Y. A. Liu

FUNDAMENTAL AND PRACTICAL DEVELOPMENTS OF MAGNETOFLUIDIZED BEDS : A REVIEW

Abstract This paper presents a review of the literature on the fundamental and practical developments of magnetofluidized beds (MFBs) for reaction, filtration and separation applications in gas—solid, liquid—solid and three-phase systems. A particular interest of our review is the discussion of the basic research and patented developments of magnetically stabilized beds (MSBs). We describe: historical developments of MSBs; the range of stabilized-bed operating conditions; MSBs of mixed-particle systems; nonconventional MSB systems; structure, solid flowability and rheological behavior of MSBs; gas-mixing behavior as well as heat and mass transfer in MSBs; countercurrent and crossflow MSBs as gas—solid contactors; and liquid—solid and three-phase MSBs. We also review the published literature on other MFBs in which the applied magnetic field may not be spatially uniform and the bed may not be stabilized (i.e., bubbling). Following a discussion of the published fundamental research results, we describe a number of recent practical developments of other MFBs, including the fluidized-bed high-gradient magnetic separation (FBHGMS), the magnetic valve for solids (MVS), the magnetic distributor—downcomer (MDD), the magnetic transporter of solids (MTS) and the magnetic recirculator of solids (MRS).

Identifying States in Shallow Vibrated Beds

Abstract Shallow particulate beds on surfaces vibrating at ca. 25 Hz and an amplitude of a few millimetres display several states, in order of increasing bed depth: two ‘Newtonian states’ in which freely bouncing particles obey simple Newtonian mechanics; a ‘coherent—expanded (CE) state’ in which particles move together in loosely organized packets to form a turbulent shallow layer; and a ‘coherent—condensed (CC) state,’ either beneath or replacing the CE state, in which particles move together in a compacted layer. In the CC state, neighboring particles tend to remain in close company. Which states are present, as well as whether a gap forms beneath the vibrated bed, depends on bed depth and particle and gas properties. Transitions between the states appear to be sharp and quantifiable, but further investigation, both theoretical and experimental, is needed before the transitions are fully understood. The CE state provides intense solid mixing and appears likely to provide superb contacting of a solid with a gas in the space above a shallow vibrated bed. The CE state may provide an opportunity for developing a family of microreactors for kinetic studies. The CC state is characterized by a bulk solid-circulation pattern, generally stable over a long time interval, as well as by a relatively low porosity.

Heat transfer in shallow vibrated beds

Abstract Coefficients are reported for transfer of heat from a horizontal cylinder and from vertical, flat surfaces (paired back-to-back, closely spaced-apart) to 30-mm deep vibrated beds (25 Hz) of alumina spheres and high- and low-density glass beds (at sizes from 63 to 707 μm). In general, the flat surfaces afford higher coefficients, but heat transfer is sensitive to pattern and vigor of powder circulation. No broadly general explanation or correlation of vibrated-bed heat transfer can be given; but the body of data provides leads for the engineer wishing to develop a vibrated-bed-heat-exchanger design for non-sticky particulate matter. The leads hold promise of coefficients beyond 500 W/m2K, even perhaps approaching 1000.

Assessment of sulfur and ash removal from coals by magnetic separation

In this paper, the recent development and current status of the magnetic removal of sulfur and ash from dry, wet and liquefied coals are critically reviewed and analyzed. A quantitative assessment of the scientific, technical and economical feasibility of applying magnetic separation to the desulfurization and deashing of coals is presented. The needs and opportunities in the future research and development work are also suggested.

A method for observing phase-dependent phenomena in cyclic systems: Application to study of dynamics of vibrated beds of granular solids

Abstract A technique is described which uses a digital circuit based on a phase-locked loop to permit viewing, photographing, or measuring phase-dependent phenomena in cyclic systems. This eliminates the need for expensive and tedious high-speed cinematographic methods that have been used thus far for visual observations of cyclic systems. The technique provides accurate quantitative data on visual cyclic phase-dependent phenomena that occur within the system with the same frequency as the forcing vibration. The technique has been used in studies of a vibrated bed. It may also be used with vibrating gas-fluidized beds. With little modifications, the technique may be applied to study phase-dependent phenomena in other cyclic systems. In direct observations at a series of phase angles, using a strobe light activated by the digital circuit, particle-free air gaps appear above and below a horizontal heat-transfer tube placed within a two-dimensional vibrated bed. The time-integrated percentage of heat-transfer surface blanketed by air, estimated from back-lit photographs, explicates trends in the heat-transfer data. A rarefied zone of reduced particle density forms at the top surface of a vibrated bed. In a bed in which solid circulates, down at walls and upward at the center, the rarefied zone is the major path for return flow of solids from the center to wall. Phase-shift photographs show that the rarefied zone develops during lift-off of the bed from the vibrating plate, indicating that rarefaction occurs because a downward flow of gas, necessary to supply gas to the gap forming between the bed and plate, exerts a lesser drag on the top layer of particles in the bed that it does on the remainder of the bed. The phase-delayed trigger system ha also been used to facilitate measurements of non-visual phase-dependent properties such as gas pressures below the bed throughout a cycle of vibration.

