Hydrate Dissociation Temperature Research Articles

On the phase behaviors of CH4-CO2 binary clathrate hydrates: Equilibrium with aqueous phase.

We explore the solubilities of guest CH4 and/or CO2 in the aqueous state coexisting with the corresponding hydrate. The equilibrium conditions are estimated by calculating the chemical potentials of water and guest species in the hydrate on the basis of a statistical mechanical theory using pairwise intermolecular potentials. This requires the least computational cost while covering a wide range of temperature, pressure, and composition of guest species, even for the binary hydrate. The nonstoichiometric nature, one of the most important characters of hydrates, is invariably taken into account when evaluating its phase behaviors and the driving force for nucleation of hydrates. The two-phase equilibrium concerning CO2 hydrate is evaluated considering a low but finite value of CO2 solubility in water. It is found that the finite solubility gives rise to a small systematic deviation of the dissociation temperature of CO2 hydrate. The solubility of CO2 coexisting with fluid CO2 decreases with temperature but the opposite temperature dependence is obtained in the presence of hydrate, as in the case of CH4. This method is applied to CH4-CO2 binary hydrates of various guest compositions. We also find a significant difference in composition of guests among the phases involved in the equilibria.

Dissociation temperature of gas hydrates through isenthalpic-isobaric molecular dynamics simulations.

Molecular simulations are a powerful tool to understand phenomena and obtain properties of gas hydrate systems. The direct coexistence method (DCM) in the NVT or NPT ensembles, the most commonly used method to determine hydrate dissociation temperatures, can be computationally expensive due to the need for several long simulations. Through an extensive set of simulations, we report here the details of the DCM within the NPH (isobaric-isenthalpic) ensemble, which require fewer and shorter trajectories. The dissociation pressure of methane hydrates is obtained for pressures of 4, 8, 15, 30, and 50 MPa. The values are in agreement with other literature simulations and experimental data. The results are further validated with the calculation of the enthalpy of dissociation, with a value of 50kJ/mol of methane, also in agreement with the literature. The complexity of a multiphase and multicomponent system presents challenges lacking in simpler water/ice systems. These are found to be dependent on energy conservation. The optimal set of parameters to achieve it is also reported, including a smaller time step and the use of double precision, along with an analysis of some factors that could affect the convergence of the method. Although these parameters require more computational cost, the NPH ensemble is successful in providing the dissociation temperature of gas hydrates in fewer simulations than other ensembles and with productions lasting only 500ns.

Thermodynamic modelling of gas hydrate dissociation conditions in porous medium in the presence of NaCl/methanol aqueous solution

AbstractDue to the growing significance of the existence of gas hydrates in natural media like the ocean floor/permafrost regions and the extraction of natural gas from hydrate reservoirs using thermodynamic hydrate inhibitors, investigating the dissociation of gas hydrates in porous media in the presence of inhibitors is crucial. This work examines a broad range of laboratory data on the dissociation conditions of gas hydrates in the porous mediums when salt/alcohol aqueous solutions are present. The temperature of gas hydrate dissociation in the presence of pure water is calculated using the van der Waals–Platteeuw solid solution theory. The water activity in the porous medium is then calculated by taking into account a number of variables, including the radius of the porous medium, molar volume, shape factor, wetting angle, and surface tension. The Pitzer and Margules activity coefficient models are used to determine the water activity in the presence of salt and alcohol, respectively. Lastly, the gas hydrate dissociation temperature in a porous medium in the presence of salt and/or alcohol aqueous solution is determined by combining Piereon's model with an enthalpy‐based correlation that was proposed by Azimi et al. The selected package can consistently correlate the gas hydrate dissociation conditions in a porous medium in the presence of alcohol or salt aqueous solution. The average absolute deviation (AAD) of 0.67 K for the whole data bank (90 experimental data points) shows the precision of the model.

Dissociation line and driving force for nucleation of the nitrogen hydrate from computer simulation. II. Effect of multiple occupancy.

