13-L1-L2-Reactor Design for engineering student

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)
• tc=corrosion allowance = 0.125 in.
c
t
P
SE
PR
t 


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
RT
E
k
T
k
C
T
k
dt
dC
r
A
o
C
i
i
j
j
i





 



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

CSTR – complete backmixing
• Outlet Conversion is used for flow sheet
mass and heat balances
k
k
ko
r
X
F
V



Review : Catalytic Reactors – Brief Introduction
Major Steps
A  B
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

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
+ C7
H8
(T) CH4
+ C6
H6
(B)
T
B
H
T
T
p
p
p
p
k
r
04
.
1
39
.
1
1
2





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)
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





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
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.

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 Reactor
Cooling
Adiabatic
Adiabatic
Q = UA ΔTlm

Reactor with Heating or Cooling
Q = UA ΔT

Kinetic Reactors - CSTR & PFR –
Temperature Effects
• 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


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

Temperature Profiles in a
Reactor
Exothermic Reaction
Recycle

Optimum Inlet Temperature
Exothermic Rxn

Inter-stage Cooler
Exothermic Equilibria
Lowers Temp.

Inter-stage Cold Feed
Exothermic Equilibria
Lowers Temp
Lowers 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+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
• CH2=CH2 + O2CH2-CH2(desired)
O

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

/
)
(
)
(
)
(
)
(










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








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 


 










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
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)

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

13-L1-L2-Reactor Design for engineering student

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