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…     

Investigation on Gas Storage in Methane Hydrate

The effect of additives (anionic surfactant sodium dodecyl sulfate (SDS), nonionic surfactantalkyl polysaccharide glycoside (APG), and liquid hydrocarbon cyclopentane (CP)) on hydrate inductiontime and formation rate, and storage capacity was studied in this work. Micelle surfactant solutions werefound to reduce hydrate induction time, increase methane hydrate formation rate and improve methanestorage capacity in hydrates. In the presence of surfactant, hydrate could form quickly in a quiescentsystem and the energy costs of hydrate formation were reduced. The critical micelle concentrations of SDS and APG water solutions were found to be 300x 10-6 and 500x 10-6 for methane hydrate formation systemrespectively. The effect of anionic surfactant (SDS) on methane storage in hydrates is more pronounced compared to a nonionic surfactant (APG). CP also reduced hydrate induction time and improved hydrateformation rate, but could not improve methane storage in hydrates.     

Formation process and fractal growth model of HCFC-141b refrigerant gas hydrate

The microscopic visualization experiment on the formation process of HCFC-141b refrigerant gas hydrate has been investigated, and the morphological photos of hydrate formation process have been obtained. The results show that gas hydrate originally nucleated on the interface of refrigerant HCFC-141b and water under the condition of supercooling, then the hydrate grows continually due to the inducement of formed nucleation and diffusion of refrigerant. The formation of gas hydrate presents an arboreous phenomenon. The fractal dimension of the hydrate formation morphology on different stages was calculated. The calculating results indicate that the initial stage of the hydrate formation belongs to fractal growth, and the dimension is about 1.52. Based on the fractal theory, an RIN-DLA (random inducement nucleation-diffusion limited aggregation) model for the HCFC-141b hydrate growth was developed. The hydrate growth process was simulated with the developed model, and the fractal dimension for the simulated     

Effect of 1-butyl-3-methylimidazolium tetrafluoroborate on the formation rate of CO2 hydrate

The effect of the addition of 1-butyl-3-methylimidazolium tetrafluoroborate （[C4mim][BF4]） on the formation rates of CO2 hydrates was investigate. The isothermal and isobaric methods were used to measure the formation rates of CO2 hydrates. As compared to those of pure water, the data of phase equilibrium changed greatly. The effects of pressure, temperature, and the concentration of [C4mim][BF4] aqueous solution on the formation rates of CO2 hydrates were investigated. With a constant concentration of [C4mim][BF4], the rate of gas consumption was enhanced with the lowering of experimental temperature. However, a decrease in pressure exerted an opposite effect on the rate of gas consumption. Moreover, the addition of [C4mim][BF4] raised the equilibrium pressure of hydrate formation at the same temperature.     

Molecular Insights into the Effect of Nitrogen Bubbles on the Formation of Tetrahydrofuran Hydrates

In this work, a molecular dynamics simulation was conducted to study the microscopic mechanism of how nitrogen bubbles affect the formation of THF hydrates at the molecular level. The results obtained reveal that the nitrogen bubble can promote the formation of THF hydrates. In the system with a nitrogen bubble, more THF-filled cages were generated, and the crystal structure was more orderly. The promotion of nitrogen bubbles on hydrate crystallization comes from the dissolution of nitrogen molecules. Some of dissolved nitrogen molecules can be enclosed in small hydrate cages near the nitrogen bubble, which can serve as stable sites for hydrate crystal growth, resulting in the fact that THF-filled cages connected with N₂-filled cages are much more stable and have a long lifetime. The results in this work can help to understand the promotion effect of micro- and nano-air bubbles on the crystallization of THF hydrates.     