In this work, we determine the dissociation line of the nitrogen (N2) hydrate by computer simulation using the TIP4P/Ice model for water and the TraPPE force field for N2. This work is the natural extension of Paper I, in which the dissociation temperature of the N2 hydrate has been obtained at 500, 1000, and 1500bar [Algaba et al., J. Chem. Phys. 159, 224707 (2023)] using the solubility method and assuming single occupancy. We extend our previous study and determine the dissociation temperature of the N2 hydrate at different pressures, from 500 to 4500bar, taking into account the single and double occupancy of the N2 molecules in the hydrate structure. We calculate the solubility of N2 in the aqueous solution as a function of temperature when it is in contact with a N2-rich liquid phase and when in contact with the hydrate phase with single and double occupancy via planar interfaces. Both curves intersect at a certain temperature that determines the dissociation temperature at a given pressure. We observe a negligible effect of occupancy on the dissociation temperature. Our findings are in very good agreement with the experimental data taken from the literature. We have also obtained the driving force for the nucleation of the hydrate as a function of temperature and occupancy at several pressures. As in the case of the dissociation line, the effect of occupancy on the driving force for nucleation is negligible. To the best of our knowledge, this is the first time that the effect of the occupancy on the driving force for nucleation of a hydrate that exhibits sII crystallographic structure is studied from computer simulation.

Three-phase equilibria of hydrates from computer simulation. III. Effect of dispersive interactions in the methane and carbon dioxide hydrates.

In this work, the effect of the range of dispersive interactions in determining the three-phase coexistence line of the CO2 and CH4 hydrates has been studied. In particular, the temperature (T3) at which solid hydrate, water, and liquid CO2/gas CH4 coexist has been determined through molecular dynamics simulations using different cutoff values (from 0.9 to 1.6nm) for dispersive interactions. The T3 of both hydrates has been determined using the direct coexistence simulation technique. Following this method, the three phases in equilibrium are put together in the same simulation box, the pressure is fixed, and simulations are performed at different temperatures T. If the hydrate melts, then T > T3. Conversely, if the hydrate grows, then T < T3. The effect of the cutoff distance on the dissociation temperature has been analyzed at three different pressures for CO2 hydrate: 100, 400, and 1000bar. Then, we have changed the guest and studied the effect of the cutoff distance on the dissociation temperature of the CH4 hydrate at 400bar. Moreover, the effect of long-range corrections for dispersive interactions has been analyzed by running simulations with homo- and inhomogeneous corrections and a cutoff value of 0.9nm. The results obtained in this work highlight that the cutoff distance for the dispersive interactions affects the stability conditions of these hydrates. This effect is enhanced when the pressure is decreased, displacing the T3 about 2-4K depending on the system and the pressure.

Experimental measurement and thermodynamic modeling of hydrate stability conditions in the methane + cyclopentane + tetra-n‑butyl ammonium chloride (TBAC) aqueous solution system

The present study investigates hydrate dissociation conditions in methane + cyclopentane (CP) and tetra-n‑butyl ammonium chloride (TBAC) aqueous solution. The experimental work was done using an isochoric vessel with varying compositions of the promoters in the pressure and temperature ranges of (1.42 to 4.90) MPa and (291.2 to 298.9) K, respectively. The methane hydrate dissociation pressure was predicted by the Gibbs minimization method. The findings denote that adding CP to the TBAC-containing system shifts the hydrate dissociation temperature of methane to higher values. The results also show an average increase of 12, 9, 5, and 3 K in hydrate dissociation temperature for 5, 10, 20, and 30 wt% TBAC in aqueous solution within the pressures. These results can be caused by the existence of more dodecahedral cages that are inhabited by methane. Thus, it makes hydrates more stable and increases the temperature of hydrate dissociation. On the other hand, adding TBAC to CP containing system has an inhibitor effect. The methane hydrate dissociation temperature was not changed sensibly when a 5 wt% TBAC was added to CP containing system. When the addition of the TBAC weight percentage to CP containing system is increased to 10, the average hydrate dissociation temperature decreases by 1 K within the pressures. When adding 20 and 30 wt% TBAC to CP containing system an average decrease of 2 K and 3 K in the hydrate dissociation temperature is resulted, respectively, within the pressures. The reduction in the hydrate dissociation temperatures can be attributed to salting-out and ion clustering. The experimental data and modeling results for the hydrate dissociation pressures exhibit an AARD of 2.4 %.

