Reactor Design S,S&L Chapter 6

Objectives De Novo Reactor Designs Plant Improvement –Debottlenecking –Increase Plant Capacity –Increase Plant Efficiency –Decrease Costs –Pollution Minimization

Reactor Types Ideal –PFR –CSTR Real –Unique design geometries and therefore RTD –Multiphase –Various regimes of momentum, mass and heat transfer

Reactors in Process Simulators Stoichiometric Model –Specify reactant conversion and extents of reaction for one or more reactions A model for multiple phases in chemical equilibrium Kinetic model for a CSTR Kinetic model for a PFR Custom-made models (UDF) Used in early stages of design

Stoichiometric Reactor C chemical Species υ i,j stoichiometric coefficient for i th species in j th reaction A j chemical formula of j th species R chemical reactions

Stoichiometric Reactor Example Reactions –1 Methane Synthesis –2 Coking Conversion, X k, of key component, k –X k =(n k-in – n k-out )/ n k-in Extent of Reaction –ξ i = (n i,j-in – n i,j-out )/ ν i,j

Reactions with low conversions? Due to slow kinetics Due to non-favorable Equilibrium –Solution Set up reactor Followed by Separator Recycle reactant to extinction

Equilibrium Reactor-1 Single Equilibrium aA +bB  rR + sS –a i activity of component I Gas Phase, a i = φ i y i P, –φ i= = fugacity coefficient of i Liquid Phase, a i = γ i x i exp[V i (P-P i s ) /RT] –γ i = activity coefficient of i –V i =Partial Molar Volume of i Van’t Hoff eq.

Equilibrium Reactor-2 Total Gibbs Free Energy is minimized at T&P –Specify components that are entering system and T&P of System –Specify possible reaction products Gives outlet composition at equilibrium

Kinetic Reactors - CSTR & PFR Used to Size the Reactor Used to determine the reactor dynamics Reaction Kinetics

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

CSTR – complete backmixing Used to Size the Reactor Outlet Conversion is used for flow sheet mass and heat balances

Catalytic Reactors Various Mechanisms depending on rate limiting step Surface Reaction Limiting Surface Adsorption Limiting Surface Desorption Limiting Combinations –Langmuir-Hinschelwood Mechanism (SR Limiting) H 2 + C 7 H 8 (T)  CH 4 + C 6 H 6 (B)

Enzyme Catalysis Enzyme Kinetics S= substrate (reactant) E= Enzyme (catalyst)

Problems Managing Heat effects Optimization –Make the most product from the least reactant

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

Optimization of Desired Product Reaction Networks –Maximize yield, moles of product formed per mole of reactant consumed –Maximize Selectivity Number of moles of desired product formed per mole of undesirable product formed –Maximum Attainable Region – see discussion in Chap’t. 6. Reactors and bypass Reactor sequences