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PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion1
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin1
WORKSHOP on 
PRODUCT AND PROCESS DESIGN
LECTURE 05: HEURISTICS FOR PROCESS 
SYNTHESIS
Daniel R. Lewin
Department of Chemical Engineering
Technion, Haifa, Israel
 Ref: Seider, Seader and Lewin (2004), Chapter 5
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin2
Introduction
Chemical reaction (to eliminate differences in molecular type)
Mixing and recycle (to distribute the chemicals)
Separation (to eliminate differences in composition) 
Temperature, pressure and phase change
Task integration (to combine tasks into unit operations)
Recalling the process operations in process synthesis:
This lecture deals with the heuristic rules that expedite 
the selection and positioning of processing operations as 
flowsheets are assembled.
These rules are based on experience and hold in general, 
but should be tested (e.g., by simulation) to ensure that 
they apply in the specific application.
Later, in Part 2, we will see how algorithmic methods are 
used to improve on design decisions.
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion2
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin3
Instructional Objectives
Understand the importance of selecting reaction paths that do 
not involve toxic or hazardous chemicals, and when unavoidable, to 
reduce their presence by shortening residence times in the 
process units and avoiding their storage in large quantities.
Be able to distribute the chemicals in a process flowsheet, to 
account for the presence of inert species, to purge species that
would otherwise build up to unacceptable concentrations, to 
achieve a high selectivity to the desired products.
Be able to apply heuristics in selecting separation processes to
separate liquids, vapors, and vapor-liquid mixtures. 
Be able to distribute the chemicals, by using excess reactants, 
inert diluents, and cold shots, to remove the exothermic heats of 
reaction.
Understand the advantages of pumping a liquid rather than 
compressing a vapor.
 When you have finished studying this unit, you should:
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin4
 Select raw materials and chemical reactions to 
avoid, or reduce, the handling and storage of 
hazardous and toxic chemicals.
 Heuristic 1:
Raw Materials and Chemical Reactions
 Example: Manufacture of Ethylene Glycol (EG).
C2H4 + O2 → CH2 - CH2
1-2
O
 (R.1)
CH2 - CH2 + H2O → CH2 - CH2
O OH OH
 (R.2)
 Since both reactions are highly exothermic, they need to be 
controlled carefully. But a water spill into an ethylene-oxide storage 
tank could lead to an accident similar to the Bhopal incident. Often 
such processes are designed with two reaction steps, with storage of 
the intermediate, to enable continuous production, even when 
maintenance problems shut down the first reaction operation.
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion3
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin5
Alternatives to the two-step EG process
 (R.3)
 OH OH
CH2=CH2 + Cl2 + 2NaOH(aq) → CH2CH2 + 2NaCl
Use chlorine and caustic in a single reaction step, to avoid 
the intermediate:
As ethylene-oxide is formed, react it with carbon dioxide 
to form ethylene-carbonate, a much less active 
intermediate that can be stored safely and hydrolyzed, 
to form the ethylene-glycol product, as needed:
 (R.4)CH2 - CH2 + CO2 → O O
O C
CH2 CH2
O
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin6
Distribution of Chemicals
 Example: Consider using excess ethylene in DCE production
 Use an excess of one chemical reactant in a 
reaction operation to completely consume a 
second valuable, toxic, or hazardous chemical 
reactant.
 Heuristic 2:
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion4
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin7
 Example:
Distribution of Chemicals (Cont’d)
When nearly pure products are required, 
eliminate inert species before the reaction 
operations, when the separations are easily 
accomplished, or when the catalyst is 
adversely affected by the inert
Do not do this when a large exothermic 
heat of reaction must be removed.
 Heuristic 3:
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin8
Distribution of Chemicals (Cont’d)
Need to decide whether to 
remove inerts before 
reaction...
Clearly, the ease and cost of the separations must be assessed. 
This can be accomplished by examining the physical properties upon 
which the separations are based, and implies the use of simulation
 … or after reaction...
