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External Transport Process inHeterogeneous ProcessCHEMICAL ENGINEERING 126:

KINETICS OF HETEROGENEOUS REACTION

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Complete Steps for Converting Reactants to Products1. Transport of reactants from bulk 

fluid to the fluid solid interface

2. Intraparticle transport of reactants in the catalyst particle

3. Adsorption of reactants at interior sited of the catalyst particle

4. Chemical reaction of adsorbedreactants to adsorbed products

5. Desorption of adsorbed products6. Transport of products from the

interior sites to the outer surfaceof the catalyst surface

7. Transport of products into the

bulk fluid stream

1

2

3,4,5

6

7

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Transport  Limitations

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External Transport  Process1

Transport of reactants from

bulk fluid to the fluid solid

interface7

Transport of products into

the bulk fluid stream

• Catalyst can only be effective if 

the reactants can reach the

surface• Transport requires a driving

force

• Transport is dependent on

1. Velocity pattern

2. Physical properties of the

fluid

3. Intrinsic rate of chemical

reactions

• Transfer Coefficients

Outer Surface of the Catalyst

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External Transport  Process1

Transport of reactants from

bulk fluid to the fluid solid

interface7

Transport of products into

the bulk fluid stream

• Transfer Coefficients

• Flow and geometric

arrangement of fluid andparticle

Outer Surface of the Catalyst

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Concentration and Temperature(with Transport  Limitations)

Endothermic Process Exothermic Process

Global rate of reaction vs Intrinsic rate of reaction

(bulk  concentration) (surface concentration)

(observed)

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Effect  of  external physical process on the global rate

Mass Transfer Mass Transfer 

Reaction

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Higher Order Reactions

2nd Order

Higher Order

2

0 5 0 51 1 1

m m m m  p m m A

m m A m m A

. k a . k ar k a C  

k a C k a C  

⎧ ⎫⎡ ⎤ ⎡ ⎤⎪ ⎪= + − + −⎨ ⎬⎢ ⎥ ⎢ ⎥

⎣ ⎦ ⎣ ⎦⎪ ⎪⎩ ⎭

n

 p

 p A

m m

r r k C 

k a

⎛ ⎞= −⎜ ⎟

⎝ ⎠

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External Heat  Transfer Limitations

A

B

C