Raman Spectroscopy Study on the Competition between CH₄ and N₂ to Form Mixed Gas Hydrate

Abstract

The mechanism of CH 4 /N 2 mixed hydrate formation was investigated by using in situ Raman Spectroscopy. The experiments were carried out in tetrahydrofuran (THF) aqueous solutions in the presence of sodium dodecyl sulfate (SDS) at concentrations varying from 0 to 1000 ppm. This work provides information that is expected to be useful for the design of an improved process to recover methane from low-concentration coalbed methane (LCCBM). It was found that during the hydrate formation process, CH 4 molecules preferentially enter the small hydrate cages and then N 2 molecules compete to fill the remaining empty cages. The presence of SDS in the THF solutions accelerates the incorporation of CH 4 and N 2 into the hydrates. A concentration of 500 ppm of SDS was found to be favorable for enhancing the cage occupancy of CH 4 molecules. The ratio of integral Raman peak areas (I) obtained at this concentration is higher than that of the other SDS concentrations. The cage occupancy competition between CH 4 and N 2 molecules is also influenced by the temperature. The temperature of 282.15 K was found to be an optimal temperature to promote the cage occupancy of CH 4 molecules. To achieve a high CH 4 separation efficiency, it is necessary to monitor the ratio of integral Raman peak areas in addition to acquiring gas consumption, the CH 4 recovery rate, and the separation factor during the hydrate formation process.