salt precipitaion

[Hamed]

Dear all,
I am currently modeling halite (NaCl) precipitation due to a temperature decrease at constant pressure. The system involves a high-salinity brine that is initially saturated with NaCl under high-temperature and high-pressure conditions. When I reduce the temperature to 60 ?C, a significant amount of halite precipitates in the model.
However, this outcome appears inconsistent with the NaCl solubility curves reported in the literature, which suggest that solubility should not decrease to this extent at 60 ?C under constant pressure. I am having difficulty interpreting this behavior and would greatly appreciate your insights.
Below is a description of my model for your reference:

Code: [Select]

SOLUTION 1   
-pressure    450
-temperature 145
-pH   7
-units   mol/L
-water    1  # kg

Na   5.5
Cl   5.5  charge

EQUILIBRIUM_PHASES 1

Ntg(g)  2.65  10
Halite 0  1
     
SAVE solution 2
END

USE solution 2

REACTION_TEMPERATURE
60
REACTION_PRESSURE
450

EQUILIBRIUM_PHASES 2

Ntg(g)  2.65  10
Halite 0 0     

END


[dlparkhurst]

Start by converting units more accurately. You need a density to convert from mol/L to mol/kgw. Add the following to the SOLUTION definition, assuming you are using pitzer.dat or phreeqc.dat:

Code: [Select]

-density 1 calc

Then you need to give a better description of what you expect compared to the modeled result. Include the experimental results that you are modeling. Also state the database that you are using.

[Hamed]

I am using the phreeqc.dat database for my simulations. In the initial equilibration step, the brine becomes saturated with respect to NaCl. However, when the temperature is reduced to 60 C, the model predicts the precipitation of more than 0.5 moles of NaCl (>20 g/L), which appears significantly higher than what is expected based on available NaCl solubility data (e.g., https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/16%3A_Solutions/16.04%3A_How_Temperature_Influences_Solubility).

I am trying to understand the cause of this unexpectedly high amount of halite precipitation.

[dlparkhurst]

I'm not sure there is a disagreement. Here is a file that compares solubility as found in Pabalan and Pitzer, 1987. Looks pretty good to me at 1 atm, which is probably what your reference presents as well. Your reference does not present the entire temperature range.

Code: [Select]

# Halite solubility f(T), from Pabalan & Pitzer, 1987, GCA 51, 2429
SOLUTION 1
EQUILIBRIUM_PHASES 1
 Halite 0 20
REACTION_TEMPERATURE 1
25 50 75 100 125 150 175 200 225 250 275 300
USER_GRAPH 1
-chart_title "Halite solubility"
-axis_titles "temperature / oC" "grams NaCl / kgw"
10 data 6.162, 6.293, 6.465, 6.677, 6.948, 7.21, 7.582, 7.983, 8.465, 9.017, 9.668, 10.43
20 dim c(20)
30 for i = 1 to 12
40   read c(i)
50 next i
60 plot_xy tc, c(step_no)*GFW("NaCl"), line_width = 0, color = Red
70 plot_xy tc, tot("Na")*GFW("NaCl"), symbol_size = 0, color = Red
-end
END