The uses of magnetic fields in the processing of solids

This paper shows how properly applied magnetic fields can advantageously be used to control the behavior of streams of solids which contain some magnetizable material. In particular, we describe the development of a new magnetic flow meter for solids, a quick acting on-off valve with no mechanical action, a sliding solids reactor which may be used for particle filtration and related operations, a positively controlled multistage moving bed contactor, fluidized beds with no gross bubbling or gas bypassing, and especially, a reliably operating distributor-downcomer for fluidized beds.The prospects are considered that these devices may open the way to new types of gas/solids contactors including non-plugging trouble-free multistage fluidized beds.     

Internally circulating fluidized bed for continuous adsorption and desorption

An internally circulating fluidized bed has been developed for continuous adsorption and desorption. This system can be used more generally for many other reaction/regeneration operations. It helps to separate and to recover gaseous pollutants and reusable compounds (e.g. CO₂, SO₂, organic solvent vapors, along with other gases). Two fluidized beds are arranged next to each other. The partition wall in the upper and lower part of the fluidized beds has horizontal openings to let solid matter pass through. As the two beds are fluidized at different rates, the bed material starts to circulate between the two beds. The bed material doubles as an adsorbent. In the adsorption zone, polluted gas is used for fluidization; in the desorption zone, heated air or steam is used. The main differences from conventional fixed-bed adsorbers are that plants can be built in a more compact manner, higher flow rates can be achieved, and separate optimization of the two zones (adsorption zone, desorption zone) is possible.     

Deterministic chaos: a new tool in fluidized bed design and operation

Deterministic chaos theory offers new and useful quantitative tools to characterize the non-linear dynamic behaviour of fluidized beds. The dimension and entropy of the fluidized bed's strange attractor can be used for various purposes, such as the classification of fluidization regimes or fluidized bed scale-up. This is illustrated by experimental and model simulation examples of deterministic chaotic behaviour in ambient gas-solids fluidized beds of Geldart B particles. It is shown that the Kolmogorov entropy is dependent on, amongst other parameters, the gas velocity and the bed aspect ratio. In dimensionless scaling of fluidized bed reactors this type of relationship can probably be of use in establishing full dynamic similarity.     

Investigation and scale-up of hot-melt coating of pharmaceuticals in fluidized beds

This study was performed to investigate and scale-up the hot-melt coating process in fluidized beds. A series of well-designed experiments was carried out in a pilot scale unit with 20 kg product capacity to investigate the effects of process variables on the efficiency of the coating of Cefuroxime Axetil with stearic acid. Results showed that the efficiency is at the highest when the fluidization air flow rate is adjusted by considering the changes in the amount of materials present in the unit as well as the changes in the terminal velocities of particles during the process.With the objective to scale-up the hot-melt coating process from pilot to production scale, a dynamic thermodynamic model based on conservation equations of mass and energy was developed. Predictive accuracy of the model was assessed by applying it to the pilot scale unit and comparing its predictions with the online measurements taken on the same unit. Results showed that the predictions of the model agree well with the measurements. Utilizing this model and taking several experiments performed in the pilot scale unit as a basis, scaling up of the hot-melt coating process was carried out. Comparisons of the model predictions with the measurements taken on the production scale unit (200 kg product capacity) revealed that the model is able to reproduce the product attributes and the outlet air temperatures across scales. Therefore, it proves to be a promising tool that can be used in the scale-up of the hot-melt coating processes in fluidized beds.     

Heat transfer within dynamically structured bubbling fluidized beds subject to vibration: A two-fluid modeling study

Bubbling fluidized beds are often used to achieve a uniform particle temperature distribution in industrial processes involving gas and particles. However, the chaotic bubble dynamics pose significant challenges in scale-up. Recent work (Guo et al., 2021, PNAS 118, e2108647118) has shown that using vibration can structure the bubbling pattern to a highly predictable manner with the characteristic bubble properties independent of system width, opening opportunities to address key issues associated with conventional bubbling fluidized beds. Herein, using two-fluid modeling simulations, we studied heat transfer characteristics within the dynamically structured bubbling fluidized bed and compared to unstructured bubbling fluidized beds and packed beds. Simulations show that the structured bubbling fluidized bed can achieve the most uniform particle temperature distribution because it can achieve the best particle mixing while maintaining a global heat transfer coefficient similar to that of a freely bubbling fluidized bed.     

