Batch model with withdrawal and injection of the gas phase

[davidsegura]

Dear PhreeqC users,

I am currently working on batch simulations of gas-water equilibrium. In my model, I would like to see how the composition of the gas phase changes when the pressure in a my reactor decreases (due to gas withdrawal) and conversely, see also the changes when gas pressure increases due to injection of a gas with a known composition.

My first problem is that the reaction between solution 1 and gas phase 2 does not occur at 30 atm but at about 1 atm, even if i use REACTION_PRESSURE and/or "-pressure 30" in the solution definition.

I have found the MIXING_GAS_PHASE keyword to be able to decrease the pressure and maintaining volume by creating a "empty" gas phase. Do you think this is a  good idea or there's a better way to do it ? Can i model the injection by simply using REACTION ?

Thanks in advance for your attention

Code: [Select]

SELECTED_OUTPUT 1
    -file                 cgt_progressive.xls
    -solution             true
    -pH                   true
    -temperature          true
    -gases                CO2(g)  Mtg(g)  H2S(g)
    -totals               Alkalinity  Ca  Na  K  Cl  F  S(6)
                          S(-2)
    -saturation_indices   CO2(g)  Mtg(g)  H2S(g)

USER_PUNCH 1
    -headings CO2_mol Mtg_mol H2S_mol
    -start
10 PUNCH GAS("CO2(g)"), GAS("Mtg(g)"), GAS("H2S(g)")
    -end

EQUILIBRIUM_PHASES 1
    CO2(g)    -0.53 10
    H2S(g)    -4.12 10
    Mtg(g)    1.47 10

SOLUTION 1
    temp      27
    pH        8
    pe        4
    redox     pe
    units     mg/l
    density   1
    Alkalinity 3000 as HCO3
    Ca        14
    Cl        24
    F         0.5
    K         8
    Na        27 charge
    S(6)      20
    -water    1 # kg

SAVE solution 1
END
############################################
### 2 — CHANGING THE GAS PHASE
###    (example : more CO2)
############################################
USE SOLUTION 1
GAS_PHASE 2
    -fixed_volume
    -pressure 30
    -volume 33
    -temperature 27
    CO2(g)    0.08
    H2S(g)    2e-06
    Mtg(g)    0.92
SAVE solution 2
SAVE gas_phase 2
END
############################################
### 3 — DECREASE PRESSURE OF GAS PHASE
###    (while having a fixed volume)
############################################
GAS_PHASE 99
    -fixed_volume
    -pressure 0
    -volume 33
    -temperature 27
    CO2(g)    0
    Mtg(g)    0
    H2S(g)    0

MIX_GAS_PHASE 3 # 50% pressure with the same volume
    2    0.5    # keeps 50% of gas
    99    0.5    # mixes with 50% empty gas
SAVE gas_phase 3
END
############################################
### 3 — EQUILIBRIUM WITH LOWER PRESSURE GAS PHASE
###    (while having a fixed volume)
############################################
USE gas_phase 3
USE SOLUTION 2
SAVE SOLUTION 3
END


[dlparkhurst]

In your simulation, you are actually removing half of the gases in GAS_PHASE 2, while keeping the volume the same. With a fixed-volume gas phase, the gas-water equilibration will then result in a new pressure.

I guess it makes sense to do the calculation this way, because you do not need to know the composition of the gas phase. There is a REACTION calculation that would have the same effect, but you need to know explicitly how much of each gas needs to be removed.

Presumably when adding gas you would use REACTION, and you will need to know the moles of each component that get added.

[davidsegura]

Dear Dr. Parkhurst,

Thank you for replying to my message. I guessed that pressure was recalculated if the volume was fixed, however i'm surprised to see total pressure of only 1 bar since partial pressure of CH4 in the solution is nearly 30 bar.

Is the gas exsolved from solution 1 'filling' the volume of the gas phase (33L) without taking into account the pressure of gas phase 2 ?
How can I make phreeqC calculate a new water-gas equilibrium between my solution and a gas phase with an initial pressure of 30 bar ?

Indeed the idea behind MIX_GAS_PHASE was being able to remove a fraction of the gas without having to know its composition, I'm happy this makes sense for you.

Best Regards,

[dlparkhurst]

Your GAS_PHASE 2 has about 1.3 moles of Mtg(g) and a partial pressure of 0.92 atm before reacting with the solution. The solution has about 0.041 moles of Mtg. Most of the dissolved Mtg exsolves, but it does not add much to the gas phase, so the pressure remains about 1 atm.

Your gas phase only had 0.92 atm of Mtg, the predominant gas component. I'm not sure what you want. Do you want to  define the gas phase to have the same composition as the EQUILIBRIUM_PHASE definition? Then give the same partial pressures in the GAS_PHASE definitions; you should get change in compositions when you equilibrate the solution and the gas.

[davidsegura]

Dr Parkhurst,

I see the error now, I thought pressure calculation took into account the total pressure of the gas phase but it is simply the sum of partial pressures after equilibrating the solution and the gas

Thank you