Jump to content

[OLD] Pressure and general comments


webmaster
 Share

Recommended Posts

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.

Link to comment
Share on other sites

  • 4 years later...

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.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...