Kinetic simulation of methane hydrate formation and dissociation in porous media

The vast amount of hydrocarbon gas deposited in the earth's crust as gas hydrates has significant implications for future energy supply and global climate. A 3-D simulator for methane hydrate formation and dissociation in porous media is developed for designing and interpreting laboratory and field hydrate experiments. Four components (hydrate, methane, water and salt) and five phases (hydrate, gas, aqueous-phase, ice and salt precipitate) are considered in the simulator. The intrinsic kinetics of hydrate formation or dissociation is considered using the Kim-Bishnoi model. Water freezing and ice melting are tracked with primary variable switch method (PVSM) by assuming equilibrium phase transition. Mass transport, including two-phase flow and molecular diffusions, and heat transfer involved in formation or dissociation of hydrates are included in the governing equations, which are discretized with finite volume difference method and are solved in a fully implicit manner. The developed simulator is used here to study the formation and the dissociation of hydrates in laboratory-scale core samples. In hydrate formation from the system of gas and ice (G+I) and in hydrate dissociation systems where ice appears, the equilibrium between aqueous-phase and ice (A-I) is found to have a “blocking” effect on heat transfer when salt is absent from the system. Increase of initial temperature (at constant outlet pressure), introduction of salt component into the system, decrease of outlet pressure, and increase of boundary heat transfer coefficient can lead to faster hydrate dissociation.     

Methane-ethane and methane-propane hydrate formation and decomposition on water droplets

Gas hydrate formation and decomposition on water droplets using an 89.4% methane—10.6% ethane mixture, and a 90.1% methane—9.9% propane mixture were carried out in a new apparatus suitable for morphology studies. As expected the induction time was found to be much shorter when the water had hydrate memory. All droplets nucleated simultaneously and the droplet size and shape had no noticeable effect on induction time and macroscopic crystal growth morphology for hydrates from the methane-ethane mixture. However, the surface of the hydrate crystals from methane-propane had a “hairy-like” appearance which changed to a smooth surface over time. Moreover, the smaller droplets during hydrate reformation showed an extensive hydrate growth and looked like snow-flakes. Sequential pictures generated by time-lapse videos showed that the time required for hydrate to cover the water droplet surface ranged from 10 to 23 s and was shorter when there was gas-phase agitation (mixing). The growth is postulated to occur in two stages. The first stage lasts about 10-23 s and growth takes place laterally. Growth takes place at the hydrate/gas and the hydrate/water interfaces during the second stage. The implication of the findings for process design of hydrate formation vessels is also discussed.     

DSC measurements and modelling of the kinetics of methane hydrate formation in water-in-oil emulsion

The kinetics of formation of clathrate hydrates of methane was investigated in a water-in-oil emulsion using high-pressure differential scanning calorimetry in the range 10-40 MPa, at various temperatures. At high driving force, the heat peak related to the formation of hydrates has a regular and symmetric shape, and its height and width depend on the gas pressure and sub cooling degree. At near equilibrium conditions, hydrate formation is delayed by more than 1 h, but is still clearly observable. A model based on crystal growth theory, coupled with a normal distribution of induction times to take into account the germination in a population of micro-sized droplets, is proposed to represent the hydrate formation rate versus time in the particular case of water-in-oil emulsions. It uses four parameters which appear strongly correlated to the experimental conditions: the growth rate constant, the over saturation of gas in the water phase, the average and standard deviation of the induction time distribution.     

Experimental data and approximate estimation for dissociation time of hydrate plugs

New experimental data coupled with a numerical model and an approximate solution are proposed to predict dissociation time of hydrate plugs in oil sub-sea pipelines. The experimental hydrate plugs are dissociated by the method of symmetric depressurisation, both in a specially designed apparatus and a classical batch reactor. The agreement between the estimation of the model and the experimental data and the simplicity of the approximate equation presents an advantage in estimating the time of hydrate plug dissociation in pipelines.     

