Molecular dynamics simulation study on the structure II clathrate-hydrates of methane + cyclic organic compounds

The ternary structure II (sII) clathrates of methane + cyclic organic compounds (COCs) are studied using molecular dynamics (MD) simulations. The results show that the COC guest molecules have a stabilizing effect on the clathrate-hydrate and stability of the unit cell decreases upon replacing more COC guest molecules in the large cages by CH₄ molecules which is confirmed by the comparison of radial distribution functions and unit cell volumes of the different occupancy cases. The maximum methane storage capacity for sII in the presence of tetrahydropyran, cyclobutanone, and cyclohexane are 11.9, 10.5, and 9.6 wt% of methane gas, respectively.     

~(13)C固体核磁共振法测定CH_4-THF二元水合物的微观结构特征

认识天然气水合物的微观结构特征对于研究其形成机理和储运技术具有重要的意义。为此,采用~(13)C固体核磁共振(SSNMR)技术分析了四氢呋喃(THF)水合物和CH_4—THF二元水合物的笼型结构特征,确定了后者的结构类型和客体分子分布特征,并获得了后者客体分子的笼占有率和水合指数。实验结果表明:①纯THF水合物的2个共振谱峰的化学位移分别为668.3和δ26.1,其THF分子填充在Ⅱ型水合物大笼(5~(12)6~4)中;②CH_4—THF二元水合物和纯THF水合物一样,同为Ⅱ型结构,其THF分子仅填充在大笼(5~(12)6~4)中,笼占有率为0.994 8,而CH_4分子仅占据小笼(5~(12)),笼占有率较低,仅为0.482 5;③由于CH_4分子填充率较低,二元水合物的水合指数为8.67,明显大干理想值(5.67),水合物储气量较小;④CH_4—THF二元水合物中客体分子笼占有率的大小与水合物的生成条件(温度、压力)及制备方法(反应状态、时间等)有关,改变水合物的形成条件,在一定程度上可以调节笼型水合物客体分子的笼占有率,从而提高水合物的储气密度。     

Experimental and Theoretical Investigations on the Enhancement of Methane Gas Hydrate Formation Rate by Using the Kinetic Additives

Effects of the most important influencing parameters on the methane gas hydrate formation kinetics at the onset of hydrate formation in the absence or presence of sodium dodecyl sulfate (SDS) kinetic promoter are investigated. At the SDS concentration of 500 ppm, the maximum methane effective diffusion coefficient and the highest conversion rate are obtained. The methane hydrate-liquid interfacial tension according to the suggested mechanism is about 18.6 mJm^–2 for all cases. The overall average absolute deviations between predicted and measured methane consumption rates are about 0.71% and 0.83% regarding nonpromoted and SDS-promoted gas hydrate formation processes, respectively.     

Analysis of natural gas hydrate formation in sodium dodecyl sulfate and quartz sand complex system under saline environment

Under natural conditions, natural gas hydrate occurs in the pores of porous media found in the sedimentary layer. Thus, the rapid formation and basic properties of the hydrate in the porous medium must be studied. Quartz sand with different particle sizes compounded with Sodium Dodecyl Sulfate solution was used to study the hydrate formation in the system at 275.15 K and 6 MPa under a saline environment. Results were as follows. 1) Methane gas uptake in a saline system with NaCl concentration of 50 mmol was higher than that in pure water. This finding indicated that although the salt is a thermodynamic inhibitor, low concentration of NaCl can promote the formation of hydrates. 2) In the initial stage of the experiment, the rate of hydrate formation in saline environment was significantly higher than that in pure water. After approximately 1 h, the formation rate of hydrate in the compound system decreased under a saltwater environment and was lower than that in the complex system under pure water. 3) The hydrogen bond network inside the high-concentration NaCl solution was seriously damaged. In this case, water molecules cannot easily form a cage structure, and the presence of chlorine ions weakens the stability of a small amount of formed hydrate cage, thereby further inhibiting the formation of hydrates. However, in the low-concentration NaCl system, mass transfer is improved, and the formation of hydrate is promoted because of the weakened hydrogen bonding at the gas–liquid interface.     

