260 J. zyxwvutsrq Chem. Eng. Data 1993,38, zyxwv 250-253 Incipient Equilibrium Data for Propane Hydrate Formation in Aqueous Solutions of NaCl, KCl, and CaC12 Peter Englezos' and Yee Tak Ngan Department of Chemical Engineering, The University of British Columbia, Vancouver, British Columbia, V6T 124 Canada Incipient equilibrium experimental data for propane hydrate formation in pure water and aqueous single, binary, and ternary solutions of NaC1, KC1, and CaClz were obtained. Forty experiments were performed in the temperature range of 261.9-278.3 K using a new apparatus which was constructed in our laboratory. The apparatus was found to provide measurementswhich are reproducibleand consistentwith values reported in the literature. The results were compared with the predictions from a hydrate equilibrium calculation method. The agreement between the data and the predictions was found to be good. Introduction Gas hydrates are nonstoichiometriccrystalline compounds. They are formedby water moleculeswhich are linkedtogether with hydrogen bonds and form a three-dimensionalstructure (lattice) with cavities. The cavitiescan be occupied by certain molecules of gases and volatile liquids. One cavity can accommodate only one molecule. These molecules should not interfere with the hydrogen bonds among the water molecules, and they should have molecular diameters which are smaller than the diameter of the cavity. The structure is by itself thermodynamically unstable (empty hydrate lattice). However, inclusion of molecules with the charac- teristicsdescribed above at suitable pressure and temperature conditionscreatesthe stable hydrate crystal lattice. The most recent comprehensive description of the properties, techno- logicalsignificance, and implicationsof gas hydrates has been presented by Sloan (I). Gas hydrate phase equilibriumdata and predictive methods are needed for the rational design of the facilities that deal with hydrates. Electrolytes are able to suppressthe formation conditions of gas hydrates. The corrosive action of the aqueous electrolyte solutions,however, prevented their wide use as inhibiting agents. In spite of their corrosivepotential, electrolytesare amongthe constituentsof drilling muds. Other reasons for obtaining the experimental data and developing predictive methods for hydrate equilibria in the presence of electrolytes are possible development of water desalination and underground gas storage facilities and occurrence of natural gas hydrate reservoirs. Knox et al. zyxwvutsrq (2) and Kubota et al. (3) studied propanehydrate formation in the presence of NaCl in order to develop a seawaterdesalination process via gashydrate formation. Sloan (I) has the complete collection of experimental hydrate formation data, including those on the inhibiting effect of electrolytes. It should be noted that all these studies were concerned with single electrolyte solutions. Englezos and Bishnoi (4) and Dholabhai et al. (5) were the first to report equilibrium data for ethane and methane hydrate formation in the presence of mixed electrolytes. These data were found to be in very good agreement with the predicted values from a method that was presented earlier (6). The objective of the present work is to report incipient equilibrium data for propane hydrate formation in the * To whom correspondence should be addressed. 0021-956819311738-0250$04.00/0 presence of single and mixed electrolytes and compare the results with the above method. The data were collected in a new apparatus that is also presented. It is shown that the experimentalhydrate formation data and the predictedvalues agree very well. Experimental Setup Apparatus. A schematic diagram of the experimental apparatus is displayed in Figure 1. The vital part of the apparatus is the equilibrium cell. The cell is immersed into a temperature controlled bath. The temperature bath holds 118 L of a liquid mixture of (5050, mass % ) water and ethylene glycol to keep a constant temperature within the system. The temperature of the glycol mixture is controlledby an external refrigeratodheater; it uses a heating/cooling coil to transfer heat in and out of the temperature bath. The apparatus uses a Forma Scientific refrigerator (model 2095) with a capacity of 28.5 L. The refrigerator utilizes another solution of (5050, mass %) of ethylene glycol and water as a heater/coolant; this solution is circulated in an enclosed loop, and heat is exchanged through the heating/coolingcoil. Copper tubing was used in the construction of the coil. A motor-driven stirring mechanism is used to maintain a uniform temperature in the glycol-water mixture. A relatively constant temper- ature (fO.10 K) can be achieved over a long period of time using this apparatus. Equilibrium Cell. The cell was machined from a solid 316 stainless steel cylindrical bar. A cross sectional view of the cell is illustrated in Figure 2. Two viewing windows are fitted ontothe front and back, while a third window is mounted at the top of the cell. The windows were machined from l/*-in.-thick Plexiglas plates. All three windows are circular, they are held in place by stainless steel bolted studs, and they are sealed with neoprene O-rings. Six bolts were used for the windows on the sides and eight for the window on the top of the cell. The cell was tested to a pressure of 1.0 MPa. By using a magnetic stir bar coupled with a set of magnets outside the equilibrium cell, good stirring was accomplished. The set of magnets is mounted on an aluminum housing connected to a dc motor. As the set of magnets spins around, the magneticforce couples the stir bar, causing it to spin and, hence, mixing the contents in the cell. The temperature insidethe cell is measuredwith twoOmega copperconstantan thermocouples. One thermocouple is situated at the top half of the chamber for the gas phase, while another thermocouple is positioned near the bottom of 0 1993 American Chemical Society