How does GWB calculate the pressure effect?

[chenxinger]

When I open the thermo.dat file, I saw there are some parameters for pressure after those for the principle temperatures. How are the pressure effect considered in GWB? I am using GWB to calculate the solution under a constant CO2 pressure of 102 atm, and at temperature of 95 deg C. Under this condition, CO2 is at supercritical phase. I am wondering how could I calculate the fugacity coefficient under this pressure?

Also, the equilibrium constant for other aqueous species will change under high pressure also, how would GWB consider this effect? thank you.

[Tom Meuzelaar]

Hello:

 

The thermo.dat database is configured for work at 1 atm system confining pressure. GWB is able to calculate fugacity values for individual gases in equilibrium with the fluid. In most cases, increasing the confining pressure of your system will have little effect on the equilibrium constants in the database. However, for CO2 this is not the case. To calculate accurate CO2 solubility at higher pressures, I recommend you look at the work of Duan et al., specifically:

 

• 
   
  
• Duan, Z., and Sun, R. (2003) An improved model calculating CO2 solubility in pure water and aqueous NaCl solutions from 273 to 533 K and from 0 to 2000 bar, Chemical Geology, v193, p. 257-271.
  
• Duan, Z., Sun, R., Zhu, C. and Chou, I. (2006) An improved model for the calculation of CO2 solubility in aqueous solutions containing Na+, K+, Ca2+, Mg2+, Cl-, and SO42-, Marine Chemistry, v98, p. 131-139
  

 

I also recommend the tools available online at: http://www.geochem-model.org/models.htm

 

I hope that helps,

 

Tom Meuzelaar

RockWare, Inc.