Effect of surfactant on the formation and dissociation kinetic behavior of methane hydrate

The effects of anionic surfactant sodium dodecyl sulfate (SDS) on the formation/dissociation kinetic behaviors of methane hydrate have been studied experimentally, with an emphasis put on dissociation kinetic behavior below ice point. The experimental results on hydrate formation show that the formation rates of methane hydrate could be speeded up by adding SDS to water and a critical SDS concentration of 650 ppm corresponding to a maximum storage capacity of 170V/V is determined. The SDS concentrations are fixed at this value in preparing hydrate samples for all dissociation tests. The dissociation experiments have been performed in two ways, at atmospheric pressure where the dissociation rates are determined by measuring the accumulative evolved gas volume, and in a closed system where the dissociation rates are determined by measuring the increasing system pressure profiles. For comparison, the dissociation tests with respect to two different cases, with and without the presence of SDS, are done in parallel. The results from tests in the first way show that the presence of SDS increases the dissociation rate of methane hydrate in whole temperature region below ice point. The results for the second way are somewhat different. The presence of SDS increases the dissociation rate and meta-stable system pressure in temperature region lower than . But when temperature is equal to or higher than , SDS speeds up the dissociation process only in beginning period, it turns to suppress the dissociation of methane hydrate several hours later and leads to a lower meta-stable system pressure compared with the case of without SDS. The experiments in closed system also demonstrate that the dissociating system approaches a meta-stable state with a pressure much lower than equilibrium dissociation pressure.     

Effect of different surfactants on methane hydrate formation rate, stability and storage capacity

The effects of anionic surfactants sodium dodecyl sulfate (SDS) and linear alkyl benzene sulfonate (LABS), cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and non-ionic surfactant ethoxylated nonylphenol (ENP) on the formation, dissociation and storage capacity of methane hydrate have been investigated. Each surfactant was tested with 3 concentrations 300, 500 and 1000 ppm and it has been found that SDS, when prepared with these three concentrations speeds up the hydrate formation rate effectively. LABS increases the hydrate formation rate at 500 and 1000 ppm but decreases it at 300 ppm. CTAB and ENP have promotion effect on hydrate formation rate at 1000 ppm but decrease it at 300 and 500 ppm. Hydrate stability tests have been performed at three temperatures 268.2, 270.2 and 272.2 K with and without surfactant promoters. The results show that all tested additives increase the dissociation rate of methane hydrate below the ice point. CTAB has the minimum and LABS the maximum effect on the methane hydrate dissociation rate. Experimental results on hydrate gas content revealed that maximum storage capacity of 165 V/V is obtained with 1000 ppm of CTAB in water.     

Effect of heavy asphaltene on stability of residual oil

An asphaltene fractionation method was developed in order to investigate the effect of heptane (n-C7) insoluble asphaltene (C7-asphaltene) on residual oil stability. C7-asphaltene was separated into heavy and light fractions by a new method using a binary solvent system of toluene and heptane (n-C7). It was found that the heavy fraction of C7-asphaltene in residual oil, extracted by this method, consisted of highly condensed polynuclear aromatics. Our new fractionation method and the Heithaus stability evaluation method were applied to hydroprocessed residual oils. The peptizability of heavy fraction of C7-asphaltene defined by the Heithaus method decreased in accordance with structural condensation of that fraction. On the other hand, light fraction of C7-asphaltene was considered to influence the peptizing power. We proposed a new conceptual model: light asphaltenes would perform as peptizing material as well as resin, and heavy asphaltene would be peptized in oil. This model introduced from our new asphaltene fractionation method could be more effective for understanding the destabilization phenomenon of residual oil.     

卵磷脂对甲烷水合物形成的影响

建立了用于测定卵磷脂(lecithin)对钻井液中水合物形成影响的实验装置及方法,以理解化学添加剂卵磷脂对北极Cascade地区钻井过程中水合物层的稳定作用。本研究旨在理解卵磷脂对纯水中甲烷水合物形成热力学和动力学的影响。结果表明,卵磷脂基本上不影响甲烷水合物生成的热力学条件,但当卵磷脂在水中的浓度超过0.003 g·g^-1时,它会影响甲烷水合物的生成速度和数量,是很好的水合物生成动力学促进剂。     

Co-adsorption behaviors of asphaltenes and different flow improvers and their impacts on the interfacial viscoelasticity

Commonly used flow improvers in oilfields, such as ethylene–vinyl acetate copolymer (EVA), poly(octadecyl acrylate) (POA), and polymethylsilsesquioxane (PMSQ) are proven to be effective to enhance the flowability of crude oil. However, the addition of these flow improvers may change the stability of the emulsion and make the crude oil treatment process challenging. In this research, the impacts of different flow improvers on the interfacial properties of the emulsions containing asphaltenes are systematically investigated. The co-adsorption behaviors of the flow improvers and asphaltenes are analyzed through dynamic interfacial tension (DIFT). The rheological properties of the interfacial layer after the adsorption are explored via dilational viscoelasticity. Significant difference is observed in the structural properties of the interface adsorbed by different flow improvers, which is attributed to different interactions between the flow improvers and asphaltenes. To investigate these interactions, conductivity, asphaltenes precipitation, dynamic light scattering (DLS), and contact angle experiments are conducted systematically. Results show that EVA and POA can alter the interfacial properties by changing the asphaltene dispersion state. The interaction between EVA and asphaltenes is stronger than that between POA and asphaltenes due to the difference in molecular structures. Unlike EVA and POA, the change of interfacial property with the addition of PMSQ is attributed to the partial adsorption of asphaltenes on PMSQ.     

