CH EN 5253 – Process Design II Dealing with Impurities in Processes and Process Simulators February 09, 2018

Books There is not chapter in the book on this subject

Impurity Effects Heat Exchange Reactors Separation Systems Recycle Loops

Impurities in Reactors Point 1: Poisons for Catalysts Kill Catalyst with time Point 2: Impurities can cause side reactions altering Reactor conversion Generating additional undesirable products Point 3: Impurities Impact Equilibrium Conversion Point 4: Impurities Impact Reaction Rates Lower concentrations Point 5: Impurities have Reaction Heat Effects Lower Cp of feed

Point 1: Poisons for Catalysts Kill Catalyst with time Lead, Sulfur, Manganese in Gasoline kill Catalytic Converter Platinum, Palladium, Rhodium Note: Vehicles equipped with catalytic converters can run only on unleaded fuel

Catalytic Reactors Various Mechanisms depending on rate limiting step Surface Reaction Limiting Surface Adsorption Limiting Surface Desorption Limiting Combinations Langmuir-Hinschelwood Mechanism (Surface Reaction Limiting) H2 + C7H8 (T) CH4 + C6H6(B) Impurities decrease the Cv concentration of active sites due to Impurity Adsorption

Point 2: Side reactions altering Reactor conversion Generating additional undesirable products Reactions

Point 3: Impurities Impact Equilibrium Conversion Temperature Effects Single Equilibrium aA +bB  rR + sS Van’t Hoff eq.

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

Point 4: Impurities Impact Reaction Rates Effect of Inert Addition Similar to Impurity Effects

Point 5: Reaction Heat Effects Heat Balance over Reactor Q = UA ΔTlm Adiabatic Adiabatic Cooling

Impact on Reactor Design

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

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

Costs Higher capital cost for Higher operating cost for Bigger reactors Cooling and heating systems Bigger Separation System Higher operating cost for Utility – hot and cold water Pumping Separation