Thermodynamic Hydrate Inhibitors Research Articles

Effects of methanol and ethylene glycol on the interactions between methane hydrate particles and water droplets

Hydrate particle agglomeration is one of the main causes of hydrate blockage in pipelines. Although methanol (MeOH) and ethylene glycol (MeG) are the most used thermodynamic hydrate inhibitors, their effects on the micromechanics behavior of natural gas hydrate particle agglomeration are not clear. In this study, the effects of MeOH and MeG on the interaction force between methane hydrate particles and droplets were investigated using a self-built high-pressure hydrate microforce measuring device. The results suggest that, with the concentration increase from 1% to 7%, for the system with MeOH, the adhesion force reduced by 11% to 33%, while for MeG, the adhesion force reduced by 15% to 43%. On the one hand, the addition of inhibitors reduces the phase equilibrium temperature, which decreases the solidification rate of water in liquid bridge, and on the other hand, they reduce the interfacial tension. both reductions lead to lower adhesion force. Furthermore, in comparison with MeG, the MeOH solutions present a lower contact angle, resulting in a larger hydrate particle-droplet contact area and thus relatively higher adhesion force. The measurement and observation in the present work provide new insight into the mechanism of methanol and ethylene glycol on inhibiting hydrate agglomeration, which is important in advancing the management of hydrate formation in pipelines.

Modeling Two-Step Mechanism of Thermodynamic Hydrate Inhibitor Injection for Gas Hydrate Plug Dissociation

Modeling Two-Step Mechanism of Thermodynamic Hydrate Inhibitor Injection for Gas Hydrate Plug Dissociation

Effect of sec-butyl alcohol on CO2 hydrate equilibrium conditions

Thermodynamic inhibitors may be used to inhibit hydrate formation in industry. Alcohol substances, due to their hydrophilic hydroxyl groups in their molecular structure, are prone to form hydrogen bond with water molecules to inhibit hydrate formation. In this work, sec-butyl alcohol is chosen to study the effects on CO2 hydrate dissociation equilibrium conditions, where the step-heating method is utilized to obtain hydrate equilibrium conditions. The equilibrium curve of carbon dioxide hydrate dissociation moves to the region of lower temperatures or higher pressures. The higher mass fraction of sec-butyl alcohol, the better effect of inhibiting hydrate formation. Sec-butyl alcohol can be used as a thermodynamic hydrate inhibitor.

Experimental Investigation Into Effects of Thermodynamic Hydrate Inhibitors on Natural Gas Hydrate Synthesis in 1D Reactor

Thermodynamic hydrate inhibitors (THIs) are often used in marine gas field operations to prevent the formation of natural gas hydrate (NGH), and it is important to study the effect of THIs on hydrate synthesis characteristics. Herein, the effects of water (H2O), sodium chloride (NaCl), and ethylene glycol (EG) on NGH synthesis characteristics under different initial pressure conditions are tested using the self‐developed 1D NGH production simulation test system. The effects of each THI on the maximum synthesis rate and maximum synthesis amount of NGH are investigated using two physical quantities, peak of temperature increment and maximum gas conversion rate. The results show that the 2.65 wt% salt solution has the minimum inhibition effect on the maximum synthesis rate of NGH, and the solution with a 3:1 mixture of H2O and EG has the maximal inhibition effect on the maximum synthesis rate of NGH. When only H2O and EG or H2O and NaCl are present in the reactor, the higher the concentration of EG or NaCl, the more significant is the inhibition effect on the maximum synthesis amount of NGH.

Phase equilibrium data for clathrate hydrates formed in the systems of urea + (methane or carbon dioxide) + water

