25
4.2.7Energy balance across the Propylene Distillation Tower
dTCnQiPi
2
1
T
T
Stream No./Name 17
18 19
Temperature(k) 302.15
299.15 338.15
Heat load (kJ/hr) 2768.57
162.825 66807.52
Q condenser = 12.82 kJ/hr
Q reboiler = -292.18kJ/hr
26
4.2.8 Energy balance across the Acrolein Distillation Tower:
dTCnQiPi
2
1
T
T
Stream No./Name 19
20 21
Temperature 338.15
325.15 378.15
Heat load (kJ/hr) 66807.52
40959.95 12662.4
Q condenser = 113.04 kJ/hr
Q reboiler = -181.8kJ/hr
27
CHAPTER NO: 5
DESIGNING OF EQUIPMENTS
5.1 Design of Shell and Tube Heat Exchanger
5.1.1Heat Exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from
one medium to another. The media may be separated by a solid wall, so that they
never mix, or they may be in direct contact.
5.1.2 Main Categories of Heat Exchangers
5.1.3 Heat Exchangers are used:
• To get fluid streams to the right temperature for the next process
• Reactions often require feeds at high temperature
• To condense vapours
Heat Exchangers
Recuperaters
Wall Separating Streams
Direct Contact
Regenerators
28
• To evaporate liquids
• To recover heat to use elsewhere
• Chemical processing etc.
5.1.4 Selection of Heat Exchanger
Exchanger
type
Maximum
pressure
range (Bar)
Temperature
approximate
range oC
Normal
area (m2)
Key features
Shell and tube 350 -200 to 700 1 to 1000 Very
adaptable and
can suitable
for gaseous
feeds
Double pipe
heat
exchanger
350
-200 to 700
.25 to 200
Suited for
small
capacities,
Pipe Coils 3 >400 1 to 2500 Pressure drop
between
fluids is
<3Mpa
Spiral tube 10 -300 to 600 2 to 600 Cannot deal
with cursive
fluids
29
5.1.5 Shell and Tube Heat Exchanger
The shell and tube exchanger is by far the most commonly used type of heat-
transfer equipment used in the chemical and allied industries.
Essentially, a shell and tube exchanger consists of a bundle of tubes enclosed in a
cylindrical shell. The ends of the tubes are fitted into tube sheets, which separate
the shell-side and tube-side fluids. Baffles are provided in the shell to direct the
fluid flow and support the tubes. The assembly of baffles and tubes is held
together by support rods and spacers.
Advantages:
1. The configuration gives a large surface area in a small volume.
2. Good mechanical layout: a good shape for pressure operation.
3. Uses well-established fabrication techniques.
4. Can be constructed from a wide range of materials.
5. Easily cleaned.
6. Well-established design procedures.
5.1.6 Types of Shell and Tube Heat Exchanger
Types of shell and tube heat exchangers are given below.
• Fixed tube heat exchanger
• U tube heat exchanger
• Floating tube heat exchanger
It may have different shell and tube passes for flow arrangements.
30
5.1.7 Design Calculations
Design/Problem Statement
Design a shell and tube heat exchanger to heat a feed mixture of,
• Propylene = 4.665kmol/hr = .0545Kg/s
• Steam = 39.77kmol/hr = .1988Kg/s
• Air = .6305kmol/hr = .0051Kg/s
From 413K (140oC) to 523K (250
oC) at pressure 203KN/m
2 (KPa). And heated
by Dowtherm oil from 673K (400oC) to 530K (257
oC).
Design Steps of Shell and Tube Heat Exchanger
The design steps of shell and tube heat exchanger are given below:
• General Design Steps. Part(A)
• Thermal Design. Part(B)
• Hydraulic Design. Part(C)
General Design Steps Part (A)
Step 1
Specification
Step 2
Obtain the necessary thermo Physical properties at mean temperature and perform
energy balance to calculate heat duties and flow rates.
Step 3
Assume suitable value of Overall coefficient.
Step 4
Decide number of shell and tube passes Calculate ΔTlm, correction factor, F, and
ΔTm.
31
Step 5
Determine heat transfer area required:
A= Q/U ΔTm
Step 6
Decide type, tube size, material layout and assign fluids to shell or tube side.
Step 7
Calculate number of tubes.
Step 8
Calculate shell diameter.
Step 9
Estimate tube-side heat transfer coefficient.
Step 10
Decide baffle spacing and estimate shell-side heat transfer coefficient.
Step 11
Calculate overall heat transfer coefficient including fouling factors, Uo.
Step 12
Estimate tube-side and shell-side pressure drops.
Thermal design of Shell & Tube Heat Exchanger Part (B)
Step 1: Specification
32
Hot Fluid[Dowtherm A]
Inlet temperature = 673k(400oC)
Outlet temperature = 530k(257oC)
Cold Fluid[Feed Mixture]
Inlet temperature = 413k(140oC)
Outlet temperature = 523k(250oC)
Step 2: Physical Properties
Mean Temperature of feed mixture:
= (523+413)/2
= 468K(195oC)
Heat Capacity (Cp):
Cp of Propylene at mean temperature = 12.25kJ/Kg.oC
Cp of Steam at mean temperature = 2.01543kJ/Kg.oC
Cp of Air at mean temperature = 1.02202kJ/Kg.oC
Density:
Density of Propylene at mean temperature = 2.21Kg/m3
Density of Steam at mean temperature = 7.106Kg/m3
Density of Air at mean temperature = .754Kg/m3
Viscosity:
Viscosity of Propylene at mean temperature =.004003Kg/m.S
Viscosity of Steam at mean temperature = .1.59x10-5
Kg/m.S
Viscosity of Air at mean temperature = .008637Kg/m.S
Thermal Conductivity (K):
“K” of Propylene at mean temperature = 3.82x10-5
KJ/m.S.oC
“K” of Steam at mean temperature =3.33x10-5
KJ/m.S.oC
“K” of Air at mean temperature =3.825x10-5
KJ/m.S.oC
Physical Properties of Dowtherm “A”:
Mean temperature = (673+530)/2 = 601.5k (328.5oC)
33
Density of Dowtherm at mean temperature = 15.60Kg/m3
Cp of Dowtherm at mean temperature = 2.049KJ/Kg.oC
“K” of Dowtherm at mean temperature = 2.99x10-5
KJ/m.S.oC
Viscosity of Dowtherm at mean temperature=1.16x10-5
Kg/m.S
Heat Duties
Heat duty of cold fluid
Heat duty can be calculated by formula given below.
Q = (m1Cp1+m2Cp2+m3Cp3)ΔT
So using the values of m1, m2,m3 and Cp1,Cp2,Cp3 & ΔT
Q = 118.08KJ/s
Mass flow rate of hot fluid
m = Q/CpΔT
Where
Q = 118.08KJ/s
Cp = 2.049KJ/Kg.oC
ΔT = 143oC m = .4031Kg/s
Step 3: Overall Heat Transfer Coefficient
As our feed is Air and Gas mixture at low pressure. So let us assume overall heat
transfer coefficient ,
U = 6W/m2.oC
OR
= .006KJ/m2S
oC
Value taken from Appendix (B), figure 3
34
Step 4: Calculation of ΔTavg
Our Heat exchanger is 1-2 pass shell and tube heat exchanger
T1= 400oC T2=257
oC
t2=250oC t1=140
oC
As (ΔT1/ ΔT1) = 1.28 which is less than 2 so we will calculate here just ΔTavg
rather than ΔTlm & ΔTm for calculation heat transfer area.
ΔTavg = (ΔT1 + ΔT2)/2 =[(T1–t2)+(T2-t1)]/2
After calculation ΔTavg = 133.5oC
Step 5: Calculation of Heat Transfer Area
Heat transfer area can be calculated by formula given below
Q = UA ΔTavg
A = Q/U ΔTavg
A = 147.4m2
Step 6: Layout, Tube Sizing & Allocation
Heat exchange fluid is allocated toward the shell & feed stream toward
tube sides due to corrosive nature.
Floating head shell and tube heat exchanger with split rings & 1-2 pass
Tubes are “Cupro-Nickel”.
Using Triangular Pitch as shell side fluid is clean.
A Iterative Selection is
(3/4inch
x14 BWG)
35
Suppose
L = 4m
O.D =di= 20mm
I.D = do=16mm
Values taken from Appendix (A), table 1
Step 7: Calculation of Number of tubes
As, Area of Single Tube = π do L
=.2512m2
No. of tubes required Nt = Total Heat Transfer Area/Area of single tube
= 604
According to TEMA standard Values taken from Appendix (A), table 2
Calculation of tube side velocity ut :
Tube cross section Area = (π/4) .di2
= 2.01x10-4
m2
Tubes per pass = Total tubes/2 = 302
Area per pass = (Tubes per pass) x (cross sectional area)
= .061m2
Volumetric flow rate = mass flow rate/density
Where,
ρ = 3.675Kg/m3
Mass flow rate= .2581Kg/s
So after adding values
Volumetric flow rate = .0704m3/S
Tube side velocity = Volumetric flow rate/Area per pass
= 1.153m/s acceptable.
36
Deduction:
According to rule of thumbs and conventions it is well known that the velocity in
the tubes should be between (.92-3.02) m/sec. So our 1-2 pass selection is
acceptable.
Step 8: Calculation of Shell Diameter
As shell side fluid id clean so we will use Triangular pitch 1.25do . So
Pt = 1.25d0
n1 = 2.207
K1 = .249
Values taken from Appendix (A), table 4
Bundle diameter Db = do(Nt/K1)1/n1
= .683m
By using split ring floating head Heat.Exchanger
Values taken from Appendix (B), figure 4
Clearance diameter = 65mm = .065m
Shell side Diameter = Bundle diameter + clearance diameter
= .748m
Step 9: Tube side heat transfer coefficient
It can be calculated from the given below formula.
hidi/Kf = jh.Re.Pr..33
.(µ/µw).14
Neglecting .(µ/µw).14
or .(µ/µw).14
=1
Where,
Kf (of mixture) Cp(of mixture) µ(of mixture) L di
3.314x10-5
KJ/m.S.oC
1.9178Kg/Kg.
0C .002881Kg/m.S 4m .016m
Re =(ρ.ut.di)/µ = 24
37
For (L/di) = 250 and Re = 24 from graph, tube side heat transfer factor is,
Jh = 3.4x10-2
Values taken from Appendix (B), figure 5
Pr = (Cp.µ)/Kf = 166.723
So after putting all these values into above formula gives the Tube side Heat
transfer coefficient is,
hi = .0507KJ/m2.S.
oC or 50.7W/m
2.oC
Step 10: Shell side heat transfer coefficient
Shell side heat transfer coefficient can be calculated by formula given below
hsde/kf = jh.Re.Pr1/3
(µ/µw).14
Neglecting (µ/µw).14 or = 1
Selecting Baffles
25%Cut Segmental Baffles.
Calculating Baffle spacing
According to the TEMA standards the allowed baffle spacing is 0.2Ds
we consider
Baffle Spacing lg= Ds/5 = .1496m
Selecting tube pitch
Tube pitch Pt = 1.25x20 = 25mm = .025m
Calculating cross flow area As
As = (Pt – d0)xDsxlg/Pt
= .0224m2
Calculating mass velocity Gs
Gs = mass of hot fluid/As
= 18.02Kg/s.m
Calculating Equivalent diameter De
De = 1.10/do(Pt2 - .917do
2)
= .014201m
38
Calculating Reynolds's Number Re
Re = Gsde/µ
=22050.09
Calculating Prandtl Number Pr
Pr = Cpµ/Kf
= .7949
Calculating Jh factor
As baffle cut is 25 so from Appendix (B), figure 6
Jh = 4x10-3
Calculating shell side heat transfer coefficient
As,
hsde/kf = jh.Re.Pr1/3
(µ/µw).14
Neglecting (µ/µw).14
and after putting values we have shell side heat transfer
coefficient.
hs = .172KJ/m2.S.
oC or 172.1W/m
2.oC
Step 11: Calculation of overall heat transfer coefficient Uo
Overall heat transfer heat transfer coefficient can be calculated from following
formula.