H型天然气水合物形成实验

大分子烃在小分子烃类气体（如甲烷、乙烷）的帮助下可形成H型天然气水合物。为此，分别进行了该型天然气水合物形成条件和形成过程实验研究。前者表明，在甲烷水合物实验体系中加入甲基环己烷，则形成的天然气水合物结构会由Ⅰ型转变为H型，水合物的形成压力降低1.0 MPa以上。后者表明，甲基环己烷可提高甲烷水合物反应体系中水合物的形成速度，但减少了水合物的生长时间和甲烷气体耗量；含甲基环己烷体系水合物的耗气量随压力的增大而增大，但随着甲基环己烷含量的增大则水合物的形成速度和耗气量逐渐减少。     

An experimental study of the synergistic effects of BMIM-BF_4,BMIM-DCA and TEACI aqueous solutions on methane hydrate formation

In this work, the effects of three ionic liquids(ILs), namely, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium dicyanamide and tetraethyl-ammonium chloride, on methane hydrate formation and dissociation kinetic parameters were studied. The kinetic parameters including the initial rate of hydrate formation, hydrate stability at atmospheric pressure and hydrate storage capacity were evaluated. The experimental measurements were performed in an initial pressure range of 3.5-7.1 MPa. It was found that both of ILs with imidazolium-based cation increase the initial methane hydrate formation rate while the IL with ammonium-based cation leads to a decrease in the initial methane hydrate formation rate. It was also interpreted from the results that all of the three studied ILs decrease methane hydrate stability at atmospheric pressure and increase methane hydrate storage capacity. Finally, both of ILs with imidazolium-based cations were found to have higher impacts on decreasing hydrate stability at atmospheric pressure and increasing the methane hydrate storage capacity than the applied IL with ammonium-based cation.     

甲烷水合物分解过程原位激光拉曼光谱观测

为了获取水合物分解过程中分子水平的信息,采用激光拉曼光谱法对冰—水转化及两种不同体系下甲烷水合物的分解过程进行了原位监测,研究了冰—水转化及甲烷水合物分解的微观动力学过程,探讨了水合物分解过程中笼型结构的变化。实验结果表明,在冰融化过程中,冰的四面体结构不断瓦解,氢键数目不断减少,O—H键的特征谱带中,带氢键的伸缩振动拉曼峰所占比例逐渐下降,不带氢键的伸缩振动拉曼峰的比例相应增大。两种不同体系下合成的甲烷水合物均为Ⅰ型水合物,大小笼中甲烷分子的拉曼峰分别为2905cm-1和2915cm-1,随着水合物的不断分解,拉曼双峰逐渐消解,并最终转化成甲烷气体单峰(2917cm-1);结构水峰的变化与冰融化的过程相类似,随着笼型结构的瓦解,氢键数目迅速减少,导致带氢键的O—H键伸缩振动峰减弱。     

An intelligent optimization–based prediction model for natural gas hydrate formation in a deepwater pipeline

Temperature, pressure, and composition of gas mixtures in deepwater pipelines promote rapid formation of gas hydrates. To avert this dilemma, it is more significant to find out the temperature and pressure limits in gas hydrates formation of the deepwater pipeline. The objective of this research is to develop an optimization method that finds the optimal temperature and pressure profile for natural gas hydrate formation conditions and an error calculation method to find the realistic approach of the hydrate formation prediction model. A newly developed correlation model is computing the hydrate formation pressure and temperature for a single component of methane (CH₄) gas. The proposed developed prediction model is based on the 2 and 15 constant coefficients and holds a wide range of temperature and pressure data about 2.64 to 46°C and 0.051 to 400 MPa for pure water and methane, respectively. The reducing error discrepancies are 1.2871, 0.35012, and 1.9052, which is assessed by GA, PSO, and GWO algorithms, respectively. The results show the newly developed optimization algorithms are in admirable compliance with the experimental data and standards of empirical models. These correlations are providing the capability to predict gas hydrate forming conditions for a wide range of hydrate formation data.     

Effect of SDS,silver nanoparticles,and SDS + silver nanoparticles on methane hydrate semicompletion time

In this research, the semicompletion time (t_95%) of gas uptake in the process of methane hydrate formation in the presence and absence of sodium dodecyl sulfate (SDS) and silver nanoparticles (SNPs) is investigated. The experiments were carried out in a 460 cm³ stirred batch reactor at temperatures of 273.65 and 275.65 K and initial pressures of 5, 5.7, and 7 MPa. The results showed that the addition of both additives at all tested conditions, noticeably, decreases the semicompletion time of methane uptake, compared to pure water. The semicompletion time of methane uptake in the presence of 500 ppm SDS, 45 μM SNPs, and 500 ppm SDS + 45 μM SNPs is 75.6, 546.3, and 129.9 min, respectively, while that for pure water it is 8729.0 min at 273.65 K and 7 MPa.     

