Fluidization characteristics of magnetic particles and determination of stable fluidization zone in magnetically fluidized bed

To determine and calculate the stable fluidization zone in a magnetically fluidized bed, the fluidization characteristics of magnetic particles are investigated. Four kinds of magnetic particles with different average diameters, ranging from 231 to 512 μm, are fluidized in the presence of magnetic fields with specified values of the intensity in the range of zero to 7330 A/m, and the particle fluidization curves are plotted. For marking the stable fluidization zone in the curves, the minimum bubbling velocities of particles are measured by the pressure-drop fluctuation. Based on the fluidization curves, the influences of the average particle diameter and magnetic field intensity on the zone are analyzed and discussed. A correlation to determine the stable fluidization zone is derived from the experimental data, using three dimensionless numbers, i. e., the ratio of magnetic potential to gravity potential, the Reynolds number and the Archimedes number. Compared with available data reported, it is shown that the correlation is more simplified to predict relative parameters for the bed operating in the state of stable fluidization under reasonable conditions.

Characteristics of non-magnetic nanoparticles in magnetically fluidized bed by adding coarse magnets

The fluidization behavior of SiO2, ZnO and TiO2 non-magnetic nanoparticles was investigated in a magnetically fluidized bed （MFB） by adding coarse magnets. The effects of both the amount of coarse magnets and the magnetic field intensity on the fluidization quality of these nanoparticles were investigated. The results show that the coarse magnets added to the bed lead to a reduction in the size of the aggregates formed naturally by the primary nanopartieles. As the macroscopic performances of improved fluidization quality, the bed expansion ratio increases whilst the minimum fluidization velocity decreases with increasing the magnetic field intensity, but for TiO2 nanoparticles there exists a suitable magnetic field intensity of 0.059 6 T. The optimal amounts of coarse magnets for SiO2, ZnO and TiO2 non-magnetic nanoparticles are 40%, 50% and 60% （mass fraction）, respectively. The bed expansion results analyzed by the Richardson-Zaki scaling law show that the exponents depend on both the amount of coarse magnets and the magnetic field intensity.

Collecting aerosol in airflow with a magnetically stabilized fluidized bed

A magnetically stabilized fluidized bed (MSB) is a highly efficient filter that takes the advantage of both fluidized beds and fixed beds. This paper presents the research to collect aerosol in airflow with a MSB. The filtering model of MSB is established with its parameters including magnetic Geld intensity, gas superficial velocity, average grain-size, and bed height on the collection efficiency of MSB. The model is verified by experiments.

Behavior of mixed ZnO and SiO_2 nano-particles in magnetic field assisted fluidization

The fluidization behavior of ZnO nano-particles in magnetic fluidized bed （MFB） by adding coarse magnetic particles was investigated, followed by the co-fluidization of mixtures of ZnO and SiO2 nano-particles. For such co-fluidization, bed expansion was found to change smoothly with gas velocity through a range of stable operation. By measuring the bed expansion ratio and pressure drop, a stability diagram for the mixture in MFB was obtained. Within this stable operation range, with increasing gas velocity the pressure drop hardly changes as the bed expands, up to an expansion ratio of more than 4.

Modeling of segregation in magnetized fluidized bed with binary mixture of Geldart-B magnetizable and nonmagnetizable particles

For the magnetized fluidized bed(MFB)with the binary mixture of Geldart-B magnetizable and nonmagnetizable particles,the magnetically induced segregation between these two kinds of particles occurs at high magnetic field intensities(H),leading to the deterioration of the fluidization quality.The critical intensity(H_(ms))above which such segregation commences varies with the gas velocity(U_g).This work focuses on establishing a segregation model to theoretically derive the H_(ms)–U_g relationship.In a magnetic field,the magnetizable particles form agglomerates.The magnetically induced segregation in essence refers to the size segregation of the binary mixture of agglomerates and nonmagnetizable particles.Consequently,the segregation model was established in two steps:first,the size of agglomerates(d_A)was calculated by the force balance model;then,the H_(ms)–U_g relationship was obtained by substituting the expression of d_Ainto the basic size segregation model for binary mixtures.As per the force balance model,the cohesive and collision forces were 1_2 orders of magnitude greater than the other forces exerted on the agglomerates.Therefore,the balance between these two forces largely determined d_A.The calculated d_A increased with increasing H and decreasing U_g,agreeing qualitatively with the experimental observation.The calculated H_(ms)–U_ g relationship agreed reasonably with the experimental data,indicating that the present segregation model could predict well the segregation behavior in the MFB with the binary mixture.

