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 Slide 2 Impurity Effects Heat Exchange Reactors
Separation Systems Recycle Loops Slide 3 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 Slide 4 Managing Heat
Effects Reaction Run Away Exothermic Reaction Dies Endothermic
Preventing Explosions Preventing Stalling Slide 5 Equilibrium
Reactor- Temperature Effects Single Equilibrium aA +bB rR + sS a i
activity of component I Gas Phase, a i = i y i P, i= = fugacity
coefficient of i Liquid Phase, a i = i x i exp[V i (P-P i s ) /RT]
i = activity coefficient of i V i =Partial Molar Volume of i Vant
Hoff eq. y i (x i ) is smaller due to Impurities Slide 6 Kinetic
Reactors - CSTR & PFR Temperature Effects Used to Size the
Reactor Used to determine the reactor dynamics Reaction Kinetics C
i is lower with Impurities Slide 7 Unfavorable Equilibrium
Increasing Temperature Increases the Rate Equilibrium Limits
Conversion Equilibrium line is repositioned and rate curves are
repositioned due to impurities Slide 8 PFR no backmixing Used to
Size the Reactor Space Time = Vol./Q Outlet Conversion is used for
flow sheet mass and heat balances r K is smaller and V is larger
due to impurities. Slide 9 CSTR complete backmixing Used to Size
the Reactor Outlet Conversion is used for flow sheet mass and heat
balances r K is smaller and V is larger due to impurities. Slide 10
Temperature Profiles in a Reactor Exothermic Reaction Impurities
effect these curves And areas under these curves =size of reactor
Slide 11 Feed Temperature, H rxn Heat Balance over Reactor Cooling
Adiabatic Q = UA T lm Impurities effect the Operating Curve same as
inert effects Slide 12 Inerts Addition Effect Similar to Impurity
Effects Slide 13 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.
Slide 14 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.
Slide 15 Failure of Flash to do its job, H 2 recycle is fed to
Reactor Both Product 1 & 2 are liquid products so there is not
place for H 2 to leave Column. Slide 16 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