Where,
Uo = the overall coefficient based on the outside area of the tube,
W/m2 0
C
ho = Outside fluid film coefficient, W/m2 o
C =172.1 W/m2 o
C
hi = Inside fluid film coefficient, W/m2 oC = 50.7 W/m
2 oC
do = Tube outside diameter, m = .02m
39
di = Tube inside diameter, m = .016m
Kw =Thermal conductivity of the tube wall material, W/moC, = 50 W/m
2 oC
hid = Inside dirt coefficient, W/m2 o
C = 5000 W/m2 o
C
hod = Outside dirt coefficient (fouling factor), W/m2 oC =5000 W/m
2 oC
Values taken from Appendix (A), table 3
So after adding values into formula we have Uo = 8.2 W/m2 o
C
We will use this Corrected heat transfer coefficient in further calculations.
Corrected Heat Transfer Area
Corrected heat transfer area is given below
A = 107.86m2
Part C: Hydraulic Design
Step 12: Calculation of Pressure Drops
Tube Side Pressure Drop
Pressure drop on tube side is calculated from given formula,
Again Neglecting (µ/µw)-m
or .(µ/µw)-m
=1
Where,
Jf =Friction factor = 3.1x10-1
Values taken from Appendix (B), figure1
Np = No.of tube passes =2
After putting values to above formula, tube side pressure drop is calculated as,
ΔPt= 3.06kpascal or .443PSi
Shell Side Pressure Drop:
Pressure drop on tube side is calculated from given formula,
40
Again Neglecting (µ/µw)-m
or .(µ/µw)-m
=1
Where,
us = Gs/ρ = 1.49m/s
lB =baffle spacing
From graph for 25% Baffle cut,
Jf = 4.8x10-2
Values taken from Appendix (B), figure 2
After putting values to above formula, shell side pressure drop is calculated as,
ΔPs= 5.43kpascal or .787PSi
41
5.1.8 Specification Sheet of shell and tube heat exchanger
Unit Shell & tube heat exchanger
No. of shell passes 1
No. of tube passes 2
Heat Transfer area 147.4m2
Diameter of shell .748m
Pitch 25mm
No. of tubes 604
Type of tube used 14BWG
No of baffles &type 12(25%cut baffle)
OD & ID of tube 20mm & 16mm
ΔPt on shell side 3.06Kpascal or .443Psi
ΔPs on tube side 5.43kpascal or .787Psi
42
5.2 Design of Reactor
Heterogeneous catalytic reactors are the most important single class of reactors
utilized by chemical industry. Whether their importance is measured by the
wholesale value of goods produced, the processing capacity or the overall
investment in the reactors and associated peripheral equipment. Our process is
continuous process so we only consider reactors for continuous and
heterogeneous processes as gas and solid phases are present.
Classification is in terms of relative motion of the catalyst particles and reactants.
Reactors in which the solid catalyst particles remain in a fixed position
relative to one another (fixed bed, trickle bed and moving bed reactors).
Reactors in which the particles are suspended in a fluid and are constantly
moving about (fluidized bed and slurry reactors).
5.2.1 Selection of Reactor Type
43
Advantages of fixed bed reactor
A fixed bed reactor has many unique and valuable advantages relative to other
reactor types.
One of its prime attributes is its simplicity.
Costs for construction, operation and maintenance relative to moving bed.
It requires a minimum of auxiliary equipment.
For economical production of large amounts of product, fixed bed reactors
are usually the first choice, particularly for gas-phase reactions.
I have selected continuous flow, adiabatic, fixed bed reactor.
i) Continuous reactor
This reaction has low residence time.
Its operating cost is low.
Production variation is not desired.
ii) Adiabatic reactor
The reaction is slightly exothermic.
Equilibrium constant remains constant with that small change in
temperature.
(iii) Fixed bed reactor
Gas vapor catalyzed reaction.
High conversion is desired.
Relatively low operating and fixed
After analyzing different configuration of fixed bed reactors we have concluded
that for our system the most suitable reactor is multi tube fixed bed reactor.
Because of the necessity of removing or adding heat, it may not be possible to use
a single large-diameter tube packed with catalyst. In this event the reactor may be
built up of a number of tubes.
44
5.2.2 Design Calculations
Preliminary Data for Reactor Design Calculations
Reactor In Reactor
Out
Temperature 2500C 327
0C
Pressure 203kpa After calculation
Mass flow rate kg/hr
Propylene 195.93 3.921
Nitrogen 1395.1 1395.1
Oxygen 423.68 180.9
Water 716.0 844.76
Acrolein ---- 187.4
Acrylic acid ---- 24.0
Carbon dioxide ---- 94.28
Total flow 2730 2730
45
Reactions
The following reactions and side reactions lead to the production of Acrolein.
C3H6 + O2 C3H4O + H2O (1)
C3H4O + 7/2O2 3CO2 + 2H2O (2)
C3H4O + 1/2O2 C3H4O2 (3)
C3H6 + 9/2O2 3CO2 + 3H2O (4)
Design steps for Reactor
Volume of reactor
Volume & weight of catalyst
Geometry of reactor
Calculation of no. of tubes required
Pressure drop along tube and shell side
Heat transfer coefficient
Heat transfer area
Available area
Specification sheet of reactor
46
Reaction Kinetics
K1=2, K2=4, K3=2
T=623 k R=1.987 kcal/kmol PC3H6 =28.1 kpa
PO2 =13.1 kpa Prexponent term= 0.108 coml./ft3hr
T0 =6230C
P.p of 02 and C3H6 are
PO2 = (5.653/108.32)×203=10.594 kpa
PC3H6= (0.0933108.32) ×203=0.1748 kpa
So rate of reaction is
ri =0.1077 kmol/ft3hr
Volume of Reactor
Design equation is
FAo=4.665 kgmol/hr
XA=0.98
VR=42.429 ft3=1.2014 m
3
Type and volume of Catalyst
A mixture of bismuth molybdate- based catalyst having average particle size of
3.5mm is used as catalyst in the process
Bulk density of catalyst, ρc = 2500 Kg/m3
Bed void fraction, = 0.4
Volume of catalyst = Vr = Vcat (1 +)
47
= 30.306ft3=0.8582 m
3
Weight of catalyst
It can be calculated as:
Weight of catalyst = ρp Volume of catalyst
= 2500 x 0.85817=2145.42 kg
Space Time
𝜏 = V/V0
V=volume of reactor
V0=initial volumetric flow rate
V=1.1808 m3
V0=initial mass/density
So for total inlet initial volumetric flow rate is 2519.10 m3/hr
Space time =1.72sec
Reactor Geometry
Assuming tube length of 12 ft or 3.6576 m and taking the diameter of tube to
prevent deviation from plug flow assumption. Dt/Dp > 10
Where,
Dt = diameter of tube
Dp = diameter of particle
Tube Dimensions: (Selected from Appendix A table 1)
Tube outside diameter do = 1.5 inch or 38.1 mm
Tube inside diameter di =1.37 inch or 34.798 mm
Plug flow test = 38.1/3.5=11 (satisfactory)
Total number of tubes Nt
48
So,
= 0.8582 / (/4 x 0.03482 x 3.6576)
= 246 tubes
(From Appendix A table 2)
Tubes available according to TEMA standards for triangular pitch=246 tubes
P = 1.25do
Where
P = tube pitch
do = outside tube diameter
P = 0.04762 m
Shell Inside Diameter
Numbers of tubes at bundle diameter are gives as:
Where, ND = number of tubes at bundle diameter
So, ND = 18.10
Shell inside diameter= Di=P [ND+1]
Di = 0.908 m
Shell Height
Length of tube=3.66 m
Leaving 20 % spacing above and below
So height of shell = 2 (0.2 3.66) + 3.66
= 5.12064 m
Pressure Drop Calculations
Tube side pressure drop
G
DDg
G
L
P
ppc
75.111501
3
49
Mass velocity G = Mass flow rate /flow area
Flow area = 1.47 inch2/tubes (kern Table 10)
Flow area = 361.62 inch2 = 0.2333 m2
G= 2730/0.2333 = 11716.73kg/hrm2
Particle diameter = DP =3.5mm =0.0035m
Average density of fluid =ρav =PM/RT= 1.6339 kg/m3
μav = 0.088 kg/m. hr
gc = 12.8 x 10 7 m.Kgm/hr2.
Kgf
Now putting all these values in equation we get
∆p = 3478.6 kgf/m2=34 kpa
Shell side pressure drop
Heat duty Q = 147975.32 kJ/hr
Water is used as cooling media having inlet temp. 25 oC and outlet 55
oC
Specific heat capacity of water = 4.318kJ/kg-C
Temperature difference, ΔT = 30K
Q = m.Cp.ΔT
147975.32=m x 4.318 x (55-25)
m=1142.31 kg/hr
Shell side flow area
Ac= π/4 [Di2 – Ntdo
2]
Di = shell inside diameter = 0.889 m
Nt = total number of tubes = 246
do = tube outside diameter = 0.0381 m
Ac = 0.340 m2
50
Equivalent Diameter:
Putting values in above formula
= 0.0510m
Shell side mass velocity = water flow rate / shell side flow area
= 1142.31 / 0.340
= 3357.2 kg/m2
- hr
Viscosity of water = 2.345 kg/m-hr
Reynolds Number Re= G De/µ
= 73
Friction factor for shell side fs = 0.0075 (from Appendix B figure7)
fs = 0.0075 x 144= 1.08
Specific gravity G=1.2
Where
ΔPS = pressure drop
Gs = shell side mass velocity= 686.34 lb/ft2/hr
L = length of tube = 12 ft
Nc = number of passes = 1
De‟= Equivalent diameter = 0.1672 ft
S = specific gravity = 1.2
= 1
Putting above values
ΔPS = 0.000906 psi (negligible)
51
Calculations of Heat Transfer Coefficient
Shell Side
For Shell side heat transfer coefficient
Where,
k = Thermal conductivity of cooling water
= 0.6315 W/m-K
De„= Shell side equivalent dia. = 0.0150 m
For Reynolds Number 73, JH = 4.2 (from Appendix B figure 8)
Shell side heat transfer coefficient, ho = 85.56 W/m2-K
Tube Side
An equation proposed by LEVA to find heat transfer co-efficient inside the tubes
filled with catalyst particles.
