Hi Brian,
What I'm trying to do is represent a "shrinking core" kinetic model. I have a rate law from experimental data that describes the initial rate of a leaching reaction that I put in the "rate law" line. And then I have another relationship (developed from multiple dissolution experiments) that describes the decrease in the leaching rate over time as a function of the progress of the leaching reaction (the overall reaction rate is decreasing due to the rate at which the reactants and products diffuse through the increasingly deep leached layer on the surface of the mineral and due to the decreasing concentration of the reactants (in this case H+) with time). This relationship I am putting in the "rate constant" line. So I'm basically using a time-variant rate constant that is based on the reaction progress to represent the shrinking core kinetics. This works well in React where I can specify the initial mass of the mineral and then hard-code that value into the rate constant equation. However, the next step is to bring this kinetic phase into X2t where the initial mass of the mineral will be heterogeneously distributed so I'll no longer be able to hard code that initial mineral mass into the rate constant equation. What I would like to be able to do is to write the rate constant equation in terms of the initial mineral mass for any given nodal block--e.g. rate constant is a function of the concentration in the fluid at each time step (e.g.[Ca]) divided by the initial mass of the mineral from which the Ca is being leached (e.g. a Ca-silicate). I tried doing this using Xi but Xi goes to one (1) at the end of the time period (which drives the rate to zero) whereas, depending on the fluid composition, there may still be Ca left in the mineral phase at the end of the model time period. So I'm wondering if there is a variable name that I can use to call that initial mineral mass as part of the rate constant equation--e.g. [Ca++ in solution]/(Initial Mass Ca-silicate).
I hope this clarifies my question. If not, I can send you the input file via email.
Thanks, Kate