Modelling gas hydrate equilibria using the electrolyte non-random two-liquid (ENRTL) model

Matthias Kwaterski (1, 2, 3) , Jean-Michel Herri (1, 2, 3)

1 SPIN-ENSMSE - Centre Sciences des Processus Industriels et Naturels
2 LPMG-EMSE - Laboratoire des Procédés en Milieux Granulaires
3 GENERIC-ENSMSE - Département Géochimie, environnement, écoulement, réacteurs industriels et cristallisation

The semi-empirical electrolyte NRTL (eNRTL) model, also referred to as the model of Chen, is a versatile model for the excess molar Gibbs energy, capable of describing multicomponent electrolyte systems over wide ranges of state conditions. The model represents the excess Gibbs molar energy as the sum of two contributions, the first one of which accounts for long range electrostatic forces between ions, and the second one for the short range forces between all species. In single solvent systems, the long range interaction contribution consists of a term originating from the Pitzer-Debye-Hückel (PDH) equation . A modified version of the Non-Random-Two-Liquid (NRTL) local composition model of Renon and Prausnitz accounts for the short range interaction between all the species in their immediate neighbourhood. The most general form of the eNRTL activity coefficient expressions for both, individual species as well as mean ionic quantities have been implemented in the JAVA language. Model parameters for different strong electrolytes are provided by means of a data bank in the xml file format. The program code of the model implementation has been incorporated into the program package "gashydyn" developed in our group and allowing for performing equilibrium calculations involving gas hydrate phases. The correctness of the program implementation of the eNRTL expressions has been verified by comparing the results of numerous examples with corresponding literature results, including the composition dependence of the mean ionic activity coefficient of binary salt + solvent mixtures as well as of ternary salt 1 + salt 2 + mixtures. For the ternary systems, the influence of different values for the salt-salt binary interaction parameter is illustrated. Calculations on HLV phase equilibria of ternary H₂O + salt + gas and quaternary H₂O + salt + gas 1 + gas 2 systems have been performed. The calculations are based upon an equation of state approach for the gas phase, the van-der-Waals and Platteeuw model for the clathrate hydrate phase and the eNRTL model to account for the liquid phase non-idealities. The results reveal that a satisfying correlation of the experimental p-T-phase equilibrium data can be achieved with results ranging from around 1 to 15 %.

Mots clés: modeling; gas hydrate; eNRTL model; electrolytes; phase equilibrium; CO₂; CH₄

Domaines: Génie des procédés

Matthias Kwaterski, Jean-Michel Herri. Modelling gas hydrate equilibria using the electrolyte non-random two-liquid (ENRTL) model. 7th International Conference on Gas Hydrates (ICGH 2011), Jul 2011, Edimbourg, United Kingdom. pp.438. ⟨hal-00617438⟩