Subsea methane hydrates are expected as a potential natural gas resource. To develop gas production methods for the subsea methane hydrates, flow assurance technologies are necessary. Urea is one of the environmentally-friendly thermodynamic hydrate inhibitors (THIs) which is expected to be applicable for subsea methane hydrate systems. To evaluate its performance as a THI, a set of phase equilibrium data are necessary. In this study, we report phase equilibrium data for systems of urea + methane + water and urea + carbon dioxide + water in the range of aqueous compositions of urea up to its solubility limits. The present data for urea + methane + water ranged in pressures from 5 to 13 MPa and in urea mass fractions from 0.05 to 0.5. The comparison with the literature found that the inhibition effect of urea is a few kelvin weaker than that of methanol in the present measurement range. With the dense urea solutions, the system reached four phase equilibrium, i.e., solid urea–aqueous solution–gas–hydrate. At 13 MPa, approximately 13 K of inhibition temperature was obtained with 0.500 of feed mass fraction where solid urea was precipitated from the aqueous phase. In the urea + carbon dioxide + water systems, urea also worked as a THI. The maximum inhibition temperature was approximately 10 K with 0.400 of urea mass feed fraction at the four phase equilibrium. Based on the present equilibrium data and seafloor conditions of the subsea methane hydrates, it was found that urea can be used for the both systems of the methane gas production and the hydrate based carbon capture and storage.

Elucidating the Impact of Thermodynamic Hydrate Inhibitors and Kinetic Hydrate Inhibitors on a Complex System of Natural Gas Hydrates: Application in Flow Assurance

To alleviate the hydrate blockage risk in oil and gas pipelines/production facilities, thermodynamic hydrate inhibitors (THIs) and kinetic hydrate inhibitors (KHIs) are the frequently used additives/chemicals. While these additives are highly effective, the impact of low dosage of THIs (under inhibited systems) and performance of KHIs at high water cut and/or high subcooling is not well understood. In this work, a high-pressure visual stirred tank reactor was employed to investigate the impact of THIs and KHIs on the kinetics of the complex hydrates of natural gas mixtures (CH4/C2H6/C3H8) and cyclopentane at high water-cut and high subcooling (∼21 °C). Various concentrations of a THI, mono-ethylene glycol (MEG) with a dosage ranging from 0.1 to 25 wt %, and multiple KHIs (polyvinylpyrrolidone (PVP k-30, PVP k-90), caprolactam, butyl glycol ether (BGE), and polyglycerin with 0.1 wt % dosage) were evaluated for hydrate formation kinetics, onset hydrate formation/nucleation temperature, and morphological changes. Gas uptake results reveal that hydrate blockage risk is higher at low MEG content (0.1 to 10 wt %) compared to the baseline, while a significant reduction in gas uptake was observed for the high MEG content trials (20–25 wt %). With 20 and 25 wt % MEG, the hydrate nucleation temperature was reduced by 10–12 °C compared to the baseline. In context to KHIs impact, 0.1 wt % BGE and caprolactam promoted the nucleation temperature and the rate of hydrate formation/gas uptake. However, PVPs not only reduced the gas uptake kinetics but also lessened the hydrate nucleation temperature by 6–8 °C compared to the baseline data. Our results may offer new insights into optimizing hydrate inhibitors dosage to diminish the risk of hydrate blockage in subsea pipelines.

Experimental Investigation to Quantify Gas Hydrate Formation during Shutdown in the Wellbore Near the Wellhead: Impact of Thermodynamic Inhibitors

In most cases, concerns about gas hydrates in hydrocarbon production are often associated with shut-in and restart of the production system associated with the fluids in the wellhead and the main flowline. The exposure of the wellhead to cold temperatures during shutdown can cause severe hydrate formation even with stagnant fluid in the wellbore. Similar conditions may exist at the top of the gas line in flowlines for hydrate formation when gas is saturated with water. This study investigated the hydrate deposition kinetics and morphology at 10 MPa (ca. 1400 psi), with the goal of preventing severe solid hydrate formation from the vapor or supercritical phase near the wellhead or a vertical conduit exposed to cold conditions such as road overpass or river crossing during shutdown and providing operating guidelines for hydrate management by understanding the impact of volatile (methanol) and non-volatile (monethylene glycol, MEG) thermodynamic hydrate inhibitors (THIs). These results may also apply to top of line hydrate and its inhibition. It was verified that non-volatile THI is ineffective in preventing hydrate deposition near the wellhead, showing growth behavior of hydrate formation in the presence of MEG similar to that with lower pressure without inhibitors. However, volatile THI prevents hydrate deposition in a cold wellhead even though the experimental condition provides sufficient convection and water condensation. In the warm wellhead, volatile THI does not prevent hydrate formation, but it delays the growth of hydrate deposits. In the presence of volatile THI, the dissociation and reformation of hydrate deposits on the vertical wall are observed. This study provides a quantifiable method for hydrate growth in real-time and insight into better management strategies for THI usage to mitigate hydrate blockage risk near the wellhead.

