Chapter 8 Part 0 – Hierarchical Design Procedures

Hierarchy of Decisions

Example Hydrodealkylation of Toluene

Purge H 2, CH 4 Benzene Dipheny1 H 2, CH 4 Toluene Input-Output Structure of the Flowsheet

Process Purge H 2, CH 4 Benzene Diphenyl H 2, CH 4 Production rate = 265 Design variables: F E and x Component H 2 F H F E CH 4 F M F M + P B /S Benzene 0 0 P B 0 0 Toluene 0 P B /S Diphenyl P B (1 - S)/(2S) 0 Temperature Pressure where S = /(1 -x) F H 2 = F E + P B (1 + S)/2S F M = (1 - y FH )[F E + P B (1 + S)/S]/ y FH F G = F H 2 + F E FIGURE Stream table Toluene Tier 1 Environmental Performance Tools!

Reactor system Separation system Gas recycle Purge H 2, CH 4 Benzene Dipheny1 H 2, CH 4 Toluene Toluene recycle Recycle Structure of the Flowsheet

Vapor recovery system Phase split Reactor system Liquid separation system Purge H 2, CH 4 Benzene Dipheny1 H 2, CH 4 Toluene Simplified Flowsheet for the Separation System

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   Purge Products & By products Feeds OR

Reactor Separation System Purge H 2, CH 4 Benzene Diphenyl H 2, CH 4 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 : reactorseparator products purge feeds Liquid ? Liquid / Vapor ? Vapor ?

LEVEL 4 : 1 ) Liquid reactor liquid separation system Liquid recycle products liquid feeds LEVEL 3

LEVEL 4 : 2 ) Liquid / Vapor reactor liquid separation system Liq. recycle products liquid feeds LEVEL 3 Phase Split Vapor Recovery system 35  C gas recycle vapor liq. vapor purge

LEVEL 4 : 3 ) Vapor reactor liquid separation system Liquid recycle products feeds LEVEL 3 Phase Split Vapor Recovery system 35  C gas recycle liquid vapor purge vapor

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.