35
Fogler-Chapter 10 Catalysis and Catalytic Reactors
Fogler-Chapter 10
Catalysis and Catalytic Reactors
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.
2
Catalysts and Catalysis
Different reaction paths
3
Catalysts affect both selectivity and yield
Catalysts and Catalysis
Different shapes and sizes of catalyst.
4
Catalysts and Catalysis
Material aktif = katalis
Support= Material yang tidak aktif tetapi dijadikan tempat menempel katalis
Promoter= Sejumlah kecil aktif material tambahan yang meningkatkan aktivitas katalis
Catalytic packed-bed reactor, schematic.
5
Catalysts and Catalysis
6
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.
7
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
9
The Adsorption Step
SASA •+
][atm /k
/ k-
1-
A
A-
AA
ASAVAASAvAAAD
kK
KCCPkCCPkr
−
••
=
−==
VAAASAAD
VAAASAD
CPkCkr
CPkCr
=
==
0/
0 :mequilibriu @
) 1( AAVVAAVSAVt PKCCPKCCCC +=+=+= •
10VAA
tV
CPK
CC
+=
1
The Adsorption Step
AA
AA
t
SA
t
AA
AASA
VAASA
VAA
tV
PK
PK
C
C
CPK
PKC
CPKC
CPK
CC
+=
+=
=
+=
•
•
•
1
1
1
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Langmuir Adsorption Isotherm
Langmuir Adsorption
Isotherm
AP
T
SA
C
C Increasing T
Slope=kA
12
AA
AA
t
SA
PK
PK
C
C
+=•
1
Langmuir Adsorption Isotherm
13
The Surface Reaction Step
The Surface Reaction Step
14
The Surface Reaction Step
15
The Surface Reaction Step
16
The Surface Reaction Step
17
18
Steps in a Catalytic Reaction
A ⎯ → ⎯ B+ C
C•S ⎯ → ⎯ ⎯ ⎯ C + S
rDC = kD CC•S −PCC
KDC
rDC = −rADC
KDC =1
KC
rDC = kD CC•S − KCPCC
(10-20)
(10-21)
Desorption from the Surface for the Reaction
20
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
21
The Rate Limiting Step:
Which step has the largest resistance?
Collecting information for catalytic reactor design
Collecting and Analyzing Data
22
23
Collecting and Analyzing Data
Normal Pentane Octane Number = 62
Iso-Pentane Octane Number = 95
24
Catalytic Reformers
n-pentane i-pentane0.75 wt% Pt
Al2O3
n-pentene i-penteneAl2O3
N I
n-pentane n-pentene-H2
Pt
Al2O3
i-pentene
+H2
Pt
i-pentane
Catalytic Reformers
25
Isomerization of n-pentene (N) to i-pentene (I) over alumina
N IAl2O3
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
26
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
27
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
28
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
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Catalytic Reformers
CO + NO → CO2 +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
31
( ) 222 NNVS•N
2
VNN
2
S•NDD2
S•NOS•COSS2
VCOCOS•CO
CO
S•COVCOCOACO
VNONOS•NO
NO
S•NO
VNONOANO
PKCCCPKCkrS2gNS•NS•N
CCkrSS•NCOS•NOS•CO
CPKCK
CCPkrS•COS•CO
CPKCK
CCPkrS•NOSNO
=−=+
→+
=++→+
=
−=
→
=
−=
→+
Catalytic Conversion of Exhaust Gas
32
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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( )
( )2
NNCOCONONO
CONO
NO
2
NNCOCONONO
CONO
k
2
tCONOSSNO
NNCOCONONO
t
V
22
22
22
PKPKPK1
PkPr
PKPKPK1
PP CKKkrr
PKPKPK1
CC
+++=−
+++==−
+++=
Catalytic Conversion of Exhaust Gas
Neglect
KN2PN2
− r NO =kPNOPCO
1+ KNOPNO + KCOPCO( )2
− r NO =kPNOPCO
1+ KNOPNO + KCOPCO + KN2PN2( )
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+ KNOPNO
KCO
Catalytic Conversion of Exhaust Gas