Experimental Study on CH4 Hydrate Dissociation by the Injection of Hot Water, Brine, and Ionic Liquids

Thermal stimulation is an important method to promote gas production and to avoid secondary hydrate formation during hydrate exploitation, but low thermal efficiency hinders its application. In this work, hydrate dissociation was carried out in synthesized hydrate-bearing sediments with 30% hydrate saturation at 6.9 MPa and 9 °C. Ionic liquids, such as 1-butyl-3-methylimidazolium chloride (BMIM-Cl) and tetramethylammonium chloride (TMACl), were injected as heat carriers, and the promotion effects were compared with the injection of hot water and brine. The results showed that the injection of brine and ionic liquids can produce higher thermal efficiencies compared to hot water. Thermodynamic hydrate inhibitors, such as NaCl, BMIM-Cl, and TMACl, were found to impair the stability of CH4 hydrate, which was conducive to hydrate dissociation. By increasing the NaCl concentration from 3.5 to 20 wt%, the thermal efficiency increased from 37.6 to 44.0%, but the thermal efficiencies experienced a fall as the concentration of either BMIM-Cl or TMACl grew from 10 to 20 wt%. In addition, increasing the injection temperature from 30 to 50 °C was found to bring a sharp decrease in thermal efficiency, which was unfavorable for the economics of gas production. Suitable running conditions for ionic liquids injection should control the concentration of ionic liquids under 10 wt% and the injection temperature should be around 10 °C, which is conducive to exerting the weakening effect of ionic liquids on hydrate stability.

Влияние метанола на кинетику нуклеации и роста гидрата метана

In the present work, we experimentally investigated the nucleation and growth kinetics of methane hydrate in the presence of aqueous methanol solutions at alcohol concentrations of 0, 1, 2.5, 5, 10, and 20 mass %. It was found that the addition of methanol statistically significantly reduces the supercooling of the methane hydrate onset ΔTo even at low concentrations. The value of ΔTo decreases bya factor of 5 when transitioning from water to 20 mass% methanol. We have observed that as the alcohol content increases, there isa correlation with an increase in the amount of hydrate at the end of the cooling stage. Adding methanol to water also increases the rate of methane hydrate growth. Thus, our experimental data indicate the role of methanol as a kinetic promoter of methane hydrate nucleation and growth, and the dual nature of methanol which is also a thermodynamic hydrate inhibitor.

New insights into methane hydrate inhibition with blends of vinyl lactam polymer and methanol, monoethylene glycol, or diethylene glycol as hybrid inhibitors

Gas hydrate inhibition is a relevant topic of flow assurance. Thermodynamic hydrate inhibitors (THIs) and kinetic hydrate inhibitors (KHIs) are applied to prevent gas hydrates formation in oil and gas production facilities. We have carried out extensive experimental studies to further develop the hybrid inhibition strategy involving KHI/THI formulations. We determined methane hydrate equilibrium conditions for an aqueous solution of methanol, MEG, and DEG (≤50mass%). Subsequently, we studied the kinetics of methane hydrate nucleation and growth for an aqueous solution of commercial vinyl lactam KHI and its blends with alcohols. KHI loses its ability to inhibit hydrate nucleation at ≥ 20 mass%methanol. Adding MeOH to KHI solution also accelerated the growth of methane hydrate. The addition of glycols to KHI increases the hydrate onset subcooling. KHI/MEG blends are optimal for hybrid inhibition as they demonstrate the enhanced delay of gas uptake and reduced rate of methane hydrate growth.

