Modification of a model for mixed hydrates to represent double cage occupancy

Abstract

Gas hydrates are subject of current research work and are also important for several industrial applications. When transporting natural gases or oil through pipelines, the formation of gas hydrates and thus a blockage of the pipeline must be avoided. There is also a great interest to understand the thermophysical properties and the phase change behavior of hydrates during liquefaction processes of natural gases, for CO2 capture and sequestration (CCS) processes, and for CH4 and C2H6 hydrates, which can be found on the seabed or in permafrost soils as a potential energy source. In this work, the model for mixed gas hydrates (mixtures of water with more than one hydrate former), which has been introduced in a previous work [Fluid Phase Equilib. 459 (2018) 170–185] and is intended for the use in CCS relevant applications, is modified to take double cage occupancy into account. The three-phase equilibrium data can be reproduced up to high pressures at which N2, O2, and Ar are known to double occupy the hydrate cavities. The influence of double occupied hydrate cages and the improved reproduction of experimental phase equilibrium data at pressures at which double cage occupancy occurs are shown for pure hydrate and for mixed hydrate phase equilibrium and composition data. In case of mixed hydrates, comparisons are shown for synthetic air (N2+O2).