G = tube side mass velocity = 11716 Kg/m2-hr
= viscosity of tube side fluid = 0.08 Kg/m-hr
k = 0.04323 W/m-K
Dp = diameter of particle = 3.5 mm
di = Inside diameter of tube = 34.798 mm
Putting values in above equation
hi = 212.95 W/m2-K
Inside & outside dirt coefficient: (from Appendix A table 3)
hid = 5000W/m2-K for organic vapors
hod = 3000W/m2-K for cooling water
D
Dpe
μ
DpG3.5
k
h4.6
0.7
p
D
52
Wall Resistance
Kw = Thermal conductivity of wall = 36 W/m-K
Rw= 4x 10-4
m2-K/W
Over all Heat Transfer Coefficient
do = tube outside diameter = 38.1 mm
di = tube inside diameter = 34.798 mm
Ui = overall heat transfer
By putting the values
Ui = 4.01 W/m2-K
Area required for Heat Transfer
∆T1 = 250-55=195 oC
∆T2 =327-25=302 o
C
∆T =∆T2 +∆T1 /2=248.5 oC
So,
Ui = 4.01W/m2-K
Q = 83506.25 W
Average Temperature = 248.5 oC
Area required for Heat Transfer= 84 m2
53
Area Available for Heat Transfer
Length of tube, Lt = 3.6576 m
Inside Diameter of tube, di = 0.034798m
Hence,
Area available = 246 x π x 0.034798 x 3.6576
= 90 m2
So, sufficient area is available for heat transfer.
54
5.2.3 Specification Sheet of reactor
Reactor
Item: Fixed Bed Multi-Tubular Reactor
Identification: Item No: PFR-101
Function: To convert gaseous mixture of propylene and air to acrolein by catalytic
oxidation.
Tube side:
Material Handled:
1) Reaction mixture consisting of
propylene and air
2) Bismith molybdate based
catalyst
Flow Rate = 2730 Kg/hr
Pressure = 203 kPa
Temperature = 250 oC
Reactor volume = 1.2014 m3
Catalyst weight = 2145.5Kg
Pellet Size = 3.5mm
Porosity = 0.4
Tubes:
Outside diameter = 38.1mm
Inside diameter = 34.80mm
Schedule No. = 40
Tube length = 3.65 m
246 tubes with triangular pitch are
aligned vertically in the shell
Shell Side:
Fluid Handled = water
Heat Duty = 147975.32 kJ/hr
Flow Rate = 1142.31 Kg/sec
Inlet Temperature = 25 oC
Temperature Change = 30 oC
Pressure = 101 kPa
Shell:
Shell Inside diameter = 0.89 m
Shell Height = 5.12m
Shell Thickness = 3.87 mm
Construction Material = Carbon Steel
55
5.3Design of Absorber
5.3.1 Packed Columns
Packed columns are used for distillation, gas absorption, and liquid-liquid
extraction; only distillation and absorption will be considered here. Stripping
(desorption) is the reverse of absorption and the same design methods will apply.
The gas liquid contact in a packed bed column is continuous, not stage-wise, as in
a plate column. The liquid flows down the column over the packing surface and
the gas or vapor, counter-currently, up the column. In some gas-absorption
columns co-current flow is used. The performance of a packed column is very
dependent on the maintenance of good liquid and gas distribution throughout the
packed bed, and this is an important consideration in packed-column design.
A schematic diagram, showing the main features of a packed absorption column,
is given in Figure 5.1
Figure 5. 1.
5.3.2 Choice of plates or packing
The choice between a plate or packed column for a particular application can only
be made with complete assurance by costing each design.
By assuring advantages and disadvantages of each type; which are listed below:
56
Plate columns can be designed to handle a wider range of liquid and gas
flow-rates than packed columns.
Packed columns are not suitable for very low liquid rates.
The efficiency of a plate can be predicted with more certainty than the
equivalent term for packing (HETP or HTU).
Plate columns can be designed with more assurance than packed columns.
There is always some doubt that good liquid distribution can be
maintained throughout a packed column under all operating conditions,
particularly in large columns.
It is easier to make provision for cooling in a plate column; coils can be
installed on the plates.
It is easier to make provision for the withdrawal of side-streams from plate
columns.
If the liquid causes fouling, or contains solids, it is easier to make
provision for cleaning in a plate column; man-ways can be installed on the
plates. With small diameter columns it may be cheaper to use packing and
replace the packing when it becomes fouled.
For corrosive liquids a packed column will usually be cheaper than the
equivalent plate column.
The liquid hold-up is appreciably lower in a packed column than a plate
column. This can be important when the inventory of toxic or flammable
liquids needs to be kept as small as possible for safety reasons.
Packed columns are more suitable for handling foaming systems.
The pressure drop per equilibrium stage (HETP) can be lower for packing
than plates; and packing should be considered for vacuum columns.
Packing should always be considered for small diameter columns, say less
than 0.6 m, where plates would be difficult to install, and expensive.
57
5.3.3 Types of packing
The principal requirements of a packing are that it should:
Provide a large surface area: a high interfacial area between the gas
and liquid.
Have an open structure: low resistance to gas flow.
Promote uniform liquid distribution on the packing surface.
Promote uniform vapor gas flow across the column cross-section.
Many diverse types and shapes of packing have been developed to satisfy these
requirements. They can be divided into two broad classes:
Packing‟s with a regular geometry: such as stacked rings, grids and
proprietary structured packing‟s.
Random packing‟s: rings, saddles and proprietary shapes, which are
dumped into the column and take up a random arrangement.
Grids have an open structure and are used for high gas rates, where low pressure
drop is essential; for example, in cooling towers. Random packings and structured
packing elements are more commonly used in the process industries.
1. Random packing
Raschig rings, are one of the oldest specially manufactured types of random
packing, and are still in general use.
Pall rings, are essentially Raschig rings in which openings have been made
by folding strips of the surface into the ring. This increases the free area and
improves the liquid distribution characteristics.
Berlsaddles, were developed to give improved liquid distribution compared
to Raschig rings,
58
Intalox saddles can be considered to be an improved type of Berl saddle;
their shape makes them easier to manufacture than Berl saddles.
The Hypac and Super Intalox packings can be considered improved types of
Pall ring and Intalox saddle, respectively.
Random Packing is Shown in Figure 5.2.
Figure 5.2.
Ring and saddle packings are available in a variety of materials: ceramics, metals,
plastics and carbon. Metal and plastics (polypropylene) rings are more efficient
than ceramic rings, as it is possible to make the walls thinner .Raschig rings are
cheaper per unit volume than Pall rings or saddles but are less efficient, and the
total cost of the column will usually be higher if Raschig rings are specified. For
new columns, the choice will normally be between Pall rings and Berl or Intalox
saddles. The choice of material will depend on the nature of the fluids and the
operating temperature. Ceramic packing will be the first choice for corrosive
59
liquids; but ceramics are unsuitable for use with strong alkalies. Plastic packings
are attacked by some organic solvents, and can only be used up to moderate
temperatures; so are unsuitable for distillation columns. Where the column
operation is likely to be unstable metal rings should be specified, as ceramic
packing is easily broken.
Packing size
In general, the largest size of packing that is suitable for the size of column should
be used, up to 50 mm. Small sizes are appreciably more expensive than the larger
sizes. Above 50 mm the lower cost per cubic-meter does not normally
compensate for the lower mass transfer efficiency. Use of too large a size in a
small column can cause poor liquid distribution.
Recommended size ranges are:Column diameter Use packing size
<0.3 m (1 ft) <25 mm (1 in.)
0.3 to 0.9 m (1 to 3 ft) 25 to 38 mm (1 to 1.5 in.)
>0.9 m 50 to 75 mm (2 to 3 in.)
2. Structured packing
The term structured packing refers to packing elements made up from wire mesh
or perforated metal sheets. The material is folded and arranged with a regular
geometry, to give a high surface area with a high void fraction as Shown in Figure
5.3
Figure5. 3. (a)
60
Figure 5.3. (b) Make-up of structured packing
The advantage of structured packings over random packing is their low HETP
(typically less than 0.5 m) and low pressure drop (around 100 Pa/m).
They are increasingly being used in the following applications:
For difficult separations, requiring many stages: such as the separation of
isotopes.
High vacuum distillation.
For column revamps: to increase capacity and reduce reflux ratio
requirements.
The applications have mainly been in distillation, but structured packings can also
be used in absorption; in applications where high efficiency and low pressure drop
are needed.
The cost of structured packings per cubic meter will be significantly higher than
that of random packings, but this is offset by their higher efficiency.
5.3.4 Column Internals
The internal fittings in a packed column are simpler than those in a plate column
but must be carefully designed to ensure good performance. As a general rule, the
standard fittings developed by the packing manufacturers should be specified.
Figure 5.4 shows the column internal structure.
61
Figure 5. 4 .Stacked packing used to support random packing
5.3.5 Packing support
The function of the support plate is to carry the weight of the wet packing, whilst
allowing free passage of the gas and liquid. These requirements conflict; a poorly
designed support will give a high pressure drop and can cause local flooding.
Simple grid and perforated plate supports are used, but in these designs the liquid
and gas have to vie for the same openings. Wide-spaced grids are used to increase
the flow area; with layers of larger size packing stacked on the grid to support the
small size random packing as shown in Figure 5.5
The best design of packing support is one in which gas inlets are provided above
the level where the liquid flows from the bed; such as the gas-injection type.
These designs have a low pressure drop and no tendency to flooding. They are
available in a wide range of sizes and materials: metals, ceramics and plastics.
62
Figure 5.5. (a) The principle of the gas-injection packing support
Figure 5.5 .(b) Typical designs of gas-injection supports (Norton Co.). (a) Small
diameter columns (b) Large diameter columns
5.3.6 Liquid distributors
The satisfactory performance of a plate column is dependent on maintaining a
uniform flow of liquid throughout the column, and good initial liquid distribution
is essential.
Various designs of distributors are used. For small-diameter columns a central
open feed pipe, or one fitted with a spray nozzle, may well be adequate; but for
larger columns more elaborate designs are needed to ensure good distribution at
63
all liquid flow-rates. The two most commonly used designs are the orifice type,
shown in Figure5. 6 (a), and the weir type, shown in Figure 5.6 (b). In the orifice
type the liquid flows through holes in the plate and the gas through short stand
pipes. The gas pipes should be sized to give sufficient area for gas flow without
creating a significant pressure drop; the holes should be small enough to ensure
that there is a level of liquid on the plate at the lowest liquid rate, but large enough
to prevent the distributor overflowing at the highest rate.
Figure 5. 6. (a) Orifice-type distributor (Norton Co.)
Figure5. 6. (b) Weir-type distributor (Norton Co.)
64
For large-diameter columns, the trough-type distributor shown in Figure 5.7 can
be used, and will give good liquid distribution with a large free area for gas flow.
All distributors which rely on the gravity flow of liquid must be installed in the
column level, or mal-distribution of liquid will occur.
Figure 5.7. Weir-trough distributors (Norton Co.)
A pipe manifold distributor, Figure 5.8 can be used when die liquid is fed to the
column under pressure and the flow-rate is reasonably constant. The distribution
pipes and orifices should be sized to give an even flow from each element.
Figure 5.8. Pipe distributor (Norton Co.)
65
5.3.7 Liquid redistributors
Redistributors are used to collect liquid that has migrated to the column walls and
redistribute it evenly over the packing. They will also even out any
maldistribution that has occurred within the packing. A full redistributor combines
the functions of a packing support and a liquid distributor; a typical design is
shown in Figure 5.9
Figure 5.9. Full redistributor
The "wal-wiper" type of re-distributor, in which a ring collects liquid from the
column wall and redirects it into the centre packing, is occasionally used in small-
diameter columns, less than 0.6 m. Care should be taken when specifying this
type to select a design that does not unduly restrict the gas flow and cause local
flooding is shown in figure 5.10
66
Figure 5.10. "Wall wiper" redistributor (Norton Co.)
