Dealing with Impurities in Processes and Process Simulators ChEN 5253 Design II Terry A. Ring There is not chapter in the book on this subject

Impurity Effects Heat Exchange Reactors Separation Systems Recycle Loops

Impurities in Reactors Poisons for Catalysts Kill Catalyst with time S in Gasoline kills Catalytic Converter Impurities can cause side reactions altering Reactor conversion Generating additional undesirable products Impurities Impact Equilibrium Conversion Impurities Impact Reaction Rates Lower concentrations Impurities have Reaction Heat Effects Lower Cp of feed in slope of operating line

Managing Heat Effects Reaction Run Away Reaction Dies Exothermic Reaction Dies Endothermic Preventing Explosions Preventing Stalling

Equilibrium Reactor- Temperature Effects Single Equilibrium aA +bB  rR + sS ai activity of component I Gas Phase, ai = φiyiP, φi== fugacity coefficient of i Liquid Phase, ai= γi xi exp[Vi (P-Pis) /RT] γi = activity coefficient of i Vi =Partial Molar Volume of i Van’t Hoff eq. yi (xi) is smaller due to Impurities

Kinetic Reactors - CSTR & PFR – Temperature Effects Used to Size the Reactor Used to determine the reactor dynamics Reaction Kinetics Ci is lower with Impurities

Unfavorable Equilibrium Increasing Temperature Increases the Rate Equilibrium Limits Conversion Equilibrium line is repositioned and rate curves are repositioned due to impurities

PFR – no backmixing Used to Size the Reactor Space Time = Vol./Q Outlet Conversion is used for flow sheet mass and heat balances rK is smaller and V is larger due to impurities.

CSTR – complete backmixing Used to Size the Reactor Outlet Conversion is used for flow sheet mass and heat balances rK is smaller and V is larger due to impurities.

Temperature Profiles in a Reactor Exothermic Reaction Impurities effect these curves And areas under these curves =size of reactor

Feed Temperature, ΔHrxn Adiabatic Adiabatic Cooling Heat Balance over Reactor Q = UA ΔTlm Impurities effect the Operating Curve same as inert effects

Inerts Addition Effect Similar to Impurity Effects

Processes are tested for Impurity Tolerance Add light and heavy impurities to feed Low concentration All impurities add to 0.1 % of feed (may need to increase Tolerance in Simulation) Medium concentration All impurities add to 1% of feed High concentration All impurities add to 10% of feed Find out where impurities end up in process Find out if process falls apart due to impurities What purges are required to return process to function.

Processes are tested for Impurity Tolerance Add light and heavy impurities to feed Low concentration All impurities add to 0.1 % of feed Medium concentration All impurities add to 1% of feed High concentration All impurities add to 10% of feed Find out where impurities end up in process Find out if process falls apart due to impurities What purges are required to return process to function.

Failure of Flash to do its job, H2 recycle is fed to Reactor Both Product 1 & 2 are liquid products so there is not place for H2 to leave Column.

Impurities in Separation Trains Non-condensible Impurities Build up in Distillation column – Big Trouble!! Condensible Impurities Cause some products to be less pure May not meet product specifications Can not sell this product – Big Trouble!! Rework cost Waste it Sell for lower price