Reactor Design for Selective Product Distribution Sieder, et.al. Chapter 15 Terry A. Ring Chemical Engineering University of Utah

Onion Model of Process Design

Overview Parallel Reactions Series Reactions Independent Reactions A+BR (desired) AS Series Reactions ABC(desired)D Independent Reactions AB (desired) CD+E Series-Parallel Reactions A+BC+D A+CE(desired) Mixing, Temperature and Pressure Effects

Examples Ethylene Oxide Synthesis CH2=CH2 + O22CO2 + 2H2O CH2=CH2 + O2CH2-CH2(desired) O Parallel

Examples Diethanolamine Synthesis Series-parallel

Examples Butadiene Synthesis, C4H6, from Ethanol Series-parallel

Rate Selectivity Parallel Reactions Rate Selectivity A+BR (desired) A+BS Rate Selectivity (αD- αU) >1 make CA as large as possible (βD –βU)>1 make CB as large as possible (kD/kU)= (koD/koU)exp[-(EA-D-EA-U)/(RT)] EA-D > EA-U T EA-D < EA-U T

Reactor Design to Maximize Desired Product

Maximize Desired Product Series Reactions AB(desired)CD Plug Flow Reactor Optimum Time in Reactor

Fractional Yield (k2/k1)=f(T)

Real Reaction Systems More complicated than either Series Reactions Parallel Reactions Effects of equilibrium must be considered Confounding heat effects All have Reactor Design Implications

Engineering Tricks Reactor types Multiple Reactors Mixtures of Reactors Bypass Recycle after Separation Split Feed Points/ Multiple Feed Points Diluents Temperature Management

Reactor Heat Effects Sieder Chapter 15 Terry A. Ring Chemical Engineering University of Utah

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

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

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.

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

Equilibrium and Kinetic Limits Increasing Temperature Increases the Rate Equilibrium Limits Conversion

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

Temperature Profiles in a Reactor Exothermic Reaction

Reactor with Heating or Cooling Q = UA ΔT Reactor is a Shell and Tube (filled with catalyst) HX

Best Temperature Path

Optimum Inlet Temperature Exothermic Rxn CSTR PFR

Various Reactors, Various Reactions

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

Reactor with Heating or Cooling Q = UA ΔT

Inerts Addition Effect

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

Inter-stage Cooler Lowers Temp. Exothermic Equilibria

Inter-stage Cold Feed Lowers Temp Lowers Conversion Exothermic Equilibria

Optimization of Desired Product Reaction Networks Heuristic 7 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 SS&L. Reactors and bypass Reactor sequences

Reactor Problem on Design I Final Exam

Feed Temperature, ΔHrxn Adiabatic Adiabatic Cooling Heat Balance over Reactor Q = UA ΔTlm

Aspen Kinetics

Aspen Units on Rate When Rate Basis is Cat (wt), substitute sec–kg catalyst for sec·m3 in each expression above. For either rate basis, the reactor volume or catalyst weight used is determined by the reactor where the reaction occurs.

HW 2 Kinetics Paper’s Rate expression P = atm (assumes 1 atm) mcat =mass of catalyst rj = Aj exp(-Ejq) a = activity of catalyst (dimensionless) Aj = atm/s/gmcat Ej=39.9 kJ/mole

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