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CO2 hydrate formation in the CO2 transmission pipelines for ocean or geological sequestration can result in pipeline blockage and rupture. Herein, monomeric substances (urea (U), acetamide (A), and glycine (G)) that are easily decomposable were evaluated as kinetic hydrate inhibitors and potential synergists for CO2 hydrate using experimental and computational approaches. The onset temperatures of CO2 + inhibitor hydrates were experimentally measured using a high-pressure autoclave reactor. G showed the best inhibition effect among single inhibitors, and combinations of U + A and A + G made the inhibition synergism for CO2 hydrate. Moreover, molecular dynamics simulations were also conducted to understand the nucleation and formation behaviors of CO2 hydrate in the presence of single and mixed inhibitors at a molecular level. The profiles of F4 order parameters and hydrate counts for each CO2 + inhibitor system followed the experimental results. Analyses of hydrogen bond distributions, radial distribution function, and mean squared displacement were carefully conducted to elucidate the inhibition mechanism of monomeric substances and their inhibition synergism for CO2 hydrate. The experimental and computational results would provide insights into the development of new biodegradable CO2 hydrate inhibitors and contribute to safe transportation and injection of CO2 for long-term storage.

Fused aromatic networks (FANs) are attracting considerable interest in the scientific community because of their intriguing electronic properties and superior physiochemical stability due to their fully fused aromatic systems. Here, a three-dimensional (3D) cage-like organic network (3D-CON) and a vertical two-dimensional (2D) layered ladder structure (designated as V2D-BBL structure) were studied as materials for gas hydrate inhibitors because of their outstanding stability in high-pressure/low-temperature and periodically incorporated molecular building blocks. The V2D-BBL structure demonstrated remarkable performance, inhibiting the formation of both methane (CH4) and carbon dioxide (CO2) hydrates, comparable to conventional lactam-based polymers. It was determined that the designed perinone moiety in the V2D-BBL structure enables synergistic interactions with the host (water) and guest (CH4) molecules involved in hydrate nucleation. Given their pre-designability and inherent stability, the FANs hold enormous potential as gas hydrate inhibitors for industrial applications.

In this study, urea (U), acetamide (A), and glycine (G), which are easily decomposable, were suggested as kinetic hydrate inhibitors (KHIs) and potential synergists for CH4 hydrate. The inhibition performance of these substances was evaluated by measuring the onset temperature of CH4 hydrate via both a stirred high-pressure autoclave and a non-stirred high-pressure micro-differential scanning calorimeter. In addition, two quantum mechanics approaches of COnductor-like Screening MOdel for Real Solvents (COSMO-RS) and quantum theory of atoms in molecules (QTAIM), were used to examine the hydrophilicity of the inhibitor candidates and to elucidate the synergism of the inhibitor mixtures. The σ-profile and σ-potential analyses using COSMO-RS indicated that the three monomeric substances have a significant potential as gas hydrate inhibitors by forming hydrogen bonds with host water molecules. The experimental results demonstrated that glycine was the best KHI among the three candidates, and either of two combinations (U + A, U + G, and A + G) had synergistic inhibition effects on CH4 hydrate. The A + G mixture showed the lowest onset temperature, i.e., the best synergism. QTAIM analysis showed that when two different inhibitor molecules were bound to the bi-cage hydrate structure, the absolute value of hydrogen bond energy between inhibitors and water molecules was larger compared to the attachment of two identical inhibitor molecules. The overall combined experimental and computational analyses not only suggest the possibility of hydrophilic monomeric substances as potent KHIs but also offer a new way to reveal the mechanism of hydrate inhibition.

In this study, the kinetics of guest exchange in sII (CH4 (90%) + C3H8 (10%)) hydrate – flue gas (CO2 (20%) + N2 (80%)) replacement was investigated to elucidate the replacement mechanism and guest exchange characteristics for hydrocarbon recovery and CO2 sequestration. The Rietveld refinement of Powder X-ray diffraction (PXRD) patterns of the replaced hydrates at two pressures (11.0 MPa and 17.0 MPa) demonstrated that sII hydrate – flue gas replacement occurred iso-structurally. The time-dependent guest compositions in the hydrate phase and the cage occupancy of guest molecules were examined to understand the effect of pressure on guest exchange behaviors. The increased CO2 inclusion in both the small (512) and large (51264) cages at a higher pressure resulted in the larger production of CH4 and C3H8 and a lower N2/CO2 ratio in the hydrate phase. The slower inclusion rate and lower N2 occupancy in the small (512) cages at a higher pressure during replacement were observed due to the competitive inclusion of CO2 and N2 in the small (512) cages. The molecular dynamics (MD) simulation demonstrated that the expansion or shrinkage of hydrate cages caused by guest exchange has a significant impact on the lattice parameter of the replaced hydrates and it was influenced by not only the average molecular size of guest molecules in each cage but also the dispersive movement of the guest molecules in each cage.

