Lecture 24

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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4

Steps in a Catalytic Reaction

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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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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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13

14

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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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