Letteratura scientifica selezionata sul tema "Ice layer crystallization"

In the present work, freeze crystallization studies, as a novel concentration method for aqueous 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]) ionic liquid solution, were conducted. In order to find the appropriate temperature and composition range for freeze crystallization, the solid–liquid equilibrium of a binary [DBNH][OAc]–water compound system was investigated with differential scanning calorimetry (DSC). Results of this analysis showed that the melting temperature of the pure ionic liquid was 58 ℃, whereas the eutectic temperature of the binary compound system was found to be −73 ℃. The activity coefficient of water was determined based on the freezing point depression data obtained in this study. In this study, the lowest freezing point was −1.28 ℃ for the aqueous 6 wt.% [DBNH][OAc] solution. Ice crystal yield and distribution coefficient were obtained for two types of aqueous solutions (3 wt.% and 6 wt.% [DBNH][OAc]), and two freezing times (40 min and 60 min) were used as the main parameters to compare the two melt crystallization methods: static layer freeze and suspension freeze crystallization. Single-step suspension freeze crystallization resulted in higher ice crystal yields and higher ice purities when compared with the single-step static layer freeze crystallization. The distribution coefficient values obtained showed that the impurity ratios in ice and in the initial solution for suspension freeze crystallization were between 0.11 and 0.36, whereas for static layer freeze crystallization these were between 0.28 and 0.46. Consequently, suspension freeze crystallization is a more efficient low-energy separation method than layer freeze crystallization for the aqueous-ionic liquid solutions studied and, therefore, this technique can be applied as a concentration method for aqueous-ionic liquid solutions.

It is shown that cryoprocesses play an important role in the pollution of watercourses during ice formation. The content of heavy metals was determined layer by layer in ice cores. The growth rate of ice crystals, determined by the temperature regime, has been established. The mechanisms of concentration of chemical compounds in supercooled water at the contact with the lower surface of the ice mass and their crystallization are considered. It is revealed that the phase transition of water from a liquid to a solid state and the growth of ice crystals contribute to the supersaturation of water with chemical compounds between ice crystals. The conditions under which the concentrations of crystallizing chemical compounds exceed their average content in under-ice water and play an important role in channel pollution during floods are described.

Cold start is one of the major issues that hinders the commercialization of polymer electrolyte membrane fuel cells (PEMFCs). In this study, a 2D transient multi-physics model is developed to simulate the cold start processes in a PEMFC. The phase change between water vapor, liquid water, and ice in the catalyst layers (CLs), micro porous layer (MPLs), and gas diffusion layers (GDLs) is also investigated, particularly the effect of ice crystallization kinetics when supercooled liquid water changes into ice. The factors affecting the different operating conditions and structural features of the membrane electrode assembly (MEA) are investigated. The results show that when the start temperature is −20 °C or higher, ice formation is delayed and the formation rate is decreased, and supercooled liquid water permeates from the CL into the MPL. For an MEA with relatively high hydrophobicity, the water permeation rate is high. These results can enable a PEMFC to start at subzero temperatures. The effect of ice crystallization kinetics is negligible when the fuel cell is started at −30 °C or below.

In conventional processes, such as evaporation, higher levels of concentration can be reached compared with freeze concentration or membrane techniques. However, the advantage of the freeze concentration technique is based on the quality of the product obtained due to the low temperatures used in the process, which makes it a very suitable technology for the processing of fruit juices. There are two basic methods for concentrating solutions by freezing: suspension and film freeze concentration. Suspension freeze concentration systems (FCS) already have operating equipment in the food industry, while film FCSs, also called layer crystallization, is still at an experimental stage. This review summarizes the most important studies relating to the suspension and film freeze concentration in fruit juices and sugar solutions, illustrating the different possibilities that freeze concentration has in the fruit juices industry; it also presents trends and suggests improvements for the future development of this technology. It is noted that most recent publications refer to the film FCS. The technology used to design, build and maintain layer crystallization equipment is simple and it can be available to any operator in the food industry, layer systems will be used in the future if their results can be improved in terms of ice purity and degree of fluid concentration.