Prediction of Optimum Hydrate Inhibitors Mass Percent for Hydrate Formation Prevention

The production tubing and pipeline have the critical task of transporting economically significant fluids. As a result, they must be meticulously designed to manage potential issues like gas hydrate formation that can lead to blockage of the pipeline. In this study, the OLGA dynamic multiphase simulator and Multiflash were employed to determine the optimum mass percentages of thermodynamic inhibitors- namely, methanol, monoethylene glycol (MEG), diethylene glycol (DEG) and triethylene glycol (TEG) that are necessary to prevent hydrate formation within the pipeline. The pipeline model was developed using OLGA and Multiflash to characterised the fluid properties and generate the requisite input files (hydrate and PVT table files) for dynamic multiphase simulation with OLGA. Sensitivity analyses were performed to determine the optimum mass percent of methanol, MEG, DEG, and TEG required to suppress hydrate formation in the pipeline. Different mass percent of 10%, 20% 30%, 40%, 50%, 60%, 70%, 80% and 90% of the four inhibitors were introduced into the pipeline to evaluate their impact on hydrate volume fractions and locations along the pipeline at where hydrate attains thermodynamic stability. Addition of the inhibitors was stopped when the system exhibited conditions of no hydrate volume fraction. Result shows that the optimum mass percent for complete hydrate inhibition was 60% for methanol, 45% for MEG, and 70% for TEG inhibitors in the pipeline. DEG showed reduced hydrate fraction but did not inhibit formation effectively even at 90%. It was observed that as the inhibitor mass percent increases, the hydrate dissociation temperature (the temperature below which the hydrate will form) is reduced for a given pressure. Consequently, Monoethylene glycol (MEG) with an optimal mass percentage of 45% was the most efficient inhibitor for curbing hydrate formation within the given pipeline, surpassing the performance of other inhibitors and offering potential cost saving.

Gas Production Potential in Geothermal-Energy-Enhanced CH4–CO2 Swapping Processes

Carbon capture and storage has become a practice to reduce the greenhouse effect of carbon dioxide (CO2) on the global climate. Recent studies have generated increasing concerns about CO2 leakage from underground structures. This has called for more research on CH4–CO2 swapping in natural gas hydrate (NGH) reservoirs to lock CO2 in a solid state in underground structures. Because the CH4–CO2 swapping is too slow to be efficient, this study proposes to use geothermal energy to accelerate the process. This paper presents a technical feasibility analysis of using geothermal energy to assist CH4–CO2 swapping for simultaneously storing CO2 in NGH reservoirs and producing the dissociated natural gas. Mathematical models were developed to compute heat transfer from geothermal zones to NGH reservoirs. A case study was carried out using the data from an NGH reservoir in the Shenhu area, Northern South China Sea. The result of the case study indicates that heat conduction dictates the heat transfer process when the heat convection flow rate is less than 0.01 m3/s over a heat-releasing borehole length of 2,000 m. Heat convection can significantly accelerate the heat transfer inside the gas hydrate reservoir. The 15°C (designed gas hydrate dissociation temperature in the studied case) heat front will propagate to the upper and lower boundaries of the gas hydrate reservoir (39 ft or 12 m) in 220 days by heat conduction only. This time can be shortened to 140 days with the aid of a fluid convection rate of 0.005 m3/s. Geothermal heating can significantly increase the initial productivity of wells in heated gas hydrate reservoirs in CO2 swapping processes. When the gas hydrate reservoir is heated from 6 to 16°C, the fold of increase is expected to exceed five in the studied case. This study shows that CH4–CO2 swapping process using geothermal stimulation is a promising method for producing natural gas and locking CO2 permanently in NGH reservoirs. Further studies should first focus on investigations of the effect of CO2-hydrate formation on the CO2 mass transfer inside reservoirs.

