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? –K eq >10,000 and with stoichiometric feed, –No Separator after the reactor is needed! –No Recycle is needed! –Example, H 2 + Cl 2 2 HCl Can Reactants and Product can be separated easily? –CO + 2H 2 CH 3 OH –Reaction with 50% conversion then Flash gives clean product as liquid and unused reactants as vapor –H 2 + N 2 NH 3 –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 2C 2 H 4 + C 4 H 10 C 8 H 18 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 Al 2 O 3 /SiO 2 catalyst As much as 10% Lost here P-DIPB