rXXXX American Chemical Society A dx.doi.org/10.1021/ie101635k | Ind. Eng. Chem. Res. XXXX, XXX, 000–000 ARTICLE pubs.acs.org/IECR Gas Hydrate Formation in Carbon Dioxide þ Nitrogen þ Water System: Compositional Analysis of Equilibrium Phases Veronica Belandria, † Ali Eslamimanesh, † Amir H. Mohammadi,* ,†,‡ and Dominique Richon † † MINES ParisTech, CEP/TEP—Centre  Energ  etique et Proc  ed es, 35 Rue Saint Honor  e, 77305 Fontainebleau, France ‡ Thermodynamics Research Unit, School of Chemical Engineering, University of KwaZulu-Natal, Howard College Campus, 6 King George V Avenue, Durban 4041, South Africa ABSTRACT: In this Article, experimental data on compositions of equilibrium phases under gas hydrate formation conditions are reported for the carbon dioxide þ nitrogen þ water system in the (273.6 to 281.7) K temperature range for pressures up to 17.628 MPa. An experimental “in-house” setup based on the “static-analytic” technique was employed for performing the measurements. An isochoric pressure-search method in combination with gas phase compositional analysis was used to measure compositions of the gas phase in equilibrium with gas hydrate and liquid water. A material balance method was used to determine compositions of the hydrate and aqueous phases. A mathematical approach based on the Newton’s numerical method and the Differential Evolution optimization strategy was applied to solve the material balance equations. The compositional data generated in this work are compared with the corresponding experimental data reported in the literature as well as the predictions of the CSMGem thermodynamic model. 1. INTRODUCTION It is believed that concentration of carbon dioxide (CO 2 ) has significantly increased over the past decades in the earth’s atmo- sphere due to human contribution through fossil fuels consumption. 1 Because of its global warming potential, CO 2 capture and sequestration (CCS) has become an important area of research to mitigate CO 2 worldwide emissions. Since CO 2 separation is the most expensive step of the CCS 2,3 process, the challenge is to evaluate and develop energy efficient and envir- onmental friendly technologies to capture the CO 2 produced in large scale power-plants, where flue gas typically contains mostly nitrogen (N 2 ) and low to medium CO 2 -content: (3-20%) or (15-40%) on the mole basis, respectively. 3 One promising approach to separate CO 2 from combustion flue gas is through gas hydrate crystallization technology. 4-7 Gas hydrates are crystals formed by combination of water and gas molecules, such as CO 2 ,N 2 , methane (CH 4 ), hydrogen (H 2 ), etc., under suitable conditions of low temperatures and specified pressures. 8 The majority of gas hydrates are known to form three typical hydrate crystal structures: structure I (sI), structure II (sII), and structure H (sH). 8 The type of crystal structure generally depends on the size of the guest molecule. In particular, as simple hydrates, CO 2 and N 2 are known to form crystal structures I and II, respectively. 8 However, the mixed CO 2 /N 2 hydrate structure is considered to be either sI or sII depending on the relative ratio of these two different gas molecules occupied in the small and large cavities. 4 Based on the difference in chemical affinity between CO 2 and N 2 in the hydrate structure, when hydrate crystals are formed from a binary mixture of the aforementioned gases, the hydrate phase is enriched in CO 2 while the concentration of N 2 is increased in the gas phase at equilibrium. The hydrate phase is then dissociated by depressur- ization and/or heating, consequently, CO 2 can be recovered as a separated gas. Despite the potential use of gas hydrate technology to capture CO 2 from combustion flue gases, the hydrate phase equilibria of the ternary carbon dioxide, nitrogen, and water have scarcely been studied. To the best of our knowledge, the experimental data on the compositions of the gas, hydrate, and liquid (aqueous) phases under hydrate-liquid water-gas equilibrium reported in the literature are limited. A summary of these data is given in Table 1. This literature review indicates that generating more equilibrium data for the CO 2 þ N 2 clathrate hydrates, elucidating the phase equilibria under three phase conditions, and also investigating the reliability of the thermodynamic models predictions for the aforementioned clathrate hydrate system are still needed. In this work, new measured compositional data of three- phases consisting of hydrate, liquid water, and gas for the CO 2 þ N 2 þ water system at various concentrations of CO 2 in the feed gas in the (273.6 to 281.7) K temperature range for pressures up to about 17.628 MPa are reported. An isochoric pressure-search method 9-12 combined with capillary gas phase sampling and compositional analysis was used to determine the molar compo- sitions of N 2 and CO 2 in the gas phase. The molar compositions of the aqueous and hydrate phases were determined by solving a system of nonlinear material balance equations. For this purpose, a mathematical algorithm based on Newton’s numerical method 13 coupled with the differential evolution (DE) optimiza- tion strategy 14 was applied to solve the corresponding system of equations. The compositional data generated in this work are compared with the corresponding experimental data reported in the literature. We also report the predictions of CSMGem 8,15 thermodynamic model for the aforementioned system. Received: July 31, 2010 Accepted: January 14, 2011 Revised: December 9, 2010