Phase Equilibrium Data on Carbon Dioxide Hvdrate in the Presence of Electrolytes, Water Soluble Polirners and Montmorillonite zyx PETER ENGLEZOS* and STEPHEN HALL Department of Chemical Engineering, The University of British Columbia, 2216 Main Mall, Vancouver, BC V6T 124, Canada Equilibrium formation conditions for carbon dioxide hydrate were measured in pure water and in aqueous polymer and electrolyte solutions. The solutions that were used include: polyethylene oxide, partially zyxw (10 and 90 percent) hydro- lyzed polyacrylamide, sodium chloride and calcium chloride. Experiments with solutions containing both electrolyte and polymers were also performed. It was found that the electrolytes exhibited a substantial inhibiting effect whereas the polymers only slightly altered the equilibrium hydrate formation conditions. The measured equilibrium formation pressures were compared with the predictions from existing hydrate equilibria methods and were found to be in good agreement. The effect of montmorillonite was also studied and it was found that it did not affect the equilibrium hydrate formation conditions. zyxwvutsr A total of 101 experimental measurements are reported. Les conditions de formation des kquilibres pour I’hydrate de gaz carbonique ont CtC mesurees dans I’eau pure et dans des solutions aqueuses de polymkres et d’electrolytes. Les solutions utiliskes sont: I’oxyde de polyCthylkne, le poly- acrylamide partiellement hydrolysk (10 et 90 p. zyxwvuts loo), le chlorure de sodium et le chlorure de calcium. Des experiences avec des solutions a base d’Clectrolytes et de polymkres ont Cgalement CtC menees. Les electrolytes se sont avCres avoir un effet d’inhibition important tandis que les polymkres n’ont que legkrement altCre les conditions de formation des hydrates zyxwvutsrqpo B I’equilibre. Les pressions de formation des Cquilibres mesurees ont kt6 comparies aux predictions de methodes d’tquilibre existantes pour les hydrates, et on a trouvC un bon accord. On a tgalement etudie I’effet de la montmoril- lonite qui ne semble pas influer sur les conditions de formation des hydrates B I’equilibre. Les resultats de 101 mesures experimentales sont present& au total. Keywords: carbon dioxide hydrate, electrolytes, water soluble polymers, montmorillonite, drilling fluids hase equilibrium data for the carbon dioxide hydrate are P required in several technological applications. For example, water and carbon dioxide are frequently found in oil reservoirs during enhanced oil recovery and they are also part of natural gas streams. The possibility of hydrate for- mation in drilling muds or decomposition during drilling through in-situ hydrate zones is always taken into consider- ation by drilling engineers. Electrolytes and polymers are constituents of water based drilling fluids. They are known to influence the equilibrium hydrate formation conditions. Furthermore, aqueous fluid inclusions containing carbon dioxide and electrolytes are encountered in various geologic environments. Finally, capture of carbon dioxide from thermal power plants and disposal in deep ocean in the form of hydrate has been proposed as a means to reduce global warming. Information on the fundamentals and the techno- logical applications of the class of compounds known as clathrate or gas hydrates is available (Sloan, 1990; Englezos, 1993). Several researchers have reported data on the equilibrium formation conditions in pure water. These data are summa- rized by Sloan (1990). Larson (1959, Vlahakis et al. (1972) and Bozzo et al. (1975) studied the effect of electrolytes on the carbon dioxide hydrate equilibrium. Experimental data on hydrate formation conditions in salt solutions were also recently presented by Dholabhai et a]. (1991), Saji et al. (1992) and by Nishikawa et al. (1992). Englezos (1992a) presented a thermodynamics based method for the predic- tion of the equilibrium formation conditions in aqueous elec- trolyte solutions. Research on the effect of components of drilling fluids such as water soluble polymers and clay on gas hydrate equilibria *Author to whom correspondence should be addressed. has been recently initiated (Cha et al., 1988; Ouar et al., 1992; Kotkoskie et a]., 1992). Because the previous equilibrium data were on hydrocarbon hydrates the objec- tive of this work was to provide data on carbon dioxide hydrate. Incipient phase equilibrium data for carbon dioxide hydrate formation in aqueous polymer and electrolyte solu- tions are reported. Data for hydrate formation in the presence of montmorillonite were also measured and are reported. Experimental apparatus and procedure The objective of an experiment is to measure, at isothermal conditions, the minimum pressure at which carbon dioxide hydrate is in equilibrium with an aqueous liquid phase and with a gaseous phase that is rich in carbon dioxide. The procedure that was followed is known as the isothermalpres- sure search procedure (Englezos and Bishnoi, 1991 ; Englezos and Ngan, 1993; 1994). It yields the incipient equilibrium hydrate formation conditions. Temperature search procedures have also been established for hydrate equilibrium measurements (Sloan, 1990). APPARATUS A schematic illustrating the main components of the experimental apparatus where all the experiments were con- ducted is shown in Figure I. The apparatus consists of a high pressure cell immersed into a temperature controlled bath. The bath contains 30 L of a liquid mixture of (50-50 mass %) water and ethylene glycol. A motor driven mechanism is used for stirring the bath contents. The temperature of the glycol mixture is controlled by an external refrigeratodheater (Forma Scientific model 2095, Caltec Scientific, Richmond, BC) with a capacity of 28.5 L. The refrigerator uses a 50-50 mass % solution of ethylene glycol and water. THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 72, OCTOBER, 1994 887