Eric Rondet

Fractal Law: A New Tool for Modelling Agglomeration Process

Wet agglomeration of granular materials is widely carried out in many industrial fields, such as the pharmaceutical and food industries. This process must be optimized in terms of yields and quality of the final product. A lot of studies have shown that agglomeration occurs under capillary, viscous and frictional forces and can be divided into four steps: nucleation, consolidation, growth and rupture. This general description of agglomeration does not take into account the simultaneous changes of agglomerates size, texture and saturation degree with respect to the increase in water content. The aim of this work is to propose a description of the agglomeration based on experimental observations and to take into account the aforementioned aspects to propose a model. In this study, experiments are carried out on kaolin in a low shear mixer for various operating conditions: three speeds of blade rotation, two wetting modes. The evolution of agglomerates granulometric and textural parameters is taken into account by image analysis and immersion in paraffin, respectively. This study makes it possible to highlight an increase in agglomerates size associated with a solid volume fraction reduction when there is an increase in water content. A power law connects solid volume fraction and median diameter of agglomerates making it possible to identify a fractal growth process between the grain and pasty-like scale. This well-know approach in colloidal aggregation is new in the powder agglomeration field. Fractal dimension defined this way is mainly affected by the drop size generated during wetting. These experiments highlight the weak influence of the other process parameters tested. For a wetting mode, an equation leading to the direct determination of the desired size of agglomerates with water content is proposed. Applied to agglomeration, fractal morphogenesis should become a potential tool in product engineering.

Psychrometry as a methodological tool for optimizing the spray drying process.

Because psychrometry takes into account a great number of variables reflecting the quality of the drying air, it is an interesting tool to improve the control and the optimization of the spray drying process. In this article the authors study the evolution of the psychrometric variables according to the values taken by four inlet parameters (inlet air temperature, liquid flow rate, solid concentration of the spray dried liquid, and nature of the product). The results highlighted the existence of mathematical models making it possible to optimize the process, but also to underline the influence of the nature of the product on the drying mechanism.

A contribution to the understanding of micro-pollutant sorption mechanisms in wastewater biological processes: case of the tributyltin

Micro-pollutant fluxes distribution throughout the physical separation and biological units of wastewater treatment plants (WWTPs) are very dependent of sorption phenomena. The understanding and the control of the sorption stage is thus essential for the optimization of micro-pollutant removal in WWTPs, and particularly in biological treatments where these mechanisms influence the bioavailability towards micro-organisms. If the influence of the micro-pollutant physicochemical characteristics (e.g. K ow, pKa) on their ability to sorb on biological media (i.e. sludge) has been demonstrated, it appears that some other parameters, like the biosorbent characteristics, have to been taken into account. The aim of this study is thus to correlate the capacities of sorption of an environmentally relevant substance (tributyltin), with a thorough characterization of different types of sludge. The characterization of three biological media (raw, sonicated and flocculated activated sludges) is proposed according to various characterization parameters related to biochemical composition, aggregate size, rheological behaviour etc. The results show first that, whatever the sludge characteristics may be, the sorption mechanisms are very rapid and that an equilibrium state is reached after a few minutes. The influence of the sludge characteristics, notably the floc size and the chemical oxygen demand partition between solid and colloidal fraction, on sorption efficiency is demonstrated. A Langmuir modelling allows giving the maximum sorption capacity, as well as the binding energy for the three studied sludges, according to their physicochemical characteristics.