Prediction of hydrate equilibrium conditions using Electrolyte Cubic Square-Well Equation of State

Abstract

Electrolyte Cubic Square-Well Equation of State, eCSW EoS [A. Haghtalab, S.H. Mazloumi, Fluid Phase Equilibr. 285 (2009) 96–104] is applied to compute the hydrate equilibrium conditions of the various systems. This equation based upon the Helmholtz free energy consists of the one non-electrolyte term and the two electrolyte terms. The non-electrolyte term is Cubic Square-Well Equation of State (CSW EoS) and the two electrolyte contributions consist of a Born energy and the mean spherical approximation terms. In this work, eCSW EoS is coupled with the van der Waals–Platteeuw model and applied to compute gas hydrate formation conditions. Equilibrium calculation is performed to regression of the Langmuir constants against temperature for N2, CH4, C2H6, C3H8, i-C4H10, n-C4H10 and CO2 hydrates using the experimental data in literature. Moreover, using eCSW EoS, some parameters such as kij in the combining rule of the EoS, the interaction parameters of CO2–water and electrolyte–water systems are adjusted. The hydrate equilibrium conditions of the various systems including single gases, mixed gases and natural gases are predicted. The model results are compared with the calculated values using SRK and PR. Also, the equilibrium conditions of various hydrate formers in the presence of electrolytes such as NaCl, KCl, CaCl2 and their mixtures are predicted. Finally, the results of the model for different systems compared with those are obtained using HWHYD and Klauda models. A good agreement between the results indicates the reliability of the present model in prediction of the gas hydrate equilibrium conditions.