Ce travail porte sur l’étude d’un procédé de purification de l’eau usée par congélation en couche sur paroi froide. Plusieurs études montrent que la croissance de la couche de glace dépend des caractéristiques de la phase liquide (concentration initiale en soluté, température du liquide, etc) et des paramètres opératoires (vitesse de refroidissement, ensemencement, sous-refroidissement et circulation de la phase liquide, etc). En revanche, la distribution du soluté dans la phase solide, un facteur crucial pour améliorer la performance du procédé, a été peu étudiée. Une conduite précise et rigoureuse du procédé est donc nécessaire pour obtenir une pureté de la glace bien maîtrisée. Pour atteindre cet objectif, cette thèse comporte une partie expérimentale et une partie de modélisation. Dans la partie expérimentale, le mélange H2O-NaCl a été choisi comme solution modèle. Un dispositif expérimental de cristallisation sur paroi froide a été spécifiquement conçu pour ce travail. Le montage est instrumenté avec des capteurs de température et de conductivité thermique, une caméra permet également de suivre in situ la croissance de la glace au cours des manipulations. Après une recherche bibliographique sur le principe de la cristallisation et la mise en œuvre du procédé, une série de manipulations pour une étude paramétrique a été réalisée. La croissance dendritique a été observée avec une vitesse de croissance élevée. Ce phénomène est dépendant de l’effet de convection, du sous-refroidissement, du gradient de température dans la phase liquide ainsi que de la concentration initiale de la solution à purifier. Pour la partie modélisation, la méthode du champ de phase a été choisie. Elle permet de simuler la morphologie et la cinétique de croissance du solide, l’inclusion de liquide (poches et interstices) dans le solide, ainsi que la phase liquide. Les simulations permettent de mieux comprendre les phénomènes intervenant lors de la cristallisation, tels que l’incorporation d’impuretés dans la glace. Cette méthode, peu utilisée dans le domaine du génie des procédés, a été d'abord appliquée à la congélation d’un corps pur (eau/glace) pour comprendre les équations et étudier les paramètres du modèle. Ce modèle a ensuite été élargi à la congélation d’un mélange binaire (H2O-NaCl), correspondant au produit choisi pour l’expérimentation. Afin de respecter la cohérence thermodynamique de cette méthode, le modèle de Pitzer a été choisi pour prédire l’équilibre et les propriétés thermodynamiques. A partir de ces bases de données thermodynamiques, les équations de bilan de matière, de bilan thermique et de champ de phases sont résolues. Les effets du sous-refroidissement, de la concentration et de l’anisotropie sur la croissance dendritique des cristaux sont comparés et discutés. Ce travail innovant a démontré la pertinence de la méthode des champs de phase, jusqu’alors peu développée en génie des procédés. Cette méthode permet de décrire les phénomènes développés à l’interface liquide/solide, de prédire le comportement de la phase solide pour limiter les phénomènes d’incorporation de soluté dans la glace et plus généralement de comprendre les phénomènes mis en jeu lors de l’étape de cristallisation
This work focuses on studying a wastewater purification process by freezing on a cold wall. Several studies show that the growth of the ice layer depends on the characteristics of the liquid phase (solute concentration, liquid temperature, etc.) and the experimental conditions (cooling rate, seeding, supercooling, and liquid phase circulation, etc.). However, the solute distribution in the solid phase, a crucial factor for improving the process performance, has been the focus of few studies. Therefore, precise and rigorous control of the process is necessary to achieve well-controlled ice purity. To achieve this goal, this thesis includes an experimental part and a modeling part. In the experimental part, the H2O-NaCl mixture was chosen as the model solution. An experimental setup for crystallization on a cold wall was specifically designed for this work. The setup is equipped with temperature and thermal conductivity sensors, and a camera is used to monitor the in-situ growth of ice during the manipulations. After a bibliographical study on the principle of crystallization and the implementation of the process, a series of manipulations for a parametric study was carried out. Dendritic growth was observed with a high growth rate. This phenomenon depends on the effect of convection, supercooling, the temperature gradient in the liquid phase, and the initial concentration of the solution to be purified. For the modeling part, the phase-field method was chosen. This method allows simulating the morphology and growth kinetics of the solid, the inclusion of liquid (pockets and interstices) in the solid, as well as the liquid phase. The simulations help better understand the phenomena occurring during crystallization, such as the incorporation of impurities into the ice. This method, rarely used in chemical engineering, was first applied to the freezing of a pure substance (water/ice) to understand the equations and study the model's parameters. This model was then extended to the freezing of a binary mixture (H2O-NaCl), corresponding to the product chosen for the experiment. To asses the thermodynamic consistency of this method, the Pitzer model was chosen to predict equilibrium and thermodynamic properties. Based on this thermodynamic data, the mass balance, thermal balance, and phase-field equations are solved. The effects of supercooling, concentration, and anisotropy on the dendritic growth of crystals are compared and discussed. This innovative work has demonstrated the relevance of the phase-field method, which has been little developed in chemical engineering until now. This method allows describing the phenomena occurring at the liquid/solid interface, predicting the behavior of the solid phase to limit the incorporation of solute into the ice, and more generally, understanding the phenomena involved during the crystallization step

