Pretreatment of saltwater by rising pH and aeration for Carbonate precipitation

[AzadGhadirian]

Hello all!


I am on my last steps of modeling some different pretreatment and posttreatment options for membrane-based brackish water desalination. Right now I am trying to simulate the pretreatment of the feed water by adding NaOH to rise the pH to 10 and simultaneously aerate the water with gas with 15% CO2 content.
I know that CO2 dissolution leads to and fall of the pH and NaOH leads to rising the pH..

Still I am trying to find out what the Precipitation potential could be.

I am using EQ_phase for aerating by adjusting the SI of CO2(g) by the partial pressure of 15% and assuming that the rest is Ntg(g).

The problem is:
- I can't raise the Moles of CO2(g) available since the program gives me an "maximum iterations exceeded"-Error.
- The amound of CO2(g) right now is not enough after the 7th Reaction-Step.


I would be very happy if someone has an idea how I can solve this problem!

Thank you very much and all the best,
Azad



SOLUTION 1
    temp      22.4
    pH        7.57
    pe        4
    redox     pe
    units     ppm
    density   1
    C(4)      1 CO2(g)     -3.374
    Ntg       1 Ntg(g)     -0.107
    Oxg       1 Oxg(g)     -0.677
    Ba     1.71
    Ca        360
    Cl        2753 charge
    K         124
    Mg        140
    Na        1757
    S(6)      1784
    Si        25.53
    -water    1 # kg


EQUILIBRIUM_PHASES 1
    CO2(g)    -0.823 5 #p(CO2)=0,15
    Ntg(g)    -0.07 4# p(N2)=0,85
   Gypsum 0 0
   Barite 0 0
   Magnesite 0 0
   SiO2(a) 0 0
   Calcite 0 0


REACTION 1
    NaOH     1
    7.3 moles in 10 steps

[dlparkhurst]

The numerical method had a hard time with this calculation. One fix is to add the following to cause the non-linear equation solver to use smaller steps to approach the solution.

Code: [Select]

KNOBS
-step 10
-pe 2

You actually run out of  CO2 before you reach the PCO2 that you specify, which I suspect is part of the reason for the failure to find a solution. If you put 10 moles of CO2 in EQUILIBRIUM_PHASES, the specified PCO2 can be achieved, and the calculation runs smoothly without adding KNOBS.

Please use the # button above the text box to enter code.

[AzadGhadirian]

Thank you very much for your reply Mr. Parkhurst!

Indeed the simulation runs when I raise the available moles of co2(g) to 10 moles. The problem is that I can't reach the aimed pH of 10 then.
It seems like in cases where I am aerating the solution with ambient air (p(co2)=0,00042) I am able to adjust the needed NaOH and moles of CO2 to have both: a pH of 10 and CO2 saturation with ambient air.

In this case I need more NaOH when I raise the amount of CO2 moles and vice versa..
Using the suggested KNOBS didn't change my simulation yet.. Do you have an another idea how I can reach both (pH and CO2 saturation)?

[dlparkhurst]

In brief, concentrations of CO2 and NaOH become unrealistically high to maintain atmospheric pCO2 above pH 9.5 or so.

See the discussion and example calculation in this thread https://phreeqcusers.org/index.php/topic,2615.msg9902.html#msg9902