Solubility of ethylene in aqueous solution of sodium dodecyl sulfate at ambient temperature and near the hydrate formation region

The solubility of ethylene in aqueous solutions of sodium dodecyl sulfate (SDS) at different concentrations was measured at temperature 298.2 K and near the hydrate formation region. The effect of SDS on the gas solubility was studied and the solubilities of ethylene in a single micelle under different conditions were evaluated. It was found that the micelle solubilization was obvious, especially in the region near hydrate formation conditions. The CMC of SDS solution was also evaluated based on the solubility vs SDS concentration curves and it was found that it decreased with decreasing temperature.     

Effect of temperature fluctuation on hydrate-based CO_2 separation from fuel gas

A new method of temperature fluctuation is proposed to promote the process of hydrate-based CO2 separation from fuel gas in this work according to the dual nature of CO2 solubility in hydrate forming and non-hydrate forming regions [1].The temperature fluctuation operated in the process of hydrate formation improves the formation of gas hydrate observably.The amount of the gas consumed with temperature fluctuation is approximately 35% more than that without temperature fluctuation.It is found that only the temperature fluctuation operated in the period of forming hydrate leads to a good effect on CO2 separation.Meanwhile,with the proceeding of hydrate formation,the effect of temperature fluctuation on the gas hydrate gradually reduces,and little effect is left in the completion term.The CO2 separation efficiencies in the separation processes with the effective temperature fluctuations are improved remarkably.     

Synergistic effect of combination of surfactant and oxide powder on enhancement of gas hydrates nucleation

In the present work we studied the induction periods of hydrate formation of natural gas in pure water, aqueous solutions of surfactants, and in the presence of surfactant together with aluminum oxide nanopowder, the activity of which as hydrate formation inducer was studied previously. Sodium dodecyl sulfate(SDS) or neonol AF-9-12 were used as the surfactants. It was demonstrated that the addition of either surfactants or aluminum oxide powder under our experimental conditions causes a decrease in the induction period of hydrate formation from ~105 min for pure water to 30–35 min for water with additives. In the case of the simultaneous presence of surfactants and aluminum oxide powder in the system, induction period decreased to ~20 min. So, the synergistic effect of the combination of surfactant and oxide powder on gas hydrate nucleation was demonstrated. Possible reasons of this effect have been discussed.     

Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation

Storage and transportation of natural gas as gas hydrate (“gas-to-solids technology”) is a promising alternative to the established liquefied natural gas (LNG) or compressed natural gas (CNG) technologies. Gas hydrates offer a relatively high gas storage capacity and mild temperature and pressure conditions for formation. Simulations based on the van der Waals–Platteeuw model and molecular dynamics (MD) are employed in this study to relate the methane gas content/occupancy in different hydrate systems with the hydrate stability conditions including temperature, pressure, and secondary clathrate stabilizing guests. Methane is chosen as a model system for natural gas. It was found that the addition of about 1% propane suffices to increase the structure II (sII) methane hydrate stability without excessively compromising methane storage capacity in hydrate. When tetrahydrofuran (THF) is used as the stabilizing agent in sII hydrate at concentration between 1% and 3%, a reasonably high methane content in hydrate can be maintained (∼85–100, v/v) without dealing with pressures more than 5 MPa and close to room temperature.     

Experimental investigation of methane hydrate decomposition by depressurizing in porous media with 3-Dimension device

In order to simulate the behavior of gas hydrate formation and decomposition, a 3-Dimension experimental device was built, consisting of a high-pressure reactor with an inner diameter of 300 mm, effective height of 100 mm, and operation pressure of 16 MPa. Eight thermal resistances were mounted in the porous media at different depthes and radiuses to detect the temperature distribution during the hydrate formation/decomposition. To collect the pressure, temperature, and flux of gas production data, the Monitor and Control Generated System (MCGS) was used. Using this device, the formation and decomposition behavior of methane hydrate in the 20～40 mesh natural sand with salinity of 3.35 wt% was examined. It was found that the front of formation or decomposition of hydrate can be judged by the temperature distribution. The amount of hydrate formation can also be evaluated by the temperature change. During the hydrate decomposition process, the temperature curves indicated that the hydrate in the top and bottom of reactor dissociated earlier than in the inner. The hydrate decomposition front gradually moved from porous media surface to inner and kept a shape of column form, with different moving speed at different surface position. The proper decomposition pressure was also determined.     