含低剂量抑制剂体系气体水合物生成动力学

水合物管道堵塞是油气工业安全生产的重要问题之一, 目前低剂量抑制剂以其经济性、环境友好性等优点, 逐步取代传统抑制剂. 文中在8.5 MPa、4 ℃条件下, 1.072 L反应釜内, 采用甲烷、乙烷和丙烷混和气, 研究了含低剂量抑制剂聚乙烯吡咯烷酮(PVP)和GHI1的水合物生成体系反应过程, 计算分析了压缩因子和自由气量随反应时间的变化, 对比了在相同反应程度下添加PVP和GHI1后水合物含气量的区别, 探讨了GHI1组合抑制剂的抑制机理. 实验结果表明PVP和GHI1能抑制水合物生长, 不能有效抑制水合物成核; 添加PVP的体系, 在实验气体组成下, 甲烷乙烷进入水合物小晶穴, 并且甲烷优先进入小晶穴; GHI1对丙烷乙烷的抑制能力强于甲烷; 对比GHI1和PVP的反应过程, 认为协同剂二乙二醇丁醚的羟基和醚类结构加强反应体系中的氢键, 和PVP结合使用, 通过氢键和空阻达到抑制效果.     

A novel correlation for estimation of hydrate forming condition of natural gases

An inherent problem with natural gas production or transmission is the formation of gas hydrates, which can lead to safety hazards to production/ transportation systems and to substantial economic risks. Therefore, an understanding of conditions where hydrates form is necessary to overcome hydrate related issues. Over the years, several models requiring more complicated and longer computations have been proposed for the prediction of hydrate formation conditions of natural gases. For these reasons, it is essential to develop a reliable and simple-to-use method for oil and gas practitioners. The purpose of this study is to formulate a novel empirical correlation for rapid estimation of hydrate formation condition of sweet natural gases. The developed correlation holds for wide range of temperatures (265–298 K), pressures (1200 to 40000 kPa) and molecular weights (16−29). New proposed correlation shows consistently accurate results across proposed pressure, temperature and molecular weight ranges. This consistency could not be matched by any of the widely accepted existing correlations within the investigated range. For all conditions, new correlation showed average absolute deviation to be less than 0.2% and provided much better results than the widely accepted existing correlations.     

Accelerated nucleation of tetrahydrofuran(THF)hydrate in presence of ZIF-61

Clathrate hydrate can be used in energy gas storage and transportation,CO 2 capture and cool storage etc.However,these technologies are difficult to be used due to the low formation rate and long induction time of hydrate formation.In this paper,ZIF-61(zeolite imidazolate framework,ZIF) was first used in hydrate formation to stimulate hydrate nucleation.As an additive of clathrate hydrate,ZIF-61 promoted obviously the acceleration of tetrahydrofuran(THF) hydrate nucleation.It shortened the induction time of THF hydrate formation from 2-5 h to 0.3-1 h mainly due to the template function of ZIF-61 by which the nucleation of THF hydrate has been promoted.     

Experimental study on flow characteristics of tetrahydrofuran hydrate slurry in pipelines

Tetrahydrofuran (THF) was selected as the substitute to study the flow behaviors and the mechanism of the hydrates blockage in pipelines. The slurrylike hydrates and slushlike hydrates are observed with the formation of hydrates in pipeline. There is a critical hydrate volume concentration of 50.6% for THF slurries and pipeline will be free of hydrate blockage while the hydrate volume concentration is lower than the critical volume concentration; otherwise, pipeline will be easy to be blocked. Fully turbulent flow occurs and friction factors tend to be constant when the velocity reaches 1.5 m/s. And then, constant values of friction factors that depend on the volume concentrations in the slurry were regressed to estimate the pressure drops of THF hydrate slurry at large mean velocity. Finally, a safe region, defined according to the critical hydrate volume concentration, was proposed for THF hydrate slurry, which may provide some insight for further studying the natural gas hydrate slurries and judge whether the pipeline can be run safely or not.     

Initial estimation of hydrate formation temperature of sweet natural gases based on new empirical correlation

Production,processing and transportation of natural gases can be significantly affected by clathrate hydrates.Knowing the gas analysis is crucial to predict the right conditions for hydrate formation.Nevertheless,Katz gas gravity method can be used for initial estimation of hydrate formation temperature (HFT) under the circumstances of indeterminate gas composition.So far several correlations have been proposed for gas gravity method,in which the most accurate and reliable one has belonged to Bahadori and Vuthaluru.The main objective of this study is to present a simple and yet accurate correlation for fast prediction of sweet natural gases HFT based on the fit to Katz gravity chart.By reviewing the error analysis results,one can discover that the new proposed correlation has the best estimation capability among the widely accepted existing correlations within the investigated range.     

