Modified h-m-w database, works at t>25 C, but not at 25C

[Anastasia]

Hi,

 

I am modeling the chemistry of a Na-Cl brine. For the calculation to be more relevant to my study area I've modified the thermodynamic database to include Al, Si, Cd, N and Si species. Activity correction is h-m-w. As usual, the majority of the Pitzer coefficients are known for 25 C, with some exceptions, therefore the results are most accurate at 25 C. Using the modified database the calculation converges when I use temperatures above 25 C (e.g. 26 C, 59 C). When I try running at 25 C I get the following error:

 

Solving for species distribution.

 

Loaded: 45 aqueous species,

162 minerals,

2 gases,

0 surface species,

12 elements,

0 oxides.

 

 

Ionic strength out of range.

Largest residual(s):

Resid Resid/Totmol Cbasis

--------------------------------------------------------

O2(aq) 0.002616 1 9e-031

--------------------------------------------------------

 

The calculation does run if I delete O2(aq) and Fe++ data entries (they are not zeros).

 

I was hoping someone can help me figure out why while using a modified h-m-w thermo database I get an error at 25 C temperature, but at other temperatures the calculation does converge. I've attached a script file with the data.

 

Thanks,

Anastasia

AGI_test.sp8

[Brian Farrell]

Hi Anastasia,

 

Sorry for the delay in response. I have been out of the office the past couple weeks conducting training. If the model converges at 26 C but not 25 C the problem is likely not due to changes in the stability of certain minerals or species, but rather from differences in what minerals and species are loaded at the different temperatures. For example, if data is available at 25 C for a particular species, but not at 60 C, then it will be considered in runs at 25 C exactly but not in runs anywhere between 25 and 60 C. You might try using the extrapolate function to test whether this fixes your problem (type "extrapolate on" in the command pane). Without your custom dataset, however, I can't test this myself.

 

Take a look at this and if it still fails to run perhaps you could post your thermo dataset or send it to support@gwb.com.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

[Anastasia]

Hello Brian,

 

Thank you so much for the reply. I did a quick test and typing in "extrapolate on" in the command does not seem to make any difference in the outcome. I'll e-mail you my files to test.

 

Thank you so much!

Anastasia

[Brian Farrell]

Hi Anastasia,

 

Thanks for sending your thermo dataset. If you look at the Results pane, you should see the list of Loaded species, minerals, gases, etc. This list is controlled by the components in the Basis pane, the temperature range of the run, and the list of suppresses species. At 25 C, I believe 46 aqueous species were loaded, but at 26 C only 28. Since your Basis is the same, and you're not suppressing anything, the only difference is that some species are not being loaded because there is no thermo data outside 25 C.

 

The "extrapolate" option can be useful here for troubleshooting because it ensures that you will consider the same species in your runs at various temperatures. In your case, using the extrapolate option does not make your model converge at 25 C. In fact, it makes the model fail at T>25 C as well. This gives me an indication that some of the species or minerals that were being considered in the 25 C run, but not the others, were the source of the problem.

 

To check this, I can make a list of all possible species that can form at 25 C (46 species), and those which form at T>25 C (28 species). The subset which are loaded at 25 C only can be suppressed (not considered/ loaded), and the model rerun, to see whether a solutions can be found. By suppressing these species, the model will converge at 25 C and higher. You typically don't want to suppress species for no good reason, however, so you might unsuppress species one at a time to find out which one(s) cause the model to fail.

 

After doing this, it looks like Fe(CO3)2-- is the culprit. With your knowledge of your system, or of the custom thermo dataset, perhaps you will be able to determine why this is the case. Is the equilibrium constant for the reaction specified in the thermo dataset (Fe(CO3)2-- + 2 H+ = Fe++ + 2 HCO3-) correct? You might consider whether Fe(CO3)2-- is likely to be important in your system, or whether it can safely be ignored (suppressed).

 

Hope this helps,

Brian

[Anastasia]

Brian,

 

Thank you so much for the solution to this issue!

 

I hope you had nice holidays,

Anastasia