The maximum bed height that should be used without liquid redistribution
depends on the type of packing and the process. Distillation is less susceptible to
maldistribution than absorption and stripping. As a general guide, the maximum
bed height should not exceed3 column diameters for Raschig rings, and 8 to 10
for Pall rings and saddles. In a large diameter column the bed height will also be
limited by the maximum weight of packing that can be supported by the packing
support and column walls; this will be around 8 m.
5.3.8 Hold-down plates
At high gas rates, or if surging occurs through mis-operation, the top layers of
packing can be fluidized. Under these conditions ceramic packing can break up
and the pieces filter down the column and plug the packing; metal and plastic
packing can be blown out of the column. Hold-down plates are used with ceramic
packing to weigh down the top layers and prevent fluidization; a typical design is
shown in Figure 5.11. Bed-limiters are sometimes used with plastics and metal
packing‟s to prevent expansion of the bed when operating at a high-pressure drop.
They are similar to hold-down plates but are of lighter construction and are fixed
67
to the column walls. The openings in hold-down plates and bed-limiters should be
small enough to retain the packing, but should not restrict the gas and liquid flow.
Figure5. 11. Hold-down plate design (Norton Co.)
If the columns must be packed dry, for instance to avoid contamination of process
fluids with water, the packing can be lowered into the column in buckets or other
containers. Ceramic packing‟s should not be dropped from a height of more than
half a meter.
5.3.9 Liquid hold-up
An estimate of the amount of liquid held up in the packing under operating
conditions is needed to calculate the total load carried by the packing support. The
liquid hold-up will depend on the liquid rate and, to some extent, on the gas flow-
rate.
The packing manufacturers' design literature should be consulted to obtain
accurate estimates. As a rough guide, a value of about 25 per cent of the packing
weight can be taken for ceramic packing‟s.
68
5.3.10Wetting rate
Wetting rate is defined as:
wetting rate = volumetric liquid rate per unit cross-sectional area
packing surface area per unit volume
LW = L
AcρLa
Where
L=Liquid Flowrate Kg/hr
Ac=Cross-Sectional area
ρL =Liquid Density Kg/m3
a=Area (m3/m
2)
5.3.11Column Auxiliaries
Operation Time, Minutes
Feed to a train of columns 10 to 20
Between columns 5 to 10
Feed to a column from storage 2 to 5
Reflux drum 5 to 15
69
Figure 5. 12. Illustrative cutaway of a packed tower, depicting an upper bed
of structured packing and a lower bed of random packing.
(Courtesy of Sulzer Chemtech.)
70
5.3.12 Design Calculations
Inlet Composition of Gases (Yb) Amounts in Kg-mole
Acrolein 187.4
Propylene 3.921
Acrylic acid 24
Nitrogen 1395.1
Carbon di-oxide 94.28
Oxygen 180.9
Water 41371.26
The Total Flow rate Of gases From reactor is 43256.86 Kg-mole/hr.
Liquid Inlet (Xa) Amounts in Kg-mole
Water 1800
The Total Flow rate of water From De-ionized water source is 1800 Kg-mole/hr.
Top Product Composition (Ya ) Amounts in Kg-mole
Nitrogen 1395.1
Oxygen 180.9
Carbon Di-oxide 94.28
Acrolein 15.3
Propylene 2.33
Water 38.6
The Total Flow rate of top Products from Absorber is 1726.51 Kg-mole/hr.
Bottom Product Compositions (Xb) Amounts in Kg-mole
Acrolein 172.1
Propylene 1.591
Acrylic acid 24
Water 43133.2
The Total Flow rate of bottom Products From Absorber is 43330.89 Kg-mole/hr.
71
Design Conditions
Basis : One Hour Operation
Iso-thermal operation (200 C and 1atm )
Its only Physical Absorption Process
92% Acrolein is absorbed
Absorbent is De-ionized Water
All Other gases are Inert Except Acrolein
Some Specified quantity of Propylene is also Absorbed
Packed-column design procedure
The design of a packed column will involve the following steps:
Select the type and size of packing.
Determine the column height required for the specified separation.
Determine the column diameter (capacity), to handle the liquid and vapor
flow rates.
Select and design the column internal features: packing support, liquid
distributor, redistributors.
The Henry Law Coefficient For Acrolein in water at 200
C
is 8.2025× 10-5
atm-
m3/mole can be converted to the slope of Equilibrium line in mole fraction units
as
P =1atm
1 m3 weights 10
6g
m = 8.2025× 10-5
atm-m3/mole × 1/atm × 10
6 mole H2O /18 m
m =4.55
And
Tan θ = 4.55
72
θ= tan-1
( 4.55)
=77.604
Height of Mass –Transfer Zone
Z= Height of mass transfer zone
Hoy =Height of transfer units ,m
Noy=Number of transfer units
As
Z= Hoy × Noy ----------------------------------------(1)
Calculation of Noy
Here Acrolein is Key Component and design will Be Based on this ,Entering De-
ionized water is free of solute Ya =0
Now
Noy =A/A-1 [ℓn (Yb/Ya)(A-1)+1/A]-------------(2)
As
A=L/mV
Here
Ya= mole Fraction of solute At Top in Gases
Yb= mole Fraction of solute At botom in Gases
m= Slope
L=Liquid Flow rate
V=Vapor Flow rate
Ya =15.3
Yb =187.4
L=1800 Kg-mole/hr
V=43205.94 Kg-mole/hr
A=L/mV=2.298
L/V= 10.459=Slope of Operating line
73
m =10.459
Tan θ =10.459
θ = Tan-1
(10.459)
θ =2.382
By Putting the values in Equation No. 2
Noy =A/A-1 [ℓn (Yb/Ya)(A-1)+1/A]-------------(2)
Noy =3.55
Calculation of Hoy
By using the Gas Film Basis
Hy=[V/S]/Kya------------------------------------------------------------(3)
Here
Kya =Overall Mass-transfer Co-efficient Based on Gas Phase (Kg-mole/m3-sec)
V=Flow rate
S=Cross-Sectional area
By Rule Of Thumb
Kya(Unknown) =Kya (Known)×( Dv Unknown/Dv Known) 0.56
...............(4)
At 200 C
Dv (unknown Acrolein)=0.4069 ft2/hr
Dv (known SO2)=0.448 ft2/hr
Kya (known SO2)=2 Kg-mole/ft3-hr-moles
By Putting the Values in above equation (4)
Kya(Unknown) =Kya (Known)×( Dv Unknown/Dv Known) 0.56
...............(4)
Kya(Unknown) =1.823 Kg-mole/ft3-hr-sec
Kya(Unknown) =0.0180 Kg-mole/m3-sec
74
Calculation of Cross-Sectional Area
As Flow Factor
Flv=Lw/Vw √ρv/ρl------------------------------------------------(5)
L=1800Kgmole/hr
V=172.1kgmole/hr
By using Formula( For Gas)
ρv =PM/RT-------------------(6)
ρv =0.775 Kg/m3
By using Formula( For Liquid)
ρl (water)= AB-(1-T/Tc)n
………………......(7)
Where
ρl (water)=998.20 Kg/m3
By Putting the values in Equation No.5
Flv=Lw/Vw √ρv/ρl------------------------------------------------(5)
Flv=0.291
For Absorber /Stripper
For Ramdom Packing ,Pressure Drop will not normally exceed 80mm of
water/m of Packing.
For Absorber and stripper Range (15------50mm)
We consider flooding velocity as 80 %
We Consider 21mm and Flv=0. 0.291
By Using Appendix B figure 9
As can be calculated by using the formula
Percentage Flooding = [K4 at Design Pressure Drop/K4 at
Flooding]×100----------(8)
Percentage Flooding = [ 0.5/ 0.8]×100
75
=62.5 %
Percentage flooding is satisfied
Type of Packing
Design data for various packings
By using Equation
K4= [13.1(Vw2) Fp (μl/ρl)0.1]/ρv(ρl-ρv)----------(9)
where
Vw, = gas mass flow-rate per unit column cross-sectional area, kg/nrs
Fp= packing factor, characteristic of the size and type of packing,
μl = liquid viscosity, Ns/m2
pL, pv = liquid and vapour densities, kg/m3
By Appendix A table 5
Selecting the Packing
CMR (Ceramic-Mini Rings ) Metal Rings
Dp=#5
a = 50 m2/m
3
Fp=26 m-1
By re-arranging equation as
Vw2 =K4 ρv (ρl-ρv) /13.1 Fp (μl/ρl )
Viscosity of Liquid ( water) at 200
C
ℓog 10 n liq = A+ B/T +CT+ DT2-----------------------------(10)
T=K
n liq =Viscosity Of Liquid (Centipose)
A,B,C=Constant
n liq =1.028 CP
76
n liq =0.00103 N.sec/m2
By putting the values in the following equation
Vw2 =K4 ρv (ρl-ρv) /13.1 Fp (μl/ρl )
Vw= 2.122 Kg/m2.sec
Gas Flow rate =43256.86 Kgmole/hr
Gas Flow rate =223.05 Kgs/sec
So
Cross-Sectional Area=223.05/2.122 Kg/sec×m2.sec/Kg
=105.11 m2
Diameter of Column
D=[4A/Π]1/2
D=11.57 m
Gas Flow rate =43256.86 Kgmole/hr
Gas Flow rate =12.001 Kgmole/sec
By using the Gas Film Basis ,Equation No. 3
Hoy=[V/S]/Kya
V/S = 0.114 Kg-mole/Sec.m2
Now Putting all values in equation No. 3
Hoy=[0.114]/0.0180
Hoy=6.34 m
By putting the values in the Equation 1
Z= Hoy × Noy
Z= 6.34×3.55
Z= 22.488 m
Wetting Rate
LW = L
AcρLa
77
Where
L=Liquid Flowrate Kg/hr
Ac=Cross-Sectional area
ρL =Liquid Density Kg/m3
a=Area (m3/m
2)
=1.7×10-6
m3/m.sec
Selecting the Packing type
Metal Pall Rings( Density for Mild steel)
Dp=3.5 in (76 mm)
a = 65 m2/m
3
Fp=16 m-1
By re-arranging equation as
Vw2 =K4 ρv (ρl-ρv) /13.1 Fp (μl/ρl )
Viscosity of Liquid ( water) at 200
C
ℓog 10 n liq = A+ B/T +CT+ DT2
T=K
n liq =Viscosity of Liquid (Centipose)
A,B,C=Constant
n liq =1.028 CP
n liq =0.00103 N.sec/m2
By putting the values in the following equation
Vw2 =K4 ρv (ρl-ρv) /13.1 Fp (μl/ρl )
Vw= 2.70 Kg/m2.sec
Gas Flow rate =43256.86 Kgmole/hr
Gas Flow rate =223.05 Kgs/sec
So
Cross-Sectional Area=223.256/2.70 Kg/sec×m2.sec/Kg
=82.68 m2
78
Diameter of Column
D=[4A/Π]1/2
D=10.26 m
Gas Flow rate =43256.86 Kgmole/hr
Gas Flow rate =12.001 Kgmole/sec
By using the Gas Film Basis ,Equation No. 3
Hoy=[V/S]/Kya
V/S = 0.145 Kg-mole/Sec.m2
Now Putting all values in equation No. 3
Hoy=[0.145]/0.0180
Hoy=8.06 m
By putting the values in the Equation 1
Z= Hoy × Noy
Z= 8.06×3.55
Z= 28.62 m
Wetting Rate
LW = L
AcρLa
Where
L=Liquid Flow rate Kg/hr
Ac=Cross-Sectional area
ρL =Liquid Density Kg/m3
a=Area (m3/m
2)
=1.6×10-6
m3/m.sec
Metal Pall Rings( Density for Mild steel)
Dp=2 in (50 mm)
a = 102m2/m
3
79
Fp=20 m-1
Satisfied
Selecting the Packing type
Plastic Pall Rings
Dp=3.5 in (88 mm)
a = 85 m2/m
3
Fp=16 m-1
By re-arranging equation as
Vw2 =K4 ρv (ρl-ρv) /13.1 Fp (μl/ρl )
Viscosity of Liquid ( water) at 200
C
ℓog 10 n liq = A+ B/T +CT+ DT2
Here
T=K
n liq =Viscosity Of Liquid (Centipose)
A,B,C=Constant
n liq =1.028 CP
n liq =0.00103 N.sec/m2
By putting the values in the following equation
Vw2 =K4 ρv (ρl-ρv) /13.1 Fp (μl/ρl )
Vw= 2.70 Kg/m2.sec
Gas Flow rate =43256.86 Kgmole/hr
Gas Flow rate =223.05 Kgs/sec
Cross-Sectional Area=223.256/2.70 Kg/sec×m2.sec/Kg
=82.51 m2
Diameter of Column
D=[4A/Π]1/2
D=10.25 m
Gas Flow rate =43256.86 Kgmole/hr
Gas Flow rate =12.001 Kgmole/sec
80
By using the Gas Film Basis ,Equation No. 3
Hoy=[V/S]/Kya
V/S = 0.114 Kg-mole/Sec.m2
Now Putting all values in equation No. 3
Hoy=[0.114]/0.0180
Hoy=6.34 m
By putting the values in the Equation 1
Z= Hoy × Noy
Z= 6.34×3.55
Z= 22.48 m
Wetting Rate
LW = L
AcρLa
Where
L=Liquid Flow rate Kg/hr
Ac=Cross-Sectional area
ρL =Liquid Density Kg/m3
a=Area (m3/m
