Characteristics and Applications of Micro Fluidized Beds (MFBs)

Fluidization of fungal pellets in a 3D-printed micro-fluidized bed

Characteristics and applications of micro fluidized beds (MFBs)

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.

https://www.mdpi.com/2227-9717/8/9/1159/pdf

Solid Circulating Velocity Measurement in a Liquid–Solid Micro-Circulating Fluidised Bed

Micro-fluidized bed has aroused much attention due to its low-cost, intensified-process and fast-screening properties. In this paper, a micro-fluidized bed (15 × 15 mm in cross-section) was designed and fabricated with the use of the stereolithography printing technique, for the investigation of bubbles’ hydrodynamics and comparison of the solids (3D-printed particles VS fungal pellets) fluidization characteristics. In a liquid–gas system, bubble flow regime started from mono-dispersed homogeneous regime, followed by poly-dispersed homogeneous regime, transition bubble regime and heterogeneous bubble regime with increasing gas flowrates from 3.7 mL/min to 32.7 mL/min. The impacts from operating parameters such as gas flowrate, superficial liquid velocity and gas sparger size on bubble size, velocity and volume fraction have been summarized. In liquid–solid fluidization, different solid fluidization regimes for both particles bed and pellets bed were identified. From the bed expansion results, much higher Umf of 7.8 mm/s from pellets fluidization was observed compared that of 2.3 mm/s in particles fluidization, because the hyphal structures of fungal pellets increased surface friction but also tended to agglomerate. The similar R–Z exponent n (5.7 and 5.5 for pellets and particles, respectively) between pellets and particles was explained by the same solid diameter, but much higher Ut of 436 µm/s in particles bed than that of 196 µm/s in pellets bed is a consequence of the higher density of solid particles. This paper gives insights on the development of MFB and its potential in solid processing.

Design and investigation of a 3D-printed micro-fluidized bed

Solid-liquid micro-fluidised beds (FBs), i.e. fluidisation of micro-particles in sub-centimetre beds, hold promise of applications in the microfluidics and micro-process technology context. This is mainly due to fluidised particles providing enhancement of mixing, mass and heat transfer under the low Reynolds number flows that dominate in micro-devices. Albeit there are quite a few studies of solid-liquid micro-fluidised beds, we are presenting the first study of a micro-circulating fluidised bed. The present experimental research was performed in a micro-circulating fluidised bed which was made by micro-machining channels of 1mm cross section in Perspex. PMMA and soda lime glass micro-particles were used as the fluidised particles and tap water as the fluidising liquid to study flow regime transition for this micro-circulating fluidised bed. The results are in line with macroscopic observation that the critical transition velocity from fluidization to circulating regime is very dependent on solid inventory but once the inventory is high enough it is approximately equal to the particle terminal velocity. However, the transitional velocity is a weakly dependent on wall effect and surface forces confirming the importance of these two properties in a micro-fluidised bed systems. Similarly the transitional velocity to transporting regime is a strong function of surface forces. Finally, combining these results with our previous result on conventional fluidization indicated that map of solid-liquid fluidisation in a micro-circulating fluidised bed system is constructed showing conventional fluidisation, circulating fluidisation and a transport regime.

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Papers

Study of Transitional Velocities of Solid-Liquid Micro-Circulating Fluidised Beds by Visual Observation