Solid circulation and gas bypassing characteristics in a square internally circulating fluidized bed with draft tube

The effects of gas velocities to draft tube (26.64–52.54 cm/s) and to annulus section (8.14–11.84 cm/s) on solid circulation rate and gas bypassing fractions were determined in a square internally circulating fluidized bed reactor with an orifice-type square draft tube. The solid circulation rate and gas bypassing fraction from the annulus section to the draft tube increase but gas bypassing fraction from the draft tube to the annulus section decreases with increasing gas velocity to the draft tube. With increasing gas velocity to the annulus section, the solid circulation rate and gas bypassing fraction from the draft tube to the annulus section increase but, gas bypassing fraction from the annulus section to the draft tube decreases. The solids circulation rate was correlated with the pressure drop across the orifice and the opening area ratio based on the orifice theory. The gas bypassing fraction was correlated with gas velocities to the fluidized and the moving beds. Based on the gas bypassing fraction data, the gas flow rates across the orifice were correlated with gas velocities to the fluidized and the moving beds, opening area ratio, particle size and solids height in the bed.     

连续操作密相流化床颗粒停留时间分布特性模拟放大研究

采用基于GPU（graphics processing unit）大规模并行的粗粒化CFD-DEM（computational fluid dynamics-discrete element method）方法,耦合多分散、非球形颗粒曳力模型,对连续操作的三维流化床进行了长时间颗粒停留时间模拟。通过对不同尺寸（长度）流化床的模拟发现不同粒径颗粒平均停留时间（mean residence time,MRT）与流化床长度呈线性关系,该关系可以用来预测更大尺寸流化床内的颗粒停留时间。随着流化床长度的增加,不同粒径颗粒MRT的差异变大,说明流化床长度的增加对不同尺寸颗粒的停留时间具有一定的调控能力。     

Mathematische Modellierung von Wirbelschichtreaktoren

Mathematical modelling of fluidized bed reactors . Among the many fluidized bed models to be found in the literature, the two-phase model originally proposed by May has proved most suitable for accommodation of recent advances in flow mechanics: this model resolves the gas/solids fluidized bed into a bubble phase and a suspension phase surrounding the bubbles. Its limitation to slow reactions is a disadvantage. On the basis of the analogy between fluidized beds and gas/liquid systems, a general two-phase model that is valid for fast reactions has therefore been developed and its validity is confirmed by comparison with the experimental results obtained by other authors. The model describes mass transfer across the phase interface with the aid of the film theory known from gas/liquid reactor technology, and the reaction occurring in the suspension phase as a pseudo-homogeneous reaction. Since the dependence of the performance of fluidized bed reactors upon geometry is accounted for, the model can also be used for scale-up calculations. Its use is illustrated with the aid of design diagrams.     

Challenges in the scale-up of particulate processes—an industrial perspective

Studies by the Rand Corporation in the 1980s identified substantial differences in the scale-up and start-up performance of plants processing particles versus those processing liquids or gases. These differences were inevitably unfavorable. Particulate process plants take longer to start up and are less likely to achieve desired production rates. Facility operators often underestimate the challenges involved. These problems generally relate to an inadequate understanding of the behavior of particle systems. Many of these behaviors are sensitive to process scale or process history in ways that would not be expected by engineers familiar only with liquid or gas systems. Empiricism must often substitute for first principles. Modeling provides some answers, but often not enough to eliminate the need to operate pilot plants. This paper reviews some of the unit operations involved in particle processing and highlights scale-up issues involved. The use of information from suppliers and other third parties is discussed, as well as scale-up strategies in competitive or regulated industries.     

Bubble control in gas-fluidized beds with applied electric fields

In some gas-fluidized bed applications the presence of bubbles and channeling is undesirable due to gas bypassing. The partial or total dissipation of such bubbles will contribute to improved fluidized bed performance in applications, such as combustion and catalytic reactors, where intimate gas—particle contact is important.Preliminary studies in this laboratory over the past two years demonstrate that, in laboratory size gas-fluidized beds, bubbles can be controlled in size and intensity by the application of suitably applied electric fields. The electric fields are shown to be effective at a gas—solid particulate interface and therefore “seek out” any existing bubbles within the bed. The required electric fields are of the order kV/cm in strength and are found to be influential on a variety of particulate materials including both good conductors (e.g. copper) and semi-insulators (e.g. coal, glass, limestone). Both a.c. and d.c. electric fields, applied with and without electrode—particle contact, are discussed on two bed geometries: rectangular and cylindrical. Data are presented for the case of a.c. electric fields with the electrodes placed externally to the bed. In a separate experiment, data collected on various kinds of particles by “gravity lifting tests” suggest that larger particles (and beds) than those fluidized here (29 – 390 μm) can also be so controlled.     