Study of CCl3F hydrate formation and dissociation in W/O emulsion by differential scanning calorimetry and X-ray diffraction

Differential scanning calorimetry (DSC) and time-resolved synchrotron X-ray diffraction as a function of temperature (XRDT) were combined in a novel way in order to study conditions of formation and the amount of gas clathrate formed in dispersed systems. The formation and dissociation of trichlorofluoromethane hydrate CCl₃F·(H₂O)₁₇ in a water-in-oil emulsion were followed by using these combined techniques. An emulsion containing 3 wt.% NaCl was submitted to a cooling and heating cycle between 20 and −50 °C. During cooling, a single exothermic peak at −43 °C, found in DCS thermograms was assigned to the freezing of under-cooled water droplets; however, no noticeable signal related to hydrate crystallisation was detected. Conversely, during subsequent heating, the progressive melting of ice was followed by an endothermic signal indicative of hydrate decomposition. From X-ray diffraction performed on an emulsion sample, it was possible to identify the exact condition of CCl₃F·(H₂O)₁₇ formation. XRDT diffraction patterns clearly demonstrated that only ice crystallised in the aqueous droplets during cooling and that the hydrate only formed during heating simultaneously with melting of ice. From the solid–liquid phase diagrams of systems H₂ONaCl and CCl₃FH₂ONaCl and from the DSC and XRDT experiments, the composition of the droplets was deduced. The upper limit of the amount of hydrate that could form in the system was calculated.     

Intensification of the performance of kinetic inhibitors in the presence of polyethylene oxide and polypropylene oxide for simple gas hydrate formation in a flow mini-loop apparatus

The main objective of the present work is enhancement of the performance of gas hydrate kinetic inhibitors in the presence of polyethylene oxide (PEO) and polypropylene oxide (PPO) for simple gas hydrate formation in a flow mini-loop apparatus. PEO and PPO are high molecular weight polymers that are not kinetic inhibitors by their self. For this investigation, a laboratory flow mini-loop apparatus was set up to measure the induction time and rate of gas hydrate formation when a hydrate-forming substance (such as C1, C3, CO₂ and i-C4) is contacted with water containing dissolved inhibitor in presence or absence of PEO or PPO under suitable temperature and pressure conditions. In each experiment, water containing inhibitors blend saturated with pure gas is circulated up to a required pressure. Pressure is maintained at a constant value during experimental runs by means of required gas make-up. The effect of PEO and PPO on induction time and gas consumption during hydrate formation is investigated in the presence or absence of PVP (polyvinylpyrrolidone) and l-tyrosine as kinetic inhibitors. Results were shown that the induction time is prolonged in the presence of PEO or PPO compared to the inhibitor only. Inclusion of PPO into a kinetic hydrate inhibitor solution shows a higher enhancement in its inhibiting performance compare to PEO. Thus, the induction time for simple gas hydrate formation in presence of kinetic hydrate inhibitor with PPO is higher, compare to kinetic hydrate inhibitor with PEO.     

Influences of different types of magnetic fields on HCFC-141b gas hydrate formation processes

Gas hydrates are crystalline compounds formedwhen gas molecules or volatile liquid molecules comein contact with water molecules through weak van derWaals force at favourable pressure and temperature.Refrigerant gas hydrates can be effectively formed atappropriate temperature (5—12℃) with a high reac-tion heat (320—380 kJ/kg). Because of their particularthermodynamic properties, refrigerant gas hydrate,especially low pressure refrigerant gas hydrate, hasbeen considered as one of the most pr…     

Theoretical temperature dependency of gas hydrate former solubility under hydrate-liquid water equilibrium

Available experimental data and current semi-empirical models suggest a positive trend for the gas hydrate former solubility in the bulk liquid phase as a function of temperature under hydrate-liquid water equilibrium. Such a trend has been widely reported without theoretical explanation. This work proposes a comprehensive derivation, based on fundamental thermodynamics, of the gas hydrate former solubility dependency on temperature for any binary system under two-phase equilibrium.     

Use of Electrical Resistance to Detect the Formation and Decomposition of Methane Hydrate

The changes of electrical resistance(R)were studied experimentally in the process of CH_4 hydrate formation and decomposition,using temperature and pressure as the auxiliary detecting methods simultaneously.The experiment results show that R increases with hydrate formation and decreases with hydrate decompositon.R is more sensitive to hydrate formation and decompositon than temperature or pressure,which indicates that the detection of R will be an effective means for detecting natural gas hydrate(NGH)quantitatively.     

