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The handling of pressure in GWB and other software


AMSwift
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This post contains what I currently know (actually, just *think* I know) about handling elevated pressure.

 

That's not very much. I'd be very grateful indeed for corrections and additions on this topic.

 

 

GWB doesn't adjust activity coefficients for pressure. All system are assumed to be either 1 bar or at the vapor pressure of water. If you're dealing with hydrothermal systems, say, this can be a bit of a problem.

 

If I want to include pressure in my GWB models, then the choices I'm aware of are:

 

1. Read "K2GWB: Utility for generating thermodynamic data files for The Geochemist's Workbench® at 0–1000 °C and 1–5000 bar from UT2K and the UNITHERM database" by Cleverley and Bastrakov at http://www.sciencedirect.com/science/article/pii/S0098300405000105. Contact Dr. Evgeniy Bastrakov and request UT2K. He will supply UT2K, but you'll have to purchase HCh (which does the actual Gibbs free energy work required) and UNITERM (the database). Cost is Australian $737 Product page. The HCh manuals are here:at Dr. Yuri Shvarov's (the author) site.I'm currently struggling to make this work for all species and minerals relevant to my application, so will NOT claim that this is a complete fix. More if I get results.

 

2. Use one of several competing software packages, PHREEQC/PHAST, TOUGHREACT, EQ3/6, or several others.

 

 

Do you have a better (or at least easier) idea?

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Pressures in hydrothermal systems aren't as big a concern as they are in say metamorphic petrology, because the deviation from the steam saturation curve isn't that large, in an absolute sense. Hydrothermal chemists are commonly more worried about error in measuring species and mineral stability (the log Ks) and in estimating activity coefficients as a function of fluid composition (the gammas).

 

If you do make an accounting of the effect of pressure on chemical stability, you generally do so by adjusting the log Ks in the thermo dataset, rather than in the activity coefficients. To my knowledge, PHREEQC, EQ3/6, etc. have no special provision for making pressure corrections to the thermo data under hydrothermal conditions. (Remember, we are talking about confining pressure here, not the partial pressures of gases, which you set in the GWB as fugacity values.)

 

If you feel that a pressure correction is important enough to your modeling to make the effort worthwhile, you can proceed along the U2TK route, or you can post a message to the forum inquiring about what databases others may have prepared for the conditions you're studying. Additionally there are a few old topics in the archive with questions about pressure - you might look there for additional sources of information.

 

But in any case keep in mind that the uncertainties in the log Ks and gammas are considerable and may well overshadow the pressure corrections you end up estimating.

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Dear all,

 

The importance of pressure on hydrothermal reaction equilibria would certainly depend on the magnitude of pressure and the P-T region that you are dealing with. For example, at 250 °C, the dissociation constant of water (H2O = H+ + OH-), admittedly the most fundamental aqueous reaction, will be 11.2, 10.9 and 10.7 at Psat, 500 bar and 1000 bar. At 350°C, the values will be 11.8, 11.1 and 10.6 for the same presures. The difference of 1.2 log units translates into 0.6 pH units, certainly not negligible. The statement that pressure does not matter in hydrothermal systems appears to be not valid IMHO.

 

The route via converting data from HCh/Unitherm via UT2K certainly works well, I have tested it and except a bit of manual editing that needs to be done on the final datafile the procedure is straightforward. A general advantage of software packages that are based on Gibbs energy minimization is that they use internally continuous equation of state functions rather than a grid of discrete log K values, resulting in more smooth behavior of calculated results. Also, one can model systems with nonideal phases such as nonideal gas mixtures or solid solution. As an alternative to HCh/UT2K you can use the following free package:

http://gems.web.psi.ch/

 

The GEMS package can do essentially all reaction path and batch equilibria calculations that GWB can do. The only drawback is that at present it does not plot activity diagrams.

 

Best regards,

 

Thomas

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