ABSTRACT The Manicouagan impact event has been the subject of multiple age determinations over the past ~50 yr, providing an ideal test site for evaluating the viability of different geochronometers. This study highlights the suitability of Manicouagan’s essentially pristine impact melt body as a medium for providing insight into the U-Pb isotope systematics of geochronometers in the absence of shock-related overprinting. We performed in situ laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb geochronology on apatite and zircon, both of which crystallized as primary phases. This study is the first application of U-Pb geochronology to apatite crystallized within a terrestrial impact melt sheet. U-Pb analyses were obtained from 200 melt-grown apatite grains (n = 222 spots), with a data subset providing a lower-intercept age of 212.5 ± 8.0 Ma. For melt-grown zircon, a total of 30 analyses from 28 grains were obtained, with a subset of the data yielding a lower-intercept age of ± 1.6 Ma. The lower precision (±8.0 Ma; ±3%) obtained from apatite is a consequence of low U and a high and variable common-Pb composition. This resulted from localized Pb*/PbC heterogeneity within the impact melt sheet that was incorporated into the apatite crystal structure during crystallization (where Pb*/PbC is the ratio of radiogenic Pb to common Pb). While considered a limitation to the precision obtainable from melt-grown apatite, its ability to record local-scale isotopic variations highlights an advantage of U-Pb studies on melt-grown apatite. The best-estimate ages from zircon and apatite overlap within error and correlate with previously determined ages for the Manicouagan impact event. An average formation age from the new determinations, combined with previous age constraints, yields a weighted mean age of 214.96 ± 0.30 Ma for the Manicouagan impact structure.

Compared with the conventional mathematical and physical models, the lattice Boltzmann (LB) method is an effective method to simulate the heat and mass transfer in porous media. Frost crystallization aggregation is a very complex process involving inconsistency of frost structures, crystal size distributions, the complex transient shapes, and other numerous influential factors. Assuming the frost is a special porous medium consists of ice crystals and humid air, a mesoscopic model is established to predict the behavior of frost formation based on the lattice Boltzmann equation. The moving boundary condition is adopted in the two-dimensional nine-speed (D2Q9) lattices. The influences of the cold flat surfaces temperature on frost formation process are investigated. The variation laws of frost density and frost layer height are obtained and discussed. Simulation results by the LB model are in agreement with the experiment data from the references.

Crystallization control by laser ablation of liquid is promising for enforcing crystal nucleation in a spatio-temporal manner. In this work, we have demonstrated ice crystallization by laser ablation of water with a single laser pulse, which allowed for the detailed study of ice crystallization with high-speed imaging. The systematic results clearly showed the ice crystallization with bubbles that were formed by laser ablation of water.

The microscopic composition of thirteen samples of epidote related to porphyry Cu mineralization was mapped using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Geological Survey of Canada. The objective of this research is to improve the indicator mineral method of mineral exploration in glaciated terrains by utilizing the trace element composition of epidote. Six bedrock samples from porphyry Cu deposits of south-central British Columbia (Gibraltar, Mount Polley and Woodjam), three bedrock samples from the Nicola Group located close (<2 km) from the intrusions host of porphyry mineralization and afar (12 km), and four epidote grains from two till samples, one at Gibraltar and a second one at Mount Polley, were analyzed. Backscattered electron (BSE) images and the LA-ICP-MS maps show an heterogeneous distribution of Fe and Al in epidote following complex and mottled patterns and consistent zoning typically with high Fe and low Al concentrations in the core progressing to low Fe and high Al concentrations in the rim. Trace elements are heterogeneously distributed in epidote following the Fe/Al zoning in some samples. Evidence of late infiltration of trace elements (e.g. Cu, Zn, and REE) along fractures in epidote is observed in some samples. The variability in epidote composition is thought to be related to the changing conditions during its crystallization including oxidation state, pH, oxygen fugacity, fluid composition, temperature and pressure. Multiple LA-ICP-MS spot analyses need to be conducted on this mineral to fully evaluate its composition as an indicator mineral of porphyry Cu mineralization.