mass transfer from fluids to membrane

Peter

Hi,

I have some difficulty in modeling mass transfer in a membrane separation process. My question is: how to couple mass diffusion in a solid-like (not porous) membrane with mass transfer near the membrane surface?

1. Case description. Binary Fluid (with species B dissolved in water) flowing through a flat slit (2mm high, 20mm wide, 190 mm long). The bottom of this rectangular geometry is a nonporous membrane. Species B will be PREFERENTIALLY permeate through membrane to reach the other side of membrane where a high vacuum is applied.

I use Gambit and Fluent 6. The difficulties I encounted included:

1) Fluent can just take membrane as SOLID, in this way I have no way to input diffusivity of B within membrane. Membrane in this case is still( not moving), yet it is not impermeable solid like steel.

2) How much B should go into membrane should be determined by concentration of B at membrane surface in contace with fluid. As Fluent seems to provide no model for mass diffusion in solid, I compromised to used surface reaction deposition model for this purpose. But in this way, the reaction rate is pretty arbitrary, though I can finally deterine the correct value by error-and-trial approach to let flux from fluid into membrane be the same as the flux through membrane to vacuum.

Thanks for your help,

Peter