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:
PR
 tc
SE  0.6 P
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.
t
–
•
•
•
•
•
•
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 Used in early stages of design
• Kinetic model for a PFR
• Custom-made models (UDF)
Kinetic Reactors - CSTR & PFR
• Used to Size the Reactor
• Used to determine the reactor dynamics
• Reaction Kinetics
 rj  
dC j
dt
C
 k (T ) Ci i
i 1
 EA
k (T )  ko exp(
/)
RT
PFR – no backmixing
• Used to Size the Reactor
Xk
dX
V  Fko 
 rk
0
• Space Time = Vol./Q
• Outlet Conversion is used for flow sheet
mass and heat balances
CSTR – complete backmixing
• Used to Size the Reactor
Fko X k
V
 rk
• Outlet Conversion is used for flow sheet
mass and heat balances
Review : Catalytic Reactors – Brief Introduction
Major Steps
B
A
Bulk Fluid
CAb
7 . Diffusion of products
from pore mouth to
bulk
1. External Diffusion
Rate = kC(CAb – CAS)
External Surface
of Catalyst Pellet
CAs
6 . Diffusion of products
from interior to pore
mouth
2. Defined by an
Effectiveness Factor
Internal Surface
of Catalyst Pellet
3. Surface Adsorption
A + S <-> A.S
Catalyst
Surface
A B
4. Surface Reaction
5. Surface Desorption
B. S <-> B + S
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)
rT  
k pT p H 2
1  1.39 p B  1.04 pT
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
rs  
k1k3CH 2OCE CS
k1CS  k 2  k3CH 2O
• S= substrate (reactant)
• E= Enzyme (catalyst)
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 ReactorTemperature Effects
• Single Equilibrium
• aA +bB  rR + sS
o
  Grxn

aRr aSs
K eq  a a  exp
,
a A aB
 RT 
Van’t Hoff eq.
o
 d ln K eq  H rxn

 
2
dT
RT


– 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
Overview of CRE – Aspects related to Process Design
Le Chatelier’s Principle
1. Levenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd ed.
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
Adiabatic
Cooling
Heat Balance over Reactor
Q = UA ΔTlm
Adiabatic
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
 rj  
dC j
dt
C
 k (T ) Cii
i 1
  EA 
k (T )  ko exp
 RT 
PFR – no backmixing
• Used to Size the Reactor
Xk
dX
V  Fko 
 rk
0
• Space Time = Vol./Q
• Outlet Conversion is used for flow sheet
mass and heat balances
CSTR – complete backmixing
• Used to Size the Reactor
Fko X k
V
 rk
• Outlet Conversion is used for flow sheet
mass and heat balances
Unfavorable Equilibrium
• Increasing Temperature Increases the
Rate
• Equilibrium Limits Conversion
Various Reactors, Various
Reactions
X
dX
V  Fko 
 rk
0
k
Fko X k
V
 rk
Reactor with Heating or Cooling
Q = UA ΔT
Temperature Profiles in a
Reactor
Exothermic Reaction
Recycle
Best Temperature Path
Optimum Inlet Temperature
Exothermic Rxn
Managing Heat Effects
• Reaction Run Away
– 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
– 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
• Ethylene Oxide Synthesis
• CH2=CH2 + 3O22CO2 + 2H2O
O
• CH2=CH2 + O2CH2-CH2(desired)
Examples
• Diethanolamine Synthesis
/
O\
CH 2  CH 2  NH 3  HOCH2CH 2 NH 2
/
O\
CH 2  CH 2  HOCH2CH 2 NH 2  ( HOCH2CH 2 ) 2 NH (desired)
/
O\
CH 2  CH 2  ( HOCH2CH 2 ) 2 NH  ( HOCH2CH 2 )3 N
Examples
• Butadiene Synthesis, C4H6, from Ethanol
C2 H 5OH  C2 H 4  H 2O
C2 H 5OH  CH 3CHO  H 2
C2 H 4  CH 3CHO  C4 H 6  H 2O
Rate Selectivity
• Parallel Reactions
– A+BR (desired)
– A+BS
SD/U 
• Rate Selectivity
rD
rU
 k D  ( D U ) (  D  U )
  CA
CB
kU 
• (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
– EA-D < EA-U
T
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
Fractional Yield
1
CH 3CH 2OH ( g )  O2  CH 3CHO  H 2O
2
5
CH 3CHO  O2  2CO2  2 H 2O
2
(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 form
lnK=A+B/T+CT+DT2
• ProMax
• Reactor type and
Kinetics must match!!
• Kinetics
– Selectable units
• Equilibrium constants