Pilot-scale studies of sulfur and ash removal from coals by high gradient magnetic separation

This paper presents the results of pilot-scale studies of sulfur and ash removal from coal by high gradient magnetic separation (HGMS). Work was done on both the liquefied coal and the raw pulverized coal in water slurries. The effects of residence time, field intensity, packing material and density, slurry concentration and recycle on the grade and recovery of the wet separation of sulfur and ash from water slurries of Illinois No. 6 coal were quantitatively examined. The HGMS was effective in reducing the weight percent of total sulfur, ash, and inorganic sulfur by as high as 40, 35, and 80%, respectively; while achieving a maximum recovery of about 95%. The results have also provided the first experimental verification of the applicability of Beans magnetic filtration model in quantitatively correlating the data obtained from the pilot-scale beneficiation of coal slurries by the HGMS. The successful verification of the model allows one to quantitatively identify the trade-off of operating parameters so as to optimize the magnetic removal of sulfur and ash. A pilot-scale HGMS system for the magnetic sedation of mineral residue from the liquefied coal has been designed and constructed. Typical results from preliminary experiments with the liquefied solvent refined coal (SRC) have been quite encouraging, indicating that the HGMS was effective in reducing the total sulfur and ash contents by as high as 70 and 76%, respectively. Finally, the liquefaction of the magnetically treated Kentucky No. 9/14 coal was studied and compared with that of the untreated coal. It was found that although the mineral matters which had been removed magnetically had a significant catalytic effect on the liquefaction behavior, the organic hydrodesulfurization remained practically the same for both the untreated and treated coals. This suggests that the magnetic removal of mineral matters prior to liquefaction may be advantageous for the SRC and other related liquefaction processes, in which the minimum hydrogenation is especially desired and the hydrodesulfurization is often limiting.

Studies in magnetochemical engineering: Part VI. An experimental study of screen-packed and conventional fluidized beds in axial and transverse magnetic fields☆

Abstract This paper describes the results from an experimental study of fluidization characteristics and magnetic stabilization of screen-packed and conventional (i.e., unpacked) beds of solids subjected to external magnetic fields applied axially or transversely relative to the fluidizing gas flow. The beds consist of either entirely ferromagnetic iron particles or admixtures of iron and paramagnetic manganese(II) oxide, MnO. Key variables investigated include superficial gas velocity, presence of screen packing, particle type and shape, magnetic field intensity and orientation, and fraction of ferromagnetic particles in the fluidizing admixture. This study of screen-packed magnetofluidized beds (SPMFBs) has fundamental importance to the development of fluidized-bed high-gradient magnetic separation (FBHGMS) and of magnetic valve for solids (MVS) and magnetic distributor-downcomer (MDD). This study also extends the concept of magnetically stabilized beds (MSBs) to admixtures of ferromagnetic and paramagnetic particles by examining the role of internal screen packing in both axial and transverse magnetic fields. The results show that screen packing significantly alters the fluidization behavior of magnetofluidized beds of iron and MnO particles. In addition, by adding a small fraction of iron, magnetic stabilization (i.e., bubble elimination) of conventional beds of MnO is achievable with an axial or a transverse field. Magnetized screens tend to serve as distributor plates in SPMFBs of iron and MnO in moderate field intensities (20–100 Oe), creating a staged bed appearance. The tendency to exhibit bed staging increase with increasing field intensity and with increasing volume fraction of iron in MnO. Gas velocities at the transition to bubbling and at the onset of staging (i.e., transition and staging velocities) increases with increasing field intensity. As the magnetic field orientation changes from transverse to axial, transition and staging velocities decrease and staged behavior is less likely. These results have two practical implications. First, in magnetofluidized-bed operations impaired by bed staging, such as dry coal desulfurization by FBHGMS, the magnetic field should be applied axially. Second, in operations aided by bed staging, such as MVS/MDD, the magnetic field should be applied transversely. To date, reported work in MVS/MDD technology employs only axial fields. This work presents quantitative data comparing the fluidization behavior and magnetic stabilization in axial and transverse fields. For beds of ferromagnetic particles and of ferromagnetic-paramagnetic admixtures, the results show a wider range of gas velocities for magnetic stabilization in an axial field compared to a transverse field. Such results validate the claim that MSB operations should utilize an axial, rather than a transverse, field to maximize the bubble-free gas throughput.

