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Amino acids (glycine and alanine) and ionic liquids ([BMIM][BF 4 ] and [BMIM][I]) were examined for their inhibition effects on CH 4 hydrates with experimental and computational approaches. Both amino acids and ionic liquids functioned well as thermodynamic hydrate inhibitors, by shifting equilibrium curves of CH 4 hydrates toward harsh conditions. However, powder X-ray diffraction patterns indicated that amino acids and ionic liquids did not affect the hydrate structure because they were not included in the hydrate cages due to their large molecular size. Gas uptake measurements showed that the conversion of water into gas hydrates and the formation rates of CH 4 hydrate were significantly influenced by inhibitors. Density functional theory calculations indicated that [BMIM][BF 4 ] had a larger potential than glycine to inhibit gas hydrate formation by giving a more negative interaction energy between a cage and an inhibitor molecule. The time-dependent Raman spectra collected during CH 4 hydrate formation demonstrated that [BMIM][BF 4 ] hindered CH 4 molecules from occupying small (512) cages, whereas glycine had a strong influence on large (51262) cages of sI hydrates. The experimental and computational results provide a better understanding of inhibition mechanisms of gas hydrates and thus can contribute to the improved control of hydrate formation in oil and gas pipelines. • We examine CH 4 hydrates in the presence of inhibitors for hydrate control. • We use amino acids and ionic liquids to examine inhibition mechanisms. • Inhibitors do not affect the structure of CH 4 hydrate. • Glycine and [BMIM][BF 4 ] inhibit large 51262 cages and small 512 cages, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mechanism of the structural transformation during replacement is investigated. • A higher CO 2 enclathration ratio causes the structural transition from sH to sI. • CO 2 molecules preferentially occupy sI large cages and sH medium cages. • A lower CO 2 enclathration ratio results in a lower extent of replacement. This study adopted experimental and computational approaches to investigate CH 4 – CO 2 replacement in the structure H (sH) CH 4 + methylcyclopentane (MCP) hydrate for its dual functions of CH 4 recovery and CO 2 sequestration. Hydrate phase equilibria, 13C NMR spectra, and molecular dynamics (MD) simulations of CH 4 + MCP – CO 2 replacement were examined and compared with those of CH 4 + neohexane (NH) – CO 2 replacement to elucidate the molecular guest exchange behaviors in both systems. The structure I (sI) hydrates were thermodynamically favored in CO 2 -rich gas mixtures (CH 4 + CO 2 + MCP systems), and a structural transformation from sH to sI occurred when CO 2 composition in the feed gas was higher than 20 %. The 13C NMR spectra indicated that the CO 2 molecules preferred to occupy the large (51262) cages of sI and the medium (435663) cages of sH compared to the small (512) cages of both sI and sH during replacement. Following CO 2 injection into the sH CH 4 + MCP hydrate, the initial sH hydrate transformed to the sI hydrate with CH 4 recovery of approximately 78 %. The MD simulations also demonstrated that structural transformation in the CH 4 + MCP – CO 2 replacement would occur at the lower CO 2 enclathration ratio compared to the CH 4 + NH – CO 2 replacement, thereby leading to a lower CO 2 concentration in the newly formed sI hydrate. This is the first study to provide both experimental and computational evidence of guest-dependent structural transformation in sH CH 4 + liquid hydrocarbons – CO 2 replacement. [ABSTRACT FROM AUTHOR]

• Waterborne polyurethanes (WPUs) were synthesized and evaluated as dual-function inhibitors. • All WPUs exhibited hydrate inhibition and anti-agglomeration effects on C1/C3 hydrates. • The corrosion inhibiting features of WPUs were revealed with several electrochemical tests. • WPUs were found to serve dual functions as hydrate and corrosion inhibitors. In this study, an effective hybrid inhibition of gas hydrates and corrosion was investigated to overcome compatibility problems between inhibitors and reduce the cost of production and transportation operations in the oil and gas industry. The newly synthesized waterborne polyurethanes (WPUs) were appraised as efficient dual-purpose gas hydrate and corrosion inhibitors through experimental and computational approaches. The onset temperatures of C1/C3 hydrates in the presence of WPUs were measured using a high-pressure autoclave to evaluate the performance of WPUs as gas hydrate inhibitors. The potential of WPUs as anti-agglomerants and the rheological behaviors of C1/C3 hydrates in the presence of WPUs were investigated using a high-pressure rheometer. All WPUs (WPU1, WPU2, WPU3, WPU4, and WPU5) were found to function as hydrate inhibitors and contribute to the prevention of agglomeration for C1/C3 hydrates. Molecular dynamics simulations revealed that the long-chained anionic parts of WPUs were adsorbed onto the hydrate surface while small cationic parts moved freely in the solution; thus, the interaction between the anions and water molecules was a dominant factor for C1/C3 hydrate inhibition. The corrosion inhibition activity of WPUs was evaluated by open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy techniques in H 2 S and CO 2 -saturated oilfield-produced water. The electrochemical measurements indicated that WPUs improved the polarization resistance of mild steel and significantly protected its surface against corrosive gases. The experimental and computational results indicated that WPUs can be used as efficient dual-function inhibitors for prevention of gas hydrate formation and CO 2 /H 2 S corrosion inside oil and gas pipelines, and also have potential as anti-agglomerant hydrate inhibitors. [ABSTRACT FROM AUTHOR]

