Modélisation d'un échangeur d'ions : possibilités et limites des modèles disponibles — Proposition d'un modèle prenant en compte les limitations diffusionnelles et la dispersion hydrodynamique

En modélisant les équilibres résine/solution à l'aide de coefficients de sélectivité supposés constants, les étapes de diffusion externe et interne par des systèmes du premier ordre et la dispersion hydrodynamique par le modèle des mélangeurs en cascade avec recyclage interne, nous proposons un modèle simple, permettant de prévoir le fonctionnement d'un échangeur d'ions. A partir de paramètres que l'on peut soit déterminer à l'aide d'expériences simples, soit estimer à partir de données de la littérature, ce modèle permet de très bien représenter des courbes de perçage obtenues expérimentalement pour diverses solutions d'alimentation

The role of vitamin E in the susceptibility of rat lung and liver microsomes to iron-stimulated peroxidation.

The production of thiobarbituric acid-reactive substances (TBA-RS) and ethane, two markers of the lipid peroxidation process, was evaluated in rat lung and liver microsomal membranes incubated in the presence of either ferrous ions or a mixture of ferric ions and ascorbate. Microsomal fractions isolated from lung tissue were more resistant than those isolated from the liver. Compared to Fe2+, the association of Fe3+/ascorbate seemed to be totally ineffective in stimulating peroxidation of lung microsomes. The fatty acid profile of lung and liver microsomal membranes could not be responsible for their different susceptibility to free radical degradation. The microsomal fraction isolated from lung showed a higher vitamin E concentration than the liver. The importance of vitamin E in protecting lung membranes was assessed by using lung and liver isolated from vitamin E-deficient and vitamin E-supplemented rats. For both lung and liver microsomal fractions an inverse relationship between vitamin E concentrations and the extent of lipid peroxidation was observed. However, although the vitamin E concentrations in lung and liver microsomes isolated from rats submitted to a vitamin E-deficient diet were not different, lung microsomes still exhibited a lower production of TBA-RS and ethane than liver. In addition to vitamin E, other factors must be involved to explain the resistance of lung microsomes to lipid peroxidation

Adsorption of complex phenolic compounds on active charcoal : Breaktrough curves

International audienceThe transient adsorption of catechol from aqueous solution (C0=5 kg/m3) on activated carbon in an upflow fixed-bed column at 293 K was studied. The critical time at which the early breakthrough of the maximum admissible concentration (Ccrit=0.3×10−3 kg/m3) occurs is deduced from a Homogeneous Surface Diffusion Model (HSDM) that accounts for adsorption equilibrium and mass-transfer kinetics. The mass-transfer coefficient is measured using a thin bed adsorption method and a correlation is proposed to account for its dependence with the flow rate. The sensitivity of the model for the prediction of the critical time to the different parameters is discussed and it is found to be mostly dependant upon the mass-transfer coefficient Kf and the adsorbant mean particle diameter dP. In addition, the critical time has been proved to increase with the adsorption capacity qmax. The existence of an optimal flow of polluted effluent through the column to achieve the removal of the pollutant with the highest efficiency is observed

Effect of oxygen concentration on production of ethane and thiobarbituric acid-reactive substances by peroxidizing lung and liver homogenates and formation of ethanol by peroxidizing docosahexaenoic acid preparations under hyperoxic conditions.

The oxygen dependence of ethane formation was investigated in rat lung and liver homogenates, incubated in sealed flasks, in which the peroxidation was stimulated by the addition of ferrous ions. For both tissues, the production of ethane was maximal under a 20% oxygenated gas phase, while hyperoxic conditions led to a decreased ethane in the gas phase. The formation of thiobarbituric acid-reactive substances (TBA-RS), another marker of the lipid peroxidation process, in the homogenates of lung and liver was strongly stimulated at 100% compared to 20% oxygen. Experiments were also carried out on iron-stimulated peroxidation of pure docosahexaenoic acid preparations, which under air led to a large production of ethane. As for tissue homogenates, the TBA-RS content was increased in the presence of 100% oxygen. Those conditions, however, did not induce an increase in ethane production but led to the formation of ethanol. Therefore, the quenching of ethyl radical by molecular oxygen seems to be a very attractive hypothesis to explain the lack of increased ethane production in favor of ethanol when iron-induced lipid peroxidation was stimulated by oxygen

Scale up of slurry bubble reactors

Bubble column reactors are finding increasing use in industrial practice. They are in particular appropriate to carry out highly exothermic reactions, such as methanol synthesis or Fischer-Tropsch synthesis of conversion of synthesis gas to liquid paraffins. Industrial process require important volumes of reactors, the reactor diameter can reach 10 m. To control the reaction temperature, internal heat-exchange tubes (vertical tubes) are inserted inside the reactor. This study deals with the effects of scale and the presence of internals on hydrodynamic characteristics, for scale-up purposes based on experiments in cold mockups. Our study shows that the liquid recirculation intensity depends strongly on the column diameter whereas the gas holdup is slightly affected. Two methods are proposed to predict scale effect on liquid velocity: an empirical correlation proposed in the literature and a phenomenological model. Internals guide liquid: the large scale recirculation increases but fluctuations of liquid velocity decrease. Therefore the mixing of liquid is significantly affected by the presence of internals and is not well described by the standard mono dimensional axial dispersion model. A two-dimensional model, taking into account a radially dependent axial velocity profile and both axial and radial dispersion, is therefore developed to describe the liquid mixing in a bubble column with internals