Conceptual Models > Design of conceptual models

Adiabatic boiling in sealed system

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Jowita_Loboda:
Hello everyone,

I am trying to model two-phase separation reaction in a sealed system of high temperature borehole, where general enthalpy is: 1620 kj/kg, flow 28.1 kg/s, pressure 13.2 bara, temp 193.0 C.

From the water analysis, done in the laboratory after cooling the sample, i know that: pH 8.67, T-pH 21.0 c, co2 91.8 mg/kg, H2S 29.90 mg/kg.
Where: steam analysis: co2 1479 mg/kg, H2S 776 mg/kg.

Furthermore, the water and the steam goes though the steam separator, which has the same enthalpy, same flow, but the temperature is 184.1 C and the pressure changes to 10.01 bara.

As i understand, at first I have to boil the data water and steam analysis to general temperature of 193.0 C and pressure of 13.2 bars, and then to the separation standards.

And here arrives the problem, while I was trying to use EQUILIBRIUM_PHASES together with REACTION_TEMPERATURE and PRESSURE the CO2 cocnentration dropped to almost similar one that i have from the other program calculations, however, CO2 in the steam phase has not risen, and the pH looks wrong too. I am not sure if I should use EQUILIBRIUM_PHASES or just GAS_PHASE. This is my code so far:

SOLUTION 1
    temp      21
    pH        8.67
    pe        4
    redox     pe
    units     mg/kgw
    density   1
    C(4)      91.8 as CO2
    S(-2)     29.9 as H2S
    Si        682.7 as SiO2
    B         2.1
    F         0.845
    Na        125
    Mg        -0.4
    Al        1.96
    Fe        0.004
    P         -0.001
    Cl        56.8
    K         20.5
    Ca        0.59
    -water    1 # kg
Save solution 1

GAS_PHASE 1
    -fixed_pressure
    -pressure     1.013
    -temperature  25
    -volume       0.001
    CO2(g) 0.0336 #steam concentration 1478 mg/kg
   H2S(g) 0.0228
SAVE gas_phase 1
USE solution 1
USE gas_phase 1
REACTION_TEMPERATURE
   193 184 in 2 steps
PRESSURE
   13.01 11.01 in 2 steps

USER_PUNCH
    -headings CO2_dissolved_mgkg CO2_gas_mgkg
    -start
    10 PUNCH TOT("C(4)") * 44.01 * 1000
    20 PUNCH GAS("CO2(g)") * 44.01 * 1000
    -end

SELECTED_OUTPUT
    -file gas_phases.csv
    -reset                false
    -pH                   true
    -temperature          true
   -gases         CO2(g)
    -molalities           CO2 H2S
    -totals               C(4) S(-2)


Thank you for your help,

Jowita

dlparkhurst:
I don't deal much with steam separators, so I am not sure how to model your situation.

I don't think you are using GAS_PHASE correctly. The values for H2S(g) and CO2(g) in GAS_PHASE should be partial pressures, not mg/kg.

To a point, I think you should use REACTION to add the CO2 and H2S back into the solution composition, but in addition, I think you should figure out how much water needs to be added to the cooled solution (that is the water lost to the gas phase) to put it back to downhole conditions.

Here is a simulation that does not add any water, only the CO2(g) and H2S(g).  You can consider how you might change the simulation to suit your situation.


--- Code: ---SOLUTION 1
    temp      21
    pH        8.67
    pe        4
    redox     pe
    units     mg/kgw
    density   1
    C(4)      91.8 as CO2
    S(-2)     29.9 as H2S
    Si        682.7 as SiO2
    B         2.1
    F         0.845
    Na        125
    Mg        -0.4
    Al        1.96
    Fe        0.004
    P         -0.001
    Cl        56.8
    K         20.5
    Ca        0.59
    -water    1 # kg
END
USE solution 1
REACTION
CO2    3.3605e-02
H2S    2.2770e-02
1 mole
REACTION_TEMPERATURE
   193 184 in 2 steps
REACTION_PRESSURE
   13.01 11.01 in 2 steps
GAS_PHASE
-fixed_pressure
CO2(g) 0
H2S(g) 0
H2O(g) 0
END

--- End code ---

Jowita_Loboda:
Hi,

thank you for your answer. So at first i have figured to use water ratio to get the mol/kg of water and i added it to the Reaction


--- Code: ---REACTION
CO2    3.3605e-02
H2S    2.2770e-02
H2O(g)     5.550e+01
1 mole
--- End code ---

then now i also have results from the Gas Analysis, therefore, i included it in the GAS_PHASE, however i am not sure it its correct initial data is:
H₂   95.00 %   
O₂   0.35 %   
N₂   4.24 %   
CH₄   0.33 %   
Ar   0.07 % and in the code:


--- Code: ---GAS_PHASE 1
    -fixed_pressure
    -pressure 1
    -volume 1
    -temperature 25
    CO2(g)    0
    H2O(g)    0
    H2S(g)    0
    CH4(g)    0.0033
    O2(g)     0.0035
    N2(g)     0.0424
    H2(g)     0.9496

--- End code ---

However, I have just noticed that as i can get amorphous silica being percipitated in the system and in the initial water analysis i have Si as285 mg/kg and SiO2 as 668 mg/kg, however, i am not sure how or where should i add as i think its importanrt to include both.

Last thing, when i change database to carbfix.dat the output shows much lower pH and different CO2 concentrations....

dlparkhurst:
I doubt you have done the calculation correctly.

It does not seem right to add a liter of water from the gas phase. The ratio of water from the liquid phase to the water in the gas phase is not likely to be 1:1.

Also, the gas phase does not seem correct. The gas phase as has a certain number of moles of each gas, which for an ideal gas is determined by PV=nRT. Your pressure and volume are 1 atm and 1 L, which seems unlikely to produce the right number of moles in the gas phase. You probably do not need to consider Ar and N2, but I'm not sure why you are treating CO2 and H2S differently from the components you added in the gas phase. You have not given your entire script, so I'm not sure what you are doing.

If you want to consider precipitation of silica, you probably want to use EQUILIBRIUM_PHASES that includes a silica phase. You could use KINETICS and RATES, but that is more complicated.

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