Cristallisation en milieu confiné : modèle pour la nucléation et le mûrissement d'Ostwald

National audienceLes modèles utilisés en cristallisation industrielle sont essentiellement conçus pour représenter la formation et l'évolution d'entités isolées dans leur solution-mère alors que dans les systèmes industriels ou naturels les cristaux sont couramment proches. A l'occasion d'une modélisation du mûrissement d'Ostwald, nous avons montré la pertinence et l'efficacité d'un modèle numérique à une dimension d'espace qui rend possible l'étude théorique de population de nombreux cristaux éventuellement très proches. Les situations de cristallisation en milieu confiné semblent se présenter de plus en plus couramment (microréacteurs, milieux poreux, milieu vivant) et nécessiter des modélisations adaptées. Nous avons voulu étendre notre expérience de la modélisation du mûrissement d'Ostwald au cas de la formation de germes en milieu d'extension réduite et étudier l'influence du confinement. Notre intérêt s'est particulièrement porté sur : - l'interaction entre nucléation et mûrissement au cours des processus ; - la validité des lois classiques de nucléation et les modifications nécessitées par le confinement au niveau de la possibilité effective de nucléation et du nombre de germes formés ; - l'influence de divers paramètres : sursaturation, cinétique de croissance, extension du milieu. La communication proposée ici présente le modèle mis en oeuvre et quelques-unes de ses applications et entend ouvrir une discussion sur ses applications potentielles

Asymptotic modelling of crystallisation in two-layer systems. Application to methane hydrate crystallization

International audienceThe problem of gas-liquid crystallization is re-visited in the framework of a two-film model. The moments of the crystal size distribution can be derived from a differential equation system. Instead of a numerical solution, we present here a general procedure to express analytically the asymptotic behaviour of the physical system. Thanks to this formulation, influence of different parameters can be easily identified and validated on experimental data which mainly concern methane hydrate crystallization

Asymptotic Modelling of Crystallisation in Two Layers Systems. Application to Methane Hydrate Formation in Batch Reactor.

6 pagesThis paper proposes to re-visit the problem of gas-liquid crystallization in the framework of a two-layer model and with the help of data coming from experiments on methane hydrate crystallization in a semi-batch reactor. Preliminary quantitative discussion of the order of magnitude of different effects makes possible realistic simplifications in the theoretical models. In particular, the role of the interfacial film is clearly defined. As previous authors did, we use a formulation in terms of moments of the crystal size distribution, however we are not interested in the numerical solution to the corresponding differential system, but we propose a general procedure to express analytically the asymptotic behaviour of the physical system. Thanks to this formulation, influence of different parameters can be easily identified and validated on available experimental dat

Light Scattering Cross Section of Hydrophilic and Hydrophobic Silica Aggregates

20 pagesInternational audienceThe aggregation dynamics of solid particles in liquid media is currently followed by optical based particle sizing methods. Because it can be used in situ and applied to a wide particle size range, turbidimetry is acknowledged as one of the best methods for this characterization. Although much work has been done about aggregation, some aspects are less known and require additional experimental and theoretical research. This is particularly the case of aggregation of hydrophobic particles. Corresponding aggregates are 3-phase objects (solid-liquid-gas) the morphology and optical properties of which are not known. Present work rests on the turbidimetric study of hydrophilic and hydrophobic silica samples in stirred aqueous solutions. Modelling involves different aspects: aggregate morphology, aggregate optical properties, and aggregation dynamics. This paper particularly emphasizes the second one. Fractal-like models are proved to be representative of the aggregate morphology even at small size. Light scattering cross-section of the aggregates is calculated from their averaged projected area; effective refractive index is proved to be a good parameter for modelling their optical properties both for hydrophilic and hydrophobic aggregates. Classical models of porous aggregate formation (Kusters theory) are used for describing the aggregation dynamics

Thermodynamic modelling of clathrate hydrates in the atmosphere of Mars

International audienceThe papers deals with the stability at low temperature and low pressure in the atmospheric conditions of March, that means a gas composition mainly dominated by CO2 plus low concentration of Argon and Nitrogen and traces of gases as methane. The paper is mainly based on the data compilation from Fray et al (2011) for pure gases equilibriums. We fit the classical van der Waals approach to compare deviation. It reveals to be acceptable and allows to simulate a Martian gas of CO2 dominated (95.3%) plus nitrogen (2.7%) and Argon (2%). The hydrate reveals to be a CO2 based hydrate which is unstable during Mars summer and stable during Mars winter time (at temperature below 154K). The proportion of methane in the hydrate is evaluated and reveals to be one fourth of the composition of the gas phase