2)
=1.2×10-6
m3/m.sec
Selecting the Packing type
Plastic Super Intalox Rings
Dp=# 3
a = 88 m2/m
3
Fp=16 m-1
By re-arranging equation as
Vw2 =K4 ρv (ρl-ρv) /13.1 Fp (μl/ρl )
Viscosity of Liquid ( water) at 200
C
ℓog 10 n liq = A+ B/T +CT+ DT2
81
T=K
n liq =Viscosity Of Liquid (Centipose)
A,B,C=Constant
n liq =1.028 CP
n liq =0.00103 N.sec/m2
By putting the values in the following equation
Vw2 =K4 ρv (ρl-ρv) /13.1 Fp (μl/ρl )
Vw= 2.70 Kg/m2.sec
Gas Flow rate =43256.86 Kgmole/hr
Gas Flow rate =223.05 Kgs/sec
So
Cross-Sectional Area=223.256/2.70 Kg/sec×m2.sec/Kg
=82.687 m2
Diameter of Column
D=[4A/Π]1/2
D=10.26 m
Gas Flow rate =43256.86 Kgmole/hr
Gas Flow rate =12.001 Kgmole/sec
By using the Gas Film Basis ,Equation No. 3
Hoy=[V/S]/Kya
V/S = 0.113 Kg-mole/Sec.m2
Now Putting all values in equation No. 3
Hoy=[0.113]/0.0180
Hoy=6.33 m
By putting the values in the Equation 1
Z= Hoy × Noy
Z= 6.33×3.55
Z= 22.48 m
82
Wetting Rate
LW = L
AcρLa
Where
L=Liquid Flow rate Kg/hr
Ac=Cross-Sectional area
ρL =Liquid Density Kg/m3
a=Area (m3/m
2
=1.2×10-6
m3/m.sec
83
5.3.13Specification Sheet of absorber
Operation Continuous
Item No.
Packed Absorption Column (T-101)
No. required 1
Function
To absorb Acrolein in Deionized water
No. of transfer units
3.55
Height of transfer units
8.06 m
Size and type of packing
Metal Pall Rings
Total height of column
28.62 m
Packing arrangement
Random
Method of packing
Float into tower filled with water
Type of packing support Gas injection support
Temperature
200
C
Pressure
1 atm
Surface area of the packing material (a) 65 m2/m
3
Absorbent fluid ( Utility) De-ionized water
84
5.4 Design of Distillation Column
In industry it is common practice to separate a liquid mixture by distillation of the
components, which have lower boiling points when they are in pure condition
from those having higher boiling points. This process is accomplished by partial
vaporization and subsequent condensation.
5.4.1Distillation
“Process in which a liquid or vapor mixture of two or more substances is
separated into its component fractions of desired purity, by the application and
removal of heat”.
.
Molecular weight water 18 Kg/Kgmole
Packing factor Fp =16 m-1
85
5.4.2 Types of Distillation Columns
There are many types of distillation columns, each designed to perform specific
types of separations, and each design differs in terms of complexity.
Batch columns
Continuous columns
Batch Columns
In batch operation, the feed to the column is introduced batch-wise. That is, the
column is charged with a 'batch' and then the distillation process is carried out.
When the desired task is achieved, a next batch of feed is introduced.
Continuous Columns
In contrast, continuous columns process a continuous feed stream. No
interruptions occur unless there is a problem with the column or surrounding
process units. They are capable of handling high throughputs and are the more
common of the two types. We shall concentrate only on this class of columns.
5.4.3 Choice between plate and packed columns
The choice between use of tray column or a packed column for a given mass
transfer operation should, theoretically, be based on a detailed cost analysis for
the two types of contactors. However, the decision can be made on the basis of a
qualitative analysis of relative advantages and disadvantages, eliminating the need
for a detailed cost comparison.
The relative merits of plate over packed column are as follows:
i) Plate columns are designed to handle wide range of liquid flow rates
without flooding.
ii) Dispersion difficulties are handled in plate column when flow rate of
liquid are low as compared to gases.
iii) For large column heights, weight of the packed column is more than
plate column.
86
iv) If periodic cleaning is required, man holes will be provided for
cleaning. In packed columns packing must be removed before
cleaning.
v) For non-foaming systems the plate column is preferred.
vi) Design information for plate column is more readily available and
more reliable than that for packed column.
vii) Inter stage cooling can be provided to remove heat of reaction or
solution in plate column.
viii) When temperature change is involved, packing may be damaged.
ix) Random packed columns are not designed with diameter greater than
1.5 m and diameter of tray column is seldom less than 0.67m.
For this particular process, I have selected plate column because:
i) System is non-foaming.
ii) Temperature change is involved.
iii) Diameter is 0.96 meter.
5.4.4 Plate Contractors
Cross flow plates are the most commonly used plate contactors in distillation. In
which liquid flows downward and vapours flow upward. The liquid move from
plate to plate via down comer. A certain level of liquid is maintained on the plates
by weir.
Three basic types of cross flow trays used are
Sieve Plate (Perforated Plate)
Bubble Cap Plates
Valve plates (floating cap plates)
5.4.5 Selection of Trays
I have selected sieve tray because:
i) They are lighter in weight and less expensive. It is easier and cheaper
to install.
87
ii) Pressure drop is low as compared to bubble cap trays.
iii) Peak efficiency is generally high.
iv) Maintenance cost is reduced due to the ease of cleaning.
5.4.6 Factors affecting Distillation Column operation
Vapour flow conditions
• Foaming
• Entrainment
• Weeping/dumping
• Flooding
Foaming
Foaming refers to the expansion of liquid due to passage of vapour or gas.
Although it provides high interfacial liquid-vapour contact, excessive foaming
often leads to liquid build-up on trays. In some cases, foaming may be so bad that
the foam mixes with liquid on the tray above. Whether foaming will occur
depends primarily on physical properties of the liquid mixtures, but is sometimes
due to tray designs and condition. Whatever the cause, separation efficiency is
always reduced.
Entrainment
Entrainment refers to the liquid carried by vapour up to the tray above and is
again caused by high vapour flow rates. It is detrimental because tray efficiency is
reduced: lower volatile material is carried to a plate holding liquid of higher
volatility. It could also contaminate high purity distillate. Excessive entrainment
can lead to flooding.
Weeping/Dumping
This phenomenon is caused by low vapour flow. The pressure exerted by the
vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to
88
leak through perforations. Excessive weeping will lead to dumping. That is the
liquid on all trays will crash (dump) through to the base of the column (via a
domino effect) and the column will have to be re-started. Weeping is indicated by
a sharp pressure drop in the column and reduced separation efficiency.
Flooding
Flooding is brought about by excessive vapour flow, causing liquid to be
entrained in the vapour up the column. The increased pressure from excessive
vapour also backs up the liquid in the down comer, causing an increase in liquid
hold-up on the plate above. Depending on the degree of flooding, the maximum
capacity of the column may be severely reduced. Flooding is detected by sharp
increases in column differential pressure and significant decrease in separation
efficiency.
Reflux Conditions
Minimum trays are required under total reflux conditions, i.e. there is no
withdrawal of distillate. On the other hand, as reflux is decreased, more and more
trays are required.
Feed Conditions
The state of the feed mixture and feed composition affects the operating lines and
hence the number of stages required for separation. It also affects the location of
feed tray.
State of Trays and Packing
Remember that the actual number of trays required for a particular separation duty
is determined by the efficiency of the plate. Thus, any factors that cause a
decrease in tray efficiency will also change the performance of the column. Tray
efficiencies are affected by fouling, wear and tear and corrosion, and the rates at
which these occur depends on the properties of the liquids being processed. Thus
appropriate materials should be specified for tray construction.
89
Column Diameter
Vapor flow velocity is dependent on column diameter. Weeping determines the
minimum vapor flow required while flooding determines the maximum vapor
flow allowed, hence column capacity. Thus, if the column diameter is not sized
properly, the column will not perform well.
5.4.7 Design Calculations of Distillation Column
Design Steps of Distillation Column
Calculation of Minimum Reflux Ratio Rm.
Calculation of optimum reflux ratio.
Calculation of theoretical number of stages.
Calculation of actual number of stages.
Calculation of diameter of the column.
Calculation of weeping point, entrainment etc.
Calculation of pressure drop.
Calculation of thickness of the shell.
Calculation of the height of the column.
90
Design Calculations of Distillation Column (T-104)
D=147.57 kg/hr
Acrolein = 0.98816
Propylene=0.000206
Water =0.01103
TOP PRODUCT
B= 81.21kg/hr
Acrolein = 0.009389
Water = 0.990610
BOTTOM PRODUCT
P=106KPa
T=650C
T =520C
P=101.325
KPa
T=105 0C
P=130kPa
Feed
F=228.78kg/hr
Acrolein = 0.64100
Propylene=0.0001332
Water = 0.35886
91
From material balance
Feed Composition & Flow Rates (F)
Component Mass Flow Rate
(Kg/hr)
Molar Flow Rate
(Kgmol/hr)
Mass Fraction
Acrolein 146.64 2.618 0.64100
Propylene 0.0304 0.0007 0.0001332
Water 82.09 4.561 0.35886
Top Product Composition and Flow Rates (D)
Component Mass Flow
Rate
(Kg/hr)
Molar Flow
Rate(Kgmol/hr)
Mass Fraction
Acrolein 145.83 2.604 0.98816
Propylene 0.0304 0.0007 0.000206
Water 1.62 0.09 0.01103
Bottom Product Composition & Flow Rates (B)
Component Mass Flow
Rate
(Kg/hr)
Molar Flow
Rate(Kgmol/hr)
Mass Fraction
Acrolein 0.762 0.013 0.009389
Propylene 0 0 0
Water 80.449 4.46 0.990610
Bottom Temperature (TB)
Bubble point calculations
PT = 130 kpa
T=105 oC (Assume)
92
Components Xb=Xi Ki Yi= KiXi
Acrolein 0.009389 3.5475 0.0333
Water 0.990610 1.0094 0.9999
Total 1.03
Top Temperature (TD)
Dew point calculations
PT = 101.325kpa
T=52 oC (Assume)
Components XD=Yi Ki Xi =Yi/ Ki
Acrolein 0.98816 0.9698 1.01
Propylene 0.000206 30.68 0.000006
Water 0.01103 0.1727 0.0638
Total 1.07
Feed Temperature (TF)
Bubble point calculations
PT = 106kpa
T =65 oC
93
Components XF=Xi Ki Yi= KiXi
Acrolein 0.64100 1.4103 0.9040
Propylene 0.0001332 37.6202 0.0050
Water 0.35886 0.2852 0.1023
Total 1.0
Since the bubble point calculations are being satisfied at feed temperature so feed
is saturated liquid.
