Unexpected rise of pH

[AzadGhadirian]

Hello all!

Happy new year!

I have problem modelling a reverse osmosis reject.

I did define the solution, put it in equilibrium with the salts precipitating and started to extract H2O (and rise the pressure and temperature) like in realitiy.
Unfortunately, the pH reaches a pH of around 5.5. In reality the feed has a pH of around 7.8 and the reject should contain a pH of around 8.0-8.2.

I did try a lot of different things to solve this problems by  my own but I couldn't find a solution..
The only thing which worked is adjusting the alkalinity.. But as I understood it Phreeqc is calculating the alkalinity precisly. Unfortunately, I don't have measured values for the alkalinity.

I thought that in the second simulation CO2 shouldn't be in a EQ phase, since there is no more contact to the atmosphere, but could it be that the CO2 can leave through the permeate and there is the need of an equilibrium phase for the gases also in the second simulation?

I am using the pitzer.dat
It would be a great help if someone knows what the issue could be.

SOLUTION 1
    temp      22.4
    pH        7.57
    pe        4
    redox     pe
    units     ppm
    density   1
    Ca        360
    Cl        2753 charge
    K         121
    Mg        140
    Na        1757
    S(6)      1784
    Si        25.53
    Ntg       14
    Oxg       8.43
    -water    1 # kg

EQUILIBRIUM_PHASES 1
    Antigorite 0 0
    CO2(g)    -3.5 10
    Quartz    0 0
    Talc      0 0
SAVE SOLUTION 2
END


USE SOLUTION 2
EQUILIBRIUM_PHASES 2
   Gypsum    0 0
   Quartz 0 0
   Antigorite   0 0
   Chrysotile 0 0
   Goergeyite 0 0
   Sepiolite 0 0
   Talc 0 0
   
REACTION_PRESSURE
20 45 in 79 steps

REACTION_TEMPERATURE
22 28 in 79 steps

REACTION 1
H2O    -1
45.5 moles in 79 steps


USER_GRAPH 100
    -headings               Volume_water pH
    -axis_titles            "Volume removed [%]" "pH"
    -chart_title            "pH"
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start
10 graph_x STEP_NO
20 GRAPH_Y -LA("H+")
  -end
    -active                 true


Thank you and all the best,
Azad

[dlparkhurst]

I think you should add CO2 to your initial solution to reach the atmospheric CO2 partial pressure at the pH of the solution. Otherwise, you are adding CO2 to the solution in the next step, which reduces the pH to the 5 range.

I'm skeptical of the phases you are precipitating. Gypsum is plausible, but crystalline quartz is unlikely to form; chalcedony or SiO2(aq) are more likely. Similarly, I'm not sure what an appropriate Mg phase would be. Perhaps you have some experimental evidence of the phases that form.

If you include Oxg and Ntg, you should probably include them in EQUILIBRIUM_PHASES, otherwise they reach unreasonable concentrations. Same for CO2(g).

This file generates pH values in the correct range. You can decide how you want to handle the minerals.
If you are having trouble, simplify. Use 1 step instead of 79.

Code: [Select]

SOLUTION 1-2
    temp      22.4
    pH        7.57
    pe        4
    redox     pe
    units     ppm
    density   1
    Ca        360
    Cl        2753 charge
    K         121
    Mg        140
    Na        1757
    S(6)      1784
    Si        25.53
    Ntg       14
    Oxg       8.43
    C(4)      1   CO2(g) -3.4
    -water    1 # kg

#EQUILIBRIUM_PHASES 1
#    Antigorite 0 0
#    CO2(g)    -3.5 10
#    Quartz    0 0
#    Talc      0 0
#SAVE SOLUTION 2
END
EQUILIBRIUM_PHASES 2
   Gypsum    0 0
 #  Quartz 0 0
 #  Antigorite   0 0
 #  Chrysotile 0 0
 #  Goergeyite 0 0
 #  Sepiolite 0 0
 #  Talc 0 0
CO2(g) -3.4 0
Ntg(g) -0.2 0
Oxg(g) -0.7 0

USE SOLUTION 2

   
REACTION_PRESSURE
20 45 #in 79 steps

REACTION_TEMPERATURE
22 28 #in 79 steps

REACTION 1
H2O    -1
45.5 moles #in 79 steps
END

[AzadGhadirian]

Thank you very much!

What you said makes totally sense!
I am researching about the kinectic behaviour of the salts. I think gypsum, a small proportion of barite, calcite and SiO2(aq) it will be.
Thanks for your help!