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

Location of Separation Units

What to do with 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 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?

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

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

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

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

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!

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

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

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

Product Specs PPM levels of impurties

Two step reactor Main Reaction Trans alkylation reactor

Separation Train Info. x x

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

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