Water auto-ionization reaction for electrolyte systems

If you plan to include liquid-phase equilibrium reactions in streams of nearly pure water in your electrolytic separation processes, we recommend that you include the water auto-ionization reaction in your equilibrium reaction submodels. Otherwise, you may experience numerical convergence issues. See Convergence issues with nearly pure streams for more information.

Typically, the contribution of water auto-ionization to electrolyte systems is negligible. However, when the composition of the fluid approaches nearly pure water, the contribution of water auto-ionization to the equilibrium calculations can be significant.

The following chemical equation shows the water auto-ionization reaction:

By definition, the equilibrium reaction constant (Keq) is the ratio of products to reactants:

where

fi is a generalized measure of composition (for example, molar fraction, mass fraction, activity, fugacity, or partial pressure)

In logarithmic form, the equation for the equilibrium reaction constant becomes:

The equilibrium reaction constant also depends on temperature. You can use the following temperature-dependent equation[1] to calculate Keq:

If you are modifying existing equilibrium reaction submodels to include the water auto-ionization reaction, you should update the number of reactions, add both equations for ln(Keq), and update the dz equations to include the contributions from the water auto-ionization reaction. You can open the RXCO2_MEA_eNRTL reaction submodel from the Fluids Library to see an example of how to update your equilibrium reaction submodels.

The Reaction Generator includes the water auto-ionization reaction in its reaction database. Therefore, you can also use the Reaction Generator to quickly create a new equilibrium reaction submodel that includes the water auto-ionization reaction.

References

1. Bishnoi, S.; Rochelle, G. T. Absorption of carbon dioxide into aqueous piperazine: reaction kinetics, mass transfer and solubility. Chem. Eng. Sci. 2000, 55 (22), 5531-5543.