Thermodynamic modelling of gas semi-clathrate hydrates using the electrolyte NRTL model
Résumé
In this work a modified version of the modelling procedure of Paricaud is presented for describing the solid-liquid equilibria encountered in aqueous solutions of tri-n-butylammonium bromide (TBAB) involving a semiclathrate hydrate phase. The theoretical framework is further applied to the description of the solid-liquid-vapour three phase p-T-lines of the ternary system water + TBAB +methane at different overall TBAB concentrations. For the calculations performed on the ternary system exhibiting a gas semiclathrate hydrate phase, model parameters gained previously for the water + TBAB mixture were used. The thermodynamics of the semiclathrate hydrate phase was modelled by means of the salt hydrate model of Paricaud. For the description of the gas semiclathrate hydrate phase a combination of the salt hydrate model of Paricaud with the Waals-Platteeuw (vdW-P) theory has been applied. An unsymmetric reference frame has been employed to treat the liquid phase, i.e., Henry's constant was adopted for the ideal solubility of methane in the aqueous phase, whereas the fugacity of pure liquid water was adopted as reference state for water. The Soave Redlich Kwong equation of state was used to calculate the fugacities in the gas phase. Whereas the model of Paricaud employs the SAFT equation of state in a φ-φ-approach to account for both, liquid and gas phase non-idealities, the electrolyte NRTL (eNRTL)-GE-model has been incorporated in our modified model to describe deviations from ideality in the liquid phase. In the calculations, the temperature dependence of the eNRTL-interaction energy parameters has been neglected and instead, ENRTL-coefficients at 298.15 K have been used. The solid-liquid T-x phase diagram of TBAB was calculated at ambient pressure up to 60% stoichiometric mass fraction of TBAB. By assuming the existence of only type B hydrate a good overall correlation of experimental data found in the literature was achieved by adjusting the values for the standard molar enthalpy of the dissociation and the temperature at the congruent melting point of the semiclathrate hydrate compound. Using these values, phase boundary HLV-lines of the ternary system H2O + TBAB + methane, calculated at different stoichiometric concentrations of TBAB in the liquid phase, are displayed and compared with measured results. Average relative deviations <|Δ p|/p> between experimental data and modeling results between 4 and 44 % show the applicability of the approach presented.
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