3A分子筛对四氢呋喃水合物生成过程的影响

用显微镜从微观角度研究了0℃以下常压四氢呋喃-水体系中加入3A分子筛粉后THF水合物的生成过程,结果表明3A分子筛的存在能诱发水合物的成核,促进THF水合物的生成,提高THF水合物的生长速率;有3A分子筛存在时,THF水合物晶体的生长速率介于0．01～0．05gm／s之间;相对于无3A分子筛存在的体系,THF水合物晶体的生长速率提高约4nm／s;在相同条件下,THF水合物晶体长成后大晶体还会发生部分变化和重组。     

基于转化率法研究冰粉生成乙烯水合物的动力学及THF非常规抑制作用

利用压缩因子Z修正理想气体状态方程,得到冰粉生成乙烯水合物过程中冰体积的转化率,并结合Avrami方程与Arrhenius方程研究温度、压力及四氢呋喃等可控因素对冰粉生成乙烯水合物的影响规律和动力学行为.结果表明,乙烯与冰粉反应生成水合物时不存在诱导期,乙烯在冰粉表面快速成核并在一维方向上等速生长.在乙烯初始压力为4.90 MPa,温度在260.05~269.75 K范围内,冰粉合成乙烯水合物活化能E_a=21.28 kJ/mol,温度为269.75 K时,冰粉的最终转化率达19.20%.随着压力(1.98~4.90 MPa)和温度(250.46~269.75 K)升高,冰粉转化率也增加.THF降低了冰粉转化率和转化效率,分子尺寸效应表明THF在冰粉生成乙烯水合物过程中存在非常规抑制作用.     

多孔介质中甲烷水合物的分解特性

利用定容降压方法测定了在不同多孔介质中甲烷水合物的分解实验数据, 所使用的多孔介质平均孔径分别为9.03, 12.95, 17.96和33.20 nm, 其中孔径为12.95 nm的多孔介质采用了3个粒径范围, 分别为0.105～0.150, 0.150～0.200和0.300～0.450 mm; 其它孔径的多孔介质的粒径范围为0.105～0.150 mm. 在封闭的条件下测定了不同温度与不同初始生成压力下甲烷水合物的分解实验数据(实验温度范围为269.15～278.15 K, 初始生成压力范围为4.1～11.0 MPa), 结果表明, 水合物的分解速度随着初始生成压力的增加和水浴温度的降低而升高, 也随孔径的增加而升高, 但随多孔介质粒径的增大而降低. 在孔径较大和分解温度较低时, 多孔介质中水合物分解引起的温度降低会使水结冰, 从而减缓水合物的分解速度.     

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.     

铈改性对HZSM-5分子筛表面性质及其催化降解甲硫醇(CH_3SH)的研究

采用浸渍法制备了Ce改性的HZSM-5分子筛催化剂,考察Ce改性前后HZSM-5分子筛催化分解甲硫醇反应中的催化活性。通过XRD,BET,H_2-TPR,NH_3-TPD和CO_2-TPD等手段对其物化性能进行表征,结果表明:Ce改性后HZSM-5分子筛的结构没有被破坏,而Ce的引入可明显提高催化剂的还原性能,进而促进甲硫醇的催化分解;同时Ce的引入还能有效调控HZSM-5分子筛表面的酸碱性,可明显增加分子筛表面碱性中心的数量,从而显著提高催化分解甲硫醇的活性。此外,探讨了在350~650℃范围内焙烧温度对Ce改性HZSM-5分子筛表面性质及催化分解甲硫醇活性的影响。实验结果表明,在550℃焙烧获得的分子筛催化分解甲硫醇的效果最好。表征结果显示:当焙烧温度较低时,Ce与HZSM-5分子筛的相互作用较弱,所以分子筛催化分解甲硫醇的活性较差,随着焙烧温度的升高,分子筛的还原性能和酸碱性能均有所降低;当焙烧温度过高时,导致Ce O_2相互堆叠形成新的介孔孔道坍塌,降低其催化分解甲硫醇的活性。     

瞬变平面热源法测定常压下四氢呋喃水合物的导热系数

采用瞬变平面热源法,测定了常压、温度233.15～273.15K下四氢呋喃水合物的导热系数,其范围在0.45～0.54W·m-1·K-1之间。实验结果表明,常压下四氢呋喃水合物的导热系数呈现随温度升高而增大的趋势,与玻璃体的变化特性相同。常压下四氢呋喃水合物的导热系数与高压下的数值相比差别很小,但常压下测得的四氢呋喃水合物导热系数的玻璃体变化特性比高压下明显。     

