calculate CO2 solubility in brine composition

[maria]

Dear all,
I have a solution concentration and based on that I calculated a brine concentration of NaCl and CaSO4 that will be used in lab experiment. I want to calculate/ predict the CO2 that can be dissolved in that brine composition. I have also CO2 concentration in solution with concentration 19.53 mmol/kg. At first, I entered CO2(aq) but I was getting an error so I tried to enter it as C(4). But still the calculations are terminating because of errors. I have used the NaCl as halite and CaSO4 as anhydrite to be in equilibrium, adding next to these phases CO2(aq), but still I am getting errors.

I would very much appreciate your help. What am I missing?

Thank you very much.

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DATABASE pitzer.dat

TITLE Geothermal Iceland

SOLUTION 1
temp 74.3
pH 7.12
units mmol/kgw
density 1
Ca 0.019
Cl 2
#F 0.097
Fe 0.0002
K 0.60
Mg 0.0002
Na 5.72
S(6) 0.58
Si 8.93
C(4) 19.53
END
REACTION 1
NaCl 2
CaSO4 0.5

USE solution 1
EQUILIBRIUM_PHASES
halite 0 CO2(aq)
anhydrite 0 CO2(aq)
END

[dlparkhurst]

Looks like you are throwing everything at the wall to see what sticks.

Your SOLUTION calculation will give you the paratial pressure of CO2(g) that would be in equilibrium with the solution. The Saturation Index for CO2(g) is -0.75 which is effectively the log10 PCO2. So, the solution would be in equilibrium with a gas phase with a partial pressure of CO2(g) of about 0.2 atm.

As an example, the calculation below calculates the solubility of CO2(g) in your solution at a partial pressure of 1 atm CO2(g), when Halite and Anhydrite are in equilibrium.

I do not know your experimental setup, but you may need to use GAS_PHASE to model your system better. EQUILIBRIUM_PHASES will provide results for a specified log10 partial pressure. GAS_PHASE can be used to model a bubble where the volume adjusts to a fixed pressure or a gas head space where the pressure adjusts to a fixed volume.

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SOLUTION 1
temp 74.3
pH 7.12
units mmol/kgw
density 1
Ca 0.019
Cl 2
#F 0.097
Fe 0.0002
K 0.60
Mg 0.0002
Na 5.72
S(6) 0.58
Si 8.93
C(4) 19.53
END

USE solution 1
EQUILIBRIUM_PHASES
Halite 0 10
Anhydrite 0 10
CO2(g) 0 10
END


[maria]

Dear David,

Thank you very much for the reply. I used the REACTION term as I wanted to specify that I want to predict the dissolved CO2 in the brine (NaCl 2 mmol/kg and CaSO4 0.5 mmol/kg). I add the equilibrium phases of this brine composition in order to show that I don't want the salts to precipitate but rather to be in equilibrium.
I followed your comment to use gas phase, instead. But I used it for the CO2. I am only puzzled whether the CO2 concentration that I get after the run is the dissolved in the brine or in the first solution. I am not sure if I can use only the salts content under the solution; I made an effort but I was getting an error that "Could not find element in database, NaCl, so the concentration was set to zero", same for CaSO4.   

Code: [Select]

SOLUTION 1
temp 74.3
pH 7.12
units mmol/kgw
density 1
Ca 0.019
Cl 2
#F 0.097
Fe 0.0002
K 0.60
Mg 0.0002
Na 5.72
S(6) 0.58
Si 8.93
C(4) 19.53

EQUILIBRIUM_PHASES 1
halite 0
anhydrite 0

GAS_PHASE 1
CO2(g) 0  10
END
Thank you very much for your help.

