Kinetic Analysis of Methane Hydrate Formation with Butterfly Turbine Impellers

Heat generation during gas hydrate formation is an important problem because it reduces the amount of water and gas that become gas hydrates. In this research work, we present a new design of an impeller to be used for hydrate formation and to overcome this concern by following the hydrodynamic literature. CH₄ hydrate formation experiments were performed in a 5.7 L continuously stirred tank reactor using a butterfly turbine (BT) impeller with no baffle (NB), full baffle (FB), half baffle (HB), and surface baffle (SB) under mixed flow conditions. Four experiments were conducted separately using single and dual impellers. In addition to the estimated induction time, the rate of hydrate formation, hydrate productivity and hydrate formation rate, constant for a maximum of 3 h, were calculated. The induction time was less for both single and dual-impeller experiments that used full baffle for less than 3 min and more than 1 h for all other experiments. In an experiment with a single impeller, a surface baffle yielded higher hydrate growth with a value of 42 × 10⁻⁸ mol/s, while in an experiment with dual impellers, a half baffle generated higher hydrate growth with a value of 28.8 × 10⁻⁸ mol/s. Both single and dual impellers achieved the highest values for the hydrate formation rates that were constant in the full-baffle experiments.     

All-Atom Molecular Dynamics of Pure Water–Methane Gas Hydrate Systems under Pre-Nucleation Conditions: A Direct Comparison between Experiments and Simulations of Transport Properties for the Tip4p/Ice Water Model

(1) Background: New technologies involving gas hydrates under pre-nucleation conditions such as gas separations and storage have become more prominent. This has necessitated the characterization and modeling of the transport properties of such systems. (2) Methodology: This work explored methane hydrate systems under pre-nucleation conditions. All-atom molecular dynamics simulations were used to quantify the performance of the TIP4P/2005 and TIP4P/Ice water models to predict the viscosity, diffusivity, and thermal conductivity using various formulations. (3) Results: Molecular simulation equilibrium was robustly demonstrated using various measures. The Green–Kubo estimation of viscosity outperformed other formulations when combined with TIP4P/Ice, and the same combination outperformed all TIP4P/2005 formulations. The Green–Kubo TIP4P/Ice estimation of viscosity overestimates (by 84% on average) the viscosity of methane hydrate systems under pre-nucleation conditions across all pressures considered (0–5 MPag). The presence of methane was found to increase the average number of hydrogen bonds over time (6.7–7.8%). TIP4P/Ice methane systems were also found to have 16–19% longer hydrogen bond lifetimes over pure water systems. (4) Conclusion: An inherent limitation in the current water force field for its application in the context of transport properties estimations for methane gas hydrate systems. A re-parametrization of the current force field is suggested as a starting point. Until then, this work may serve as a characterization of the deviance in viscosity prediction.     

Phase Transition of a Structure II Cubic Clathrate Hydrate to a Tetragonal Form

The crystal structure and phase transition of cubic structure II (sII) binary clathrate hydrates of methane (CH₄) and propanol are reported from powder X‐ray diffraction measurements. The deformation of host water cages at the cubic–tetragonal phase transition of 2‐propanol+CH₄ hydrate, but not 1‐propanol+CH₄ hydrate, was observed below about 110 K. It is shown that the deformation of the host water cages of 2‐propanol+CH₄ hydrate can be explained by the restriction of the motion of 2‐propanol within the 5¹²6⁴ host water cages. This result provides a low‐temperature structure due to a temperature‐induced symmetry‐lowering transition of clathrate hydrate. This is the first example of a cubic structure of the common clathrate hydrate families at a fixed composition.     

Spectroscopic Observation of the Hydroxy Position in Butanol Hydrates and Its Effect on Hydrate Stability

In this study, we investigate the crystal structures and phase equilibria of butanols+CH₄+H₂O systems to reveal the hydroxy group positioning and its effects on hydrate stability. Four clathrate hydrates formed by structural butanol isomers are identified with powder X‐ray diffraction (PXRD). In addition, Raman spectroscopy is used to analyze the guest distributions and inclusion behaviors of large alcohol molecules in these hydrate systems. The existence of a free OH indicates that guest molecules can be captured in the large cages of structure II hydrates without any hydrogen‐bonding interactions between the hydroxy group of the guests and the water‐host framework. However, Raman spectra of the binary (1‐butanol+CH₄) hydrate do not show the free OH signal, indicating that there could be possible hydrogen‐bonding interactions between the guests and hosts. We also measure the four‐phase equilibrium conditions of the butanols+CH₄+H₂O systems.     