A water droplet size distribution dependent modeling of hydrate formation in water/oil emulsion

Experimental data on chord length distributions and growth rate during methane hydrate formation in water‐in‐oil emulsions were obtained in a high pressure stirring reactor using focused beam reflectance measurement and particle video microscope. The experiments were carried out at 274.2 K for 10–30% water cuts and agitation rates ranging from 200 to 500 rpm initially at 7.72 MPa. Rapid growth was accompanied by gradually decrease in rate. Free water was observed to become depleted during rapid growth while some water remained encapsulated inside hydrate layers constituting a mass transfer barrier. The apparent kinetic constants of methane hydrate formation and free‐water fractions were determined using a newly developed kinetic model independent of the dissolution rate at the gas–oil interface. It was illustrated that continued growth depends on distribution and transfer of water in oil‐dominated systems. This perception accords with observations of hydrate film growth on suspended water droplet in oil and clarifies transfer limits in kinetics. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1010–1023, 2017     

A study on formation and stability of epoxy resin inverse concentrated water/oil emulsion

Inverse concentrated emulsions were prepared using aqueous colloidal silica suspension as the hydrophilic dispersed phase and a solution of diglycidyl ether of bisphenol‐A (DGEBA), its curing agent polyamide resin, low molecular weight 650, surfactant nonyl phenol polyoxyethylene ether (NPE‐4) in 4‐methyl‐2‐pentanon as the continuous phase, which was expected to be used as the precursors of preparation of porous epoxy resins. The stability, i.e., the resistance to phase separation was studied. The effects of various parameters on the stability of the concentrated emulsions were investigated. The colloidal silica can strengthen the steric repulsion in the system and improve the stability. Viscosity of both phases played a major role in the stability. Precuring of the continuous phase provided an increased initial viscosity and enhanced the stability. Lower volume fraction of the dispersed phase can help to maintain stability of the concentrated emulsions. Properly increasing the curing rate, the concentrated emulsions may acquire a high viscosity in a short time, which retarded the phase separation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Experimental and modeling study of the kinetics of methane hydrate formation and dissociation

In this work, several experiments were conducted at isobaric and isothermal condition in a CSTR reactor to study the kinetics of methane hydrate formation and dissociation. Experiments were performed at five temperatures and three pressure levels (corresponding to equilibrium pressure). Methane hydrate formation and dissociation rates were modeled using mass transfer limited kinetic models and mass transfer coefficients for both formation and dissociation were calculated. Comparison of results, shows that mass transfer coefficients for methane hydrate dissociation are one order greater than formation conditions. Mass transfer coefficients were correlated by polynomials as relations of pressure and temperature. The results and the method can be applied for prediction of methane production from naturally occurring methane hydrate deposits.     

油水乳液中水合物法分离甲烷/乙烯生成动力学

低碳烯烃的分离在石化行业有着重要意义。使用恒容的釜式反应器进行了水合物法分离甲烷和乙烯混合气动力学实验,并研究了不同温度和乳液用量对于水合分离效果和生成动力学的影响。在温度为273.35 K、100 ml乳液用量条件下,混合气中乙烯含量从34%降至9.97%,回收率达到90.36%,温度越低、乳液量越多,乙烯回收率越高。提出了一套以逸度差为推动力的动力学模型,能够较好地拟合实验数据,同时根据参数能够较好地预测实验结果。     

Hydrodynamic cavitation as an efficient method for the formation of sub-100 nm O/W emulsions with high stability

Hydrodynamic cavitation, a newly developed process intensification technique, has demonstrated immense po-tential for intensifying diverse physical and chemical processes. In this study, hydrodynamic cavitation was ex-plored as an efficient method for the formation of sub-100 nm oil-in-water (O/W) emulsions with high stability. O/W emulsion with an average droplet size of 27 nm was successful y prepared. The average droplet size of O/W emulsions decreased with the increase of the inlet pressure, number of cavitation passes and surfac-tant concentration. The formed emulsion exhibited admirable physical stability during 8 months. Moreover, the hydrodynamic cavitation method can be generalized to fabricate large varieties of O/W emulsions, which showed great potential for large-scale formation of O/W emulsions with lower energy consumption.     