不同晶粒ZSM-5沸石分子筛对合成气羰基化反应性能的影响

对沸石分子筛而言,分子筛的孔道、孔径、孔容和微孔等物理性质和酸性影响它的活性.通过调节和控制以上物理性质能够提高产物选择性和收率,降低副产物,从而促进反应性能提高活性.我们考察了ZSM-5分子筛的晶粒度和硅铝比对合成气羰基化反应性能的影响.结果表明,晶粒度小及具有一定比例的中强酸中心的ZSM-5沸石分子筛对反应有利,但晶粒度比较大即1和3μm的ZSM-5沸石分子筛目标产物选择性比较低.纳米级的ZSM-5沸石分子筛催化剂在反应中表现出较高的活性及较低的副产物选择性,是适宜的合成气羰基化反应催化剂载体.温度考察结果可知,反应温度为300℃时,效果为最佳.其中, 30～50 nm的ZSM-5沸石分子筛催化剂CO转化率为55%,乙酸甲酯和甲酸甲酯选择性之和为52%,而晶粒度3μm时, CO转化率仅为25%,乙酸甲酯和甲酸甲酯选择性之和为20%,是30～50 nm沸石分子筛的一半.当反应继续升温时,副产物的选择性也随之增加,是因为所生成的中间产物和甲醇等继续进行各种反应生成二甲醚、芳烃、烷烃以及裂解生成CO_2等干气.     

磁场特性与作用方式对HCFC-141b气体水合物生成过程的影响

通过实验研究考察了静态磁场和旋转磁场下磁场特性与作用方式对低压制冷剂HCFC-141b气体水合物生成过程特性影响, 重点研究在不同磁场条件下的气体水合物结晶形态、生成温度和引导时间三个重要因素. 试验结果发现, 适当的旋转磁场对提高水合物生成温度、降低过冷度、减少引导时间、提高生成速度有显著的作用, 使结晶形态混杂致密; 而适当的静态磁场则使水合物形态规整, 有利于提高传热性能.     

H-ZSM-5分子筛催化二甲苯异构化的反应机理

采用密度泛函理论(DFT)和ONIOM方法, 研究了H-ZSM-5分子筛上二甲苯异构化机理. 描述了中间体物种和过渡态的结构. 反应物吸附和产物脱附对二甲苯异构化的反应趋势有重要影响. 反应活化能的计算结果表明, 在H-ZSM-5分子筛延伸的孔道结构中, 异构化反应沿着生成间二甲苯的方向进行. 但是较高的脱附能使生成的间二甲苯滞留在分子筛孔道中,其进一步异构化生成对二甲苯具有动力学优势. 对二甲苯产物在分子筛孔道的酸中心上可选择性生成. 在H-ZSM-5分子筛外表面, 不受延伸孔道结构的静电限制时, 二甲苯异构化生成间二甲苯产物, 其可以很容易从活性位上脱附. 非选择性异构化降低了对二甲苯的选择性. 因此, 对H-ZSM-5分子筛外表面改性能够抑制二甲苯的非选择性异构化, 因此限制了反应在分子筛孔道中进行, 提高了对二甲苯的选择性. 二甲苯异构化相对反应速率常数的计算结果也表明, 在分子筛外表面上, 生成间二甲苯的异构化反应速率较快. 升高反应温度会降低对二甲苯的选择性.     

水分子对α相CL-20热分解机理影响的分子动力学研究

研究六硝基六氮杂异伍兹烷(CL-20)晶体不同晶型在不同温度下的反应机理, 对于深入认识含能材料在极端条件下的冲击起爆、冲击点火和爆轰过程等具有重要意义. 基于反应力场, 研究水分子在纯α相CL-20及其水合物的晶体结构中数量随时间的变换, 分析水分子对两种体系的初始分解和第二阶段的分解路径的影响. 计算结果表明: CL-20 分子的初始分解路径与水分子无关, 第二阶段的分解反应与水分子有关. 在低温(T<1500 K)下, 水分子对两种体系没有影响, 二者的初始分解路径均为N-NO₂键生成NO₂自由基; 在1500 K≤T≤2500 K时, 水分子作为反应物或与NO₂、、OH自由基等组成催化体系, 生成O₂、H₂O₂等产物, 加速水合物体系在高温下的第二阶段反应, 使得高温下水合物体系的化学反应速率和反应生成的NO₂自由基的数量比纯CL-20体系的化学反应速率和反应生成的NO₂自由基的数量大; 在T>2500 K时, 水分子的催化反应抑制CL-20初始分解反应, 使得在3000 K时纯CL-20体系的反应速率大于水合物体系中CL-20的反应速率.     

