Experimental Study of Mixed Gas Hydrates from Gas Feed Containing CH 4 , CO 2 and N 2 : Phase Equilibrium in the Presence of Excess Water and Gas Exchange

This article presents gas hydrate experimental measurements for mixtures containing methane (CH 4 ), carbon dioxide (CO 2 ) and nitrogen (N 2 ) with the aim to better understand the impact of water (H 2 O) on the phase equilibrium. Some of these phase equilibrium experiments were carried out with a very high water-to-gas ratio that shifts the gas hydrate dissociation points to higher pressures. This is due to the significantly different solubilities of the different guest molecules in liquid H 2 O. A second experiment focused on CH 4 -CO 2 exchange between the hydrate and the vapor phases at moderate pressures. The results show a high retention of CO 2 in the gas hydrate phase with small pressure variations within the first hours. However, for our system containing 10.2 g of H 2 O full conversion of the CH 4 hydrate grains to CO 2 hydrate is estimated to require 40 days. This delay is attributed to the shrinking core effect, where initially an outer layer of CO 2 -rich hydrate is formed that effectively slows down the further gas exchange between the vapor phase and the inner core of the CH 4 -rich hydrate grain.