pH and PCO2 changes with time

[serhatttt]

Hi Parkhurst;

I want to simulate changes in pH and PCO2 with respect to time (216 seconds).

I prepared the code, but the results are unchanged, and they are unreasonable.

Please help me with this

Thank you.

Here is the code:

SOLUTION 1
-temp 188
-pressure 9
-water 0.065
END
GAS_PHASE 1
-fixed_pressure
CO2(g) 0
END
USE solution 1
USE gas_phase 1
REACTION_PRESSURE
9
REACTION_TEMPERATURE
188
REACTION 1
CO2 1
5.89-2 in 20 #dissolved CO2 in the reactor.

USER_PUNCH
-headings Seconds P(CO2) pH
10 PUNCH SIM_TIME / 3600 / 24, SI("CO2(g)")/PR_PHI("CO2(g)"), -LA("H+")


USER_GRAPH 1
    -headings               _time_ P(CO2) pH
    -axis_titles            "Time, in days" "P(CO2)" "pH"
    -axis_scale              secondary_y_axis 4.0 7.0 1.0 0.5
    #-initial_solutions      false
    #-connect_simulations    true
    #-plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME / 3600 / 24
20 GRAPH_Y SI("CO2(g)")/PR_PHI("CO2(g)")
30 GRAPH_SY -LA("H+")
  -end
END

[dlparkhurst]

If you want to model the pH and PCO2 of the solution, a first approximation is simply to add CO2 with REACTION. You can see the PCO2 increase as CO2 dissolves.

If you want to use Peng-Robinson, you must include a GAS_PHASE. The GAS_PHASE will always be in equilibrium with the solution, so the CO2 that you add will automatically partition between the solution and the gas phase. If you use a fixed-voume gas phase, you can see the pressure increasing as you add CO2; if you specify a small volume, you can minimize the effect of CO2 partitioning to the gas phase. If you use a fixed-pressure gas phase, you will see the increase in solution to a point where a gas bubble forms, and no change after that as you add more CO2 (it will simply create a bigger volume in the gas phase).

If you want to model kinetic dissolution of CO2, you can look at the following thread:
https://phreeqcusers.org/index.php/topic,2260.msg8452.html#msg8452