Reactor-Separator-Recycle Networks

Chapter 8

Terry Ring

Location of Separation Units

What to do with Low Reactor Conversion?

• Low Reactor Conversion– Recycle to extinction

• Overall process conversion– Reactor/separator/recycle– Goes to ~100%

Location of Separation Network

• After Reactor– Products are traditionally separated– Reactants Recycled

• Before Reactor– Reactants are typically purified before Reaction

• But– Could reactor be run so that no separation is

needed?– Could reactor be run so that a small/simple

separation system could be used?– Could products also be effectively separated

before the reactor with one separation system?– Could other hybrid configurations be used?

Location of Separation System

• Can reactor produce nearly pure products?– Keq>10,000 and with stoichiometric feed,– No Separator after the reactor is needed!– No Recycle is needed!– Example, H2 + Cl2 2 HCl

• Can Reactants and Product can be separated easily?– CO + 2H2 CH3OH– Reaction with 50% conversion then Flash gives clean product as

liquid and unused reactants as vapor– H2 + N2 NH3 – Reaction with 40% conversion then Cryo-Flash gives clean

product as liquid and unused reactants as vapor• Note, Reactants do not need to be separated into pure

component streams to be recycled.

Trade-off between Reactor and Separator

• Factors– Reactor Conversion of limiting reactant

• Effects cost and size of Separation Train

– Reactor Temperature and mode of operation (adiabatic, isothermal, etc.)

• Effect utility costs for both separation and reaction• Effect impurities from side reactions

– High Reactor Pressure for Le Chatlier cases (less moles of product)

• Higher cost for recycle compression

Trade-off between Reactor and Separator

• Factors, cont.– Use of excess of one or more reactant to increase

equilibrium conversion and/or reaction rate• Increases cost of separation train

– Use of diluents in adiabatic reactor to control temperature in reactor

• Increases cost of separations train

– Use of purge to avoid difficult separation.• Decreases the cost of separations• Loss of reactants – increase cost of reactants• May increase cost of reactor, depending on the purge-to-

recycle ratio

Factors that affect recycle/purge

• Factor– Excess reactants

• Increases recycle flow• Increases separation costs• Increase feed stream costs

– Raw Materials– Heat up and Pressure up requirements

– Concentration of impurities to be purged• Effects the recycle-to-purge ratio

– High Reactor outlet temperature and pressure• Increase cost of utilities in separation• Increase cost to recycle - compressor

Preliminary Flow Sheet

•2C2H4 + C4H10 C8H18

Reactor ΔP

Flash ΔP= 2 psi

Distillation ΔP= 10 psi

Purge Stream

Recycle Stream

Reactor/Separation/Recycle Networks

• Suggestions for efficient operation– Make reactor hit high conversion– Minimize side reactions with

• Temperature profile• Pressure used• Excess reactants• Impurities added to the feed

– Streamline separation train– Use purge for impurities in any recycle stream– Understand trade-offs

• Impacts on operating costs• Impacts on capital cost

Feedback effects of Recycle Loop

• Small disturbance on feed

• Large effect on recycle flow– rate/composition

• Snowball effect on reactor/separator

Cumene Process

Main Reaction over Al2O3/SiO2 catalyst

As much as 10%Lost hereP-DIPB

Product Specs

Two step reactor

• Main Reaction

• Trans alkylation reactor

Separation Train Info.

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DIPB