Study on the influence of SDS and THF on hydrate-based gas separation performance

A Molecular Dynamics Study on Xe/Kr Separation Mechanisms Using Crystal Growth Method

The separation of xenon/krypton gas mixtures is a valuable but challenging endeavor in the gas industry due to their similar physical characteristics and closely sized molecules. To address this, we investigated the effectiveness of the hydrate-based gas separation method for mixed Xe-Kr gas via molecular dynamics (MD) simulations. The formation process of hydrates facilitates the encapsulation of guest molecules within hydrate cages, offering a potential strategy for gas separation. Higher temperatures and pressures are advantageous for accelerating the hydrate growth rate. The final occupancy of guest molecules and empty cages within 512, 51264, and all hydrate cages were thoroughly examined. An increase in the pressure and temperature enhanced the occupancy rates of Xe in both 512 and 51264 cages, whereas elevated pressure alone improved the occupancy of Kr in 51264 cages. However, the impact of temperature and pressure on Kr occupancy within 512 cages was found to be minimal. Elevated temperature and pressure resulted in a reduced occupancy of empty cages. Predominantly, 51264 cages were occupied by Xe, whereas Kr showed a propensity to occupy the 512 cages. With increasing simulated pressure, the final occupancy of Xe molecules in all cages rose from 0.37 to 0.41 for simulations at 260 K, while the final occupancy of empty cages decreased from 0.24 to 0.2.

Understanding the effect of moderate concentration SDS on CO2 hydrates growth in the presence of THF

Hypothesis Additives like Tetrahydrofuran (THF) and Sodium Dodecylsulfate (SDS) improve Carbon Dioxide (CO2) hydrates thermal stability and growth rate when used separately. It has been hypothesised that combining them could improve the kinetics of growth and the thermodynamic stability of CO2 hydrates. Simulations and Experiments We exploit atomistic molecular dynamics simulations to investigate the combined impact of THF and SDS under different temperatures and concentrations. The simulation insights are verified experimentally using pendant drop tensiometry conducted at ambient pressures and high-pressure differential scanning calorimetry. Findings Our simulations revealed that the combination of both additives is synergistic at low temperatures but antagonistic at temperatures above 274.1 K due to the aggregation of SDS molecules induced by THF molecules. These aggregates effectively remove THF and CO2 from the hydrate-liquid interface, thereby reducing the driving force for hydrates growth. Experiments revealed that the critical micelle concentration of SDS in water decreases by 20% upon the addition of THF. Further experiments in the presence of THF showed that only small amounts of SDS are sufficient to increase the CO2 storage efficiency by over 40% compared to results obtained without promoters. Overall, our results provide microscopic insights into the mechanisms of THF and SDS promoters on CO2 hydrates, useful for determining the optimal conditions for hydrate growth.

Rapid Growth of the CO2 Hydrate Induced by Mixing Trace Tetrafluoroethane

Rapid formation of the CO2 hydrate can be significantly induced by the gaseous thermodynamic promoter 1,1,1,2-tetrafluoroethane(R134a) due to the mild phase equilibrium conditions, although the formation mechanism and dynamic behavior are not clear. Therefore, a visual experimental system was developed to study the effects of different concentrations of R134a on the induction time, gas consumption, and growth morphology of the CO2 hydrate. At the same time, the combined effects under stirring and sodium dodecyl sulfate (SDS) systems were also studied. In addition, visualization and experimental model diagrams were combined to explain the fast formation mechanism of the R134a/CO2 hydrate. The results show that the CO2 hydrate average conversion rate was increased by more than 63% with the addition of mixed trace R134a(7%). A special phenomenon is found that two temperature peaks appear on the hydrate formation temperature curve, corresponding to two different stages of hydrate formation when stirring or SDS is added to the mixed gas reaction system. Furthermore, the gas consumption in stirring and SDS systems increases by 9 and 44%, respectively. Finally, it is also found that the R134a/CO2 mixed hydrate formed under the action of SDS has a "capillary" mechanism, which provides a gas-liquid phase exchange channel and a large number of nucleation sites for CO2 hydrate, thus promoting the formation of CO2 hydrate. This paper provides a novel, simple, and efficient method for CO2 hydrate gas storage technology.