Machine learning models for fast selection of amino acids as green thermodynamic inhibitors for natural gas hydrate

Natural amino acids are non-toxic thermodynamic hydrate inhibitors without negative environmental impact, but it is difficult to accurately select the appropriate amino acid as a quick response to the operational conditions changes in the natural gas pipeline. The objective of this study was to develop mathematical models to predict the hydrate formation temperature (HFT) in presence of amino acids, capture the relationship between amino acid structure properties and their hydrate inhibition strength, and determine the optimal type and concentration to use. The HFT prediction was evaluated using multiple linear regression (MLR) and three machine learning methods including random forest (RF), M5 Rule (M5R) and support vector machine (SVM). After parameter optimization using the trial-and-error method, the coefficient of determination (R2) of the four models were 0.9328, 0.9793, 0.9795 and 0.9980, respectively. The SVM prediction of HFT outperformed other models as the root mean square error (RMSE) was 83%, 76% and 69% lower than that of the MLR, RF and M5R, respectively. Results also demonstrated that the relative importance of the amino acid concentration to the hydrate phase equilibrium was 5-fold higher than that of the intrinsic properties of the amino acid molecular. The SVM model proposed in this study served an easy-to-use tool for reliable prediction of HFT by just providing a new set of input data. This made it possible to accurately determine the minimum concentration of amino acids to be used during the gas pipeline transportation.

Towards Gas Hydrate-Free Pipelines: A Comprehensive Review of Gas Hydrate Inhibition Techniques

Gas hydrate blockage is a major issue that the production and transportation processes in the oil/gas industry faces. The formation of gas hydrates in pipelines results in significant financial losses and serious safety risks. To tackle the flow assurance issues caused by gas hydrate formation in the pipelines, some physical methods and chemical inhibitors are applied by the oil/gas industry. The physical techniques involve subjecting the gas hydrates to thermal heating and depressurization. The alternative method, on the other hand, relies on injecting chemical inhibitors into the pipelines, which affects gas hydrate formation. Chemical inhibitors are classified into high dosage hydrate inhibitors (thermodynamic hydrate inhibitors (THI)) and low dosage hydrate inhibitors (kinetic hydrate inhibitors (KHI) and anti-agglomerates (AAs)). Each chemical inhibitor affects the gas hydrate from a different perspective. The use of physical techniques (thermal heating and depressurization) to inhibit hydrate formation is studied briefly in this review paper. Furthermore, the application of various THIs (alcohols and electrolytes), KHIs (polymeric compounds), and dual function hydrate inhibitors (amino acids, ionic liquids, and nanoparticles) are discussed thoroughly in this study. This review paper aims to provide a complete and comprehensive outlook on the fundamental principles of gas hydrates, and the recent mitigation techniques used by the oil/gas industry to tackle the gas hydrate formation issue. It hopes to provide the chemical engineering platform with ultimate and effective techniques for gas hydrate inhibition.

Experimental investigation and modelling of synergistic thermodynamic inhibition of Diethylene Glycol and glycine mixture on CO2 gas hydrates

In this experimental and modelling study, Diethylene glycol (DEG) and Glycine (Gly) mixtures are introduced to hinder carbon dioxide hydrate formation by pushing the phase boundaries on the lower temperature side. The mixture of DEG and Gly with the ratio of 1:1 is experimented at 15, 10, and 5 wt% concentrations and the pressure vary from 2.5 to 4.0 MPa. The T-cycle method is employed to assess the effect of the studied blends on the CO2 hydrate by evaluating the hydrate dissociation temperature. Varied compositions of pure DEG and Gly as well as their mixtures are used to compute the synergistic effect. The studied system's thermodynamic hydrate inhibition (THI) influence is a concentration-driven phenomenon. Higher concentration can shift the hydrate liquid vapor equilibrium (HLVE) curve to lower temperatures and high-pressure regions. The outcomes depict that mixture of DEG and Gly at 15 wt%. Shows comparatively better results than the mixtures at 5 and 10 wt%, respectively. The obtained 10 wt% mixture results have also been compared with the conventional hydrate inhibitors and other THIs systems and provide a significant hydrate average suppression (ΔT) of 2.4 K. Furthermore, the freezing point-based Dickens and Quint Hunt model was also applied to predict the HLVE data of CO2 hydrates and satisfactory agreement found with maximum mean absolute error (MAE) of 0.498 K. A better inhibitory performance was seen when diethylene glycol and glycine were combined, demonstrating the potential of amino acids as synergistic inhibitors in the exploitation of hydrates, transportation of oil and gas, and flow assurance.