Influence of Injecting Some Chemicals on MEG Inhibition Performance for Less Recycled MEG

Monoethylene glycol (MEG) is often injected into submarine transportation pipelines, from offshore oil and gas platforms, to suppress hydrate formation. MEG can be recovered, for recycling, through a MEG regeneration plant, onshore. The costs related to hydrate inhibition operations are significant; thus, introducing new methods of reducing the volumes of MEG required by the system potentially will provide significant economic benefits for the operation. The objectives of this study were to evaluate the influence of ammonium hydroxide (NH4OH), sodium hydroxide (NaOH), potassium hydroxide (KOH), and methyl diethanolamine (MDEA) on inhibition performance when separately combined with a MEG 20 wt % solution in a methane gas system. The pH levels applied were 10.96, 10.84, 10.62, and 10.07 for NaOH, KOH, NH4OH, and MDEA, respectively. The results indicated that these compounds suppressed the hydrate dissociation temperatures, to different degrees, when compared with a normal MEG 20 wt % solution. The temperatures were reduced by an average, across the range of pressures, of 0.72, 0.78, and 0.94 °C in the case of MDEA, KOH, and NaOH, respectively. However, when combined with NH4OH, the reduction in dissociation temperature was even more significant, averaging 1.68 °C. An economical assessment of the benefits of reducing the inhibition temperature by this amount, through the combination with NH4OH, indicated an operational cost saving of about 17% within the MEG regeneration process. This level of cost reduction will be of significant interest to the offshore gas-producing industry.

Dual inhibition effect of reline deep eutectic solvent on methane hydrate nucleation and formation

The search for environmentally friendly hydrate inhibitors is indispensable for flow assurance to prevent hydrate formation in gas or oil pipeline. Deep eutectic solvents (DESs) are classified as a new form of green solvents due to their non-toxic and biodegradable properties. In this work, the dissociation conditions of methane hydrates influenced by reline DES, urea, and choline chloride (ChCl) were experimentally determined. Reline is derived from urea and ChCl in a 2-to-1 M ratio. The isochoric method was applied to evaluate the hydrate-liquid water–vapor-three-phase-coexisting phase boundary for ChCl (3.09 and 5.89 wt%), urea (2.58 and 5.11 wt%), and reline (5.58 and 10.43 wt%) aqueous solutions in the temperature ranging from 272.7 to 287.7 K and the pressure ranging from 2.96 to 12.78 MPa. All these three additives consistently demonstrated an inhibitory effect on methane hydrate formation. At 3.07 MPa, the dissociation temperature of methane hydrate in the presence of reline (10.43 wt%) is lower than that of the pure water system by 2.2 K. For the kinetic studies, a high-pressure micro-differential scanning calorimeter was employed for the measurement of the induction time of hydrate formation. Compared to pure water, all the studied samples demonstrated the potential to prolong the induction time of hydrate nucleation. The reline could strongly hinder the nucleation of hydrate crystals for more than 48 h. The reline DES is identified to act as both kinetic and thermodynamic inhibitors, exhibiting a dual inhibition effect on nucleation and formation of methane hydrates.

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.

Experimental and modeling investigations of hydrate phase equilibria in natural clayey-silty sediments

