Xuqiang Guo

Molecular dynamics simulation was employed to reveal the influence mechanism of H2S on the hydrate inhibition performance of two typical kinetic hydrate inhibitors (i.e., polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap)). The results indicate that the presence of hydrogen sulfide significantly weakens the inhibitory performance of PVP and PVCap on methane hydrates, and in the growth stage, PVCap appeared to lose its inhibitory effect in our simulations. Because hydrogen sulfide itself promotes hydrate formation, it weakens the performance of the inhibitors primarily by altering the bulk hydrate-forming environment. Hydrogen sulfide and water molecules form transient “pseudo-cyclic” complexes. This structure weaken the adsorption of methane molecules on the inhibitor surface and promote the migration of methane to the hydrate clusters, which may contribute to the self-assembly and growth of hydrates. The simulations also showed a greater reduction in the inhibition performance of PVCap than of PVP. However, condensed-phase analysis indicates that this difference does not result from direct interactions between H2S and the inhibitors. The oxygen-containing functional groups remain coordinated by water molecules, and H2S is not enriched around the hydrophobic groups in either inhibitor system. Instead, the reduced inhibition performance is primarily associated with the influence of H2S on the bulk hydrate-forming environment, including enhanced methane mobility. Together, these effects lead to a significant reduction in the effectiveness of kinetic hydrate inhibitors in H2S-containing systems and provide a molecular-level explanation for the interference caused by H2S. These conclusions are based on a limited number of simulation trajectories and should therefore be regarded as representative of the dominant trends.

Renewable methanol has gained global attention for its carbon-neutral potential, yet a unified definition and standardized evaluation framework are still lacking internationally. This study employs life cycle assessment (LCA) to quantify CO2 emissions across five representative methanol synthesis pathways, including coal-to-methanol (CTM), natural gas-to-methanol (NGTM), biomass-to-methanol (BTM), biomass and green hydrogen-to-methanol (BHTM), and CO2 and green hydrogen-to-methanol (CHTM), under both cradle-to-gate and cradle-to-grave boundaries. The results show that fossil-based pathways exhibit higher life-cycle CO2-eq emissions than renewable pathways. Within the cradle-to-grave boundary, CTM and NGTM show 4.75 and 2.83 kg CO2-eq/kg CH3OH, respectively, whereas BTM and BHTM decrease emissions to 0.81 and 0.67 kg CO2-eq/kg CH3OH. For CHTM, the cradle-to-grave emissions range from 0.58 to 1.29 kg CO2-eq/kg CH3OH depending on the CO2 source scenario. A normalized Z factor was introduced as a unified metric for comparing methanol synthesis pathways. It links life-cycle CO2-eq emissions to the amount of CO2 temporarily stored in methanol. Overall, this study provides a consistent basis for comparing the carbon performance of different methanol production routes.

As a finite strategic resource, helium is extracted from natural gas (NG). The concentration of helium in NG is very low, which makes helium hard to separate. The hydrate-based gas separation (HBGS) was proposed as a promising method for the separation of the NG with low helium content in this work. This work systematically investigated the HBGS of helium from simulated NG. The thermodynamic analysis reveals that the existence of 5.00 mol% tetrahydrofuran (THF) in the liquid phase decreased the gas–liquid–hydrate equilibrium pressure by 92.11%, compared to the deionized water system. The single-stage HBGS experimental results show that high THF concentration, low temperature, and high pressure benefited the gas processing capacity and helium purification, but they led to a low helium recovery rate. The best HBGS performance was limited by the “hydrate shell effect”. The decrease in gas–liquid ratio led to an increase in helium concentration without losing the gas processing capacity, but it caused a decrease in the helium recovery rate. Through three-stage HBGS optimization, the helium concentration was increased from 0.54 mol% to 13.54 mol% (a 25.07-fold enrichment), and a total helium recovery of 87.34% was achieved. The mathematical model proposed in this work accurately predicts the performance of HGBS with 2.09% average relative error compared to the experimental data.

Sour natural gas holds significant economic value, but its pipeline transportation is challenged by H₂S accelerating natural gas hydrate nucleation/growth and exacerbating pipeline blockages under low temperature. Through molecular dynamics simulations, our study reveals that H₂S promotes nanobubble dissolution and increases local guest molecule concentration, thereby shortening induction time and accelerating nucleation. However, high H₂S concentrations inhibit stable cage growth by disrupting water's hydrogen-bond network, causing the decomposition of unstable/large cages. Simultaneously, H₂S alters hydrate cage composition: reducing H₂S unstable/large cages while significantly increasing stable 5¹²6² cages, thus promoting Structure I (sI) hydrate formation. Mechanistically, H₂S acts as a hydrogen atom donor to form hydrogen-bond-like interactions with water, stabilizing the H-bond network and repairing lattice defects─collectively accelerating hydrate nucleation and growth. These findings provide a fundamental understanding that could advance research on sour natural gas transportation.

