lecture 24
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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 24
Today’s lectureNormal Pentene Iso-PentaneLangmuir Adsorption IsothermCatalytic Converters for Automobiles
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Last Lecture Catalytic Reforming Reaction (Gasoline)
Normal Pentane Octane Number=62Iso-Pentane Octane Number=90Normal Pentane Iso-Pentane
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Steps in a Catalytic Reaction
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Ethylidyne as chemisorbed on platinum (Adapted from G.A.Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994)6
Steps in a Catalytic ReactionBA
Adsorption on Surface SASA
Surface Reaction Single Site SBSA
Dual Site SSBSSA
Desorption from Surface SBSB
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Platinum on alumina (Figure from R.I. Masel, Chemical Kinetics and Catalysis, Wiley , New York, 2001, p 700)8
n-pentanei-pentane
0.75 wt% Pt Al2O3
n-pentenei-pentene Al2O3
N
I
n-pentane n-pentene
-H2
PtAl2O3 i-pentene+H2
Pti-pentane
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2
11NN
P1N
k
N21S
SN PKPK1K/PPKCkrr
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Isomerization of n-pentene (N) to i-pentene (I) over alumina
N I
Al2O3
1. Select a mechanism (Mechanism Dual Site)
Adsorption on Surface: SNSN
Surface Reaction: SSISSN
Desorption: SISI
Treat each reaction step as an elementary reaction when writing rate laws.
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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
S1SNSSN K
CCCCkrr
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3. Find the expression for concentration of the adsorbed species CI.S. Use the other steps that are not limiting to solve for CI.S (e.g., CN.S and CI.S). For this reaction,
CKPC NNSN:0kr
A
AD From
CPKKCPC II
D
ISI:0
kr
D
D From
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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:
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IINN
PIN
k
N2Is
SN PKPK1KPPKCkrr
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Placed inline with exhaust system (~$500)
NOx plays big role in smog formation
NO
CO
CO2
'NOr
)N2/1CO( productsinert NOCO 22
Automotive Catalytic Converters
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SSNCOSNOSCO
SCOSCO
SNOSNO
2
SNOSCOSS
NOSCOVCOCOADCO
NOSNOVNONOADNO
NCkr
k/CCPkr
k/CCPkr
1) Propose a mechanismAutomotive Catalytic Converters
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(from graph data, we see CO2 is not on the surface and that the surface reaction is irreversible) S2NSNSN 2
22 NVN
2SND kCPCkr
22 NNVSNND
VCOCOSCOCOADCO
VNONOSNONOADNO
PkCC0k/r
CPkC0k/r
CPkC0k/r
Automotive Catalytic Converters
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2NNCOCONONO
2TNOCOCONOS'
NO
22PkkPPk1
CPPkkkr
VNOCOCONOSS'NO CPPkkkrr
SNSCOSNOVT CCCCC
22 NNCOCONONO
TV PkkPPk1
CC
Automotive Catalytic Converters
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2COCONONO
NOCO'NO PkPk1
PkPr
Is this consistent with the given data?-No CO2 in rate law -rate law decreases as PN2 increasesThe contribution of PN2 is very small and can be
neglected
Automotive Catalytic Converters
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@ optimum : drNOdPCO
0 PCO opt
1 kNOPNOkCO
NOr
optimum
Automotive Catalytic Converters
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SISI
SISN
SNSN
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End of Lecture 24
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