CO2 Reaction Rate

[serhatttt]

Hi Everyone.

I am trying to model batch reaction experiments in COMSOL Multiphysics.

In the batch reaction experiment, I had 65 mL of geothermal water, and I allowed 3L/min CO2 to enter the reactor.
In this experiment, my aim is to observe changes in the chemical compositions of the geothermal water due to CO2 dissolution.

In the reactor, the pressure is 9 bar, and the temperature is 188 Celcius degree.

In addition to this, I want to simulate CO2 dissolution in the reactor for 1500 seconds in COMSOL Multiphysics.

But I need the CO2 reaction rate in units of mol/m^3*s in the Transport of Diluted species tool in COMSOL.

How can I calculate the CO2 reaction rate in Phreeqc from the results of this experiment?

I mean, is it possible to determine the CO2 reaction rate from the PhreeqC code?

Thank you.

[dlparkhurst]

You haven't said what you are measuring in your experiment.

Here is a script that calculates how the pH should change as CO2 is added to 0.065 kg pure water at 188 C and 9 atm. Ultimately, the P(CO2) of the solution reaches 9 atm and the solution is in equilibrium with the gas phase at 9 atm. Perhaps you can adjust the script to your needs, possibly by replacing the REACTION with KINETICS and RATES.

Code: [Select]

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
0.6e-2 in 20
USER_GRAPH 1
    -headings               rxn P(CO2) pH
    -axis_titles            "CO2 added" "P(CO2)" "pH"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y SI("CO2(g)")/PR_PHI("CO2(g)")
30 GRAPH_SY -LA("H+")
  -end
END


Here is a thread that considers the rate of hydration of CO2, but I would think that reaction is fast at 188 C. https://phreeqcusers.org/index.php/topic,2260.msg8413.html#msg8413

[serhatttt]

Hi David,

Thank you for the code and quick reply. The code is very helpful for me.

Actually, My experiment includes 3 different steps. T and P are constant, but I change the CO2 flow rate that enters the reactor. (I am sending CO2 to the reactor during 25 min, 1500sec).

First, 188 C, 9 bar, 3L/min CO2
Second, 188 C, 9 bar, 4L/min CO2
Third, 188 C, 9 bar, 7L/min CO2

From these experiments, I have two aims.
First, I need to calculate how much CO2 dissolves (in mol) in my batch reaction experiment in every different step. (Actually, When I look the output file, I see Total CO2 (mol/kg).
Second, I need the CO2 reaction rate in units of mol/m^3*s. (Probably, I need to look link you sent).

Finally, please accept my apologies for not understanding 0.6e-2 in 20 in the Reaction data block.
I suppose 0.6e-2 is moles of CO2 entering the reactor based on 3L/min, right?

Again, thank you so much for your valuable time and response.

[dlparkhurst]

Conceptually, CO2 is added to 0.065 kg of water until the partial pressure of CO2 is equal to 9 atm. The total amount of CO2 added is 0.06 moles, and it is added in 20 equal steps. A gas phase forms after about 0.05 moles have been added, and the solution composition stops changing; the additional CO2 goes into the gas phase.

[serhatttt]

Hi David;

Again thank you so much for your valuable comment.

But, I couldn’t understand how we calculated the total amount of CO2 added, which is 0.06 moles.

Sorry for asking this.

I have one more quesiton. For the dissolved CO2, i need to look total CO2 (mol/kg) in the output file, right ?

[dlparkhurst]

I did it by trial and error, but you could use EQUILIBRIUM_PHASES to equilibrate with CO2(g) with a log partial pressure of log10(9). That would give the amount of CO2 gas that could dissolve in your solution to produce a partial pressure of 9 atm.

The Henry's law equation is K = a(CO2(aq))/f(CO2(g)).

If we ignore fugacity coefficients, P(CO2(g)) = a(CO2(aq))/K. Here CO2(aq) represents an aqueous species and a represents activity.

So, I am not sure what your question is. The concentration of CO2(aq) is probably not the amount of CO2(g) that would dissolve to reach a given P(CO2(g)).

[serhatttt]

Hi David,

Actually, I want to know the concentration of CO2(aq) and CO2(g). In addition, I want to show dissolved CO2 concentration in my solution with respect to pH in phreeqC for 25 min because I sent CO2 to the geothermal water for 25 min.

My reactor parameters are 9 atm and 188 degrees Celsius, and I have 65 mL of geothermal water.

I know that I can find the dissolved CO2 (CO2(aq)) in my solution using Henry's Law so I calculated the dissolved CO2 in my solution 1.8316E-4 moles. PCO2 is 0.95 at 9 atm

I added EQUILIBRIUM_PHASES to the code you said, but I'm not sure whether it is correct or not


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

EQUILIBRIUM_PHASES 1
      CO2(g) 0.95  #Partial pressure of carbondioxide at 9 atm

REACTION 1
CO2 1
0.6e-2 in 20
USER_GRAPH 1
    -headings               rxn P(CO2) pH
    -axis_titles            "CO2 added" "P(CO2)" "pH"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y SI("CO2(g)")/PR_PHI("CO2(g)")
30 GRAPH_SY -LA("H+")
  -end
END

[dlparkhurst]

I am not sure how important the flow rate of gas actually is; that is for you to investigate.

Here is the calculation that I implied to find the amount of CO2 that must dissolve to reach a partial pressure of 9 atm.

Code: [Select]

SOLUTION 1
-temp 188
-pressure 9
-water 0.065
END
USE solution 1
EQUILIBRIUM_PHASES 1
      CO2(g) 0.95  10 #Partial pressure of carbondioxide at 9 atm
END


[serhatttt]

Dear David;

Thank you so much!!

These are very helpful for me.