[Display omitted] • Hydrophilic monomeric substances were tested as kinetic hydrate inhibitors for CH 4 hydrate. • Both experimental and computational approaches were used to reveal the inhibition mechanism. • Glycine (G) showed better inhibition performance than urea (U) and acetamide (A). • The A + G mixture had the lowest onset temperature and the best synergism. • U, A, and G were potent hydrate inhibitors as well as powerful synergists for CH 4 hydrate. 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 CH 4 hydrate. The inhibition performance of these substances was evaluated by measuring the onset temperature of CH 4 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 CH 4 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Kinetic guest exchange in sII hydrate-flue gas replacement was investigated. • The sII hydrate-flue gas replacement occurred iso-structurally. • The higher pressure enhanced guest exchange with CO 2 in both small and large cages. • The competitive inclusion of CO 2 and N 2 in the small (512) cages was observed. • The size and dispersive movement of the guest molecules affected the lattice parameter. In this study, the kinetics of guest exchange in sII (CH 4 (90%) + C 3 H 8 (10%)) hydrate – flue gas (CO 2 (20%) + N 2 (80%)) replacement was investigated to elucidate the replacement mechanism and guest exchange characteristics for hydrocarbon recovery and CO 2 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 CO 2 inclusion in both the small (512) and large (51264) cages at a higher pressure resulted in the larger production of CH 4 and C 3 H 8 and a lower N 2 /CO 2 ratio in the hydrate phase. The slower inclusion rate and lower N 2 occupancy in the small (512) cages at a higher pressure during replacement were observed due to the competitive inclusion of CO 2 and N 2 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. [ABSTRACT FROM AUTHOR]

• σ-profiles of substances by COSMO-RS were used to pre-screen potential inhibitors. • ChCl, urea, and DES contributed to the shift of hydrate equilibrium temperature. • The QTAIM analysis validated the experimental results of potential inhibitors. • ChCl, urea, and DES had great potential for use as hydrate inhibitors. 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 CH 4 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 (L w) – vapor (V)) equilibria of CH 4 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 CH 4 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. [ABSTRACT FROM AUTHOR]

• The σ profiles from COSMO-RS were used for the pre-screening of potential KHIs. • Both a non-stirred DSC and a stirred autoclave were used for the evaluation of KHIs. • The onset temperature of the glycine and [BMIM][BF 4 ] mixture was lower than that of PVCap. • The slow growth rate of CH 4 + glycine + [BMIM][BF 4 ] hydrate was confirmed via Raman. • The cage-specific inhibition of glycine and [BMIM][BF 4 ] caused synergism. In this study, the kinetic inhibition effects of amino acids and ionic liquids and the potential synergism of their mixtures on CH 4 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][BF 4 ] (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][BF 4 ] (0.5 wt%) mixture showed a synergism for CH 4 hydrate as a KHI. The kinetic inhibition synergism of the glycine (0.5 wt%) and [BMIM][BF 4 ] (0.5 wt%) mixture on CH 4 hydrate was double-checked using a stirred high-pressure autoclave. The onset temperature of the glycine (0.5 wt%) and [BMIM][BF 4 ] (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][BF 4 ] caused kinetic inhibition synergism and the consequent slow growth rate of CH 4 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fused aromatic networks are unique subclasses of porous organic polymers. • Fused aromatic networks were examined as kinetic hydrate inhibitors (KHIs). • The V2D-BBL structure showed remarkable performances as a KHI for gas hydrates. • The designed perinone moiety enabled synergistic interactions with host and guest. 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 (CH 4) and carbon dioxide (CO 2) 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 (CH 4) molecules involved in hydrate nucleation. Given their pre-designability and inherent stability, the FANs hold enormous potential as gas hydrate inhibitors for industrial applications. [ABSTRACT FROM AUTHOR]