Cristallisation non-stoechiométrique et modélisation d’un flash thermodynamique : Cas des hydrates mixtes de gaz

National audienceClathrate Hydrates are ice-like compounds that can be formed under high pressure and low temperature. They are composed of water and small molecules of ‘’gas’’. Hence they are usually called gas hydrates. They are involved in a significant issue of the oil industry, the hydrate plugs in pipelines (flow-assurance), as well as gas capture and storage, air conditioning… Moreover, methane hydrates can be found in sediments in deep sea and permafrost. That is why they are also considered as a significant methane resource on earth.Since they are non-stoichiometric compounds, it is difficult to model these crystals in process simulation. Furthermore, the speed of crystallization seems to influence the hydrate composition. Therefore, a modeling of the hydrate crystallization taking into account the history of the solid formation could be an interesting tool.In this work, a successive thermodynamic flash approach is presented according to two different hypotheses: heterogeneous hydrate phase during the crystal growth, and homogeneous hydrate phase. The main idea of these procedures is to discretize the crystal growth while the hydrate volume is increasing. Hence, three phase flash calculations are performed on the system. Each time, the previous amount of hydrate that has been formed is removed (at each iteration).The results of such algorithms are compared to batch experiments at low and quick crystallization rates (Duyen et al. 2016). The flash algorithms at given temperature (only one degree of freedom) give accurate results. The predicted final pressure and the hydrate volume are calculated within 7% accuracy. Moreover, the flash calculation results with no hydrate reorganization are closer to experiments at quick crystallization rate, whereas the experiment at low crystallization rate is better predicted with the second hypothesis (reorganization of the hydrate phase during growth). This work and its results provide a more realistic and comprehensive view of gas hydrate crystallization (more details in Bouillot and Herri, 2016).Les hydrates mixtes de gaz sont des cristaux solides dont la composition n’est pas stoechiométrique. Cette spécificité entraine des difficultés de modélisation lorsque qu’un volume significatif peut être formé. Il faut dans ce cas faire l’hypothèse d’un solide homogène (cas le plus simple), ou tenir compte de l’historique de la cristallisation si on suppose qu’un cristal formé à un temps t1 ne se met pas en équilibre rapidement avec sa solution, dont la composition peut avoir varié, à un temps t2.Dans le présent travail, une approche de modélisation de la cristallisation d’hydrates mixtes de gaz est proposée. Cette approche tient compte de l’évolution du milieu au cours de la cristallisation par des calculs successifs de flash thermodynamique.Deux hypothèses sont considérées. Dans la première, les cristaux d’hydrates mixtes croissent à l’équilibre thermodynamique à chaque instant. Il en résulte un cristal non homogène. La deuxième hypothèse forte est une ré-homogénéisation de la phase hydrate au cours de la cristallisation.Ces deux approches sont comparées à des mesures expérimentales obtenues par cristallisation lente ou rapide à partir de mélange d’hydrocarbures (CO2, CH4, C2H6). Les données importantes sont notamment : la pression finale, la composition des cristaux d’hydrates et leur volume.Les résultats obtenus par simulation s’accordent bien avec les données expérimentales. Les erreurs obtenues sur la pression d’équilibre et le volume d’hydrate sont généralement inférieures à 7%. Plus particulièrement, les résultats montrent qu’une cristallisation lente est plus proche d’un cas où le cristal se réorganise (équilibre thermodynamique), tandis qu’une cristallisation rapide est mieux simulée par une croissance non homogène. Une cristallisation rapide des hydrates mixtes de gaz peut donc se produire hors équilibre thermodynamique

Coalescing Cellular Automata

We say that a Cellular Automata (CA) is coalescing when its execution on two distinct (random) initial configurations in the same asynchronous mode (the same cells are updated in each configuration at each time step) makes both configurations become identical after a reasonable time. We prove coalescence for two elementary rules and show that there exists infinitely many coalescing CA. We then conduct an experimental study on all elementary CA and show that some rules exhibit a phase transition, which belongs to the universality class of directed percolation

Single File Diffusion of particles with long ranged interactions: damping and finite size effects

We study the Single File Diffusion (SFD) of a cyclic chain of particles that cannot cross each other, in a thermal bath, with long ranged interactions, and arbitrary damping. We present simulations that exhibit new behaviors specifically associated to systems of small number of particles and to small damping. In order to understand those results, we present an original analysis based on the decomposition of the particles motion in the normal modes of the chain. Our model explains all dynamic regimes observed in our simulations, and provides convincing estimates of the crossover times between those regimes.Comment: 30 pages, 9 figure

A PIONIER View on Mass-Transferring Red Giants

Symbiotic stars display absorption lines of a cool red giant together with emission lines of a nebula ionized by a hotter star, indicative of an active binary star system in which mass transfer is occurring. PIONIER at the VLT has been used to combine the light of four telescopes at a time to study in unprecedented detail how mass is transferred in symbiotic stars. The results of a mini-survey of symbiotic stars with PIONIER are summarised and some tentative general results about the role of Roche lobe overflow are presented.Comment: Report for the ESO Messenger June issu

Triagem enzimática e caracterização morfológica de fungos filamentosos isolados de amostras de indústria têxtil

Anais do VI Encontro de Iniciação Científica e II Encontro Anual de Iniciação ao Desenvolvimento Tecnológico e Inovação – EICTI 2017 - 04 a 06 de outubro de 2017 - temática Ciências BiológicasA atividade industrial para a produção têxtil utiliza-se de etapas de operações unitárias geradoras de grandes cargas de efluentes com alto potencial poluidor. Apesar de tais efluentes receberem um tratamento que precede o lançamento em um corpo hídrico, muitas vezes o mesmo apresenta resultados insatisfatórios na remoção de determinados agentes poluentes, devido ao desconhecimento por parte dos operadores da natureza química dos produtos utilizados, bem como a infinidade de produtos utilizados durante cada etapa industrial. Desta forma, efluentes originários da produção de tecidos têm se apresentado como um causador de impactos ambientais diversos em corpos d’agua e em suas áreas de entornoUniversidade Federal da Integração Latino-Americana (Unila); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Fundação Araucária; Parque Tecnológico Itaipu (PTI) e Companhia de Saneamento do Paraná (SANEPAR

Two experimental set-ups designed for investigation of friction stir spot welding process

International audienceThe effects of positioning and clamping conditions of a specimen of friction stir spot welding are investigated in this paper in terms of axial force and torque generated during the process. For this purpose, two special designs of experimental set-ups embedding different positioning and clamping conditions are presented. A four-component mechanical sensor is used for the measurements. First, the effects of the rotational speed of the spindle and the plunge depth of the tool on the axial force and torque are studied. Second, the effects of positioning and clamping conditions are investigated through both set-ups designed, varying the spindle rotation speed. It is shown that the axial force and torque exhibit an important dependence with respect to the rotation speed of the tool and that their maxima depend on positioning and clamping conditions of the specimen