不同浓度聚乙烯醇对甲烷水合物分解作用的分子动力学模拟

采用分子动力学方法,模拟了273.15K下聚乙烯醇(PVA)对甲烷水合物的分解作用.研究发现,PVA浓度为2.7wt%时,水分子始终在其平衡位置附近波动,扩散系数仅为1.04×10-11m2/s;PVA浓度为5.2wt%时,水合物笼型结构坍塌,水分子以液态水的形式存在,甲烷分子从孔穴中逸出,聚乙烯醇的羟基在分子内部形成氢键,形成团簇,产生空间位阻,阻止了水分子再生成水合物,水分子的扩散系数1.61×10-9m2/s;PVA浓度为7.6wt%时,甲烷水合物周围有部分笼型结构被破坏,部分甲烷分子从孔穴中逸出,水分子扩散系数为3.55×10-10m2/s.得出聚乙烯醇对甲烷水合物的分解作用大小为:5.2wt%7.6wt%2.7wt%,为PVA溶液促进甲烷水合物分解实验研究提供参考.     

热力学抑制剂作用下甲烷水合物分解过程的分子动力学模拟

采用正则系综(NVT)分子动力学方法模拟研究277.0 K、11.45 mol·L-1的热力学抑制剂乙二醇(EG)溶液作用下甲烷水合物分解微观过程. 模拟显示甲烷水合物的分解从甲烷水合物固体表面开始, 逐渐向内部推移, 固态水合物在分解过程中逐渐缩小, 直至消失. 固态水合物的分解从晶格扭曲变形开始, 之后笼形框架结构破裂, 最后形成笼形结构碎片. 同时已经分解的甲烷水合物在外层形成水膜, 包裹里层正在分解的甲烷水合物, 增大里层甲烷水合物分解传质阻力.     

过氧化氢水溶液作用下甲烷水合物分解特性的分子动力学模拟研究

采用分子动力学模拟方法研究过氧化氢水(HP)溶液作用下结构I型(SI)甲烷水合物晶体分解特性. 系统分析甲烷水合物在过氧化氢水溶液作用下由晶态向液态转变过程的机理, 对比相同摩尔浓度乙二醇(EG)溶液作用下甲烷水合物分解变化规律, 得出HP与水合物热力学抑制剂EG一样对甲烷水合物分解具有促进作用, 为HP溶液促进甲烷水合物分解实验研究提供参考.     

用分子动力学模拟天然气水合物的抑制效应

用分子动力学模拟方法确定了结构H型(SH)天然气水合物的稳定晶体生长面为(001), 系统研究了277 K时三种动力学抑制剂对此晶面的影响. 模拟显示抑制剂中的氧与表面水分子形成氢键, 从而破坏原有的稳定结构, 造成水合物笼型结构坍塌, 达到抑制水合物形成的效果. 比较三种不同动力学抑制剂对SH的抑制效果得出: PVCap＞PEO＞PVP. 在此基础上研究了PVCap对天然气水合物结构I型(SI), 结构II型(SII)和SH三种不同晶型的抑制效应. 模拟发现抑制效果的次序为: SH＞SI＞SII.     

客体分子所处晶穴结构不同对甲烷水合物稳定性的影响

采用分子动力学方法模拟了SⅠ型甲烷水合物受热分解微观过程,并对水合物分解过程中不同晶穴结构内客体分子对甲烷水合物稳定性的作用进行了研究.通过最终构象、均方位移和势能等性质的变化规律对分别缺失大晶穴和小晶穴中客体分子的2种水合物体系随模拟温度升高稳定性的变化进行了分析.模拟结果显示,随温度的上升,水合物稳定性逐渐下降直至彻底分解;而水合物分解速度与2种晶穴各自部分晶穴占有率相关,不能简单的通过整体晶穴占有率表示.对比相同注热过程中2种水合物体系分解状况,发现位于大晶穴内的客体分子对水合物稳定性影响更大,缺失大晶穴内客体分子的水合物更容易随温度升高而分解.     