Selection of key components
Light key Acrolein
Heavy key Water
Calculation of Relative Volatility
Component Top Bottom Average
α Ki αDi=Ki/KHK Ki αBi=Ki/KHK
Acrolein 0.9698 5.615 3.5475 3.514 4.442
Propylene 30.68 177.6 59.818 59.216 102.5
Water
0.1727 1 1.0094 1 1
94
Calculation of minimum reflux ratio (Rm)
Using Underwood equation
q1θα
α
θα
α
θα
α
C
fCC
B
fBB
A
fAA
xxx
As feed is at its bubble point so q = 1
By trial = 1.4
Using equation of min. reflux ratio,
Where,
α = Relative volatility of component with respect to some reference usually
heavy key
xd = Concentration of component in top product
xf= Concentration of component in feed
= Root of equation at Rm
R m = .1414
Actual reflux ratio (R)
R =(1.2 -- 1.5) R m
R = 1.5 R m
R= 0.621
1Rθα
α
θα
α
θα
αm
C
dCC
B
dBB
A
dAA
xxx
95
Minimum Number of Plates (Nm)
By Fenske Equation
Nm = avAB
sA
B
dB
A
αln
x
x
x
xln
(AB)av = Average relative volatility of light key with respect to heavy key= 4.442
A = Light key
B = Heavy key
Nm = 6.41
Theoretical no. of Plates
Gilliland related the number of equilibrium stages and the minimum reflux ratio
and the no. of equilibrium stages with a plot that was transformed by Eduljee into
the relation;
From “Kirk bride” relation
566.0
1175.0
1 R
RR mm
N= 14
Calculation of actual number of stages
Using O‟ Connell‟s Correlation for overall tray efficiency
Average temperature of column = 351.65k
Feed viscosity at average temperature = avg = 0.267 mNs/m2
So,
Eo = 49%
avgavgoE .log5.3251
96
So,
No. of actual trays = Nact = 14-1/0.49= 27
Location of feed Plate
log [ND/NB] = 0.206 log
2
HK
LK
LK
HK
x
x
D
B
DF
ND = No. of stages above the feed plate
NB = No. of stages below the feed plate
B = molar flow rates of bottom
D = molar flow rate of distillate
XLK=mole fraction of light key component
XHK=mole fraction of heavy key component
From which,
NB=16 ND=11
Determination of Column Diameter
Top Conditions Bottom Conditions
Ln = 91.641 kg/hr Lm= Ln +F=320.421 kg/hr
Vn=239.211 kg/hr Vm= Vn =239.211 kg/hr
TD=325.15 k TB=378.15 k
Mavg = 55.5 Mavg =18.35
Liquid density = L = 808.4 kg/m3 Liquid density = L = 953.7kg/m
3
Vapor density = V =1.8 kg/m3 Vapor density = V =0.60 kg/m
3
Surface Tension = σ = 19.88 Dynes/cm
or
0.01988 N/m
Surface Tension=σ= 57.98 Dynes/cm
or
0.05798N/m
97
Flow Parameter
Ln = liquid flow rate in kg/sec
Vn= Vapor flow rate in kg/sec
FLV = Liquid Vapor Factor (Top) = 0.018
FLV = Liquid Vapor Factor (Bottoms) = 0.033
Calculation of flooding velocity
Assumed tray spacing = 0.45
Uf = k1(L -V/V)0.5
Where,
Uf = flooding velocity in m/sec
K1= constant
From Appendix B figure 10
Top K1=0.08
Bottom K1=0.082
Correction for surface tension K1 × [σ/0.02]0.2
Where σ in N/m
Top K1=0.0801
Bottom K1=0.1
Top Uf = 1.695 m/sec
Bottom Uf = 3.985 m/sec
Assuming 90% flooding
So actual vapor velocity (U)
At Top U = 1.525 m/sec
At Bottom U = 3.586 m/sec
0.5
L
v
n
nLV
ρ
ρ
V
LF
98
Maximum volumetric flow rate of vapors = mv = mass vapor flow rate
(3600) × vapor density)
= 0.78 m3
/ s (Top)
mv = 2.226 m3
/ s (Bottom)
Net area required = An = mv/ U
=0.511 m2 (Top)
An= 0.621 m2 ( Bottom)
Assume that downcommers occupies 15% of cross sectional Area (Ac) of column.
Ac = An + Ad
Where, Ad = downcommer area
Ac = An + 0.15(Ac)
Ac = An / 0.85
Ac=0.601m2
(Top)
Ac=0.730m2
(Bottom)
Ac =π/4D2
D = (4Ac/π)
D = 0.87meter (Top)
D = 0.96meter (Bottom)
Since bottom diameter is larger so calculations will be based on bottom conditions.
Liquid flow arrangement
Maximum liquid flow rate = (Liquid mass rate)/ (3600) × (Liquid density)
Max Liquid Rate is at the bottom of column
So, Maximum liquid flow rate = 0.0030m3/sec
From Appendix B figure 11, cross flow single pass plate is selected.
Provisional Plate Design
Column Diameter Dc= 0.96 m
99
Column Cross-sectional Area (Ac)= 0.730 m2
Down comer area Ad = 0.15Ac = 0.109 m2
Net Area (An) = Ac - Ad =0.621 m2
Active area Aa=Ac-2Ad = 0.512 m2
Hole area Ah take 6% Aa = 0.06 × 0.512 = 0.0307 m2
Weir length
Ad / Ac = 0.109 / 0.730 = 0.149
From Appendix B figure 13 ,
Lw / dc = 0.80
Lw = 0.96*0.80= 0.768 m
Weir length should be 60 to 85% of column diameter which is satisfactory.
Take weir height, hw = 50 mm
Hole diameter, dh = 5 mm
Plate thickness = 5 mm (Carbon Steel)
Check Weeping
Uh(min) = [K2-0.9(25.4-dh)]/ v 0.5
Where Uh(min) is the minimum design vapor velocity.
The vapor velocity at weeping point is the minimum velocity for the stable operation.
In order to have K2 value we have to 1st find how (depth of the crest of liquid over
the weir)
Where how is calculated by following formula:
how(max) = 750 (Lm/LLw)2/3
Taking minimum liquid rate at 70% turn down ratio of maximum liquid rate
At Maximum rate (how) = 16.170 mm Liquid
At Minimum rate (how) = 12.73mm Liquid
hw + how = 50 + 12.73 = 62.73 mm Liquid
100
From Appendix B figure 12 ,
K2 = 30.2
So,
U (min) = 15.66 m/sec
Now taking maximum volumetric flow rate (vapors) at 70% turn down ratio
Actual minimum vapor velocity =minimum vapor rate / Ah
= 21.8 m/sec
So minimum vapor rate will be well above the weep point.
Plate Pressure Drop (P.D)
Consist of
Dry plate P.D (orifice loss)
P.D due to static head of liquid and
Residual P.D (bubbles formation result in energy loss + froth formed in
operating plates)
Dry Plate Drop
Max. Vapor velocity through holes (Uh) = 29.7 m/sec
Active Area = Aa = 0.512 m2
Ah/Aa = Ah/Ap = 0.059
Where Ap is the perforated area.
From Appendix B figure 14, C0 = 0.82
hd = 51(Uh / Co)2 (v / L)
= 42.09 mm liquid
Reisdual Drop
hr = 12.5 × 1000 / L
= 13.1 mm liquid
Total Plate Pressure Drop
ht = hd + hr + (hw +how)
= 117.92 mm liquid
101
Total pressure drop ∆Pt = 9.81 × 10-3
×(ht) ×L× Nact
= 29787.36 Pa = 29.78 KPa
Assumed and calculated pressure drop are almost equal.
Downcomer Liquid backup/ Liquid height in downcomer
Caused by P.D over the plate and resistance to flow in the downcomer itself.
hdc = 166 ×(Lw /L × Aap)2
Take hap = hw-10 = 40 mm = 0.04
Area under apron = hap×Lw
= 0.031 m
2
As Aap is less than Ad = 0.109 m2 so use this value of Aap in the following equation:
hdc = 166 ×(Lw /L × Aap)2
= 1.041 mm
hb = (hw+ how) + ht + hdc = 241mm = 0.241m
hb < ½ (Tray spacing + weir height)
0.241 m < 0.25 m
So tray spacing of 0.45m is acceptable
Residence time
tr =Ad hbc ρL/L(max)
tr = 8.00 sec
It should be > 3 sec.
so, result is satisfactory.
Entrainment
(un) actual velocity (based on net area) = Maximum volumetric flow rate/ Net area
(un) actual velocity = 2.871 m/sec
Velocity at flooding condition uf = 3.586 m/sec
So Percent flooding =un/ uf = 0.80 = 80%
102
Liquid flow factor = FLv =0.033
From Appendix B figure 15 ,
Fractional entrainment (ψ) = 0.05
Well below the upper limit of (ψ) which is 0.1.
Below this effect of entrainment on efficiency is small.
Number of holes
Area of 1 Hole = (π/4) Dhole2
= 0.0000196 m2
Area of N Holes = 0.0307 m2
Number of Holes = 1566.3
Height of Distillation Column
Height of column Hc = (Nact -1)Hs+ ∆H+ plates thickness
No. of plates = 27
Tray spacing Hs = 0.45 m
∆H= liquid hold up and vapor disengagement
∆H=0.55+0.55=1.1 m
Total thickness of trays = 0.005× 27 = 0.135 m
Height of column = (26 ×0.45) + 1.1+0.135
= 12.9 meters
103
5.4.8 Specification Sheet of Distillation Column
Identification:
Item Distillation column
Equipment-Code T-104
Tray type Sieve tray
Function: Separation of Acrolein from propylene and water
Operation: Continuous
Design Data
No. of trays 27 Weir height 50mm
Pressure drop per
tray
1.1kPa Weir length 0.7688 m
No of Holes 1566.3 Minimum Reflux
Ratio
0.414
Height of column 12.9m Reflux ratio 0.621
Column-Diameter 0.96m Hole size 5mm
Tray spacing 0.45m Entrainment 0.05
Tray thickness 5mm Hole area 0.0307 m2
Flooding 80 % Active area 0.512m2
104
CHAPTER NO: 6
MECHANICAL DESIGN OF REACTOR
6.1Mechanical Design
Shell Thickness
Shell thickness can be calculated by following relationship
𝑒 = 𝑃 𝐷
2𝑓𝐽 − 𝑃+ 𝐶
Where,
e = Design thickness of shell in mm
f = Design stress = 137895 k Pa for carbon steel
J = 1
D = Shell diameter = 0.908 m=908mm
P = Maximum allowable pressure = 205 k Pa
C = Corrosion allowance = 3.2 mm under sever conditions
Shell thickness = 3.87 mm
Material of construction
For the reactor shell, carbon steel is proposed as material of construction as it is
both cheap and also compatible with water. The reactor tubes are suggested to be
of stainless steel so that any contamination of maleic anhydride due to corrosion
products is avoided.
