calcite dissolution in a pipe

[davide]

Hi, i'm performing a model in order to evaluate the dissolution of calcite in a vertical pipe inside the ocean, as a function of water and CO2.
Since the reaction pressure is linear increasing along the pipe, and the dissolution of calcite increases with increasing pressure, it is possible to define an increase of reaction pressure with time?

Thank you.

[dlparkhurst]

There is no capability to change pressure (or temperature) in KINETICS.

However, you can specify a sequence of reaction pressures that correspond to a sequence of time steps in KINETICS. The following would increase the pressure by 1 atmosphere each day.

Code: [Select]

KINETICS
..
-step 864000 in 10 steps # 1 day steps
REACTION_PRESSURE
1 10 in 10 steps
END

It may also be useful to know that if you mix two solutions, the pressure gets mixed linearly, so a 50:50 mixture of a solution of 10 atm pressure with one of 20 atm, would have a pressure of 15 atm.

[davide]

Thank you, i will try in this way

[Charlie]

This paper might be of interest (precipitation though)

https://www.sciencedirect.com/science/article/pii/S0920410519306199?via%3Dihub

Estimating CO2 degassing effect on CaCO3 precipitation under oil well conditions

[davide]

Thank you, i will read it.

I have a problem with my model. Whenever value of pH i choose the model tends to a value of 6, and also the amonuts of CO2 and CaCO3 in solution don't go to zero, but more than the half of the initial concentrations don't dissolve, even if the time and the pressure increase a lot.
This is my model

RATES 1  #use parameters from Plummer et al (1978) as shown in phreeqc.dat
    Calcite
-start
  1   REM   PARM(1) = specific surface area of calcite, cm^2/mol calcite
  2   REM   PARM(2) = exponent for M/M0
 10  si_cc = SI("Calcite")
 30  IF (M <= 0  and si_cc < 0) THEN GOTO 200
 40  k1 = 10^(0.198 - 444.0 / TK )
 50  k2 = 10^(2.84 - 2177.0 /TK )
 60  IF TC <= 25 THEN k3 = 10^(-5.86 - 317.0 / TK)
 70  IF TC > 25 THEN k3 = 10^(-1.1 - 1737.0 / TK )
 80  IF M0 > 0 THEN area = PARM(1)*M0*(M/M0)^PARM(2) ELSE area = PARM(1)*M
110 rate = area * (k1 * ACT("H+") + k2 * ACT("CO2") + k3 * ACT("H2O"))
120 rate = rate * (1 - 10^(2/3*si_cc))
130 moles = rate * 0.001 * TIME
200 SAVE moles
-end
END

SOLUTION 1
    temp      5
    pH        7
    pe        4
    redox     pe
    units     g/kgw
    density   1
    Cl        18.9799
    S(6)      2.6486
    Alkalinity 0.1397
    Br        0.0646
    F         0.0013
    Na        10.5561
    Mg        1.272
    Ca        0.4
    Sr        0.0133
    K         0.38
    -water    1 # kg
END

EQUILIBRIUM_PHASES 1
    CO2(g) 0.3 0.3

KINETICS 1
Calcite
    -formula  Calcite  1
    -m        0.113
    -m0       0.113
    -parms    2e5 0.6
    -tol      1e-08
-steps        4000 in 100 steps # seconds
-step_divide 1
-runge_kutta 3
-bad_step_max 500
#INCREMENTAL_REACTIONS

USE solution 1

REACTION_PRESSURE 1
    1 400 in 100 steps

USER_GRAPH 1
    -headings               Time CaCO3 pH
    -axis_titles            "Time [min]" "CaCO3 in solution [kg]" ""
    -chart_title            "kg CaCO3 not reacted"
    -axis_scale sy_axis     0 8 auto auto
    -initial_solutions      true
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME/60
20 GRAPH_y M*100*200
30 pH = -LA("H+")
40 graph_sy pH
  -end
    -active                 true
USER_GRAPH 2
    -headings               Time CO2
    -axis_titles            "Time [min]" "CO2 in solution [kg]" ""
    -chart_title            "kg CO2 not reacted"
    -axis_scale sy_axis     0 8 auto auto
    -initial_solutions      true
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME/60
20 GRAPH_y MOL("CO2")*44*200
30 pH = -LA("H+")
40 graph_sy pH
  -end
    -active                 true

SELECTED_OUTPUT 1
    -file                 co2 dissolution double time.txt
    -pH                   true
    -molalities           CaCO3 CO2
    -kinetic_reactants    Calcite

Since I'm new to this software there are probably some mistakes that i don't notice. What do you think I should change?

[dlparkhurst]

Seems like you are simply approaching equilibrium with calcite at the specified CO2 partial pressure. See equilibrium calculation below. If you had less calcite available, it would dissolve completely.

If your conceptual model is water moving through a pipe, I'm not sure you should have it open to a fixed partial pressure of CO2.

(INCREMENTAL_REACTIONS true will speed the calculation.)

