Effects of magnetic fields on HCFC-141b refrigerant gas hydrate formation

Gas hydrates are crystalline compounds formed (usually above 0℃) by water reacting with some gases or volatile liquids (hydrate former). Guest molecules, such as gas or volatile liquid molecules, are enclosed firmly inside the host cavities and act with water molecules in weak van der Waals force. Gas hydrate usually includes natural gas hydrate, refrigerant gas hydrate and CO2 gas hydrate. Refrigerant hydrates can be formed above 0℃, and their crystallization is similar to the ordinary ice…     

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…     

多孔介质中气体水合物降压分解的分形理论研究

通过将水合物的分解过程看作是无固态产物层生成的气固反应过程, 结合粒径缩小的收缩核反应模型和分形理论, 建立了多孔介质中水合物降压分解的分数维动力学模型, 提出了基于水合物分解实验数据计算多孔介质分形维数的方法. 分别利用前人的甲烷水合物和CO2水合物降压分解实验数据, 对上述分数维动力学模型进行了验证. 计算结果表明, 用提出的方法所计算得到的多孔介质分形维数与前人的测定结果基本符合; 对甲烷水合物和CO2水合物的降压分解过程, 提出的分数维动力学分解模型得出了和实验结果基本一致的预测, 绝对平均误差(AAD)小于10%.     

A model for estimating flow assurance of hydrate slurry in pipelines

The problem of hydrate blockage of pipelines in offshore production is becoming ever-increasing severe because oil fields in ever-increasing unusual environments have been brought in production.HCFC-141b and THF were selected as the substitutes to study the flow assurance of the hydrates in pipelines.There are critical hydrate volume concentrations for these two slurries.Hydrate slurries behave like Bingham fluids and have high agglomerating tendency when the hydrate volume concentrations are larger than the critical ones.Based on theological behaviors of these two hydrates,a non-dimensional parameter is proposed through studying the driving forces of agglomeration among hydrate particles,which shows the agglomerating probability of hydrate particles in pipeline and can be used to judge the safety of the pipeline.Moreover,a safe model to judge the safely flow hydrate slurries was presented and verified with the experimental data,which demonstrates that the model is effective to judge whether the pipeline can be run safely or not.     

Experimental study and modeling of the kinetics of refrigerant hydrate formation

The purpose of this study was to identify compatible hydrate forming-refrigerants suitable for air-conditioning systems. The main challenge in designing an air conditioning system which utilises refrigerant hydrates as a media for storage of cold energy is the rate of formation and dissociation of the refrigerant hydrates. Hence, in this experimental study the kinetics of hydrate formation of three refrigerant blends, viz. R407C, R410A and R507C have been investigated. The induction time for hydrate formation, apparent rate constant of the hydrate reaction, water to hydrate conversion during hydrate growth, storage capacity, and the rate of hydrate formation of these refrigerants at various pressures and temperatures have been obtained using a kinetic model. The effect of sodium dodecyl sulfate (SDS) on the hydrate nucleation rate was also investigated.     

Gas hydrate fast nucleation from melting ice and quiescent growth along vertical heat transfer tube

Gas hydrates, or clathrate hydrates, are ice-likecrystal, composed of host lattice (cavities) formed byhydrogen-bonded water molecules, and other guestmolecules called guest molecules. The guest mole-cules act with host lattice in weak van der Waals force…     

天然气水合物的导热系数

分析介绍了关于水合物导热系数类玻璃体变化规律的三种模型及其相关测试手段、样品制备方法和测试结果。针对物性和测试手段的特点，笔者指出在制备测试样品的时候就要注意其生成品质。最后对比了国外相关实验结果和本实验室对制冷剂水合物的测量结果。     

Sorption kinetics of 1-fluoro-1,1-dichloroethane in vinylidene chloride-acrylonitrile-styrene terpolymer

The sorption kinetics of liquid 1-fluoro-1,1-dichloroethane (HCFC-141b) and its vapor in vinylidene chloride (VDC)-acrylonitrile (AN)-styrene (St) terpolymers were investigated by weight gain method. It was found that the sequence of the effect of monomer unit on the barrier property for HCFC-141b is AN>VDC>St at the same molar content. The diffusion mechanism of HCFC-141b in VDC-AN-St terpolymer depends on the glass transition temperature (T_g) of VDC terpolymers, sorption temperature, vapor pressure and so on. For Fickian diffusion, the diffusivity (D) is exponential function of vapor pressure at the same temperature. A quantitative relationship between D in terms of HCFC-141b in VDC-AN-St terpolymer with composition VDC/AN/St=55.7/12.9/31.4 (wt), temperature and activity was obtained, and the activation energy of diffusion is 37.68 KJ/mol.     

