C isotopes with transport using rates

[Pete_Scott]

I'm trying to use examples 2 and 3 (from http://www.hydrochemistry.eu/exmpls/istp.html) to perform isotopic fractionation calculations using rates commands. We have to do this because of constraints imposed by coupling with a reservoir simulator, which means I cannot use the isotopes database, and we require a species or phase to pass between the two.

My problem has been trying to create a dummy phase for [C13]O2 (labelled as ChO2), and to model the fractionation between the gas phase and solution during dissolution. I have tried modifying the analytical expression for [C13]O2 dissolution, but I am not sure if this is the best approach, as in most cases only [C12]O2 dissolves. Is there a easier way to do this? Thanks. Code attached below.

SOLUTION_MASTER_SPECIES
 Ch Ch 0 1 1
 
SOLUTION_SPECIES
 Ch = Ch; -log_k 0
 
PHASES  # add dummy phase for mores
 Calcite13
      H+ = H+   #dummy
      log_k  0.0
 Ch(g)
      Ch(g) + CO2 = Ch + CO2(g)
      -log_k   -1.468 #Overriden by analytical expression
      -delta_h -4.776 kcal
     -analytic   10.5624   -2.3547e-2  -3972.8  0  5.8746e5  1.9194e-5 #Original CO2 expression
#   -analytic   10.56153  -2.3547e-2  -3972.8    0  5.8746e5  1.9194e-5 #Modified CO2 expression alpha = 1.002
      -T_c  304.2 # critical T, K
      -P_c   72.86 # critical P, atm
      -Omega 0.225 # acentric factor



SOLUTION 1
 pressure   1
 units mmol/kgw
 pH 4.59 charge
 Na 1
 Cl 1
 Ca 10
 C(4) 1000; Ch 11.12463  # d13C = -10

EQUILIBRIUM_PHASES 1
CO2(g) 0 0
Ch(g) 0 0