SKKC 4143 Plant Design 1
HEURISTICS ( Rules of thumb) FOR
PROCESS SYNTHESIS
Edited and Presented by
Assoc. Prof. Dr. Agus Arsad
Department of Bioprocess & Polymer Engineering
UTM
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Introduction
 Recalling the process operations in process synthesis:





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)
 We have applied heuristics in the synthesis of vinyl
chloride PFD such as using pump instead of compressor in
order to increase stream pressure.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
 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.
 Heuristics usually lead to profitable design, but we need
to be watchful for situation in which they might lead to
suboptimal design.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Instructional Objectives
When you have finished studying this unit, you should:
 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.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Raw Materials and Chemical Reactions
Heuristic 1: Select raw materials and chemical reactions to
avoid, or reduce, the handling and storage of
hazardous and toxic chemicals.
Example:
Two-step Manufacture of Ethylene Glycol (EG).
O
(R.1)
1
C2H4 + -2 O2  CH2 - CH2
O
OH
OH
CH2 - CH2 + H2O  CH2 - CH2
(R.2)
Remember: Inherent safety
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
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.
So the main issue here is the risk associated with the storage of
hazardous intermediate (ethylene-oxide).
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Two alternatives to the two-step EG process
 Use chlorine and caustic soda in a single reaction step, to
avoid the storage for intermediate but higher raw material
cost:
(R.3)
 Or as ethylene-oxide is formed in R.1, 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:
O
O
CH2 - CH2 + CO2 
C
O
O
(R.4)
CH2 CH2
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals
Heuristic 2:
Use an excess of one chemical reactant in a
reaction operation to completely consume a
second valuable, toxic, or hazardous chemical
reactant.
Example: Consider using excess ethylene in DiChloroethane
production
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Advantages of excess ethylene
To completely consume the hazardous and toxic reactant
(chlorine)
To absorb excess heat of reaction hence maintaining
moderate temperature
Minimize side reactions.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals (Cont’d)
Heuristic 3:  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.
Example:
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals (Cont’d)
Need to decide whether
to remove inerts (i.e. D)
before reaction...
…or after reaction...
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
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
oVolatility difference for distillation
oDifference in freezing point for crystallization
oPermeability and selectivity for membrane separation
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals (Cont’d)
Heuristic 4: 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.
Example: NH3 Synthesis Loop.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Recirculation w/o purging will lead to build-up of
Ar and CH4.
Here the purge stream contains Ar, CH4, N2, H2
Alternatives for separating traces species cannot
be totally rule out
Note: Purge flow rate selection depends on economics!
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals (Cont’d)
Heuristic 5: 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.
Example: Catalytic converter in car exhaust system.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals (Cont’d)
Heuristic 6: By-products that are produced in reversible
reactions, in small quantities, are usually not
recovered in separators or purged. Instead,
they are usually recycled to extinction.
When the reaction proceeds irreversibly, small quantities of
by-products must be purged, otherwise they will buildup in
the process continuously until the process must be shut
down.
When, however, the reaction proceeds reversibly, it
becomes possible to achieve an equilibrium conversion at
steady state by recycling product species without removing
them from the process. In so doing, it is often said that
undesired byproducts are recycled to extinction.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals (Cont’d)
Heuristic 7:
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For competing series or parallel reactions,
adjust the T, P, 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.
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals (Cont’d)
Example: Manufacture of allyl-chloride
-An example of series-parallel rxn
-The desired product is allyl-chloride .
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Allyl Chloride Manufacture (Cont’d)
Kinetic data
HR
ko
Btu/lbmole
lbmole/(hr ft atm )
206,000
13,600
2
-79,200
11.7
3,430
3
-91,800
4.6 x 108
21,300
Reaction
1
19
-4,800
3
2
E/R (oR)
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
1.02E-03
1.01E-03
1.00E-03
9.90E-04
9.80E-04
9.70E-04
9.60E-04
Allyl Chloride Manufacture (Cont’d)
-0.4
ln(k)
-0.8
-1.2
ln(k1)
ln(k2)
-1.6
1/T (980<T<1042 deg R)
ln(k3)
What range of operating temperatures favor
production of Allyl Chloride ?
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Distribution of Chemicals (Cont’d)
Heuristic 8: 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 reactionseparation operations lead to very different
distributions of chemicals.
Example Manufacture of Methyl-acetate using
:
reactive distillation, a combined operation.
Conventionally, this would call for reaction:
MeOH + HOAc


MeOAc + H2O,
followed by separation of products using a
sequence of separation towers.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
MeOAc Manufacture using Reactive Distillation
MeOAc
HOAc
Reaction
zone
MeOH

H2O
MeOH + HOAc  MeOAc + H2O
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
No need excess reactant
Higher conversion
No need vary pressure for gas phase rxn
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Separations
Heuristic 9: Separate liquid mixtures using distillation and
stripping towers, liquid-liquid extractors,
crystallization and adsorption.
Ref: Douglas (1988)
Example:
Product from reactor is liquid
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Select from
distillation, enhanced
distillation, stripping
towers, liquid-liquid
extraction, etc.
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Separations (Cont’d)
Heuristic 10: Attempt to condense vapor mixtures with
cooling water. Then, use Heuristic 9.
Heuristic 11:
Separate vapor mixtures using partial
condensers, cryogenic distillation, absorption
towers, adsorbers, and/or membrane devices.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Separations (Cont’d)
Example: Product from reactor is vapor
Ref: Douglas (1988)
Select from partial
condensation,
cryogenic distillation,
absorption, adsorption,
membrane separation,
etc.
Attempt to cool
reactor products
using cooling water
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Select from
distillation, enhanced
distillation, stripping
towers, liquid-liquid
extraction, etc.
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Separations (Cont’d)
Example: Products from rxtor are in liquid and vapor phases
Ref: Douglas (1988)
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Heat Transfer in Reactors
Although heat transfer in reactors is better discussed in the
context of heat and power integration, it is treated here
because many methods dealing with heat transfer in reactors
also affect the distribution of chemicals. Treated first are
exothermic reactors.
Heuristic 21:
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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.
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Heat Transfer in Reactors (Cont’d)
Heuristic 21: To remove a highly-exothermic heat of
reaction, consider the use of…
excess reactant
an inert diluent
cold shots.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Heat Transfer in Reactors (Cont’d)
Heuristic 22: 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.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Heat Transfer in Reactors (Cont’d)
An Example of Diabatic Operation:
TVA design for NH3 synthesis converters
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Heat Transfer in Reactors (Cont’d)
Endothermic reactors are treated similarly:
Heuristic 23: To control temperature for a highlyendothermic heat of reaction, consider the use
of excess reactant, an inert diluent, and hot
shots. These affect the distribution of
chemicals and should be inserted early in
process synthesis.
Heuristic 24: For less endothermic heats of reaction,
circulate reactor fluid to an external heater,
or use a jacketed vessel or heating coils. Also,
consider the use of interheaters.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Pumping and Compression
Heuristic 43: 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.
Since work done by pumping or compressions is given by:
   P2VdP
W
P
1
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Pumping and Compression
It follows that it is more efficient (cheaper)) 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.
For example: If we want to turn low pressure liquid stream to a
high pressure vapor stream, we should increase the pressure
first with a pump and then vaporize the high pressure liquid.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
Process Design Heuristics - Summary
We have covered several design heuristics, enabling you to:
 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.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics
What you need to do……….
Make sure to review all 53 Heuristics in Chapter 6
» ……….Thank you.
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DESIGN AND ANALYSIS II - (c) Daniel R. Lewin
Heuristics