1 - 27/09/2012 Department of Chemical Engineering Lecture 11 Kjemisk reaksjonsteknikk Chemical Reaction Engineering Review of previous lectures Kinetic data analysis of heterogeneous reactions 1. Characterization of Pt catalysts 2. Kinetic study, find a kinetic expression and kinetic parameter 3. Catalytic reactor design
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1 - 27/09/2012
Departm
ent of Chem
ical Engineering
Lecture 11Kjemisk
reaksjonsteknikk
Chemical Reaction Engineering
Review of previous lectures
Kinetic data analysis of heterogeneous reactions
1.
Characterization of Pt catalysts2.
Kinetic study, find a kinetic expression and kinetic parameter3.
Catalytic reactor design
2 - 27/09/2012
Departm
ent of Chem
ical Engineering
7-Step Procedure for CRE Data analyses (1)
1. Postulate a rate law
A. Power law models fro homogeneous reactions
B. Langmuir-Hinshelwood models for heterogeneous reactions
2. Select reactor type and corresponding mole balance A.If batch reactor, use mole balance on Reactant A
B.If differential PBR, use mole balance on product P (A →P)
BAA CkCr
2)1( BBAA
BAA PKPK
PkPr
dtdCr A
A
WC
WFr PP
A
0
dtdCr A
A
WC
WFr PP
A
0
1. Postulate a rate law
A. Power law models for homogeneous reactions
B. Langmuir-Hinshelwood models for heterogeneous reactions
2. Select reactor type and corresponding mole balance A.If batch reactor, use mole balance on Reactant A
B.If differential PBR, use mole balance on product P (A →P)
WC
WFr PP
A
0
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Departm
ent of Chem
ical Engineering
7-Step Procedure for CRE Data analyses (2)
3. Process your data in terms of the measured variables (e.g. NA
, CA
, or PA
). If necessary, rewrite your mole balance in terms of your measured variables
4. Look for simplification For example, if one of the reactants is in excess, assume its concentration is constant,. If the gas phase mole fraction of reactant A is small, set ε=0
5. For a batch reactor, calculate –rA
as a function of concentration CA
to determine the reaction order:A.Differential analysisB.Integral methodC.Nonlinear regression
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Departm
ent of Chem
ical Engineering
7-Step Procedure for CRE Data analyses (3)
6.
For differential PBR, calculate –rA
as a function of CA
or PA
A. Calculate as a function of reactant concentration CA
or partial pressure PA
B. Choose a model, e.g.,
C. Use nonlinear regression to find the best model and model
parameters
7. Analyze your rate law model for “goodness of fit”. Calculate a correlation coefficient.
WC
WFr PP
A
0
AA
AA PK
kPr
1
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Departm
ent of Chem
ical Engineering
Example: Scaling up of toluene hydrogen process
Hydrogen and toluene are reacted over a Pt catalysts supported on crystalline silica-alumina to form methane and benzene.
C3
H5
CH3
+H2
→
C6
H6
+ CH4
We wish to design a packed bed reactor to process a feed consisting of 20% toluene and 80 % hydrogen. Toluene is fed at a rate of 50 mol/min at a temperature of 640 oC
and
a pressure of 40 atm.
Step 1, Characterization of Pt catalystsStep 2, Kinetic study, find a kinetic expression and kinetic
parameterStep 3, Catalytic reactor design
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Departm
ent of Chem
ical Engineering
Determination of active site numbers
The concentration of active sites on the catalyst surface is typically determined by H2
or CO chemisorption.
The chemisorption
of H2
at 298 K on Pt catalysts 1) determine Langmuir-isotherm. 2) determine the Pt surface area or dispersion
0
0.5
1
1.5
2
2.5
0 50 100 150 200 250
P (mmHg)
H2
ads (
CM
3 STP
)
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Departm
ent of Chem
ical Engineering
Associative adsorption
2
2
1 H
HA KP
KP
2
111
HV
t
KPCC
** 22 HH ka
2*2 HDHa kPk
1* A
Linearization of Langmuir-isotherm
0
5
10
15
20
25
0 0.02 0.04 0.06 0.08 0.1 0.12
1/P (mmHP-1)
1/C
v (g/
mm
mol
)
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Departm
ent of Chem
ical Engineering
Dissociative adsorption
2/12
2/12
)(1)(
H
HH KP
KP
2/12 )(
111
HV
t
H KPCC
*2*22 HH ka
22*2 HDHa kPk
1* A
Linearization of Langmuir-isotherm
0
5
10
15
20
25
0 0.1 0.2 0.3 0.4
1/P0.5 (mmHP-1)
1/C
v (g/
mm
mol
)1/CT
slop=1/(CT
K0.5)
CT
=0.137 mmol/gcat
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Departm
ent of Chem
ical Engineering
Kinetic Modeling and Analysis
1) Select a proper reactor for kinetic study
2) Perform kinetic study and design kinetic experiments
3) Developing an algebraic rate expression consistent with experimental observations
2) Analyzing the rate expression in a such manner that the rate expression parameters can readily be determined from experimental data
3) Find a mechanism and rate determining step consistent with the experimental data (for
non-elementary reactions)
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Departm
ent of Chem
ical Engineering
We select differential fixed bed reactor for the kinetic study
viscosity of gas passing through the bed kg/m.sZ length down the packed bed mu, superficial velocity m/sρ
gas density
kg/m3
G= ρu =superficial mass velocity kg/m2,s
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Departm
ent of Chem
ical Engineering
TURBULENT
LAMINAR
p3
pc
G75.1D11501
DgG
dzdPErgun Equation:
Pressure Drop in Packed Bed Reactors
22 0
00 T
TPP)X1(
0
0
0T
T0 T
TPP
FF
00
00
0mm Constant mass flow:
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Departm
ent of Chem
ical Engineering
0T
T
0
0
p3
pc0 FF
TT
PPG75.1
D11501
DgG
dzdP
T
0T0
00 F
FTT
PP
Variable Density
G75.1
D11501
DgG
p3
pc00Let
Pressure Drop in Packed Bed Reactors
23
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Departm
ent of Chem
ical Engineering
0T
T
0
0
cc
0
FF
TT
PP
1AdWdP
ccbc 1zAzAW Catalyst Weight
0cc
0
P1
1A2
Let
Pressure Drop in Packed Bed Reactors
24
b bulk density c solid catalyst density porosity (a.k .a., void fraction )
Where
Ac , cross section area , z length of the reactor
25 - 27/09/2012
Departm
ent of Chem
ical Engineering
We will use this form for single reactions:
X1
TT
PP1
2dWPPd
00
0
0T
T
0 FF
TT
y2dWdy
0P
Py
X1TT
y2dWdy
0
X1y2dW
dy
Isothermal
case
Pressure Drop in Packed Bed Reactors
25
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Departm
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ical Engineering
The two expressions are coupled ordinary differential equations. We can only solve them simultaneously using an ODE solver such as Polymath. For the special case of isothermal operation and epsilon ε= 0, we can obtain an analytical solution.
Polymath will combine the mole balance, rate law and stoichiometry.