Mass extraction and saturation of bulk solution

Hi all,

I am fairly new to COMSOL. I am after solving a non steady-state packed bed extractor with fluid flow convection in the axial direction of the extractor and diffusion of soluble material from the solid particles.

The first thing I am trying is to solve the diffusion equation in 2D with different boundary conditions, as if the particles were in a stirred vessel.

My question is:

When a forced convection boundary condition is imposed, how can be the saturation of the bulk fluid be accounted for?

Manually the process would be

1) Bulk concentration of the fluid is 0 at time 0
2) After the first time step has elapsed, the solution of the diffusion equation gives a total flow (mass released) out of the particle. That mass divided by the volume of the vessel gives the bulk concentration for the next time step.

In COMSOL, how can I update the bulk concentration by imposing that it would be the volume integral over the particle divided by the volume of the vessel for each time step? Is there a way to create a for loop of should I combine this problem with Matlab?

Thanks in advance

B