The electroreduction of oxygen to hydrogen peroxide on fluidized cathodes

The cathodic reduction of oxygen to hydrogen peroxide on fluidized beds of graphite has been studied. The cathodes were fluidized by an oxygen saturated solution of 0.1M NaOH, or by the simultaneous introduction of oxygen gas and hydroxide solution. With increasing current density, the current efficiency always decreased while the product peroxide concentration went through a maximum. In the two-phase system the maximum peroxide concentration increased with bed height. Both current efficiency and the rate of peroxide production generally decreased with catholyte flowrate. For the three-phase fluidized cathode the rate of peroxide production and the current efficiency increased with both catholyte and oxygen flowrate. Possible rate controlling steps are discussed. Current densities for both two phase and three phase fluidized beds were too low to be of commercial use.     

The electroreduction of oxygen to hydrogen peroxide on fluidized cathodes

The cathodic reduction of oxygen to hydrogen peroxide on fluidized beds of graphite has been studied. The cathodes were fluidized by an oxygen saturated solution of 0.1M NaOH, or by the simultaneous introduction of oxygen gas and hydroxide solution. With increasing current density, the current efficiency always decreased while the product peroxide concentration went through a maximum. In the two-phase system the maximum peroxide concentration increased with bed height. Both current efficiency and the rate of peroxide production generally decreased with catholyte flowrate. For the three-phase fluidized cathode the rate of peroxide production and the current efficiency increased with both catholyte and oxygen flowrate. Possible rate controlling steps are discussed. Current densities for both two phase and three phase fluidized beds were too low to be of commercial use.     

Convective solids transport in large diameter gas fluidized beds

It is demonstrated that the convective solids transport occurring in large diameter gas fluidized beds can be predicted quantitatively on the basis of measured properties of the bubble phase. Based on the fundamental findings of Rowe and co-workers [5], who have shown the solids mixing in gas fluidized beds for particle diameters greater than 100 μm to be caused solely by the action of rising bubbles, an equation has been derive from extensive measurements of the bubble development in a 1 m diam. fluidized bed of quartz sand which relates the convective solids mass flow due to solids transport in the bubble wakes to easily determinable parameters. The predictions of this relationship are found to bein good agreement with direct measurements of the convective solids transport carried out by Schmalfeld [21] on a pilot scale in a semicylindrical bed of 0.8 m diam.     

Practical scaling considerations for dense gas fluidized beds interacting with the air-supply system

The present study addresses the problem of the scale-up/down of dense gas fluidized beds that may interact with their air-supply system (restricted here to the plenum and the distributor). An abundant literature about scaling issues yielded various sets of dimensionless numbers to be matched in order to ensure the invariance of the bed dynamics at different scales. All these sets embed the first indispensable requirement that is the geometrical similarity between scaled beds. Despite its self-evident character, this letter highlights important and new considerations about the applicability of this requirement for the design of the distributor and the plenum. For the distributor, it is shown that the geometrical similarity introduced by some authors is not always relevant and should only be used under particular conditions. For the scaling of the plenum, to our knowledge, nothing is mentioned in the literature. The present work shows that the ‘by-default’ geometrical scaling of the plenum is completely irrelevant. A practical rule to determine the plenum size at different scales is given so that similarity of the dynamic interaction between the bed and its air-supply system is ensured.     

RAINING OF PARTICLES FROM AN EMULSION-GAS INTERFACE IN A FLUIDIZED BED

This paper deals with the raining of particles from an interface between a dense fluidized phase and a gas phase with the fluidized phase uppermost. Such interfaces occur at the upper surfaces of gas bubbles and slugs in fluidized beds. Particle rain in these cases would enhance contact between gas and particles within the bubbles and slugs.

The rise velocities of single square-nosed slugs injected in incipiently fluidized beds of different diameters were measured. Relatively small columns of internal diameters of 0.0125, 0.019 and 0.0254 m were employed in the experiments; In such beds, square-nosed slugs are formed which span the entire cross-section of the beds and rise entirely due to raining of particles from their top surfaces. Since the upper surface of such slugs is flat, their motion can be analyzed using the one-dimensional hydrodynamic theory. Glass ballotini and sand of different sizes were used as bed particles. Comparison of theory and experiment has enabled the determination of the dimensionless gradient diffusivity characterizing the motion of particles induced by a gradient in the void fraction. The results confirm the scaling proposed by Batchelor (1988). The use of the calculated gradient diffusivity in the criterion for stability of a gas fluidized bed predicts the systems under consideration to be always unstable.     

