Chapter 10 Lecture 1

7/31/2019 Chapter 10 Lecture 1

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Chemical Reaction Engineering

Week 10 (S1-2012)Fogler Chapter 10

Catalysis and Catalytic Reactors

Lecture 1

Gia Hung Pham


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Objectives

- Catalysts and catalytic reactor design

- Steps in a heterogeneous reaction

- Reaction mechanism

- Rate law

- Catalyst decay

- Reactor types


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Catalysts

Definition

- Fogler: A substance that affects the rate of a reaction but

emerges from the process unchanged.

- Ostwald (1895): A catalyst accelerates a chemicalreaction without affecting the position of the equilibrium.


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Phenomenon

Reaction A P

Catalyst changes reaction pathway.

A

Cat Cat-A

Chemical bonding

Cat-PP


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- Positive catalyst increases reaction rate.

- Negative catalyst decreases reaction rate.

Importance of catalysis

- About 75% of all chemicals are produced by catalytic process.

Catalyst can be:- Gas (NOx)

- Liquid (H2SO4)

- Solid (Ni)

Note: In this chapter only solid catalysts will be considered.


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Reaction energetic

Energy

A

P

Ea (without cat)

H

Ea (with cat)Cat-A

Cat-P

adsorption

reaction

desorption

Reaction progress


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Heterogeneous catalyst

PBR

Reactants (gas or liquid)

Solid catalyst

Products


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PBR

Catalyst Bed


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PBR

Catalyst Bed Catalyst pellet


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PBR

Catalyst Bed Catalyst pelletCatalyst pore

Active site


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Catalyst surface area

- Specific surface area of silica-alumina cracking catalyst is

300 m2/g.

- External surface area of 1mm alumina sphere is

0.0015m2

/g.- A catalyst that has large area resulting from pores

(internal surface area) is called a porous catalyst.


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Film theory

Fogler. Page 773


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Individual steps in heterogeneous catalysis ( A P )

CAb

CAs

Cat-A

Cat-P

A

A

AA

PP

P

4

Boundary layer

P

Catalyst pellet

Catalyst pore

1

2 3

56

7


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Individual steps in heterogeneous catalysis (Mass transfer and

chemical reaction)

Step 1. Diffusion of the reactant A from the fluid to the catalystsurface (External mass transfer).

Step 2. Diffusion of the reactant A from the pore mouth into thepore (Internal mass transfer or pore diffusion).

Step 3. Adsorption of the reactant onto the surface of the pore

(Reactant adsorption).Step 4. Chemical reaction on the catalyst surface to form product

P (Surface reaction).

Step 5. Desorption of the product P from the catalyst surface(Product desorption).

Step 6. Diffusion of the product P out of the pore (Internal masstransfer or pore diffusion).

Step 7. Diffusion of the product from the pore mouth through theboundary layer into the bulk fluid (External mass transfer).


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Catalytic reaction Kinetics

Mass transfer

and Effective reaction raterA,eff

Catalytic reaction

Reaction kinetics without mass transfer effects.

We choose the conditions, under which the mass transfers do not affect the effective

reaction rate.

For example: High feed flow rate and /or small catalyst particle size


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No mass transfer limiting CA,b = CA,s = CA

Conc.

CAs

Catalyst pore

bulk

CAb CA

Concentration of A in catalyst pore does NOT change

with pore length L.

L


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Assumptions: - Uniform surface

- Active site has the same attraction

Conc.

CAs

Catalyst pore

s s s s s

bulk

Active site

Catalyst surface

CAb CA


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Steps in a heterogeneous catalytic without mass transfer effects

- Step 3 : Adsorption of reactants- Step 4 : Surface reaction catalytic chemical reaction

- Step 5 : Desorption of products

Reaction A P

s s s s s

A P

Catalyst surface

3

45


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Adsorption Isotherms

Concentration of active site

Active site balance: Ct = Cv + CAs + CPs

s s s s s s

PA

Occupied sitesVacant sites

Total active si te per gram catalyst (mol/gcat)

Vacant site per gram catalyst (mol/gcat)

Occupied sites by species A and B (mol/gcat)


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Two adsorption models

- Model 1: Molecular or nondissociated adsorption.

Example: Adsorption of CO on Ni-catalyst

CO

- Model 2: Dissociative adsorption

Example: Adsorption of CO on Fe-catalyst

ss s s s

CO

Molecule COS = Ni

CO + S COS

CO

s s s s

C O

Atom C and O

S = Fe

CO + 2S C-S +OS


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Model 1

Rate law for the reaction

Rate of attachment = kA PCO Cv

Rate of detachment = k-A CCOS

Net rate of adsorption rAD = kAPCOCv k-A CCOS

or

with (adsorption equilibrium constant)

CO + S COS

kA

k-A

A

A

A

k

kK

A

SCO

vCOAAD

K

CCPkr


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Site balance : Ct = CV + CCOSAt equilibrium rAD = 0 we have CCOS = KACVPCO

This adsorption isotherm equation is called Langmuir Isotherm.

