1. 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

2. Impurity Effects Heat Exchange Reactors Separation Systems
Recycle Loops

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

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

5. 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

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

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

8. 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.

9. 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.

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

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

12. Inerts Addition Effect Similar to Impurity Effects

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.

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.

15. 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.

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