甲烷水合物导热机理的分子动力学模拟

甲烷水合物导热系数是甲烷水合物勘探、开采、储运以及其他应用过程中一个十分重要的物理参数.我们采用平衡分子动力学（EMD）方法Green-Kubo理论计算温度203.15～263.15K、压力范围3～100MPa、晶穴占有率为0～1的sI甲烷水合物的导热系数,采用的水分子模型包括TIP4P、TIP4P-Ew、TIP4P-FQ、TIP4P/2005、TIP4P/Ice.研究了主客体分子、外界温压条件等对甲烷水合物导热性能的影响.研究结果显示甲烷水合物的低导热性能由主体分子构建的sI笼型结构决定,而客体分子进入笼型结构后,使得笼型结构导热性能增强,同时进入笼型结构的客体分子越多,甲烷水合物导热性能越强.研究结果还显示在高温区域（T〉TDebye/3）内不同温度作用下,所有sI水合物具有相似的导热规律.压力对导热系数有一定影响,尤其是在较高压力条件下,压力越高,导热系数越大.而在不同温度和不同压力作用过程中,密度的改变对导热系数的增大或减小几乎没有影响.     

甲烷水合物分解及自保护效应的分子动力学模拟

采用分子动力学(MD)方法, 在温度T = 240, 260, 280和300 K的条件下模拟了Ⅰ型甲烷水合物晶体的分解过程. 研究发现,水合物分解后将在相界面上形成一层“准液膜”,准液膜中水分子的结构性质、空间取向和动力学性质均出现由“似晶”到“似液”的渐变过程. 在水合物分解过程中, 准液膜的存在对水分子和甲烷分子的扩散形成传质阻力. 由于甲烷分子必须穿过准液膜才能进入气相, 准液膜的传质阻力抑制了甲烷分子向气相的扩散过程, 致使水合物的分解速率随之降低, 从而产生自保护效应. 当温度低于水的冰点时, 准液膜中水分子的“似晶”程度较高, 准液膜的传质阻力较大, 自保护效应较明显. 当温度高于水的冰点时, 准液膜中水分子的“似液”程度较高, 准液膜的传质阻力显著下降, 水合物的自保护效应明显减弱.     

甲烷水合物分子间势能的量子化学研究

用Hartree-Fock SCF和密度泛函(BLYP,B3LYP,MPW1PW91)方法对以结构-Ⅰ为单元的甲烷水合物进行了分子间势能的理论研究.该结构单元为正十二面体,其中包括20个水分子,甲烷分子在其中心.采用从头算HF/6-31G(d,p)对甲烷分子进行几何优化,采用ST2模型对水分子作几何优化.水-水间氢键势能Ehb(l)和水-甲烷间范德华势能Evdw(l)作为边长l的函数进行计算,计算时固定水和甲烷分子的几何形状.所有计算中均使用6-31G(d,p)基组.基组重叠误差(BSSE)经校正其上限和下限为水-水氢键能加以确定.由B3LYP经基组重叠误差(BSSE)校正得到的O—O距离为RO—O=0.280 nm,C—O距离RC—O=0.392 nm,比其他方法更接近实验值的0.282和0.395 nm.结果表明,在天然气水合物结构-Ⅰ中水-水分子对的氢键能(30~36 kJ/mol)大于水的二聚体(H2O)2氢键能(-22.6±2.9)kJ/mol,亦大于六角形冰的(-21.7±0.5)kJ/mol,十二面体结构为一稳定单元.以上分子间相互作用势能的结果为得出Lennard-Jones和Kihara势能参数提供了坚实的基础,此参数对分子动力学模拟天然气水合物是非常有用的.     

南海神狐海域及祁连山冻土区天然气水合物的拉曼光谱特征

采用显微激光拉曼光谱对我国在南海神狐海域及祁连山冻土区首次钻获的天然气水合物实物样品进行了详细的研究, 探讨了其笼型结构特征及其气体组成. 结果表明, 南海神狐海域天然气水合物样品是典型的I型结构(sI)水合物, 气体组分主要是甲烷, 占99%以上; 水合物大笼的甲烷占有率大于99%, 小笼为86%, 水合指数为5.99. 祁连山冻土区天然气水合物气体组分相对复杂, 主要成分除甲烷外(70%左右), 还有相当数量的乙烷、丙烷及丁烷等烃类气体, 从拉曼谱图上可初步判断其为II型结构(sII)水合物; 水合物的小、大笼的甲烷占有率的比值(θ_S/θ_L)为26.38, 远远大于南海神弧海域水合物的0.87, 这主要是由于祁连山水合物气体组分中的大分子(乙烷、丙烷及丁烷等)优先占据水合物的大笼, 大大减少了大笼中甲烷分子的数量.     