有机相变乳液中HCFC–141b水合物生成及稳定性

为了促进水合物形成，在HCFC–141b、有机相变材料（正癸酸和十二醇）和水体系中添加表面活性剂Tween 80和Span 80作为乳化剂，采用高速搅拌的方法制备了有机相变材料-表面活性剂-制冷剂-水乳液体系，增大水分子与制冷剂的接触面积。实验研究了静态条件下有机相变材料和表面活性剂添加量对水合物形成的影响。研究结果表明添加乳化剂可以有效提高水合物的蓄冷量，减少水合物形成诱导时间，降低水合物生成的随机性；温度越低，水合物促进效果越好。水合物生成/分解循环实验表明，添加Tween 80的乳液体系的稳定性好，有机相变乳液提高了水合物生成/分解循环过程的稳定性。     

Effect of additives on formation of natural gas hydrate

The formation of natural gas hydrate (NGH) is studied in this work. Kinetics data of hydrate formation with no agitation were collected at various concentrations of the aqueous solutions with different additives such as alkylpolyglucside, sodium dodecyl benzene sulfonate and potassium oxalate monohydrate. Various kinds of additive increased the formation rates of NGH and its storage capacity and reduced the induction time of NGH formation. Moreover, the storage capacity, the induction time and the hydrate formation rate were influenced by the concentration of the aqueous solution.     

天然气水合物的生成对浆液流动稳定性影响综述

目前在海底混输管道的水合物风险控制策略中,允许水合物在管道内的生成,以液固浆液流动的形式对海底油气产物进行输送。其中主要通过控制浆液中水合物的生成量和聚集程度,来实现对海底集输管线的流动安全保障。液固浆液流动具有相当复杂的流动特性,固相颗粒的引入对于流体的流动特性影响很大。本文分别综述了拟单相流动体系和气液多相流动体系中水合物颗粒对于管输体系流动稳定性的影响以及水合物对混输管道堵管特性的影响。着重讨论了水合物在管道壁面的生长和沉积特性、水合物与气液流型的耦合关系以及不同体系中水合物的堵管机理。此外,对软件模拟在水合物生成及浆液流动特性研究中的应用做了简单介绍。最后,根据对相关研究结果的总结,指出水合物在壁面生长沉积的微观特性和定量表述、颗粒不同分散形式的临界流速、不同气液流型条件下的水合物生成特性和颗粒行为等是今后水合物相关研究中需要进一步深入探究和明确的问题。     

添加剂对柴油机润滑油油水界面性质及乳状液稳定性的影响

测定了柴油机油模拟油/蒸馏水体系的界面张力、界面剪切粘度和乳状液稳定性。结果表明,含有添加剂T154的柴油机油模拟油与蒸馏水间的界面张力明显低于无添加剂的体系的界面张力,界面剪切粘度增加,乳状液特性值比模拟油/蒸馏水体系稍大,乳状液稳定性增加;添加剂T109却使模拟油与蒸馏水体系的界面张力增大,界面剪切粘度降低,乳状液特性值比模拟油/蒸馏水体系小,乳状液稳定性降低,有一定的破乳作用。     

Effect of Soy-Derived Phospholipid on the Autoxidation of Canola Oil in a Water/Oil Emulsion

The effects of addition of soy‐derived phosphatidylcholine (PC), phosphatidylethanolamine (PE), or phosphatidylinositol (PI) and the contribution of their structural segment during iron‐catalyzed autoxidation of canola oil in a water/oil (W/O) (1:1, w/w) emulsion were studied by headspace oxygen consumption using gas chromatography and hydroperoxide production by the ferric thiocyanate method. The phospholipid content was monitored by high performance liquid chromatography. Addition of PC and PE significantly (p < 0.05) improved the oxidative stability of the oil in the emulsion by decelerating headspace oxygen consumption and hydroperoxide production, with the PC having higher antioxidant effect. All phospholipids were degraded during autoxidation of the emulsion, with higher sensitivities of the PE and PC than the PI. Among the structural segments, ethanolamine and phosphoric acid significantly contributed to the antioxidant activity, while inositol showed little effect. Linoleic acid and choline showed the highest antioxidant activity at 350 mg/kg. The results suggest that hydrogen donation and a physical barrier to oxygen contribute to the antioxidant activity of PE and PC in the W/O emulsion, respectively.