Modelling of the rheological behaviour of slurries used for cold transportation

National audienceSuspensions of solid crystals are used for cold transportation purposes with the aim of reducing the use of classical refrigerants. From a general point of view, the understanding and the mastering of the rheological properties of the crystal suspensions is essential for controlling the operating conditions. Water ice and Tetra-ButylAmmonium Bromide (TBAB) hydrate aqueous give rise to non-Newtonian flows. In particular Bingham-like behaviours are frequently observed. Two reasons at least can explain deviation from Newtonian characteristics: crystal agglomeration and particle

Crystallisation and rheology of an hydrate slurry as secondary two-phase refrigerant for air-conditioning application

8 pagesUnder atmospheric pressure condition and temperatures between 0°C and +12°C, Tetra-n- ButylAmmonium Bromide (TBAB) aqueous solutions crystallise into hydrate slurries. These slurries seems to be well appropriate for cold storage and transportation in the case of air conditioning applications. We focus here on the crystallisation and the rheological properties of TBAB hydrates slurries. Once the crystallisation conditions described, we propose an experimental analysis of their flow behaviour. The experimental device is made up of a brushed surface heat exchanger in which the hydrates slurry is generated, and of a measurement loop. The flow behaviour of the slurry is characterised through flow rates and pressure drops measurements. We obtain flow curves of hydrates slurries depending on the volume fraction of hydrate solid particles. In the range of shear rate investigated, hydrate slurries behave as Bingham fluids. We propose a method of determining their apparent viscosities and yield shear stress versus the volume fraction of hydrates

Rheological study of an hydrate slurry for air conditionning application

Fifth International Conference on Gas Hydrates (ICGH 5), Tromdheim, Norvège, 13 au 16 juin 2005International audienceUnder atmospheric pressure condition and tempertures between 0°C and +12°C, Tetra-n-ButylAmmonium bromide (TBAB) aqueous solutions crystallise into hydrate slurries. These slurries seem to be well appropriate for cold storage and transportation in the case of air-conditionning applications. We focus here on the crystallisation and the rheological properties of TBAB hydrates slurries. Once the crystallisation conditions described, we propose an experimental analysis of their flow behaviour. The experimental device is made up of a brushed surface heat exchanger in which the hydrates slurry is generated, and of a mesasurement loop. The flow behaviour of the slurry is characterised through flow rates and pressure drops measurements. We obtain flow curves of hydrates slurries depending on the volume fraction of hydrate solid particles. In the range of shear rate investigated, hydrate slurries behave as Bingham fluids. We propose a method of determining their apparent viscosities and yield shear stress versus the volume fraction hydrates

Influence of non-wetting conditions on aggregation of divided solids in liquid phase

National audienceWe present here the main results of an experimental study of aggregation which was performed on two silica granular samples: one is naturally hydrophilic; the other has been made hydrophobic by superficial grafting of hydrophobic groups. Aggregation is studied in water-ethanol mixtures of variable compositions and followed by in situ turbidimetry at different stirring rates. The aggregation behaviors of the two samples are quite different. Aggregates of hydrophilic particles are small and fragile whereas aggregates of hydrophobic particles are large and particularly solid. Interpretation of these differences is presented; it is based on several elements: modeling of hydrophobic interaction and of aggregate morphology, modeling of the aggregate optical properties and determination of the aggregation and fragmentation rates. Essential modification introduced by non-wetting consists of the formation of gas bridges around contact points between particles in aggregates

Modelling and simulation of condensation phenomena of acid gases in an industrial chimney

National audienceCoal power stations as well as waste incinerators produce humid acid gases which condensate in industrial chimneys. These condensates may cause corrosion of the internal cladding made of stainless steels, nickel base alloys or non metallic materials. In the aim of polluting emission reduction and material optimal choice, it is necessary to determine all the phenomena which occur throughout the chimney such as condensation and dissolution of acid gases (in this particular case, sulphur dioxide SO2). The production of energy from fossil fuels (coal, petroleum, natural gas, etc.) brings about the emission of gas containing sulphur compounds (SO2, SO3) as well as chlorine and fluorine compounds. To avoid this atmospheric pollution and its harmful effects (acid rains, impact on the health) due to hydrochloric, sulphuric and hydrofluoric acids produced in the presence of air, it is necessary to steam these flue gases. Nevertheless, a considerable quantity of residual acid gases remains in the gas discharge which also contains large amounts of water vapour. Thus, condensation may occur and by the way an acid attack of the internal cladding of the chimney. This results in high costs of maintenance and a reduction of their structural stability. The knowledge of the phenomena of heat and mass transfer during the condensation of these acid gases in chimney is essential for their conception and materials choice