十二烷基硫酸钠对甲烷水合物生成过程影响研究

根据甲烷水合物含气率高、分解速度慢等特性,提出利用高压注水技术和表面活性剂促进作用促使矿井瓦斯水合化以预防煤与瓦斯突出的思路。进行了3·6~12℃、7·82~12·26MPa条件下两种浓度体系(10mmol/L和0·3mmol/L)中十二烷基硫酸钠(SDS)对甲烷水合物作用效果的实验研究,结合水合物诱导时间、生成速度及含气率等计算对实验数据进行了分析,并运用表面张力法测得8℃时SDS溶液的临界胶束浓度(CMC)为2·5mmol/L。结果表明,高浓度体系对水合物生成速度、含气率的影响较之低浓度体系的更强,但是低浓度体系中水合物生成的诱导时间却较短,表面活性剂溶液浓度超过其CMC后对水合物的生成影响显著。     

Molecular dynamics simulations for the growth of CH_4-CO_2 mixed hydrate

Molecular dynamics simulations are performed to study the growth mechanism of CH4-CO2 mixed hydrate in xCO2= 75%, xCO2= 50%, and xCO2= 25% systems at T = 250 K, 255 K and 260 K, respectively. Our simulation results show that the growth rate of CH4-CO2 mixed hydrate increases as the CO2 concentration in the initial solution phase increases and the temperature decreases. Via hydrate formation, the composition of CO2 in hydrate phase is higher than that in initial solution phase and the encaging capacity of CO2 in hydrates increases with the decrease in temperature. By analysis of the cage occupancy ratio of CH4 molecules and CO2 molecules in large cages to small cages, we find that CO2 molecules are preferably encaged into the large cages of the hydrate crystal as compared with CH4 molecules. Interestingly, CH4 molecules and CO2 molecules frequently replace with each other in some particular cage sites adjacent to hydrate/solution interface during the crystal growth process. These two species of guest molecules eventually act to stabilize the newly formed hydrates, with CO2 molecules occupying large cages and CH4 molecules occupying small cages in hydrate.     

Molecular dynamics simulations for the growth of CH4-CO2 mixed hydrate

Molecular dynamics simulations are performed to study the growth mechanism of CH4-CO2 mixed hydrate in xco2 = 75%, xco2 = 50%, and zco2 = 25% systems at T = 250 K, 255 K and 260 K, respectively. Our simulation results show that the growth rate of CH4-CO2 mixed hydrate increases as the CO2 concentration in the initial solution phase increases and the temperature decreases. Via hydrate formation, the composition of CO2 in hydrate phase is higher than that in initial solution phase and the encaging capacity of CO2 in hydrates increases with the decrease in temperature. By analysis of the cage occupancy ratio of CH4 molecules and CO2 molecules in large cages to small cages, we find that CO2 molecules are preferably encaged into the large cages of the hydrate crystal as compared with CH4 molecules. Interestingly, CH4 molecules and CO2 molecules frequently replace with each other in some particular cage sites adjacent to hydrate/solution interface during the crystal growth process. These two species of guest molecules eventually act to stabilize the newly formed hydrates, with CO2 molecules occupying large cages and CH4 molecules occupying small cages in hydrate.     

水合物CH4-CO2置换的双重机理:空位辅助与竞争

过去的CO2置换甲烷水合物的微观机理研究,主要集中在客体分子(CH4、CO2)之间的交换、占据状态,孤立地研究分解过程或生成过程,忽视主体-客体之间的作用、主体分子(H2O)的空位辅助和客体分子的多重竞争通道。本文基于水合物分解的过冷水及其水空位辅助,以及水合物生成的串滴链及其客体分子竞争的研究,进一步评论水合物CH4-CO2置换的双重机理。然后,对微观机理的动态性和未来研究的相关问题进行讨论。通过综述和评论,文章得出以下初步结果:过冷水通过水空位推动客体分子的跳跃、扩散,实现置换过程的自组装;CO2分子在分解前沿形成一个有序结构的CO2串滴链,其动态性伴随水的组织到获取包合物笼的结构,以及非晶形包合物转变成晶形包合物的生长过程;CO2和CH4在中晶穴中必然产生竞争,并且存在多种竞争类型;成核过程中,不稳定簇导致竞争结构,且有多重竞争通道。最后,结果表明水合物的CH4-CO2置换机理具有双重性,即主体分子的空位辅助和客体分子的竞争,是分解过程和生成过程的自然统一。     

Raman spectra of vibrational and librational modes in methane clathrate hydrates using density functional theory

