Continuous powder mixing of segregating mixtures under steady and unsteady state regimes: Homogeneity assessment by real-time on-line image analysis

Abstract

Continuous powder processes, such as continuous powder mixing, are more than ever envisioned as a viable alternative to batch equipment, in various industries such as pharmaceuticals, specialty chemicals (zeolites, SiC), bio-renewables or food. In the present work we have implemented an on-line image analysis setup that is able to capture all the images of the particles at the outlet of a continuous pilot-scale mixer. This allows the determination of the homogeneity of mixtures of two different compositions, as well as the analysis of their evolution during steady-state and transitory regimes. The importance of a proper definition of the scale of scrutiny of the mixture is emphasized by providing homogeneity results obtained at four different scales. Evidence of segregation by percolation giving rise to the enrichment of the mixer's bed with fine particles is given and commented. The impact of the stirrer's rotational speed on the quality of the mixtures, as provided by the coefficient of variation CV, is reported. Up to 20 Hz, CV's are extremely high, while above 30 Hz, the influence of the impeller speed is much weaker. Finally, the influence of impeller speed's step perturbations is measured and commented. Due to the size-segregation phenomenon inside the mixer, negative steps are deeply detrimental to the mixing process. On the contrary, positive steps can be absorbed by the equipment without degradation of the quality of the mixtures.

Powder flow and mixing in a continuous mixer operating in either transitory or steady-state regimes: Mesoscopic Markov chain models

“…17 shows the fine particle content in the five cells at steady-state for the two mixtures. As commented in Ammarcha et al [3], the first four cells are richer in fine particles than the last cell, whatever the conditions under which the blender operates, pointing out the presence of a segregation effect. As for the coarse product alone, it can be shown that the steady hold-up weight of the mixture in each cell can be assimilated to that of coarse couscous, which means that (Eq.…”

“…Part of the presented work will deal with mixtures of fine and coarse couscous. The analysis of these will be performed on-line by an image analysis ring that has been fully presented in Ammarcha et al [3]. A belt conveyor is placed at the mixer's outlet so as to form a single layer of particles by adjusting its speed to the inflow rate considered.…”

“…We focused only on the bulk particle flow (no mixture) and derived empirical correlations through the frame of a single cell Markov chain model. Later on [3], we developed a system for on-line image analysis of simple mixtures, able to capture all the particles flowing out of the equipment and calculate mixture homogeneity in real-time. The influences of the scale of scrutiny, as well as that of N, on the quality of the mixtures were demonstrated.…”

Review of noninvasive methods to characterize granular mixing

“…Optical methods typically have high spatial and temporal resolutions that allow these methods to be readily extended to advanced methods such as PIV or particle tracking velocimetry (PTV) [35]. As such, optical methods are found extensively in lab scale systems [36] and easily extendable to pilot scale systems [37]. The greatest limitation of the digital imaging techniques however, is that only wall or surface phenomenon can be investigated.…”

Industrial Mixing of Particulate Solids: Present Practices and Future Evolution

“…Basically, the idea is to develop and industrialise, either on-line techniques or in-situ techniques for measuring powder blend homogeneity during processing, and use this directly for product validation. For this, electrical methods -such as capacitance-can be used (Ehrhardt et al [11]), but also image analysis (Muerza et al [12], Berthiaux et al [13], Ammarcha et al [14]), NIR spectroscopy (Hailey et al [15]; Koller et al [16]; Martinez et al [17]; Vanarase et al [18]), FT-Raman (Vergote et al [19], De Beer et al [20]), Laser Induced Fluorescence (Lai et al [21]), NIR chemical imaging, etc. Of course, because of mixtures variety, there is not a single technique able to treat all the cases, and not all the methods cited have the same running price.…”