CH EN 5253 – Process Design II Effects of Impurities on Reactions and Reactor Design February 11, 2019.

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Presentation transcript:

CH EN 5253 – Process Design II Effects of Impurities on Reactions and Reactor Design February 11, 2019

Books There is no chapter in the book on this subject

Impurity Effects Reactors Heat Exchange Separation Systems Recycle Loops

Location of Separation Units

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 Increase Cp

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

Review: Mechanism of Heterogeneous Catalysis

Catalytic Reactors Various Mechanisms depending on rate limiting step Surface Reaction Limiting Surface Adsorption Limiting Surface Desorption Limiting Combinations

Catalytic Reactors Toluene Hydrodealkylation (HDA)Process H2 + C7Hi (T) CH4 + C6H6(B) Catalyst: Chromium or molybdenum or platinum oxides Langmuir-Hinshelwood Mechanism (Surface Reaction Limiting) Impurities are adsorbed on catalytic sites Decrease the Cv concentration of active sites

Point 2: Side reactions altering Reactor conversion Generating additional undesirable products (undesired) (undesired)

Point 3: Impurities Impact Equilibrium Conversion Temperature Effects Single Equilibrium aA +bB  rR + sS 𝐾 𝑒𝑞 = 𝑎 𝑅 𝑟 𝑎 𝑆 𝑠 𝑎 𝐴 𝑎 𝑎 𝐵 𝑏 = exp −Δ 𝐺 𝑟𝑥𝑛 𝑜 𝑅𝑇 , 𝑑 𝑙𝑛 𝐾 𝑒𝑞 𝑑𝑇 = Δ 𝐻 𝑟𝑥𝑛 𝑜 𝑅 𝑇 2 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

Point 5: Reaction Heat Effects Effect of Inert Addition Similar to Impurity Effects Adiabatic ∆T=T2-T1 Q=external heat added With increased inerts Cp rises, and these curves become more closely vertical.

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

Impact on Reactor Design

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

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

Consideration of Impurities in Catalytic Reactor Design How the surface adsorption and surface desorption influence the rate law? Whether the surface reaction occurs by a single-site or dual-site reaction between adsorbed molecule and molecular gas? How does the reaction heat generated get dissipated by reactor design?