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion5
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin9
 Introduce liquid or vapor purge streams to 
provide exits for species that
– enter the process as impurities in the feed
– produced by irreversible side-reactions
 when these species are in trace quantities 
and/or are difficult to separate from the 
other chemicals. 
 Heuristic 4:
Distribution of Chemicals (Cont’d)
 Example: NH3 Synthesis Loop.
 Note: Purge flow rate selection depends on economics!
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin10
Distribution of Chemicals (Cont’d)
 Do not purge valuable species or species that 
are toxic and hazardous, even in small 
concentrations. 
– Add separators to recover valuable species.
– Add reactors to eliminate toxic and hazardous 
species.
 Heuristic 5:
 Example: Catalytic converter in car exhaust system.
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion6
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin11
Distribution of Chemicals (Cont’d)
 For competing series or parallel reactions, 
adjust the temperature, pressure, and catalyst 
to obtain high yields of the desired products. 
In the initial distribution of chemicals, assume 
that these conditions can be satisfied - obtain 
kinetics data and check this assumption before 
developing a base-case design.
 Heuristic 7:
 Example: Manufacture of allyl-chloride.
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin12
Allyl Chloride Manufacture (Cont’d)
 Example: Manufacture of allyl-chloride.
 Reaction
 ∆HR
 Btu/lbmole
ko
lbmole/(hr ft3atm2)
 E/R (oR) 
 1 -4,800 206,000 13,600
 2 -79,200 11.7 3,430
 3 -91,800 4.6 x 108 21,300
 
 Kinetic data
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion7
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin13
Allyl Chloride Manufacture (Cont’d)
-1.6
-1.2
-0.8
-0.4
9.
60
E-
04
9.
70
E-
04
9.
80
E-
04
9.
90
E-
04
1.0
0E
-0
3
1.0
1E
-0
3
1.0
2E
-0
3
1/T (980<T<1042 deg R)
ln
(k
)
ln(k1)
ln(k2)
ln(k3)
 What range of operating temperatures favor 
production of Allyl Chloride ?
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin14
 For reversible reactions, especially, consider 
conducting them in a separation device capable 
of removing the products, and hence, driving 
the reactions to the right. Such reaction-
separation operations lead to very different 
distributions of chemicals.
 Heuristic 8:
Distribution of Chemicals (Cont’d)
 Example: Manufacture of Ethyl-acetate using reactive 
distillation.
 Conventionally, this would call for reaction:
 followed by separation of products using a 
sequence of separation towers.
 MeOH + HOAc MeOAc + H2O,←
→
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion8
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin15
MeOAc Manufacture using Reactive Distillation
Reaction
zone
MeOAc
HOAc
MeOH
H2O
 MeOH + HOAc MeOAc + H2O←
→
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin16
 Ref: Douglas (1988)
 Separate liquid mixtures using distillation and 
stripping towers, and liquid-liquidextractors, 
among similar operations.
 Heuristic 9:
Separations
 Select from 
distillation, enhanced 
distillation, stripping 
towers, liquid-liquid 
extraction, etc.
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion9
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin17
 Ref: Douglas (1988)
Separations (Cont’d)
 Attempt to condense vapor mixtures with 
cooling water. Then, use Heuristic 9.
 Heuristic 10:
 Select from partial 
condensation, 
cryogenic distillation, 
absorption, adsorption, 
membrane separation, 
etc.
 Select from 
distillation, enhanced 
distillation, stripping 
towers, liquid-liquid 
extraction, etc.
 Attempt to cool 
reactor products 
using cooling water
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin18
 Ref: Douglas (1988)
Separations (Cont’d)
 Separate vapor mixtures using partial 
condensers, cryogenic distillation, absorption 
towers, adsorbers, and/or membrane devices.
 Heuristic 11:
 Combination of the 
previous two flowsheets
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion10
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin19
 To remove a highly-exothermic heat of 
reaction, consider the use of excess reactant, 
an inert diluent, and cold shots. These affect 
the distribution of chemicals and should be 
inserted early in process synthesis.
 Heuristic 21:
Heat Transfer in Reactors
 For less exothermic heats of reaction, 
circulate reactor fluid to an external cooler, 
or use a jacketed vessel or cooling coils. Also, 
consider the use of intercoolers.