Enhanced Hydrate-Based Geological CO2 Capture and Sequestration as a Mitigation Strategy to Address Climate Change

Geological sequestration of CO2-rich gas as a CO2 capture and storage technique has a lower technical and cost barrier compared to industrial scale-up. In this study, we have proposed CO2 capture and storage via hydrate in geological formation within the hydrate stability zone as a novel technique to contribute to global warming mitigation strategies, including carbon capture, utilization, and storage (CCUS) and to prevent vast methane release into the atmosphere caused by hydrate melting. We have attempted to enhance total gas uptake and CO2 capture efficiency in hydrate in the presence of kinetic promoters while using diluted CO2 gas (CO2-N2 mixture). Experiments are performed using unfrozen sands within hydrate stability zone condition and in the presence of low dosage surfactant and amino acids. Hydrate formation parameters, including sub-cooling temperature, induction time, total gas uptake, and split fraction, are calculated during the single-step formation and dissociation process. The effect of sands with varying particle sizes (160–630 µm, 1400–5000 µm), low dosage promoter (500–3000 ppm) and CO2 concentration in feed gas (20–30 mol%) on formation kinetic parameters was investigated. Enhanced formation kinetics are observed in the presence of surfactant (1000–3000 ppm) and hydrophobic amino acids (3000 ppm) at 120 bar and 1 ℃ experimental conditions. We report induction time in the range of 7–170 min and CO2 split fraction (0.60–0.90) in hydrate for 120 bar initial injection pressure. CO2 split fraction can be enhanced by reducing sand particle size or increasing the CO2 mol% in incoming feed gas at given injection pressure. This study also reports that formation kinetics in a porous medium are influenced by hydrate morphology. Hydrate morphology influences gas and water migration within sediments and controls pore space or particle surface correlation with the formation kinetics within coarse sediments. This investigation demonstrates the potential application of bio-friendly amino acids as promoters to enhance CO2 capture and storage within hydrate. Sufficient contact time at gas-liquid interface and higher CO2 separation efficiency is recorded in the presence of amino acids. The findings of this study could be useful in exploring the promoter-driven pore habitat of CO2-rich hydrates in sediments to address climate change.

Screening of Amino Acids and Surfactant as Hydrate Promoter for CO2 Capture from Flue Gas

In this study, the kinetics of flue gas hydrate formation in bulk water in the presence of selected amino acids and surfactants are investigated. Four amino acids (3000 ppm) are selected based on different hydropathy index. Constant-ramping and isothermal experiments at 120 bar pressure and 1 °C temperature are carried out to compare their hydrate promotion capabilities with surfactant sodium dodecyl sulfate (SDS) (500–3000 ppm) and water. Based on experimental results, we report the correlation between hydrate promotion capability of amino acids and their hydrophobicity. Hydrophobic amino acids show stronger flue gas hydrate promotion capability than water and hydrophilic amino acids. We discuss the controlling mechanisms to differentiate between promoters and inhibitors’ roles among the amino acids. Between 2000–3000 ppm concentrations, hydrophobic amino acids have near similar promotion capabilities as SDS. This research highlights the potential use of amino acids as promoters or inhibitors for various applications.

Structure, mechanism, and performance evaluation of natural gas hydrate kinetic inhibitors

Ice-like crystal compounds, which are formed in low-temperature and high-pressure thermodynamic conditions and composed of a combination of water molecules and guest gas molecules, are called gas hydrates. Since its discovery and recognition as the responsible component for blockage of oil and gas transformation line, hydrate has been under extensive review by scientists. In particular, the inhibition techniques of hydrate crystals have been updated in order to reach the more economically and practically feasible methods. So far, kinetic hydrate inhibition has been considered as one of the most effective techniques over the past decade. This review is intended to classify the recent studies regarding kinetic hydrate inhibitors, their structure, mechanism, and techniques for their performance evaluation. In addition, this communication further analyzes the areas that are more in demand to be considered in future research.

Monte Carlo simulations of the separation of a binary gas mixture (CH4 + CO2) using hydrates

The current study employs Grand Canonical Monte Carlo simulations in order to calculate the process efficiency of separating CH₄ + CO₂ gas mixtures by utilizing structure sI clathrate hydrates.

Direct phase coexistence molecular dynamics study of the phase equilibria of the ternary methane–carbon dioxide–water hydrate system

Grown mixed carbon dioxide–methane hydrate with molecular dynamics.