Experimental studies of the formation and dissociation of methane hydrate in loess

In order to study the nature of gas hydrate in porous media, the formation and dissociation processes of methane hydrate in loess were investigated. Five cooling rates were applied to form methane hydrate. The nucleation times of methane hydrate formation at each cooling rate were measured for comparison. The experimental results show that cooling rate is a significant factor affecting the nucleation of methane hydrate and gas conversion. Under the same initial conditions, the faster the cooling rate, the shorter the nucleation time, and the lower the methane gas conversion. Five dissociating temperatures were applied to conduct the dissociation experiment of methane hydrate formed in loess. The experimental results indicated that the temperature evidently controlled the dissociation of methane hydrate in loess and the higher the dissociating temperature, the faster the dissociating rates of methane hydrate.     

Experimental and modeling study on hydrate formation in wet activated carbon

The formation of methane hydrate in wet activated carbon was studied. The experimental results demonstrated that the formation of methane hydrate could be enhanced by immersing activated carbon in water. A maximum actual storage capacity of 212 standard volumes of gas per volume of water was achieved. The apparent storage capacity of the activated carbon + hydrate bed increased with the increasing of mass ratio of water to carbon until reaching a maximum, then decreased drastically as the bulk water phase emerged above the wet carbon bed. The highest apparent storage capacity achieved was 140 v/v. A hydrate formation mechanism in the wet activated carbon was proposed and a mathematical model was developed. It has been shown that the proposed model is adequate for describing the hydrate formation kinetics in wet activated carbon. The kinetic model and the measured kinetic data were used to determine the formation conditions of methane hydrate in wet carbon, which are in good agreement with literature values and demonstrate that hydrate formation in wet carbon requires lower temperature or higher pressure than in the free water system.     

Tuning ionic liquids for hydrate inhibition

Pyrrolidinium cation-based ionic liquids were synthesized, and their inhibition effects on methane hydrate formation were investigated. It was found that the ionic liquids shifted the hydrate equilibrium line to a lower temperature at a specific pressure, while simultaneously delaying gas hydrate formation.     

Experimental study on geochemical characteristic of methane hydrate formed in porous media

The natural occurrence of methane hydrates in marine sediments has been intensively studied over the past decades, and geochemical charac-teristic of hydrate is one of the most attractive research fields. In this paper, we discussed the geochemical anomaly during hydrate formation in porous media. By doing so, we also investigated the temperature influence on hydrate formation under isobaric condition. It turns out that sub-cooling is an important factor to dominate hydrate formation. Larger subcooling provides more powerful driving force for hydrate formation. During the geochemical anomaly research, six kinds of ions and the total dissolved salt (TDS) were measured before and after the experiment in different porous media. The result is that all kinds of ionic concentration increased after hydrate formation which can be defined as salting out effect mainly affected by gas consumption. But the variation ratio of different ions is not equal. Ca2+ seems to be the most significantly influenced one, and its variation ratio is up to 80%. Finally, we theoretically made a model to calculate the TDS variation, the result is in good accordance with measured one, especially when gas consumption is large.     

基于量子化学计算方法的天然气水合物稳定性研究进展

天然气水合物以资源丰富、优质、洁净等特点,被视为21世纪新能源。天然气水合物稳定性的研究对天然气水合物资源勘探开发具有重要意义。本文简述了微观、介观、宏观、矿藏四个尺度天然气水合物稳定性的研究,重点从微观量子尺度介绍了量子化学计算方法对水合物晶体结构及其稳定性以及水合物宏观物理特性微观表征的计算研究。应用量子化学计算方法可以对天然气水合物的晶体结构、电子轨道分布、振动光谱、成键特性及主客体相互作用进行计算研究,其结果能够为天然气水合物在油气储运、水合物成藏、开采及其综合利用等方面的研究提供理论支持。目前,量子化学计算方法的优化与分子动力学模拟、分子力学模拟等方法的结合将有助于水合物形成和分解微观机理研究的发展,提升计算精度和扩大研究体系,为矿场尺度的天然气水合物资源开采利用提供理论支持。     

A new approach in modelling phase equilibria and gas solubility in electrolyte solutions and its applications to gas hydrates