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.     

Tetrahydrofuran hydrate decomposition characteristics in porous media

Many tetrahydrofuran (THF) hydrate properties are similar to those of gas hydrates. In the present work THF hydrate dissociation in four types of porous media is studied. THF solution was cooled to 275.15 K with formation of the hydrate under ambient pressure, and then it dissociated under ambient conditions. THF hydrate dissociation experiments in each porous medium were conducted three times. Magnetic resonance imaging (MRI) was used to obtain images. Decomposition time, THF hydrate saturation and MRI mean intensity (MI) were measured and analyzed. The experimental results showed that the hydrate decomposition time in BZ-4 and BZ-3 was similar and longer than that in BZ-02. In each dissociation process, the hydrate decomposition time of the second and third cycles was shorter than that of the first cycle in BZ-4, BZ-3, and BZ-02. The relationship between THF hydrate saturation and time is almost linear.     

Influence of 3A molecular sieve on tetrahydrofuran (THF) hydrate formation

Visual observation of the THF hydrate formation process in the presence of a 3A molecular sieve has been made at normal atmosphere and below a temperature of zero by microscopy. The results indicate that a 3A molecular sieve can induce the nucleation of the THF hydrate and promote the THF hydrate growth. With the existence of a 3A molecular sieve, the growth rate of THF hydrate is between 0.01 and 0.05 μm/s. In comparison with the system without any 3A molecular sieve, the growth rate increases about 4 nm/s. After the THF hydrate grows into megacryst, the crystals will recombine and partially change under the same condition.     

NMR/MRI study of clathrate hydrate mechanisms

Clathrate hydrates are of great importance in many aspects. However, hydrate formation and dissociation mechanisms, essential to all hydrate applications, are still not well understood due to the limitations of experimental techniques capable of providing dynamic and structural information on a molecular level. NMR has been shown to be a powerful tool to noninvasively measure molecular level dynamic information. In this work, we measured nuclear magnetic resonance (NMR) spin lattice relaxation times (T1's) of tetrahydrofuran (THF) in liquid deuterium oxide (D2O) during THF hydrate formation and dissociation. At the same time, we also used magnetic resonance imaging (MRI) to monitor hydrate formation and dissociation patterns. The results showed that solid hydrate significantly influences coexisting fluid structure. Molecular evidence of residual structure was identified. Hydrate formation and dissociation mechanisms were proposed based on the NMR/MRI observations.     

Effect of antifreeze proteins on the nucleation, growth, and the memory effect during tetrahydrofuran clathrate hydrate formation

The inhibition activities of two antifreeze proteins (AFPs) on the formation of tetrahydrofuran (THF) clathrate hydrate have been tested. AFPs from fish (wfAFP) and insect (CfAFP) changed the morphology of growing THF hydrate crystals. Also, both AFPs showed higher activities in inhibiting the formation THF hydrate than a commercial kinetic inhibitor, poly(vinylpyrrolidone) (PVP). Strikingly, both AFPs also showed the ability to eliminate the "memory effect" in which the crystallization of hydrate occurs more quickly after the initial formation. This is the first report of molecules that can inhibit the memory effect. Since the homogeneous nucleation temperature for THF hydrate was measured to be 237 K, close to that observed for ice itself, the action of kinetic inhibitors must involve heterogeneous nucleation. On the basis of our results, we postulate a mechanism for heterogeneous nucleation, the memory effect and its elimination by antifreeze proteins.     

Time-resolved in situ neutron diffraction studies of gas hydrate: transformation of structure II (sII) to structure I (sI).

