Jitendra Sangwai

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 assumed 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. Thus, 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.

Thermodynamic hydrate inhibitors (THIs) are water-loving additives that in an aqueous phase preferentially interact with water and inhibit hydrate nucleation at given temperature and pressure conditions. Hydrate inhibition is one of the flow assurance challenges in oil and gas facilities, primarily for the production and transmission pipelines. In this era of a shift towards offshore production of oil and gas that is usually accompanied by large water cuts in production pipelines, the use of THIs requires a huge onboard inventory (of these additives); further, associated higher recovery costs and environmental issues has resulted in the identification and use of low dosage hydrate inhibitors (LDHIs). Thus, as the name suggests they are used in lower concentrations and do not present any space/storage problem on the offshore platforms. However, LDHI works differently, it primarily inhibits the growth and agglomeration of hydrate crystals thus ensuring flow assurance for a given period of time. Hence, an effective LDHI, may or may not delay hydrate nucleation, however, it should certainly reduce the kinetics of hydrate growth and should act as an anti-agglomerant. This work comparatively investigates the effect of three such additives on methane gas hydrate formation kinetics at very low concentrations. Sodium Dodecyl Sulphonate (SDS) is a well-studied molecule for gas hydrate formation kinetics, while SDS is a good anti-agglomerant (and has been used as anti-agglomerant in multiple gas hydrate studies), it is also a well-known hydrate promoter. In this work, additives having some similar molecular fragments to that of SDS were tested for better gas hydrate inhibition insights at molecular level. Sodium Benzene Sulphonate (SBS), Sodium Dodecyl Benzene Sulphonate (SDBS) and SDS was screened for methane gas hydrate formation experiments. A comparative kinetic study with these additives at two different concentrations (0.05 M and 0.1 M) has been presented for methane gas hydrate formation at 5 MPa (at the beginning of the experiments) and 274.15 K in n-heptane with 50% water cut, in a stirred tank isothermal reactor. The presence of a benzene ring in the molecular structure of the additive along with varying lipophilic tail and hydrophilic head on the methane gas hydrate formation kinetics are thoroughly investigated.