Evaporation steps linked to time

[Charlie]

We are trying to simulate lake water evaporation (closed system for now), and are using the stepped method:

REACTION 1
H20 -1.0
50 moles in 25 steps # approx. 90% evaporation

However, we later want to add some KINETIC rates to the minerals. 
Is it possible to relate the REACTION steps to a time step? i.e. make an evaporation rate, for instance a loss of 10% water per year, and therefore be in sync with the KINETICS time?

Thanks

[dlparkhurst]

Use KINETICS for the evaporation. You would add an evaporation reaction to your KINETICS definitions.

[Charlie]

Thanks David for the advice.  Would this be the method to use for the evaporation?


INCREMENTAL_REACTIONS True
KINETICS 1
Water       #evaporation
-formula -H2O
-m       55.0
-m0     55.0
-tol     1.e-8
-steps       1000 in 55 steps # seconds

[dlparkhurst]

Here is an example with a constant rate of evaporation. You will need to be careful not to remove all the water. Alternatively, you could make the rate first order with moles of water in solution, which would give an exponential decrease in moles of water.

Code: [Select]

SOLUTION 1
Na 1
Cl 1
END
INCREMENTAL_REACTIONS True
USE solution 1
RATES
Water
10 REM constant rate of evaporation
20 k = 1/86400
30 moles = -k * 55 * TIME
40 save moles
KINETICS 1
Water       #evaporation
-formula H2O 1
-m       100
-steps      86400 in 24 steps # seconds
USER_GRAPH 1
    -axis_titles            "Days" "Na, mol/kgw" "H2O, kg"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x TOTAL_TIME / 86400
20 graph_y TOT("Na")
30 graph_sy TOT("water")
  -end
    -active                 true



[Charlie]

Thanks David that's helpful and logical.

How best would I add rates of precipitation of mineral phases in this?  For example I want to look at the precipitation of calcite under this system. I'm not too sure how that would be possible.
Thanks

[dlparkhurst]

You can have multiple kinetic reactions in one KINETICS data block. Simply add Calcite in addition to the water evaporation.

KINETICS
Water
...
Calcite
...
END

[Charlie]

In addition to the above, is it possible to have mixing of several solutions in step with the kinetic evaporation time steps? Thanks

[dlparkhurst]

To be honest, I am not quite sure how to do it, but a solution is actually just the number of moles of H, O, charge, and all the other elements. So, I guess you could define the formula for a KINETIC reaction to be the moles of elements in a solution. You probably want to charge balance the solution (because you cannot define charge in the formula), and you may need to have eight or more places of accuracy, especially for H and O.

You can get the total moles of an element with the DUMP keyword or the TOTMOL function. To get more decimal places with TOTMOL, you can use the STR_E$ function or -high_precision in SELECTED_OUTPUT/USER_PUNCH.

Once you get the -formula defined, the rate expression would add or remove that "solution" from the reaction beaker.

For example, the user print output should be sufficient to define this sodium bicarbonate solution in -formula of KINETICS.

Code: [Select]

SOLUTION
Na 1
C  1 charge
USER_PRINT
10 t = SYS("elements", count, name$, type$, moles)
20 FOR i = 1 to count
30   IF (MID$(name$(i), LEN(name$(i))) <> ")") THEN PRINT PAD(name$(i), 10), STR_E$(moles(i), 20, 10)
40 NEXT i