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Author Topic: Help with kinetic Calcite precipitation  (Read 3924 times)

floydian86

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  • Posts: 7
Help with kinetic Calcite precipitation
« on: 06/02/20 14:35 »
Hello Everyone!

I'm trying to model calcite precipitation upon changes in the system such as pressure, temperature and CO2 degassification. I also need to take kinetics into account in order to know how much calcite would precipitate after a certain amount of time.
I've been reading the documentation and after learning the basics I also searched the forum for similar examples. The main issue I found so far is that most people are trying to model kinetic dissolution and not precipitation, so I'm not sure if KINETIC definitions are correct for precipitation.

Here's a description of what I did:
1. First I defined calcite RATES, SOLUTION_SPECIES for CO2 and then CO2(g) using PHASES.
2. Then I set up a solution. I used the sit database because it includes all the solid phases that are being used to make the solution in the lab, so it's easier to just introduce all the species to pure water using REACTION.
3. I introduce CO2(g) using GAS_PHASES and let the software calculate how much CO2 will be dissolved in the solution. I chose this method instead of EQUILIBRIUM_PHASES because I want to "save" the "remaining" gas phase for later in order for CO2 to return to it when degassification occurs upon a pressure drop.
4. I define KINETICS for calcite and an abrupt pressure drop and let the simulation run.

I used the calcite rates already included in the database. After reading some topics in the forum, I undertood that this works for both dissolution and precipitation, so no changes would be needed.
For KINETICS, I used the model shared by the user pia said. I'll later change parms, m0, m and time steps when I can get the correct data for my model. Still, this is where I think I should work on because pia said's kinetic model was for dissolution and not precipitation.

Leaving the tuning of the model aside for now, I believe that my simulation is adding calcite to the system instead of working with the species that are already present in the solution and that's why some of the results I'm getting make no sense. Is my assumption correct? Could anyone please point me in the right direction regarding precipitation kinetics?


Here's a part of the model I've built so far. There are still many things I need to fix, so I'm not expecting it to do exactly what I want yet.
For instance, I'm always getting a positive SI for calcite, which is something I'm not expecting to happen at least at low pH, but I'll work on that later. First I'd like to know how to correctly model kinetic calcite precipitation.

Thanks!!!

Code: [Select]
RATES
    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")
 20  IF (M <= 0  and si_cc < 0) THEN GOTO 200
 30  k1 = 10^(0.198 - 444.0 / TK )
 40  k2 = 10^(2.84 - 2177.0 /TK )
 50  IF TC <= 25 THEN k3 = 10^(-5.86 - 317.0 / TK)
 60  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_SPECIES
CO3-2 + 2 H+ = CO2 + H2O
   -log_k   16.681
   -delta_h -5.738   kcal
   -analytic   464.1965   0.09344813   -26986.16   -165.75951   2248628.9
   -dw    1.92e-9
   -Vm   7.29  0.92  2.07  -1.23  -1.60 # ref. 1 + McBride et al. 2015, JCED 60, 171
2CO2 = (CO2)2 # activity correction for CO2 solubility at high P, T
   -log_k -1.8
   -analytical_expression  8.68  -0.0103  -2190
   -Vm   14.58  1.84  4.14  -2.46  -3.20
END

PHASES
CO2(g)
   CO2 = CO2
   -log_k   -1.468
   -delta_h -4.776 kcal
   -analytic   10.5624  -2.3547e-2  -3972.8  0  5.8746e5  1.9194e-5
   -T_c  304.2 # critical T, K
   -P_c   72.86 # critical P, atm
   -Omega 0.225 # acentric factor
END

SOLUTION 1
temp 25
pressure 1
pure water

USE SOLUTION 1
REACTION 1 Add minerals to water
Halite 0.85639
Bischofite 0.75033
Portlandite 0.29999
Nahcolite 0.01939
Thernardite 0.00364
CaCl2:2H2O(cr) 0.30003
SAVE solution 2
END

USE SOLUTION 2
REACTION_TEMPERATURE
110
#REACTION_PRESSURE
#500
SAVE solution 3
END

USE SOLUTION 3
GAS_PHASE 1
    -fixed_pressure
    -pressure 500
    -volume 1.53
    -temperature 110
    CO2(g)    500
SAVE gas_phase 2
SAVE solution 4
END

KINETICS 1
Calcite
    -formula  Calcite  1
    -m        0.0625
    -m0       0.0625
    -parms    380000 0.6
    -tol      1e-08
-steps        200 in 10 steps # seconds
-step_divide 1
-runge_kutta 3
-bad_step_max 500
#INCREMENTAL_REACTIONS

USE SOLUTION 4
USE GAS_PHASE 2
REACTION_PRESSURE
300
END
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dlparkhurst

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  • Posts: 4030
Re: Help with kinetic Calcite precipitation
« Reply #1 on: 06/02/20 16:20 »
You are on your own to develop your simulation.

It appears you are using llnl.dat, although you have misspelled thenardite and CaCl2:2H2O(c) is not included. pitzer.dat is probably the best database for modeling the high concentrations associated with these minerals. If you do use pitzer.dat, you cannot easily define new SOLUTION_SPECIES because of all the parameters that are needed to ensure consistency.

