vietnam ecat cinetique reelle 2

8/6/2019 Vietnam ECat Cinetique Reelle 2

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Ecole Franco-Vietnamienne 2005 de Catalyse

Cintique et Raffinage

IRCInstitut de Recherches

sur la Catalyse

Module Cintique et Catalyse htrogne

Dr. Michel VRINAT

PART 2

Real kinetic


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Real kinetic

W y kinetic studies?

Use of a catalyst to increase t e rate of a reaction and (or) to modify t edirection of t e transformation (selectivity).

All catalysts development need a kinetic study in order to quantify and to

understand t e effects of experimental parameters over t e reaction.


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Real kinetic

Power rate law

Reactor (T)R1(P1)

R2 (P2)Products

...22

1

1

2

2

1

1

/ FFEE

PPrr

RTE PPPPekv !

Useful for engineer in the building of a unit

No information on the mechanism of the reaction

Need of models


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Real kinetic

UNIMOLECULAR REACTIONS

k1

a) A ' A B

k1

k2

Stationary state : [ A ] = constant

rate of formation = rate of disappearance

k1

[ A ] [ S ] = k1

[ A ] + k2

[ A ]

equation of conservation of catalytic sites:

[ S ]0 = [ S ] + [ A ]

resolution : ? A? A ? A ? A_ aASAkAkk ! 012'1 ? A ? A? A ? AAkkk

SAkA1

'12

01

!

? AAkv 2! ? A ? A? AAKAK

Skv'

'

21

!2

'1

1'

kk

kK

!with


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6/31

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Real kinetic

b) A ' A B ' B

k1

k1

k2 k3

k3

Stationary state for A: [ A ] = constante

? A? A ? A ? AAkAkSAk '121 ! ? A ? A? ASAkk

kA

2'1

1

!

? A ? A? A ? ABkSBkAk '332 ! ? A ? A? A ? A? ASBkk

SA

kk

k

k

kB

'3

3

2'1

1

'3

2

!

Stationary state for B: [ B ] = constante

Equation of conservation of t e catalytic sites

? A ? A ? A ? ASBAS !0

? A ? A ? A ? A ? A

! Bk

kA

kk

k

k

kA

kk

kSS

'3

3

2'1

1

'3

2

2'1

10 1


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15

Real kinetic

? AAkv 2!? A? A

? A ? ABKAKSAKk

v

BA

A

!

"

0'

2

1

With2'1

1'

kk

kKA

!

!

'3

2'"1

k

kKK AA

Rate of reaction:

and

Remark: if the surface reaction is slow as compared to the adsorption-desorption

steps, we have

et

'12 kk and

'3k

'1

1"'

k

kKKK AAA !!!

? A ? A? A ? ABKAK

AKSkv

BA

A

!

1

02and


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Real kinetic

BIMOLECULAR REACTIONS

1) Langmuir-Hins elwood model:

reaction between two species adsorbed on the catalyst surface

a) only one type of catalytic sites

CA

(

B

+

(

C

(

A

B kSurface equilibrium adsorption established and slow surface reaction

? A? A? ASA

AKA ! BK, CK

? A ? A ? A ? A ? ACBASS !0

? A? ABAkv ! ? A ? A ? A? A ? A ? A_ a2

20

1 CKBKAK

SBKAKkv

CBA

BA

!


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Real kinetic

Remarks:

1. if KC[ C] is low (few adsorption of t e product)

? A ? A? A? A ? A_ a22

0

1 BKAKSBKAKkv

BA

BA

!

2. adsorption of C important its an in ibitor

Ex: H2S produced during t e HDS of sulfur compounds


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Real kinetic

b) two type of catalytic sites

CAS1

(

B

+(

A

BS2k

? A? A21 SS

BAkv !

? A? A? A1

1

SA

AK

S

A ! ? A ? A_ aAKSS A! 1101 ? A ? A ? A? A? AAKSAK

AAA

AS

!!1

01

1

!BK .

? A? A

? A? A

? A ? A0201

11SS

BK

BK

AK

AKkv

B

B

A

A

!


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Real kinetic

c) dissociative adsorption of a reactant

C2A

(+2S

B

+(

C

(

A2

B

? A? A? A2

2

SA

AKA ! , KB , KC

? A ? A? AAKSA A! ? A ? A ? A ? A ? A_ aCKBKAKSS CBA ! 10

? A Afv !


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Real kinetic

Exemple 1: Study of a Pt-R /Al2O t ree ways catalyst

In a detail kinetic study of a Pt-Rh/Al2O3 catalyst used for CO and NO

reduction, Lecomte proposed that the reaction proceeds according to

the following mechanisms:

Determine the rate equations corresponding to such mechanisms, and

discriminate between both proposals using experimental values given

in Table 1.

