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Compressibility of Powders and Granular Media with the Powder Cell: Bulk Density and Flowability Indices Compared with Cohesion Strength

Understanding of the properties and behavior of powders and granular media is essential for different industrial applications such as pneumatic transport, tableting processes, or design of hoppers and silos. This application report describes a characterization method for measuring compressibility with the Anton Paar Powder Cell and compares the method and results to the cohesion strength measurement. Compressibility can be used to analyze the aerated bulk density, but also the consolidated bulk density, while the Carr index and Hausner ratio are often used as indicators for flowability of powders.

Introduction

A wide range of characterization methods for powders and granular media is available with the Anton Paar Powder Cell. This application report discusses a method for measuring compressibility. Furthermore, the method and its results are compared with the cohesion strength method.

Compressibility

The first method concerns measurement of compressibility. Compressibility is a measure of the relative volume change a sample undergoes when pressure is applied or changes. It describes the relationship of bulk density to applied pressure. There are many factors, e.g. size, form, elasticity, water content and temperature, which influence compressibility. Despite being a simple test of compressibility, it can be useful for identification of powder flow behavior. Knowledge about bulk density of materials is essential for avoiding ratholing and arching effects in silos and hoppers. Furthermore, the Carr compressibility index and Hausner ratio can also be determined. The Carr index is a characteristic value of a powder’s compressibility, which is often used as an indicator for its flowability.

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