CO<inf>2</inf>-CH<inf>4</inf>Hydrate Formation Using l -Tryptophan and Cyclooctane Employing a Conventional Stirred Tank Reactor

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.