Application of results to real cases

[dat]

Hi all,

I am here posting a general question. In Phreeqc, equations in databases are defined with a given K and given enthalpy. So can we use the results in real cases, will those happen?

For example, let's consider the reaction  CO2 + 4H2 = CH4 + 2H2O. For this to happen following conditions are given in the literature.

            High temperature and pressure -400-600°C, 20-50 bar
            electrochemical reduction of CO2
            Biological conditions

But when I added H2 and CO2 in the PHREEQC gas phase under low pressure and temperature conditions, it was found that CH4 formation by reduction of H2 and CO2. Are there any other conditions for this reaction to happen?

Or else is PHREEQC considering any other factors for reactions?

Really appreciate your help.
Thank you.

[dlparkhurst]

By default, PHREEQC calculates thermodynamic equilibrium; so, theoretically, CO2 and H2 should react to form methane. The world is not always in equilibrium, otherwise, the atmosphere would be nitric acid and all organic matter would cease to exist.

Many reactions occur kinetically, where the rate of transformation is not known a priori. In most cases, rates of reactions need to be determined empirically. The PHREEQC calculation tells you where the reaction is headed if you have sufficient time, a catalyst, or suitable microbes. For your case, I suspect that the abiological transformation of H2 is slow, but there seems to be a lot of activity in the field of H2 storage, so perhaps there are papers that address the kinetics of H2 reactions.

One approach to kinetics makes use of instantaneous equilibrium between H2 and carbonate. You can define gaseous and dissolved hydrogen as Hdg, which is inert. Kinetically, you can remove Hdg and add H2. Because the H2 reacts instantaneously, you will be modeling the kinetic formation of methane, which depends only on the rate you assign to the Hdg->H2 reaction.