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Chapter 8
Part 0 – Hierarchical Design
Procedures
Hierarchy of Decisions
1. Batch versus continuous
2. Input-output structure of the flowsheet
3. Recycle structure of the flowsheet
4. General structure of the separation system
a. Vapor recovery system
b. Liquid recovery system
5. Heat-exchanger network
Example Hydrodealkylation of Toluene
CH3
1
＋ CH4
＋ H2
Toluene
Benzene
2
＋ H2
Benzene
Diphenyl
Purge
H2 , CH4
H2 , CH4
Benzene
Toluene
Dipheny1
Input-Output Structure of the Flowsheet
5
H2 , CH4
1
Purge
H2 , CH4
3
Process
2
4
Toluene
Benzene
Diphenyl
Production rate = 265
Design variables: FE and x
Component
1
2
H2
FH2
0
CH4
FM
0
Benzene
0
0
Toluene
0
PB/S
Diphenyl
0
0
Temperature
100
100
Pressure
550
15
where S = 1 - 0.0036/(1 -x)1.544
3
0
4
0
5
FE
0
0
FM + PB/S
PB
0
0
0
0
0
0
PB(1 - S)/(2S) 0
100
100
100
15
15
465
FH2 = FE + PB(1 + S)/2S
FM = (1 - yFH)[FE + PB(1 + S)/S]/ yFH
FG = FH2 + FE
FIGURE 5.2-1
Stream table
.
Tier 1 Environmental Performance Tools!
Gas recycle
H2 , CH4
Toluene
Purge
H2 , CH4
Benzene
Reactor
system
Separation
system
Dipheny1
Toluene recycle
Recycle Structure of the Flowsheet
Vapor recovery
system
H2 , CH4
Reactor
system
Purge
H2 , CH4
Phase
split
Toluene
Liquid separation
system
Benzene
Dipheny1
Simplified Flowsheet for the Separation System
Vapor Recovery System
H2 Feed
Purge
compressor
Reactor
Toluene
Feed
Flash
Drum
Benzene
Toluene
Recycle
Distillation Train
Stablizer
Benzene Col.
Toluene Col.
Diphenyl
Liquid
recycle
H2, CH4
Benzene
product
Purge
compressor
Gas
recycle
Diphenyl
furnace
Reactor
CW
FIGURE 1.2-2
Hydrodealkylation of toluene; maximum energy recovery.
ENERGY INTEGRATION
Flash
hh
H2, feed
Toluene
feed
LEVEL 2 DECISIONS:
1 ) Should we purify the feed streams before they enter the process?
2 ) Should we remove or recycle a reversible by-product?
3 ) Should we use a gas recycle and purge stream?
4 ) Should we not bother to recover and recycle some reactants?
5 ) How many product streams will there be?
6 ) What are the design variables for the input/output structure?
What economic trade-offs are associated with these variables?
Feeds



PROCESS
Products
&
By products



OR
Feeds



PROCESS



Purge
Products
&
By products
Purge
H2 , CH4
H2 , CH4
Toluene
LEVEL 2
Reactor
Separation
System
Benzene
Diphenyl
LEVEL 3 DECISIONS
1 ) How many reactors are required ?
Is there any separation between the reactors ?
2 ) How many recycle streams are required ?
3 ) Do we want to use an excess of one reactant at the reactor inlet ? Is there a
need to separate product partway or recycle byproduct ?
4 ) Should the reactor be operated adiabatically or with direct heating or cooling ?
Is a diluent or heat carrier required ?
What are the proper operating temperature and pressure ?
5 ) Is a gas compressor required ? costs ?
6 ) Which reactor model should be used ?
7 ) How do the reactor/compressor costs affect the economic potential ?
LEVEL 3 :
purge
feeds
reactor
separator
Liquid ?
Liquid / Vapor ?
Vapor ?
products
LEVEL 4
:
1 ) Liquid
LEVEL 3
feeds
reactor
liquid
Liquid recycle
liquid
separation
system
products
purge
LEVEL 4
:
2 ) Liquid / Vapor
gas recycle
vapor
feeds
reactor
vapor
35C
liquid
Liq. recycle
LEVEL 3
Phase
Split
Vapor
Recovery
system
liq.
liquid
separation
system
liq.
products
purge
LEVEL 4
:
3 ) Vapor
vapor
gas recycle
35C
Vapor
Recovery
system
Phase
liquid
liquid
Split
vapor
feeds
liquid
separation
system
reactor
Liquid recycle
LEVEL 3
products
Vapor Recovery System
H2 Feed
Purge
compressor
Reactor
Toluene
Feed
Flash
Drum
Benzene
Toluene
Recycle
Distillation Train
Stablizer
Benzene Col.
Toluene Col.
Diphenyl
RULES FOR PLACING THE VAPOR RECOVERY SYSTEM
1 ) Place the vapor recovery system on the purge stream if significant amounts
of valuable materials are being lost in the purge.
2 ) Place the vapor recovery system on the gas-recycle stream if materials that
are deleterious to the reactor operation (catalyst poisoning, etc.) are present
in this stream or if recycling of some components degrades the product
distribution.
3 ) Place the vapor recovery system on the flash vapor stream if both 1 ) and 2 )
are valid.
4 ) Do not use a vapor recovery system if neither 1 ) nor 2 ) are important.
TYPE OF VAPOR RECOVERY SYSTEM
1 ) Condensation - low. T and/or high P
2 ) Absorption
3 ) Adsorption
4 ) Membrane separation
5 ) Reaction
ADJUST MATERIAL BALANCE ?
DECISION IN SYNTHESIZING LIQUID SEPARATION
SYSTEM
1. How should light ends be removed, if they might contaminate
the product ?
2. What should be the destination of light end ?
3. Do we recycle components that from azeotropes with reactants,
or do we split the azeotropes ?
4. What separations can be made by distillation ?
5. What sequence of columns do we use ?
6. How should we accomplish separations if distillation is not
feasible ?
P2 Rationale
• Some of the decisions made at each level
might change the streams leaving the
process. Some of the exit streams may
cause pollution problems.
• If we can make decisions that do not
introduce exit streams to the flowsheet that
have environmental impacts, the P2 goal is
achieved.