GAS liquid mixing

[jerome]

Dear users,

I try to simulate a Liquid-Gas Mixing (gas bubble like described in the section: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/html/final-54.html)

The Gas phase is Air, at fixed pressure 0.2 Atm 55°C but also with a fixed Gas-Liquid volumic ratio  at this pressure : 619L Gas/Lsolution by fixing volume
I adjusted the partial pressure of Air components from 1 atm to 0.2 Atm.
In order to prevent N2 gas reactions I added a new  Phase  called "Nzero"  like suggested in a previous topic on the forum.

My issue is the following one : After calculation the Total pressure is :  0.37 atmospheres (Peng-Robinson calculation) and not fixed at 0.2 Atm mainly due to H2O(g) partial pressure adjustment. (Hereafter the code for this calculation)

Is there another way to calculate such equilibrium at 0.2Atm by respecting the gas /liquid ratio?

Thanks by advance for any comments.

Regards,

Jerome



 #Using Database: Amm.dat

SOLUTION_MASTER_SPECIES

Nzero      Nzero2  0.0     Nzero      14.0067

SOLUTION_SPECIES

Nzero2 = Nzero2
         log_k 0.0

PHASES
Nzero2(g)
      Nzero2 = Nzero2
      -log_k       -3.1864
      -analytic -58.453 1.818e-3  3199  17.909 -27460
      -T_c  126.2
      -P_c   33.50
      -Omega 0.039

SOLUTION   1      
   units   mol/kgw      
   Temp   25.0      
   pH   8.35      
   pe   -4.23      
   density   1.0098515      
   Ca   0.003807969      
   Mg   0.00025118      
   K   0.020298779      
   Na   0.029210219      
   Cl   0.055561361      
   C(4)   0.294120034   as   CO3-2
   Amm   0.337755107      
   -water   0.973   kg   
   pressure   1.000      
   END         
            
# REACTIONS :            
   # Temperature increase :         
   USE Solution   1      
   REACTION_TEMPERATURE    1      
      55.00      
      SAVE solution   2   
   END         
            
   # Pressure decrease :         
   USE Solution   2      
   REACTION_PRESSURE    1      
      0.200      
      SAVE solution   3   
   END         
            
# Liquid gas equilibrium  :             
   GAS_PHASE   10      
   -fixed_pressure         
   -pressure   0.200      
   -fixed_volume         
   -Volume   619      
   -temperature   55.00      
      CO2(g)   0.00008   
      Nzero2(g)   0.15616   
      O2(g)         0.0419   
      Amm(g)   0   
      H2O(g)       0   
            
   USE solution   3      
   USE gas_phase   10      
   SAVE solution   4               

   END      






 

[dlparkhurst]

You must pick whether you want a fixed-pressure gas phase or a fixed-volume gas phase. In your file, you first define -fixed-pressure, but then you have -fixed-pressure. The second definition will prevail, and you keep a fixed volume of 619 L, but the pressure is determined to be 0.37 atm. If you remove -fixed_volume, then the pressure will remain 0.2 atm, but the volume will be 3000 L. You cannot fix the pressure, the volume, and the initial moles of each gas. Basically, the calculation must satisfy

PV = nRT

If you want the pressures of each gas to be fixed, then you should use EQUILIBRIUM_PHASES, but that basically adjusts n, the number of moles available.

Note that Ntg and Ntg(g) (unreactive nitrogen, dissolved and gas) are defined in Amm.dat and phreeqc.dat, so you can use Ntg instead of Nzero.

[jerome]

Thank you Dr Parkhurst for your reply,

I understand that we need a degree of freedom and I was wondering if the number of mole could vary proportionally by fixing both pressure and volume. I will review my approach. I need at least to fix the volumic gas/liquid ratio but it does not seem possible with EQUILIBRIUM_PHASE.
Thank you also for the comment on Amm.dat and Nt(g) which is a really usefull database to prevent from unwanted redox reactions in such cases.

Is there a way to "PUNCH" the partial pressures of each gas components in a "SELECTED_OUTPUT" file after a GAS_PHASE reaction?

Have a nice Week end,

Regards,

Jerome

[dlparkhurst]

If you use both EQUILIBRIUM_PHASES and GAS_PHASE. The volume could be set with GAS_PHASE and the gas components would adjust to the pressures defined by the same components in EQUILIBRIUM_PHASES. Don't know if it makes sense to do this, but it may be a way to define the gas phase that you want, which could then be used in other calculations.

SI("CO2(g)") gives the log10 fugacity of CO2(g) (Peng-Robinson gas).

PR_P("CO2(g)") gives the partial pressure of CO2(g) (Peng-Robinson gas).

PR_PHI("CO2(g)") gives the fugacity coefficient (Peng-Robinson gas).

For a non-Peng Robinson gas, SI will be equal to the log10 partial pressure (atm).