Characterization of granular mixing in vertical axis bladed mixers using X-ray imaging systems

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2021-08
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Nadeem, Humair
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Heindel, Theodore J
Subramaniam, Shankar
Hu, Hui
Michael, James
Passalacqua, Alberto
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Mechanical Engineering
Abstract
Granular materials are used in a wide variety of products and form an integral part of many industries such as cosmetics, food processing and pharmaceuticals. Frequently, granular materials need to be mixed to meet product specifications, for which vertically bladed mixers are a popular choice. Vertically bladed mixers can be employed to handle not just free flowing granular material but also cohesive powders, pastes and slurries. Despite being a common process in various industries, granular mixing in bladed mixers is still not completely understood. In the pharmaceutical industry, where granular mixing is used in the preparation of critical medicinal products, recent recommendations from regulatory bodies have pushed for greater monitoring and control of the mixing process. As a result, there is a pressing need for the development of monitoring tools that can provide real time measurements of mixture homogeneity as well as offer a greater degree of accuracy in terms of determining final mixing outcomes. Noninvasive methods of quantifying mixing are typically superior to conventional sampling. Over the years, several noninvasive techniques have been devised to characterize granular mixing, and in this work, a comprehensive review is provided of these systems. The advantages and limitations of the various methods are discussed and their viability in different applications is reported. It has been noted that the final mixing state is dependent on the particle characteristics of the mixture constituents and the operating parameters of the mixing equipment. In an initial study, single tagged intruder particles were introduced in a 150 mm inner diameter, bladed mixer consisting of native particles, and the trajectories of the tagged particles were noted relative to native particle motion. Data were acquired via visual particle tracking using a camera recording the top free surface of the mixer bed. The effects of density difference between the intruder and native particles, as well as blade speed and initial particle location, were varied and their effect on the intruder particle motion were investigated. It was observed that there was no qualitative difference in the particle location distribution due to changes in blade speed while initial particle location was only important for the first few rotations of the blade, after which the intruder particles “lost all memory” of their initial location. Particle density differences were observed to have a significant impact on intruder particle motion and the mean radial location of the intruder particle could be predicted based on the density ratio between the native and the intruder particles. While visual particle tracking supplied valuable insights into tagged particle behavior, data acquisition was restricted to the free surface only. Recent developments in X-ray imaging technology however, have enabled high-quality data and image acquisition, even in opaque mixer beds. Preliminary investigations suggest that X-ray computed tomography (CT) is a promising candidate to provide detailed information about particle distribution in a mixture, in turn yielding an accurate measure of homogeneity. Whereas X-ray CT can provide highly detailed mixture information; the mixing process needs to be interrupted for data acquisition. For such cases, X-ray radiography has been identified as a promising alternate. X-ray radiography follows the trajectory of a single particle, and if observed for a suitable amount of time, the time averaged location data can be used to infer spatial distribution of one specie in a binary mixture and provide a measure of mixture homogeneity. In this work, data analysis methods to be used with both experimental techniques are discussed. The effects of different particle properties such as particle density, size, and volume fraction, as well as the operating conditions such as blade speeds, on the segregation in binary mixtures are to be investigated using X-ray CT. X-ray radiography is proposed as an in-line technique and investigations are thus proposed to demonstrate that results comparable to CT measurements can be obtained. The need to identify if appropriate mixing indices such that the respective strengths of both techniques can be fully exploited is also discussed.
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