(212b) Relating Particle Scale Properties to Bulk Behavior and Then to Process Behavior (A guide for product design: Bulletin 321 – a primer)

In the world of powders there exists a complex relationship between the particle scale properties and the behavior that one observes on a plant scale as powders are processed, packaged, or even used by the consumer. Over the years the powder flow community has developed a means of measuring bulk properties of powders and, through scientific and theory-based procedures and analysis, use the bulk flow properties to describe behavior during processing, handling and packaging. This has led to useful design criteria which can be used to design a generally successful powder handling process. If bulk flow properties are known, the criteria provides a good set of basic rules to guide engineers in configuring and specifying equipment to handle powders. However, it is not completely accurate and still has some limitations. When it comes to process design, as long as the process is built of simple and basic structures, we understand the rules that govern good design. However, the same cannot be said when trying to establish how a powder will work in a given handling system when only basic information about the size, shape and particle surface morphology, and other particle-based measurements are known. To achieve a truly robust/successful process the design, the formulator and the process design engineer must work together. There are two barriers to developing an analysis or design basis to relate particle scale properties to process design. First, the problem is a two scale process. The formulator must understand how a change, on the particle scale, will change a set of key bulk scale flow properties. Second, the designer must understand, select and use a set of bulk scale properties to design the process of interest.

The second step in this process is reasonably well recognized. Our understanding of this part of the design process began some pioneering work in the 1960’s and continues today. However, the first step in this process (particle to bulk scale) still requires some work to understand how basic particle scale properties will influence bulk properties. Perhaps one of the most important bulk properties which must be understood is the unconfined yield strength of the material. Understanding the influence of this property can allow for design of containers, bins, hoppers, feeders, blenders, fluid beds, milling operations, roll presses, and agglomeration units which operate without flow problems. Granted, for each unit operation in the aforementioned list there are other key properties that must be understood so as to create a reliable design, but unconfined yield strength (or cohesion) is common to all of these unit operations. Thus, if the formulator and design engineer understand and use the relationship between particle scale properties and cohesive flow properties, then this could be a major step forward in relating particle scale properties to process behavior. Although important, cohesive flow issues are not the only issues that plague processes that handle powders. Sometimes particle scale properties also influence the ability of a mixture of particles to remain well mixed during processing. The following paper discusses how particle scale properties of a bulk powder material influence both the cohesive flow properties and the ability of a mixture of powder to remain well mixed.