 Heuristic 22:
 Although heat transfer in reactors is better discussed in the 
context of heat and power integration (see Lectures 6- 8), it 
is treated here because many methods dealing with heat 
transfer in reactors also affect the distribution of chemicals. 
Treated first are exothermic reactors. 
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin20
Heat Transfer in Reactors (Cont’d)
 To remove a highly-exothermic heat of 
reaction, consider the use of…
 Heuristic 21:
 excess reactant
 cold shots. 
 an inert diluent
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion11
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin21
Heat Transfer in Reactors (Cont’d)
 For less exothermic heats of reaction, 
circulate reactor fluid to an external cooler, 
or use a jacketed vessel or cooling coils. Also, 
consider the use of intercoolers.
 Heuristic 22:
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin22
Heat Transfer in Reactors (Cont’d)
 TVA design for NH3 synthesis converters Example:
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion12
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin23
Heat Transfer in Reactors (Cont’d)
 NH3 synthesis converter cold-shot optimization Example:
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin24
Heat Transfer in Reactors (Cont’d)
 NH3 synthesis converter cold-shot optimization Example:
 Before optimization – φφφφ = [0.100, 0.100]T After optimization – φφφφ = [0.277, 0.240]T
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion13
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin25
 To increase the pressure of a stream, pump a 
liquid rather than compress a gas; that is, 
condense a vapor, as long as refrigeration (and 
compression) is not needed, before pumping.
 Heuristic 43:
Pumping and Compression
∫= 2
1
P 
P dPVW
 Since work done by pumping or compressions is given by:
 It follows that it is more 
efficient to pump a liquid than 
to compress a gas. Thus, it is 
almost always preferable to 
condense a vapor, pump it, and 
vaporize it, rather than 
compress it. 
Exception: if condensation 
requires refrigeration.
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin26
Example : Feed Preparation of Ethylbenzene
Ethylbenzene is to be taken from storage at
25 °C and 1 atm and fed to a styrene 
reactor at 400°C and 5 atm at 100,000 lb/h.
Show two alternatives for positioning the 
temperature and pressure-increase 
operations. 
Pumping and Compression
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion14
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin27
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin28
Process Design Heuristics - Summary
Understand the importance of selecting reaction paths that do 
not involve toxic or hazardous chemicals, or to reduce their 
presence by shortening residence times in the process units and 
avoiding their storage in large quantities.
Be able to distribute the chemicals in a process flowsheet, to 
account for the presence of inert species, to purge species that
would otherwise build up to unacceptable concentrations, to 
achieve a high selectivity to the desired products.
Be able to apply heuristics in selecting separation processes to
separate liquids, vapors, and vapor-liquid mixtures. 
Be able to distribute the chemicals to remove exothermic heats 
of reaction.
Understand the advantages of pumping a liquid rather than 
compressing a vapor.
We have focused on 16 design heuristics (out of 53 in the 
book), enabling you to:
PRODUCT and PROCESS DESIGNLECTURE 05
Daniel R. Lewin, Technion15
Heuristics PRODUCT AND PROCESS DESIGN - (c) Daniel R. Lewin29
Parting Thought – When working on design projects,
students need to be made aware of important sources of 
heuristics;e.g.,
Walas, S. M., Chemical Process Equipment – Selection and Design, 
Butterworths, Stoneham, MA, 1988.
Turton, R., R. C. Bailie, W. B. Whiting, and J. A. Shaeiwitz, Analysis, 
Synthesis, and Design of Chemical Processes, Second Edition, 
Prentice- Hall, 2003. Chapter 9 –Utilizing Experience-based Principles 
to Confirm the Suitability of a Process Design.
Happel, J., and D. G. Jordan, Chemical Process Economics, Second 
Edition, Marcel Dekker, New York, 1975 – Appendix C. 
Ulrich, G. D., A Guide to Chemical Engineering Process Design and 
Economics, Wiley, 1984 – Appendix B.
Process Design Heuristics - Sources

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