Study on the recovery of hydrogen from refinery (hydrogen + methane) gas mixtures using hydrate technology

A novel technique for separating hydrogen from (H2 CH4) gas mixtures through hydrate formation/dissociation was proposed. In this work, a systematic experimental study was performed on the separation of hydrogen from (H2 CH4) feed mixtures with various hydrogen contents (mole fraction x = 40%-90%). The experimental results showed that the hydrogen content could be enriched to as high as ~94% for various feed mixtures using the proposed hydrate technology under a temperature slightly above 0℃ and a pressure below 5.0 MPa. With the addition of a small amount of suitable additives, the rate of hydrate formation could be increased significantly. Anti-agglomeration was used to disperse hydrate particles into the condensate phase. Instead of preventing hydrate growth (as in the kinetic inhibitor tests), hydrates were allowed to form, but only as small dispersed particles. Anti-agglomeration could keep hydrate particles suspended in a range of condensate types at 1℃ and 5 MPa in the water-in-oil emulsion.     

Kinetic and thermodynamic behaviour of CF4 clathrate hydrates

This study presents experimental kinetic and thermodynamic data for CF₄ clathrate hydrates. Experimental measurements were undertaken in a high pressure equilibrium cell with a 40 cm³ inner volume. The measurements of experimental hydrate dissociation conditions were performed in the temperature range of (273.8 to 278.3) K and pressures ranging from (4.55 to 11.57) MPa. A thermodynamic model based on van der Waals and Platteeuw (vdW–P) solid solution theory was used for prediction and comparison of hydrate dissociation conditions and the Langmuir constant parameters for CF₄ based on Parrish and Prausnitz equation are reported. For the kinetics, the effect of initial pressure and temperature on the induction time, CF₄ hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion during the hydrate formation were studied. Kinetic experiments were performed at initial temperatures of (275.3, 276.1 and 276.6) K and initial pressures of (7.08, 7.92, 9.11, 11.47 and 11.83) MPa. Results show that increasing the initial pressure at constant temperature decreases the induction time, while CF₄ hydrate formation rate, the apparent rate constant of reaction, storage capacity, and water to hydrate conversion increase. The same trends are observed with a decrease in the initial temperature at constant pressure.     

Intensification of methane and hydrogen storage in clathrate hydrate and future prospect

Gas hydrate is a new technology for energy gas (methane/hydrogen) storage due to its large capacity of gas storage and safe. But industrial application of hydrate storage process was hindered by some problems. For methane, the main problems are low formation rate and storage capacity, which can be solved by strengthening mass and heat transfer, such as adding additives, stirring, bubbling, etc. One kind of additives can change the equilibrium curve to reduce the formation pressure of methane hydrate, and the other kind of additives is surfactant, which can form micelle with water and increase the interface of water-gas. Dry water has the similar effects on the methane hydrate as surfactant. Additionally, stirring, bubbling, and spraying can increase formation rate and storage capacity due to mass transfer strengthened. Inserting internal or external heat exchange also can improve formation rate because of good heat transfer. For hydrogen, the main difficulties are very high pressure for hydrate formed. Tetrahydrofuran (THF), tetrabutylammonium bromide (TBAB) and tetrabutylammonium fluoride (TBAF) have been proved to be able to decrease the hydrogen hydrate formation pressure significantly.     

I型和II型结构气体水合物的记忆效应

利用水合物二次生成实验装置, 采用“定容法”对I型(甲烷、二氧化碳)和II型(丙烷)结构气体水合物的二次生成进行了实验, 研究了不同结构水合物(I型、II型)彼此间的记忆效应, 发现水合物生成过程存在明显的诱导期, I型结构水合物间在二次生成过程中存在着记忆效应. I型与II型结构水合物之间在相互二次生成过程中存在着显著的记忆效应.     

