Simulating closed batch system w/ mass transfer of O2 from headspace to solution

[jeekay]

Hello Fellow Users,

I'm a beginner to using PHREEQC but I'm proficient in reactive transport modeling and have used other RTM codes like CrunchFlow and PFLOTRAN for my work. I'd like to start using PHREEQC with simulating a simple batch system involving a 160 ml vial with 100 ml of solution (water) and 60 ml of headspace. The headspace consists of air with oxygen at 21% v/v. Since this is a closed system, the amount (mass or volume) of the oxygen in the system (headspace) is finite. In my experiments, there appears to be O2 mass transfer limitations from headspace to the solution, since there's no vigorous mixing involved. I'm interested in knowing whether PHREEQC allows for simulating O2 mass transfer kinetics from headspace to the solution, say, following a first-order rate kinetics?

If so, could somebody give me a kickstart with the input file to simulate the same? I can work off of it as a starting point with increasing complexity.

Thanks so much!


   

[dlparkhurst]

Here is a script to get you started. It defines an insoluble form of O2(g) as [O]2(g), where [O] is separate from normal oxygen. The kinetic reaction transforms [O]2 to normal O2. O2 equilibrates between the solution and the gas phase. Eventually all [O]2 converts to O2, and the O2 is in equilibrium between the gas and the water. I have used a first-order rate constant of 1 per second, which you can adjust.

Code: [Select]

SOLUTION_MASTER_SPECIES
[O]         [O]2     0   [O]             16      #   O2  gas
SOLUTION_SPECIES
[O]2 = [O]2 # O2
-Vm 5.7889 6.3536 3.2528 -3.0417 -0.3943 # supcrt
-dw 2.35e-9
PHASES
[O]2(g)
[O]2 = [O]2
#-analytic -7.5001 7.8981e-3 0 0 2.0027e5
      log_k -100
-T_c 154.6; -P_c 49.8; -Omega 0.021
RATES
O2_dissolution
10 k = 1 # 1/s
20 rate = k * GAS("[O]2(g)")
30 moles = rate * TIME
40 SAVE moles
END
SOLUTION 1
-water 0.1
END
GAS_PHASE 1
-fixed_volume
-temp 25
-volume 0.060
[O]2(g) 0.21
O2(g) 0
END
USE solution 1
USE gas_phase 1
KINETICS
O2_dissolution
-formula [O]2 -1 O2 +1
-M 1
-steps 6 in 10 # sec
USER_GRAPH 1
    -headings               time [O]2(g) O2(g) O2(aq) [O]2](g)+O2(g)+O2(aq)
    -axis_titles            "Seconds" "Moles" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME
20 GRAPH_Y GAS("[O]2(g)"), GAS("O2(g)")
30 GRAPH_Y MOL("O2")*TOT("water")
40 GRAPH_Y GAS("[O]2(g)") + GAS("O2(g)") + MOL("O2")*TOT("water")
  -end
    -active                 true
END