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

H2＋

CH3

CH4＋

Toluene Benzene

1

2 H2＋

Benzene Diphenyl

Purge

H2 , CH4

Benzene

Dipheny1

H2 , CH4

Toluene

Input-Output Structure of the Flowsheet

Process1

2

3

4

5 PurgeH2 , CH4

Benzene

Diphenyl

H2 , CH4

Production rate = 265Design variables: FE and x

Component 1 2 3 4 5

H2 FH2 0 0 0 FE

CH4 FM 0 0 0 FM + PB/S Benzene 0 0 PB 0 0 Toluene 0 PB/S 0 0 0 Diphenyl 0 0 0 PB(1 - S)/(2S) 0 Temperature 100 100 100 100 100 Pressure 550 15 15 15 465

where S = 1 - 0.0036/(1 -x)1.544 FH2 = FE + PB(1 + S)/2S

FM = (1 - yFH)[FE + PB(1 + S)/S]/ yFH FG = FH2 + FE

FIGURE 5.2-1

.

Stream table

Toluene

Tier 1 Environmental Performance Tools!

Reactorsystem

Separationsystem

Gas recycle PurgeH2 , CH4

Benzene

Dipheny1

H2 , CH4

Toluene

Toluene recycle

Recycle Structure of the Flowsheet

Vapor recoverysystem

Phasesplit

Reactorsystem

Liquid separationsystem

Purge

H2 , CH4

Benzene

Dipheny1

H2 , CH4

Toluene

Simplified Flowsheet for the Separation System

Reactor

compressor

Vapor Recovery System

Purge

FlashDrum

Benzene

Toluene Col.

Benzene C

ol.

Stablizer

H2 Feed

Toluene Feed

TolueneRecycle

Diphenyl

Distillation Train

compressor

Flashhh

Reactorfurnace

Purge

Liquidrecycle

Gasrecycle

H2, feed

CW

Benzeneproduct

H2, CH4

Diphenyl

FIGURE 1.2-2Hydrodealkylation of toluene; maximum energy recovery.

ENERGY INTEGRATION

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?

PROCESS

Products&By products

Feeds

PROCESS

PurgeProducts&By products

Feeds

OR

ReactorSeparation

System

Purge

H2 , CH4

Benzene

Diphenyl

H2 , CH4

Toluene

LEVEL 2

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 :

reactor separator products

purge

feeds

Liquid ?Liquid / Vapor ?

Vapor ?

LEVEL 4 :

1 ) Liquid

reactorliquid

separationsystem

Liquid recycle

productsliquidfeeds

LEVEL 3

LEVEL 4 :

2 ) Liquid / Vapor

reactorliquid

separationsystem

Liq. recycle

productsliquid

feeds

LEVEL 3

PhaseSplit

VaporRecovery

system35Cgas recycle

vapor liq.liq.

vapor

purge

LEVEL 4 :

3 ) Vapor

reactorliquid

separationsystem

Liquid recycle

productsfeeds

LEVEL 3

PhaseSplit

VaporRecovery

system35Cgas recycle

liquidliquid

vapor

purge

vapor

Reactor

compressor

Vapor Recovery System

Purge

FlashDrum

Benzene

Toluene Col.

Benzene C

ol.

Stablizer

H2 Feed

Toluene Feed

TolueneRecycle

Diphenyl

Distillation Train

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.