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Lecture 17. 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. Web Lecture 17 Class Lecture 22–Thursday 4/4/2013. Introduction to Catalysts and Catalysis Interstage cooling - PowerPoint PPT Presentation
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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 17Class Lecture 22–Thursday 4/4/2013Introduction to Catalysts and Catalysis Interstage coolingNoble Prize 2007Catalytic 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
Different reaction paths
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Catalysts affect both selectivity and yield
Catalysts and Catalysis
Different shapes and sizes of catalyst.
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Catalysts and Catalysis
Catalytic packed-bed reactor, schematic.
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Catalysts and Catalysis
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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.
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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
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The Adsorption Step
SASA
][atm /k
/ k- 1-
A
A-
AA
ASAVAASAvAAAD
kK
KCCPkCCPkr
VAAASAAD
VAAASAD
CPkCkrCPkCr
0/ 0 :mequilibriu @
) 1( AAVVAAVSAVt PKCCPKCCCC
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The Adsorption Step
𝐶𝑉=𝐶𝑡
1+𝐾 𝐴𝑃 𝐴
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Langmuir Adsorption Isotherm
Langmuir Adsorption Isotherm
AP
T
SA
CC
Increasing T
Slope=kA
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AA
AA
t
SA
PKPK
CC
1
Langmuir Adsorption Isotherm
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The Surface Reaction Step
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
A B C
C S C S
rDC kD CCS PCCKDC
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
kSA
vAAdAdACCPkrr
SBSA
SBSB
C
k
SBSASSACCkrr
BBBSBDDA CPkCkrr
Steps in a Single-Site Catalytic Reactor
Electrical analog to heterogeneous reactions
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The Rate Limiting Step:Which step has the largest resistance?
Collecting information for catalytic reactor design
Collecting and Analyzing Data
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Collecting and Analyzing Data
Normal Pentane Octane Number = 62Iso-Pentane Octane Number = 95
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Catalytic Reformers
n-pentanei-pentane
0.75 wt% Pt
Al2O3
n-pentenei-pentene
Al2O3
N
I
n-pentane n-pentene -H2
Pt
Al2O3i-pentene
+H2
Pti-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
SISNSS
'IN K
CCkrrr
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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:0kr
A
AD From
CPKKCPC II
D
ISI:0
kr
D
D From
Catalytic Reformers
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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
PKPKKPPkrr
PKPKKPPKCkrr
1
1
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Catalytic Reformers
CO NO CO2 12
N2
1994 2004 2008HC 0.41 0.125 0.10CO 3.4 3.4 3.4NO 0.4 0.4 0.14
Catalytic Conversion of Exhaust Gas
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222 NNVS•N
2VNN
2S•NDD2
S•NOS•COSS2
VCOCOS•COCO
S•COVCOCOACO
VNONOS•NONO
S•NOVNONOANO
PKCCCPKCkrS2gNS•NS•N
CCkrSS•NCOS•NOS•CO
CPKCKC
CPkrS•COS•CO
CPKCKC
CPkrS•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 KN 2PN 2
Catalytic Conversion of Exhaust Gas
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2NNCOCONONO
CONONO
2
NNCOCONONO
CONO
k
2tCONOS
SNO
NNCOCONONO
tV
22
22
22
PKPKPK1
PkPr
PKPKPK1
PP CKKkrr
PKPKPK1C
C
Catalytic Conversion of Exhaust Gas
Neglect
KN2PN 2
r NO kPNOPCO
1 KNOPNO KCOPCO 2
r NO kPNOPCO
1 KNOPNO KCOPCO KN 2PN2 2
Catalytic Conversion of Exhaust Gas
Find optimum partial pressure of CO
r NO kPNOPCO
1 KNOPNO KCOPCO 2
d rNO dPCO
0
PCO 1 KNOPNOKCO
Catalytic Conversion of Exhaust Gas
End of Web Lecture 17
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