Equilibrium Hydrate Formation Data for Carbon Dioxide in Aqueous Glycerol Solutions Ed Breland and Peter Englezos* Department of Chemical Engineering, The University of British Columbia, Vancouver, British Columbia, V6T 1Z4, Canada Equilibrium conditions for carbon dioxide hydrate in pure water and aqueous glycerol solutions were measured in order to evaluate the effectiveness of glycerol as an inhibiting agent. The isothermal pressure search method was employed. Twenty-two experiments were conducted in the temperature range of 270.1-280.2 K using glycerol concentrations of 0, 10, 20, and 30 mass % in water. Glycerol was shown to have a considerable inhibiting effect on carbon dioxide hydrate formation though it was not as effective as an inhibiting agent as sodium chloride or methanol. Introduction Carbon dioxide hydrate is a solid solution of water and carbon dioxide. Carbon dioxide hydrate is a clathrate compound also known as gas hydrate. Clathrates are inclusion compounds which are formed as a result of physical interactions between two molecular species. One species (host) forms a lattice which physically traps the other species (guest). No chemical bonds are formed between the host molecules and the guest molecules. The carbon dioxide + water system will form clathrate or gas hydrate at certain temperature and pressure conditions. The term hydrate refers to the presence of water as the host species. The ability of water to form clathrate compounds is due to the hydrogen bonding of water molecules. There are more than 100 chemicals which can form clathrate hydrates. Information on the structure, physical and thermodynamic properties, and technological significance of gas hydrates is available in several publica- tions (Sloan, 1990; Englezos, 1993; Sloan et al., 1994). It has been known since the 1930s that natural gas and water can form gas hydrate at temperatures above the normal freezing point of water and thus plug a pipeline that is used for gas transport (Hammerschmidt, 1934). Because the formation of hydrates has severe economic consequences, prevention of formation is therefore a major concern. One of the techniques to prevent hydrate forma- tion is to alter the partial phase diagram of the gas-water system in the hydrating region by injecting methanol, glycol, or electrolytes (inhibiting substances). Methanol and glycol injections are used extensively in gas and oil systems. Incipient equilibrium data and predictive methods for gas hydrates are needed for the design of the facilities that deal with mixtures capable of forming hydrates. Data are also needed for the development and testing of predictive methods for hydrate equilibria. Incipient equilibrium data for carbon dioxide hydrate in aqueous glycerol solutions are needed for the design of facilities that use carbon dioxide in miscible flooding (Blytas, 1987). The CO 2 used in enhanced oil recovery is produced from underground formations in which it occurs saturated with water. This water must be removed in order to avoid formation of carbon dioxide hydrate in the pipeline that carries it from the point of production to the points of injection. Hydrate formation must also be inhibited at the CO 2 production site from the point of production to the point of dehydration. Prevention of hydrate formation can be accomplished by using ethylene glycol, glycerol, diethyl glycol, or triethyl glycol. It is noted that when diethyl or triethyl glycol is used in the drying of carbon dioxide high losses are encountered due to the solubilization in the compressed CO 2 . Thus, the use of glycerol is advantageous because it leads to lower solubility losses. Glycerol is also used in water-based drilling fluid formulations (Bland, 1991) and is considered an environmentally less hazardous compound than methanol and glycol at offshore platforms (Ng and Robinson, 1994). Ng and Robinson (1994) reported three measurements of hydrate-vapor CO 2 -liquid water equi- librium and two measurements of hydrate-liquid CO 2 - liquid water equilibrium. The concentration of glycerol was 25 mass %. The objective of this work is to report additional incipient equilibrium data for carbon dioxide hydrate in aqueous glycerol solutions. Solutions with glycerol concentrations of 10, 20, and 30 mass % were studied. Apparatus A schematic of the apparatus to be used in this experi- ment appears in Figure 1. The apparatus is described in detail by Englezos and Ngan (1994). A high-pressure cell is immersed in a temperature-controlled bath. The cell was constructed from a 316 stainless steel rod. Two circular viewing windows were fitted onto the front and back. The top of the cell can be removed and is held in place by six stainless steel bolts. The top is sealed with a neoprene O-ring. The temperature control bath contains 30 L of a solution consisting of approximately 50/50 (mass %) water and ethylene glycol. A motor-driven mechanism is used to stir the contents of the bath. The temperature of the bath is controlled by an external refrigerator/heater (Forma Scientific Model 2095, Caltech Scientific, Richmond, British * To whom correspondence should be addressed. Figure 1. Schematic of the apparatus. 11 J. Chem. Eng. Data 1996, 41, 11-13 0021-9568/96/1741-0011$12.00/0 © 1996 American Chemical Society