We report the in situ observation from diffraction data of the conversion of a gas hydrate with the structure II (sII) lattice to one with the structure I (sI) lattice. Initially, the in situ formation, dissociation, and reactivity of argon gas clathrate hydrate was investigated by time-of-flight neutron powder diffraction at temperatures ranging from 230 to 263 K and pressures up to 5000 psi (34.5 MPa). These samples were prepared from deuterated ice crystals and transformed to hydrate by pressurizing the system with argon gas. Complete transformation from D(2)O ice to sII Ar hydrate was observed as the sample temperature was slowly increased through the D(2)O ice melting point. The transformation of sII argon hydrate to sI hydrate was achieved by removing excess Ar gas and exposing the hydrate to liquid CO(2) by pressurizing the Ar hydrate with CO(2). Results suggest the sI hydrate formed from CO(2) exchange in argon sII hydrate is a mixed Ar/CO(2) hydrate. The proposed exchange mechanism is consistent with clathrate hydrate being an equilibrium system in which guest molecules are exchanging between encapsulated molecules in the solid hydrate and free molecules in the surrounding gas or liquid phase.     

The detection of tetrahydrofuran hydrate formation and saturation using magnetic resonance imaging technique

Tetrahydrofuran (THF) hydrate was formed in bulk as well as in glass beads pack with a mean diameter of 3.0 mm by controlling the temperature under ambient pressure. Images of THF hydrate formation procedure were obtained using the magnetic resonance imaging (MRI) technique. The experiment results showed that MRI is an effective method for the detection of hydrate formation. Saturation of hydrate formed both in bulk and glass beads can be confirmed by intensity integration of MRI images.     

Memory effect on the pressure-temperature condition and induction time of gas hydrate nucleation

The focus of this study is to investigate the influence of memory effect and the relation of its existence with the dissociation temperature, using gas hydrate formation and dissociation experiments. This is beneficial because memory effect is considered as an effective approach to promote the thermodynamic and dynamic conditions of gas hydrate nucleation. Seven experimental systems (twenty tests in total) were performed in a 1 L pressure cell. Three types of hydrate morphology, namely massive, whiskery and jelly crystals were present in the experiments. The pressures and temperatures at the time when visual hydrate crystals appeared were measured. Furthermore, the influence of memory effect was quantified in terms of pressure-temperature-time (p-T-t) relations. The results revealed that memory effect could promote the thermodynamic conditions and shorten the induction time when the dissociation temperature was not higher than 25 ℃. In this study, the nucleation superpressure and induction time decrease gradually with time of tests, when the earlier and the later tests are compared. It is assumed that the residual structure of hydrate dissociation, as the source of the memory effect, provides a site for mass transfer between host and guest molecules. Therefore, a driving force is created between the residual structures and its surrounding bulk phase to promote the hydrate nucleation. However, when the dissociation temperature was higher than 25 ℃, the memory effect vanished. These findings provide references for the application of memory effect in hydrate-based technology.     

Experimental studies of the formation and dissociation of methane hydrate in loess

In order to study the nature of gas hydrate in porous media, the formation and dissociation processes of methane hydrate in loess were investigated. Five cooling rates were applied to form methane hydrate. The nucleation times of methane hydrate formation at each cooling rate were measured for comparison. The experimental results show that cooling rate is a significant factor affecting the nucleation of methane hydrate and gas conversion. Under the same initial conditions, the faster the cooling rate, the shorter the nucleation time, and the lower the methane gas conversion. Five dissociating temperatures were applied to conduct the dissociation experiment of methane hydrate formed in loess. The experimental results indicated that the temperature evidently controlled the dissociation of methane hydrate in loess and the higher the dissociating temperature, the faster the dissociating rates of methane hydrate.     

Promoting effect of super absorbent polymer on hydrate formation

The effect of super absorbent polymer (SAP) on the formation of tetrahydrofuran (THF) hydrate was studied by the successional cooling method. It was found that THF solution samples with 0.004 wt% and 0.03 wt% of SAP formed THF hydrate completely during the same cooling process. The corresponding induction time was 16-29 min, 14-31 min, respectively, which was obviously shorter than that of THF solution samples without SAP (25-62 min). It indicated that SAP accelerated the formation of THF hydrate. At the same time, the pictures of hydrate formation with and without SAP had been compared. It was found that SAP did not change the morphology of the hydrate. Finally, the mechanism of SAP promoting effect on the formation of THF hydrate was suggested.