Reactor-Separator-Recycle Networks

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? • 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.

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

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

8. Factors that effect 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 • 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

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

14. Two step reactor • Main Reaction • Trans alkylation reactor

15. Separation Train Info. x x

16. DIPB Reactor --> Separation Train --> Reactor

17. DIPB Reactor --> Separation Train --> Reactor