Processes > Dissolution and precipitation

Deviation of hydrogen solubilities from experimental values at higher pressures

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kennedyantwi1:

--- Code: ---Dear Dr. Parkhurst,
I'm trying to compare hydrogen solubilities in pure water between numerical and experimental models. At higher pressures (eg., > 300 bars), the numerical values tend to deviate from the experimental. Please, is there a way i may fix these deviations up to about 600 bars? The attempted code is attached, thanks:



[SOLUTION 1
    temp  30.00
    pH    7.0 charge
    units  mol/kgw
    -water 1.0 #1.0 kg
    -redox pe
    pe 4.0

GAS_PHASE 1
    -fixed_pressure
    -pressure 592.154
    -volume 10.0
    -temperature 30.00
    Hdg(g) 1.0

REACTION_PRESSURE
0.1 592.154 in 10

USER_GRAPH 1
    -headings Pressure Mol(Hdg) Gas_molar_volume
    -chart_title "Solubility of Hydrogen in Water as a Function of Pressure"
    -axis_titles "Pressure, atm", "Hdg(aq), mol/kgw" "Gas molar volume, L/mol"
    -plot_concentration_vs x
    -start
10 GRAPH_X PRESSURE
20 GRAPH_Y MOL("Hdg")
30 GRAPH_SY GAS_VM
    -end
ENDcode]
--- End code ---

dlparkhurst:
For the databases pitzer.dat and phreeqc.dat, the parameters that affect the solubility of Hdg(g) are defined in PHASES and SOLUTION_SPECIES. By defining T_c, P_c, and Omega (critical T, critical P, and eccentricity), the Peng-Robinson equation of state is used. The values affect the relation between T, P, and V for the gas. The molar volume of Hdg(aq) (-Vm definition) affects the pressure dependence of the solubility. See the manual, and Appelo, Parkhurst and Post, 2014. Geochim. Cosmochim. Acta 125, 49?67 for more details. You would have to decide if it is reasonable to adjust any of these parameters.


--- Code: ---PHASES
Hdg(g)
Hdg = Hdg
-analytic -9.3114e+0 4.6473e-3 -4.9335e+1 1.4341e+0 1.2815e+5
-T_c 33.2; -P_c 12.8; -Omega -0.225
SOLUTION_SPECIES
Hdg = Hdg # H2
-Vm 6.52 0.78 0.12 # supcrt
-dw 5.13e-9
END
--- End code ---

Alternatively, you can use the ideal gas law instead of Peng-Robinson. The ideal gas law will be used if T_c, P_c, and Omega are not defined. Vm will still affect the pressure dependence, but could be eliminated. The Henry's Law constant will determine the solubility as follows:

Kh = act(Hdg(aq)) / P(Hdg(g))

kennedyantwi1:
Thanks very much, Dr Parkhurst, it worked perfectly well. I'm closest to the Experimental values!
My second polite request: Can you guide me on writing the code (in User Graph as well) to determine the change in Ionic Strength as the Hydrogen Solubility changes.
Very much appreciated in advance.


--- Code: ---Dear Dr. Parkhurst,
I'm trying to compare hydrogen solubilities in pure water between numerical and experimental models. At higher pressures (eg., > 300 bars), the numerical values tend to deviate from the experimental. Please, is there a way i may fix these deviations up to about 600 bars? The attempted code is attached, thanks:



[SOLUTION 1
    temp  30.00
    pH    7.0 charge
    units  mol/kgw
    -water 1.0 #1.0 kg
    -redox pe
    pe 4.0

GAS_PHASE 1
    -fixed_pressure
    -pressure 592.154
    -volume 10.0
    -temperature 30.00
    Hdg(g) 1.0

REACTION_PRESSURE
0.1 592.154 in 10

USER_GRAPH 1
    -headings Pressure Mol(Hdg) Gas_molar_volume
    -chart_title "Solubility of Hydrogen in Water as a Function of Pressure"
    -axis_titles "Pressure, atm", "Hdg(aq), mol/kgw" "Gas molar volume, L/mol"
    -plot_concentration_vs x
    -start
10 GRAPH_X PRESSURE
20 GRAPH_Y MOL("Hdg")
30 GRAPH_SY GAS_VM
    -end
ENDcode]
--- End code ---

dlparkhurst:
The ionic strength is derived only from H+ and OH-, and so it is quite small for all pressures--on the order of 1e-7.

You can plot the ionic strength with the Basic function MU. Look at the section The Basic Interpreter in the manual for version 3 for all Basic functions.

kennedyantwi1:
Dear Mr. Parkhurst, thanks for your guidance.

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