Theoretically achievable efficiency of hydrate-based desalination and its significance for evaluating kinetic desalination performance of gaseous hydrate formers

Abstract

Freshwater can be obtained through gas hydrate formation from saline water, which is called hydrate-based desalination (HBD). In this study, the thermodynamic and kinetic features of propane, R134a, R22, and R152a hydrates in the presence of NaCl were examined to determine the energy-efficient gaseous hydrate former. Structure I hydrate formers (R22 and R152) gave lower hydrate depression temperatures than structure II hydrate formers (propane and R134a) at a given salinity. The theoretically achievable salinity and water yield of HBD at each given thermodynamic condition were calculated using the Hu-Lee-Sum correlation. The theoretical HBD efficiency increased as the initial salinity decreased, the operating pressure decreased, and the initial subcooling temperature increased. At a fixed initial subcooling (2 K), R134a gave faster formation kinetics in the early stage, but R22 eventually offered highest hydrate conversion. At a fixed temperature (272 K), R152a showed fastest formation kinetics and highest HBD efficiency due to its milder hydrate equilibrium conditions. The overall results demonstrated that the thermodynamic stability and the intrinsic formation kinetics of gas hydrates significantly impact HBD efficiency and that the theoretical HBD efficiency can be the quantitative criterion for evaluating the kinetic performance and desalination capability of hydrate formers.