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X1t convergence problem


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Sorry posted this on the archive thread...


Hi, I am running a simulation of a flow through column experiment and slowly working up to the full rtm. I run the initial solution through for 5 days then the contaminated solution. calcite reacts based on kinetic constraints, precipitation of gypsum, gibbsite, Fe(OH)3 at equilibrium all else suppressed, adsorption using the D&M model and the FeOH database (decreased site densities of w and s sites by an order of magnitude). after adding adsorption I cannot get it to finish. current error is "Step is constrained by rate of increase in length of reaction step at node 0." after 8 days of 10 day run. Any ideas?



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Ok, sorted some out. Turns out the contaminated fluid was undersaturated with respect to Fe(OH)3 so by day 8 it has exhausted the Fe(OH)3 and that made convergence not possible for some reason that seems to be related to increase in the length of the reaction step in the first cell. So using goethite instead, seems to be working.... So I am wondering about the why of it. Fe(OH)3 is the adsorbant and is the breakdown because there is no adsorbant left , other minerals (calcite for instance) do go to 0 in the first cell, so does that mean that there has to be some adsorbant left in a cell in order for simulation to go forward? 

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Hello Dirk,

I am glad you got your model up and running. It sounds like your convergence issues might be linked to your model running out of minerals. When you run out of mineral(s) or some other reactant in your system, the model will take smaller time steps to try to calculate the numerical solution, which might explain why you received the error “Step is constrained by rate of increase in length of reaction step at node 0”. In this case, you were trying to sorb solutes onto Fe(OH)3 when it is running out, which is probably why your model couldn’t converge. However, I can’t be sure unless I take a look at your input file but that would be my guess. If you would like someone to help take a closer look, please attach your input and thermo file.

Hope this helps,

Jia Wang

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