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# L23 - Membrane Seperation Processes More A less B A B Less...

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4/27/2004 1 Membrane Seperation Processes. A + B More A, less B Less A, more B

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4/27/2004 2 Ex Gases: Air- Æ O 2 ,N 2 Liquids A + Water -- Æ water with less salt + salty Two kinds of Membranes: -Porous Membranes : Diffusion -Polymeric Membranes : Sublimation + Diffusion Seperation of Gases through a porous membrane: Pores are narrow, r = radius of pore r<1um Knudsen Diffusion, λ = mean free path of any molecule >r For O 2 around room T and P , λ ∼ 1000A~1um - Knudsen diffusivity D A = 9700* r *(T/M)^0.5 where r = pore radius Bigger molecules diffuse faster D e,A = D A * ε / τ , ε = porosity, τ = tortuosity. L L
4/27/2004 3 J A = D e,A C A ./ z = D e,A P A / zRT Assumed C A = P A /RT ; Ideal Gas J B = D e,B P/ zRT Say steady state diffusion Composition of the Permeate, y Az = J A / J A + J B = P A /M A ^0.5/( P A /M A ^0.5+ P B /M B ^0.5 Example PA1 = PB1 = 1atm (P1 = 2atm, x A = 0.5) PA2 = 0.6atm PB2= 0.4 atm implies P CA1, PA1 CA2,PA2 z PA1, PB1 = 1atm PA1=0 .6 PB2=0 .4 PA y A2 = J A2 /(J A2 + J B2 ) = P A2 / P A2 + P B2 = 0.6 =(0.4/M A ^0.5/(0.4/M A ^0.5+ 0.4/M B ^0.5) = 0.6 (M A /M B )^0.5 = 9/4 If, M A /M B =16/81 yA2 = 0.6 if XA1 =0.5

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L23 - Membrane Seperation Processes More A less B A B Less...

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