Thermodynamic and Raman spectroscopic studies on hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrates

Thermodynamic stability and hydrogen occupancy on the hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate have been investigated by means of phase equilibrium (pressure-temperature) measurements and Raman spectroscopic analyses for two mole fractions, 0.018 and 0.034 (stoichiometric for the cubic structure) of tetra-n-butyl ammonium fluoride aqueous solutions. In the case of higher concentration (0.034), the stability boundary curve of hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate locates at about 23 K higher temperature than that of hydrogen+tetrahydrofuran mixed gas hydrate. The storage capacity of hydrogen in the cubic structure for the hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate is smaller than that of hydrogen+tetrahydrofuran mixed gas hydrate. In the case of hydrate prepared from the lower concentration (0.018) of aqueous solution, the Raman spectra and phase behavior reveal that the cubic structure of semi-clathrate hydrate is changed to a different one at about 9 MPa and 299.2 K. The new structure can entrap larger amount of hydrogen than the cubic one. The stability boundary curve of hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate obtained in the aqueous solution of lower mole fraction (0.018) is shifted to slightly low-temperature or high-pressure side from that of higher mole fraction (0.034).     

Notes on the dissolution rate of gas hydrates in undersaturated water

An elementary model for the dissolution of pure hydrate in undersaturated water is proposed that combines intrinsic decomposition within a desorption film and the subsequent diffusion of the released hydrate guest species into bulk water. Applying the proposed approach to recently published measurements of the decomposition rates of methane (CH₄) and carbon dioxide (CO₂) hydrates in deep seawater suggests that the concentration of the hydrate guest species at the interface between desorption film and diffusive boundary layer may be much lower than ambient solubility. Calculations, however, fail to account for the observed proportionality of decomposition rate with solubility for both CH₄ and CO₂ hydrates. This may indicate a limitation in the range of applicability of published formulas for intrinsic hydrate decomposition rates.     

Icosahedron cage occupancy of structure-H hydrate helped by Xe -1,1-, cis -1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes

Four mixtures of 1,1-, cis-1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes (hereafter abbreviated DMCH) including H₂O and Xe have been investigated in a temperature range over 274.5 K and a pressure range up to 2.7 MPa. The 1,1-DMCH and cis-1,2-DMCH generate the structure-H hydrate in the temperature range up to 295.2 and 280.2 K, respectively. Especially, very large depression of equilibrium pressure has been observed in the structure-H 1,1-DMCH hydrate system. On the other hand, neither trans-1,2-DMCH nor cis-1,4-DMCH generates the structure-H hydrate in the present temperature range. It is an important finding that the cis-1,4-DMCH does not generate the structure-H hydrate in the presence of Xe, while the mixture of cis-1,4-DMCH and methane generates the structure-H hydrate.     

Stability boundaries of gas hydrates helped by methane—structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane

The four-phase coexistence curves for the structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane in the presence of methane are measured in the temperature range 274.09- and pressure range 1.42-. Very large pressure reductions from the pure methane hydrate are observed by forming structure-H hydrates. The present investigation on the trans-1,2-dimethylcyclohexane system reveals that the limit of the largest-cage occupancy for the structure-H hydrate is laid between the 1,2-dimethylcyclohexane stereo-isomers.     

Gas hydration of trans-1,2-dimethylcyclohexane with cooperative assistance of methane and cis-1,2-dimethylcyclohexane

It has been regarded that the limit of the largest cage occupancy for the structure-H hydrate is between the 1,2-dimethylcyclohexane stereo-isomers, because the cis-isomer is able to generate the structure-H hydrate in the presence of methane while the trans-isomer is not. In the present study, gas hydration of trans-1,2-dimethylcyclohexane in the presence of methane and cis-1,2-dimethylcyclohexane is found from stability boundaries for the structure-H hydrate system.     

Storage capacity of hydrogen in tetrahydrofuran hydrate

The storage capacity of hydrogen in tetrahydrofuran hydrate was investigated by means of pressure-volume-temperature (p-V-T) measurement and Raman spectroscopic analysis. We carried out two measurement strategies using Raman spectroscopic analysis. One was isothermal pressure-swing absorption using tetrahydrofuran hydrate at 277.15 K, and the other was the preparation of a single crystal of hydrogen+tetrahydrofuran mixed gas hydrate from compressed hydrogen and tetrahydrofuran aqueous solutions along the stability boundary. The storage amount of hydrogen at 277.15 K obtained from the p-V-T measurement is about 1.6 mol (hydrogen)/mol (tetrahydrofuran) (about 0.8 mass%) at 70 MPa, and isothermal Raman spectroscopic measurement reveals that it reaches the maximum value of 2.0 mol (hydrogen)/mol (tetrahydrofuran) at about 85 MPa. These results agree well with those for a single crystal of hydrogen+tetrahydrofuran hydrate.     

Large pressure depression of methane hydrate by adding 1,1-dimethylcyclohexane

The structure-H hydrate of 1,1-dimethylcyclohexane (DMCH) helped by methane has been investigated in a temperature range of 274.6-289.3 K and pressure range up to 6.7 MPa. The present results suggest that 1,1-DMCH is a suitable additive which makes a mild-pressure handling of natural-gas hydrate possible.     

