Aeration of water to remove carbon dioxide

[temo]

Beginners question :)
Is it possible to simulate addition of oxygen by aeration in a closed system on a waterplant? The goal is to remove carbondioxide (to avoid korrosion) from the water without adding NaOH. I would like to calculate the CCPP value (delta (calcite)) as a result of a higher pH.

SOLUTION 1                     #Hern Vand
   temp   10.2                  #Temperatur
   units               mg/L
   pH 7.71 
   F 0.09           
   Ca 42          
   Mg 4.7       
   Na 27      
   K   1.5   
   Alkalinity   163         as HCO3
   Cl   34   
   N(5)   0.4            as NO3
   S(6) 6.5       
   Fe   0.97      
   P   0.15   
#REACTION_TEMPERATURE 1
   # 10
EQUILIBRIUM_PHASES 1
    Calcite  0
#REACTION 1
   # O2(g)      1
    #1 moles in 1 steps
END

[dlparkhurst]

The result depends on how effectively you can strip CO2 from the solution. In theory, if you had an open system, you could remove all of the carbon from the solution by stripping with enough zero-CO2 gas. In a closed system, with a gas head space, CO2 would accumulate in the gas phase, which would limit the effectiveness of the gas stripping.

The first simulation below considers the results if varying amount of CO2 were removed from solution. At some point calcite will start to precipitate ; as more CO2 is removed, more calcite precipitates, but eventually, as there is less carbon available, less calcite will precipitate.

The second simulation considers a gas phase that forms at 1 atm pressure as oxygen is added to the solution. Oxg is in phreeqc.dat as an inert gas, so any oxidation reactions are ignored. As more Oxg is added a gas phase forms and CO2 partitions between the gas phase and the solution. After 20 mol of Oxg is added, the gas phase has a volume of 464 L compared to 1 L of solution.

You will have to decide which of these approaches is better for your situation, and how much CO2 can reasonably stripped from your solutions. Perhaps stripping with air is more reasonable (log10 PCO2 -3.4), in which case, I get a pH of 8.3 and 0.5 mol calcite precipitated. I leave that as an exercise for you.

Code: [Select]

SOLUTION 1                     #Hern Vand
   temp   10.2                  #Temperatur
   units               mg/L
   pH 7.71
   F 0.09           
   Ca 42         
   Mg 4.7       
   Na 27     
   K   1.5   
   Alkalinity   163         as HCO3
   Cl   34   
   N(5)   0.4            as NO3
   S(6) 6.5       
   Fe   0.97     
   P   0.15   
EQUILIBRIUM_PHASES 1
    Calcite  0 0
REACTION
CO2 1
-2 mmol in 10 steps
USER_GRAPH 1
    -headings               CO2 Calcite pH CO2
    -axis_titles            "CO2 removed, moles" "Calcite precipitated" "pH, log10PCO2"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X -RXN
20 GRAPH_Y EQUI("Calcite")
30 GRAPH_SY -LA("H+"), SI("CO2(g)")
  -end
    -active                 true
END


Code: [Select]

SOLUTION 1                     #Hern Vand
   temp   10.2                  #Temperatur
   units               mg/L
   pH 7.71
   F 0.09           
   Ca 42         
   Mg 4.7       
   Na 27     
   K   1.5   
   Alkalinity   163         as HCO3
   Cl   34   
   N(5)   0.4            as NO3
   S(6) 6.5       
   Fe   0.97     
   P   0.15
EQUILIBRIUM_PHASES 1
    Calcite  0 0
GAS_PHASE
-fixed_pressure
Oxg(g) 0
CO2(g) 0
REACTION
Oxg 1
20 mol in 10 steps
USER_GRAPH 1
    -headings               O2 Calcite pH CO2
    -axis_titles            "O2 added, moles" "Calcite precipitated" "pH, log10PCO2"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y EQUI("Calcite")
30 GRAPH_SY -LA("H+"), SI("CO2(g)")
  -end
END


[temo]

@dlparkhurst - thank you for the response. You are right, I have to use the open system/stripping with air. So my input-file should be like below. Can I use REACTION to calculate the amount of air added in order to remove a small amount of corrosive CO2 (6 mg/l)? When I try I get a fixed delta calcite for all my steps.
Great tip you gave me on how to use USER_GRAPH, thank you.

SOLUTION 1                     #Hern Vand
   temp   10.2                  #Temperatur
   units               mg/L
   pH 7.71 
   F 0.09           
   Ca 42          
   Mg 4.7       
   Na 27      
   K   1.5   
   Alkalinity   163         as HCO3
   Cl   34   
   N(5)   0.4            as NO3
   S(6) 6.5       
   Fe   0.97      
   P   0.15   
EQUILIBRIUM_PHASES 1
    Calcite   0 0
CO2(g)   -3.4
END

[dlparkhurst]

I don't think there is a simple relation between air added and CO2 stripped. Your calculation shows the end result when the water comes to equilibrium from the air, but there are kinetic factors to the liquid-gas exchange.