Reactor Design
S,S&L Chapter 7Terry 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)• tc=corrosion allowance = 0.125 in.
ctPSEPRt
6.0
Reactor Cost
• Pressure Vessel – Material of Construction gives ρmetal
– Mass of vessel = ρmetal (VC+2VHead)• Vc = πDL
• VHead – from tables that are based upon D
– Cp= FMCv(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
/)exp()(
)(1
RTEkTk
CTkdtdC
r
Ao
C
ii
jj
i
PFR – no backmixing
• Used to Size the Reactor
• Space Time = Vol./Q• Outlet Conversion is used for flow sheet
mass and heat balances
kX
kko r
dXFV0
CSTR – complete backmixing
• Used to Size the Reactor
• Outlet Conversion is used for flow sheet mass and heat balances
k
kko
rXFV
Review : Catalytic Reactors – Brief Introduction
Major Steps
A B
Bulk Fluid
External Surfaceof Catalyst Pellet
Catalyst Surface
Internal Surfaceof Catalyst Pellet
CAb
CAs
2. Defined by an Effectiveness Factor
1. External DiffusionRate = kC(CAb – CAS)
3. Surface AdsorptionA + S <-> A.S
4. Surface Reaction5. Surface DesorptionB. 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)
• H2 + C7H8 (T) CH4 + C6H6(B)
TB
HTT pp
ppkr
04.139.112
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?
Enzyme Catalysis
• Enzyme Kinetics
• S= substrate (reactant)• E= Enzyme (catalyst)
OHS
SEOHs CkkCk
CCCkkr
2
2
321
31
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
Temperature Effects
• On Equilibrium• On Kinetics
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-Pi
s) /RT]– γi = activity coefficient of i – Vi =Partial Molar Volume of i
2
ln,exp
RTH
dTKd
RTG
aaaaK
orxneq
orxn
aB
aA
sS
rR
eq
Van’t Hoff eq.
Overview of CRE – Aspects related to Process Design
1. Levenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd 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 , 3rd ed.
Feed Temperature, ΔHrxn
Heat Balance over ReactorCooling
Adiabatic
Adiabatic
Q = UA ΔTlm
Reactor with Heating or Cooling
Q = UA ΔT
Kinetic Reactors - CSTR & PFR – Temperature Effects
• Used to Size the Reactor• Used to determine the reactor dynamics• Reaction Kinetics
RTEkTk
CTkdtdC
r
Ao
C
ii
jj
i
exp)(
)(1
PFR – no backmixing
• Used to Size the Reactor
• Space Time = Vol./Q• Outlet Conversion is used for flow sheet
mass and heat balances
kX
kko r
dXFV0
CSTR – complete backmixing
• Used to Size the Reactor
• Outlet Conversion is used for flow sheet mass and heat balances
k
kko
rXFV
Unfavorable Equilibrium
• Increasing Temperature Increases the Rate• Equilibrium Limits Conversion
Various Reactors, Various Reactions
kX
kko r
dXFV0
k
kko
rXFV
Reactor with Heating or Cooling
Q = UA ΔT
Temperature Profiles in a Reactor
Exothermic Reaction
Recycle
Best Temperature Path
Optimum Inlet TemperatureExothermic Rxn
Managing Heat Effects
• Reaction Run Away– Exothermic
• Reaction Dies– Endothermic
• Preventing Explosions• Preventing Stalling
Inter-stage Cooler
Exothermic Equilibria
Lowers Temp.
Inter-stage Cold Feed
Exothermic Equilibria
Lowers TempLowers Conversion
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+BR (desired)– AS
• Series Reactions– ABC(desired)D
• Independent Reactions– AB (desired)– CD+E
• Series Parallel Reactions– A+BC+D– A+CE(desired)
• Mixing, Temperature and Pressure Effects
Examples
• Ethylene Oxide Synthesis• CH2=CH2 + 3O22CO2 + 2H2O
• CH2=CH2 + O2CH2-CH2(desired)
O
Examples
• Diethanolamine Synthesis
NCHHOCHNHCHHOCHCHCH
OdesiredNHCHHOCHNHCHHOCHCHCH
ONHCHHOCHNHCHCH
O
32222222
\/
22222222
\/
222322
\/
)()(
)()(
Examples
• Butadiene Synthesis, C4H6, from Ethanol
OHHCCHOCHHCHCHOCHOHHCOHHCOHHC
264342
2352
24252
Rate Selectivity
• Parallel Reactions– A+BR (desired)– A+BS
• 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
)()(A
U
Drr
D/UD
U
D CkkS UDU
BC
Reactor Design to Maximize Desired Product for Parallel Rxns.
Maximize Desired Product
• Series Reactions– AB(desired)CD
• Plug Flow Reactor• Optimum Time in Reactor
Fractional Yield
OHCOOCHOCH
OHCHOCHOgOHCHCH
2223
23223
2225
21)(
(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 with interstage
Cooling/Heating
A few words about simulators
• Aspen• Kinetics
– Must put in with “Aspen Units”
• Equilibrium constants– Must put in in the formlnK=A+B/T+CT+DT2
• ProMax• Reactor type and
Kinetics must match!!• Kinetics
– Selectable units• Equilibrium constants