Liquid phase CO2 in PHREEQC?

[floydian86]

Hello.

I've just recently started using PHREEQC and I'm trying to learn the basics. My goal is to model a process that required the presence of liquid CO2 (superctitical fluid, not dissolved CO2(g)).

But, as far as I've seen, CO2 in only available in gas phase or dissolved in water, but not in liquid phase. Is CO2(l) available in any of the databases? Or would I have to incorporate thermodynamic information to a database myself?
And even so, would PHREEQC be able to manage aqueous, liquid CO2, gas phase and solid phase at the same time?

Thanks!!!

[dlparkhurst]

PHREEQC allows mass-action equilibrium between a phase and the aqueous phase. You can write a reaction

CO2(l) -> CO2(aq)

so, if you have an equilibrium constant for that reaction, PHREEQC can calculate the equilibrium distribution.

That said, you also need to stay within the limits of the phreeqc.dat and pitzer.dat databases, liberally 0-200 C and 0-1000 atm.

[floydian86]

PHREEQC allows mass-action equilibrium between a phase and the aqueous phase. You can write a reaction

CO2(l) -> CO2(aq)

so, if you have an equilibrium constant for that reaction, PHREEQC can calculate the equilibrium distribution.

That said, you also need to stay within the limits of the phreeqc.dat and pitzer.dat databases, liberally 0-200 C and 0-1000 atm.

Thanks!
The pressure and temperature limits are not an issue. The lowest temperature is -4ºC, but I can work with 0ºC. Max pressure is 550 atm.

Supposing I do have the equilibrium constant, would it make sense to input a reaction with CO2(l) just like that? Wouldn't I have to define that species using SOLUTION_MASTER_SPECIES?

Also, if I wanted pure CO2(l) - no water present - I was thinking of building a solution with a certain amount of CO2(l) at certain pressure and temperature conditions and then "evaporate" all the water using REACTION. I could then mix that pure liquid CO2 with different solutions.

[dlparkhurst]

All you can do is have a pure CO2(l) phase, analogous to a calcite phase. You could define CO2(l) in PHASES, and include CO2(l) in EQUILIBRIUM_PHASES.

Sorry, you cannot have an aqueous phase that is pure CO2. There is an implicit assumption that H2O is the solvent, and numerical issues arise if the amount of solvent is too small. You can always add CO2 to a solution with REACTION or KINETICS (or with EQUILIBRIUM_PHASES).

[floydian86]

All you can do is have a pure CO2(l) phase, analogous to a calcite phase. You could define CO2(l) in PHASES, and include CO2(l) in EQUILIBRIUM_PHASES.

Sorry, you cannot have an aqueous phase that is pure CO2. There is an implicit assumption that H2O is the solvent, and numerical issues arise if the amount of solvent is too small. You can always add CO2 to a solution with REACTION or KINETICS (or with EQUILIBRIUM_PHASES).

Thank you very much.
I'll define the CO2(l) phase and see how it goes. I think I can work with not being able to define a pure CO2 phase because it would then mix with an aqueous phase.