Use of PC-SAFT for Global Phase Diagrams in Binary Mixtures Relevant to Natural Gases. 3. Alkane + Non-Hydrocarbons Santiago Aparicio-Martı ´nez* and Kenneth R. Hall Artie McFerrin Department of Chemical Engineering, Texas A&M UniVersity, College Station, Texas 77843-3122 This work continues the systematic study of global phase diagrams in binary systems relevant for the description of natural gases using the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EOS). Reservoirs frequently contain non-hydrocarbon compounds such as CO 2 ,N 2 , or H 2 S that have strong effects upon the thermodynamic properties of natural gases. Thus, we study the behavior of their binary mixtures with n-alkanes over wide pressure and temperature ranges to get a deeper insight into the intermolecular pair interactions reflected in the phase behavior of the mixtures. These systems show complex patterns within the van Konynenburg classification, and the theoretical model accurately reproduces them. In the case of H 2 S containing mixtures (H 2 S is a compound that has autoassociation hydrogen-bonding ability), we analyze several association models to obtain the most reliable approach by comparing the predicted association degree of H 2 S to available experimental data. A conclusion from this series of papers is that PC-SAFT is a reliable and accurate model to predict the behavior of the different types of binary mixtures involved in complex multicomponent natural gases, with the ability to describe the different patterns of fluid- phase behavior presented by the studied systems over wide pressure and temperature ranges. Introduction Natural gases are mainly hydrocarbon molecules, but they also contain some light non-hydrocarbon compounds in sig- nificant proportions. 1,2 The presence of these non-hydrocarbon molecules increases the structural complexity of natural gas multicomponent mixtures, because their sizes and shapes are very different from the hydrocarbons and because they give rise to stronger intermolecular forces in the mixtures because of their polar character or hydrogen bonding. This complexity appears in the strong effect of these compounds upon the phase behavior of natural gases; 3 thus, the knowledge and systematic analysis of the phase-equilibria patterns of the binary systems n-alkane + non-hydrocarbon is a valuable tool to analyze the character- istics of molecular-level binary structural effects, which are useful for understanding the properties of the more-complex multicomponent mixtures, and to test the abilities of theoretical models to reproduce the intermolecular pair interactions in the binary systems reflected by the phase behavior of the mixtures. Although the composition of natural gases is a function of the reservoir from which they come, 1 the most frequently found non-hydrocarbon molecules in natural gas reservoirs are CO 2 , N 2 , or H 2 S. 2 N 2 and CO 2 systems are important, even though commercial natural gases must contain small quantities of these compounds. A significant proportion of natural gas reservoirs contain high levels of nitrogen, which decreases the quality of the gas and must be reduced before entering pipeline transporta- tion systems. This is frequently an expensive operation that can cause uneconomic scenarios for the development of new fields. Accurate knowledge of phase equilibria may help in the development of more-efficient removal procedures. It is also important to remark that N 2 , like methane, is always supercritical under reservoir conditions. 2,3 Thus, its behavior must be described accurately by any model to give good predictions for the multicomponent natural gas mixtures. Carbon dioxide binary systems are interesting both from academic and industrial viewpoints. 4 CO 2 has no permanent dipole moment, but its large quadrupole 5 moment gives rise to complex interactions with hydrocarbons that are difficult to predict theoretically and that result in complex phase behaviors, with the appearance of azeotropy in some systems such as ethane + CO 2 . Use of CO 2 rich natural gas reservoirs has increased recently because of the rising demand for natural gas, and knowledge of their phase behavior is necessary for adequate development. Hydrogen sulfide is generally an undesirable component of natural gases, but it may be present up to relatively high percentages, making the gas sour. When present, H 2 S not only can affect the economic proportion of hydrocarbon gas in the reservoir, but it is highly toxic and corrosive for production equipment and for the environment. Thus, it should be elimi- nated or reduced before most uses by means of suitable technologies. As with CO 2 , the need for greater natural gas production has increased the production of acid reservoirs, and removal of hydrogen sulfide has become increasingly important. The procedures for gas sweetening require accurate knowledge of complex mixtures phase behavior, 6,7 which is affected by the hydrogen-bonding ability of H 2 S. Mixtures such as n-alkane + CO 2 , +N 2 , or +H 2 S considered in this work are difficult tests for theoretical models because the complex intermolecular effects 8 rising from the quadrupolar character of the molecules or from the existence of autoasso- ciative hydrogen bonding, together with size and shape effects, cause complex phase-equilibria patterns. In this work, we apply the perturbed-chain statistical associating fluid theory (PC- SAFT) 9 equation of state (EOS) to predict phase equilibria for mixtures over wide temperature and pressure ranges and to test the model capabilities. Although the most frequently used EOSs for design purposes are cubic, 10 it is obvious that these EOSs are not reliable for prediction of phase equilibria in complex systems 11 such as the ones studied in this work. PC-SAFT, which has stronger molecular foundations and contains associa- tive terms in the EOS, should produce more accurate predictions * Corresponding author. Permanent address: Department of Chem- istry, University of Burgos, 09001 Burgos, Spain. Phone: +34 947 258 062. Fax: +34 947 258 831. E-mail: sapar@ubu.es. 291 Ind. Eng. Chem. Res. 2007, 46, 291-296 10.1021/ie060711r CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2006