Modelling the effect of methanol,glycol inhibitors and electrolytes on the equilibrium stability of hydrates with the SAFT-VR approach

In this work we integrate the statistical associating fluid theory for fluids interacting through potentials of variable range (SAFT-VR) into a traditional van der Waals and Platteeuw framework for modelling clathrate hydrates. We incorporate a new water–guest cell potential for the hydrate phase that can be related to the potential adopted in the familiar SAFT-VR equation of state for modelling fluids. We show how the ability of this equation of state to treat a wide range of complex fluids increases the scope of hydrate modelling to incorporate, in a single framework, the presence of various inhibitors (alcohols, glycols) or brines – or, indeed, any fluid for which a model is available (for use within SAFT-VR) or can be conveniently obtained. Agreement with experimental results is good throughout and, in many cases, excellent.     

Raman Spectroscopic Studies of Methane Gas Hydrates

Abstract

A brief review of the Raman spectroscopic studies of methane gas hydrates is given, supported by some new measurements done in our laboratory.     

超声波作用下天然气水合物的形成

实验搭建了一套高压条件下超声波作用于天然气水合物反应的装置系统，该系统中超声波的频率为20kHz，功率为0-150W可调。利用该装置研究了超声波对天然气水合物生成过程和引导时间的影响。实验发现声场中生成特征曲线诸如温度、压力、流量等随着时间的推迟与静态反应时不同，反应引导时间约缩短至静态时的1／6，分解后重新反应引导时间甚至缩短至静态的1／10。水合物成核速率提高是由于超声波提高了传质系数、成核点浓度和过饱和度，而降低了界面能。     

Clathrate Hydrates of Sulfur Hexafluoride at High Pressures

The pressure dependence (0.4 Mpa–1.3 GPa) of the hydrate decomposition temperatures in the sulfur hexafluoride-water system has been studied. In addition to the known low-pressure hydrate SF₆17H₂O of Cubic Structure II, two new high-pressure hydrates have been found. X-ray analysis in situ showed the gas hydrate forming in the sulfur hexafluoride-water system above 50 MPa at room temperature to be of Cubic Structure I. The ability of water to form hydrates whose structures depend on the guest molecule size under normal conditions and at high pressures is discussed.     

Studies on the synthesis and thermal decomposition mechanisms of rare-earth metal (Pr,Nd, Sm) salt hydrates of 3-nitro-1,2,4-triazol-5-one

Three new rare-earth metal (Pr, Nd and Sm) salt hydrates of 3-nitro-1,2,4-triazol-5-one (NTO) were prepared and characterized. The thermal behaviour of the three salt hydrates, M(NTO)₃·nH₂O (M=Pr and Nd,n=9;M=Sm,n=8) were studied by means of TG and DSC under conditions of linear temperature increase. The thermal decomposition intermediates were determined by means of IR, MS and X-ray diffraction spectrometry. The thermal decomposition mechanisms of these hydrates were proposed as follows: We express our thanks to Professor Zhu Chunhua, Associate Professor Fu Xiayun, and Lecturers Fan Tao and Liang Yanjun for their help in this work.     

Solid-state of pharmaceutical compounds

This article summarizes the different steps needed for a proper design and monitoring of the solid-state in pharmaceutical industry in order to fulfill the requirements of the guideline dealing with polymorphism of the International Conference of Harmonization. This revised version was published online in July 2006 with corrections to the Cover Date.     

Use of sub-ambient DSC to complement conventional DSC and TG

Several drug substances or excipients are hygroscopic. The uptake or loss of water of such substances is generally difficult to control during processing or storage of drug products. DSC instruments with sub-ambient temperature equipment allow the determination of the amount of freezable water by measuring the corresponding melting enthalpy.The determination of freezable water adds valuable information complementary to TG analysis for understanding the processing and storage of raw materials and drug products. Several substances were tested as is, without treatment, after storage at 92% r.h. and after equilibration with water. The results of these experiments showed that it was possible to demonstrate defined hydrate formation, to determine the upper level of binding of water in amorphous substances and to confirm reversible hydrate formations demonstrated by temperature resolved X-ray diffraction.     

Structures and functions of reaction interfaces developed during solid-state dehydrations

The literature reveals that the mechanisms of some solid-state dehydrations are more complicated than has been generally accepted. Reactions at a thin advancing reactant-product interface provide the geometric models on which the most widely employed rate equations are based. For some systems, this thin interface model is a simplification of observed behaviour. Elimination of water from crystallographic sites may occur to a significant extent within a much thicker zone of reactant towards which the active interface is progressing. Consequently the region of chemical change may not coincide with the region of structural transformation. Limited initial dehydration may occur across all crystal faces prior to the onset of a nucleation and growth process that is usually regarded as the dominant rate process in the dehydrations of many large crystals. Experimental observations for solid-state dehydrations are discussed and reaction mechanisms with different rate controlling processes are distinguished. Studies of dehydrations have contributed substantially to the theory of solid-state reactivity, and advances in understanding may have wider application to other solid-state reactants.     

Proton mobility in hydrates of inorganic acids and acid salts

Published data on the mechanisms of hydrogen ion transport in solids and aqueous solutions are described systematically, including defect formation, rotational mobility of proton-containing groups, proton hopping along a hydrogen bond, proton translational mobility, and proton conduction. Resorting to the authors" theoretical results and published data, the main criteria for the selection of systems possessing high proton mobility are formulated.     

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.