Abstract
Liquid–solid circulating fluidised beds (CFB) possess many qualities which makes them useful for industrial operations where particle–liquid contact is vital, e.g., improved heat transfer performance, and consequent uniform temperature, limited back mixing, exceptional solid–liquid contact. Despite this, circulating fluidised beds have seen no application in the micro-technology context. Liquid–solid micro circulating fluidised bed (µCFBs), which basically involves micro-particles fluidisation in fluidised beds within the bed of cross-section or inner diameter at the millimetre scale, could find potential applications in the area of micro-process and microfluidics technology. From an engineering standpoint, it is vital to know the solid circulating velocity, since that dictates the bed capability and operability as processing equipment. Albeit there are several studies on solid circulating velocity measurement in CFBs, this article is introducing the first experimental study on solid circulating velocity measurement in a CFB at micro-scale. The experimental studies were done in a novel micro-CFB which was fabricated by micro milling machining 1 mm2 cross-section channels in Perspex and in a 4 mm2 cross-section micro-CFB made by additive manufacturing technology. Soda-lime glass and polymethyl methacrylate (PMMA) micro-particles were employed as solid materials and tap water as the liquid medium. The digital particle image velocimetry (PIV) method was used as a measurement technique to determine the particle velocity in the micro-CFB system and validated by the valve accumulation technique using a novel magnetic micro-valve. The measured critical transition velocity, Ucr, is comparable to the particle terminal velocity, i.e., the normalised transition velocity is approximately 1 in line with macroscopic systems results and our previous study using simple visual observation. As in macroscopic CFB systems, Ucr decreased with solid inventory (1–9%) and finally becomes stable when the solid inventory is high enough (10–25%) and it increases with a reduction in particle size and density.
Highlights
Liquid–solid circulating fluidised beds have become the preferred liquid–solid contactors in several industrial processes due to their advantages such as improved heat transfer performance, and consequent uniform temperature, reduced back mixing, good liquid–solid contact, and good control of reaction and regeneration of biosolids or catalyst at the same time [1,2]
The pressure drop across the micro fluidised bed was not measured, because it isSolid veryCirculation difficult due toby very fine resolution required, the pressure drop across the bed is only of the Method as Validation
Overall the results obtained from these techniques agreed well with each other, and this agreement provided a strong basis for the deployment of a particle image velocimetry (PIV) software PIVlab method as a novel, simpler, and faster approach instead of an accumulation technique to determine the solid circulation rate in a micro-circulating fluidised bed
Summary
Liquid–solid circulating fluidised beds have become the preferred liquid–solid contactors in several industrial processes due to their advantages such as improved heat transfer performance, and consequent uniform temperature, reduced back mixing, good liquid–solid contact, and good control of reaction and regeneration of biosolids or catalyst at the same time [1,2]. Micro-fluidised beds combine the advantages of fluidised beds and micro-technology systems such as reduced pollution, waste and by-products, less energy and resources consumption, lower operational and capital cost, increased safety, intensive mass and heat transport, increased chemical reaction conversion rates, and exhibiting good mixing and temperature uniformity All these important qualities make micro-fluidised beds more efficient and sustainable fluid-solid processing equipment, but the major application so far is in a fast screening of various solid particles chemical processes like solid reaction kinetics measurements, catalyst, and adsorbent screening [15,16,17,18]. Measurement of solid circulating velocity in a micro-circulating fluidised bed is extremely hard to be implement using current measurement technology such as butterfly valve, electrical capacitance, X-ray, magnetic resonance imaging, as they are expensive, and it is very difficult to scale down these techniques for application in a microfluidics context [10]. On solid circulating velocity have been carefully studied using PIVlab and MATLAB
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