Effect of precipitation process parameters on boehmite properties: in situ optical monitoring

Alumina, which is a catalyst support, is obtained by boehmite calcination. This work aims to study boehmite precipitation and the impact of this step on alumina quality. Precipitation of boehmite is studied in a double-jet stirred tank and with a micro-mixer. These devices induce different mixing of the reagents and different evolution of the supersaturation. Precipitation is followed on-line with an optical apparatus based on multiple light scattering. Alumina obtained with micro-mixer presents significantly lower specific surface area, pore volume and mean pore diameter than the one obtained with the double jet reactor, showing a totally different crystallite aggregation. Using different types of micromixers at isosupersaturation, the results show that Reynolds number in the micro-mixer, stirring power in the receiving vessel and micro-mixer type have no effect on porosity and thus on aggregation, which could be explained by a supersaturation that is sufficiently high to prevent other parameters from having effects. Using double jet reactor, feeding time directly affects initial supersaturation, which level is lower than the one created in micro-mixer. For double jet precipitation, the textural properties are directly correlated to the initial supersaturation. Alumina obtained with the highest value of supersaturation with double-jet stirred tank has similar porosity value than the one obtained in micro-mixer. This supersaturation level would thus be a threshold beyond which other operating parameters have no effect. Aggregation would be directly controlled by initial supersaturation. Furthermore, it was shown that differences of alumina textures are correlated with different behaviors of backscattering levels. This technique allows the in situ characterization of aggregation differences, during precipitation, before the impact of filtration–washing–drying steps on aggregation

Maîtrise de la précipitation des polymorphes du carbonate de calcium en vue de la conception d’un procédé de purification en réacteur à lit fluidisé

The purification process of sodium chloride solutions consists of the precipitation of Ca2+ and Mg2+ ions as calcium carbonate and magnesium hydroxide. The impurity content is low, so the best results can be obtained when the precipitation takes place in a fluidized bed reactor where the solid concentration is high. The improvement of the precipitate quality, mainly composed of CaCO3, is necessary for the good running of the process, thus the control of the calcium carbonate polymorphism is essential. First, a reliable method for the quantitative determination of the polymorphic composition using Raman spectroscopy combined with multivariate curve resolution is developed. Then the key parameters which control the polymorphic. composition are identified through the experiments carried out in a batch reactor. Based on these results, a new process using a fluidized bed reactor is designed and experimentally tested. From these experiments, the optimal operating conditions allowing a high level of purification and a very good solid/liquid separation are determinedLe procédé industriel pour purifier des solutions concentrées de chlorure de sodium consiste en la précipitation des ions calcium sous forme de carbonate de calcium et des ions magnésium sous forme d'hydroxyde de magnésium. Ces impuretés étant en faible concentration, la précipitation est menée en réacteur à lit fluidisé en présence d’une forte charge solide. La maîtrise de la qualité du précipité, constitué. Dans un premier temps, une méthode analytique fiable est développée pour déterminer quantitativement la composition d’un mélange de polymorphes : la spectroscopie Raman associée à une méthode de résolution de courbes. Ensuite, la précipitation du carbonate de calcium en réacteur fermé permet d’identifier les paramètres clés du procédé pour maîtriser la composition polymorphique. A partir de ces résultats, un nouveau procédé de purification en réacteur continu à lit fluidisé est conçu et mis en œuvre. L’étude expérimentale réalisée dans celui-ci permet de définir les paramètres opératoires conduisant à une parfaite maîtrise de la qualité du précipité et une conduite optimale de la purificatio

Kinetic modelling of Pt/gamma-Al2O3-Cl catalysts formulation changes in n-heptane reforming

International audienceBridging the gap between kinetic model conception and catalyst design is targeted in catalytic naphtha reforming process development. New catalysts are continuously optimised in order to achieve higher selectivity in C5+ products. An adequate description of catalytic transformations by the kinetic models would therefore provide clues for catalyst design and accelerate the time to market implementation of process simulators. This study investigates the influence of site density and location on n-heptane reforming selectivity. It identifies the nature of the limiting steps for the different reforming pathways on a broad range of catalyst formulations. A common lumped model using power law kinetics is developed to describe already published experimental observations on the set of selected catalysts. Linear free energy relationships are used in order to handle a reduced number of statistically relevant adjustable parameters. The dependence between reference rate constants and active phase formulation is then unravelled. Unexpected results indicate that chlorine content and repartition at the crystallite scale affects the hydrogenolysis activity. Within the range of tested formulations, this study suggests that hydroisomerisation reactions are limited by acid sites transformations whereas the aromatisation pathways seem to proceed through a metal/acid bi-functional scheme. The further elaboration of a kinetic model that is able to predict the effect of an industrial catalyst active phase formulation change in full naphtha cut reforming lies beyond the scope of this article