The sI type methane clathrate hydrate lattice is formed during the process of nucleation where methane gas molecules are encapsulated in the form of dodecahedron (5(12)CH(4)) and tetrakaidecahedron (5(12)6(2)CH(4)) water cages. The characterization of change in the vibrational modes which occur on the encapsulation of CH(4) in these cages plays a key role in understanding the formation of these cages and subsequent growth to form the hydrate lattice. In this present work, we have chosen the density functional theory (DFT) using the dispersion corrected B97-D functional to characterize the Raman frequency vibrational modes of CH(4) and surrounding water molecules in these cages. The symmetric and asymmetric C-H stretch in the 5(12)CH(4) cage is found to shift to higher frequency due to dispersion interaction of the encapsulated CH(4) molecule with the water molecules of the cages. However, the symmetric and asymmetric O-H stretch of water molecules in 5(12)CH(4) and 5(12)6(2)CH(4) cages are shifted towards lower frequency due to hydrogen bonding, and interactions with the encapsulated CH(4) molecules. The CH(4) bending modes in the 5(12)CH(4) and 5(12)6(2)CH(4) cages are blueshifted, though the magnitude of the shifts is lower compared to modes in the high frequency region which suggests bending modes are less affected on encapsulation of CH(4). The low frequency librational modes which are collective motion of the water molecules and CH(4) in these cages show a broad range of frequencies which suggests that these modes largely contribute to the formation of the hydrate lattice.     

Nucleation of the CO2 hydrate from three-phase contact lines

Using molecular dynamics simulations on the microsecond time scale, we investigate the nucleation and growth mechanisms of CO(2) hydrates in a water/CO(2)/silica three-phase system. Our simulation results indicate that the CO(2) hydrate nucleates near the three-phase contact line rather than at the two-phase interfaces and then grows along the contact line to form an amorphous crystal. In the nucleation stage, the hydroxylated silica surface can be understand as a stabilizer to prolong the lifetime of adsorbed hydrate cages that interact with the silica surface by hydrogen bonding, and the adsorbed cages behave as the nucleation sites for the formation of an amorphous CO(2) hydrate. After nucleation, the nucleus grows along the three-phase contact line and prefers to develop toward the CO(2) phase as a result of the hydrophilic nature of the modified solid surface and the easy availability of CO(2) molecules. During the growth process, the population of sI cages in the formed amorphous crystal is found to increase much faster than that of sII cages, being in agreement with the fact that only the sI hydrate can be formed in nature for CO(2) molecules.     

Phase equilibrium measurements and crystallographic analyses on structure-H type gas hydrate formed from the CH4-CO2-neohexane-water system

Phase equilibrium conditions and the crystallographic properties of structure-H type gas hydrates containing various amounts of methane (CH4), carbon dioxide (CO2), neohexane (2,2-dimethylbutane; NH), and liquid water were investigated. When the CH4 concentration was as high as approximately 70%, the phase equilibrium pressure of the structure-H hydrate, which included NH, was about 1 MPa lower at a given temperature than that of the structure-I hydrate with the same composition (except for a lack of NH). However, as the CO2 concentration increased, the pressure difference between the structures became smaller and, at CO2 concentrations below 50%, the phase equilibrium line for the structure-H hydrate crossed that for the structure I. This cross point occurred at a lower temperature at higher CO2 concentration. Extrapolating this relation between the cross point and the CO2 concentration to 100% CO2 suggests that the cross-point temperature would be far below 273.2 K. It is then difficult to form structure-H hydrates in the CO2-NH-liquid water system. To examine the structure, guest composition, and formation process of structure-H hydrates at various CH4-CO2 compositions, we used the methods of Raman spectroscopy, X-ray diffraction, and gas chromatography. Raman spectroscopic analyses indicated that the CH4 molecules were found to occupy both 5(12) and 4(3)5(6)6(3) cages, but they preferably occupied only the 5(12) cages. On the other hand, the CO2 molecules appeared to be trapped only in the 4(3)5(6)6(3) cages. Thus, the CO2 molecules aided the formation of structure-H hydrates even though they reduced the stability of that structure. This encaged condition of guest molecules was also compared with the theoretical calculations. In the batch-type reactor, this process may cause the fractionation of the remaining vapor composition in the opposite sense as that for CH4-CO2 hydrate (structure-I), and thus may result in an alternating formation of structure-H hydrates and structure-I in the same batch-type reactor.     