L‐Lysine: Exploiting Powder X‐ray Diffraction to Complete the Set of Crystal Structures of the 20 Directly Encoded Proteinogenic Amino Acids

During the last 75 years, crystal structures have been reported for 19 of the 20 directly encoded proteinogenic amino acids in their natural (enantiomerically pure) form. The crystal structure is now reported for the final member of this set: L ‐lysine. As crystalline L ‐lysine has a strong propensity to incorporate water under ambient atmospheric conditions to form a hydrate phase, the pure (non‐hydrate) crystalline phase can be obtained only by dehydration under rigorously anhydrous conditions, resulting in a microcrystalline powder sample. For this reason, modern powder X‐ray diffraction methods have been exploited to determine the crystal structure in this final, elusive case.     

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.     

Origins for epitaxial order of sexiphenyl crystals on muscovite(001)

The epitaxial order of sexiphenyl crystals on muscovite(001) is investigated by x‐ray diffraction, lattice misfit calculations and atomic force microscopy. Depending on the substrate temperature during the thin film growth process, different epitaxial orientations are formed. Sexiphenyl thin films prepared at 370 K preferentially form crystals with the crystallographic (11‐1) planes parallel to the substrate surface while at 434 K a strong fraction of crystals with (11‐2) orientations is grown. The epitaxial orders of sexiphenyl crystals are compared with lattice misfit calculations. The in‐plane order of the {11‐1} crystals can be explained by a point‐on‐line coincidence I, which reveals that the interface is formed by undisturbed crystal surfaces. The epitaxial order of the {11‐2} oriented crystals is characterised by the experimental observation that low indexed crystal directions in the sexiphenyl(11‐2) plane and the muscovite(001) surface coincide with each other, forming a near‐coincidence case. Corrugations of the substrate surface are responsible for this second type of epitaxial order. Characteristic features in the thin film morphology could be correlated to the two observed epitaxial orientations of the sexiphenyl crystals. Copyright © 2009 John Wiley & Sons, Ltd.     

Molecular dynamics study of the stereostructure of 1,4‐linked poly(cyclohexa‐1,3‐diene) obtained with π‐allylnickel‐based catalysts

Temperature‐constant and pressure‐constant molecular dynamics simulations of crystalline 1,4‐linked poly(cyclohexa‐1,3‐diene) (CHD) were performed using the COMPASS force field. Powder X‐ray diffraction spectra calculated from the simulated atomic coordinates were compared with the measured spectrum of the crystal of 1,4‐linked poly(CHD), obtained using a bis(allylnickel bromide) (ANiBr)/methylaluminoxane (MAO) catalyst. As a result of the comparison, the geometrical isomerism of the 1,4‐linked poly(CHD) obtained with the ANiBr/MAO catalyst was found to be cis syndiotactic. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 973–978, 2001     

Molecular dynamics (MD) simulations and large-angle X-ray scattering (LAXS) studies of the solid-state structure and assembly of isotactic (R)-poly(2,2'-dioxy-1,1'-binaphthyl-)phosphazene in the bulk state and in the cast film

The intrachain conformation, molecular structure and interchain assembly of isotactic (R)-poly(2,2'-dioxy-1,1'-binaphthyl)phosphazene (P-DBNP) both in the bulk state (I) and in the cast film (II) were studied by molecular dynamics (MD) simulations of models, as implemented by a bias potential for the analysis of the radial distribution function (RDF) obtained from large-angle X-ray scattering (LAXS) data. The microscopic structure and order extension of the polymer changed from I to II, as qualitatively shown in the shapes of their experimentally measured RDF curves. With the use of a bias potential, the MD simulations provided a much more accurate analysis of the models, as seen in the reproduction of the RDFs. The chiral P-DBNP chain was found to be consistent with helix conformations in both the I and the II samples. The predominant interchain clustering motif was best reproduced with a seven-chain model. In the case of I, the maximum chain length was 18 monomeric -R(2)NP- units, while in the case of the cast film II the chain was more elongated, up to distances of approximately 100 A, equivalent to over 48 monomeric -R(2)NP- units. The seven-chain assembly was accounted for in terms of nonbonded interactions favouring the minimum voids area between the seven tubular structures of the material. The results validate our earlier finding that MD analysis with implementation of a biasing potential for the RDFs can provide quantitative information on the structural and conformational features of amorphous solids. The combined theoretical and experimental approach was found to be a useful tool to detect, locate and evaluate the intra- and intermolecular modifications of materials subsequent to their phase transformation and, as in the present case, changes in their microscopic structures or preparation methods.     

