Article The Need to Accurately Define and Measure the Properties of Particles Yimin Deng 1 , Raf Dewil 1 , Lise Appels 1 , Huili Zhang 2 , Shuo Li 3 and Jan Baeyens 1,3, *   Citation: Deng, Y.; Dewil, R.; Appels, L.; Zhang, H.; Li, S.; Baeyens, J. The Need to Accurately Define and Measure the Properties of Particles. Standards 2021, 1, 19–38. https://doi.org/10.3390/ standards1010004 Academic Editor: Peter Glaviˇ c Received: 9 July 2021 Accepted: 5 August 2021 Published: 12 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Process and Environmental Technology Lab, Department of Chemical Engineering, KU Leuven, J. De Nayerlaan 5, 2860 Sint-Katelijne-Waver, Belgium; yimin.deng@kuleuven.be (Y.D.); raf.dewil@kuleuven.be (R.D.); lise.appels@kuleuven.be (L.A.) 2 School of Life Science and Technology, Beijing University of Chemical Technology, Chaoyang District, Beijing 100029, China; zhhl@mail.buct.edu.cn 3 Beijing Advanced Innovation Centre of Smart Matter Science and Technology, Beijing University of Chemical Technology, Chaoyang District, Beijing 100029, China; ssurel@mail.buct.edu.cn * Correspondence: Baeyens.j@gmail.com Abstract: When dealing with powders, a fundamental knowledge of their physical parameters is indispensable, with different methods and approaches proposed in literature. Results obtained differ widely and it is important to define standards to be applied, both toward the methods of investigation and the interpretation of experimental results. The present research intends to propose such standards, while defining general rules to be respected. Firstly, the problem of defining the particle size is inspected. It was found that describing the size of a particle is not as straightforward as one might suspect. Factors of non-sphericity and size distributions make it impossible to put ‘size’ in just one number. Whereas sieving can be used for coarser particles of a size in excess of about 50 μm, instrumental techniques span a wide size range. For fine particles, the occurrence of cohesive forces needs to be overcome and solvents, dispersants and sample mixing need to be applied. Secondly, the shape of the particles is examined. By defining sphericity, irregularly shaped particles are described. Finally, the density of particles, of particle assemblies and their voidage (volume fraction of voids) and the different ways to investigate them are explored. Keywords: particles; size; size distribution; density; bed voidage 1. Introduction The design of fluid–solid processes often relies on using empirical correlations that include characteristic powder properties such as the particle size and its size spread, the particle shape and density and the bed voidage for particle assemblies [1–7]. Unfortunately, most correlations do only provide predictions within a range of ±25%. The reasons for this inaccuracy are not due to the lack of engineering skills, but in the complexity of unambiguously defining and measuring even these fundamental particle parameters. As a first guideline, ISO standards can be consulted. A first set of documents indeed provides guidance on instrument qualification and particle size and its size distribution measurements, whereas a second set deals with the representation of the results of the particle size analysis. These standards are periodically reviewed and confirmed. Within the first set of standards, we refer to the measurement techniques by gravitational methods [8], by laser diffraction method [9] and by sieving analysis [10], among others. The second set includes several parts of the ISO 9276 standard that specifically deal with the representation of results of particle size analysis in Part 2 [11] and with the descriptive and quantitative representation of particle shape and morphology in Part 6 [12]. Whereas in Part 2 particle shape factors are not taken into account, Part 6 recognizes the ineffectiveness of averaging the shape over all particles and restricts the methods to those that can be correlated with physical properties in industrial applications. Although particle shape and morphology are Standards 2021, 1, 19–38. https://doi.org/10.3390/standards1010004 https://www.mdpi.com/journal/standards