lecture 17 - university of michigan · web lecture 17 class lecture 22–thursday 4/4/2013...

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 17

Class Lecture 22–Thursday 4/4/2013

Introduction to Catalysts and Catalysis

Interstage cooling

Noble Prize 2007

Catalytic steps

2

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.

3

Catalysts and Catalysis

Different reaction paths

4

Catalysts affect both selectivity and yield


Different shapes and sizes of catalyst.

5


Catalytic packed-bed reactor, schematic.

6


7

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.

8

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

10

The Adsorption Step

SASA

][atm /k

/ k-

1-

A

A-

AA

ASAVAASAvAAAD

kK

KCCPkCCPkr

VAAASAAD

VAAASAD

CPkCkr

CPkCr

0/

0 :mequilibriu @

) 1( AAVVAAVSAVt PKCCPKCCCC

11

The Adsorption Step

𝐶𝑉 =𝐶𝑡

1 + 𝐾𝐴𝑃𝐴

12

Langmuir Adsorption Isotherm

𝐶𝑉 =𝐶𝑡


𝐶𝐴∙𝑆 = 𝐾𝐴𝑃𝐴𝐶𝑉

𝐶𝐴∙𝑆 = 𝐾𝐴𝑃𝐴

1+𝐾𝐴𝑃𝐴𝐶𝑡

𝐶𝐴∙𝑆𝐶𝑡

= 𝐾𝐴𝑃𝐴


Langmuir Adsorption

Isotherm

AP

T

SA

C

C Increasing T

Slope=kA

13

AA

AA

t

SA

PK

PK

C

C

1

Langmuir Adsorption Isotherm

14

The Surface Reaction Step


15


16


17


18

19

Steps in a Catalytic Reaction

A BC

CS CS

rDC kD CCS PCC

KDC

rDC rADC

KDC 1

KC

rDC kD CCS KCPCC

(10-20)

(10-21)

Desorption from the Surface for the Reaction

21

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

22

The Rate Limiting Step:

Which step has the largest resistance?

Collecting information for catalytic reactor design

Collecting and Analyzing Data

23

24

Collecting and Analyzing Data

Normal Pentane Octane Number = 62

Iso-Pentane Octane Number = 95

25

Catalytic Reformers

n-pentane i-pentane 0.75 wt% Pt

Al2O3

n-pentene i-pentene Al2O3

N I

n-pentane n-pentene -H2

Pt

Al2O3 i-pentene

+H2

Pt

i-pentane

Catalytic Reformers

26

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

27

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

28

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

29

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

30

Catalytic Reformers

CO NOCO2 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

32

222 NNVS•N

2

VNN

2

S•NDD2

S•NOS•COSS2

VCOCOS•CO

CO

S•CO

VCOCOACO

VNONOS•NO

NO

S•NOVNONOANO

PKCCCPKCkrS2gNS•NS•N

CCkrSS•NCOS•NOS•CO

CPKCK

CCPkrS•COS•CO

CPKCK

CCPkrS•NOSNO


33

rS kS CNO•SCCO•S

rS kSKNOKCOPNOPCOCV2

CT CV CNO•S CCO•S CN•S

CV CVKNOPNO CVKCOPCO CV KN2PN2


34

2NNCOCONONO

CONO

NO

2

NNCOCONONO

CONO

k

2

tCONOS

SNO

NNCOCONONO

t

V

22

22

22

PKPKPK1

PkPr

PKPKPK1

PP CKKkrr

PKPKPK1

CC


Neglect

KN2PN2

r NO kPNOPCO

1KNOPNO KCOPCO 2

r NO kPNOPCO

1KNOPNO KCOPCO KN2PN2

2


Find optimum partial pressure of CO

r NO kPNOPCO

1KNOPNO KCOPCO 2

d rNO dPCO

0

PCO 1KNOPNO

KCO


End of Web Lecture 17

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lecture 17 - university of michigan · web lecture 17 class lecture 22–thursday 4/4/2013...

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