ABSTRACT:

To describe the phase equilibria of gas hydrate by means of a macroscopic approach, hydrate phase equilibria for the CH4 + water, CO2 + water and CH4 + CO2 + water mixtures in silica gel pores were measured and compared with the calculated results based on van der Waals and Platteeuw model. At a specified temperature, three phase HLW-V equilibrium curves of pore hydrates were shifted to higher pressure region depending on pore sizes when compared with those of bulk hydrates. The activities of water in porous silica gels were expressed with a correction term to account for both capillary effect and activity decrease. In microscopic analysis of gas hydrates, the CH4 + CO2 and CO2 + N2 mixed gas hydrate were measured by spectroscopic methods such as NMR and XRD. With these measurements, we can analyze and obtain some critical information on gas hydrates such as hydration number and cage occupancy. Accordingly, it becomes important to interconnect various physicochemical properties determined from both macroscopic and microscopic methods.

INTRODUCTION

Gas hydrates are non-stoichiometric crystalline compounds formed when "guest" molecules of suitable size and shape are incorporated in the well-defined cages in the "host" lattice made up of hydrogenbonded water molecules. Gas hydrates are of particular interest in the petroleum industry as well as in energy and environmental field. Initial interest in gas hydrates began with the discovery that hydrate formation could plug natural gas pipelines. Large masses of natural gas hydrates exist both on-shore buried under the permafrost and off-shore buried under the oceanic and deep lake sediments. Since each volume of hydrate can contain as much as 170 volumes of gas at standard temperature and pressure conditions, naturally occurring gas hydrates in the earth containing mostly CH4 are regarded as future energy resources (Sloan, 1998).

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