1. Reactor-Separator-Recycle Networks
Chapter 8 S,S&L
Terry Ring
Chemical Engineering
University of Utah

2. Location of Separation Units

3. What to do with Low Reactor Conversion?
Recycle to extinction
Overall process conversion
Reactor/separator/recycle
Goes to ~100%

4. 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 separation system could be used?
Could products also be effectively separated before the reactor with one separation system up front?
Could other configurations be used?

5. Location of Separation System
Can reactor produce nearly pure products?
Keq>10,000 and with stoichiometric feed
Reactor big enough to reach equilibrium
No Separator after the reactor is needed!
No Recycle is needed!
Example, H2 + Cl2  2 HCl
Can Reactants and Product be separated easily?
CO + 2H2  CH3OH
Reaction with 50% conversion then Flash gives clean product as liquid and unused reactants as vapor for recycle
H2 + N2  NH3
Reaction with 40% conversion then Cryo-Flash gives clean product as liquid and unused reactants as vapor for recycle
Note, Reactants do not need to be separated into pure component streams to be recycled.
BP CO -191C
BP CH3OH +65C

6. Trade-off between Reactor and Separator
Factors
Reactor Conversion of limiting reactant
Affects cost and size of Separation Train
Both capital and operating expenses
Reactor Temperature and mode of operation (adiabatic, isothermal, etc.)
Affect utility costs for both separation and reaction
Affect impurities from side reactions
High Reactor Pressure for Le Chatlier cases
2A + B C, 3 moles reactants to one mole product
Higher cost for recycle compression
High Adiabatic output Temp requires cooling for flash or distillation
High reactor temp can give more impurities

7. Trade-off between Reactor and Separator
Factors, cont.
Use of excess reactants 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
A +B  C CA >> CB
Suppose that B and C are difficult to separate then excess A so that little B is in product will make the separation easy – maybe use purge to remove the little B from the recycle loop

8. Factors that effect recycle/purge
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
Purge at highest concentration of impurities in recycle loop
High Reactor outlet temperature and pressure
Increases cost of utilities in separation and recycle
Increases cost to recycle - compressor

9. Preliminary Flow Sheet
2C2H4 + C4H10  C8H18
Flash
ΔP= 2 psi
Distillation
ΔP= 10 psi
Purge Stream
Recycle Stream
What is recycled?
What is purged?
Le Chatlier effects! That is why the reactor is at pressure. Reaction decreases pressure significantly.
Note partial condenser and only vapor stream is recycled.
Why use flash at all?
Heat exchanger before and after reactor not in place here!

10. Reactor/Separation/Recycle Networks
Suggestions for efficient operation
Make reactor hit highest conversion
Minimize side reactions with
Temperature profile
Pressure used
Excess reactants
Impurities added to the feed
Streamline separation train
Use purge for impurities in all recycle streams!!!
Purge from point of highest impurity concentration
Understand trade-offs
Impacts on operating costs
Impacts on capital cost

11. Feedback effects of Recycle Loop
Small disturbance on feed
Large effect on recycle flow
rate/composition and recycle compression
Snowball effect on reactor/separator
Trouble with operations
Trouble with simulations

12. Cumene Process Main Reaction over Al2O3/SiO2 catalyst As much as 10%
Lost here
(p-

13. Product Specs
PPM levels of impurties

14. Two step reactor
Main Reaction
Trans alkylation reactor

15. Separation Train Info. x x

16. Reactor --> Separation Train --> Reactor
What happens to n-propyl benzene? It has a similar BP to cumene (iso-propyl benzene)?
What happens to the heavies t butyl benzene and cymene?
Do we need a purge? Where
DIPB
Reactor --> Separation Train --> Reactor

17. Reactor --> Separation Train --> Reactor
Do we need a purge? where
DIPB
Reactor --> Separation Train --> Reactor