Process intensification in micro-fluidized bed systems: A review
Graphical abstract
Introduction
Process intensification (PI), is originally based on the concept of physical miniaturization of process equipment or reduction of the number of operating units but retaining process efficiency and performance [1]. Further research and industrial development in PI has also resulted in the novel apparatus and technologies leading to higher production capacity, less energy consumption and waste formation per unit vessel volume [2]. Therefore, the definition of PI can be generally classified into: (a) process-intensifying equipment; (b) process-intensifying methodologies [3]. PI equipment comprises of different functional reactors (e.g. oscillatory baffled reactors, spinning disc reactor, rotating packed-bed reactor, membrane reactor, etc.) and processing equipment such as static mixers, compact heat exchanger, centrifugal adsorber and others. PI methods may also involve multifunctional reactors or make use of external energy such as magnetism and ultrasonic force to achieve intensified processing.
Besides the functional reactors mentioned above, fluidized bed as another type of reactor has also been widely used in chemical and process industrial due to its excellent multi-phases contact, minimum diffusional resistance, good heat and mass transfer [4], [5], [6], [7]. With regards to PI, the micro- fluidized bed (MFB) is getting more attention as a good tool for fast screening of solid process and bioprocesses with low experimental costs. The miniaturization of fluidized bed also promotes the solid-fluid contact intensity and consequently increasing both fluid and solid mixing, which is of high importance for potential industrial production. This importance is exemplified in bioengineering, as sufficient mixing in fluidized bed bioreactors ensure good nutrient medium distribution and dissolved oxygen (DO) supply for cell growth, while other reactors such as stirred tank bioreactor requires external energy for agitated impellers to improve mixing. Apart from its implementation in traditional chemical and biological engineering, the MFB finds application in environmental sustainability, such as CO2 capture [8, 9] and wastewater treatment [10], [11], [12].
Since its re-introduction by Potic et al. [13] in mid 2000s, MFB was either defined by the principle of small hydraulic diameter, or as general small-scale fluidized beds in conventional engineering context showing apparent changes in hydrodynamics properties. Definitions of MFB based on hydraulic diameter of the bed vary from sub-500 µm [14] to several centimetres [13,15], although the widely asserted boundary between macro- and micro-fluidization is 1 mm [14,16]. From the degree of gas back-mixing, Xu et al. [17] defined the MFB with the inner diameter of 21 mm and the static bed height in the range of 20−50 mm. Besides, Xu et al. also argued to combine the bed diameter and particle size (namely, bed-to-particle ratio) when determining a micro-fluidization. For instance, their group suggested a homogenous micro-fluidization with the combination of MFB (20 mm ID) and silica sand particles (242.1 µm diameter) to keep the bed-to-particle ratio (dB/dP) within 100 [18]. Therefore, some cases where the fluidized bed had 50 mm inner diameter (ID) and particle size of 2–3 mm [19,20] should be also considered as micro-fluidized beds even if not specifically referred as such. Another factor to distinguish micro-fluidization from macro-fluidization is the wall effect and surface forces relative to volumetric forces such as gravity, which is not only related to bed size but also connected with bed material, particle size and shape, liquid density/viscosity, etc. [21,22]. Besides, the low initial height in the centimetre-scale bed can also contribute to the characteristics of MFB. As a result, the asserted boundary of 1 mm in hydraulic bed diameter is too narrow to conclude the micro-fluidization. Instead, this review paper implemented a fuzzy boundary of 50 mm ID with dB/dP lower than < 100 to determine the micro-fluidized system.
Based on the fuzzy boundary, Fig. 1 summarizes the bibliographic network of MFB studies from the Scopus database. According to the bibliographic network, it is obvious that MFB techniques have been intensively studied in terms of the fundamental characteristics (illustrated by green and orange clusters), as well as applications including chemical conversions (indicated from yellow and blue clusters) and bioprocessing (presented by red clusters). Based on the bibliographic network, this paper aims to review the fundamental characteristics of MFB, as well as the recent progress of MFB applications from the perspective of process intensification. Finally, the current bottlenecks and potential future work regarding the micro-fluidized bed systems are also addressed.
Section snippets
Characteristics of MFB
In this section, the fundamental research on the minimum fluidized velocity, mixing performance and mass transfer of MFB are discussed with regard to the hydrodynamics performance from micro-fluidized bed to macro-fluidized bed, but also principally addresses the significance to process intensification.
Applications
The adoption of PI-based techniques has led to rapid advancement with the application of MFB reactors in conventional chemical processing, as well as in areas such as environmental issues and biological industrial. In this section, the miniaturized fluidized bed techniques are discussed from a PI perspective.
Conclusions and prospect
In this review, the process intensification using micro-fluidized bed techniques have been discussed from the hydrodynamic properties of this miniaturized reactor. The discussions of MFB applications have covered solid screening, chemical conversion, CO2 capture, wastewater treatment and microbial processing. The miniaturized size enables the fast screening, controllable safety and low capital cost. Besides, the advantages of good mixing, excellent mass and heat transfer broaden the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.
Acknowledgement
Financial supports from Newcastle University (Gant No. LOC/150025720/400382711) and SIFBI of A*STAR (under IAFPP3-H20H6a0028), Singapore to Yi Zhang is fully acknowledged.
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