喷雾反应器对生成甲烷水合物影响研究

对喷雾制备甲烷水合物进行实验研究,考察制备水合物的工艺过程和特点,分析喷嘴喷雾角、气液空间体积比等影响因素,探讨甲烷水合物的生长机理。实验表明:采用实心45°喷雾角的喷嘴的雾化效果最好,反应容器最佳气液相空间体积比约为1∶1;甲烷气体消耗量随着过冷度的增大而增大,而且水合物晶体生长形态差异明显;系统水过冷度作为甲烷水合物生长驱动力,拟合出甲烷水合物生长速率与过冷度的关系式。     

煤层中甲烷水合物存在可能性探讨

之所以进行此项研究，就是力图使煤层气和瓦斯研究者重视煤层中甲烷水合物的存在，拓展煤层气或天然气水合物研究领域。为此，依据目前形成甲烷或天然气水合物的物质基础和温度压力的最新研究成果，结合煤层气形成及组成、含气煤层条件和煤的孔隙特征，探讨了煤层中甲烷水合物形成的可能性，并对煤层中甲烷水合物的相关问题进行了初步讨论。研究表明：①煤层中存在着甲烷水合物形成的物质、储存和温压条件；②煤的微孔结构和煤层气组成，特别是重烃组分有利于甲烷水合物在煤中的形成；③煤层中不仅存在着吸附态、游离态和溶解态甲烷，而且还存在着固态水合物甲烷；④煤层周围的岩层中也可能存在非常规天然气水合物。煤层气勘探开发和瓦斯防治过程中应重视煤层及其围岩中甲烷水合物的存在，对此还有大量的研究工作要做。     

PAM-AA水凝胶体系中甲烷水合物生成过程的实验研究

作为储量巨大的清洁能源和应用广泛的工业原料,天然气水合物越来越多地受到学者和工业界的关注。实验研究了含聚丙烯酰胺-丙烯酸（PAM-AA）盐溶液中甲烷水合物在反应釜中的生成过程,揭示了PAM-AA水凝胶体系对生成甲烷水合物的作用。结果表明:2 ℃、8 MPa时,水凝胶体系促进甲烷水合物生成;2 ℃、4.5 MPa时,水凝胶体系抑制甲烷水合物生成;PAM-AA水凝胶体系能够扰乱甲烷水合物的正常生成,表现为缩短甲烷水合物大量生成的时间（促进）和阻碍气液间的传质（抑制）;实验生成的甲烷水合物形态分为两种,即疏松且含有大量孔隙或坚硬且致密,分别对应抑制/促进甲烷水合物生成的体系。通过探究PAM-AA水凝胶体系中甲烷水合物的生成过程,能够为工业领域内甲烷水合物的生成、防治、储存和运输提供借鉴。     

Kinetics of (TBAF + CO2) semi-clathrate hydrate formation in the presence and absence of SDS

In this communication, the impacts of adding SDS (sodium dodecyl sulfate), TBAF (tetra-n-butylammonium fluoride) and the mixture of SDS + TBAF on the main kinetic parameters of CO₂ hydrate formation (induction time, the quantity and rate of gas uptake, and storage capacity) were investigated. The tests were performed under stirring conditions at T = 5 ℃ and P = 3.8 MPa in a 169 cm³ batch reactor. The results show that adding SDS with a concentration of 400 ppm, TBAF with a concentration of 1–5 wt%, and the mixture of SDS + TBAF, would increase the storage capacity of CO₂ hydrate and the quantity of gas uptake, and decrease the induction time of hydrate formation process. The addition of 5 wt% of TBAF and 400 ppm of SDS would increase the CO₂ hydrate storage capacity by 86.1% and 81.6%, respectively, compared to pure water. Investigation of the impact of SDS, TBAF and their mixture on the rate of gas uptake indicates that the mixture of SDS + TBAF does not have a significant effect on the rate of gas uptake during hydrate formation process.     

Experimental study of separation of ammonia synthesis vent gas by hydrate formation

Termodynamic data on methane hydrate formation in the presence of ammonia are very important for upgrading of ammonia synthesis vent gas using hydrate formation. This paper is focused on the formation conditions of methane hydrate in the presence of ammonia and the effects of gas-liquid ratio and temperature on the separation of vent gas by hydrate formation. Equilibrium data for methane hydrate within an ammonia mole concentration range from 1% to 5 % were obtained. The experimental results indicated that ammonia has an inhibitive effect on hydrate formation. The higher the ammonia concentration, the higher is the pressure reguired for methane hydrate formation would be. The primary experimental results showed that when volume ratio of gas to liquid was 80：1 and temperature was 283.15 K, total mole fraction of （H2＋N2） in gas phase could reach 96.9 %.     

