(563c) Correlating Density Measurements to Flow Behavior of Cohesive Granular Materials
Reliable flow of particulate solids or powders from storage devices is of prime concern, among others, in agriculture, ceramic, food, mineral, mining and pharmaceutical industries. Knowledge of powder flow properties is very important when developing powder processes and handling procedures such as flow from hoppers and silos, transportation, mixing, compression and packaging. It is well known that flow characteristics of cohesive powders (i.e., powders where interparticle forces greatly exceed average particle weight) are highly dependent on their densification (consolidation) states, i.e. powders can be more or less dilated or compressed when stressed, thus leading to a large variability of interparticle contact density, resulting in widely different bulk flow properties.
The majority of the research on powder flowability has been carried out for cohesionless materials with particle size larger than 100ìm. However, as particle size decreases, the inter-particle forces (e.g. van der Waals forces) become stronger leading to an increase in cohesion. The increase in cohesion plays a dominant role in flow dynamics as it directly impacts the bulk flowability of solid material. Increased cohesiveness can cause jamming of the flow of granular material, even under conditions where the cohesionless material flows. Powder flow characteristics are commonly investigated under gravity loading conditions. The compressibility of a powder is a commonly used indicator of flowability and is often expressed using the Hausner Ratio, which is the ratio between the tapped and the loose-packed bulk densities of the powder. Compressibility is also one of the tests proposed by Carr for the assessment of powder properties. Another commonly used flow indicator is the time it takes for powder to flow out of a funnel with a standard orifice size. Such measurements have demonstrated the dependence of powder flowability on particles shape and size distribution.
In recent years, researchers have attempted to quantify cohesion and flow characteristics of granular materials by primarily analyzing powder behavior in its consolidated state. The most common method is the shear tester in which the force required to shear a powder under well-defined conditions is measured. Shear testers have served successfully as an engineering tool for design of silo flow and work well with incipient failures; however no single method has been accepted as a standard. As powder cohesion increases, it becomes increasingly difficult to maintain reproducibility due to the powders ability to consolidate differently leading to substantial fluctuations in local (micro) density. An important drawback of these testers is the difficulty in characterizing the bulk flow properties of unconfined powders in motion, which is important under a variety of processing conditions.
In this talk, we develop a correlation between the ?flow index' achieved using the Gravitational Displacement Rheometer (GDR) under unconfined conditions, and the degree of bed expansion (dynamic dilation) for an extremely wide range of cohesive behaviors, ranging all the way from 3 mm glass beads (extremely flowable) to micronized lactose (highly cohesive). In our previous work we concluded that the Flow Index is a function of the powder cohesiveness. As powder cohesion increases, it becomes increasingly difficult for it to flow, which is captured by an increase in the index, which also correlates with an increase in the dynamic dilation. For example, glass beads and Fast Flo Lactose, which exhibit low cohesion, dilated by 3% and 8% of the bed volume. On the other hand, micronized Lactose, the most cohesive material, dilated by as much as 25%. These results are further confirmed by analyzing local density fluctuations and coordination numbers using analysis of the DEM simulations, which decrease with increasing cohesion. The lowering of the dynamic density with cohesion can be attributed to the formation of rigid agglomerates in the powder bed. As cohesion increases, the agglomerates become larger, allowing for a larger pore size distribution. As a result, the flow index is linearly related to the bed expansion. Density, which is a simple property to measure on line, can help in predicting the flow behavior of powders in a dynamic and unconfined environment. The variations in density can help in predicting flow through hoppers and can serve in minimizing or eliminating many flow problems. Furthermore, the variations in density from a hopper discharge on to a tablet die during the filling process can help in minimizing the problems caused during processing of tablets. A simple measure of density of pharmaceutical material can help in preventing numerous processing problems that are associated to powder flow behavior.