Transesterification of palm oil using supercritical methanol with co-solvent HCFC-141b

The transesterification of palm oil in supercritical methanol has been investigated without using any catalyst. HCFC-141b was used as co-solvent to reduce the molar ratio of methanol to palm oil under the milder conditions. The reaction was carried out in a flow-type tubular reactor. The residence time was fixed at 40 min. When the molar ratio of methanol to palm oil was set to 20:1 at 325 °C and 35 MPa, the optimum molar ratio of methanol to co-solvent was found to be 20:1. Addition of HCFC-141b increased FAME production even at the lower molar ratio of methanol to palm oil. In addition, a similar FAME content was obtained under the milder conditions (5 MPa lower pressure) compared with conditions without co-solvent at higher pressure. The role of HCFC-141b in the transesterification reaction under supercritical conditions was investigated.     

Study of the permeability characteristics of porous media with methane hydrate by pore network model

The permeability in the methane hydrate reservoir is one of the key parameters in estimating the gas production performance and the flow behavior of gas and water during dissociation. In this paper, a three-dimensional cubic pore-network model based on invasion percolation is developed to study the effect of hydrate particle formation and growth habit on the permeability. The variation of permeability in porous media with different hydrate saturation is studied by solving the network problem. The simulation results are well consistent with the experimental data. The proposed model predicts that the permeability will reduce exponentially with the increase of hydrate saturation, which is crucial in developing a deeper understanding of the mechanism of hydrate formation and dissociation in porous media.     

Experimental studies on a mixture of HFC-32/125/161 as an alternative refrigerant to HCFC-22 in the presence of polyol ester

In this work, experiment was conducted to examine the thermo-physical properties of an alternative refrigerant to HCFC-22 in the presence of polyol ester (POE). The new alternative refrigerant is a mixture of HFC-32/125/161, whose physical properties are similar to HCFC-22 but has a lower global warming potential (GWP) than that of R407C. POE is used as the tested lubricating oil in the experiment. The saturated vapor pressure data and vapor–liquid equilibrium data of nine different mass fractions of the new refrigerant and polyol ester (POE) in the temperature range of 253–323 K were measured by single-phase cycle method. The experiment results showed that there was no stratification, no sediment generation in the liquid phase of the mixture, and the color of liquid phase of the mixture had no change in the equilibrium cell before and after the experiment with the POE concentration greater than 20% and the temperature higher than 258 K; with POE concentration lower than 20% and temperature lower than 258 K, stratification began to appear. Meanwhile, when POE and the refrigerant were miscible, the saturated pressure data of the mixture (HFC-32/125/161 + POE) revealed that POE had a very small impact on saturated vapor pressure of the mixture (almost negligible) when POE was less than 10% of the mixture; POE has an obvious effect on the saturated vapor pressure of the mixture when there is more than 10% POE in the mixture, especially when the temperature is higher than 283.15 K. Experimental data were correlated by Flory–Huggins model, Heil model, NRTL model and Wilson model. The results showed that to the average and maximum pressure deviation, the results were better with considering the effects of temperature on the energy parameters. Among the above models, the NRTL activity coefficient model was the best, the Heil and Wilson models followed and the Flory–Huggins model had the largest deviation from the experimental data.     

Kinetic of hydrate formation of propane and its mixture with methane in a circulating flow reactor

Kinetics of hydrate formation for propane and its mixture with 73% methane have been studied experimentally and theoretically at pressure up to 2 MPa and temperature up to 277.65 K in a 10 m circulating flow reactor. A mathematical model has been developed for the process of hydrate formation based on crystallization, mass transfer and thermodynamics concepts. The amounts of gas consumptions due to hydrate formation are measured experimentally and predicted by the model. The agreement between the experimental measured gas consumptions and predicted values by the mathematical model are very good and the average deviation errors in the prediction of gas consumption are less than 10%.     

Measurement and modeling of the effective thermal conductivity for porous methane hydrate samples

The effective thermal conductivities of gas-saturated porous methane hydrates were measured by a single-sided transient plane source (TPS) technique and simulated by a generalized fractal model of porous media that based on self-similarity.The density of porous hydrate,measured by the volume of the sample in the experimental system,was used to evaluate the porosity of methane hydrate samples.The fractal model was based on Sierpinski carpet,a thermal-electrical analogy technique and one-dimensional heat flow assumption.Both the experimental and computational results show the effective thermal conductivity of methane hydrate decreases with the porosity increase.The porosity of 0.3 can reduce the thermal conductivity of the methane hydrate by 25%.By analysis of the experimental data and the simulative result,the optimized thermal conductivity of the zero-porosity methane hydrate is about 0.7 W m-1K-1.     

