External Transport Process In Heterogeous Reactions

External Transport Process in Heterogeneous Process CHEMICAL ENGINEERING 126: KINETICS OF HETEROGENEOUS REACTION 1

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Complete Steps for Converting Reactants to Products 1. 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 adsorbed reactants to adsorbed products 5. Desorption of adsorbed products 6. Transport of products from the interior sites to the outer surface of the catalyst surface 7. Transport of products into the bulk fluid stream

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1

7

6 2 3,4,5

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Transport Limitations

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External Transport Process 1 Transport of reactants from bulk fluid to the fluid solid interface

7 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 Process 1 Transport of reactants from bulk fluid to the fluid solid interface

7 Transport of products into the bulk fluid stream

•

Transfer Coefficients • Flow and geometric  arrangement of fluid and  particle

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) 1/24/2009 1/24/2009 University of the Philippines

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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

⎧ 0 .5 k m a m ⎪⎡ rp = k m a m ⎨ ⎢1 + k m am C A ⎪⎩ ⎣

⎤ ⎥− ⎦

⎫ ⎡ 0 .5 k m a m ⎤ ⎪ + − 1 1 ⎬CA ⎢ ⎥ km am C A ⎦ ⎣ ⎭⎪ 2

Higher Order

rp ⎞ ⎛ rp = k ⎜ C A − ⎟ k a m m ⎠ ⎝ 1/24/2009 1/24/2009 University of the Philippines

n

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External Heat Transfer Limitations

B

C

A

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