PVTx measurements of mixed clathrate hydrates in batch conditions under different crystallization rates: Influence on equilibrium

Abstract

Notwithstanding the numerous studies about classical temperature and pressure equilibrium of mixed gas hydrates, few provide hydrate composition and volume at local and final equilibrium under batch conditions. Therefore, required new phase equilibrium data for mixed gas hydrates including CO 2-C 3 H 8 , C 2 H 6-nC 4 H 10 , CH 4-nC 4 H 10 , CO 2-C 2 H 6-C 3 H 8 , CH 4-C 2 H 6-nC 4 H 10 and CH 4-C 2 H 6-C 3 H 8-nC 4 H 10 are presented in this paper. Moreover, results contain hydrate phase properties such as hydrate volume and density, storage capacity, water conversion and guest composition in all phases (vapor, liquid and hydrate). Importantly, effect of crystallization rate on final state has been evaluated. Finally the experimental results were compared to the thermodynamic model of van der Waals and Platteeuw to investigate kinetic effects. Experimental results show that equilibrium pressures at final state are dissimilar with respect to the crystallization rate. Furthermore, the hydrate volume formed at slow crystallization rate is noticeably lower than at quick. Noticeably, storage capacity in the case of slow crystallization is higher which could be crucial for industry. Also, the guest composition in hydrate phase at final state differs. Enclathration of heavier molecules is more significant at slow crystallization rate, and the hydrate phase seems to be more homogeneous according to the thermodynamic study. Indeed, modeling results, that assume the hydrate phase to be homogeneous in composition, show better agreement with the slow crystallization results for both equilibrium pressure and guest distribution in hydrate phase. This elucidates that, at quick crystallization rate, thermodynamic equilibrium cannot be reached. In conclusion, how kinetics influence the design of clathrate hydrate based industrial applications such as energy storage and transportation, carbon capture sequestration etc. are demonstrated.

Recent Advances in Experimental Measurements of Mixed-Gas Three-Phase Hydrate Equilibria for Gas Mixture Separation and Energy-Related Applications

“…The binary gas mixture ethane (C 2 H 6 ) + propane (C 3 H 8 ) is of significant importance primarily to flow assurance problems. Therefore, a significant number of experimental measurements for three-phase equilibrium conditions have been reported by Holder and Hand, Nixdorf and Oellrich, Babakhani et al, , and Ballard et al Ballard et al used two independent experimental methods and clearly demonstrated the importance of stirring during the experiments, particularly in regions with a simultaneous presence of a liquid hydrocarbon phase in addition to the vapor phase. The authors also presented an extensive discussion of the pseudo-retrograde hydrate dissociation phenomena that exist at low pressures (10–15 atm) near 287 K for the system C 2 H 6 + C 3 H 8 + H 2 O, for example, the phenomenon of sII hydrate dissociation, which is observed for a wide water-free composition range when slight increases in pressure occur.…”