Effect of ammonium hydroxide-based ionic liquids' freezing point and hydrogen bonding on suppression temperature of different gas hydrates

The study presents the effect of freezing point depression and hydrogen bonding energy interaction on four ammonium hydroxide-based ionic liquids (AHILs) of gas hydrate systems. The AHILs investigated are tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetrabutylammonium hydroxide. The considered hydrate system includes methane (CH4), carbon dioxide (CO2), and three binary mixed gas hydrates (70-30 CO2 + CH4, 50-50 CO2 + CH4, 30–70 CO2 + CH4), which are often encountered in the flow assurance pipelines. The experimental temperature range is between 274.0 and 285.0 K, corresponding to pipeline pressures for different gas systems. The thermodynamic influence, i.e., average suppression temperature (ΔŦ) of the studied system, was reported for different mass concentrations (1, 5, and 10 wt%) and correlated with the freezing point depression and hydrogen bonding energy interaction of AHILs. The study also covers the structural impact of AHILs (in the form of alkyl chain variation) on the thermodynamic hydrate inhibition (THI) behaviour via freezing point and hydrogen bonding energy interactions. Findings revealed that the increased alkyl chain length of AHILs reduced the ΔŦ due to a decrease in hydrogen bonding ability. The highest THI inhibition (ΔŦ = 2.27 K) is attained from the lower alkyl chain AHIL, i.e., TMAOH (10 wt%) for the CO2 hydrate system. The freezing point depression of AHILs is a concentration-dependent phenomenon. Increased concentration of the AHILs in the system yielded lower freezing point temperature, positively influencing hydrate mitigation. Although the study provided the initial insight between the freezing point tendency and hydrogen bonding energies of AHILs on thermodynamic inhibition (ΔŦ). Based on the freezing point depression and hydrogen bonding energy interaction, a more generalized correlation should be developed to predict any potential ionic liquids regarded as promising hydrate inhibitors.

Investigation the partitioning amount while the simultaneous use of thermodynamic and kinetic hydrate inhibitors by UV-spectrometery analysis

In this paper the amount of partitioning while the simultaneous use of THIS and a KHI inhibitors is examined and, results were investigated. The effects of adding oil, alcohols and salts on the efficiency of kinetic inhibitors were investigated using UV spectrometry and Beer-Lamberdt law. Samples of the mentioned inhibitors were prepared using a kinetic inhibitor (Poly Vinyl Pyrrolidone) and then spectrometry was conducted. Results indicated positive impacts whenever salt was added to the solution; since salt addition caused stronger bonds between kinetic inhibitor and water molecules. It was found that between salts NaNO3 with 70% had the highest and KCl with 39% had the lowest percentage of partitioning. Addition of oil showed fewer negative impacts. Bangestan oil with 2% of partitioning had the least negative effect and n-heptane with 7% had the most negative effect. Normal butanol (16%), acetone(11%) and ethylene glycol with 18% did not have major negative influences.

Dual functionality of ultralow levels of a model kinetic hydrate inhibitor on hydrate particle morphology and interparticle force

