Thierry Ruiz

Experimental study of vertical stress profiles of a confined granular bed under static and dynamic conditions

In a wet agglomeration process inside a low shear mixer, the blade function is to induce i) homogenization of the liquid sprayed on the powder surface and ii) a stress field able to transfer the mechanical energy at the particle scale. In this work we study the mechanical state of a confined powder bed through the analysis of stress distributions (by force measurements) in a rectangular cell in two cases: for a classical model powder (i.e. glass beads) and a complex powder (i.e. wheat semolina). Two types of vertical stress profiles are obtained according to the type of measurements carried out in the powder bed, either locally (at different positions in the cell) or globally (at the entire base). The global vertical stress profile follows Janssens model and the local vertical stress profile highlights a critical length, identified as the percolation threshold of the force network, and a shielding length near the bottom, which is similar to an influence length of the side walls. In the context of wet agglomeration, the results allow to consider the role of the characteristic lengths in the mixing bowl under vertical mechanical solicitation.

Preparation of granular systems by extrusion/spheronization: A twin product/process approach

The extrusion/ spheronization technique makes a notable contribution to the existing range of pharmaceutical forms, especially in the area of oral controlled-release product. Analysis of this technique leads the authors to make a parallel study of the influences of the aspects linked to formulation (water content) and to processing used (extruder screw speed) with the aim of optimizing the product quality. Textural variables, porosity, saturation degree and water content were monitored throughout the elaboration process. The kneading operation brings the material to a state in which porosity is linked to water content. The extrusion operation densities the material to saturation point, while spheronization is simply a shaping process which maintains the hygrotextural state. Also emphasized is the presence of solid/liquid ratios optimizing pellet quality independently of the operating variables. In parallel to this, a range of water content for which the influence of the process parameter is relevant is identified.

Advances in food powder agglomeration engineering.

Food powders are used in everyday life in many ways and offer technological solutions to the problem of food production. The natural origin of food powders, diversity in their chemical composition, variability of the raw materials, heterogeneity of the native structures, and physicochemical reactivity under hydrothermal stresses contribute to the complexity in their behavior. Food powder agglomeration has recently been considered according to a multiscale approach, which is followed in the chapter layout: (i) at the particle scale, by a presentation of particle properties and surface reactivity in connection with the agglomeration mechanisms, (ii) at the mechanisms scale, by describing the structuration dynamics of agglomerates, (iii) at the process scale, by a presentation of agglomeration technologies and sensors and by studying the stress transmission mode in the powder bed, and finally (iv) by an integration of the acquired knowledge, thanks to a dimensional analysis carried out at each scale.

Agglomeration/granulation in food powder production

Abstract: Agglomeration is a unit operation during which native particles are assembled to form bigger agglomerates, in which the original particle can still be distinguished. Agglomeration is considered to improve the functionality of food powders. Both physical and physicochemical phenomena must be considered when describing agglomeration. Food powders react when exposed to high temperatures and/or the addition of liquid, that is, their molecules can undergo irreversible physicochemical changes under these conditions. In this chapter, the agglomeration of food powders is described, from the generic concepts through to specificities associated with their physicochemical reactivity.

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.