Note also that only phreeqc.dat, Amm.dat, and pitzer.dat have the molar volumes needed to model the effects of pressure.

Look carefully at the output. Calcite is indeed supersaturated, but the pH of the final solution is 10.4.
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John Mahoney

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  • Posts: 73
Re: Help with kinetic Calcite precipitation
« Reply #2 on: 06/02/20 18:08 »
In my searches on Google earlier today I came across this document.

https://www.vtt.fi/inf/pdf/technology/2018/T321.pdf 


VTT TECHNOLOGY 321
FINLAND

Industrial and mine water chemistry
Advanced aqueous database for modelling industrial processes

Risto Pajarre is the first author

Appendix D is a kinetically constrained CaCO3 Precipitation model. I have not looked at it in any detail, but I thought it may be helpful.
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floydian86

  • Contributor
  • Posts: 7
Re: Help with kinetic Calcite precipitation
« Reply #3 on: 06/02/20 18:59 »
Quote from: dlparkhurst on 06/02/20 16:20
You are on your own to develop your simulation.

It appears you are using llnl.dat, although you have misspelled thenardite and CaCl2:2H2O(c) is not included. pitzer.dat is probably the best database for modeling the high concentrations associated with these minerals. If you do use pitzer.dat, you cannot easily define new SOLUTION_SPECIES because of all the parameters that are needed to ensure consistency.

Note also that only phreeqc.dat, Amm.dat, and pitzer.dat have the molar volumes needed to model the effects of pressure.

Look carefully at the output. Calcite is indeed supersaturated, but the pH of the final solution is 10.4.

Thank you for your answer, Mr. Parkhurst.
The development of my simulation is on me, of course. The details I gave are only for the readers to better understand what I'm trying to do.

My request for help is only regarding calcite precipitation kinetics. I'm certain I can use the RATES included in the database for both dissolution and precipitation, but I'm not sure if anything should be different under KINETICS when modelling precipitation instead of dissolution, such as a negative sign for the stoichiometric coefficient... but maybe I'm just talking nonsense here.

Thanks for your suggestions about databases. I'll take them into consideration and see if I can make some improvements there.
I'm actually using sit.dat and not llnl.dat. CaCl2:2H2O is included there.
I reviewed the databases before chosing one, so I believe Thenardite is actually misspelled in sit.dat (I'm not a native english speaker and I don't usually deal with the names of these minerals, so I wrote it just as it is spelled in the database):

Code: [Select]
Thernardite
Na2SO4     = 2.000Na+     + 1.000SO4-2
     log_k    -0.360     
     delta_h  -2.200     #kJ/mol       
     # Enthalpy of formation:           -1387.82        #kJ/mol        #98CHA
     -analytic -7.45423E-1 0E+0 1.14914E+2 0E+0 0E+0
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floydian86

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  • Posts: 7
Re: Help with kinetic Calcite precipitation
« Reply #4 on: 06/02/20 19:12 »
Quote from: John Mahoney on 06/02/20 18:08
In my searches on Google earlier today I came across this document.

https://www.vtt.fi/inf/pdf/technology/2018/T321.pdf 


VTT TECHNOLOGY 321
FINLAND

Industrial and mine water chemistry
Advanced aqueous database for modelling industrial processes

Risto Pajarre is the first author

Appendix D is a kinetically constrained CaCO3 Precipitation model. I have not looked at it in any detail, but I thought it may be helpful.

Thank you, John, for your contribution. I'll take a look at the document.
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dlparkhurst

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  • Posts: 4030
Re: Help with kinetic Calcite precipitation
« Reply #5 on: 06/02/20 19:32 »
Sorry, didn't realize you were using sit.dat. You do need to define the mineral names as they appear in the database.

The stoichiometry for the KINETICS definition should be the same for dissolution and precipitation. The calculated mole transfer in the RATES definition will be negative for precipitation and positive for dissolution, but the -formula in KINETICS remains the same. The convention is that if the product of the quantity SAVEd in RATES times the stoichiometric coefficient in the -formula is positive, concentrations increase in the solution (dissolution); if negative, concentrations decrease in solution (precipitation).
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floydian86

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  • Posts: 7
Re: Help with kinetic Calcite precipitation
« Reply #6 on: 07/02/20 11:37 »
Thank you very much!
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Charlie

  • Admin
  • Posts: 158
Re: Help with kinetic Calcite precipitation
« Reply #7 on: 19/06/20 09:42 »
Hi - did you manage to achieve your objective for this?

I am also looking at precipitation of calcite (kinetically) so am interested to learn more about your approach.

Thanks!
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MichaelZ20

  • Top Contributor
  • Posts: 162
Re: Help with kinetic Calcite precipitation
« Reply #8 on: 19/06/20 14:56 »
I paid attention that specific surface area of calcite in the example included in phreeqc.dat is about 4 times lower as compared to this in THERMODDEM.dat database. Available internet data are for granulated/powdered calcite. What may be the representative range for natural sediment?
« Last Edit: 20/06/20 16:47 by MichaelZ20 »
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