Gas hydrate plug formation is a major concern in oil and gas exploitation efforts, wherein line blockages can pose major safety, economic, and environmental risks. Kinetic hydrate inhibitors (KHIs) are a promising class of hydrate management chemicals, which are potentially cleaner, cheaper, and greener than traditional thermodynamic hydrate inhibitors (THIs). Therefore, understanding the effects that KHIs have on hydrate particles is vital to their application. In this study, polyvinylpyrrolidone (PVP), a model KHI, was investigated at ultralow concentrations to determine its effect on the properties of hydrates and elucidate when nucleation and growth inhibition begins. It was found that PVP can adsorb at the hydrate particle surface to reduce interparticle force by 40–54 %. Low concentration PVP continues to affect interparticle forces at prolonged contact times, reducing forces at 30-minutes to 1-hour of contact by 20–40 % and reducing sintering rate. PVP also reduces film growth rates by 30–50 % depending on the concentration of PVP in the water phase. The onset of major nucleation and growth effects was observed to occur at 0.01 wt% PVP in the water phase, two orders of magnitude below concentrations typically employed in hydrate management. It was discovered that low dosage PVP can cause major morphological changes to the hydrate particles in both the short and long term, which can influence interparticle forces and particle agglomeration, and may serve as a morphological screening tool for KHIs. A proposed mechanism for the observed morphology changes explains how the heterogenous adsorption of chemicals at the particle surface can lead directly to the newly observed particle morphology. The results presented in this paper show that ultralow concentrations of KHIs (0.0005 wt%) can have combined effects on the interfacial activity and crystal growth and morphology of hydrates, showing KHIs to be a dual function inhibitor of both interparticle interactions and hydrate growth. These results can inform KHI applications from industrial flow assurance to carbon dioxide transport for unimpeded carbon capture and sequestration.

Thermodynamic evaluation of inhibitors for methane-hydrate formation

The present paper presents an experimental study on the thermodynamic effect of three amines including (3-Aminopropyl)triethoxysilane (APTES), (3-Aminopropyl)triethoxysilane (APTMS), and Triethylenetetramine (TETA) on methane hydrates. Hydrate-Liquid-Vapour Equilibrium (HLwVE) data for methane hydrate were evaluated in the presence of 7 wt% and 20 wt% APTES, APTMS, and TETA, and with pressure ranging from 2.0 to 9.0 MPa. The thermodynamic effect of these amines on methane-hydrate formation has been investigated via measuring the methane-hydrate dissociation temperature, in applying an isochoric pressure-search method. The results reveal that APTES, APTMS, and TETA effectively shift the hydrate equilibrium curve to higher-pressure and lower-temperature regions for the methane + amine + water system. All of these three show thermodynamic-inhibition effects similar to the lower-concentration case (7 wt%), but with increasing concentration to 20 wt% for the thermodynamic-inhibition effect in the order APTES > APTMS > TETA. The average hydrate-suppression temperature (ΔT), as a ‘signature’ of thermodynamic-inhibition effect on methane hydrates measured for 7 wt% APTES, APTMS and TETA was 1.5, 1.5, and 1.3 K, respectively. ΔT increased with increasing amine concentration; these values were 5, 4.3, and 3.2 for 20 wt% APTES, APTMS, and TETA, respectively. Hydrate-onset time was further delayed due to the synergistic inhibition effect of inhibitors. The results show that these amines have a kinetic effect on gas-hydrate formation, such as for methane-hydrate formation in presence of APTMS: here, the hydrate formed after several cycles with the help of the memory effect. Moreover, the inhibition impact of these amines was compared with conventional hydrate thermodynamics, and found to be in the range of some and similar to ethylene glycol at 5 wt%. Furthermore, the addition of 5 wt% APTMS to sea salt not only enhances the thermodynamic inhibition effect, but also inhibits kinetically the formation of methane hydrate. For modelling of methane-hydrate dissociation conditions, the solid-solution theory of van der Waals and Platteeuw was applied for the hydrate phase along with the Peng–Robinson equation of state for the gas phase. Taken together, this study elucidates a novel class of thermodynamic hydrate inhibitors, which can control methane-hydrate formation efficiently.

Atomistic insights into the performance of thermodynamic inhibitors in the nucleation of methane hydrate

