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
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
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
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
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
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+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
D
r
r
D/U
D
U
D
C
k
k
S U
D
U
B
C
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
