This document discusses reactor design and catalytic reactions. It begins by describing different reactor types including ideal plug flow reactors (PFRs) and continuous stirred tank reactors (CSTRs), as well as real reactors with complex geometries. It then discusses factors that influence reactor cost such as vessel material and pressure requirements. The document also covers topics like temperature effects on kinetics and equilibrium, heat transfer considerations, and optimization of desired product formation through various reactor configurations and sequences.

1. Reactor Design
S,S&L Chapter 7
Terry A. Ring
ChE

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

3. Reactor Cost
• Reactor is
– PRF
• Pressure vessel
– CSTR
• Storage tank with mixer
• Pressure vessel
– Hydrostatic head gives the pressure to design for

4. 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.
c
t
P
SE
PR
t 


6
.
0

5. 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)

6. 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

7. Kinetic Reactors - CSTR & PFR
• Used to Size the Reactor
• Used to determine the reactor dynamics
• Reaction Kinetics
/)
exp(
)
(
)
(
1
RT
E
k
T
k
C
T
k
dt
dC
r
A
o
C
i
i
j
j
i





 



8. PFR – no backmixing
• Used to Size the Reactor
• Space Time = Vol./Q
• Outlet Conversion is used for flow sheet
mass and heat balances
 

k
X
k
ko
r
dX
F
V
0

9. CSTR – complete backmixing
• Used to Size the Reactor
• Outlet Conversion is used for flow sheet
mass and heat balances
k
k
ko
r
X
F
V



10. Review : Catalytic Reactors – Brief Introduction
Major Steps
A  B
A
Bulk Fluid
External Surface
of Catalyst Pellet
Catalyst
Surface
Internal Surface
of Catalyst Pellet
CAb
CAs
2. Defined by an
Effectiveness Factor
1. External Diffusion
Rate = kC(CAb – CAS)
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

11. 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)
T
B
H
T
T
p
p
p
p
k
r
04
.
1
39
.
1
1
2





12. 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?

13. Enzyme Catalysis
• Enzyme Kinetics
• S= substrate (reactant)
• E= Enzyme (catalyst)
O
H
S
S
E
O
H
s
C
k
k
C
k
C
C
C
k
k
r
2
2
3
2
1
3
1





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

15. 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

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

17. Temperature Effects
• On Equilibrium
• On Kinetics

18. 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
RT
H
dT
K
d
RT
G
a
a
a
a
K
o
rxn
eq
o
rxn
a
B
a
A
s
S
r
R
eq















 



Van’t Hoff eq.

19. 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

20. Unfavorable Equilibrium
• Increasing Temperature Increases the
Rate
• Equilibrium Limits Conversion

21. Overview of CRE – Aspects related to Process Design
1. Levenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd ed.

22. Feed Temperature, ΔHrxn
Heat Balance over Reactor
Cooling
Adiabatic
Adiabatic
Q = UAΔTlm

23. Reactor with Heating or Cooling
Q = UAΔT

24. Kinetic Reactors - CSTR & PFR –
Temperature Effects
• Used to Size the Reactor
• Used to determine the reactor dynamics
• Reaction Kinetics










 

RT
E
k
T
k
C
T
k
dt
dC
r
A
o
C
i
i
j
j
i
exp
)
(
)
(
1


25. PFR – no backmixing
• Used to Size the Reactor
• Space Time = Vol./Q
• Outlet Conversion is used for flow sheet
mass and heat balances
 

k
X
k
ko
r
dX
F
V
0

26. CSTR – complete backmixing
• Used to Size the Reactor
• Outlet Conversion is used for flow sheet
mass and heat balances
k
k
ko
r
X
F
V



27. Unfavorable Equilibrium
• Increasing Temperature Increases the
Rate
• Equilibrium Limits Conversion

28. Various Reactors, Various
Reactions
 

k
X
k
ko
r
dX
F
V
0
k
k
ko
r
X
F
V



29. Reactor with Heating or Cooling
Q = UAΔT

30. Temperature Profiles in a
Reactor
Exothermic Reaction
Recycle

31. Best Temperature Path

32. Optimum Inlet Temperature
Exothermic Rxn

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

34. Inter-stage Cooler
Exothermic Equilibria
Lowers Temp.

35. Inter-stage Cold Feed
Exothermic Equilibria
Lowers Temp
Lowers Conversion

36. 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

37. Reactor Design for Selective
Product Distribution
S,S&L Chapt. 7

38. 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

39. Examples
• Ethylene Oxide Synthesis
• CH2=CH2 + 3O22CO2 + 2H2O
• CH2=CH2 + O2CH2-CH2(desired)
O

40. Examples
• Diethanolamine Synthesis
N
CH
HOCH
NH
CH
HOCH
CH
CH
O
desired
NH
CH
HOCH
NH
CH
HOCH
CH
CH
O
NH
CH
HOCH
NH
CH
CH
O
3
2
2
2
2
2
2
2
/
2
2
2
2
2
2
2
2
/
2
2
2
3
2
2
/
)
(
)
(
)
(
)
(










41. Examples
• Butadiene Synthesis, C4H6, from Ethanol
O
H
H
C
CHO
CH
H
C
H
CHO
CH
OH
H
C
O
H
H
C
OH
H
C
2
6
4
3
4
2
2
3
5
2
2
4
2
5
2








42. Rate Selectivity
• Parallel Reactions
– 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
)
(
)
(
A
U
D
r
r
D/U
D
U
D
C
k
k
S U
D
U
B
C 


 










43. Reactor Design to Maximize
Desired Product for Parallel Rxns.

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

45. Fractional Yield
O
H
CO
O
CHO
CH
O
H
CHO
CH
O
g
OH
CH
CH
2
2
2
3
2
3
2
2
3
2
2
2
5
2
1
)
(






(k2/k1)=f(T)

46. 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

47. 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

48. 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