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DESIGN METHODOLOGY II:
INPUT-OUTPUT DECISIONS
GOAL -- Choose the process chemistry and
feedstock most appropriate to the plant to be
designed and determine the associated
economic potential.
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BASIC LEVEL II DECISIONS
u Choice of reaction chemistry
u Choice of raw material feedstocks
u Determination of need for vapor recycle
u Evaluation of economic potential for process
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CHOICE OF REACTION CHEMISTRY
The choice of reaction chemistry involves the following
considerations:
u Source of the chemistry to be employed
eDeveloped in-house
eFreely available (patents expired)
e Available only through a license
u Availability of feed stocks
u Choice of process type
u Environmental considerations
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EXAMPLE #1
Chemistries available to manufacture Vinyl chloride (from
Seider et al,Process Design Principles)
uDirect chlorination of Ethylene
C2H4 + Cl2 --> C2H3Cl + HCl
(Only half the Cl2 converted to vinyl chloride)
u Thermal cracking of Dichloroethane via
Oxychlorination of Ethyelene
C2H4 + 2HCl + 1/2 O2 --> C2H4Cl2 + H2O
C2H4Cl2 --> C2H3Cl + HCl
(Uses HCl instead of Cl2,but requires two reaction steps)
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EXAMPLE #1 (Cont’d.)
u Balanced Process for Chlorination of of Ethylene
C2H4 + Cl2 --> C2H4Cl2
C2H4 + 2HCl + 1/2 O2 --> C2H4Cl2 + H2O
2 C2H4Cl2 --> 2 C2H3Cl +2 HCl
(Converts all Cl atoms to Vinyl Chloride)
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EXAMPLE #2
Chemistries available for the manufacture of maleic
anhydride:
u From benzene
C6H6 + 3 1/2 O2 --> <(.CHCO)2>O + 2 CO + 2 H2O
C6H6 + 4 1/2 O2 --> 6 CO2 + 3 H2O
u From butane
C4H10 + 3 1/2 O2 --> <(.CHCO)2>O + 4 H2O
C4H10 + 4 1/2 O2 --> 4 CO + 5 H2O
C4H10 + 6 1/2 O2 --> 4 CO2 + 5 H2O
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EXAMPLE #2 (Cont’d.)
u Maleic Anhydride from Benzene
e High selectivity at almost complete conversion - no
recycle required
e Benzene is expensive and is a carcinogen
u Maleic Anhydride from Butane
e Lower selectivity - cannot operate at complete
conversion
e Butane is relatively cheap
Almost all new plants use butane as the feedstock.
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CHOICE OF FEEDSTOCK
u Availability
e What feedstocks are available at the potential plant
site?
e At what price?
u Purity
e At what purities is are the feedstocks available?
e Are the impurities
Inert?
Affect reaction rates or separation system performance?
Catalyst poisons or otherwise foul the process equipment?
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PROCESS TYPE
u Type I
e All components in reactor effluent must be
condensable at a reasonable temperature and pressure
using cooling water or moderate refrigeration.
e Can assume that all components can be recovered
and purified using distillation or liquid-liquid
extraction.
e Can further assume that all (100%) of the limiting
raw material fed to the process can be converted to
products or byproducts as a good approximation..
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Raw
Materials
Products
Byproducts
Type I Process
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PROCESS TYPE (Cont’d)
u Type 2
e Cannot conveniently condense all components in
reactor effluent
e Cannot cheaply separate non-condensable raw
materials from non-condensable inerts or byproducts.
e Vapor purge of inerts or byproducts also purges
some fraction of the raw materials fed to the process.
e Overall conversion of limiting raw materials is less
than 100%,
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Raw
Materials
Products
Byproducts
Type 2 Process
Purge
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ECONOMIC POTENTIAL
u A major fraction of the total cost of producing most
large-scale commodity chemicals is the cost of the raw
materials (25-50%)
u A useful measure of the profitability of a proposed
process design the economic potential EP.
EP = Sales revenue - cost of raw materials
If EP < 0 for all possible design alternatives,then the
project,baring improvements in the technology,should be
abandoned.
If EP < twice the cost of raw materials,then the profitability
is suspect (rule of thumb).