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

The mechanism of CH4/N2 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, CH4 molecules preferentially enter the small hydrate cages and then N2 molecules compete to fill the remaining empty cages. The presence of SDS in the THF solutions accelerates the incorporation of CH4 and N2 into the hydrates. A concentration of 500 ppm of SDS was found to be favorable for enhancing the cage occupancy of CH4 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 CH4 and N2 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 CH4 molecules. To achieve a high CH4 separation efficiency, it is necessary to monitor the ratio of integral Raman peak areas in addition to acquiring gas consumption, the CH4 recovery rate, and the separation factor during the hydrate formation process.