Kind regards,
Maria

[dlparkhurst]

First, let's talk about anhydrite and halite. You were right to use REACTION if you want to add these compounds to your solution in those amounts. However, when you specify EQUILIBRIUM_PHASES as follows

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EQUILIBRIUM_PHASES 1
halite 0
anhydrite 0

the default amount of halite and anhydrite available to react is 10 moles. This is a sufficient amount to ensure the solution is at equilibrium with the two minerals. You will get the same solution composition whether you add the REACTION or not. Perhaps what you want is the following:

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EQUILIBRIUM_PHASES 1
Halite 0 0
Anhydrite 0 0

where the second zero on each line means there is no Halite or Anhydrite available to react with the solution. In this case, each mineral can only precipitate. In fact, Anhydrite is supersaturated after the addition of 0.5 mol CaSO4, and should precipitate. Halite will be undersaturated and will not precipitate (and cannot dissolve because the moles of halite in equilibrium phases is zero).

As for solubility of CO2(g), you need to define some more constraints.

(1) You can specify the partial pressure of CO2(g) in EQUILIBRIUM_PHASES. This would imply that you have a large gas phase in equilibrium with the solution that fixes the PCO2. Below is a script that assumes the partial pressure of CO2(g) in this gas phase is 10 atm. The results indicate that anhydrite precipitates and 0.08 mol of CO2 dissolve into the solution.

(2) You can let the solution equilibrate with a fixed-volume GAS_PHASE. CO2 (and possibly other gases) will partition between the head space and the solution.

(3) You can let the solution equilibrate with a fixed-pressure GAS_PHASE. If the sum of the partial pressures of the components defined for the gas phase exceed the specified pressure, a gas bubble will form. If the sum of the partial pressures of the components are less than the specified pressure, no gas phase will be present.

Those are your choices. You must decide which fits your system.

In your definition

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GAS_PHASE 1
CO2(g) 0  10
END

by default it is a fixed-pressure gas phase at 1 atm. You are specifying a partial pressure of 0, so no moles of CO2(g) are present initially in the gas phase. [The 10 is ignored because only one number is read for the partial pressure of CO2(g).] Under the temperature and pressure conditions, the partial pressure of CO2(g) calculated for the solution will be less than 1 atm, so no gas phase will be present.

Code: [Select]

SOLUTION 1
temp 74.3
pH 7.12
units mmol/kgw
density 1
Ca 0.019
Cl 2
#F 0.097
Fe 0.0002
K 0.60
Mg 0.0002
Na 5.72
S(6) 0.58
Si 8.93
C(4) 19.53
END
REACTION 1
NaCl 2
CaSO4 0.5
1 mol
END
EQUILIBRIUM_PHASES 1
Halite 0 0
Anhydrite 0 0
CO2(g)  1
END
TITLE PCO2=10 atm, Halite, Anhydrite precipitate if necessary
USE solution 1
USE reaction 1
USE equilibrium_phases 1
END


[maria]

Dear David,

Thank you very much. I actually used both equilibrium_phases and phase_gas for CO2(g). Under the equilbrium phase, at first I used the partial pressure that the solution calculated (-0.75).

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GAS_PHASE 1
CO2(g) 0

EQUILIBRIUM_PHASES 1
CO2(g) -0.75
That gave me a positive delta. However, when I used a partial pressure CO2(g) 0, that gave me a negative delta -3.8e-03. I believe that the negative one means that 0.0038 CO2 moles will be dissolved. I am only puzzled about the positive delta. Does it mean the same?

Many thanks.

Maria

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-------------------------------Phase assemblage--------------------------------

                                                      Moles in assemblage
Phase                  SI log IAP  log KT      Initial       Final       Delta

CO2(g)              -0.75   -2.62   -1.87    1.000e+01   1.000e+01   3.821e-03

[dlparkhurst]

I don't know why you are using both EQUILIBRIUM_PHASES and GAS_PHASE; normally one would use one or the other. For a fixed-pressure gas phase, which is the default in your definition, there should be no gas phase unless the CO2(g) partial pressure is greater than 1 (log10PCO2 > 0). Please look at the output file carefully.

In the following, a positive value for Delta means that CO2 moves from the solution to the gas phase. A negative value means that CO2 moves from the gas phase to the solution (dissolution).

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-------------------------------Phase assemblage--------------------------------

                                                      Moles in assemblage
Phase                  SI log IAP  log KT      Initial       Final       Delta

CO2(g)              -0.75   -2.62   -1.87    1.000e+01   1.000e+01   3.821e-03