A naphtha reforming process development methodology based on the identification of catalytic reactivity descriptors

SSCI-VIDE+ING+OSA:DFAInternational audienceSeveral major refining catalytic processes show a strong dependence of overall performances upon catalyst formulations and their structure. This observation is particularly true in naphtha catalytic reforming for which the product distribution is sensitive to slight changes in active phase formulations. This review presents different issues encountered in naphtha reforming that are related to active phase formulation changes. Two research and development approaches are usually proposed. (i) Rationalise the development of new catalytic formulations in order to increase selectivity towads valuable products. (ii) Design simulators that would guide operation in order to maintain process performances. Current limitations to a faster process development come from the disjunction between kinetic modelling and catalyst optimisation. This critical review is not an extensive analysis of existing naphtha reforming kinetic modelling methodologies or advances in catalytic behaviour elucidation. Rather, it proposes an original and pragmatic process development approach aimed at merging catalyst development with kinetic modelling through the identification of “effective” and “measurable” catalytic descriptors. The specificities of the structure–property relationships of naphtha reforming catalysts are reviewed and taken into account in order to list potential catalytic descriptors for this process. A focus is made on a current bottleneck faced in the description of the active phase which lies in understanding the role of the proximity and the interaction between acid and metallic sites in the balance between different bi-functional pathways. A variety of experimental approaches that can be used to measure these naphtha reforming active phase catalytic descriptors are presented and compared

Estimation of kinetic parameters and diffusion coefficients for the transesterification of triolein with methanol on a solid ZnAl2O4 catalyst.

International audienceThe transesterification of vegetable oils by methanol implies a set of three parallel–series reversible reactions. Over a heterogeneous catalyst, reactions are limited by molecular diffusion, resulting in the use of a large excess of methanol to achieve a high conversion. This excess is detrimental to process efficiency because it involves large energy requirement for downstream separation. In order to identify the coupled physico-chemical phenomena and to optimize the process, a pilot unit was developed to perform transesterification of triglycerides with methanol over a solid ZnAl2O4 catalyst in a tubular fixed bed reactor under high pressure and temperature. The effects of different parameters such as particle diameter, temperature, residence time and molar feed ratio of methanol to triglyceride on fatty acid methyl ester (FAME) yield and triglyceride (TG) conversion were studied. A pseudo-homogeneous reactor model has been developed to identify rate laws and kinetic parameters by exploiting the experimental data in stationary regime. A set of kinetic constants was also determined for the given catalyst considering two kinetic models: an Eley–Rideal model with the reaction between TG and methanol as the rate determining step and a classical second order model taking into account thermodynamic equilibrium without adsorption. A heterogeneous model has also been established to determine molecular diffusion coefficients for each species in the mixture, through the use of experimental results in the transient regime. These molecular diffusion coefficients are dependent on the mixture viscosity and temperature. As a result, it was determined that diffusion of triglycerides and the first reaction kinetic rates are limiting the global conversion of the system

Spray drying of colloidal suspensions: Coupling of particle drying and transport models with experimental validations

International audienceA numerically effective approach was developed for the modeling of spray-drying of colloidal suspensions. This approach was based on the integration of two models. The first is a phenomenological and radially symmetric model accounting for the drying of single-droplets, while the second employs computational fluid dynamics (CFD) simulations to account for the gas flow conditions and atomization in a spray dryer. Experiments were also conducted on single suspension droplets trapped in an acoustic field as well as on droplets in a mini-spray dryer. The predictions of the models were found to be in reasonable agreement with the experimental data, in terms of droplet shrinking and buckling, particle yield, and spatial distribution in the spray dryer mockup

Non Monotonous Product Distribution Dependence on Pt/gamma-Al2O3-Cl Catalysts Formulation in n-Heptane Reforming.

SSCI-VIDE+ING+OSA:VPT:DFA:FMOInternational audienceThe synthesis of 19 carefully selected Pt/γ‐Al2O3−Cl formulations was followed by high‐throughput catalytic testing in order to unravel the effect of an active phase formulation change on n ‐heptane reforming performances. Pt/γ‐Al2O3−Cl catalysts were prepared with different Pt (0.3–1 %wt) and Cl (0.1–1.4 %wt) contents and using two γ‐Al2O3 supports so that both sites concentrations and sites locations at the crystallite surface vary among the catalyst pool. Catalytic tests were conducted in mild conditions for a comparison of catalysts in kinetic regime. Results show that Pt and Cl concentrations control the competition between hydroisomerisation, hydrogenolysis and hydrocracking pathways. Aromatisation, on the contrary, is poorly affected by formulation changes. Non‐monotonous trends linking Pt/Cl ratio to isomerisation selectivity are found for both γ‐Al2O3 supports. This study provides new insights for the description of bi‐functional transformations in catalytic naphtha reforming