Research Note pubs.acs.org/IECR
Phase Equilibria of Propane and Ethane Simple Hydrates in the Presence of Methanol or Ethylene Glycol Aqueous Solutions Amir H. Mohammadi* and Dominique Richon MINES ParisTech, CEP-TEP, Centre Énergétique et Procèdes, 35 Rue Saint Honoré, 77305 Fontainebleau, France ABSTRACT: We report dissociation pressures of propane and ethane simple hydrates in the presence of methanol or ethylene glycol aqueous solutions at different temperatures and various concentrations of inhibitor in aqueous solutions. The equilibrium data were generated using an isochoric pressure-search method. These data are compared with some selected experimental data from the literature on the dissociation conditions of propane and ethane simple hydrates in the presence of pure water to show the inhibition effects of the above-mentioned aqueous solutions. Comparisons are finally made between our experimental data and the corresponding literature data. Table 1. Purities and Suppliers of Chemicalsa
1. INTRODUCTION Gas hydrates, or clathrate hydrates, are icelike crystalline compounds formed through combination of water and small molecule(s), such as methane, ethane, propane, etc., under lowtemperatures and elevated pressures.1 In gas hydrates, small molecule(s) is (are) trapped inside the cavities formed by water molecules.1 Formation of gas hydrates can cause pipeline blockage and other operational problems in the petroleum industry.1 To inhibit the formation of gas hydrates, methanol or ethylene glycol aqueous solutions are normally used.1−7 Therefore, knowledge of gas hydrate phase equilibrium in the presence of the latter aqueous solutions is of interest to estimate boundary of gas hydrate formation.1−7 A preliminary study shows that most of experimental measurements have been reported for lowconcentration methanol or ethylene glycol aqueous solutions, while the hydrate dissociation data in the presence of highconcentration methanol or ethylene glycol aqueous solutions are limited.1−7 In this work, we report the dissociation conditions of propane and ethane simple hydrates in the presence of methanol or ethylene glycol aqueous solutions at different temperatures and various concentrations of inhibitor in aqueous solutions. The equilibrium data were measured using an isochoric pressure-search method.8 The experimental data reported in this work are compared with some selected experimental data from the literature on dissociation conditions of propane and ethane simple hydrates in the presence of pure water9−13 to show the inhibition effects of the above-mentioned aqueous solutions. A discussion is finally made on the reliability of the literature data.2−7
a
supplier
purity
ethane propane ethylene glycol methanol
Messer Griesheim Messer Griesheim Aldrich Aldrich
99.995 (mol %) 99.995 (mol %) 99.0 (GC,%) 99.9 (GC,%)
Deionized (DI) water was used in all experiments.
up to 60 MPa. The equilibrium cell has an inner volume of 201.5 cm3 and two sapphire windows. A motor-driven turbine agitation system (Top Industrie) ensures sufficient agitation to facilitate reaching equilibrium even during the hydrate formation. Temperature of equilibrium cell is controlled using a thermostatic bath (Tamson Instruments, Model TV400LT), which allows the visual observation of the cell content throughout the experiments. One platinum resistance sensor (Pt100) inserted in the cell interior is used to in situ measure the temperature within ±0.02 K uncertainty estimated after calibration against a 25-Ω reference platinum resistance thermometer. This 25-Ω reference probe was calibrated, following the ITS 90 protocol, by Laboratoire National d’essais (Paris). Pressure is measured using a pressure transducer (Druck, type PTX611) for pressures up to 6 MPa. As a result of calibration against a dead weight balance (Desgranges & Huot 5202S CP, Aubervilliers, France), the pressure measurement uncertainty is estimated to be within ±0.002 MPa. 2.3. Experimental Method. The hydrate dissociation points were measured with isochoric pressure-search procedure.8,17 The vessel containing the aqueous solution (10% by volume of the vessel was filled with the aqueous solution) was immersed into the temperature controlled bath, and the gas was supplied from a cylinder through a pressure regulating valve into the vessel. Note that the vessel was evacuated (down to 0.8 kPa for at least 2 h) before introducing any aqueous solution and gas. After attaining temperature and pressure
2. EXPERIMENTAL SECTION 2.1. Chemicals. Table 1 reports the purities and suppliers of materials used in this work. Aqueous solutions were prepared following the gravimetric method, using an accurate analytical balance. Consequently, uncertainties on the basis of mole fraction are estimated to be
