Published: May 20, 2011 r2011 American Chemical Society 2971 dx.doi.org/10.1021/je200261a | J. Chem. Eng. Data 2011, 56, 2971–2975 ARTICLE pubs.acs.org/jced Measurement and Modeling of CO 2 Frost Points in the CO 2 ÀMethane Systems Longman Zhang, † Rod Burgass, ‡ Antonin Chapoy, ‡, * Bahman Tohidi, ‡ and Even Solbraa § † Norwegian University of Science and Technology, Department of Energy and Process Engineering, NO-7491 Trondheim, Norway ‡ Centre for Gas Hydrate Research, Institute of Petroleum Engineering Heriot-Watt University, Edinburgh, EH14 4AS United Kingdom § Statoil, Research and Development NO-7005 Trondheim, Norway ABSTRACT: Technology is being developed to separate carbon dioxide (CO 2 ) from natural gas by frosting CO 2 out from the mixture. VaporÀsolid phase equilibrium data in the CO 2 Àmethane systems are important in developing such processes. In this work, new experimental data are reported for the frost points in the CO 2 Àmethane systems for a wide range of CO 2 range concentration (i.e., CO 2 mole fraction 0.108 to 0.542). The SoaveÀRedlichÀKwong (SRK) equation of state (EoS) is employed to calculate the fugacity of the fluid phase. The CO 2 solid-forming conditions are modeled by a solid fugacity model based on the sublimation pressure of pure CO 2 . The thermodynamic model was used to predict the CO 2 frost points in the presence of methane. Predictions of the developed model are validated against independent experimental data and the data generated in this work. A good agreement between predictions and experimental data is observed, supporting the reliability of the developed model. ’ INTRODUCTION Removal of CO 2 from high carbon dioxide (CO 2 ) content natural gas fields is important to the gas industry development in some countries. For example, in Malaysia alone, over 13 Tscf of hydro- carbon gas remains undeveloped in high CO 2 content fields, where the CO 2 mole fraction is even higher than 0.70 in some gas fields. 1 One challenge in developing such gas fields is the economical separation of CO 2 from the feed gas. A technique has been suggested based on frosting CO 2 at low temperature and separating the CO 2 solid from the natural gas; hence, technologies are being developed to efficiently separate the CO 2 especially in high CO 2 content feed gases. 2,3 Therefore understanding of the vaporÀsolid equilibrium at low temperature is critical in designing such separation processes. Existing experimental data on CO 2 frost from CO 2 Àmethane and other gas mixtures are scarce and normally focused on low CO 2 content systems. Pikaar 4 measured the frost points in CO 2 Àmethane systems for the (0.01 to 0.20) CO 2 mole fraction concentration range. Agrawal 5 measured the frost points in the CO 2 ÀN 2 Àmethane system for the (0.0012 to 0.1067) CO 2 mole fraction concentration range. More recently, Le 6 measured the frost points in CO 2 Àmethane, CO 2 ÀmethaneÀN 2 , and CO 2 ÀmethaneÀethane systems for the (0.01 to 0.0293) CO 2 mole fraction concentration range. In this work, the frost points have been measured in the CO 2 Àmethane systems for the CO 2 content of 0.108, 0.178, 0.334, 0.424, and 0.542 mol fraction, these data are important for evaluating thermodynamic models for process simulation. A thermodynamic model using the well-proven SRK equation of state 7 has been employed to model the phase equilibria. The thermodynamic model is based on uniformity of fugacity of each component throughout all the phases. The CO 2 -solid phase is modeled by a solid fugacity model based on the sublimation pressure of pure CO 2 . Experimental data both from this work and literature have been compared to the modeling work and good agreement between experimental data and predictions is observed. ’ EXPERIMENTAL SECTION The frost temperature is measured based on detecting CO 2 solid melting point during heating of a CO 2 Àmethane solidÀ vapor mixture in a constant volume equilibrium cell. Materials. Ultra high pure grade methane (99.995 % pure) and CO 2 (99.99 % pure) supplied by BOC were used. Each synthetic mixture was made up by gravimetric means using the above pure components. The gas composition was checked using gas chro- matography (GC). The GC (VARIAN model CP-3800) is equipped with two detectors in series, a thermal conductivity detector (TCD) and a flame ionization detector (FID). The TCD was used to detect CO 2 . It was repeatedly calibrated by introdu- cing known amounts of CO 2 through a gas syringe in the injector of the gas chromatograph. The CO 2 calibration uncertainty is estimated to be within ( 0.8 %. The FID was used to detect methane and the same calibration procedure was used. The methane calibration uncertainty is estimated to be within ( 0.7 %. Figure 1. Schematic of the experimental apparatus. Received: March 24, 2011 Accepted: May 12, 2011