Thermodynamic modeling of phase equilibria of semi-clathrate hydrates of CO₂, CH₄, or N₂+tetra-n-butylammonium bromide aqueous solution

Prediction of phase equilibria of semi-clathrate hydrates has been very rarely investigated in the literature. In this work, a thermodynamic model is proposed for representation/prediction of phase equilibria of semi-clathrate hydrates of the CO2, CH4, or N2+tetra-n-butylammonium bromide (TBAB) aqueous solution.

For modeling the hydrate phase, the van der Waals–Platteeuw (vdW–P) solid solution theory is used, revised with two modifications for evaluations of Langmuir constants and vapor pressure of water in the empty hydrate lattice, in which these values are supposed to be a function of TBAB concentration in aqueous solution.

The Peng–Robinson (PR-EoS) equation of state along with re-tuned parameters of Mathias–Copeman alpha function is applied for calculation of the fugacity of gaseous hydrate former. For determination of the activity coefficient of the non-electrolyte species in the aqueous phase, the Non-Random Two-Liquid (NRTL) activity model is used.

To calculate the mean activity coefficients of the electrolyte portion, a correlation on the basis of existing osmotic coefficient and activity coefficient values is employed. It is shown that the presented model results are in acceptable agreement with the experimental semi-clathrate hydrate dissociation data investigated in this work.