天然气水合物勘探及结构H型水合物的特性研究

天然气水合物发展的实施障碍主要在于其勘探开发技术，文章介绍了目前天然气钻探的几种基本方法即地球化学方法、保压取心法、BSR技术及其它方法；同时也介绍了用震动室装置反应器研究甲烷气结构H型水合物的储藏性能，假定甲烷气要占据5个小格(3个512和2个435663)，而最大的格子(1个51268)由一个单位结构大分子(如新己烷)占据。把这种将甲烷气储藏于小格子的方法进行理论比较，表明一个单位体积的结构H型水合物能储藏160～200体积的甲烷气。进行了三种实验来测量甲烷气结构H型水合物储藏。发现用重量比0．1％的卵磷脂和PVP溶液可促使结构H型水合物的形成。与纯水合物相比，当这些水合物在较高温度下熔化时，促进剂起到了水合物的稳定性作用。     

温度梯度对粗砂中甲烷水合物形成和分解过程的影响及电阻率响应

评价甲烷水合物形成和分解过程中电阻率的变化对多年冻土区天然气水合物的勘测具有重要意义。利用本实验室自主研发设计的测量冻土相变温度和电阻率分布的装置,研究温度梯度对粗砂中甲烷水合物形成和分解过程的影响以及在此过程中的电阻率响应。实验表明,该装置可以准确有效地探测出水合物成核、形成、聚集及分解的过程。同时温度梯度的大小对多孔介质中水合物的形成和分布具有很大影响,随着温度梯度的增大,水合物的分布越不均匀,在高温端富集的水合物越多,水合物发生富集的时间间隔就越短。随着反应过程中水合物饱和度的增大,电阻率随之也增大.     

冰点以下不同粒径冰颗粒形成甲烷水合物的实验

冰点以下甲烷水合物形成动力学研究一直是天然气水合物实验室研究的重点。为此,采用6种不同粒径的冰颗粒开展了甲烷水合物的形成实验研究,研究冰点以下甲烷水合物形成过程中,诱导初始压力、补充压力、诱导生成反应时间的变化、冰颗粒的粒径等因素对甲烷水合物形成的影响。实验结果表明：在相同的实验条件下,初始诱导压力越大越容易形成甲烷水合物;冰颗粒粒径对甲烷水合物形成有较大影响,粒径较小的冰颗粒越容易形成甲烷水合物,形成反应的时间也较快。     

甲烷水合物在石英砂中生成过程研究

在初始压力Pin=13．0MPa±0．2MPa,过冷度Twup=14．2K±0．1K条件下,研究了7种不同粒径石英砂中甲烷水合物生成诱导期、生成时间以及孔隙水“记忆效应”的影响。结果显示,石英砂中诱导期较纯水中明显缩短;石英砂粒径减小,诱导期变长。孔隙水“记忆效应”极大缩短甲烷水合物诱导期,并表现出随机性。石英砂中甲烷水合物生成时间与石英砂粒径、孔隙水“记忆效应”的关系与诱导期类似。分析认为石英砂颗粒提供更多成核中心,而孔隙减小阻碍了甲烷的扩散。     

不同颗粒介质内甲烷水合物形成反应特征

在半饱和含水态具有不同粒径、孔隙半径的人工、天然介质--硅胶、粉土内形成甲烷水合物,3支pF-meter探头测量反应过程中介质内垂直方向上不同位置处的基质势,据此计算分析介质内由水合物形成引起的水分消耗规律,进而探讨不同介质内甲烷水合物形成反应特征。通过实验发现,多孔介质的颗粒、孔径物理性质明显影响甲烷水合物的形成反应特征：水合物在具天然介质性质的粉土内的形成反应是一个长期持续过程,相反地,在硅胶内的形成反应过程较短且在硅胶内均匀形成;人工多孔介质--硅胶粉内的水合物形成反应速率维持恒定,天然多孔介质--粉土内的水合物形成速率由快减慢;甲烷气体由反应釜顶部注入,气体扩散进入介质顶层的速率最     

沉积物体系中甲烷水合物平衡温度、压力条件实验模拟

研究沉积物中甲烷水合物的平衡温度和压力条件，对认识甲烷水合物的稳定性以及对未来准确评价和利用甲烷水合物能源非常重要。在温度为270．9～278．2K、压力为2．47～4．31 MPa条件下，分别对平均孔径为53．2nm、27．2nm和15．5nm的沉积物体系中甲烷水合物的分解平衡温度和压力条件(即相平衡)进行了测定，结果表明，近自然沉积物的平均孔径大小会影响甲烷水合物的相平衡条件。相同温度条件下，纯水体系中甲烷水合物的平衡压力比在沉积物体系中的低；随着孔径的增大，甲烷水合物在相图中稳定区的面积逐渐增加，但当体系中温度降低到冰点以下或者沉积物孔径增大到超过60nm时，沉积物孔隙毛细管对甲烷水合物稳定性的影响非常小，与纯水中的重合或者接近。甲烷水合物在沉积物体系中的相平衡数据可以用经验热力学方程拟合，拟合结果能反映孔径大小与甲烷水合物平衡压力、温度条件的相互关系。图3表1参20