Heads for reactor shell
Standard torispherical heads are most commonly used for pressure up to 15bar.
Thus as ASME standard torispherical heads have been designed for the reactor.
The proposed material of construction is plain carbon steel.
105
Thickness of the head
Cs = Stress concentration factor torispherical head
Rc = Crown radius
Rc = 2.15 m
Rk= Knuckle radius
= 0.06 x Rc= 0.129 m
Cs = 1.77
Thickness = 6.2 mm
Reactor Support
The types of support used for vessels are:
Saddle support
Skirt support
Bracket support
Saddle supports are used for horizontal vessels while other two types are used for
vertical vessels. For the reactor in this case, a skirt support is proposed as it is
safer than bracket support and can more efficiently bear the weight of the reactor
and water as a cooling media circulating through the reactor.
)2.0(2
sCiPJf
sCcRiPe
kRcR /34
1
106
CHAPTER NO: 7
INSTRUMENTATION AND CONTROL
7.1 Instrumentation and Process Control
Measurement is a fundamental requisite to process control. Either the control can
be affected automatically, semi automatically or manually. The quality of control
obtainable also bears a relationship to accuracy, reproducibility and reliability of
measurement methods, which are employed. Therefore, selection of the most
effective means of measurements is an important first step in design and
formulation of any process control system.
Design of control system involves large number of theoretical and practical
consideration such as quality of controlled response, stability, the safety of
operating plant, the reliability of control system, the range of control, easy of start
up, shutdown or changeover, the ease of the operation and cost of control system.
Traditionally one under takes the design of control system for chemical plant only
after the process flow sheet has been synthesized and designed. This allows the
control designer to know
What units are in plant and their sizes
How they are interconnected
The range of the operating conditions
Possible disturbance, available measurements and manipulations
What problem may arise during shutdown and start up
7.2 Process instrument
Process instrument is a device used directly or indirectly to perform one or more
of the following three functions
Measurement
Control
107
Manipulation
The primary purpose of control in process industry is to aid in the economics of
industrial operations by improving quality of product and efficiency of
production.
7.3 Control
Control means methods to force parameters in environment to have specific value.
There is some control on different parameters as follows
7.3.1Temperature measurement and control
Temperature measurement is used to control the temperature of outlet
and inlet streams in heat exchangers, reactors, etc.
Most temperature measurements in the industry are made by means of
thermocouple to facilitate bringing the measurements to centralized location. For
local measurements at the equipment bimetallic or filled system thermometers are
used to a lesser extent. Usually, for high measurement accuracy, resistance
thermometers are used.
All these measurements are installed with thermo wells when used locally. This
provides protection against atmosphere and other physical elements.
7.3.2 Pressure measurement and control
Like temperature, pressure is a valuable indication of material state and
composition.
In fact, these two measurements considered together are the primary evaluating
devices of industrial materials.
Pumps, compressors and other process equipments associated with pressure
changes in the process material are furnished with pressure measuring devices.
Thus pressure measurement becomes an indication of an energy decrease or
increase.
108
Most pressure measuring devices in industry are elastic element devices, either
directly connected for local use or transmission type to centralized location. Most
extensively used industrial pressure measuring device is the Bourdon Tube or a
Diaphragm or Bellow gauges.
7.3.3 Flow measurement and control
Flow indicator is used to control the amount of liquid. Also all manually
set streams require some flow indication or some easy means for occasional
sample measurement.
For accounting purposes, feed and product streams are metered. In addition
utilities to individual and grouped equipments are also metered.
Most flow measuring devices in the industry are Variable Head devices. To a
lesser extent variable area is used as many types are available as special metering
situation arise.
7.4 Control scheme of distillation column
Objectives
In distillation column any of following may be the goals to achieve.
1. Overhead composition
2. Bottom composition
3. Constant over head product rate
4. Constant bottom product rate
Manipulated variables
Any one or any combination of following may be the manipulated variables.
1. Steam flow rate to reboiler
2. Reflux rate
3. Overhead product with drawn rate
4. Bottom product withdrawn rate
109
5. Water flow rate to condenser
Loads or disturbances
Following are typical disturbances.
1. Flow rate of feed.
2. Composition of feed.
3. Temperature of feed.
4. Pressure drop of steam across reboiler.
5. Inlet temperature of water for condenser.
Control scheme
Here is control scheme on acrolein distillation column. Consider the feed to this
column as binary mixture composed of acorlein and water. We can specify four
control variable for this distillation column are
Acrolein product quality
Fractional recovery of acroelin in overhead product (distillate rate)
Liquid level in overhead accumulator
Liquid level at bottom of column
Overall product rate is fixed and any change in feed must be absorbed by
changing bottom product rate. The change in product rate is accomplished by
direct level control of reboiler if the stream rate is fixed, feed rate increases then
vapor rate is approximately constant and the internal reflux flow must increase.
Trying to control the liquid level at the bottom of column with reflux flow or
distillate flow rate involves very long time response because action of
manipulated variable must travel the whole length of distillation column before it
is felt by the controller variable so it cannot be done.
A long time response is involved when we try to control the level in the overhead
accumulator by manipulating the bottoms flow rate & stream flow rate. It is quite
complicated to control the distillate composition or flow rate with bottom flow
110
rate. Since an increase in feed rate increases reflux rate with vapor rate being
approximately constant, then purity of top product increases.
Explanation
First on the cold day or in rainstorm the temperature of cold water in overhead
condenser drops and overhead vapors passing through condenser produces sub
cooled liquid. When sub cooled liquid returns back from reflux to the top tray of
distillation column it causes less vapors to go overhead. Low vapors in overhead
causes less liquid level in accumulators. If the accumulator level is controlled by
reflux flow the latter will decrease thus the disturbance causes by the cooling
water temperature drop does not propagate down the column in terms of increased
liquid level overflow. The acrolein product composition is controlled by distillate
flow. The scheme shown is cascade scheme for distillation column.
Figure 7.1. Control scheme of distillation column
111
7.5 Heat exchanger control
Figure 7.2. Control scheme of heat exchanger
The control objective is to maintain the temperature at desired value and to allow
particulate heat exchange. The manipulated variable is flow rate of utility stream.
The external disturbance that will affect the operation of heat exchanger is
surrounding temperature, inlet temperature and steam pressure and steam
temperature or its flow rate in case when utility is steam. The output variable is
the temperature of outlet process stream and temperature of outlet utility stream.
The above is feedback control scheme for heat exchanger.
The control system of complete plant must permit smooth, safe and relatively fast
startup and shutdown of plant operation.
7.6 Control Scheme of PFR
Objectives
In PFR control any of following may be the goals to achieve
1. Constant Temperature inside the reactor
2. High quality of Product
112
Reactor Variable
The independent variables for the PFR may be divided into following categories
1. Uncontrolled variables
2. Manipulated variables
3. Controlled Variables
Uncontrolled Variables
The variables, which cannot be controlled by controller, are called uncontrolled
variables. The Uncontrolled variables include
1.Vent gases rate
2.Temperature of feed, etc
Manipulated Variables
The independent manipulated inputs are variables, which are adjusted to control
the chemical reaction. Any one or any combination of following may be the
manipulated variables
1.Flow rate of cooling water
2.Flow rate of Feed
3.Flow rate of Product stream
Controlled Variables
Any process variable that is selected to be maintained by a control system is
called a controlled variable. Following are the controlled variables
1.Inside reactor Temperature
2.Inside reactor Pressure
Temperature Control Scheme
The simplest method of cooling a PFR is shown in diagram. Here we measure the
reactor temperature and manipulated variable the flow of cooling water to the
shell side in shell and tube type reactor. Using a shell side for cooling has two
advantages. First, it minimizes the risk of leaks and thereby cross contamination
between the cooling system and the process. Second, heat transfer rate is
increased by using baffles.
113
A temperature sensor measures the inside reactor temperature and transfer signal
to temperature transducer, transducer converts these signals in other form and the
output of transducer is accepted by controller and controller transfer its signal to
final control element. Final control element takes step to overcome these
disturbances.
PFR Control Configuration
Figure 7.3. Control Scheme of PFR
114
CHAPTER NO: 8
HAZOP STUDY
8.1Introduction
A HAZOP survey is one of the most common and widely accepted methods of
systematic qualitative hazard analysis. It is used for both new or 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.
8.2 Background
The technique originated in the Heavy Organic Chemicals Division of ICI, which
was then a major British and international chemical company. The history has
been described by Trevor Kletz .
In 1963 a team of 3 people met for 3 days a week for 4 months to study the design
of a new phenol plant. They started with a technique called critical
examination which asked for alternatives, but changed this to look for deviations.
The method was further refined within the company, under the name operability
studies, and became the third stage of its hazard analysis procedure (the first two
being done at the conceptual and specification stages) when the first detailed
design was produced.
In 1974 a one-week safety course including this procedure was offered by
the Institution of Chemical Engineers (IChemE) at Teesside Polytechnic.Coming
115
shortly after the Flixborough disaster, the course was fully booked, as were ones
in the next few years.
In the same year the first paper in the open literature was also published. In 1977
the Chemical Industries Association published a guide .Up to this time the
term HAZOP had not been used in formal publications. The first to do this was
Kletz in 1983, with what were essentially the course notes (revised and updated)
from the IChemE courses. By this time, hazard and operability studies had
become an expected part of chemical engineering degree courses in the UK.
8.3Types of HAZOP
1. Process HAZOP
The HAZOP technique which was originally developed to assess plants and
process systems
2. Human HAZOP
It is a family of specialized HAZOPs that are more focused on human errors
rather than technical failures.
3. Procedure HAZOP
It is a review of procedures or operational sequences, sometimes
also denoted as SAFOP, SAFE Operation Study.
4. Software HAZOP
It deals with the identification of possible errors in the
development of software.
Advantages
1. Systematic examination
2. Multidisciplinary study
3. Utilizes operational experience
116
4. Solutions to the problems identified may be indicated
5. Reduces risks
6. Better contingency
7. More efficient operations
8. Considers operational procedures
8.4 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
8.5 HAZOP study of an Absorber
Item
No.
Deviation
Causes Consequences Safeguards Actions
AB1
Low pressure Unsuitable
packing
High liquid
loading
Low flooding
efficiency
Flood can
occur
Use pressure
controller at
above stream
of absorber
Use blower
upstream
and also use
suitable
packing for
absorber
High
pressure
Low
pressure
Good
absorption
Use pressure
controller
Use blower
working
117
drop
Low
temperature
Chocking
can occur in
the packing
Efficiency of
absorber
reduces also
pressure drop
increases
Use
temperature
controller for
the
measurement
of
temperature
of inlet gases
and stream
Use control
valve and
controller at
upstream of
absorber
High
temperature
Quencher is
not working
properly
Low
absorption
Damage to the
packing
Use
temperature
controller for
temperature
measuring of
inlet gases
and stream
Use control
valve and
controller at
upstream of
absorber
High
concentration
of CO2
Change in
wood
composition
Increase in
CO2
More water is
required to
remove CO2
Increase in
operating cost
and vice versa
Check CO2
concentration
after cracker,
use wood of
constant
composition
Use
controller
for
controlling
composition
of CO2
Low
concentration
of CO2
Less
conversion
in cracker,
more carbon
remains as it
is
118
CHAPTER NO: 9
ENVIRONMENTAL IMPACT ANALYSIS
OF ACROLEIN
9.1Hazards Identification
9.1.1Potential Acute Health Effects
Acrolein is very hazardous in case of skin contact (irritant), of eye contact
(irritant), of ingestion, of inhalation. Liquid or spray mist may produce tissue
damage particularly on mucous membranes of eyes, mouth and respiratory tract.
