Experimental Measurement and Thermodynamic Modeling of Water Content in Methane and Ethane Systems Amir H. Mohammadi, † Antonin Chapoy, ‡ Dominique Richon, ‡ and Bahman Tohidi* ,† Centre for Gas Hydrate Research, Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, U.K., and Centre d’Energe ´ tique, Ecole Nationale Supe ´ rieure des Mines de Paris, CENERG/TEP, 35 Rue Saint Honore ´ , 77305 Fontainebleau, France In this article, we first report a summary of experimental methods used for measuring water content and water dew point of gaseous systems. After reviewing the available water content data in the literature, new experimental data and thermodynamic modeling on the amount of water in methane and ethane systems are reported. Equilibrium measurements are conducted at 282.98-313.12 K and 282.93-293.10 K and pressures up to 2.846 and 2.99 MPa, respectively. A static-analytic apparatus has been used in the experimental measurements, taking advantage of a pneumatic capillary sampler in combination with an exponential dilutor. The Valderrama modification of Patel-Teja equation of state with the nondensity dependent mixing rules are used for modeling the fluid phases with the previously reported binary interaction parameters. The hydrate phase is modeled by the solid solution theory of van der Waals and Platteeuw, using the previously reported Kihara potential parameters. The fugacity of ice is calculated by correcting the saturation fugacity of water at the same temperature by using the Poynting correction. The experimental data generated in this work were compared with predictions of the thermodynamic model as well as other predictive methods. The predictions were in good agreement with the experimental data, demonstrating the reliability of experimental techniques and thermodynamic modeling used in this work. 1. Introduction Natural gases are generally saturated with water at reservoir conditions. During production, transportation, and processing, some of the dissolved water in the vapor phase may condense. The condensed water may con- tribute to gas hydrates and/or ice formation under specific temperature and pressure conditions. This phenomenon can arise during transportation in pipe- lines with large temperature gradients. Forming a condensed water phase may lead to cor- rosion and/or two-phase flow problems. The formation of gas hydrates and/or ice could result in pipelines blockage and shutdown. To avoid these problems, ac- curate knowledge of water-hydrocarbon phase behavior is of great interest to the petroleum industry. On the other hand, estimating water content is crucial in the design and operation of natural gas facilities. However, most of experimental data on water content for hydro- carbons and non-hydrocarbon gases (e.g., nitrogen, carbon dioxide, and hydrogen sulfide) at low tempera- ture conditions are scarce and often rather dispersed. It seems that achieving equilibrium at low temperature conditions, especially near and inside hydrate forming conditions, is a very slow process and requires a long time. At rather low temperatures and high pressures conditions, the water content of a gas is indeed very low. It is well-known that the determination of water traces in gases is one of the most difficult problems of trace analyses, and their accurate measurements require very specialized techniques. To give a qualified estimate of the amount of water in the gas phase, predictive methods are required. General methods of calculation include the use of the following: (1) empirical or semiempirical equations and plots of water content versus pressure and tempera- ture and corrections for the presence of acid gases such as hydrogen sulfide and carbon dioxide or heavy hydrocarbons and salts (e.g. Ideal model, Ideal model + Poynting correction, Bukacek correlation, 1 Sharma-Campbell method, 2 Robinson et al. chart, 3 Maddox et al. correlation, 4 Wichert-Wichert correla- tion, 5 McKetta-Wehe chart, 6 and Ning et al. correla- tion 7 ) and (2) thermodynamic models which are based on equality of chemical potential of various components in different phases. The aim of this work is to study phase equilibria in water-methane and water-ethane systems by generat- ing new experimental data as well as extending a thermodynamic model to low-temperature conditions. For this purpose a review is made on popular methods for measuring water content/water dew point of gases. Then the water content data for natural gas main components are gathered from the literature. An ap- paratus based on a static-analytic method combined with a dilutor apparatus to calibrate the gas chromato- graph (GC) detectors with water is used to measure the water content of methane and ethane. A thermodynamic model based on the Valderrama modification of the Patel and Teja equation of state (VPT-EoS) 8 with the nondensity dependent (NDD) mixing rules 9 is used for predicting phase equilibrium. In this model, the fugacity of ice is calculated by correcting the saturation fugacity of water at the same temperature by an exponential factor (the Poynting correction). The hydrate phase is modeled by the solid * To whom correspondence should be addressed. Tel.: +44 (0)131 451 3672. Fax: +44 (0)131 451 3127. E-mail: Bahman.Tohidi@pet.hw.ac.uk. † Heriot-Watt University. ‡ Ecole Nationale Supe ´rieure des Mines de Paris. 7148 Ind. Eng. Chem. Res. 2004, 43, 7148-7162 10.1021/ie049843f CCC: $27.50 © 2004 American Chemical Society Published on Web 09/30/2004