Rapid and accurate acquisition of hydrate phase equilibria in sediments is crucial for gas hydrate resource assessment and exploitation and hydrate-based technological applications. However, current models have significant limitations and deficiencies in predicting hydrate phase equilibria in sediments, especially clayey-silty sediments prevalent in nature. To this end, a unified hydrate phase equilibrium model was developed, which considers the combined inhibitions of surface adsorption, capillary effect, and salt solution. It is derived that water content and salinity can be used as direct input parameters to the model to estimate the hydrate dissociation temperature depression. Using the stepwise heating method, the hydrate dissociation conditions in various natural sediments were measured. It suggests that montmorillonite has the greatest dissociation temperature depression, followed by SA sediments and kaolin, while pore hydrates in silts exhibit almost the same thermodynamic equilibrium conditions as bulk hydrates unless the water content is below 15%. Special attention was also paid to the additional inhibition of surface adsorption on hydrate phase equilibria in sediments with salt solutions. This inhibition was found to be quite pronounced in clay-rich sediments, especially expansive clays, but essentially negligible in silty or sandy sediments. The maximum average error of 3.29% was observed between model predictions and measured data. Additionally, the amounts of clathrated water (CW), non-equilibrium unclathrated water (NEUW), and equilibrium unclathrated water (EUW) in sediments after the end of hydrate formation were determined using the model. It reveals that CW has the highest proportion in silts, reaching 68.9%, while only 0.231 in montmorillonite; in contrast, EUW is dominant in montmorillonite; NEUW seems to be independent of lithology but shows a strong dependence on the water content. It implies that silty sediments are more conductive to hydrate-based gas storage and CO2 sequestration than clayey sediments. This study provides some practical insights and implications for hydrate phase behaviors in natural sediments and can potentially be employed as a tool for phase equilibrium prediction with universality and accuracy.

Rheology of Cyclopentane sII Hydrate Slurry in Water-in-Model Oil Emulsions: Effect of Surfactant Concentration and Water Droplet Size Relevant for Flow Assurance

Investigation on the effect of surfactant concentration and droplet size distribution in a water-in-oil emulsion system on the hydrate suspension rheology is vital to delineate the antiagglomeration role and formulate a suitable mitigation strategy for flow assurance issues. Though several studies have been reported on the effect of surfactants on hydrate kinetics, rheological characterization of hydrate systems with surfactants and their role as antiagglomerant are scarce. In this study, rheological measurements are carried out to understand the effect of a mixed surfactant system (Span80 and Tween80 with HLB value of 5.9) at varying concentrations (0.5, 1, 1.5, 2, and 3 wt %) on the structure sII cyclopentane hydrate slurry formed in a water-in-model oil emulsion with varied droplet size distribution at 265.15 K, atmospheric pressure, and γ̇ = 100 s–1. The model oil consists of light paraffin oil and toluene to represent the saturate and aromatic fractions of the crude oil. Viscosity profiles are reported during formation and dissociation of hydrate slurries, while the flow curve and viscoelasticity tests are reported for the stable hydrate slurries. The presence of a mixed surfactant system is observed to increase the maximum peak and steady-state viscosities of the hydrate slurries up to 1.5 wt % surfactant concentration due to an enhanced oil–water interface and the water bridging between hydrate particles and droplets. At higher surfactant concentration (>1.5 to 3 wt %), capillary bridge between hydrate particles in contact is diminished due to steric hindrance provided by the hydrophobic group of the surfactant adsorbed on the hydrate surface, thus reducing the hydrate slurry viscosity and acting as antiagglomerant. At higher surfactant concentration, the yield stress reduction is observed in the range of ∼44–87% from the base value. The dissociation experiments show a small depression in the hydrate dissociation temperature from 0.5 wt % to 2 wt %. For 3 wt %, the higher dissociation temperature could be due to metastable hydrate formation in addition to the surfactant molecules surrounding the hydrate crystals, delaying their dissociation.

Experimental determination of dissociation temperatures and enthalpies of methane, ethane and carbon dioxide hydrates up to 90 MPa by using a multicycle calorimetric procedure

This work presents the application of a multicycle procedure to determine the dissociation temperature and enthalpy of gas hydrates using high-pressure microcalorimetry (HP-µDSC). In the multicycle procedure, a sample of water in contact with the gas undergoes successive cooling and warming cycles below hydrate dissociation temperature, increasing the conversion of water into hydrate. This technique has been previously used in literature to increase water conversion during carbon dioxide hydrate formation up to 2.0 MPa, but its applicability has not been assessed for higher pressures and with different guest molecules. New experimental equilibrium data for methane, ethane, and carbon dioxide hydrates were obtained through HP-µDSC up to 90 MPa using the multicycle procedure. The advantages and limitations of the method are discussed. Dissociation temperatures are in good agreement with data previously obtained from a HP-µDSC standard method, which confirms the reliability to determine this property by both methods. Nevertheless, the enthalpy of hydrate dissociation obtained by the direct integration of thermograms provided by the HP-µDSC standard method is not accurate. Thus, it is usually calculated by using the Clapeyron equation from equilibrium temperature and pressure data obtained by the HP-µDSC standard method, as presented in previous work. The new dissociation enthalpies presented in this work were obtained experimentally by direct integration of thermograms using the multicycle procedure, which provides accurate data as the conversion is very high (above 97% of water converts into hydrate), the baselines are clearly established, and no exothermic peak related to metastable phases is observed.

