Gas hydrates can be formed at low-temperature and high-pressure conditions in the coexistence of gas and water molecules. The electrolyte-exclusion nature of gas hydrates has enabled it a promising technology for desalination. A successful design of desalination by gas hydrates requires the knowledge of equilibrium phase boundary of gas hydrates in electrolyte solutions. In this work, a new model has been proposed to predict the equilibrium phase boundary of gas hydrates in sodium chloride (NaCl) solutions. A single electrolyte-equation-of-state (e-EoS) is implemented for both vapor and liquid phases while the van der Waals-Platteeuw model is used for the hydrate phase. The effect of electrolytes on the phase boundary is incorporated by molecular-scale ion-water and ion-ion interactions, which are quantified by a Born term and a mean spherical approximation (MSA) approach, respectively. The reliability of the newly proposed model is enhanced by using sequential stability test and three-phase flash calculation. The newly developed model is validated with two sets of well-documented experimental phase boundary data for pure CH4 hydrates and CH4-CO2 hydrates in NaCl solutions. The ratio of CH4 to CO2 in the loading gas for the CH4-CO2 hydrate is 80 mol% over 20 mol%. The NaCl concentration is from 5 wt% to 20 wt% for both of them. The calculated equilibrium pressures at the phase boundary are compared with the measured ones at various temperatures. Consequently, the average absolute relative deviation (AARD) for the two systems are determined as 3.7% and 5.4%, respectively. It indicates that the newly proposed model is capable of predicting the equilibrium phase boundary of CH4-CO2 hydrates in NaCl solutions. Furthermore, the equilibrium phase boundaries of CH4-CO2 hydrates under various conditions have been obtained and plotted in a template for practical reference. The favorable phase boundary of gas hydrates for desalination can be obtained by increasing the CO2 concentration. Overall, the newly developed model not only greatly reduces the experimental cost for such a complex system but also provides valuable data for the desalination design using gas hydrates.

Phase boundary of gas hydrates: Application in desalination

2019 Fall National Meeting

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