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[OLD] Pressure and general comments


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From: Joel Brugger

Subject: Pressure - and general comments

Correct me if I am wrong, but I understand that GWB can only work under vapour saturated pressures, up to 300 C. There is really no theoretical reason for this limitation - only historical ones. We have ways to estimate properties over a wide range of temperatures and pressures. I suggest that the inclusion of the SupCrt database and algorithms into GWB would be a very valuable addition to the program!!!

From: Craig Bethke

Subject: Re: Pressure - and general comments

The GWB can work at any pressure, and pressure can vary with temperature. The pressure curve is set within the thermo database, in a table near the top. As you note, geochemists generally compile thermo data at 1 atm (below 100 C) or along the steam saturation curve (100 C and above), because these are the conditions at which the data is collected. Furthermore, there's little impetus for compiling data at other pressures because the change in log K with pressure for most reactions is commonly considered small compared to the uncertainty in this value (there are notable exceptions). But you can create a database for any pressure conditions by correcting the stability constants for species and minerals (according to the well-known formula involving the standard volume change of reaction) in the dataset, and then setting the pressure table at the top of the file. You can use a program like SUPCRT to do this, but it's a simple calculation to do by hand or with a spreadsheet. I might note that when people inquire about the effects of "pressure", they are almost invariably referring to the partial pressure of a gas such as P-CO2. Such variables of course can have dramatic effects on solution chemistry. In the GWB, you set partial pressure (actually fugacity) directly with the "fugacity" command; the discussion above applies only to the more subtle effects of confining pressure on mineral and species stability.

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From: Joel Brugger

Subject: Pressure - and general comments

Correct me if I am wrong, but I understand that GWB can only work under vapour saturated pressures, up to 300 C. There is really no theoretical reason for this limitation - only historical ones. We have ways to estimate properties over a wide range of temperatures and pressures. I suggest that the inclusion of the SupCrt database and algorithms into GWB would be a very valuable addition to the program!!!

From: Craig Bethke

Subject: Re: Pressure - and general comments

The GWB can work at any pressure, and pressure can vary with temperature. The pressure curve is set within the thermo database, in a table near the top. As you note, geochemists generally compile thermo data at 1 atm (below 100 C) or along the steam saturation curve (100 C and above), because these are the conditions at which the data is collected. Furthermore, there's little impetus for compiling data at other pressures because the change in log K with pressure for most reactions is commonly considered small compared to the uncertainty in this value (there are notable exceptions). But you can create a database for any pressure conditions by correcting the stability constants for species and minerals (according to the well-known formula involving the standard volume change of reaction) in the dataset, and then setting the pressure table at the top of the file. You can use a program like SUPCRT to do this, but it's a simple calculation to do by hand or with a spreadsheet. I might note that when people inquire about the effects of "pressure", they are almost invariably referring to the partial pressure of a gas such as P-CO2. Such variables of course can have dramatic effects on solution chemistry. In the GWB, you set partial pressure (actually fugacity) directly with the "fugacity" command; the discussion above applies only to the more subtle effects of confining pressure on mineral and species stability.

Hi,

I want to model a boiling process varying the pressure, so I want to use the command "pressure" or "P" as a polythermal model but it do not recognize this command. Thanks for your help.

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