Magnetic field assisted fluidization—Dimensional analysis addressing the physical basis

This paper originates a discussion on dimensional analysis and scaling in magnetically assisted fluidized beds. Basic examination of process variables, merging mechanical and magnetic units, allows the conversion of mixed sets of variables into unified terms representing surface forces as effects of the fields contributing to the assisted fluidization behaviour. This transformation is termed ＂pressure transform＂ since the new variables are all characteristic pressures generated by three basic fields： gravity, magnetic and fluid flow. This approach addresses the physical basis in terms of dimensionless groups rather than formal algebraic manipulations pertinent to classical dimensional analysis. Basic dimensionless group termed granular magnetic Bond number is introduced as the ratio of characteristic pressures of gravity and of magnetic field. This analysis also provides a set of named dimensionless numbers characterizing magnetic field assisted fluidization such as Filippov number, Rosensweig number, Kwauk number and Siegell number, derived as ratios of characteristic pressures.

Magnetically assisted gas-solid fluidization in a tapered vessel:Part II Dimensionless bed expansion scaling

The article presents an effort to create dimensionless scaling correlations of the overall bed porosity in the case of magnetically assisted fluidization in a tapered vessel with external transverse magnetic field. This is a stand of portion of new branch in the magnetically assisted fluidization recently created concerning employment of tapered vessels. Dimensional analysis based on ＂pressure transform＂ of the initial set of variables and involving the magnetic granular Bond number has been applied to develop scaling relationships of dimensionless groups representing ratios of pressures created by the fluid flow, gravity and the magnetic field over an elementary volume of the fluidized bed. Special attention has been paid on the existing data correlations developed for non-magnetic beds and the links to the new ones especially developed for tapered magnetic counterparts. A special dimensionless variable Xp = （Ar△Dbt）1/3√RgMQ combining Archimedes and Rosensweig numbers has been conceived for porosity correlation. Data correlations have been performed by power-law, exponential decay and asymptotic functions with analysis of their adequacies and accuracies of approximation.

Removal of dust from flue gas in magnetically stabilized fluidized bed

A magnetically stabilized fluidized bed （MSFB,Ф 500mm×2100mm） was designed to study dust removal from flue gas. Based on the mechanism of dust removal in a fixed bed, the effects on collection efficiency of magnetic field intensity, ratio of flue gas velocity to minimum fluidization velocity, bed height, and particle average diameter, were investigated. Then feasible methods for MSFB to better remove dust were proposed. Over 95 % of dust removal with MSFB can be achieved, when stable fluidization is maintained and when magnetic particles are frequently renewed.

Operating range of magnetic stabilization flow regime for magnetized fluidized bed with Geldart-B magnetizable and nonmagnetizable particles

The magnetic stabilization flow regime could also be created forGeldart-Bnonmagnetizable particles pro-vided some magnetizable particles are introduced and the magnetic field is applied.This study aimed toexplore the size(d_(pM))and density(ρ_(pM))effects of magnetizable particles on its operating range.The upperlimit(Umb;)could not be determined from the △P_(b)-U_(g)↓curve but could from analyzing the variation of △P_(b)-fluctuation with increasing U_(g).Due to the variation of U_(mfH)(lower limit)with d_(pM) and ppw,both U_(mbH)-U_(mfH) and(U_(mbH)-U_(mfH))/U_(mfH) were used to quantify the operating range of magnetic stabilization.U_(mbH)-U_(mfH) varied hardly with ρ_(pM) but increased significantly with decreasing ρ_(pM).(U_(mbH)-U_(mfH))/U_(mfH)increased as d_(pM) or ρ_(pM) decreased.lt was more difficult for the nonmagnetizable particles to escape fromthe network formed by the smaller/lighter magnetizable particles.For the same magnitude of change,dp had a stronger effect than ρ_(pM) on(U_(mbH)-U_(mfH))/U_(mfH).Neither U_(mbH)-U_(mfH) nor(U_(mbH)-U_(mfH):)/Uma variedmonotonously with the minimum fluidization velocity of the magnetizable particles,indicating that nostraightforward criterion for matching the magnetizable particles to the given nonmagnetizable particlescould be established based on their minimum fluidization velocities to maximize the operating range ofmagnetic stabilization.

