Mass Transfer by Diffusion General Considerations Mass transfer
Mass Transfer by Diffusion General Considerations Mass transfer refers to mass in transit due to a species concentration gradient in a mixture.
Units in Mass Transfer
Property Relations • Mixture Concentration: • Mixture Density: • Mixture of Ideal Gases: Dalton’s law for gas mixture
Units, Dimensions Molar concentration of species i. Mass density (kg/m 3) of species i. Molecular weight (kg/kmol) of species i. Molar flux of species i due to diffusion. Ø Transport of i relative to molar average velocity (v*) of mixture. Absolute molar flux of species i. Ø Transport of i relative to a fixed reference frame. Mass flux of species i due to diffusion. Ø Transport of i relative to mass-average velocity (v) of mixture. ni¢¢: Absolute mass flux of species i. Ø Transport of i relative to a fixed reference frame. Mole fraction of species i Mass fraction of species i
Analogy (Fick’s law) Molar and Mass Fluxes of Species A due to Diffusion in a Binary Mixture of Species A and B • Mass Flux of Species A: Binary diffusion coefficient or mass diffusivity (m 2/s) • Molar Flux of Species A:
Mass Transfer in Moving Media
Velocities and Fluxes
Absolute Molar and Mass Fluxes • Molar Flux of Species A: • Mass Flux of Species A: • Special Case of Stationary Medium: Ø Ø
Evaporation in a Column
For steady, one dimensional conditions with no chemical reaction
Solution
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