Code: [Select]

SOLUTION 1
    temp      5
    pH        7
    pe        4
    redox     pe
    units     g/kgw
    density   1
    Cl        18.9799
    S(6)      2.6486
    Alkalinity 0.1397
    Br        0.0646
    F         0.0013
    Na        10.5561
    Mg        1.272
    Ca        0.4
    Sr        0.0133
    K         0.38
    -water    1 # kg
END
USE solution 1
EQUILIBRIUM_PHASES 1
    CO2(g) 0.3 0.3
    Calcite 0 0.113
REACTION_PRESSURE 1
    400
END


[davide]

But in this way i can't see the amount of CO2 and CaCO3 that dissolve, I only get the final equibrium.
Also the pressure is not increasing with time, it's like reacting all at 400 bar.

[dlparkhurst]

I'm only pointing out why all of the calcite does not dissolve and the pH tends to 6. ("the model tends to a value of 6, and also the amounts of CO2 and CaCO3 in solution don't go to zero, but more than the half of the initial concentrations don't dissolve, even if the time and the pressure increase a lot.")

Yes, your file shows a path toward final equilibrium at a pressure of 400 atm, I'm just showing where it will end up if you eventually reach equilibrium. The calculation shows the amount of CO2 and Calcite that will dissolve (Phase assemblage in the output) to reach equilibrium at the given CO2 partial pressure and final pressure of 400 atm. Given the amounts of CO2 and calcite available to react and the specified partial pressure of CO2, you will tend toward this point regardless of your path in kinetics and pressure.

[davide]

Ok thank you, I didn't get it.

Now I'm reducing the amount of CaCO3 and with a value ten times lower, respect to the initial one, all the CaCO3 is dissolved, but only half of the CO2 dissolves independent from the initial value.
In theory the dissolution of CO2 in water is fast, and i more than an hour all of the initial gas should be dissolved.
Is this maybe a problem with the funtion EQUILIBRIUM_PHASES?
I should use instead of it the function GAS_PHASE?

[dlparkhurst]

The simulation calculates the amount of CO2(g) that has to dissolve to reach a partial pressure of 2 atm (log10 PCO2 = 0.3, the first 0.3 in EQUILIBRIUM_PHASES). Not sure where this CO2 would come from in a pipe, but you have allowed 0.3 moles of CO2 to be available to react (the second 0.3 in EQUILIBRIUM_PHASES). That is a sufficient amount to attain the specified PCO2. So, again, the reaction stops because equilibrium is reached.

Whether you use EQUILIBRIUM_PHASES or GAS_PHASE depends on your conceptual model. If you have an infinitely large gas reservoir with a given PCO2 (like the atmosphere, possibly a soil zone, or other environment), then use EQUILIBRIUM_PHASES. If you have a finite gas bubble under a fixed pressure, then use a fixed-pressure gas phase (where the gas bubble may dissolve completely); if you have a finite gas bubble that has a fixed volume (like head space in an experiment), then use a fixed-volume gas phase (the gas head space is always present). If you have a closed system (like your pipe?) then you would not use CO2 in either EQUILIBRIUM_PHASE or GAS_PHASE.

[davide]

Ok now it’s clear thank you.
The idea is a way to sequester CO2 from power plant waste gas, so In the pipe are inserted CO2, water and CaCO3 that will react along the pipe and the solution is discharged in the ocean. The CaCO3 is used to increase the final pH of the solution.
The aim of the model is to see if at the end of the pipe all the CO2 has reacted and then is safely stored.

[dlparkhurst]

OK, I understand better what you are trying to do.

Inserting myself where I probably don't belong, here is an analysis of your problem. The basic question as I see it is how much CO2 can you sequester per liter (kg) of water. In this simulation, the amount of CO2 available to the kilogram of water is varied from 0 to 3 moles. An excess of calcite is available to react to equilibrium in all cases. A gas phase is available to form if the partial pressure of CO2 exceeds 400 atm. You want conditions such that gas bubbles do not form at 400 atm and calcite equilibrium. I used pitzer.dat database.

The simulation indicates that a gas bubble will not form (at 400 atm) if less than about 1.7 mol/kgw CO2 is introduced with a kilogram of water. Up to about 0.063 mol of calcite react. More CO2, and some gas will escape as gas bubbles, and no additional calcite will react. This is an idealized, equilibrium calculation. You would need to consider the uncertainties, and there still may be kinetic issues. The calculation does give some expected limits to CO2 and calcite feed ratios.

Code: [Select]

SOLUTION 1
    temp      5
    pH        7
    pe        4
    redox     pe
    units     g/kgw
    density   1
    Cl        18.9799
    S(6)      2.6486
    Alkalinity 0.1397
    Br        0.0646
    F         0.0013
    Na        10.5561
    Mg        1.272
    Ca        0.4
    Sr        0.0133
    K         0.38
    -water    1 # kg
END
USE solution 1
EQUILIBRIUM_PHASES
Calcite 0 10
GAS_PHASE 1
-fixed_pressure
CO2(g) 0
REACTION_PRESSURE
400
REACTION
CO2(g) 1
3 mol in 30 steps
USER_GRAPH 1
    -headings               CO2 CO2_gas Calcite_dissolved
    -axis_titles            "Moles of CO2 per kilogram of water" "Moles of CO2 in the gas phase" "Moles of calcite dissolved"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x rxn
20 graph_y GAS("CO2(g)")
30 graph_sy 10-EQUI("Calcite")
  -end
    -active                 true
END


[davide]

Thank you, this is a good starting point for the analysis. Now i will try to model the kinetics with this data.