氨合成回路的相平衡计算

根据化工热力学理论 ,应用改进的R K状态方程和VanLaar型活度系数方程 ,在前人工作的基础上建立H2 N2 CH4 Ar NH3 五元体系的气、液相平衡数学模型 ,完成其程序设计 ,计算该体系在不同条件下的气、液相平衡常数K值和气、液相组成 ,取得满意的结果。     

Dual function inhibitors for methane hydrate

The performance of five imidazolium-based ionic liquids as a new class of gas hydrate inhibitors has been investigated. Their effects on the equilibrium hydrate dissociation curve in a pressure range of 30-110 bar and the induction time of hydrate formation at 114 bar and a high degree of supercooling, i.e., about 25 °C, are measured in a high-pressure micro differential scanning calorimeter. It is found that these ionic liquids, due to their strong electrostatic charges and hydrogen bond with water, could shift the equilibrium hydrate dissociation/stability curve to a lower temperature and, at the same time, retard the hydrate formation by slowing down the hydrate nucleation rate, thus are able to act as both thermodynamic and kinetic inhibitors. This dual function is expected to make this type of inhibitors perform more effectively than the existing inhibitors.     

CO2/离子液体混合物相平衡的研究进展

离子液体对二氧化碳有良好的溶解性能,可以实现二氧化碳的固定与转化。超临界二氧化碳可以从离子液体/有机物体系中选择性萃取有机物,避免相间的交叉污染,实现离子液体的回收。从CO2在离子液体中的溶解度实验测定方法、CO2/离子液体二元体系高压相平衡测定、SC-CO2/离子液体/有机物的三元体系相平衡研究以及模型预测四个方面介绍了CO2/离子液体体系相平衡研究的最近进展,分析了这一研究领域的发展方向。     

Evidence of liquid water formation during methane hydrates dissociation below the ice point

Dissociation of small methane hydrate samples formed from water droplets of size 0.25-2.5 mm has been investigated below the ice melting point in the temperature range of 240-273 K, where the self-preservation effect is observed for bulk hydrates. The experiments included optical microscopy observations combined with P-T measurements of the dissociation conditions for the methane hydrates. For the first time, the formation of supercooled liquid water during the hydrate dissociation was reliably detected in the temperature range of 253-273 K. The formation of the liquid phase was visually observed. The induction time of the ice nucleation for the metastable liquid water depended from the dissociation temperature and a size of water droplets formed during the hydrate dissociation. It was found that in the temperature range of 253-273 K values of the dissociation pressure for the small hydrate samples fall on the extension of the water-hydrate-gas equilibrium curve into the metastable region where supercooled water exist. The average molar enthalpy of 51.7 kJ/mol for the dissociation of the small methane hydrate samples in the temperature range of 253-273 K was calculated using Clausius-Clapeyron equation. This value agrees with the enthalpy of dissociation of bulk methane hydrates into water and gas at temperatures above 273 K.     

Structural phase transitions of methane+ethane mixed-gas hydrate induced by 1,1-dimethylcyclohexane

Methane+ethane+1,1-dimethylcyclohexane+water system was investigated by using Raman spectroscopy and isothermal phase equilibrium measurements under four-phase (gas+aqueous+large guest species+hydrate phases) equilibrium conditions at 288.15 K. The results suggest that three kinds of hydrate structures emerge at 288.15 K in the methane+ethane+1,1-dimethylcyclohexane+water system. The hydrate structure for this system changed from structure-H to structure-I via structure-II with increase in the mole ratio of ethane to methane.     

Concentration of sodium chloride in aqueous solution by chlorodifluoromethane gas hydrate

The decomposition temperature and pressure (quadruple point) of chlorodifluoromethane (R22) gas hydrate in aqueous sodium chloride (NaCl) solution was measured at different NaCl concentrations in the solution as a phase diagram. The operative concentration curve of NaCl was obtained as a function of temperature. The maximum decomposition temperature of R22 hydrate was about 290 K at 0.9 MPa pressure, and it decreased as the NaCl concentration increased in the solution. R22 hydrate caused supercooling, and the supercooling occurrence temperature was much lower than the decomposition temperature. The ultrasonic charge reduced the supercooling of hydration effectively even though the ultrasonic charge did not change the decomposition temperature at all. The concentration experimental results from the several NaCl solutions having different NaCl concentrations were in good agreement with the theoretical operative concentration curve for NaCl.     

