Predicting the phase equilibria of refrigerant gas hydrate systems in the presence of electrolytes using molecular theory

Abstract

Hydrate-based desalination (HyDesal) has been proposed as one of the promising technologies to remove salts from seawater and brackish water. At the appropriate temperature and pressure, water molecules in brine can form cages around a guest molecule, thereby forming a separate ice-like solid phase that is free of salt. The process can be energy intensive due to the high refrigeration load required to cool the brine and gas feed below the hydrate formation temperature. Fluorinated refrigerants have been proposed as potential guest molecules that can form hydrates near ambient conditions, thus minimizing the energy requirement of the process. Hence, understanding the inhibition effect of electrolytes on the phase behavior of refrigerant hydrates is vital for the design and optimization of HyDesal processes. We present a molecular model that is based on the electrolyte, polar, and perturbed chain form of the statistical associating fluid theory (e-Polar PC-SAFT) combined with the solid solution theory of van der Waals and Platteeuw to fully predict the three-phase equilibria of pure and mixed hydrochlorofluorocarbon and hydrofluorocarbon systems in the presence of NaCl, KCl, CaCl2, and MgCl2. The model captures the effect of the refrigerant structure on the hydrate dissociation conditions and the effect of cation size and charge number on the thermodynamic inhibition of gas hydrates well. Based on a detailed thermodynamic and environmental analysis, three refrigerants (R-32, R-152a, and R-410A) are shortlisted as viable options for HyDesal. All three refrigerants can form hydrates under reasonable conditions, and their phase equilibria show weak dependence on salt concentration.