Gas-Solid Fluidization ;Fine particles and nanoparticles

Fluidization is the process of bringing a granular phase into contact with a fluid phase, keeping the particles in suspension. The term "fluidization" comes from the fact that the gas-solid suspension is brought into a liquid-like state. The major advantage of fluidization lies in the quality of the intimate contact between the fluid phase and the solid particles. The intensity of material and heat transfer (both between phases, and between the bed and submerged surfaces) results in uniform temperatures and concentrations within the fluidized bed.

However, not all powders have the same ability to be fluidized, and consequently may behave differently with regard to fluidization. Based on average particle diameter and density, Geldart's classification gives the type of fluidization that will be achieved with air under ambient conditions. Four groups of particles emerge, each characterized by a different fluidization regime. These four groups are described in articles [J 4 100] and [J 1 065] . This article focuses on the fluidization of powders belonging to Geldart's group C (fine, cohesive particles) and nanoparticles.

Fluidization of fine powders (group C) and nanoparticles is currently finding a growing number of applications in various industries (semiconductors, catalysts, pharmaceuticals, cosmetics, food products, plastics, powder metallurgy, etc.). Generally speaking, these powders are characterized by their small diameter and large specific surface area, but also by high inter-particle cohesive forces. The influence of these interaction forces on fluidization is still poorly understood, and the current state of knowledge is such that it remains difficult to predict the overall behavior of a powder from the individual characteristics of its constituent particles. It should be noted that the term "nanoparticles" is generally used to designate primary particles smaller than 100 nm (more precisely, granular materials with at least one dimension smaller than 100 nm). Group C corresponds to particles with a diameter of less than 20 to 80 μm, depending on their density (the boundary between groups A and C in Geldart's classification).

According to Geldart's...