Local heat transfer properties in co- and counter-current G-L-S magnetically stabilized fluidized beds

Heat transfer coefficients were measured by immersed probes in co- and counter-current G-L-S magnetically stabilized fluidized beds （MSFBs） using air, water and nickel-alloy particles as the gas, liquid and solid phases. Influences of major factors, including magnetic field intensity, superficial gas and liquid velocities, liquid viscosity and surface tension, on heat-transfer properties were studied experimentally, indicating that both co- and counter-current G-L-S MSFB can provide relatively uniform radial distribution of heat transfer coefficients under appropriate operation conditions, thus controlling operation temperature for highly exothermic multi-phase reaction systems. Two correlations were provided to estimate accurately heat transfer properties in both co- and counter-current G-L-S MSFB systems, with an average error of less than 10%.

High-efficiency removal of NOx by a novel integrated chemical absorption and two-stage bioreduction process using magnetically stabilized fluidized bed reactors

To enhance the bioregeneration of Fe(II)EDTA and to avoid the inhibition of the components in nitrogen oxides(NOx) scrubbing solution, a novel integrated process of metal chelate absorption and two-stage bioreduction was developed. In this process, magnetically stabilized fluidized beds(MSFB) were used as the bioreactors, and the phase diagram for the MSFB operation was determined. Factors including inlet NO, O2 and SO2 concentrations, magnetic field intensity, gas flow rate and liquid circulation rate, were studied experimentally to investigate their effects on NO removal. In addition, a mathematical model for NO removal in this integrated system was developed. The results revealed that the integrated system could be steadily operated with a high NO removal efficiency and elimination capacity, even under the condition of high NO and O2 shock-loading. The established model showed that NO removal efficiency was related to the spray column property and the active Fe(II)EDTA concentration, while the latter depends on the bioregeneration of the disabled absorbent in the MSFB.

Hydrodynamic behavior of magnetized fluidized beds with admixtures of Geldart-B magnetizable and nonmagnetizable particles

This work focuses on the hydrodynamic behavior of admixtures of Geldart-B magnetizable and non- magnetizable particles in a magnetized fluidized bed. The applied magnetic field was axial, uniform, and steady. In operating the beds, the magnetization-LAST mode was adopted under which four distinct flow regimes exist： fixed, magnetized-bubbling, partial segregation-bubbling, and total segregation-bubbling, The operational phase diagram was drawn to display the transitions between flow regimes in an intuitive manner. Only in the magnetized-bubbling regime could the magnetic field reduce the bubble size and improve fluidization quality. In the segregation-bubbling regimes, fluidization quality deteriorated as segregation developed. The segregation of the binary mixture was quantitatively studied by observing pressure drops in the local bed. Reasons for the improvement in fluidization quality as well as the occur- rence of segregation were analyzed. Furthermore. the flow regime transition under magnetization-LAST operation mode was different from that under magnetization-FIRST mode. The magnetically stabilized bed （MSB） flow regime, which could be easily created under magnetization-FIRST mode, could no longer be achieved under magnetization-LAST mode. With the admixture, the MSB was proved to be a metastable equilibrium state. Under the magnetization-LAST mode, the admixture bed reached directly the stable equilibrium state-bubbling with segregation.

Identification of flow regimes and determination of the boundaries for magnetized fluidized bed with Geldart-B particles

The magnetized fluidized bed(MFB)with Geldart-B particles exhibits many distinct flow regimes depend-ing on the magnetic field intensity(H)and gas velocity(U_(g)).The identification of these regimes was reviewed for the MFB with magnetizable particles and that with binary admixture of magnetizable and nonmagnetizable particles.Meanwhile,methods for determining the boundaries between two adjacent flow regimes were clarified.The MFB state was found to depend not only on H and Ug but also on their application sequence(i.e.,operation mode)within certain operating zones.The dependence feature arose from that the MFB therein could have different equilibrium states for the same combination of H and Ug.Furthermore,such a polymorphic characteristic of the MFB was revealed to result from the internal friction among the particles that were in unfluidized/packed state.Many of the MFB states were demon-strated to be in metastable equilibrium.Nevertheless,they differed significantly from the metastates well-known in the discipline of physical chemistry,such as supercooling and superheated.In fact,they belonged to the amorphous/glass state.This review will deepen our hydrodynamic understanding of the MFB and further promote its commercial application in the chemical and biochemical industries.