Chapter 14 Handouts

Chapter 14 Handouts - Diffusion Mass Transfer General Considerations General Considerations Mass transfer refers to mass in transit due to a Must

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Diffusion Mass Transfer
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General Considerations General Considerations Mass transfer refers to mass in transit due to a ______________________________ __________________ . ¾ Must have a mixture of two or more species for mass transfer to occur. ¾ The ___________________________ is the driving potential for transfer. ¾ Equations describing mass transfer by diffusion can be ________________ ___________________________ . • Physical Origins of Diffusion: ¾ Transfer is due to ___________________________ . ¾ Consider two species A and B at the same T and p , but initially separated by a partition. Diffusion in the direction of decreasing concentration dictates net transport of A molecules to the right and B molecules to the left. In time, uniform concentrations of A and B are achieved.
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Definitions Definitions : i C Molar concentration of species i. ( ) 3 kmol/m : i ρ Mass density (kg/m 3 ) of species i. : i M Molecular weight (kg/kmol) of species i. ii i C = M * : i J Molar flux of species i due to diffusion . () 2 kmol/s m ¾ Transport of i relative to molar average velocity (v*) of mixture. : i N ′′ Absolute molar flux of species i. ( ) 2 kmol/s m ¾ Transport of i relative to a fixed reference frame. : i j Mass flux of species i due to diffusion . 2 kg/s m ¾ Transport of i relative to mass-average velocity (v) of mixture. ¾ Transport of i relative to a fixed reference frame. : i x Mole fraction of species i ( ) / . xC C = : i m Mass fraction of species i ( ) / . m ρρ = Absolute mass flux of species i. ( ) 2 kg/s m : i n ′′
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Property Relations Property Relations Mixture Concentration : i i CC = 1 i i x →= Mixture Density : i i ρ = 1 i i m Mixture of Ideal Gases : i i i p C T = i i i p RT = i i p p = ii i Cp x ==
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Diffusion Fluxes Molar and Mass Fluxes of Species A due to Diffusion in a Binary Mixture of Species A and B Molar Flux of Species A: ¾ By definition: () A vv AA JC ∗∗ =− AB vvv xx =+ ¾ From Fick’s law (mass transfer analog to Fourier’s law ): B A J CD x Binary diffusion coefficient or mass diffusivity (m 2 /s) Mass Flux of Species A: ¾ By definition: A j ρ A v = v v B mm + ¾ From Fick’s law: B A jD m
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Special Case of a Stationary Medium • Valid for many problems involving mass diffusion in solids or stationary liquids.
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This note was uploaded on 03/11/2011 for the course CHEME 333 taught by Professor Anthony during the Spring '11 term at UNL.

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Chapter 14 Handouts - Diffusion Mass Transfer General Considerations General Considerations Mass transfer refers to mass in transit due to a Must

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