Formation of a tetragonal Cu3Au alloy at gold/copper interfaces

A gold–copper alloy with a nominal composition of Cu₃Au but with a tetragonal (c = 4a) structure is observed to form at Au/Cu interfaces of gold/copper multilayers deposited on amorphous substrates by d.c. magnetron sputtering. The formation of this non‐equilibrium structure (tentatively D0₂₃) under‐ambient conditions is detected by secondary ion mass spectrometry, x‐ray diffraction and high‐resolution cross‐sectional transmission electron microscopy. Co‐sputtering of Au and Cu under similar conditions produces only conventional fcc Cu₃Au alloy phases, suggesting that interfacial confinement plays a significant role in producing the novel Cu₃Au alloy phase in gold/copper multilayers. Copyright © 2003 John Wiley & Sons, Ltd.     

In Situ Total X‐Ray Scattering Study of WO3 Nanoparticle Formation under Hydrothermal Conditions

Pair distribution function analysis of in situ total scattering data recorded during formation of WO₃ nanocrystals under hydrothermal conditions reveal that a complex precursor structure exists in solution. The WO₆ polyhedra of the precursor cluster undergo reorientation before forming the nanocrystal. This reorientation is the critical element in the formation of different hexagonal polymporphs of WO₃.     

Formation and Structural Characterization of Metal Complexes derived from Thiosalicylic Acid

The formation and structural aspects of some metal complexes of thiosalicylic acid (TSA) were studied. The μ‐bridging tetra‐coordinated Ru complex, [Ru(C₆H₄(CO₂)(μ‐S)(H₂O)]₂ ( 1 ) was formed by hydrothermal reaction of TSA with RuCl₃. The complexes [M(dtdb)(phen)(H₂O)]_n ( 2 – 4 ) (M = Zn^II, Co^II, Ni^II, dtdb = 2,2′‐dithiodibenzoate anion, phen = 1,10‐phenanthroline) were obtained by the slow diffusion technique and the in situ S–S bond formation was confirmed by elemental, spectral and X‐ray analysis. Reaction of TSA with CuCl₂ and 2,2′‐bipyridine (bipy) under the slow diffusion technique yielded the dimer [Cu(tdb)(bipy)] ( 5 ) (tdb = thiodibenzoic acid), where the in situ generation of 2,2′‐thiodibenzoic acid was observed.     

Dynamics of Monolayer–Island Transitions in 2,7‐Dioctyl‐benzothienobenzthiophene Thin Films

The order in molecular monolayers is a crucial aspect for their technological application. However, the preparation of defined monolayers by spin‐coating is a challenge, since the involved processes are far from thermodynamic equilibrium. In the work reported herein, the dynamic formation of dioctyl‐benzothienobenzothiophene monolayers is explored as a function of temperature by using X‐ray scattering techniques and atomic force microscopy. Starting with a disordered monolayer after the spin‐coating process, post‐deposition self‐reassembly at room temperature transforms the initially amorphous layer into a well‐ordered bilayer structure with a molecular herringbone packing, whereas at elevated temperature the formation of crystalline islands occurs. At the temperature of the liquid‐crystalline crystal–smectic transition, rewetting of the surface follows resulting in a complete homogeneous monolayer. By subsequent controlled cooling to room temperature, cooling‐rate‐dependent kinetics is observed; at rapid cooling, a stable monolayer is preserved at room temperature, whereas slow cooling causes bilayer structures. Increasing the understanding and control of monolayer formation is of high relevance for achieving ordered functional monolayers with defined two‐dimensional packing, for future applications in the field of organic electronics.