Internal Transport Process In Heterogeous Reactions Part1

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

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1 Chemical Engineering 126

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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6 2 3,4,5

2Chemical Engineering 126 2

Transport Limitations

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3Chemical Engineering 126 3

Single Cylindrical Pore Model Mantle area where  reaction is occurring

S = 2πr Δx

Δx

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4Chemical Engineering 126 4

Single Cylindrical Pore Model

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5Chemical Engineering 126 5

Spherical Catalyst Δr

r

R

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6Chemical Engineering 126 6

Spherical Catalyst

Concentration profile in a spherical catalyst 1/29/2009 1/29/2009 University of the Philippines

7Chemical Engineering 126 7

Spherical Catalyst

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8Chemical Engineering 126 8

Understanding Thiele Modulus

Characteristic Length

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9Chemical Engineering 126 9

Understanding Thiele Modulus

For small particles • Intraparticle mass transport has  no effect on the rate of per pellet • Chemical reaction step controls  the rate • Small values of the Thiele  Modulus

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10 10 Chemical Engineering 126

Understanding Thiele Modulus For large particles • Intraparticle diffusion has  large effect on the rate • Diffusion into the pellet is  relatively slow • Large values of the Thiele  Modules

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11 11 Chemical Engineering 126

Understanding the Effectiveness Factor • Constant rate • The rate at the center is  the same as the rate at  the outer surface – all  the surface is fully  effective

η →1

η →0

CA

• Only the surface in the  periphery of the pellet is  effective • Concentration drops to near  zero • Central portion of the catalyst  is not utilized • Large pellet, low De or high k  (very active catalyst) • Intrapellet diffusion  resistance will reduce the  global rate

radial distance

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12 12 Chemical Engineering 126