Hydrate phase equilibria of multi-guest systems containing carbon dioxide and methane

Abstract

Clathrate hydrates are crystalline compounds formed by the physically stable interaction between water and relatively small guest molecules. Under suitable conditions of pressure and temperature, water molecules, via hydrogen bonding, form into polyhedral cavities that stack to space-filling frameworks. Because of partial cage filling, these crystalline compounds are non-stoichiometric. They can be divided into three distinct structural families I (sI), II (sII), and H (sH), which differ in the combination of cavities of different sizes and shapes. Since each volume of hydrate was known to contain as much as 184 volume of gas, it has been suggested the potential application of crystal hydrate nature to large-scale storage of greenhouse gases and removal of organic pollutants from aqueous solutions. The thermodynamic phase equilibria and structural characteristics of clathrate hydrate provide the fundamental variables contributed to develop the industrial processes. In this study, the phase equilibrium conditions of pure and mixed hydrates were determined to identify the thermodynamic stability region in hydrate structure. These data can be used to separate $CO_2$ from the recovered $CH_4$ hydrates in the feasibility natural gas exploitation in connection with $CO_2$ sequestration. In addition Tetrahydrofuran (THF) which was used as the promoter in mixed $CH_4$ and $CO_2$ hydrate system decreased the pressure and increased the temperature, and so the economic advantages could be got in process. To examine the feasibility of natural gas exploitation in connection with $CO_2$ sequestration the complex phase behavior of gas hydrate systems in porous media was measured and compared with the thermodynamic modeling. Three-phase H-Lw-V equilibria for the binary $CH_4$ + water, $CO_2$ + water mixtures in silica gel pores with nominal diameters of 6.0, 15.0 and 30.0 nm were measured and compared with the pore hydrate model calculations. In addition, three-phase H-Lw-...