Processes > Reactive transport modelling

Kinetic Reactions Rate EQUATIONS


Dear Phreeqc Users,
I'm new to Phreeqc and trying to model the geological storage of hydrogen using phreeqc.
I've been reading a fellow researcher's published work on the same topic and came across the reaction RATE EQUATION as shown below:

20 rate = 2.3e-09*(TOT("C(4)")/1e-3 + TOT("C(4)")) + 9.26e-8*(TOT("S(6)")/(1.e-4 + TOT("S(6)")))

Please, would anyone be kind enough to throw some light on the following?
1. what RATE EQUATION has been used
2. how the numbers 2.3e-09; 1e-3; 9.26e-8; 1.e-4 were obtained

I'm not sure you transcribed the equation exactly. I think the C(4) term should be analogous to the S(6) term. If you examine this term, it is constant at 9.26e-8, probably with units mol/sec, until the S(6) concentration decreases to the range of about 1e-4. At lower concentrations, the rate tends to zero.

--- Code: ---9.26e-8*(TOT("S(6)")/(1.e-4 + TOT("S(6)")))

--- End code ---

You don't give the entire rate script or the KINETICS definition, but maybe it is a rate expression for consumption of H2? I don't know where the rate constants come from, perhaps from fitting empirical data or the literature. The numbers 1e-3 and 1e-4 are somewhat arbitrary to designate where the rates start to diminish.

You cite the Hemme et al. paper from 2018. Go to Eqn. (4) of that paper (Monod equation).
It has the general form y = ymax (X/(a+X)); right under that equation you fill find the input values for carbon=C(4) and sulphur=S(6), respectively:
Ymax = 2.3e-9 and 9.26e-8 for C and S
a = 1e-3 and 1e-4 for C and S
X = C or S; both terms are added up for overall growth
I am sure by looking closer at the equation and the few written lines underneath it, it will all become clear to you


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