A molecular dynamics study of ethanol-water hydrogen bonding in binary structure I clathrate hydrate with CO2

Guest-host hydrogen bonding in clathrate hydrates occurs when in addition to the hydrophilic moiety which causes the molecule to form hydrates under high pressure-low temperature conditions, the guests contain a hydrophilic, hydrogen bonding functional group. In the presence of carbon dioxide, ethanol clathrate hydrate has been synthesized with 10% of large structure I (sI) cages occupied by ethanol. In this work, we use molecular dynamics simulations to study hydrogen bonding structure and dynamics in this binary sI clathrate hydrate in the temperature range of 100-250 K. We observe that ethanol forms long-lived (>500 ps) proton-donating and accepting hydrogen bonds with cage water molecules from both hexagonal and pentagonal faces of the large cages while maintaining the general cage integrity of the sI clathrate hydrate. The presence of the nondipolar CO(2) molecules stabilizes the hydrate phase, despite the strong and prevalent alcohol-water hydrogen bonding. The distortions of the large cages from the ideal form, the radial distribution functions of the guest-host interactions, and the ethanol guest dynamics are characterized in this study. In previous work through dielectric and NMR relaxation time studies, single crystal x-ray diffraction, and molecular dynamics simulations we have observed guest-water hydrogen bonding in structure II and structure H clathrate hydrates. The present work extends the observation of hydrogen bonding to structure I hydrates.     

Effect of small cage guests on hydrogen bonding of tetrahydrofuran in binary structure II clathrate hydrates

Molecular dynamics simulations of the pure structure II tetrahydrofuran clathrate hydrate and binary structure II tetrahydrofuran clathrate hydrate with CO(2), CH(4), H(2)S, and Xe small cage guests are performed to study the effect of the shape, size, and intermolecular forces of the small cages guests on the structure and dynamics of the hydrate. The simulations show that the number and nature of the guest in the small cage affects the probability of hydrogen bonding of the tetrahydrofuran guest with the large cage water molecules. The effect on hydrogen bonding of tetrahydrofuran occurs despite the fact that the guests in the small cage do not themselves form hydrogen bonds with water. These results indicate that nearest neighbour guest-guest interactions (mediated through the water lattice framework) can affect the clathrate structure and stability. The implications of these subtle small guest effects on clathrate hydrate stability are discussed.     

The conversion process of hydrocarbon hydrates into CO2 hydrates and vice versa: thermodynamic considerations

Microscopy, confocal Raman spectroscopy and powder X-ray diffraction (PXRD) were used for in situ investigations of the CO(2)-hydrocarbon exchange process in gas hydrates and its driving forces. The study comprises the exposure of simple structure I CH(4) hydrate and mixed structure II CH(4)-C(2)H(6) and CH(4)-C(3)H(8) hydrates to gaseous CO(2) as well as the reverse reaction, i.e., the conversion of CO(2)-rich structure I hydrate into structure II mixed hydrate. In the case of CH(4)-C(3)H(8) hydrates, a conversion in the presence of gaseous CO(2) from a supposedly more stable structure II hydrate to a less stable structure I CO(2)-rich hydrate was observed. PXRD data show that the reverse process requires longer initiation times, and structural changes seem to be less complete. Generally, the exchange process can be described as a decomposition and reformation process, in terms of a rearrangement of molecules, and is primarily induced by the chemical potential gradient between hydrate phase and the provided gas phase. The results show furthermore the dependency of the conversion rate on the surface area of the hydrate phase, the thermodynamic stability of the original and resulting hydrate phase, as well as the mobility of guest molecules and formation kinetics of the resulting hydrate phase.     

Heterogeneous crystal growth of methane hydrate on its sII [001] crystallographic face

This paper presents a systematic molecular simulation study of the heterogeneous crystal growth of methane hydrate sII from supersaturated aqueous methane solutions. The growth of sII hydrate on the [001] crystallographic face is achieved through utilization of a recently proposed methodology, and rates of crystal growth of 1 A/ns were sustained for the molecular models and specific conditions employed in this work. Characteristics of the crystals grown as well as properties and structure of the interface are examined. Water cages with a 5(12)6(3) arrangement, which are improper to both sI and sII structures, are identified during the heterogeneous growth of sII methane hydrate. We show that the growth of a [001] face of sII hydrate can produce an sI crystalline structure, confirming that cross-nucleation of methane hydrate structures is possible. Defects consisting of two methane molecules trapped in large 5(12)6(4) cages and water molecules trapped in small and large cages are observed, where in one instance we have found a large 5(12)6(4) cage containing three water molecules.