Methane hydrate nucleation and growth from the bulk phase: Further insights into their mechanisms

Methane hydrate nucleation and growth from a bulk phase has been investigated using a quiescent high-pressure view cell. Several hydrate formation/dissociation cycles have been performed at two different initial pressures (10 MPa and 19.5 MPa). Every experiment was performed with a maximum of five consecutive cycles of cooling/heating. For each cycle, the induction time of incipient hydrate formation has been determined. On one hand, results obtained from cycles using fresh water led to the conclusion that the hydrate formation process is rather stochastic, with induction times varying over a large scale. On the other hand, the whole dataset enabled us to investigate on the controversial memory effect of water which may have the ability to fasten the hydrate formation. Moreover, video monitoring has been performed for most of the hydrate formation/dissociation cycles and pictures were taken at different steps of the hydrate formation. The analysis of the results allowed a better understanding of the hydrate nucleation and growth. Two different mechanisms have been observed according to the initial pressure. At initial pressure around 10 MPa, a thick layer of hydrates was created within a couple seconds at the water–gas interface. This layer hinders the gas diffusion and considerably slows down their growth. At 19.5 MPa, the hydrate formation occurs within a larger volume of the bulk phase, and still close to the water–gas interface. The small hydrate crystals are rather dispersed, allowing the diffusion of gas and enhancing the hydrate growth until the formation of a hard layer at the interface of both phases

Solubilidades de gases en líquidos : nuevo equipo para sistemas CO2 + lubricante

En este trabajo se han analizado diferentes m´etodos experimentales utilizados en la bibliograf´ýa para la investigaci´on del equilibrio de fases a alta presi´on y se presenta un nuevo equipo isoc´orico que ha sido implementado en nuestro laboratorio para la determinaci´on de la solubilidad de gases en l´ýquidos, similar al empleado por Wahlstr¨om y Vamling [1,2]. Adem´as, se ha determinado la solubilidad de CO2 en dos ´esteres de pentaeritritol (PEs), tetrapentanoato de pentaeritritol, PEC5, y tetra-2-etilhexanoato de pentaeritritol, PEBE8, en el rango de (283-333) K y hasta 7 MPa. Nuestros resultados han sido comparados con los obtenidos por Bobbo et al. [3,4] para los is´omeros de estos lubricantes, tetra- 2-metilbutanoato de pentaeritritol, PEBM5 y tetraoctanoato de pentaeritritol, PEC8. Se ha observado como la solubilidad del CO2 expresada en porcentaje en peso del refrigerante disminuye con el tama˜no de la mol´ecula del ´ester de pentaeritritol, mientras que el efecto de la ramificaci´on de las mol´eculas es muy d´ebil

Standard partial molar heat capacities and volumes of aqueous N-methylpiperidine and N-methylpiperidinium chloride from 283 K to 393 K

International audienceDensities and volumetric heat capacities for dilute solutions of N-methylpiperidine (N-MP, 0.2 mol·kg−1) and N-methylpiperidinium chloride (N-MPH+Cl−, 0.1 mol·kg−1) were measured relative to water at 0.1 MPa using a vibrating-tube densimeter, and at 0.4 MPa using a twin fixed-cell, power-compensation, differential temperature-scanning nano calorimeter, over the temperature ranges 283.15 K ≤ T ≤ 363.15 K and 283.15 K ≤ T ≤ 393.15 K, respectively. The resulting apparent molar volumes, Vϕ, and heat capacities, Cp,ϕ, were corrected for speciation effects using Young’s rule and extrapolated to infinite dilution using the Debye-Hückel limiting law, Yo ≈ Yϕ − AYIs1/2, to yield the standard partial molar properties, Yo, for N-MP(aq) and N-MPH+Cl−(aq). The standard partial molar volumes, Vo, and heat capacities, Cpo, were represented by a semi-empirical function of solvent density and temperature, Yo(ρw,T), the “density” model, which can be used to extrapolate these results, as well as other thermodynamic properties for the ionization of N-methylpiperidine, to higher temperatures

Methane Hydrate Formation and Dissociation in Sand Media: Effect of Water Saturation, Gas Flowrate and Particle Size

