L16 - Lecture 16: Diffusion & Mass Transfer...

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Unformatted text preview: Lecture 16: Diffusion & Mass Transfer Diffusion & Mass Transfer Diffusion Extract Raffinate conc Diffusion is movement of a component through a mixture not associated with bulk flow or convection. Causes of Diffusion: Chemical potential gradient- concentration gradient- pressure gradient- temperature gradient- force-field (centrifuge) - Similar to conduction C ~ T But unlike heat, molecules occupy space. Diffusion makes molecules move. 2 Types of Situations : 1. One component mass transfer: Only one component is being transferred from one phase to the other. Example: Absorption Gas Liquid A 2. Counter-Current Diffusion B Gas Liquid Example: Distillation A (a) Equimolar counter-current diffusion Flux A = -Flux B Net flux = 0 (b) Counter-current diffusion with Flux A = Flux B Total Flux = 0 Variables 1. Flux across a fixed plane: N A (moles / area-time) Molar average velocity N A N A = C A u A Fixed Plane b N B N B = C B u B N = N A + N B = C A u A + C B u B = m u o J A J B Definition of J A J A C A u A- C A u o Definition of J B J B C B u B- C B u o 2. Flux relative to a plane that moves with the molar average velocity (u o ): J A (moles / area - time) 3. Molar concentration of A: C...
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This note was uploaded on 04/15/2008 for the course CHEE 3462 taught by Professor Mohanty during the Spring '07 term at University of Houston.

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L16 - Lecture 16: Diffusion & Mass Transfer...

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