Presentation on theme: "Reactor Design S,S&L Chapter 7 Terry A. Ring ChE."— Presentation transcript:
Reactor Design S,S&L Chapter 7 Terry A. Ring ChE
Reactor Types Ideal –PFR –CSTR Real –Unique design geometries and therefore RTD –Multiphase –Various regimes of momentum, mass and heat transfer
Reactor Cost Reactor is –PRF Pressure vessel –CSTR Storage tank with mixer Pressure vessel –Hydrostatic head gives the pressure to design for
Reactor Cost PFR –Reactor Volume (various L and D) from reactor kinetics –hoop-stress formula for wall thickness: – t= vessel wall thickness, in. P= design pressure difference between inside and outside of vessel, psig R= inside radius of steel vessel, in. S= maximum allowable stress for the steel. E= joint efficiency (≈0.9) t c =corrosion allowance = 0.125 in.
Reactor Cost Pressure Vessel – Material of Construction gives ρ metal –Mass of vessel = ρ metal (V C +2V Head ) V c = πDL V Head – from tables that are based upon D –C p = F M C v (W)
Reactors in Process Simulators Stoichiometric Model –Specify reactant conversion and extents of reaction for one or more reactions Two Models 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
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
Review : Catalytic Reactors – Brief Introduction Major Steps A B A Bulk Fluid External Surface of Catalyst Pellet Catalyst Surface Internal Surface of Catalyst Pellet C Ab C As 2. Defined by an Effectiveness Factor 1.External Diffusion Rate = k C (C Ab – C AS ) 3. Surface Adsorption A + S A.S 4. Surface Reaction 5. Surface Desorption B. S B + S 6. Diffusion of products from interior to pore mouth B 7. Diffusion of products from pore mouth to bulk
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)
Catalytic Reactors – Implications on design 1.What effects do the particle diameter and the fluid velocity above the catalyst surface play? 2.What is the effect of particle diameter on pore diffusion ? 3.How the surface adsorption and surface desorption influence the rate law? 4.Whether the surface reaction occurs by a single-site/dual –site / reaction between adsorbed molecule and molecular gas? 5.How does the reaction heat generated get dissipated by reactor design?
Problems Managing Heat effects Optimization –Make the most product from the least reactant
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. 7. Reactors (pfrs &cstrs in series) and bypass Reactor sequences –Which come first
Managing Heat Effects Reaction Run Away –Exothermic Reaction Dies –Endothermic Preventing Explosions Preventing Stalling
Equilibrium Reactor- Temperature Effects 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.
Overview of CRE – Aspects related to Process Design 1.Levenspiel, O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons, 3 rd ed. Le Chatelier’s Principle
Unfavorable Equilibrium Increasing Temperature Increases the Rate Equilibrium Limits Conversion
Overview of CRE – Aspects related to Process Design 1.Levenspiel, O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons, 3 rd ed.
Feed Temperature, ΔH rxn Heat Balance over Reactor Cooling Adiabatic Q = UA ΔT lm
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
Reactor Design for Selective Product Distribution S,S&L Chapt. 7
Overview Parallel 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 Butadiene Synthesis, C 4 H 6, from Ethanol
Rate Selectivity Parallel Reactions –A+B R (desired) –A+B S Rate Selectivity (α D - α U ) >1 make C A as large as possible (β D –β U )>1 make C B as large as possible (k D /k U )= (k oD /k oU )exp[-(E A-D -E A-U )/(RT)] –E A-D > E A-U T –E A-D < E A-U T
Reactor Design to Maximize Desired Product for Parallel Rxns.
Maximize Desired Product Series Reactions –A B(desired) C D Plug Flow Reactor Optimum Time in Reactor
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 with interstage Cooling/Heating
A few words about simulators Aspen Kinetics –Must put in with “Aspen Units” Equilibrium constants –Must put in in the form lnK=A+B/T+CT+DT 2 ProMax Reactor type and Kinetics must match!! Kinetics –Selectable units Equilibrium constants