Assessing the influence of key parameters governing the formation of hydrates and determining the capacity of the latter to store gaseous molecules is needed to improve our understanding of the role of natural gas hydrates in the oceanic methane cycle. Such knowledge will also support the development of new industrial processes and technologies such as those related to thermal energy storage. In this study, high-pressure laboratory methane hydrate formation and dissociation experiments were carried out in a sandy matrix at a temperature around 276.65 K. Methane was continuously injected at constant flowrate to allow hydrate formation over the course of the injection step. The influence of water saturation, methane injection flowrate and particle size on hydrate formation kinetics and methane storage capacity were investigated. Six water saturations (10.8%, 21.6%, 33%, 43.9%, 55% and 66.3%), three gas flowrates (29, 58 and 78 mLn·min−1) and three classes of particle size (80–140, 315–450 and 80–450 µm) were tested, and the resulting data were tabulated. Overall, the measured induction time obtained at 53–57% water saturation has an average value of 58 ± 14 min minutes with clear discrepancies that express the stochastic nature of hydrate nucleation, and/or results from the heterogeneity in the porosity and permeability fields of the sandy core due to heterogeneous particles. Besides, the results emphasize a clear link between the gas injection flowrate and the induction time whatever the particle size and water saturation. An increase in the gas flowrate from 29 to 78 mLn·min−1 is accompanied by a decrease in the induction time up to ~100 min (i.e., ~77% decrease). However, such clear behaviour is less conspicuous when varying either the particle size or the water saturation. Likewise, the volume of hydrate-bound methane increases with increasing water saturation. This study showed that water is not totally converted into hydrates and most of the calculated conversion ratios are around 74–84%, with the lowest value of 49.5% conversion at 54% of water saturation and the highest values of 97.8% for the lowest water saturation (10.8%). Comparison with similar experiments in the literature is also carried out herein

Calorimetric study of carbon dioxide (CO2) hydrate formation and dissociation processes in porous media

International audienceUnderstanding the formation and dissociation mechanisms of gas hydrate in porous media is important for the development of new energy-efficient and environmentally friendly technologies related to cold storage as they provide significant latent heat and energy density at suitable phase change temperature. The challenge is to understand the interactions between gas hydrates and the chosen storage media in order to assess the operating conditions likely to optimize time and energy consumption in cold production and storage systems. In this work, CO 2 hydrates formation and dissociation are investigated in two morphologically different porous materials: sand and silica gels. A calorimetric approach is applied to study both the CO 2 hydrate formation kinetics, particularly the induction time, and the amount of hydrate formed for each of the two porous materials. The experiments are performed using a 2 differential thermal analysis device with two identical measuring cells. The present work is focused on assessing the effect of key factors like water saturation, particle size and the morphology of porous media on CO 2 hydrate formation and dissociation processes. Overall, the results do not show a statistically significant correlation between these factors and the induction time. Interestingly, the results obtained with dual porous silica gel showed a higher amount of hydrate formed compared to those with sand for similar initial pressure, temperature and water content conditions. This result may be due to the fact that silica gels provide higher surface area due to their smaller particle size (20-45 µm vs. 80-450 µm for sand), and the presence of internal pore volume in silica gel particles

Influence of Clay-Containing Sediments on Methane Hydrate Formation: Impacts on Kinetic Behavior and Gas Storage Capacity

On Earth, natural hydrates are mostly encountered in clay‐rich sediments. Yet their formation processes in such matrices remain poorly understood. Achieving an in‐depth understanding of how methane hydrates accumulate on continental margins is key to accurately assess (1) their role in sustaining the development of some chemosynthetic communities at cold seeps, (2) their potential in terms of energy resources and geohazards, and (3) the fate of the methane releases, a powerful greenhouse gas, in this changing climate. This study investigated the formation of methane hydrates and their gas storage capacity in clay‐rich sediments. A set of hydrate experiments were performed in matrices composed of sand, illite‐rich clay and montmorillonite‐rich clay at different proportions aiming to determine the role of mineralogy on hydrate formation processes. The experiments demonstrate that a clay content of 10% in a partially water saturated sand/clay mixture increases the induction time by ∼60%, irrespective of the nature of the clay used. The increase in water saturation in the two matrices promotes hydrate formation. Micro‐Raman spectroscopic analyses reveal that increasing the clay content leads to a decrease in the hydrate small‐cage occupancy, with an impact on the storage capacity. Finally, the analyses of collected natural samples from the Black Sea (off Romania) enable us to estimate the gas storage capacity of the deposit. Our estimates is different from previous ones, and supports the importance of coupling multiscale properties, from the microscale to the geological scale, to accurately assess the total amount of methane hosts in hydrate deposits worldwide

