Onset Of Hydrate Formation Research Articles

Raman Spectroscopic Study of the Effect of Aqueous Salt Solutions on the Inhibition of Carbon Dioxide Gas Hydrates.

We present an experimental Raman study on the thermodynamic inhibition effect of different salts (NaCl, KCl, MgCl2, and CaCl2 from 2.5 to 11 wt %) on the formation of carbon dioxide gas hydrates. We performed the experiments in a high-pressure vessel with two phases: a water-rich phase and a CO2-rich phase. We investigated the changes the inhibitors induce in the water-rich phase before the onset of hydrate formation. This includes a study of the change in molar reaction enthalpy between strongly and weakly hydrogen-bonded water and the decrease in solubility of carbon dioxide in water. Additionally, the growth mechanisms of carbon dioxide hydrates were investigated by determining the amount of solid hydrates formed and the reaction constant. The results show that the molar reaction enthalpy, the solubility of CO2, and the amount of solid hydrates formed can be correlated with the effective mole fraction, whereas the reaction constant is not affected by the addition of salts. The decrease of the molar reaction enthalpy can be directly correlated with the equilibrium temperature of the gas hydrates.

Method for the monitoring of hydrate formation process in intrafield flowlines

During the production of gas in the Far North and the Arctic, the formation of hydrate and ice plugs in intrafield flowlines is a major concern. The existing methods for determining the onset of hydrate formation are mostly based on the analysis of pressure-and-temperature conditions and therefore they only allow to detect the occurrence of conditions for hydrate formation. They do not allow to localize the specific place where hydrates start to form. The recently developed methods based on echolocation technology have a number of limitations due to the physical nature of the radiation used in them. The proposed method for the monitoring of hydrate formation processes in intrafield flowlines is based on a combination of analysis of pressure-and-temperature conditions in the flowline and the results of flowlines echolocation obtained by means of periodic generation of scanning pressure waves at the end of the flowline (from the side of the switching valve building). The flowlines are divided into characteristic sections bound by characteristic points linked to the structure of the flowline. The propagation speed of the scanning pressure wave is determined in each measuring cycle within the reference section. This can be any section between the characteristic points to which the distance is precisely known and which produce well-defined waveforms and time-stable reflections of scanning pressure waves, for example, the first section located adjacent to the switching valve building. The obtained echograms are compared with the model echogram, which is obtained from a flowline, which is known to be unclogged. Any abnormal change in the signal amplitude is indicative of an onset of the formation of a new local resistance. The proposed recursion formula makes it possible to calculate the temperature in the proximity of this local resistance and, taking into account the pressure value using the diagram of three-phase equilibria for hydrate-forming gases, to diagnose the possibility of existence of crystalline hydrates at a given point. Since pressure-and-temperature conditions for the formation of ice and hydrates are different, the proposed method is selective and enables accurate prediction of the nature of potential buildups.

Formation of Methane Clathrate Hydrates in Coal Moisture: Implications for Coalbed Methane Resources and Reservoir Pressures