This paper presents a new predictive model for phase equilibria and gas solubility calculations in the presence of electrolyte solutions. It treats salts as pseudo-components in an equation of state (EoS) by defining the critical properties and acentric factor for each salt. The water–salt, gas–salt and salt–salt binary interaction parameters (BIP) have been determined by using available experimental data on freezing point depression and boiling point elevation as well as gas solubility and salt solubility data in saline solutions.The methodology has been applied in modelling sodium chloride, potassium chloride and their mixtures, as well as solubility of methane and carbon dioxide in aqueous single and mixed electrolyte solutions.The developed model is capable of accurately predicting the phase behaviour, gas hydrate stability zone and potential salt precipitation in single and mixed electrolyte solutions. The model predictions are compared with available independent experimental data, including hydrate inhibition characteristics of single and mixed electrolyte solutions, and good agreement is demonstrated.     

Solubility of 1,3,5,7-tetra azatricyclo[3.3.1.1]decane (HMT) in water from 275.15 K to 313.15 K

The water solubility of 1,3,5,7-tetra azatricyclo[3.3.1.1^3,7]decane, hexamethylenetetramine (HMT) or urotropine was determined at temperatures between 275.15 K and 313.15 K. Van’t Hoff graphs of ln m_sat versus 1/T gave three different straight lines, suggesting the existence of three different forms of the substance in the temperature region studied.

Previous work reported both increasing and decreasing solubility with temperature. This work found the solubility does not have a simple behavior.     

Advances in the Study of Gas Hydrates by Dielectric Spectroscopy

The influence of kinetic hydrate inhibitors on the process of natural gas hydrate nucleation was studied using the method of dielectric spectroscopy. The processes of gas hydrate formation and decomposition were monitored using the temperature dependence of the real component of the dielectric constant ε′(T). Analysis of the relaxation times τ and activation energy ΔE of the dielectric relaxation process revealed the inhibitor was involved in hydrogen bonding and the disruption of the local structures of water molecules.     

Detecting gas hydrate behavior in crude oil using NMR

Because of the associated experimental difficulties, natural gas hydrate behavior in black oil is poorly understood despite its grave importance in deep-water flow assurance. Since the hydrate cannot be visually observed in black oil, traditional methods often rely on gas pressure changes to monitor hydrate formation and dissociation. Because gases have to diffuse through the liquid phase for hydrate behavior to create pressure responses, the complication of gas mass transfer is involved and hydrate behavior is only indirectly observed. This pressure monitoring technique encounters difficulties when the oil phase is too viscous, the amount of water is too small, or the gas phase is absent. In this work we employ proton nuclear magnetic resonance (NMR) spectroscopy to observe directly the liquid-to-solid conversion of the water component in black oil emulsions. The technique relies on two facts. The first, well-known, is that water becomes essentially invisible to liquid state NMR as it becomes immobile, as in hydrate or ice formation. The second, our recent finding, is that in high magnetic fields of sufficient homogeneity, it is possible to distinguish water from black oil spectrally by their chemical shifts. By following changes in the area of the water peak, the process of hydrate conversion can be measured, and, at lower temperatures, the formation of ice. Taking only seconds to accomplish, this measurement is nearly direct in contrast to conventional techniques that measure the pressure changes of the whole system and assume these changes represent formation or dissociation of hydrates - rather than simply changes in solubility. This new technique clearly can provide accurate hydrate thermodynamic data in black oils. Because the technique measures the total mobile water with rapidity, extensions should prove valuable in studying the dynamics of phase transitions in emulsions.     

Formation and decomposition of ethane, propane, and carbon dioxide hydrates in silica gel mesopores under high pressure

The experimental data on decomposition temperatures for the gas hydrates of ethane, propane, and carbon dioxide dispersed in silica gel mesopores are reported. The studies were performed at pressures up to 1 GPa. It is shown that the experimental dependence of hydrate decomposition temperature on the size of pores that limit the size of hydrate particles can be described on the basis of the Gibbs-Thomson equation only if one takes into account changes in the shape coefficient that is present in the equation; in turn, the value of this coefficient depends on a method of mesopore size determination. A mechanism of hydrate formation in mesoporous medium is proposed. Experimental data providing evidence of the possibility of the formation of hydrate compounds in hydrophobic matrixes under high pressure are reported. Decomposition temperature of those hydrate compounds is higher than that for the bulk hydrates of the corresponding gases.