Couple diffusion and convection

Hi,

I have a packed bed with spherical elements from which material is being extracted. I would like to know the concentration profile in the particles and in the axial direction of the packed bed.

I can solve the diffusion equation for a single particle and I can solve laminar flow without particles but I dont know how to couple both (flux out of the particles determines the concentration in the bed).

Any help?


Thanks in advance