Tim Freeman's Powder Flow Feature: What Makes a Powder Flow?

By Tim Freeman, Managing Director, Freeman Technology

This editorial considers some of the many variables that influence powder behavior. The main focus will be the impact of different parameters on how powders flow, given that this is such a critical performance attribute. Poor flowability lies at the heart of many powder processing problems, including sub-optimal throughput, unplanned shutdowns, erratic operation and inconsistent product. So, in many instances, achieving the desired flowability is an extremely productive way to enhance process efficiency.

The complexity of powders

Powders are more than just the particles alone, they are bulk assemblies containing particles. It is often the case that the terms ‘particle’ and ‘powder’ are used interchangeably, but this can be misleading. Powders also consist of gases, normally in the form of air, and liquid, usually water, on the surface of the particle or within its structure.

It is the properties of each phase of a powder, and the interactions between them, that define bulk powder behavior. This means that behavior is influenced by several variables and an array of potential interactions, as well as process or ‘external’ influences, which is why they are complex and why powder performance cannot be accurately predicted from measurements of physical properties alone.

Powders perform in different ways, depending on how they are formulated and manufactured, as well as the environment to which they are subjected. While this can make them difficult to predict it is ultimately what underpins their industrial value.

Knowledge-driven approach

A knowledge-driven approach to powder processing would suggest a strategy of manipulating variables that are known to influence powder behaviour. This relies on understanding two things. Firstly, which powder properties are important in defining and optimising process performance, and secondly, which variables can be manipulated and how.

Better understanding

For many years, there has been some understanding of which properties influence powder behaviour. A classic example is the widely-recognised link between particle size and powder flowability, with finer powders generally flowing less freely. However, the degree of aeration/consolidation, and amount of moisture, can also have a marked impact but this understanding is often still qualitative rather than quantitative. There are relatively few published correlations that relate either primary particle characteristics such as size, shape, surface roughness and charge, or system variables such as air or moisture content, to powder flow properties. This is true even for commonly used powders, such as lactose.

This lack of information can largely be attributed to historical difficulties in securing reproducible, reliable powder property data. Without such data, detailed investigation of the impact of different variables is, of course, impossible, so qualitative relationships have had to suffice. However, this problem has been solved by the advent of modern, more sophisticated testing instruments, with the development of dynamic powder testing a particularly important milestone.

Powder flowability

Dynamic powder testing involves precisely measuring the axial and rotational forces acting on a specially shaped blade as it rotates along a helical path through a powder sample, to generate values of flow energy which directly quantify powder flowability. Well-defined methodologies and a high degree of automation make dynamic testing, carried out using instruments such as the FT4 Powder Rheometer®, highly reproducible. Furthermore, tests can be carried out on consolidated, conditioned, aerated or even fluidised powders.

Modern instrumentation has also brought greater precision to shear and bulk property measurement. In other editorials I’ll be looking at studies of some of the most influential variables: particle size; particle shape; aeration; moisture content (humidity) and surface charge, where I will illustrate that effective powder testing delivers a substantial opportunity to develop a thorough understanding of how to control powder behaviour, and consequently how to access much higher levels of manufacturing performance.

Author Biography

Tim Freeman, Managing Director, Freeman Technology

Tim Freeman is Managing Director of powder characterisation company Freeman Technology for whom he has worked since the late 1990s. He was instrumental in the design and continuing development of the FT4 Powder Rheometer® and the Uniaxial Powder Tester. Through his work with various professional bodies, and involvement in industry initiatives, Tim is an established contributor to wider developments in powder processing.

Tim has a degree in Mechatronics from the University of Sussex in the UK. He is a mentor on a number of project groups for the Engineering Research Center for Structured Organic Particulate Systems in the US and a frequent contributor to industry conferences in the area of powder characterisation and processing. A past Chair of the American Association of Pharmaceutical Scientists (AAPS) Process Analytical Technology Focus Group Tim is a member of the Editorial Advisory Board of Pharmaceutical Technology and features on the Industry Expert Panel in European Pharmaceutical Review magazine.  Tim is also a committee member of the Particle Technology Special Interest Group at the Institute of Chemical Engineers, Vice-Chair of the D18.24 sub-committee on the Characterisation and Handling of Powders and Bulk Solids at ASTM and a member of the United States Pharmacopeial (USP) General Chapters Physical Analysis Expert Committee (GC-PA EC).

www.freemantech.com.cn

info@freemantech.com.cn

Introduction

Following Tim Freeman's articles on Powder Flow,

understanding powder behaviour to optimise process performance, increase productivity and improve quality.

More Articles by Tim Freeman:

Tim Freeman's Powder Flow Feature: Understanding Powders