G.M.H. Meesters

Population balances for particulate processes—a volume approach

Abstract Population balance models have been used in chemical engineering since the 1960s and have evolved to become the most important tools for design and control of particulate processes. In this paper we show that the intrinsic particle parameter that determines changes in the process and should thus be included in the population balance is the particle volume. The basic population that is modeled should be the mass distribution, or the volume distribution if the density is constant. The population balance thus describes the change of the volume distribution of volume with time. Furthermore, we suggest that the “birth” and “death” terms that are often used to describe discrete events in particulate processes can almost always be replaced by a rate of change term. To design and control existing and future processes, a multi-dimensional population balance model is required. We propose a volume-based model in which the particle properties that are modeled are the volumes of solid, liquid, and air, respectively. In the most general case the model will consist of a properties vector and a distribution tensor. Depending on the complexity of the process, one or more of the properties may be omitted from the model. This is shown in three examples of increasing complexity: comminution, sintering, and granulation.

Growth and compaction behaviour of copper concentrate granules in a rotating drum

In order to understand the growth and compaction behaviour of chalcopyrite (copper concentrate), batch granulation tests were carried out using a rotating drum. The granule growth exhibited induction-type behaviour, as defined by Iveson and Litster [AIChE J. 44 (1998) 15 10]. There were two consecutive stages during granulation: the induction stage, during which the granules are gradually being compacted and little or no growth occurs, and the rapid growth stage, which starts when the granules have become surface wet and are rapidly growing. In agreement with earlier findings. an increased amount of binder liquid shortened the induction time. The compaction behaviour was also investigated. A displaced volume method was adopted to determine the porosity of the granules. It was shown that this technique had a limitation as it was unable to detect the reduction of the volumes of the granule pores after the granules had become surface wet. Due to this, some of the measurements were not suited for fitting a three-parameter empirical model. Attempts were made to determine whether the rapid growth stage started with the pore saturation exceeding a certain critical value, but due to the scatter in the porosity measurements and the fact that some of the measurements could not be used, it was not possible to determine a critical pore saturation, However, the porosity measurements clearly demonstrated that the porosity of the granules decreased during the induction stage of an experiment and that when rapid growth occurred, the granules had a pore saturation was around 0.85. This value was slightly lower than unity, which is most likely due to trapped air bubbles

Determining granule strength as a function of moisture content

Abstract A repeated impact test was used to determine the breakage behavior of granules containing an organic material which had a narrow size distribution of 600–700 μm and a moisture content (mass fraction) ranging from 0.02 to 0.32. In this test, a sample of approximately 200 granules encounters 100 particle–wall impacts per second with an adjustable impact velocity. The strength of the granules was thus determined by observing the breakage caused. Over the range of moisture contents tested, the granules have a maximum strength at a moisture content of between 0.20 and 0.25. From the amount of material sticking to the walls of the particle container, qualitative insight into the attrition strength of the granules is obtained. Using image analysis, the change of the particle size distribution and of the shape of the granules was determined. From these changes, appropriate breakage mechanisms are proposed: the breakage of granules with a high moisture content is mainly by attrition, while at low moisture contents, fracture of the granules into smaller pieces is the dominating mechanism.

Increased Antimicrobial Activity of Cheese Coatings Through Particle Size Reduction

This chapter discusses the results of reducing the particle size distribution of an anti-microbial solid to facilitate a better use of the material. The product, an antifungal agent is size reduced using a wet-stirred media mill. The obtained product has the size of around 180 nm. The way of grinding is described and the used sizing techniques discussed. In the last part the product is characterized and tested in its application. Here it is shown that the reduced size gives a better performance against fungal anti-microbial infections than a product containing larger particles. This is explained by discussing the coverage, as well as the diffusion of the product through a layer of material, in this case a cheese surface.

A Study of the Combined Compaction and Drying of a Wet Crystal Mass

The combined compaction and drying of a wet crystal mass was studied using an industrial food processor and a custom built open turbo dryer setup. From extrapolation of an empirical relation for the porosity of a packed particle bed a qualitative idea of the influence of the constituent crystals an the minimum attainable porosity was obtained. It increases with increasing constituent crystal size and decreasing width of the crystal size distribution. From the experiments it was found that the compaction and size reduction of the wet crystal mass is mainly influenced by the characteristics of the premix. The compactibility and strength of the premix may be influenced by changing the saturation and the constituent crystal size and size distribution.