Kinetic Analysis of Methane Hydrate Formation with Butterfly Turbine Impellers

Heat generation during gas hydrate formation is an important problem because it reduces the amount of water and gas that become gas hydrates. In this research work, we present a new design of an impeller to be used for hydrate formation and to overcome this concern by following the hydrodynamic literature. CH₄ hydrate formation experiments were performed in a 5.7 L continuously stirred tank reactor using a butterfly turbine (BT) impeller with no baffle (NB), full baffle (FB), half baffle (HB), and surface baffle (SB) under mixed flow conditions. Four experiments were conducted separately using single and dual impellers. In addition to the estimated induction time, the rate of hydrate formation, hydrate productivity and hydrate formation rate, constant for a maximum of 3 h, were calculated. The induction time was less for both single and dual-impeller experiments that used full baffle for less than 3 min and more than 1 h for all other experiments. In an experiment with a single impeller, a surface baffle yielded higher hydrate growth with a value of 42 × 10⁻⁸ mol/s, while in an experiment with dual impellers, a half baffle generated higher hydrate growth with a value of 28.8 × 10⁻⁸ mol/s. Both single and dual impellers achieved the highest values for the hydrate formation rates that were constant in the full-baffle experiments.     

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.     

Formation of methane hydrate from polydisperse ice powders

Neutron diffraction runs and gas-consumption experiments based on pressure-volume-temperature measurements are conducted to study the kinetics of methane hydrate formation from hydrogenated and deuterated ice powder samples in the temperature range of 245-270 K up to high degrees of transformation. An improved theory of the hydrate growth in a polydisperse ensemble of randomly packed ice spheres is developed to provide a quantitative interpretation of the data in terms of kinetic model parameters. This paper continues the research line of our earlier study which was limited to the monodisperse case and shorter reaction times (Staykova et al., 2003). As before, we distinguish the process of initial hydrate film spreading over the ice particle surface (stage I) and the subsequent hydrate shell growth (stage II) which includes two steps, i.e., an interfacial clathration reaction and the gas and water transport (diffusion) through the hydrate layer surrounding the shrinking ice cores. Although kinetics of hydrate formation at stage II is clearly dominated by the diffusion mechanism which becomes the limiting step at temperatures above 263 K, both steps are shown to be essential at lower temperatures. The permeation coefficient D is estimated as (1.46 +/- 0.44) x 10(-12) m2/h at 263 K with an activation energy Q(D) approximately 52.1 kJ/mol. This value is close to the energy of breaking hydrogen bonds in ice Ih and suggests that this process is the rate-limiting step in hydrate formation from ice in the slower diffusion-controlled part of the reaction.     

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.     

Method for calculating the parameters of formation of hydrates from multicomponent gases

A model of hydrate formation in multicomponent gas–liquid water or ice systems including the exo- and endothermic processes has been suggested. Based on this model, a method for calculating the molecular and energy parameters such as the hydration number, amount of moles of hydrate, amount of gas and water in it, its density and molar mass, and the energy and rate of hydrate formation was developed. A comparison of the calculated and experimental values of the parameters revealed that the difference between them varied from 0 to 5.46%.     

Fractal Study of Mesoporous Structure in Water Molecular Condensed System

Bis-(4-stearoylaminophenyl) ether (BSADE) can aggregate and self-assemble in water. Transmission electron microscopy (TEM) indicated that the morphology of BSADE aggregates in water was entanglement and thin fiber-like, and that a three dimensional network structure was formed. Water molecules were entrapped in this three dimensional network structure and formed a new type of condensed system (so-called water molecular gel). Water molecular gel is a typical mesoporous material which can be characterized by the fractal dimension D. Using gas adsorption method along with measurement of porosity and specific surface area, the fractal dimension D of the complicated pores was 2.1-2.2 for this water molecular gel. Using viscosity method and the Cayley fractal tree, the fractal dimension D of the fiber-like three dimensional network was determined to be 1.98. The formation process of water molecular gel can be described as nucleation followed by a repeated growing and branching cycle.     

Influence of Gas Supply Changes on the Formation Process of Complex Mixed Gas Hydrates

Natural gas hydrate occurrences contain predominantly methane; however, there are increasing reports of complex mixed gas hydrates and coexisting hydrate phases. Changes in the feed gas composition due to the preferred incorporation of certain components into the hydrate phase and an inadequate gas supply is often assumed to be the cause of coexisting hydrate phases. This could also be the case for the gas hydrate system in Qilian Mountain permafrost (QMP), which is mainly controlled by pores and fractures with complex gas compositions. This study is dedicated to the experimental investigations on the formation process of mixed gas hydrates based on the reservoir conditions in QMP. Hydrates were synthesized from water and a gas mixture under different gas supply conditions to study the effects on the hydrate formation process. In situ Raman spectroscopic measurements and microscopic observations were applied to record changes in both gas and hydrate phase over the whole formation process. The results demonstrated the effects of gas flow on the composition of the resulting hydrate phase, indicating a competitive enclathration of guest molecules into the hydrate lattice depending on their properties. Another observation was that despite significant changes in the gas composition, no coexisting hydrate phases were formed.