Hydrogen and air storage in salt caverns: a thermodynamic model for phase equilibrium calculations

Abstract

When storing gas in a salt cavern, it occupies most of the excavated volume, but the lower part of the cavern inevitably contains residual brine, in contact with the gas. The design of hydrogen and compressed air storage in salt caverns requires to have a thermodynamic model able to accurately predict both phase properties such as densities, and phase equilibrium (gas solubility and water content of the vapour phase). This work proposes a parameterization of the e-PPC-SAFT equation of state in this context. Experimental data of pure components and mixtures of light gas + pure water and light gas + salted water are reviewed and used to fit pure component parameters for hydrogen, nitrogen, oxygen, and the brine, and binary interaction parameters between H2, O2, N2 + water and H2, O2, N2 + ions (Na+ and Cl−), for temperature ranging from 273 to 473 K and salinities up to NaCl saturation (6 mol/kg). The model developed delivers good accuracy in reproducing data: the average deviation between experiments and calculated data is between 3% and 9% for gas solubility in saturated brine. More interestingly, the model has been validated on its capability to predict data not included in the parameterization database, including the composition of the vapor phase, and its extension to a mixture, such as air. Finally, it has been used in a case study of Compressed Air Energy Storage (CAES) to evaluate the water content of the gas produced during injection-withdrawing cycles.