Mass balance GAS_PHASE Equilibrium (Ntg)

[Olivier]

Hello Phreeqc users!

I want to put in equilibrium a water with Air. In order to achieve this, I describe my gas phase like this :

GAS_PHASE 1
    -fixed_pressure
    -pressure 1
    -volume 43.21
    -temperature 15
      
    Ntg(g)    0.79
    Oxg(g)    0.21

And I use a solution describe like this :
SOLUTION 1
    temp      30
    density 1 calculated
    pH        7
    units     mg/l
    pressure 1
    water 1
    Na    114.89
    Cl    0.21 charge

I use a data base without Peng-Robison model but only with perfect gas law model. Regarding the Phreeqc help, I can calculate the number of moles in the initial gas phase:

Ntg : P*V / (R*T) 0.79*43.21 / (0.08207 * (15 + 273)) *1000 = 1444.223596 mmoles
Oxg : P*V / (R*T) 0.79*43.21 / (0.08207 * (15 + 273)) *1000 = 383.907538 mmoles

After calculation, I want to check the mass balance of Ntg. For this, I get the calculated value in a specific file:
SELECTED_OUTPUT 1
-high_precision true
-file         GP.txt
-time false
-sim false
-pe false
-step false
-state false
-solution false
-distance false
-water
-totals Na Cl Ntg Oxg
-molalities    H2O
-gases Ntg(g)     Oxg(g)   

USER_PUNCH
-headings density volume Molar_Volume Ntg(g)
10 PUNCH RHO
20 PUNCH SOLN_VOL
30 PUNCH GAS_VM
40 GAS("Ntg(g)")


The number of mole in the final gas phase is : 1443.159778617 mmoles (Ntg)
The number of mole in the final liquid phase is : 4.931917778762e-01 mmoles (Ntg)
Total number of Ntg moles :  1443.65297039488 mmoles

If I compare initial and final value, I can find a difference of 0.5706 mmoles.

Could you explain me the difference on the mass balance?

Are there a possibility to ask the number of initial mole in gas phase with Selected_Out keyword?

Thanks you very much for your help and time.

[dlparkhurst]

The sum of Ntg would be the amount in the initial gas phase. You can calculate that with the SYS function.

You also could use a small volume of water (say, -water 1e-6 in SOLUTION) so that the amount of gas partitioning into the water is negligible. The gas composition would then represent the initial gas phase reasonable well. You can use GAS("Ntg(g)") to get the number of moles to more precision.

If you include the input file and state the database, I will look at your calculation.

[Olivier]

I agree with you, that the sum of Ntg after equilibrium should be the amount in the initial gas phase, but I can't find it with the calculation.

SYS function give only the quantity of total Ntg after equilibrium. If I use this keyword, the result is Ntg = 1443.652970395 mmoles. This values match with my calculation of Ntg in the gas phase + liquid phase (1443.159778617 (gas) + 4.931917778762e-01 (liquid) = 1443.65297039488 mmoles).

My understanding is on the initial calculation of Ntg in the gas phase. Like is impossible to fetch the initial value, I want to calculate it to do a mass balance.

In my system, Phreeqc give me a total quantity of Ntg = 1443.652970395 mmoles and with the perfect gas law  Ntg = 1444.223596 mmoles

Like I have only Ntg in the gas phase initially, I could suppose that mass balance should be work.

Please find attached the files.

Thanks

[dlparkhurst]

Your conversion to Kelvin should be 273.15, and the value of R used in PHREEQC is 0.0820597.

P   0.79
V   43.21
R   0.0820597
C   15
C to K   273.15
   
millimoles   1443.65297

There is no direct way to get the initial moles of the gas. You can make a calculation equilibrating the gas phase with pure water and use SYS, or you can calculate the initial moles as you have with the ideal gas law (provided you use the same values PHREEQC does).

[Olivier]

Thanks a lot.

With the right value of R and temperature conversion factor, it's OK.