Development of a new system for circulating fluidized particles within a single vessel

A new system for circulating fluidized particles within a single vessel has been developed which has the same advantages as the dual-bed systems widely used for the FCC process and the Fluid Coker process. The interior of the vessel was divided into four sections by intersecting two flat vertical plates at right angles. Two sections were used for the upflowing bubbling fluidized beds and the other two sections were used for the downflowing bubble-free fluidized beds. Fluidized particles were circulated between the two upflowing beds, which were to be used as reactors, through the two downflowing beds, which were to be used as downcomers. Solid particles were fluidized by several streams of gas injected at several stages of the bed. The effects of these gas injection rates on the circulation rate of solids were investigated. The circulation rate of solids was measured using both a measuring box, which collected the overflowing particles, and the downward motion of a gauze net induced by the descending particles. The static pressure distribution within the vessel and the residence time distribution for coarse foreign solids in the system were measured. The present research demonstrated that the proposed system potentially had the same advantages as the conventional dual-bed system, and that the system can be applied to the simple gasification of biomass and solid wastes.     

Structured bubbling in layered gas-fluidized beds subject to vibration: A CFD-DEM study

Bubble dynamics in gas fluidized beds are mathematically chaotic and difficult to predict. Various ways have been proposed in the past to alter the overall bubble dynamics to improve particular processes. In particular, it has been shown that pulsed gas flow and vibration can be used to transform the chaotic motion of gas bubbles into a dynamically structured pattern. The structured bubbling pattern does not change with the system width, opening opportunities to address key issues in scaling up gas-solids bubbling fluidized beds. However, the pattern can only maintain for a limited particle height, well below the height of most industrial fluidized beds. Herein, we proposed to use a layered configuration with multiple stages of gas distributors to maintain the ordered bubbling structure to a higher particle height. Computational fluid dynamics-discrete element method (CFD-DEM) simulations performed here demonstrate the effectiveness and key parameters for maintaining structure in the proposed design, providing potential for industrial use.     

The clustered dense phase model for group A fluidization: I. Dense phase hydrodynamics

A new hydrodynamic model is proposed to represent the gas flow in the dense phase of fluidized Group A powders. The model views that the particles form clusters under the influence of inter-particle forces, giving rise to the formation of a heterogeneous void structure consisting of clusters of particles and interstitial cavities. The model contains two parameters, one representing the intrinsic void structure of the clusters and the other representing their size.The model predictions have been tested against reported experimental dense phase hydrodynamic data of Group A powders, both in bubble-free beds and in freely bubbling beds. The results show 2-3 particle clusters in bubble-free beds, but considerably larger clusters, containing 100 particles or so, in the dense phase of freely bubbling beds. The model also provides predictions for bubble through-flow, bubble splitting from below, dense-phase solids circulation, and interstitial gas bypassing in freely bubbling beds.     

Hydrodynamics and scale-up of liquid-solid circulating fluidized beds: Similitude method vs. CFD

Hydrodynamics and scale-up of liquid-solid circulating fluidized beds (LSCFBs) are investigated using similitude method and computational fluid dynamics (CFD) technique. Similitude method is applied to establish the dynamic similarity among LSCFBs by tuning physical properties of liquids and solids, operating conditions and bed dimensions to match several scaling sets of dimensionless groups. The hydrodynamic behaviors in these constructed LSCFBs are simulated by a validated CFD model [Cheng, Y., Zhu, J., 2005. CFD modeling and simulation of hydrodynamics in liquid-solid circulating fluidized beds. Canadian Journal of Chemical Engineering 83, 177-185] and compared in terms of the axial and radial flow structures characterized by the solids fraction, particle and liquid velocities and solids mass flux. The results demonstrate that only the full set scaling parameters obtained from similitude method, i.e., five dimensionless groups together with fixed bed geometry, particle sphericity, particle size distribution as well as particle collision properties, can ensure the similarity of hydrodynamics in the fully developed region of different LSCFBs. Developing flow structures in LSCFBs are strongly influenced by some parameters such as turbulent kinetic energy at the inlet so that the proposed similitude method may not always be applicable.     

Flow phenomena in a two-dimensional fluidized bed

The hydrodynamic behavior of a ‘two-dimensional’ model of an industrial-scale fluidized bed, with gas entering through both a grid and a downward-facing sparger, was investigated. Observed values of jet penetration depth, initial and final bubble diameters, bed expansion ratio, and TDH agreed well with values predicted from established correlations for three-dimensional bubbling fluidized beds. This provides further confidence in the use of such correlations in the scale-up of fluidized-bed reactors.