How to use different CO2(g) model?

[ats5482]

Hi,

How do I tell React to use a different CO2(g) activity model?

I created a high-pressure (1 kbar) SUPCRTBL database (attached) using a Python package called PyGCC. It uses the Duan & Sun (2003) CO2 activity method. According to Section 3.4.2 of the Reference manual, I need to set the relevant ion size, a0, values to -0.5. CO2(g) is of course not an ion, so I'm not sure what I'm supposed to change. HCO3- and CO3--?

Thanks!

-- Adam

1kbar_db_test_2.tdat

[Jia Wang]

Hello Adam,

The ion size parameter, a0, in the debye huckel (or bdot) activity model has a special meaning as you had learned in the Reference manual for neutral species. The equation used to calculate the neutral species' activity coefficient may change depending on a0. For charged species, like HCO3- and CO3--, the ion size is used in the bdot equation (an extended form of the debye-huckel equation) for calculating species activity coefficients. Please see this post for a more detailed explanation of what the ion size parameter represents.

Taking a look at the introduction of the paper cited, I noticed that they mention that the model developed uses the specific interaction model of Pitzer. The dataset you attached uses the debye-huckel model, which is very different. I suggest double checking that you are generating the correct dataset. The GWB can use the Harvie-Moller-Weare(HMW) formalism of the Pitzer equations for modeling high ionic strength solutions. Unfortunately, I don't have access to Duan etal. 2003 and can't compare it to the HMW model. For more information on activity models in the GWB, please see section 2.3 in the GWB Essentials User Guide. You can open GWB datasets in TEdit and find more information regarding its origins in the Headers Pane. In particular, you may be interested in thermo_hmw.tdat.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[ats5482]

Hi Jia,

I think I figured out the ion size part. In TEdit, I was looking at CO2(g), which doesn't have an a0 parameter. However, CO2(aq) does. This is the a0 that I should change, correct?

Thanks for checking out the paper; I didn't realize the Pitzer model is different. When I made this database, I verified that I did it correctly with one of the authors of PyGCC. He recommended I use the Duan Sun method (because I'm doing high-P models), and to set a0 to -0.5. Given that Duan & Sun (2003) use the Pitzer model, I'm now not sure where the CO2 activity coefficient terms in the database I made come from. I will ask him for clarification.

Thanks,

Adam

[Jia Wang]

Hello Adam,

There are special values that you can set a0 that that tells the software to use a specific equation for calculating the activity coefficients of neutral species. When a0 = -0.5, the activity coefficient is calculated from the "CO2" coefficients in the data table section of the dataset according to equation 8.6 in the “Geochemical and Biogeochemical Reaction Modeling”. When a0 = -0.6, an alternate calculation from Drummond (1981) is used. If your dataset's CO2 coefficients were adjusted by the Duan Sun, then you can set a0 to use the CO2 coefficients to calculate your CO2 ion activity. You should check what PyGCC does when it generates the dataset and how it accounts for the Duan Sun method for activity coefficient calculations.

Best regards,
Jia

[ats5482]

Hi Jia,

That clears everything up, thanks! I'll go back and verify how PyGCC is generating the CO2 activity coefficients.

Best,

Adam