Difference between - SetTimeStep in PhreeqcRM, time in KINETIC RATE

[SaiP]

Hi Phreeqc users,

I am trying to benchmark calcite dissolution in a column (discussed in Paper – Comp. Geosci (2015), Mingliang Xie et.al.). I am using PhreeqcRM for solving the chemistry and a CFD package for transport.

I tried to be consistent with all the units and values presented in the paper, and the outcome is that: I am either dissolving calcite too fast or too slow but not anywhere close to the published results.

The flow velocity is as expected at the start of the run. I am inclined towards thinking if my RATE KINETICS are causing the discrepancy.

I use the following for calcite Kinetic dissolution;

Code: [Select]

RATES
calcite
   -start
1   REM   PARM(1) = specific surface area of calcite, mol/dm3/s     [units – mol/dm3/s]
10  si_cc = SR("Calcite")                 [units - -]
20  rate = PARM(1)*(1 - si_cc)                 [units – mol/dm3/s]
30  mole = rate*time                         [units – mol/dm3]
40  if(mole > m) then mole = m
50  save mole 
   -end

Note: I have taken the effect of surface area depreciation due to dissolution in Parm1.

1.   In PhreeqcRM there is a function “SetTimeStep”. For this, I use the time step of the transport code (21600 s).
2.   In the KINETIC RATE block, I multiply the dissolution rate with time to get the amount of moles in ‘mol/dm3’ [line 30] as explained in the online manual.

A few questions popped up for me;
-> Considering 1 and 2, am I using the time as a multiplication factor twice while solving the kinetic part (I do notice that there is a substantial difference in the dissolution chemistry if I turn these two parameters on or off)?

-> Also I am not sure what ‘time’ is in the Kinetic Rate block? Is the value equal to “SetTimeStep” value or is it different and independent with respect to SetTimeStep?

Thanks.

[dlparkhurst]

The value of "time" is set internally as part of the integration calculation. It is a sub interval of the time step.

The time step for RunCells used in the integration is the one set for PhreeqcRM.

I think a discrepancy may be related to the representative volume, 1 L by default. If your porosity is less than one, then the volume of solution is less than 1 L. To calculate the appropriate rate, you should multiply your rate mol/L/s by the solution volume (Basic function SOLN_VOL), then by time, to achieve the result in moles.

Note also, there is a conversion in PhreeqcRM to calculate the number of moles of calcite in a cell. You have choices whether the moles defined in a KINETICS block read by the initial phreeqc instance is transferred to the workers as moles per liter of solution in a cell, the number of moles per liter of representative volume, or the number of moles per liter of rock. See  SetUnitsKinetics of PhreeqcRM.