Vincent
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Posts posted by Vincent
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Hello,
I am unsure what the meaning of "free volume %" versus "volume %" in the React software is. I am trying to figure out if it is a percentage of the total mineral mass (as I tried to input it as in my screenshot) or a percentage of the solvent mass / volume. The values I input for each mineral come from "The Crust of Venus: Theoretical Models of Chemical and Mineral Composition" by Barsukov et. al. (1982) which is shown in the first column of the second screenshot. We are trying to run React on a mixture of minerals in equilibrium with water to see what precipitates. In the past we have successfully done this with a starting liquid, but now we get errors when minerals get in the mix. I believe our first issue comes from our use of volume %, but any suggestions would be appreciated. A screenshot of the error message is included as well.
Thank you,
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Helge
I think I checked aphi in the past and I had not seen any significant change or that abrupt at 239 K (I am still trying to figure out why this temperature since none of my values is defined at this temperature, I go by 20 K increments from 193 to 333 K).
How can I send you the database privately?
Thank you again for your help
Vincent
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Helge,
To answer your comments:
- I am aware of the martian geochemistry papers (that one and a whole collection of similar papers). This is the reason why I am trying to develop this database. Marion (the paper you cite) uses a code called FREZCHEM which works under 0oC and can model freezing and evaporation. Interestingly I tried to include ice precipitation in the database but this generates an error ("phase rule violation") when running the code but it still shows the saturation index for water ice, which is all you need to see when ice precipitates.
- No database is designed to run at such low temperatures. My aim was not to run simulations that low, more around -40oC but I wanted a smooth set of Pitzer parameters over this range of temperature. Some of the parameters I use have been tested this low (some down to -50 -60oC).
- I know some parameters can behave weirdly beyond their temperature range. This is due to the fact they are fitted with polynomial equations which can be difficult to extrapolate. But in the absence of measurements at such low temperatures, I either used constant values or values measured at these temperatures.
My issue is not a "weird" Pitzer parameter (at least from what I can see) because the transition is way too sharp (figure I initially attached), unless I am missing something somewhere.
Sincerely
Vincent
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Hello,
I have encountered an issue when running the react code I can't explain. For my research I need to run evaporation simulations well below the freezing point of pure liquid water. Therefore, I designed an extension of the Thermo_phrqpitz database down to -80oC (60, 40, 20, 0, -20, -40, -60 and -80oC). I added numerous minerals and aqueous species along with their pitzer parameters, from recent literature publications. However, when I run the code, I encounter a sudden jump in water activity between 239 and 239.1 K, which I can't explain in any way. This jump affects the water activity, the ionic activity and therefore the saturation index, generating precipitations of minerals at much higher water activity than they should (basically from 273 down to 239, the results "make sense" but not below, the water activity is way too high). I attach a figure showing such jump in water activity for a 50/50 mol% Mg(ClO3)2 / Mg(ClO4)2.
Does anyone has any idea why such jump occurs?
Thank you
Sincerely
Vincent
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Hello,
In my laboratory we have developed a dynamic model using Matlab. Our code requires calculating some thermodynamic parameters like the concentration of some elements during evaporation and we thought about using GWB.
However, I don't know how to create the pipes mentioned in the Reference Manual of GWB to be able to control GWB from our code.
Does anyone has some experience in creating programs or routines that allow controlling GWB from Matlab and would be willing to share their experience?
Also does someone knows if the helper functions provided for C++ or TCL exist for Matlab?
Thank you very much
Best regards
Vincent
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"Admittedly reading the data in from database file and converting it to the right format might be more tricky (and could pose a compatibility problem with old database files, but perhaps one can just append the new coefficients after the existing ones and just read treat missing coefficients as zeroes)."
The GWB thermodynamic databases based on the Pitzer model (Phrqpitz) can be modified to add new Pitzer coefficients. I have been doing this for a while and it work pretty well.
"If this is programmed in a modular way with derivatives of the original T function propagating into activity coefficient (and osmotic coefficient) expressions, replacing the original T dependence and its derivatives by a new one should be no big deal actually."
Sorry but I am not sure I understand what you mean. The program is defined not to read various values of the coefficients at various temperatures (the way the solubility constants are defined) but to read the fit coefficients X0, c1, c2, c3 and c4 and recalculate the parameters from these and the equation. This is why I have difficulties defining the parameters if the temperature dependency does not follow the predefined fit equation.
Vincent
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Thanks for both answers. I agree it may look a bit complicated to redesign the program, I was just wondering if there was a simple solution.
I also agree that 8 different terms can look a bit like over-complicating the situation. I realized I forgot to precise that not all of them are used. It is a general equation used by some author to fit the data. I think the highest number of parameters I have seen is 4. However, in some cases the T3 or the T4 or the 1/T2 were used.
Therefore, even if I did not necessarily need the complete equation, I needed a way to define them in GWB.
The whole idea is that I use compilations of parameters and their temperature dependency is based of this general equation. I could track down the original data, and do the fit with the GWB equation, but I preferred to save a lot of time and use these parameters.
In any case it would be useful to have custom defined virial coefficients, just in case, and especially because their temperature dependency is always based on empirical equations.
Best regards
Vincent
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Hello,
I am currently creating a new database, based on the phrqpitz database. In the database, the virial coefficients (beta_0, beta_1, beta_2, C_Phi, theta, lambda and psi) temperature dependency is defined by a polynomial equation of the form:
X = X0 + c1*(T - T0) + c2*(1/T - 1/T0) + c3*ln(T/T0) + c4*(T2-T02) where T0 = 298.15K, T is the temperature in K and X0 is the value of the parameter at 25oC.
For the coefficients I am using I need more parameters since the coefficients are fitted by a general equation of the form:
X = X0 + c1*(T - T0) + c2*(1/T - 1/T0) + c3*ln(T/T0) + c4*(T2-T02) + c5*(T3-T03) + c6*(T4-T04) + C7*(1/T2-1/T02)
Is there any way I can modify the equation used in GWB to adapt it to my database, rather than recalculating all the parameters to find a temperature dependency fitted by the standard GWB equation? Although this is feasible, it is very time consuming and often I can't find a standard equation fitting as well my general equation.
Thank you very much in advance
Vincent
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Hello,
I'm relatively new in the use of GWB (version 6.0.3). I'm especially interested in React to trace the evolutions of evaporating solutions and as function of temperature.
I know that for example it is possible to determine the precipitating parageneses with increasing temperature. However, my models aim at simulating surface processes on Mars. Therfore, I'd like to simulate the processes occuring under 0 C. I know that saline solutions at low temperatire have a lower eutectic temperature. In some cases some eutectic solutions can remain liquid down to -70 C (Ferric sulfate for example). I previously modelled such behaviors on simple salts using the Pitzer model. I know there is a Pitzer database working very well for evaporation but I would like to be able to use it in models at low temperature. Is there a way I could model/program this in the command section?
Also two quick questions:
- How can I calculate the water-to-rock ratio during evaporation other than manually by adding all the precipitated phases and dividing by the amount of water
- How can I extract the water activity (for example as a text file) calculated using the Pitzer model after running a model?
Thank you very much.
Vincent
Reference for GWB Reaction Modeling Guide
in The Geochemist's Workbench
Posted
Hello,
In Release 12 of the GWB Reaction Modeling Guide, an example calculation on Feldspar is used with a hypothetical water composition. The composition is listed in Section 2.2 "Example Calculation" and I've attached it. I used this as a guide to get me started for my latest model. Does anyone know if this is simply hypothetical, or if there is literature that supports the numbers used in this example? In other words, what reference (if any) was used for these numbers?
Thank you,