Metastable states during dissociation of carbon dioxide hydrates below 273 K

In this study, the dissociation of isolated carbon dioxide hydrate particles of sizes in the range 0.25–2.5 mm was investigated. It was found that below the ice melting point, the hydrates dissociated into supercooled water (metastable liquid) and gas. The formation of the liquid phase during CO₂ hydrate dissociation was visually observed, and the pressures of the hydrate dissociation into supercooled water and gas were measured in the temperature range 249–273 K. These pressures agreed well with the calculated data for the supercooled water–hydrate–gas metastable equilibrium (Istomin et al., 2006). In the P–T area on the phase diagram bounded by the ice–hydrate–gas equilibrium curve and the supercooled water–hydrate–gas metastable equilibrium curve, hydrates could exist for a long time because the metastable phase and their stability are not connected to the self-preservation effect. The growth of the metastable CO₂ hydrate film on the surface of supercooled water droplets formed during the hydrate dissociation was observed at pressure above the three-phase supercooled water–hydrate–gas metastable equilibrium pressure but still below the three-phase ice–hydrate–gas equilibrium pressure. It was found that the growth rate of the metastable CO₂ hydrate film was higher by a factor of 25 and 50 than that for methane hydrate and propane hydrate, respectively.     

A reverse osmosis desalting plant operated by gas turbines

Power plants in Kuwait use gas turbines (GT) only for a few hours to produce power at peak load times. Peak loadoccurs in the summer due to the air-conditioning load. As an example, the average number of operating hours for six gas turbines in the Doha East power plant was 16 in the summer of 2001. There is little concern about efficiency of these GT since they work for a very short time during the year. However, a recent increase in desalted seawater demand suggests the use of these GT to operate reverse osmosis (RO) desalting systems all year around. The summer outside design temperature in Kuwait for air-conditioning calculations is 48°C dry bulb temperature (DBT), and 28°C wet bulb temperature (WBT); but the ambient temperature can easily reach 60°C. Gas turbine power output and efficiency are drastically reduced by the increase in temperature of intake air to the gas turbine's compressor, especially during harsh Kuwaiti summer conditions. Thus, it is essential to investigate cooling of air intake to the GT compressor. The performance of a typical GT unit and its ability to produce desalted waterby a RO desalting system at different ambient temperatures are presented. Calculation of needed capacities for the cooling of intake air to the GT compressor was performed for evaporative cooling, single and multiple mechanical vapor compression cycles, and combined indirect evaporation cooling with the refrigeration system. The improvements of power output and efficiency due to the cooling of air intake of the GT and the resulting increase in desalted water are also presented.     

Sugaring-out: A novel phase separation and extraction system

In this study, we made a novel observation that by introducing a monomeric sugar or a disaccharide into an acetonitrile-water solution, the acetonitrile (ACN) can be separated from water to form a new phase. The two-phase formation triggered by sugar addition was visualized with Sudan I. The ability of different sugars to form an ACN-water two-phase system and the effect of glucose and xylose concentration on the phase separation were studied. The distribution of syringic acid, furfural, para-coumaric acid, ferulic acid and 5-hydroxymethyl furfural in the upper ACN phase and lower water phase was examined. The lower concentration limit for the two-phase formation for glucose and xylose at 1 °C was 15 and 25 g/L, respectively. At higher temperatures, the concentration needed for phase separation increased. Addition of polysaccharides (starch and dextran) did not result in phase separation. The distribution coefficient of the five organic compounds in the ACN-water two-phase system was in the range of 1.7-8.9 when the corresponding sugar concentration was 15-50 g/L. The phase ratio of the five organic compounds in the two-phase system was in the range of 0.1-0.5. The new two-phase system may find applications in the separation of chemicals having different solubility in water and in ACN.     

Storage capacity of hydrogen in tetrahydrothiophene and furan clathrate hydrates

The storage capacity of hydrogen in the tetrahydrothiophene and furan hydrates was investigated by means of pressure-volume-temperature measurement. The hydrogen-absorption rate of tetrahydrothiophene and furan hydrates is much larger than that of tetrahydrofuran hydrate in spite of same crystal structure (structure-II). The storage amount of hydrogen at 275.1 K is about 1.2 mol (hydrogen)/mol (tetrahydrothiophene or furan hydrate) (∼0.6 mass%) at 41.5 MPa, which is coincident with that of tetrahydrofuran hydrate.     

Solid Phase Extraction for Determination of Polycyclic Aromatic Hydrocarbons from Atmospheric Wet and Dry Deposition Samples

Solid phase extraction with C-18 sorbent tubes was employed for extraction and preconcentration of trace level (ng/l) of polycyclic aromatic hydrocarbons (PAHs) from water samples obtained by collecting wet and dry atmospheric deposition. Recoveries of spiked PAHs from 0.2 to 10.0 l water samples ranged from 60% to 90%. Five deuterated PAHs (naphthalene- d 8 , acenaphthene- d 10 , anthracene- d 10 , chrysene- d 12 , and perylene- d 12 ) were added to the samples as surrogates. The recoveries of surrogates were in the same range as the spiked PAHs. The recoveries of surrogates were used to estimate the recoveries of ambient PAHs with a similar ring structure. Factors that contributed to the relatively low recoveries include breakthrough of the sorbates, adsorption on the container surface, and degradation in water during storage.     

汽液平衡二相组成与温度间的相互关系

汽液平衡时液相分子仍不断地从液相蒸发到汽相,那些从液相蒸发到汽相中的分子与滞留在液相中的同类分子相比具有较高的能量。通过考虑液相中分子能量的分布以及分子相互之间碰撞对汽液相分子分布的影响,从新的角度探讨了汽液二相平衡时汽液相组成与温度之间的相互关系。