Separation of coal mine methane gas mixture via sII and sH hydrate formation

Abstract

The release of coal mine methane (CMM) or coal bed methane (CBM) to the atmosphere leads to the wastage of a valuable energy resource and contributes to the greenhouse effect. In the present work, CMM has been considered as a mixture of methane and nitrogen (CH4-N2, 30:70 mol%), and this gas mixture has been separated by the hydrate-based gas separation (HBGS) process. Formation of sI hydrate with 70% N2 in the mixture requires significantly higher pressure and thus not suitable for scale-up. Therefore, in this work with a suitable thermodynamic promoter, sII and sH hydrates were formed to study the methane separation from CMM gas mixture at moderate temperature and pressure conditions. sII hydrate formation was carried out using polar THF (Tetrahydrofuran) and non-polar cyclopentane (CP) at different molar concentrations and at 274.2 K temperature. sH hydrate formation was facilitated using polar tert-butyl-methyl-ether (TBME) and non-polar neo-hexane (NH) at different molar concentrations and at 274.2 K temperature. Further, water-soluble hydrate promoter sodium dodecyl sulfate (SDS) was used at 500 ppm to enhance the rate of hydrate formation and thus to achieve better separation efficiency in a given time. For the CMM gas mixture, sII hydrate formation showed better methane recovery compared to sH hydrate formation, whereas sH hydrate formation showed a better separation factor compared to sII hydrate formation. Hydrate dissociation was also carried out to recover the hydrated gas via depressurization and thermal stimulation to compare the effect of polar and non-polar hydrate formers.