A thermodynamic framework for determination of gas hydrate stability conditions and water activity in ionic liquid aqueous solution

The formation of gas hydrates in pipelines and plugging of the gas flow path are a major cause of operating expenses, safety issues, pressure drop, and fatal accidents. Therefore, the inhibition of gas hydrate formation is of paramount importance. The utilization of a new class of inhibitors such as ionic liquids (ILs) is currently of great interest. In this regard, adjusting water activity in the inhibitor blend (water + IL) is a vital factor. Several models have been developed to calculate the activity of water in the presence of IL(s). The major disadvantage of these models is their correlative basis. This study aimed to propose a rigorous predictive model for the calculation of water activity in the presence of IL(s), which would then be applied in the prediction of the gas hydrate stability conditions. The model is made up of a molecular term (the short-range interactions from Free-Volume-Flory-Huggins (FVFH) activity model) as well as a contribution from ionic interactions as a result of IL(s) dissociation in water (the long-range electrostatic interactions from the extended Debye–Hückel (EDH) model). The overall absolute temperature deviation and the average absolute relative deviation percent in the calculated gas (methane and carbon dioxide) hydrate dissociation temperatures for the whole databank (500 data points including 37 ILs) were found to be 0.61 K and 0.22%, respectively. This proves the superiority of the model over the previous correlative-basis ones. Finally, it is concluded that the higher temperature, the higher the IL(s) concentration, and the lower IL(s) molecular weight(s) result in larger model deviations.

Effect of HFE254 or Cyclopentanone on Phase Equilibrium Dissociation Conditions for Carbon Dioxide Hydrate

Hydrate equilibrium conditions of the CO2 + HFE254 or cyclopentanone + water system are determined by the isometric method. The presence of cyclopentanone promotes CO2 hydrate formation, while HFE254 does not affect the equilibrium pressure of CO2 hydrate. The hydrate dissociation temperature of the CO2 + cyclopentanone system increases as the pressure increases. However, the hydrate equilibrium temperature gradually decreases when pressure is above 3.2 MPa and a mixed hydrate of structure I and structure II forms. It can be seen that cyclopentanone is a thermodynamic accelerator that can promote the formation of CO2 hydrate.

Equilibrium Conditions of Methane Hydrates in Seawater From Krishna-Godavari Basin, India

Abstract Immense gas hydrate reservoirs have been reported in the Krishna-Godavari Basin, India. They mostly constitute methane gas and could serve as an alternative energy source. For efficient exploitation of methane from hydrates, it is crucial to know the region's stability conditions. The present study reports the stability and equilibrium conditions of methane hydrates, synthesized with seawater obtained from the Krishna-Godavari Basin. At Station MD161/02/GH, the water samples are collected at depths ranging from 500 to 1,500 m. The influence of salinity on methane hydrate formation and dissociation in the presence of seawater is established. The hydrate dissociation patterns in seawater and saline water (4 wt% NaCl) are similar and follow the phase equilibrium around 6 wt% NaCl. The identical dissociation behavior of the two systems ascertains seawater to have ~4 wt% salinity. The salinity concentration varies little with depth because the hydrate dissociation temperatures are the same for all the samples collected at the three depths. Using the Clausius-Clapeyron equation, dissociation enthalpies are calculated. The dissociation enthalpy in saline systems is about 6% higher. The hydrate growth kinetics is marginally faster in the saline system.

