lecture 17 - university of michigan · web lecture 17 class lecture 22–thursday 4/4/2013...
TRANSCRIPT
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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
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Web Lecture 17
Class Lecture 22–Thursday 4/4/2013
Introduction to Catalysts and Catalysis
Interstage cooling
Noble Prize 2007
Catalytic steps
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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.
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Catalysts and Catalysis
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Different reaction paths
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Catalysts affect both selectivity and yield
Catalysts and Catalysis
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Different shapes and sizes of catalyst.
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Catalysts and Catalysis
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Catalytic packed-bed reactor, schematic.
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Catalysts and Catalysis
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Steps in a Catalytic Reaction
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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.
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Active Sites
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Active Sites – Ethylidyne on Platinum
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Vacant and occupied sites
For the system shown, the total concentration of sites is
Ct = Cv + CA.S + CB.S
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The Adsorption Step
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SASA
][atm /k
/ k-
1-
A
A-
AA
ASAVAASAvAAAD
kK
KCCPkCCPkr
VAAASAAD
VAAASAD
CPkCkr
CPkCr
0/
0 :mequilibriu @
) 1( AAVVAAVSAVt PKCCPKCCCC
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The Adsorption Step
𝐶𝑉 =𝐶𝑡
1 + 𝐾𝐴𝑃𝐴
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Langmuir Adsorption Isotherm
𝐶𝑉 =𝐶𝑡
1 + 𝐾𝐴𝑃𝐴
𝐶𝐴∙𝑆 = 𝐾𝐴𝑃𝐴𝐶𝑉
𝐶𝐴∙𝑆 = 𝐾𝐴𝑃𝐴
1+𝐾𝐴𝑃𝐴𝐶𝑡
𝐶𝐴∙𝑆𝐶𝑡
= 𝐾𝐴𝑃𝐴
1 + 𝐾𝐴𝑃𝐴
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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
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The Surface Reaction Step
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The Surface Reaction Step
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The Surface Reaction Step
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The Surface Reaction Step
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The Surface Reaction Step
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Steps in a Catalytic Reaction
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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
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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
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Electrical analog to heterogeneous reactions
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The Rate Limiting Step:
Which step has the largest resistance?
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Collecting information for catalytic reactor design
Collecting and Analyzing Data
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Collecting and Analyzing Data
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Normal Pentane Octane Number = 62
Iso-Pentane Octane Number = 95
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Catalytic Reformers
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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
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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
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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
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SSISSN
Catalytic Reformers
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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
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SNSN
SISI
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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
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Catalytic Reformers
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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
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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
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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
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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
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Neglect
KN2PN2
r NO kPNOPCO
1KNOPNO KCOPCO 2
r NO kPNOPCO
1KNOPNO KCOPCO KN2PN2
2
Catalytic Conversion of Exhaust Gas
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Find optimum partial pressure of CO
r NO kPNOPCO
1KNOPNO KCOPCO 2
d rNO dPCO
0
PCO 1KNOPNO
KCO
Catalytic Conversion of Exhaust Gas
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End of Web Lecture 17
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