Jitendra Sangwai

A fundamental study on hydrate formation from an equimolar CO2-CH4 gas mixture has been carried out with two focal points: accelerating the kinetics of hydrate formation and enhancing the gas separation efficiency of the process. To this effect, the impact of inducing different hydrate structures from the same gas mixture by introducing suitable additives into the system has been investigated, and experiments are being carried out in a horizontal packed bed reactor at two different initial pressures, 3.5 and 5.0 MPa, to study the effect of driving forces on the kinetics of hydrate formation and the separation efficiency of the process. sH hydrate former cyclooctane (Cyclo-O) induces rapid nucleation of hydrate and also yields significant gas uptake in hydrates, 29.55% higher compared to the water system. This may be attributed to the simultaneous formation of sH and sI hydrates when Cyclo-O is present in the system. It was observed that the environmentally benign hydrophobic amino acid tryptophan in low concentration (1 wt %) can effectively accelerate the kinetics of hydrate formation, with 90% water to hydrate conversion being obtained within the first 30 min of hydrate formation. Further, the use of Cyclo-O and tryptophan together shows a synergistic effect, resulting in the highest gas uptake among all the systems studied. Although the problem of slow kinetics of hydrate formation from CO2-CH4 gas mixtures has been satisfactorily solved through this work, there are still significant strides that need to be made toward improving the separation efficiency of the process. The formation of the mixed hydrate is unable to return a satisfactorily high efficiency for gas separation.

The present study investigates the effect of l-tryptophan on the kinetics of CO2-CH4 (50:50 mol %) hydrate formation in a gas-water system by employing a stirred tank reactor. The concentration of tryptophan was varied in the range from 0.01 to 1 wt %, and its impact on the kinetics of CO2-CH4 (50:50 mol %) hydrate formation was investigated at 5.0 MPa and 274.15 K. The time required for 90% completion of hydrate formation was reduced from 100.89 ± 11.03 min to 19.11 ± 1.68 min when the tryptophan concentration increased from 0.01 wt % to 1 wt %. The gas uptake achieved using 1 wt % tryptophan (0.1045 ± 0.0023 mol per mol) was approximately 5 times higher than that of 0.01 wt % tryptophan (0.0227 ± 0.0017 mol per mol). Thus, the addition of tryptophan resulted in rapid and extensive hydrate formation. The hydrate formation kinetics were assessed based on gas uptake of hydrate growth, t90 time, and rate of gas uptake of hydrate growth, and 1 wt % tryptophan concentration was found to be the optimum concentration in the gas-water system. Further, experiments were performed to understand the kinetics of CO2-CH4 (50:50 mol %) hydrate formation by inducing sH hydrate using 2.86 mol % cyclooctane (Cyclo-O), a thermodynamic promoter in a gas-liquid hydrocarbon (LHC)-water system at 5.0 MPa and 274.15 K. After that, the synergistic effect using Cyclo-O with 0.1 and 1 wt % tryptophan separately was evaluated. Application of 1 wt % tryptophan helps in crossing the resistance of immiscible Cyclo-O phase, resulting in higher gas uptake of hydrate growth compared to 0.1 wt % tryptophan with Cyclo-O. Visual macroscopic morphology observations coupled with kinetic experiments were also performed for gas-water and gas-LHC-water systems. Visual observations showed that the hydrate could form and grow below the gas-liquid interface for experiments conducted with (i) pure water, (ii) water and 0.01 wt % tryptophan, and (iii) water and 2.86 mol % Cyclo-O, whereas the experiments with tryptophan concentration > 0.01 wt % in the gas-water system and with tryptophan concentration ≥ 0.1 wt % with Cyclo-O in gas-LHC-water resulted in hydrate formation above the gas-liquid interface. Thus, the capability of tryptophan in forming hydrates above the interface was attributed to different hydrate morphology; an altered morphology resulted in a capillary effect through which the mass transfer of hydrate-forming components improved.

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.

This study investigates the kinetics and morphology of an equimolar CO2-CH4 gas mixture by inducing sI and sH hydrates in gas-water and gas-liquid hydrocarbon (LHC)-water systems using pure water and sH hydrate former cyclooctane (Cyclo-O) at 274 K and 5.0 MPa in a quiescent system. Further, the effect of promoter l-tryptophan in enhancing the kinetics is evaluated for both systems. The visual morphology observations provide mechanistic insights into the hydrate crystal nucleation and growth kinetics in the presence of pure water, 1 wt % tryptophan, 2.86 mol % Cyclo-O, and 1 wt % tryptophan with 2.86 mol % Cyclo-O. Distinct variations in the kinetics and morphology of hydrate crystal growth in aqueous bulk solution depend on the type of additive used. Swordlike elongated polygons were observed in the pure water system at the gas-liquid interface. As time progressed, the smooth polygonal shape was observed in the gas phase, and the evolution of swordlike to larger polygons was observed at the gas-liquid interface. The addition of 1 wt % tryptophan to the gas-liquid system led to significant gas uptake and rapid hydrate formation rate compared to the pure water system and that was evident by morphology study as well. Whereas, in the case of the gas-LHC-water system, the presence of water-insoluble Cyclo-O provides resistance to mass transfer between the gas and the bulk water phase; however, hydrate formation involves only dissolved guest gas molecules that could travel through the Cyclo-O layer in quiescent conditions, showing cloudlike hydrate formation. The addition of 1 wt % tryptophan with 2.86 mol % Cyclo-O aids inducing a growing front by bridging the hydrophobic Cyclo-O layer and enhances gas uptake and hydrate formation rate significantly compared to the water-Cyclo-O system. The significant increase in gas uptake in the presence of tryptophan was assigned to the porous hydrate formation, which enhanced the gas-water contact.