COA

tCOA

SCO PK

CPKC

1

CCOS

PCO

Linear


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Model 2

CO + 2S CS + OS

Net reaction rate

At equilibrium rAD

= 0

Site balance Ct = CV + COS + CCS

kA

K-A

SCSOAVCOAADCCkCPkr

2

A

SOSC

VCOAAD

K

CCCPkr

2


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2/1

2/1

21 COA

COAt

SO

PK

PKCC

COS

PCO

parabolic


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Inverse of both sides and multiply2/1

COP

t

CO

AtOS

CO

C

P

KCC

P2/1

2/1

2/121

2/1

COP

OS

CO

C

P2/1


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Adsorption isotherm equation of A in the presence of adsorbate B

s s s s s

A B

Ct = CV + CAS + CBS

BBAA

tAA

SA

PKPK

CPKC

1


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Surface reaction

* Model 1: Single site

A S BS

s s

A B

kS

k-S

S

SB

SASs

K

CCkr

ks

k-s


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* Model 2: Dual site

AS + S BS + S

AB

S

VSB

VSASs

K

CCCCkr

ks

k-s

ks

k-s


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* Model 3: Eley Rideal

AS + B(g) CS

B(g)

A C

S

SC

BSASs

K

CPCkr

s s

ks

k-s

ks

k-s


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Desorption

CS C + S

With KDC = 1/KC

s s

CC

DC

VC

SCDDC K

CPCkr

VCCSCDDC

CPKCkr

kD

k-DkD

k-D


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Synthesizing a rate law

Mechanism and rate limiting step

Foglers Example (page 671): Reaction

C6H5CH(CH3)2 C6H6 + C3H6Cumene Benzene PropyleneC B P


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Reaction mechanism

C + S CS (Adsorption)

CS BS + P(gas) (Surface reaction)

BS B + S (Desorption)

kA

k-A

kS

k-S

kD

k-D


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Reaction rate

Adsorption

rAD = kA Pc Cv k-A CCS

Surface reaction

rS = kS C CS k-S PP CBS

Desorption

rD = kDCBS k-D PB CV

S

SBPSCSS

K

CPCkr

VBBSBDD CPKCkr

C

SC

VCAAD

K

C

CPkr


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Case 1

The reaction rate of three discussed reactions are equal

DSADC rrrr '


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Case 2

Reaction rate limiting

One or two of three discussed reactions has/have very slow reaction rate.

We assume the adsorption of cumene is rate limiting (slowest step).

What is the rate law if it is true?

Adsorption rate

We need to express Cv and CCS in terms of PC, PB and PP

C

SC

VCAAD

K

CCPkr

Unknown

Measurable


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kA : small

kS : large or rS/kS 0

kD : large or rD/kD 0

Surface reaction

S

SBP

SCSS

KCPCkr

S

PSB

SC

K

PC

C

unknown


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Desorption VBBSBDD

CPKCkr VBBSB

CPKC

V

S

PB

BSCC

K

PPKC

CS

VPBB

VCAAD

KK

CPPKCPkr

Overall reaction C B + P

B

CS

PK

KKK


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Site balance Ct = CV + CCS + CBS

Solving for CV we have

VBBVPB

S

B

VtCPKCPP

K

KCC

BBBP

S

B

t

V

PKPP

K

K

CC

1


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Adsorption control rate law

At t = 0 min we have Pc = Pc,0

PP = 0 (no product)

PB = 0 (no product)

BPBP

S

B

P

PB

CAt

AD

PKPPK

K

K

PPPkC

r

1


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Adsorption control rate law at t = 0 min

Plot initial rate versus

.

0,0,

'

, CCAtADOC PkPkCrr '

,OCr

0,CP

'

,OCr

0,CP


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We assume the surface reaction is rate limiting (slowest step).


Surface reaction

* kS : small

* kA : large or rAD/kA 0

* kD : large or rD/kD 0

S

SBP

SCSS K

CPCkr

We get these concentrations from

adsorption and desorption reaction.


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Adsorption reaction

Desorption reaction

C

SC

VCAAD K

CCPkr

VCCSCCPKC

VBBSBDD

CPKCkr

VBBSBCPKC


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Surface reaction rate limiting

V

P

BP

CCSSC

K

PPPKkr

We get this one from site balance

CCBBt

V

PKPK

CC

1

CCBB

B

PB

CtCS

S

PKPK

K

PPPCKk

r

1


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At t = 0 min

At low partial pressure of C we have 1 >> KC PC,0

Initial rate increase linearly with the partial pressure of C

0,

0,,

0,

1 CC

C

SC

PK

Pkrr

tCSCKkkwith

0,

,

0, CSC Pkrr


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At high initial partial pressure of C we have KC PC,0 >>1

tconsK

krr

CSC tan

,

0,

'

,OCr

0,CP

k


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We assume the desorption of Benzene reaction is rate limiting (slowest step).


Desorption reaction

* kD : small

* kA : large or rAD/kA 0

* kS : large or rS/kS 0

VBBSBDD

CPKCkr

Not measureable but we can get this one from adsorption and surface reaction.


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Surface reaction

Adsorption reaction

Site balance

P

SC

SSBP

C

KC

,

VCCSCCPKC

CC

P

C

SC

t

V

PK

P

PKK

CC

1


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Desorption limiting

At t= 0 min

PCCCSCP

B

PB

C

tSCDDC

PPKPKKP

K

PP

P

CKKkrr,

0,

tconsCkrtDC

tan,

0,

'

,OCr

0,CP

Foglers algorithm for determining reaction mechanism and rate


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Fogler s algorithm for determining reaction mechanism and rate

limiting step.

1. Select a mechanism

2. Assume a rate-limiting step

3. Find the expression for concentration of the

adsorbed species Ci,S

4. Write a site balance

5. Derive the rate law

6. Compare with data


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In-class exercise

The following reaction is carried out over a solid catalyst, the reactionmechanism is believed to be

We assume step 3 is the slowest step, derive the rate law for thereaction.

SDSD

SCSC

SDSCSBSA

SBSB

SASA

)5

)4

)3

)2

)1

Chapter 10 Lecture 1

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