Initial mass variable

[kateduke]

Hi All,

 

What is the name of the variable that represents the initial mass of a reactant? That is, if I start with 0.5 grams of calcite, and I want to calculate the reaction progress as a function of the Ca in solution divided by the original Ca mass (in the calcite) how do I represent the initial Ca or calcite mass in an equation or script?

 

Thanks, Kate

[Brian Farrell]

Hi Kate,

 

I guess I'm not sure what you're trying to do. Where is it that you're trying to use an equation or script?

 

Regards,

 

Brian Farrell

Aqueous Solutions

[kateduke]

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

[Brian Farrell]

Hi Kate,

 

Thanks for the clarification. I'm looking into this.

 

Regards,

Brian

[Brian Farrell]

Hi Kate,

 

You can return the mass of mineral at the current time step, but there's no command to return the initial mass of a mineral. However, it might be possible to write some code to retrieve the mass of a mineral at each node during the first timestep, then evaluate your rate law using those saved values.

 

I'm out of the office all this week, so please understand response times will be a little slow.

 

Regards,

Brian

[kateduke]

Hi Brian,

 

That would be great. No worries about response time--I appreciate you being willing to try to find a solution.

 

Thanks, Kate