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CP302 Separation Process Principles Mass Transfer / Set 1 (Worked) Examples in Mass Transfer by Diffusion in Binary Systems Example 1: Diffusion through a stagnant gas film Oxygen is diffusing in... a mixture of oxygen-nitrogen at 1 std atm, 25C. Concentration of oxygen at planes 2 mm apart are 10 and 20 volume % respectively. Nitrogen is non-diffusing. (a) Derive the appropriate expression to calculate the flux oxygen. Define units of each term clearly. (b) Calculate the flux of oxygen. Diffusivity of oxygen in nitrogen = 1.89x10 –5 m 2/sec. Solution: Let us denote oxygen as A and nitrogen as B. Flux of A (= N A) is made up of two components, namely that resulting from the bulk motion of A (= NxA) and that resulting from molecular diffusion J A: A A A N Nx  J (1.1) From Fick’s law of diffusion, d z d C J D A A AB  (1.2) Substituting (1.2) in (1.1), we get d z d C N Nx D A A A AB   (1.3) Since N = N A + N B and x A = C A / C, (1.3) becomes   d z d C D C C N N N A AB A A A B    Rearranging the terms and integrating between the planes between 1 and 2,        2 1 A A C C A A A B A AB N C C N N dC CD d z (1.4) Since B is non-diffusing N B = 0. Also, the total concentration C remains constant. Therefore, (1.4) becomes [Show More]

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