The effect of process conditions on powder rheology

The industrial practice indicates that process conditions can significantly affect the flowability of powders and their fluidization behavior in process units. Temperature, moisture and consolidation level are some of the main process parameters playing an important role. This review reports recent experimental and modeling work aiming at assessing the effect of process conditions and at providing a physical interpretation by linking interparticle forces and bulk level behavior. Changes of cohesive flow properties of powders are observed at high temperature in fluidized bed reactors, granulators and dryers. Several studies highlighted that a change of the fluidization behavior with the temperature cannot be explained by considering only fluid dynamic forces. Temperature can cause an increase of interparticle forces and of their relative weight with respect to body forces. This paper reports recent work on the indirect characterization of this effect in interparticle interactions by means of the measurement of rheological properties at the bulk level at high temperature. A procedure is proposed to estimate the interparticle forces from the bulk flow properties as measured with standardized techniques. Powders flow properties are strongly dependent on their moisture content too. The latter can be controlled by the environmental conditions, namely the humidity of the process unit atmosphere which the powder is exposed to. In fact, capillary condensation can cause the formation of liquid bridges between particles giving rise to interparticle capillary forces. These capillary forces cause an increase of powder cohesion and tensile strength and, thus, negatively affect the powder flowability. Moreover, capillary forces modify also other powder bulk properties such as voidage, aggregative structure and permeability. These properties and the relative importance of capillary forces with respect to fluid dynamic forces seriously affect also the fluidization behavior of fine powders. This review reports recent advancements in the understanding of the effect of air humidity on the flow properties of fine powders. In particular, experiments by means of a shear cell and a Couette device are presented. On modeling grounds the link between capillary condensation and cohesive properties at the bulk scale is addressed. Finally, the flow properties of fine powders at very low consolidation levels are relevant to small scale industrial application of powder flow, such as in small process hoppers and mixers of pharmaceutical powders, or in every day applications such as toner flow in cartridges and dosing and dispersion in dry powder inhalers. Conventional shear testers are not suitable for testing flow properties of loosely consolidated powders unless special procedures are adopted. Previous studies demonstrated that aeration does not affect the intrinsic flow properties of easy flowing and moderately cohesive powders. As a result, aeration below the fluidization threshold allows to carry out experiments with powders at very low consolidation levels. This paper reports a rheological model using the torque measurement in a mechanically stirred aerated bed rheometer to derive the powder flow propertie