The thermodynamic characters of mixture gas (CO2/N2) hydrates were investigated with five kinds of N2 concentrations. Hydrate phase equilibrium conditions were obtained using isochoric method in a high pressure vessel. The experimental results indicated that the presence of N2 gas brings higher equilibrium pressure for mixture gas hydrates than pure CO2 hydrate at the same temperature. The equilibrium pressure increased with the increase of N2 concentration. An improved model was proposed to predict CO2/N2 hydrates phase equilibrium in porous medium, which is based on the traditional thermodynamic model of van der Waals & Platteeuw.
The amount of Carbon dioxide (CO2) in atmosphere has increased markedly with the tremendous consumptions of fossil fuels, and the "greenhouse effect" is influencing the daily life of human being. The disposal of CO2 in atmosphere has become an issue of worldwide concern, and the target of climate changes control cannot be obtained without Carbon dioxide capture and storage (CCS) in short order (Freund et al., 1997).
The first step of CCS is CO2 capture. There are lots of methods for CO2 capture from fossil fuel power plant (Yang et al., 2008). Gas hydrate technology is based on ice-like crystalline compounds, where gas molecules are enclathrated in cavities formed by water molecules. Hydrate-based CO2 capture as a promising option for fossil fuel power plant is identified as the most promising long-term CO2 capture technology by Department of Energy (USA) (Elewell et al., 2006). Wong et al. (2002) proposed that the cost of the developed hydratebased CO2 capture technology will cause a sharply decrease when comparing to that of the current chemical absorption technology. The basic principle of hydrate based CO2 capture for conventional coal-fired power plant is the phase equilibrium pressure differences for CO2 hydrate and N2 hydrate at the same temperature. Englezos et al., (1994), Breland et al., (1996); Dholabhai et al., (1996, 1997), Yang et al., (2000), Linga et al., (2007) have studied the CO2 hydrate phase equilibrium characters with the presence of polymer or electrolyte. The investigations of CO2 hydrate equilibrium in porous medium were carried out by Uchida et al., (1999) and Zatsepina et al., (2002), and they found that hydrate formation process was restricted by the transfer of CO2 from liquid water. Smith et al., (2002) measured CO2 hydrate equilibrium conditions in silica gel pores with nominal radii 7.5, 5.0, and 3.0 nm, and they calculated the pore size at hydrate equilibrium conditions. Kumar (2005) collected lots of experimental equilibrium conditions data for CO2 hydrate in porous medium and measured the permeability of the porous medium in the presence of hydrate. By the way, hydrate based gas purification is the further development for gas separation. In recently, the captured CO2 form fuel or flue gas usually contains more than 5 mol% gas impurities, such as N2, H2 or H2O.