Influence of Clay-Containing Sediments on Methane Hydrate Formation: Impacts on Kinetic Behavior and Gas Storage Capacity

On Earth, natural hydrates are mostly encountered in clay‐rich sediments. Yet their formation processes in such matrices remain poorly understood. Achieving an in‐depth understanding of how methane hydrates accumulate on continental margins is key to accurately assess (1) their role in sustaining the development of some chemosynthetic communities at cold seeps, (2) their potential in terms of energy resources and geohazards, and (3) the fate of the methane releases, a powerful greenhouse gas, in this changing climate. This study investigated the formation of methane hydrates and their gas storage capacity in clay‐rich sediments. A set of hydrate experiments were performed in matrices composed of sand, illite‐rich clay and montmorillonite‐rich clay at different proportions aiming to determine the role of mineralogy on hydrate formation processes. The experiments demonstrate that a clay content of 10% in a partially water saturated sand/clay mixture increases the induction time by ∼60%, irrespective of the nature of the clay used. The increase in water saturation in the two matrices promotes hydrate formation. Micro‐Raman spectroscopic analyses reveal that increasing the clay content leads to a decrease in the hydrate small‐cage occupancy, with an impact on the storage capacity. Finally, the analyses of collected natural samples from the Black Sea (off Romania) enable us to estimate the gas storage capacity of the deposit. Our estimates is different from previous ones, and supports the importance of coupling multiscale properties, from the microscale to the geological scale, to accurately assess the total amount of methane hosts in hydrate deposits worldwide

Strong geochemical anomalies following active submarine eruption offshore Mayotte

Submarine volcanic activity releases large amounts of gases and metals in the water column, affecting biogeochemical cycles and ecosystems at a regional and local scale. In 2018, Fani Maoré submarine volcano erupted 50 km offshore Mayotte Island (Comoros Archipelago, Indian Ocean). Active eruptive plumes were observed in May 2019 at and around the summit with acoustic plumes rising 2 km into the water column coupled to strong geochemical anomalies. Between May 2019 and October 2020, three research cruises monitored the eruptive activity. Here, we report spatial and temporal variability of water column chemistry above the volcano, focusing on dissolved gases, trace metal concentrations, and physico-chemical parameters. In May 2019, concentrations above 800 nM in CH4 and H2 were measured throughout the water column, with Total Dissolvable Mn and Total Dissolvable Fe concentrations above 500 nM, and CO2 values of 265 μM. Strong water column acidification was measured (0.6 pH unit) compared to the regional background. From May 2019 to October 2020, we observed a general decrease in gas concentrations, and an evolution of the TDMn/TDFe ratios similar to previously reported values in other submarine volcanic contexts, and consistent with a decrease of the eruptive activity at the volcano. In October 2020, a rebound of high H2 concentrations resulted from new lava flows, which were identified by seafloor observation using deep-towed camera, 5 km further than the volcano summit. During 2 years timespan of our observations (2019–2020), He, CO2 and CH4 concentrations correlate highlighting a magmatic origin of dissolved gases. δ13C-CH4 values of −34‰ vs. vPDB might suggest magma/sediments interaction during the magma ascent, and potential thermal cracking of organic matter, although abiotic methane generation cannot be ruled out given the volcanic context. Weak correlations between H2 and excess of 3He suggest complex processes of H2 from magmatic degassing, lava/seawater interaction, and oxidation processes in the water column. Strong and correlated Fe, Mn and Si water column anomalies are also consistent with fluid-rock reactions induced by acidic fluids rich in magmatic volatiles. Water column acidification appears to be associated with the release of CO2-rich fluids. A year after the main eruptive event, the system seems to be back to steady-state highlighting the buffer capacity and resilience of the seawater column environment