Adsorption, X-ray diffraction, and nuclear magnetic resonance analysis of subbituminous to high-volatile bituminous coal at in situ moisture content, low temperatures, and moderate pressures demonstrate that a significant proportion of the inherent moisture (nonmobile water) is available to form methane clathrate hydrates. These results have implications for coal gas resources and reservoir pressures in current areas of permafrost and the much larger regions that were glaciated during the Pleistocene. Methane adsorption tests indicate the clathrates form comparatively rapidly in coal micro- and mesopores from an immobile water phase, at lower pressures than those formed in macroporous materials. At successively higher experimental pressures, hydrates nucleate and grow rapidly on the scale of minutes to hours, until an “apparent” equilibrium pressure is reached. The onset of hydrate formation at 0 °C is at about 3.25 MPa for a Tarn coal with 33% moisture and at slightly higher pressures for the other coals with lower inherent moisture. The amount of gas consumed in hydrate formation, in excess of that attributed to sorption, is 11.6 cm3/g coal for the Tarn coal (Alaska) with 33% moisture, 8.15 cm3/g coal for a Dietz coal (Wyoming) with 22% moisture, and 1.85 cm3/g coal for a Texas coal with about 8% moisture. On a volume of gas (STP) per volume of water basis at 8 MPa, the Tarn and Dietz coal have similar values (35 and 37 cm3/cm3), whereas the Texas coal is measureably less (24 cm3/cm3). At 8 MPa and 0 °C, about 20% of the inherent moisture has participated in hydrate formation for the Tarn and Deitz coals and 13% for the Texas coal. If hydrate formation strips methane from the sorbed state, the proportion of water contributing to hydrate calculates was over 100% for the Texas coal, which suggests that methane is mainly forming from free gas. The methane hydrates analyzed by X-ray diffraction and nuclear magnetic resonance (NMR) spectroscopy are cubic (Pm3n̅ space group) and have small and large cage sI structures. NMR and PXRD spectra indicates that the small cages are about 90% occupied, while the large cages are completely full, yielding a stoichiometry of ca. CH4·6H2O, which is consistent with other natural hydrates. There is some evidence that some methane gas remains trapped in the smallest pores, whereas the hydrates occupy large pores, which may be due to the suppression of hydrate formation by high capillary pressures. The formation of methane hydrates, particularly in low-rank coals, markedly increases the capacity of the coal to store gas. Depending on the coal moisture content and coal rank, methane storage capacity in gas hydrates is up to 2 orders of magnitude greater than the gas storage capacity of the coal by sorption alone. Since low-rank coals invariably have high moisture content, if the strata lie with the hydrate stability zone, significant gas storage in hydrates is anticipated, if gas is available. At the low temperature required for hydrate formation, however, self-sourced methane from methanogenesis or thermal alteration is not anticipated. Successive formation and dissociation of methane hydrates during glacial and interglacial times in the Pleistocene can be anticipated to have impacted shallow gas reservoirs, including coals, to depth up to about 800 m, depending on the surface temperature and geothermal gradient. It is speculated that during the Pleistocene free and sorbed coal gas was scavenged during the formation of hydrates, and this process may explain the low gas content of some high-latitude coals. Similarly, lower than anticipated pore pressure and lack of free water in the Horseshoe Canyon coals in western Canada may be the result of dissociation of hydrates forming overpressures, which potentially could hydraulically fracture the coal and flush the coal of free water. Subsequent dissipation of the gas pressure would lead to the current low reservoir pressure.

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.

Manipulating Theophylline Monohydrate Formation During High-Shear Wet Granulation Through Improved Understanding of the Role of Pharmaceutical Excipients

To investigate the effect of common pharmaceutical excipients on the kinetics of theophylline monohydrate formation during high-shear wet granulation. A mixture of anhydrous theophylline and the excipient was granulated in a high-shear granulator, using water as the granulation liquid. Non-contact Raman spectroscopy was used to monitor the rate of transformation of anhydrate to hydrate during the granulation process. The kinetics of conversion was also monitored in slurries of theophylline whereby the excipients were added to the aqueous phase. Optical microscopy was used to visualize the transformation and to measure the linear growth rates of hydrate crystals in the presence and absence of the excipients. At pharmaceutically relevant amounts of excipient, the transformation kinetics of theophylline was unchanged for the majority of excipients tested. However, when granulating with low concentrations of some commonly used polymeric binders, the transformation kinetics could be significantly retarded. For example, methylcellulose polymers delayed both the onset of hydrate formation as well as retarding the transformation rate. When 0.3% (w/w) of hydroxypropyl methylcellulose was added to a model formulation containing 30% (w/w) theophylline anhydrous, the formation of the monohydrate could be completely prevented over the time period of the granulation experiment, without significantly affecting the granular properties. Microscopic observations of hydrate formation in the presence of the polymer revealed that the polymers that inhibited hydrate formation reduced the hydrate crystal growth rates and influenced hydrate morphology. Raman spectroscopy is a useful technique to monitor hydrate formation during wet granulation. Some commonly used polymeric pharmaceutical excipients can be used to manipulate theophylline hydrate formation in aqueous pharmaceutical environments. These excipients may affect either the nucleation and/or the growth of the hydrate phase.

Equilibrium and Kinetic Properties of Methane Hydrate

During methane dissolution in water in a closed space, the pressure varies exponentially with time until the formation of methane hydrate. A model of this process is proposed that fits experimental data well. The methane concentration at the onset of hydrate formation is calculated as a function of temperature and pressure.