Hydrate-based gas separation (HBGS) is a new method for extracting helium from helium-rich natural gas (HNG). The ethane in HNG affects the thermodynamic equilibrium hydrate formation pressure (Peq), and Peq is crucial to the application of HBGS for extracting helium. In this work, the Peq of the HNGs with different ethane contents (0.5 mol%, 1.0 mol%, and 10 mol%) and the solutions with different tetrahydrofuran (THF) contents (5 wt%, 10 wt%, and 19 mol%) at different temperatures were experimentally investigated through the isothermal pressure search method. Ethane and THF have a competitive effect on hydrate formation. A new thermodynamic model was proposed to predict the Peq of different HNG–THF solution systems. The effect of ethane on Peq can be quantitatively described, and the Peq of HNGs can be accurately predicted by the model in this work. The average relative deviation of the model for predicting Peq of HNGs in different THF aqueous solution systems is less than 3%. The results of this study can guide the operating conditions for the optimization of extracting helium from HNGs by the HBGS process.

Carbon capture, utilization and storage are facilitated through carbon dioxide (CO₂) transport. Pipe transportation is the main method for transporting CO₂. However, hydrate blockages reduce transport efficiency in the pipelines, and the throttling devices are the main location of hydrate blockages. In this paper, the mechanism of hydrate formation in the throttling of CO₂-containing trace moisture was investigated. The throttling device in a pipe was mimicked using a cylindrical orifice plate. The work also studied the effects of moisture content, upstream pressure and upstream temperature on hydrate formation. The results indicate that the Joule–Thomson cooling effect is a key contributor, and promotes the condensation of trace moisture, resulting in the free water necessary for hydrate nucleation. Under the effect of gas flow back-mixing, it is easy for the hydrate to adhere to the inner surface of the pipe behind the orifice plate. When the moisture content in the gas increases from 123 μmol/mol to 1024 μmol/mol, the hydrate induction time decreases from infinity to 792 s. However, the moisture content has no effect on the adhesion strength of the hydrate to the inner surface of the pipe. When the initial upstream pressure increases from 2.0 MPa to 3.5 MPa, the hydrate induction time decreases from infinity to 306 s. When the upstream temperature decreases from 291.15 K to 285.15 K, the hydrate induction time decreases from infinity to 330 s. With the decrease in the initial upstream temperature, the adhesion of hydrate particles to the inner surface of the pipe is promoted. This study provides experimental evidence for the characteristics of hydrate formation in the process of CO₂ throttling.

Hydrate-based gas separation (HBGS) has good potential in the separation of helium from helium-rich natural gas. HBGS should be carried out under a pressure higher than the thermodynamic equilibrium hydrate formation pressure (P_eq) to ensure the formation of hydrate so that the accurate prediction of P_eq is the basis of the determination of HBGS pressure. In this work, the P_eq of the helium-rich natural gases with different helium contents (1 mol%, 10 mol%, and 50 mol%) in gas and different tetrahydrofuran (THF) contents (5 wt%, 10 wt%, and 19 wt%) in liquid at different temperatures were experimentally investigated through the isothermal pressure search method. A new thermodynamic model was proposed to predict the P_eq of helium-rich natural gas. This model can quantitatively describe the effects of THF and helium on P_eq, and it predicts the P_eq of the helium-rich natural gases in this work accurately. The average relative deviation of the model is less than 3%. This model can guide the determination of the operating condition of the HBGS of helium-rich natural gas.

The adhesion between a hydrate and a pipe wall is the main cause of hydrate deposition and blockage. In this study, the adhesion strength of semi-clathrate hydrate (tetrabutylammonium bromide hydrate) to four kinds of solid surfaces (E235B carbon steel, E355CC low alloy steel, SUS304 stainless steel, and polytetrafluoroethylene) was measured. This investigation reveals that the adhesion strength of the hydrate to a solid surface is negatively correlated with the wettability of the solid surface, which suggests that hydrophobic materials effectively reduced the hydrate adhesion to the pipe wall. The surface roughness showed different effects on the adhesion of the hydrate to hydrophilic or hydrophobic surfaces. To be specific, when the surface roughness increased from 3.2 µm to 12.5 µm, the hydrate adhesion strength to the hydrophilic surface of SUS304 increased by 123.6%, whereas the hydrate adhesion strength to the hydrophobic surface of polytetrafluoroethylene only increased by 21.5%. This study shows that low wettability and low surface roughness effectively reduce the critical rate required to remove hydrate deposition, which achieves the self-removal of hydrates. At the same time, it was found that the adhesion strength of the hydrate to surfaces increases with increasing subcooling. This investigation holds significant theoretical implications for designing self-cleaning surfaces for oil and gas pipes.