Catalytic Oxidative Desulfurization: A Comparison of Reactors
Catalysis and Catalytic Reactors
Transcript of Catalysis and Catalytic Reactors
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Fogler-Chapter 10
Catalysis and Catalytic Reactors
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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
Material aktif = katalis
Support= Material yang tidak aktif tetapi dijadikan tempat menempel katalis
Promoter= Sejumlah kecil aktif material tambahan yang meningkatkan aktivitas katalis
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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 +=+=+= •
10VAA
tV
CPK
CC
+=
1
The Adsorption Step
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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
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Langmuir Adsorption
Isotherm
AP
T
SA
C
C Increasing T
Slope=kA
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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 ⎯ → ⎯ 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
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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-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
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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
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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 + 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
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( ) 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
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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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( )
( )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
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Neglect
KN2PN2
− r NO =kPNOPCO
1+ KNOPNO + KCOPCO( )2
− r NO =kPNOPCO
1+ KNOPNO + KCOPCO + KN2PN2( )
2
Catalytic Conversion of Exhaust Gas
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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