Different result between normal and sliding temperature

[Kohei AKAKU]

I have two different results from the attached input file with normal temperature setting at. 25 deg. C, and with sliding temperature 25-50 deg. C. The results are from the same fluid composition.

One is pH 8.17and 0.0004686 bars in CO2 partial pressure at 25 deg. C. The other is pH=8.36 and PCO2=0.00032 bar  at 25 deg. C with the sliding temperature from 25-50 deg. C. 

I would like you to advise me where the difference comes from.

GSS_Seawater Nozaki 1997_ERROR.rea React_output_GSS_Seawater Nozaki 1997_T25ERROR.txt React_output_GSS_Seawater Nozaki 1997_Tslide25-50ERROR.txt

[Brian Farrell]

Hi,

With T-table thermodynamic datasets, it is expected that the results of an isothermal calculation at a principal temperature (e.g. 25 C) can be somewhat different from a polythermal run at the same temperature. In isothermal calculations at a principal temperature, the program uses the log Ks specified at that temperature directly. But in a polythermal path, it will use a polynomial fit to the data. And where data isn’t available to span the full range of temperatures considered in a polythermal calculation, a species might not be loaded. So, an isothermal calculation at 25 C could have a different set of species considered than a polythermal run spanning 25 – 50 C.

You can use the span feature (e.g. “span 25 C to 50 C”) to limit the species considered to make them equivalent in both runs. Alternatively, you can use the extrapolate feature to estimate log K values for species with limited data. For more information, please see the “span” and “extrapolate” commands in the React chapter of the GWB Command Reference. Please see as well section 3.2 Temperature expansions in the Thermo Datasets chapter in the GWB Reference Manual.

Also note that you describe and attach results for a reaction path that goes from 25 to 50 C, but the input file you attach ends at 0 C, not 50 C. The solution - use a consistent “span” or “extrapolate” - still applies.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC
 

[Kohei AKAKU]

Hello Brian,

Thanks for your quick response.

I understood that difference may happen in polythernal from isothermal calculations due to the logKs regression error. I am just surprised at the difference at 25 deg.C for typical seawater.

To solve the problem I should check all logKs in thermo.tdat by using "span" and "extrapolate" commands? It seems to be a big task for me.

Regards,

Kohei