SAJJAD KHUDHUR ABBASCeo , Founder & Head of SHacademyChemical Engineering , Al-Muthanna University, IraqOil & Gas Safety and Health Professional – OSHACADEMYTrainer of Trainers (TOT) - Canadian Center of Human Development

Episode 49 : Selectivity of Cumene Produced from Alkylation of Benzene

Selectivity of Cumene Produced from Alkylation of Benzene

1. Reaction informationa. Reactions stoichiometry

Cumene is produced by alkylation of benzene over a Lewis acid calayst

C3H6

(propylene)+ C6H6

(benzene) C9H12

(cumene)

The most important side reaction is the reaction fo cumene with propylene to produce di-iso-propyl benzene

C3H6 + C9H12 C12H18

(propylene) (cumene) (p-diisopropyl benzene)

b. Reaction kinetics

The reaction can be carried in either gas phase (older process) or liquid phase (newer process). For gas phase reaction, the reaction kinetics is given by:

r1 k1C pCb

(1)

r2 k2C pCc

(2)

where r1 and r2 are the rates of reaction of reactions 1 dan 2 in kmole (kg catalyst)-1 s-1, Cp is the concentration of propylene in kmole m-3, Cb is the concentration benzene in kmole m-3 and Cc

is the concentration of cumene in kmole m-3 , k1 and k2 in kmole-1 s-1 m-3 are given by

k k eksp RT

RT

E10

1 1 (3)

k k eksp

E2 2 20 (4)

with k1 dan k2 is the reaction rate coefficient, E1 and E2 is the activation energies, R is the gas constanto o

(=8.314 kJ kmole-1 K-1) and T is temperature in K. Table 3.2 values of the parameters.

Table 3.2

Data from Turton et al. 2003

Reactionki

o

(kmole-1 s-1 m-3)

Ei

(kJ kmole-1)

1 2.8 x 107 - 1.04 x 105

2 2.32 x 109 - 1.47 x 105

c. Reaction conditions:

The liquid phase reaction is carried out at between 120 to 240C and 20 to 30 bar whereas the gas phase reaction is carried out at a higher temperature of 350C at similar pressures

2. Cumene selectivity

Liquid Phase

Reaction:Material balance for a batch reactor will yield similar results for material balance of a plug flow packedbed reactor.Since the reaction is in liquid phase, there will be no changes in volume because of mole changes in the reaction stoichiometry.Propylene

1 dnP 1 d CPV dCP rP 1 2 1 P B 2 P C r r k C C k C CV dt V dt dt

(5)

Benzene 1 dnB 1 d CBV dCB rB 1 1 P

B r k C CV dt V dt dt

(6)

Cumene

1 dnC 1 d CCV dCC

r

C 1 2 1 P B 2 P C

r r k C C k C CV dt V dt dt

(7)

p-diisopropyl benzene

1 dnI 1 d CIV dCI r k C

C

2 2 P CV dt V dt dt

(8)

By substituting the expression for propylene concentration in terms of its conversion, CP CPo 1 X P

in equations (5), (6), (7) and (8), then

dCP d CPo 1 X P

C

dX P k C 1 X C k C 1 X

C1 Po P B 2 Po P CPo dtdt dt

dX P k

1 X

C k

1 X C

P CP Bdt 21 (9)

dCB k C

1 X C

P B1 Podt(10)

dCC k C

1 X C k C

1 X C

P CP B 2 Po

1 Podt(11)

dC I k C

1 X C

P C2 Podt(12)

The degree of freedom is decreased by application of chain rule to convert the time derivative of equations (10), (11) and (12) to derivatives of the con version of propylene.Then equation (10) becomes

k1CPo 1 X P

CBdt dX P dX P k1 1 X P CB k 2 1 X P CC k1CB k 2CC

k 1CPoCB dCB dCB dt (13)

In the same way, equations (11) and (12) become

dCC k1CPo 1 X P CB k 2CPo 1 X P CC k1CPoCB k

2CPoCCk1 1 X P CB k 2 1 X P

CC

k1CB k 2CCdX P

(14)

k 2CPo 1 X P

CCdX P k1 1 X P CB k 2 1 X P CC k1CB k 2CC

k 2CPoCCdCI (15)

Mole balance of propylene, benzene and

cumene PropylenenP n1 n2

(16)

BenzenenB n1

(17)

Cumene

nC n1 n2

(18)

p-diisopropyl benzene

nI n2

(19)

From equation (17)n1 nB

Subtracting equation (16) by equation (17)

(20)

n2 nP nB (21)

nC 2nB nP (22)

nI nP nB

Reformulating in terms of concentrations

(23)

CC 2CBo CB CPo X P (24)

CI CPo X P CBo CB

(25)

Substituting equation (24) in equation (13)

dX P k1CB k 2 CPo X P 2CBo CB

k 1CPoCB dCB (29)

Substituting equation (25) in equation (14)

dCC k1CPo 1 X P CB k 2CPo 1 X P CC k1CPoCB k

2CPoCCk1 1 X P CB k 2 1 X P

CC

k1CB k 2CCdX P

(30)

Substituting equations (27) and (28) in equation (15)

k 2CPo CPo X P 2CI

dX P k1 CBo CI CPo X P k 2 CPo X P 2CI

dCI (31)

Equations (19), (30) and (30) can be solved for CB , CC and C I as a function of Runge Kutta algorithm available in MathLab (ODE45).

X P using 4 Order

th

Selectivity of cumene SC (mole of cumene produced/mole of propylene converted) for plug flow reactor or batch reactor is given by

P

C

Po P

CCC C X X

CS

*

(35)

3. Feed conditions

Liquid benzene feed at 28C and pressure 1 bar is assumed to be pure.Liquid propylene feed at 28C and pressure 12 bar contains 95.2 % propylene and 4.8 % propane.

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