J. Lecomte , Thse Lille


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Real kineticTable 1:Results of kinetic study at 300C

Mechanism A:

KCO


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Real kinetic

Exemple 2: CO met anation over Ni/Al2O

CO + H2 CH4 + H2O Berty reactor, N2+ CO + H2 ; 47 -62 K

r = A exp(E/RT)PH2xPCO

y

Bartholomew etal., Appl. Catal., 1982


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Real kinetic

Exemple 2


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Real kinetic

Exemple 2


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Real kinetic

Exemple 2

Rate determining step could change with the temperature


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Real kinetic

Exemple : Hydrodesulfurization of sulfur compounds


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Real kinetic

S

R R

S

H RR

S

R R

S

R R R R

R R

DDS

SH

R R

HYD

H+

B-

Exemple


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Real kinetic

HDS: new approac

S

R R

S

H R

R

S

R R

S

R R R R

R R

DDS

SH

R R

HYD

H+

B-

Exemple

S. Kasztelan , D. Guillaume, I.E.C. Res. 1994

E.Olguin M. Vrinat, Appl. Catal., 1998


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Real kinetic

Exemple

S S

S

DBT-DH

DBT-TH DBT-HH

BP

S

DBT

R kD H D H H S

D H H S

!

.( ) ( )

( )

E E E E E

E E E

2 2 2

2 2

1

1

12

12

2

R k H HD H

H H S H D H D H H D H

!

.( ( ) ( ) )

E E

E E E E E E E

2

2 2 2 21 2

Rds= addition ofH- (and afterH+)

Under lowH2S partial pressure

Rds = elimination reaction E2, under high H2S pressure

= iPi


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Real kinetic

2- Eley-Rideal model:

reaction between one species adsorbed on the catalyst surface and

one molecule of the gas phase

CA

B

+(

C

(B

k

? A? ABAkv !? A ? A? A

? A ? ACKBKSBKA

kvCB

B ! 1 0


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Real kinetic

Exemple : V2O5/TiO2 catalyst for Catalytic Selective Reduction of NO

OHNONONH 222 6444 p

DeNOx Process by reduction of NO using NH3 has been performed

over a V2O5/TiO2 and the solid characterized by TPD and FTIR.

a) IR studies indicated adsorbed NH3 species, but no NO adsorbed species,suggesting an Eley-Rideal mechanism:

MNH 3 ' 3NH

3NH-M + NO p 2k Produits + M (slow)

-M (fast)

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12

NHNH

NHNHNO

PK

PKPkv

vv!Rate equation could be written:

Dumesic etal., ICC1992


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Real kinetic

b) Such an equation could not fit the experimental values. Thereforethe authors proposed a second model in which NH3 adsorption would be slow,

according to:

k1

NH3 + M NH3-M (slow)

NH3-M+ NO Produits + M (slow)

Rate equation:

p n

1k

p

2k

NONONH

NONHNO

PK

PPK

PPKPkv

21

1

2

3

3

1 vv!

212211 ; kkKkkK !!

Good fit with experimental kinetic results


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Real kinetic

C) Previous mechanism does not agree with TPD and FTIR studies which indicatea rapid adsorption of NH3 and adsorption of this reactant over2 type of sites.

Therefore the authors proposed a new mechanism:

NH3-M+ S

p n

2k NH3-S+ M (slow)

MNH 3 ' 3NH -M (fast)

k2

NH3-S+ NO p 3

k Produits + S (slow)

Corresponding to the rate equation:

333

3

21

13

1 NHNHNONONH

NONHNO

PKPKPPK

PPKPkv

v

vvv!

322321 ;3 kkKkkKK NH !!with


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Real kineticHow to differenciate L-H and E-R ?

? A ? A? A? A ? ACKBK

SBKAkv

CB

B

!1

0? A ? A? A? A ? A_ a2

20

1 BKAK

SBKAKkv

BA

BA

!


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Real kinetic

3- Stationary state Model:Mechanism in which two reactions are r.d.s.

Introduced by Mars and Van Krevelen for catalytic mild oxidation

Hydrocarbon + cata oxidic state Produit + reduced catalyst

Reduced cata. + oxygene cata in oxidic stateoxyd

UAPkv 11 ! (U : O2coverage )

nOPkv 2122 U!

21 vv ! n

OA

n

OA

PkPk

PkPkv

2

2

21

21

!n

OA PKPkv 221

111!

ki , n


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Real kinetic

Exemple: production of tetra ydrofuran


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Real kinetic


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Real kinetic

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