In this study, the thermodynamic inhibition effects of choline chloride (ChCl, a hydrogen bond acceptor), urea (a hydrogen bond donor), and their mixture (deep eutectic solvent, DES) on CH4 hydrates were investigated with both experimental and computational approaches. The synthesis of DES from the mixture of ChCl and urea was confirmed by measuring its melting temperature through a high-pressure micro-differential scanning calorimeter. Sigma (σ)-profiles of ChCl, urea, and DES obtained by the COSMO-RS software indicated that these substances have great potential to be used as thermodynamic hydrate inhibitors (THIs). The three-phase (gas hydrate (H) – liquid water (Lw) – vapor (V)) equilibria of CH4 hydrates in the presence of ChCl (1.0, 3.0, and 5.0 mol%), urea (1.0, 3.0, and 5.0 mol%), and DES (3.0 mol%) demonstrated that ChCl, urea, and DES contributed to the significant shift of hydrate equilibrium temperature to inhibition regions at a specified pressure and thus can function as effective THIs. Moreover, quantum theory of atoms in molecules (QTAIM) analysis also demonstrated that the strength of hydrogen bonding between inhibitor molecules and hydrate cages (small 512 and large 51262 cages) of CH4 hydrates was in the order of ChCl > DES > urea, which was consistent with experimental results. The experimental and computational results from this study would be helpful for a better understanding of the inhibition mechanism of ChCl, urea, and DES and for their possible application to flow assurance in oil and gas pipelines.

This study examined the synergistic inhibition effect of glycine (an amino acid) and [BMIM][BF4] (an ionic liquid) on the thermodynamic phase equilibria and growth behaviors of CH4 hydrates. Hydrate phase equilibria indicated that there was no thermodynamic synergism of inhibitor mixtures on CH4 hydrates. Powder X-ray diffraction (PXRD) patterns demonstrated that the presence of inhibitor mixtures did not affect the structural characteristics of CH4 hydrates. However, the glycine (1.5 mol%) + [BMIM][BF4] (1.5 mol%) system showed significantly less growth of CH4 hydrate, less final gas uptake, and less conversion of water into hydrate than a pure water system. Time-dependent Raman spectra revealed that [BMIM][BF4] inhibited CH4 molecules from occupying small 512 cages at the initial stage of hydrate formation, whereas glycine was effective in preventing CH4 molecules from entering large 51262 cages for the duration of hydrate formation. The cage-specific inhibition mechanism of the glycine and [BMIM][BF4] mixture had a synergistic effect, significantly reducing the growth of CH4 hydrate. The results of this study provide a better understanding of the inhibition mechanism and the synergistic potential of various inhibitors and could contribute to an expansion in the types of inhibitors that could be used for flow assurance in the pipelines of natural gas production and transportation.

In this study, the kinetic inhibition effects of amino acids and ionic liquids and the potential synergism of their mixtures on CH4 hydrates were investigated by adopting three different approaches. The sigma (σ) profiles of inhibitor molecules obtained from the Conductor-Like Screening Model for Real Solvents (COSMO-RS) software were used to estimate inhibitor–water interactions for the pre-screening of potential inhibitors, and thus glycine (an amino acid) and [BMIM][BF4] (an ionic liquid) were selected as candidates for kinetic hydrate inhibitors (KHIs). A non-stirred high-pressure micro-differential scanning calorimeter (HP μ-DSC) revealed that the glycine (0.5 wt%) and [BMIM][BF4] (0.5 wt%) mixture showed a synergism for CH4 hydrate as a KHI. The kinetic inhibition synergism of the glycine (0.5 wt%) and [BMIM][BF4] (0.5 wt%) mixture on CH4 hydrate was double-checked using a stirred high-pressure autoclave. The onset temperature of the glycine (0.5 wt%) and [BMIM][BF4] (0.5 wt%) mixture was found to be lower than that of poly(N-vinylcaprolactam) (PVCap; 1.0 wt%). Time-dependent Raman spectra demonstrated that the cage-specific inhibition of glycine and [BMIM][BF4] caused kinetic inhibition synergism and the consequent slow growth rate of CH4 hydrate. The synergistic kinetic inhibition of amino acids and ionic liquids will be helpful in understanding the accurate roles of inhibitors in inhibitor–water interactions and will contribute to opening up a new field related to the synergism of eco-friendly inhibitors for flow assurance in oil and gas pipelines.