Figure TOC Comparison of methane hydrate nucleation process (a) without or (b) with EG molecules. Critical nucleus size increase under the influence of Thermodynamic hydrate inhibitors. • Hydrophobic groups from the alcohols could approach the initial hydrate cages and destroy initial clusters. • The –OH groups were observed to form hydrogen bonds with the cage clusters and even initiate a new hydrate cage. • Alcohols can increase the critical nucleus size of gas hydrate. Thermodynamic inhibitors are intensively used in oil and gas pipelines to modify the formation conditions of gas hydrate, preventing the blockage of pipelines. However, their kinetic performance in hydrate nucleation is largely unclear. Herein, the molecular interactions of the small molecules of ethylene glycol and methanol with hydrate clusters were investigated. It was found that the hydrophobic groups from the glycols could approach the initial hydrate cages, thereby constricting the regional distribution of adjacent water molecules; thus, they functioned in the same way as the guest molecules that transiently helped to stabilize the water framework. However, these glycols would eventually dissolve into the solution to stay at the water/nanobubble interface; this would consequently destroy the initial cages upon losing the constraint from the surrounding hydrophobic molecules. In addition, the –OH groups were observed to form hydrogen bonds with the cage clusters and even initiate a new hydrate cage; however, they cannot stably reside there due to different atom coordination. Notably, these glycols can increase the critical nucleus size, which is considered the intrinsic mechanism of thermodynamic inhibition. Our results provide atomistic insights into the performance of glycols in the kinetic nucleation of gas hydrates and can help understand the interactions between guest and host molecules in the presence of hydrophobic and hydrophilic functional groups.

Ionic liquids for the inhibition of gas hydrates. A review

The formation of gas hydrates is a major issue during the operation of oil and gas pipelines, because gas hydrates cause plugging, thereby disrupting the normal oil and gas flows. A solution is to inject gas hydrate inhibitors such as ionic liquids. Contrary to classical inhibitors, ionic liquids act both as thermodynamic inhibitors and hydrate inhibitors, and as anti-agglomerates. Imidazolium-based ionic liquids have been found efficient for the inhibition of CO2 and CH4 hydrates. For CO2 gas hydrates, N-ethyl-N-methylmorpholinium bromide showed an average depression temperature of 1.72 K at 10 wt% concentration. The induction time of 1-ethyl-3-methyl imidazolium bromide is 36.3 h for CO2 hydrates at 1 wt% concentration. For CH4 hydrates, 1-ethyl-3-methyl-imidazolium chloride showed average depression temperature of 4.80 K at 40 wt%. For mixed gas hydrates of CO2 and CH4, only quaternary ammonium salts have been studied. Tetramethyl ammonium hydroxide shifted the hydrate liquid vapour equilibrium to 1.56 K at 10 wt%, while tetrabutylammonium hydroxide showed an induction time of 0.74 h at 1 wt% concentration.

Amino acid-based ionic liquids as dual kinetic-thermodynamic methane hydrate inhibitor

Three amino acid-based ionic liquids (AAILs) are synthesized to evaluate their performance as inhibitors. They are 1-ethyl-3-methyl-imidazolium-glutamate (EMIMGlu), 1-(3-cyanopropyl)-3-methyl-imidazolium-glutamate (CPMIMGlu) and 1-butyl-3-methyl-imidazolium-glutamate (BMIMGlu). The structures are clarified using Nuclear Magnetic Resonance. Evaluation of their methane hydrate inhibitor performance is performed by micro-differential scanning calorimeter at 5–15 MPa. As a baseline, the hydrate dissociation in water is also evaluated. A standard correlation of methane hydrate dissociation in water is successfully developed with a low average absolute error. Additionally, the AAILs behave as both thermodynamic (THI) and kinetic (KHI) hydrate inhibitors. They simultaneously shift the HLVE curve to a lower temperature and decelerate the hydrate formation by reducing the hydrate nucleation rate. EMIMGlu shows the highest THI performance by producing an average temperature shift of 1.14 K, followed by CPMIMGlu (0.91 K) and BMIMGlu (0.87 K). Furthermore, the addition of the nitrile group in CPMIMGlu IL has enhanced the kinetic inhibition process. The kinetic inhibition performance represented by the relative inhibition power (RIP) decreases in the trend of CPMIMGlu (1.31), EMIMGlu (1.30) and BMIMGlu (0.063). The mechanism of the inhibition is further studied by utilizing COSMO-RS software through σ-profile and σ-potential to understand the inhibition process at the molecular level. The experimental results and computational studies reveal that AAILs behave as THI and KHI through the existence of four oxygen atoms in their anions and cyano group in the CPMIM cation. Thermodynamic inhibition properties of AAILs are found to be influenced by the polarity of AAILs while the kinetic inhibition properties of AAILs are found to be influenced by the hydrogen-bonding acceptor value of the AAILs.