New to FEM - have started using COMSOL. Trying to adapt a "stiff-spring" method that was used in this example: http://www.engineering.uiowa.edu/~cbe_217/Misc/dialysis.pdf Questions: 1. How does this "stiff-spring" approximation method work? I got the books that where listed as references, but I still can't seem to piece it together. I guess I am wondering how it's derived. 2. Could I use the "stiff-spring" boundary conditions at only one boundary (i.e. create two domains, with different saturation values, and then link them with a stiff-spring boundary condition in order to solve for steady state?) 3. The partition coefficient confuses me a little. It's set as the ratio of concentrations? But which concentrations? Apparently it's a constant - so where is it coming from? If you guys and gals could help me on any of the following questions (or just point me in the right direction) I'd be grateful. Thanks.