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si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure
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Topic: si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure (Read 526 times)
zsszabo86
Frequent Contributor
Posts: 16
si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure
«
on:
November 05, 2019, 09:38:37 AM »
Good day!
I have run equilibrium models in which I have a gas phase of CO2 and Helium, a carbonate rock, and a solution.
I decrease the pressure step by step with the REACTION_PRESSURE keyword.
I have three questions/difficulties:
1. If in the original gas phase I give 200 bars which is divided on a certain way among CO2 and Helium (giving their partial pressures summing up to 200 bars) and then later I change the pressure by REACTION_PRESSURE, then the partial pressures will change in the models following the same ratio what I given originally? For example 200 bars is originally from 199 bars CO2 ans 1 bar Helium, then if the pressure gets 100 bar, then it will be from 99.5 bar CO2 and 0.5 bar Helium? Is it automatic? Are there any limitations if there are many decimals?
2. In the SELECTED_OUTPUT I print out si_CO2(g) and g_CO2(g) as well. As I understand si_CO2(g) is the log10 partial pressure of CO2, whereas the g_CO2(g) is the moles of CO2. However, if I calculate the partial pressure to moles, I get a very different value. An example:
si_CO2(g)
2.0439
this is 110.6 bars
then 110.6/(0.08206*343.65)=3.92 (n=p/(R*T), R is given in L atm K−1 mol−1 units)
But
g_CO2(g)
1.47E+01 which is far not equal to 3.92
Why these numbers are not the same?
3. If I run a model for example with 250 bars, I print out si of all gas phases, also H2O, then I sum up their partial pressures, I even calculate the partial pressures for which the dissolved and mineralized gases could be responsible for, I am not getting 250 bars, but just around 125 bars. Is there any experience with this? Why these numbers are not matching either?
Thanks for any replies for any of the points!
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dlparkhurst
Top Contributor
Posts: 2301
Re: si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure
«
Reply #1 on:
November 05, 2019, 02:53:32 PM »
You don't give quite enough information, but here are a few comments.
There are two equations of state that may be used for GAS_PHASE, depending on the database that you use. phreeqc.dat, Amm.dat, and pitzer.dat use the Peng Robinson equation of state (provided parameters are provided for all of the gases); see Appelo, Parkhurst and Post, 2014. Geochim. Cosmochim. Acta 125, 49–67. He is not included in these database by default, so I don't think you are using this equation of state. At 200 atm, the non ideal effects are probably significant.
The ideal gas law is used for other databases, which is probably what you are doing. In this case, yes PV =nRT for the gas phase. There are non-ideal effects through activity coefficients in the solution, which will have a minor effect on uncharged aqueous species.
The other piece of information is whether the gas phase is fixed volume or fixed pressure. With fixed volume, the gas phase always exists, and the pressure of the gas phase will vary; REACTION_PRESSURE will have an effect on the equilibrium constants that are calculated. For a fixed-pressure gas phase, the pressure of the gas phase and equilibrium constants are set by REACTION_PRESSURE, and the volume of the gas phase will vary. I did not see you take explicit account of the volume of the gas phase in your calculation, which will vary in the fixed pressure gas phase.
I need to see the input file to answer some of your other questions.
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zsszabo86
Frequent Contributor
Posts: 16
Re: si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure
«
Reply #2 on:
November 05, 2019, 03:36:27 PM »
Thank you for the reply!
The difference looks so big to me that I cannot imagine that the problem can be resolved by considering the non-ideal effects. I might be wrong, I am aware.
Thank you in advance, if you have a look at my input file. I also attach an excel sheet detailing my thinking.
Please advice me at any points of the file. For example there might be a better solution for including the isotope fractionation as well.
My aim is to model the CO2/He3 ratio as the pressure decreases underground around a CO2 gas bubble. Plus, I am interested in the mineralization processes as well.
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dlparkhurst
Top Contributor
Posts: 2301
Re: si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure
«
Reply #3 on:
November 05, 2019, 05:21:50 PM »
I think the non-ideal effects with Peng Robinson may be greater than you think.
With the ideal gas law n/V = P/RT. The Basic function GAS_VM gives Liter/mol for the Peng-Robinson equation of state. [Note there is also the function PR_P that gives the partial pressure of a gas in the gas phase, and PR_PHI that give the fugacity coefficient of a gas, where K = a/(lambda*P), where a is activity in solution, lambda is the fugacity coefficient and P is the partial pressure.]
The attached file is a simplified version of your file that compares P/RT with 1/GAS_VM to show the non-ideal effects. I am not sure if this resolves your concerns, but consider running the calculations without Peng-Robinson for comparison.
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zsszabo86
Frequent Contributor
Posts: 16
Re: si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure
«
Reply #4 on:
November 07, 2019, 03:25:47 PM »
Many thanks for the reply!
Let me have another question. Why can it be that if I sum up the partial pressures, I am not getting the total pressure? I even calculate the partial pressures for which the dissolved and mineralized gases could be responsible for. You may see that in the column "SUM partial pressures+dissolved and mineralized" of the excel file attached earlier. I cannot figure out the reason.
Regards,
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dlparkhurst
Top Contributor
Posts: 2301
Re: si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure
«
Reply #5 on:
November 07, 2019, 07:01:25 PM »
The value given in the list of saturation indices is log10(fugacity). The partial pressure of one of the GAS_PHASE gases is given with PR_P. Again PR_PHI is the fugacity coefficient.
The example in the previous post sums the values of PR_P to show they sum to the REACTION_PRESSURE.
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si_CO2(g) vs. g_CO2(g) in SELECTED_OUTPUT; decreasing pressure