Skin contact may produce burns. Inhalation of the spray mist may produce severe
irritation of respiratory tract, characterized by coughing, choking, or shortness of
breath. Severe over-exposure can result in death. Inflammation of the eye is
characterized by redness, watering, and itching. Skin inflammation is
characterized by itching, scaling, reddening, or, occasionally, blistering.
9.1.2 Potential Chronic Health Effects
Acrolein is mutagenic for mammalian somatic cells and for bacteria and/or yeast.
The substance is toxic to lungs, upper respiratory tract. The substance may be
toxic to skin, eyes. Repeated or prolonged exposure to the substance can produce
target organs damage. Repeated or prolonged contact with spray mist may
produce chronic eye irritation and severe skin irritation. Repeated or prolonged
exposure to spray mist may produce respiratory tract irritation leading to frequent
attacks of bronchial infection. Repeated exposure to a highly toxic material may
produce general deterioration of health by an accumulation in one or many human
organs.
119
9.2Fire and Explosion Data
Flammability of the Product: Flammable.
Auto-Ignition Temperature: 220°C (428°F)
Flammable Limits: LOWER: 2.8% UPPER: 31%
Products of Combustion: These products are carbon oxides (CO, CO2).
Fire Hazards in Presence of Various Substances:
Acrolein is highly flammable in presence of open flames and sparks, of heat also
in presence of oxidizing materials.
Explosion Hazards in Presence of Various Substances:
There is a risk of explosion of the product in presence of mechanical impact and
slightly explosive in presence of heat.
Fire Fighting Media and Instructions:
Flammable liquid, soluble or dispersed in water. In case of small fire use dry
chemical powder while for large fire alcohol foam, water spray or fog may be
used.
Special Remarks on Fire Hazards:
Vapors may form explosive mixtures with air. Vapor may travel considerable
distance to source of ignition and flash back. When heated to decomposition it
emits toxic fumes of carbon monoxide, peroxides.
Special Remarks on Explosion Hazards:
Vapors may form explosive mixtures with air.
9.3Accidental Release Measures
Small Spill:
Dilute with water and mop up, or absorb with an inert dry material and place in an
appropriate waste disposal container.
120
Large Spill:
Acrolein is flammable, corrosive and Poisonous liquid. Keep it away from heat
also from sources of ignition. Absorb with dry earth, sand or other non-
combustible material. Do not get water inside container. Do not touch spilled
material. Use water spray curtain to divert vapor drift. Use water spray to reduce
vapors. Prevent entry into sewers, basements or confined areas; dike if needed.
Call for assistance on disposal.
9.4 Handling and Storage
Precautions:
Acrolein should be kept away from sources of ignition. Ground all equipment
containing material. Do not ingest. Do not breathe gas/fumes/ vapor/spray. Never
add water to this product. In case of insufficient ventilation, wear suitable
respiratory equipment. If ingested, seek medical advice immediately and show the
container or the label. Avoid contact with skin and eyes. Keep away from
incompatibles such as oxidizing agents, acids, alkalis.
Storage:
It should be stored in a segregated and approved area. Keep container in a cool,
well-ventilated area also keep it tightly closed and sealed until ready for use.
Avoid all possible sources of ignition (spark or flame). Do not store above 8°C
(46.4°F).
9.5Exposure Controls/Personal Protection
Engineering Controls:
Provide exhaust ventilation or other engineering controls to keep the airborne
concentrations of vapors below their respective threshold limit value. Ensure that
eyewash stations and safety showers are proximal to the work-station location.
121
Personal Protection:
Face shield, full suit and vapor respirator should be used. Be sure to use an
approved/certified respirator with gloves and boots.
Personal Protection in Case of a Large Spill:
A self contained breathing apparatus should be used to avoid inhalation of the
product. Suggested protective clothing might not be sufficient; consult a specialist
before handling this product.
9.6First Aid Measures
Eye Contact:
Check for and remove any contact lenses. Immediately flush eyes with running
water for at least 15 minutes, keeping eyelids open. Cold water may be used. Get
medical attention immediately.
Skin Contact:
In case of contact, immediately flush skin with plenty of water for at least 15
minutes while removing contaminated clothing and shoes. Cover the irritated skin
with an emollient. Cold water may be used. Wash clothing before reuse.
Thoroughly clean shoes before reuse. Get medical attention immediately.
Serious Skin Contact:
Wash with a disinfectant soap and cover the contaminated skin with an anti-
bacterial cream. Seek immediate medical attention.
Inhalation:
If inhaled, remove to fresh air. If no breathing is possible, give artificial
respiration. If breathing is difficult, give oxygen. Get medical attention
immediately.
Serious Inhalation:
Evacuate the victim to a safe area as soon as possible. Loosen tight clothing such
as a collar, tie, belt or waistband. If breathing is difficult, administer oxygen. If
the victim is not breathing, perform mouth-to-mouth resuscitation.
122
WARNING: It may be hazardous to the person providing aid to give mouth-to-
mouth resuscitation when the inhaled material is toxic, infectious or corrosive.
Seek immediate medical attention.
Ingestion:
If swallowed, do not induce vomiting unless directed to do so by medical
personnel. Never give anything by mouth to an unconscious person. Loosen tight
clothing such as a collar, tie, belt or waistband. Get medical attention
immediately.
123
CHAPTER NO: 10
COST ESTIMATION
A plant design must present a process as capable of operating under conditions
which will yield a profit and net profit equals total income minus all expenses.
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.
10.1Cost Indexes
All cost-estimating methods use historical data and are themselves forecasts of
future costs. The prices of the materials of construction and the costs of labor
considerably increase with time due to changes in economic conditions .Therefore
the cost index is used to update the historical cost data available .A cost index is
merely an index value for a given point in time showing the cost at that time
relative to a certain base time. If the cost at some time in the past is known, the
equivalent cost at the present time can be determined by use of cost indexes.
Cost in year A = Cost in year B × (Cost Index in year A/Cost Index in year B)
The common indexes permit fairly accurate estimates if the time period involved
is less than 10 years. Many different types of cost indexes are published regularly
in Chemical Engineering Journal .The most common of these indexes are the
Marshall and Swift all-industry and process-industry equipment indexes, the
Engineering News-Record construction index, the Nelson-Farrar refinery
construction index, and the Chemical Engineering plant cost index.
124
10.2Cost of designed equipments
Cost is being calculated by using following formula
Cost of equipment in year A=Cost of equipment in year B × Cost index in year A
Cost index in year B
Using Marshall and Swift Equipment Cost Index (MS)
Heat Exchanger
From appendix B figure 16,
For carbon steel shell, stainless steel tubes and floating head,
Material adjustment factor = 1
Pressure adjustment factor = 1
Bare cost = $ 140,000
Purchased cost of shell & tube Condenser (Mid 2004) = 140000 × 1 × 1
= $ 140,000
From appendix B figure 17, using Marshall & Swift equipment cost index
Cost index in year 2004 = 1200
Cost index in year 2012 = 1700
Cost in 2012=140000 × 1700/1200
= $ 198,333
125
Reactor
From appendix B figure 16,
For carbon steel shell, stainless steel tubes and fixed head,
Material adjustment factor for fixed tube sheet= 0.8
Pressure adjustment factor for 2.05 bar = 1
Bare cost = $ 31,000
Purchased cost of muti tubular reactor (Mid 2004) = 31000 × 0.8 × 1
= $ 24,800
From appendix B figure 17, using Marshall & Swift equipment cost index
Cost index in year 2004 = 1200
Cost index in year 2012 = 1700
𝐶𝑜𝑠𝑡 𝑖𝑛 2012 = 1700
1200 × 24800
= $ 35,133
126
Absorber
The purchased cost of packed column can be divided into the
following components;
Cost for column shell, including heads, skirts,
manholes and nozzles.
Cost for internals including packing, support and
distribution plates.
Diameter = D = 10.26 m
Height = H = 28.62 m
From Appendix B figure 18,
Material of Construction =C.S(Carbon Steel)
Material Adjustment factor =1
Pressure Adjustment factor =0.5
Bare cost of Absorber = 3×105× 0.5×1
= $150000
From Appendix B figure 19,
Material of Construction =C.S (Carbon Steel)
Packing Material Adjustment factor =1.2
Packed Height =28.26 m
Cost of Absorber (Includes column internal support and distribution) = 5×105× 1.2
= $600000
127
Total Cost of Absorber Column =$150000+$600000
=$210000
Distillation Column
Diameter of column = D = 0.96 m
Height of column = H = 12.9m
Plate type = Sieve plate
Total pressure drop =29787.36pa
Number of plates = 27
Material of construction = Carbon steel
Cost of distillation column= cost of vertical column+ cost of sieve plates
From Appendix B figure 20
Cost of column in 1998 = (bare cost from fig) ×material factor ×pressure factor
Cost of column in 1998 = (7×1000) ×1×1
Cost of column in 1998 = $7000
From Appendix B figure 21
Cost of plate in 1998 = (bare cost from fig) ×material factor
Cost of plate in 1998 = (320) ×1
Cost of plate in 1998= $320
128
Cost of plate in 1998 = 320×27 = $8640
Cost of distillation column in 1998 = 8640+7000
=$15640
Marshall and Swift Equipment Cost Index using Appendix B figure 17,
Cost index in 1998 = 1092
Cost index in 2012=1500
Cost of column in 2012=Cost of column in 1998× Cost index in 2012
Cost index in 1998
=15640× (1500/1092)
=$21483.
129
APPENDICES
APPENDIX A
Table 1. Heat exchanger and condenser tube data
130
Table 2. Tube sheet layouts.(Tube counts)
Triangular Pitch
131
Table 3.Fouling factor (Coefficients) typical values
132
Table 4.Fouling factor (Coefficients) typical values
Table 5.Data for different packings
133
Table 5. Continued
134
APPENDIX B
Figure 1. Relation between Reynolds number and
friction factor
135
Figure 2. Relation between Reynolds number and friction factor
with respect to baffle cut
136
Figure 3. Overall Coefficients
137
Figure 4. Tube patterns
138
Figure 5. Tube side heat transfer factor
139
Figure 6. Shell side heat transfer factor, segmental baffles
140
Figure 7. Shell side friction factor
141
Figure 8. Shell side heat transfer curve
142
Figure 9. Generalized pressure drop correlation, adapted from a
figure by the Norton Co. with permission
143
Figure 10. Flooding velocity, sieve plates
144
Figure 11. Selection of liquid flow arrangement
145
Figure 12. Weep point correlation (Eduljee, 1959)
146
Figure 13. Relation between downcomer area and weir length
147
Figure 14.Discharge coefficients, sieve plates
148
Figure 15. Entrainment correlation for sieve plates
149
Figure 16. Purchased cost of shell and tube heat exchanger
150
Figure 17. Variation of cost indices
151
Figure 18. Purchased cost of absorber column
152
Figure 19. Purchased cost of packing
153
Figure 20. Purchaesd cost of distillation column
154
Figure 21.Purchaesd cost of column plates
155
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