Experimental and modeling studies on enhancing the thermodynamic hydrate inhibition performance of monoethylene glycol via synergistic green material

This paper presents an experimental and modeling studies on the thermodynamic inhibition effects of the mixture of monoethlyene glycol (MEG) and glycine (Gly) on the carbon dioxide hydrate phase boundary condition. The monoethlyene glycol and glycine (1:1) mixture inhibition effects were investigated at concentrations of 5, 10, and 15 wt.% and pressure ranges from 2.0–4.0 MPa. The effects of the proposed mixture on the carbon dioxide hydrate phase boundary were evaluated by measuring the dissociation temperature of carbon dioxide hydrate using a T-cycle method. The synergistic effect was evaluated based on comparison with pure MEG and Gly data. The results show that 15 wt.% of MEG and Gly mixture displays the highest inhibition effect compared to the 5 and 10 wt.% mixtures, respectively. However, the synergistic effect is higher at 10 wt.%. Dickens' model was also adopted to predict the phase equilibrium data of CO2 hydrates in the presence of the mixture. The modified model successfully predicted the data within a maximum error of ± 0.52 K.

Nucleation probability and memory effect of methane-propane mixed gas hydrate

The hydrate memory effect is one of the top puzzles in the field of gas hydrate. We developed a sapphire cell to test the memory effect of hydrate under linear cooling ramp conditions. The methane-propane mixed gas with a mass ratio of 9:1 was used to form hydrate at a constant cooling rate of 0.1 K/min. Influences of superheating, heating duration, and dissociation pressure on the strength and certainty of memory effect were studied. Our experiments show little evidence of the strengthening of the memory effect with the increase of cooling/heating circles. The nucleation probability of C1/C3 mixed gas hydrate would be weakened by excessive superheating temperatures and vanishes once hydrate was forced to dissociate at a superheating temperature of 14 K for 60 min. The memory effect increases linearly with the decrease of hydrate dissociation temperature and dissociation duration. However, the dissociation temperature and duration contribute unequally to the strength of the memory effect. The hydrate nucleation subcooling temperature is found to be more predictable (i.e. less stochastic) at a relatively lower dissociation temperature, whereas the dissociation pressure has little influence on the certainties of the hydrate nucleation subcooling. However, the certainty of the memory effect becomes higher with the increase of dissociation duration, although the strength of the memory effect becomes weaker. The strength and certainty of the memory effect are supposed to be affected by the behaviors of dipoles surrounding MNBs.

Generation of methane gas hydrate equilibrium curve for the thermodynamic gas hydrate inhibitor propylene glycol

Use of thermodynamic hydrate inhibitors (THIs) for prevention of gas hydrate formation in the oil and gas field operations is a well-known and accepted process all over the world. Although glycols are reported as THIs, but only monoethylene glycol is studied in detail and applied conventionally. In this study, gas hydrate inhibition efficiency of propylene glycol is determined. The data is acquired using a stirred cryogenic PVT sapphire cell and the hydrate dissociation temperature measured at isobaric condition (constant pressure) is considered as the thermodynamic equilibrium temperature, because although dissociation temperature depends on hydrate particle size and heat flux, but, it is unlike the random nature and unpredictability of formation temperature. The key objective of this study is to establish the equilibrium condition of methane hydrate in presence of 20 wt% propylene glycol as thermodynamic inhibitor. The identification of dissociation temperature is based on visual method. The gas hydrate equilibrium curve for 20 wt% propylene glycol in methane-water system is established by determining the hydrate equilibrium dissociation temperatures for a pressure range 70–190 bar. Also performance comparison of propylene glycol with monoethylene glycol is done to gauge the inhibition efficiency of propylene glycol, the results of which have shown better inhibition efficiency of propylene glycol. Hydrate equilibrium curve of 20 wt% propylene glycol system showed an average of 10.42 °C deviation towards lower temperature compared to that of the system without the inhibitor. The fitted curve for hydrate dissociation temperatures showed co-efficient of determinants (R2) value ≥ 0.97, suggesting the reliability of the experimental observations.