lecture 17 - university of michigan · web lecture 17 class lecture 22–thursday 4/4/2013...
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Chemical Reaction Engineering (CRE) is the field that studies the rates and mechanisms of
chemical reactions and the design of the reactors in which they take place.
Lecture 17
Web Lecture 17
Class Lecture 22–Thursday 4/4/2013
Introduction to Catalysts and Catalysis
Interstage cooling
Noble Prize 2007
Catalytic steps
2
A Catalyst is a substance that affects the rate of
chemical reaction but emerges from the process
unchanged.
Catalysis is the occurrence, study, and use of
catalysts and catalytic processes.
Approximately 1/3 of the GNP of materials
produced in the U.S. involves a catalytic process.
3
Catalysts and Catalysis
Different reaction paths
4
Catalysts affect both selectivity and yield
Catalysts and Catalysis
Different shapes and sizes of catalyst.
5
Catalysts and Catalysis
Catalytic packed-bed reactor, schematic.
6
Catalysts and Catalysis
7
Steps in a Catalytic Reaction
Reactions are not catalyzed over the entire
surface but only at certain active sites or centers
that result from unsaturated atoms in the surface.
An active site is a point on the surface that can
form strong chemical bonds with an adsorbed
atom or molecule.
8
Active Sites
Active Sites – Ethylidyne on Platinum
Vacant and occupied sites
For the system shown, the total concentration of sites is
Ct = Cv + CA.S + CB.S
10
The Adsorption Step
SASA
][atm /k
/ k-
1-
A
A-
AA
ASAVAASAvAAAD
kK
KCCPkCCPkr
VAAASAAD
VAAASAD
CPkCkr
CPkCr
0/
0 :mequilibriu @
) 1( AAVVAAVSAVt PKCCPKCCCC
11
The Adsorption Step
𝐶𝑉 =𝐶𝑡
1 + 𝐾𝐴𝑃𝐴
12
Langmuir Adsorption Isotherm
𝐶𝑉 =𝐶𝑡
1 + 𝐾𝐴𝑃𝐴
𝐶𝐴∙𝑆 = 𝐾𝐴𝑃𝐴𝐶𝑉
𝐶𝐴∙𝑆 = 𝐾𝐴𝑃𝐴
1+𝐾𝐴𝑃𝐴𝐶𝑡
𝐶𝐴∙𝑆𝐶𝑡
= 𝐾𝐴𝑃𝐴
1 + 𝐾𝐴𝑃𝐴
Langmuir Adsorption
Isotherm
AP
T
SA
C
C Increasing T
Slope=kA
13
AA
AA
t
SA
PK
PK
C
C
1
Langmuir Adsorption Isotherm
14
The Surface Reaction Step
The Surface Reaction Step
15
The Surface Reaction Step
16
The Surface Reaction Step
17
The Surface Reaction Step
18
19
Steps in a Catalytic Reaction
A BC
CS CS
rDC kD CCS PCC
KDC
rDC rADC
KDC 1
KC
rDC kD CCS KCPCC
(10-20)
(10-21)
Desorption from the Surface for the Reaction
21
Adsorption
Surface Reaction
Desorption
Which step is the Rate Limiting Step (RLS)?
SASA
A
k
SAvAAdAdA
CCPkrr
SBSA
SBSB
C
k
SBSASSA
CCkrr
BBBSBDDA CPkCkrr
Steps in a Single-Site Catalytic Reactor
Electrical analog to heterogeneous reactions
22
The Rate Limiting Step:
Which step has the largest resistance?
Collecting information for catalytic reactor design
Collecting and Analyzing Data
23
24
Collecting and Analyzing Data
Normal Pentane Octane Number = 62
Iso-Pentane Octane Number = 95
25
Catalytic Reformers
n-pentane i-pentane 0.75 wt% Pt
Al2O3
n-pentene i-pentene Al2O3
N I
n-pentane n-pentene -H2
Pt
Al2O3 i-pentene
+H2
Pt
i-pentane
Catalytic Reformers
26
Isomerization of n-pentene (N) to i-pentene (I) over alumina
N I Al2O3
1. Select a mechanism (Mechanism Single Site)
Adsorption on Surface: SNSN
Surface Reaction: SISN
Desorption: SISI
Treat each reaction step as an elementary reaction when writing rate laws.
Catalytic Reformers
27
2. Assume a rate-limiting step. Choose the surface reaction first, since more than 75%
of all heterogenous reactions that are not diffusion-
limited are surface-reaction-limited. The rate law for the
surface reaction step is:
S
SI
SNSS
'
INK
CCkrrr
28
SSISSN
Catalytic Reformers
3. Find the expression for the concentrations of
the adsorbed species CN.S and CI.S. Use the other steps that are not limiting to
solve for CN.S and CI.S. For this reaction:
CKPC NNSN:0k
r
A
AD From
CPKK
CPC II
D
ISI:0
k
r
D
D From
Catalytic Reformers
29
SNSN
SISI
4. Write a Site Balance.
SISNt CCCC
5. Derive the rate law. Combine steps 2, 3 and 4 to
arrive at the rate law :
IINN
PINSN
IINN
PIN
k
NtsSN
PKPK
KPPkrr
PKPK
KPPKCkrr
1
1
30
Catalytic Reformers
CO NOCO2 1
2N2
1994 2004 2008
HC 0.41 0.125 0.10
CO 3.4 3.4 3.4
NO 0.4 0.4 0.14
Catalytic Conversion of Exhaust Gas
32
222 NNVS•N
2
VNN
2
S•NDD2
S•NOS•COSS2
VCOCOS•CO
CO
S•CO
VCOCOACO
VNONOS•NO
NO
S•NOVNONOANO
PKCCCPKCkrS2gNS•NS•N
CCkrSS•NCOS•NOS•CO
CPKCK
CCPkrS•COS•CO
CPKCK
CCPkrS•NOSNO
Catalytic Conversion of Exhaust Gas
33
rS kS CNO•SCCO•S
rS kSKNOKCOPNOPCOCV2
CT CV CNO•S CCO•S CN•S
CV CVKNOPNO CVKCOPCO CV KN2PN2
Catalytic Conversion of Exhaust Gas
34
2NNCOCONONO
CONO
NO
2
NNCOCONONO
CONO
k
2
tCONOS
SNO
NNCOCONONO
t
V
22
22
22
PKPKPK1
PkPr
PKPKPK1
PP CKKkrr
PKPKPK1
CC
Catalytic Conversion of Exhaust Gas
Neglect
KN2PN2
r NO kPNOPCO
1KNOPNO KCOPCO 2
r NO kPNOPCO
1KNOPNO KCOPCO KN2PN2
2
Catalytic Conversion of Exhaust Gas
Find optimum partial pressure of CO
r NO kPNOPCO
1KNOPNO KCOPCO 2
d rNO dPCO
0
PCO 1KNOPNO
KCO
Catalytic Conversion of Exhaust Gas
End of Web Lecture 17
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