Numerical Simulation on the Dissociation,Formation, and Recovery of Gas Hydrates on Microscale Approach

Investigations into the structures of gas hydrates, the mechanisms of formation, and dissociation with modern instruments on the experimental aspects, including Raman, X-ray, XRD, X-CT, MRI, and pore networks, and numerical analyses, including CFD, LBM, and MD, were carried out. The gas hydrate characteristics for dissociation and formation are multi-phase and multi-component complexes. Therefore, it was important to carry out a comprehensive investigation to improve the concept of mechanisms involved in microscale porous media, emphasizing micro-modeling experiments, 3D imaging, and pore network modeling. This article reviewed the studies, carried out to date, regarding conditions surrounding hydrate dissociation, hydrate formation, and hydrate recovery, especially at the pore-scale phase in numerical simulations. The purpose of visualizing pores in microscale sediments is to obtain a robust analysis to apply the gas hydrate exploitation technique. The observed parameters, including temperature, pressure, concentration, porosity, saturation rate, and permeability, etc., present an interrelationship, to achieve an accurate production process method and recovery of gas hydrates.     

常见客体分子对笼型水合物晶格常数的影响

Natural gas hydrates are considered as ideal alternative energy resources for the future, and the relevant basic and applied research has become more attractive in recent years. The influence of guest molecules on the hydrate crystal lattice parameters is of great significances to the understanding of hydrate structural characteristics, hydrate formation/decomposition mechanisms, and phase stability behaviors. In this study, we test a series of artificial hydrate samples containing different guest molecules (e.g. methane, ethane, propane, iso-butane, carbon dioxide, tetrahydrofuran, methane + 2, 2-dimethylbutane, and methane + methyl cyclohexane) by a low-temperature powder X-ray diffraction (PXRD). Results show that PXRD effectively elucidates structural characteristics of the natural gas hydrate samples, including crystal lattice parameters and structure types. The relationships between guest molecule sizes and crystal lattice parameters reveal that different guest molecules have different controlling behaviors on the hydrate types and crystal lattice constants. First, a positive correlation between the lattice constants and the van der Waals diameters of homologous hydrocarbon gases was observed in the single-guest-component hydrates. Small hydrocarbon homologous gases, such as methane and ethane, tended to form sI hydrates, whereas relatively larger molecules, such as propane and iso-butane, generated sⅡ hydrates. The hydrate crystal lattice constants increased with increasing guest molecule size. The types of hydrates composed of oxygen-containing guest molecules (such as CO₂ and THF) were also controlled by the van der Waals diameters. However, no positive correlation between the lattice constants and the van der Waals diameters of guest molecules in hydrocarbon hydrates was observed for CO₂ hydrate and THF hydrate, probably due to the special interactions between the guest oxygen atoms and hydrate "cages". Furthermore, the influences of the macromolecules and auxiliary small molecules on the lengths of the different crystal axes of the sH hydrates showed inverse trends. Compared to the methane + 2, 2-dimethylbutane hydrate sample, the length of the a-axis direction of the methane + methyl cyclohexane hydrate sample was slightly smaller, whereas the length of the c-axis direction was slightly longer. The crystal a-axis length of the sH hydrate sample formed with nitrogen molecules was slightly longer, whereas the c-axis was shorter than that of the methane + 2, 2-dimethylbutane hydrate sample at the same temperature.     

Experimental and theoretical investigation of simple gas hydrate formation with or without presence of kinetic inhibitors in a flow mini-loop apparatus

Gas hydrates are ice-like crystalline compounds, which form through a combination of water and suitably sized guest molecules under low temperature and elevated pressure conditions. These solid compounds give rise to problems in the natural gas oil industry because they can plug pipelines and process equipment. Low dosage hydrate inhibitors are a recently developed hydrate control technology, which can be more cost-effective than traditional practices such as methanol and glycols. The kinetics of hydrate growth has been modeled by numerous authors who have measured the gas consumption rate during hydrate formation in batch agitator reactors.     

13C固体核磁共振测定气体水合物结构实验研究

采用高功率1H去偶结合魔角旋转13C固体核磁共振技术,在低温常压条件下对合成的乙烷和丙烷气体水合物进行了测试,获得了两种纯气体水合物的13C核磁共振谱图,初步建立了固体核磁共振波谱法测定天然气水合物的实验方法.实验表明:乙烷水合物的13C核磁共振谱图中仅有一条谱线(δ7.7),结构类型为sI,且乙烷分子仅填充在大笼中(...     

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.     

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…