Studies in magnetochemical engineering: Part V. An experimental study of fluidized beds with screen packing and applied magnetic field

Abstract An experimental study has been made of the characteristics of fluidized beds of ferromagnetic iron and/or paramagnetic manganese(II) oxide (MnO) particles subjected to both external magnetic field and internal screen packing. Key variables investigated include particle size and shape, magnetic properties of particles, superficial gas velocity, screen packing density, and applied field intensity. This study of screen-packed magnetofluidized beds (SPMFBs) has fundamental importance to the development of fluidized-bed high-gradient magnetic separation (FBHGMS) described in Part IV. The primary objective of the study is to gain some basic understanding of SPMFBs, focusing on the practical implications of experimental results to improve the technical performance of FBHBMS for dry coal desulfurization. This study also extends the concept of magnetic stabilization (that is, bubble elimination) in fluidized beds by examining the role of internal screen packing, in addition to that of external magnetic field. The results show that in SPMFBs of ferromagnetic particles, magnetic stabilization can be achieved, but both the minimum fluidization velocity and the pressure drop at minimum fluidization significantly increase with increasing field intensity. By adding a small fraction of iron, SPMFBs of paramagnetic MnO particles can also be stabilized. Of particular interest to FBHGMS in the observation that magnetized screens tend to serve as distributor plates in SPMFBs, creating a staged bed appearance. The tendency to exhibit staging increases with increasing field intensity and increasing gas velocity. In addition, screens appear to promote the field-induced particle agglomeration. Both bed staging and particle agglomeration are undesirable in many applications of SPMFBs, and their occurrence can be effectively avoided by auxiliary mechanical vibrations as is done in FBHGMS. This study also shows that there exists a trade-off with increasing screen packing density in SPMFBs, particularly in FBHGMS. Denser screen packing increases the surface area for magnetic particle capture, but also increases the occurrence of particle agglomeration as well as bed staging and slugging, which degrades the performance of FBHGMS for dry coal desulfurization.

Pressure drop across shallow fluidized beds: Theory and experiment

A special manometer system has been developed to permit an accurate measurement of average pressure drop across shallow fluidized beds, despite the possible pressure fluctuations and oscillations caused by the vigorous solid mixing and gas bubbling commonly observed in such beds. Quantitative measurements have been made of bed pressure drop over wide ranges of superficial gas velocities, static-bed heights, distributor design characteristics, and particle types and properties. By making a macroscopic momentum balance over the fluidized-bed control volume, a simple model for correlating and predicting the pressure-drop ratio (i.e., the ratio of bed pressure drop to static-bed pressure) has been developed. The model indicates that the pressure-drop ratio PR is linear in the reciprocal of static-bed height (Hs): PR = ‡ α — β/Hs, where α and β are constants which can be predicted a priori from known experimental variables and common dimensionless groups such as Reynolds and Froude numbers based on the gas velocity passing through the distributor. A comparison between model predictions and experimental data shows that the proposed model can accurately correlate and predict the bed pressure drop across shallow fluidized beds.

Studies in magnetochemical engineering: Part IV. A fluidized-bed superconducting magnetic separation process for dry coal desulfurization☆

Abstract This paper describes the development and demonstration of a fluidized-bed, superconducting magnetic separation process for desulfurization of dry pulverized coal for utility boiler applications (typically ground to 70 – 80 minus 200-mesh, or below 0.074 mm). Experimental results have shown significant advantages for several processing methods and equipment innovations, such as using a superconducting magnetic field, a fluidized-bed separator matrix, and an automatic internal vibration/washing system, in maximizing the sulfur reduction, Btu recovery and processing throughput. Without any external size classification and ultra-fines removal prior to magnetic separation, the process can reduce the sulfur dioxide emission level (lb SO2 per million Btu, or g SO2 per million joule) of several pulverized Eastern coals by 30 to 55% and achieve an average Btu recovery of 85 to 90%. Ranges of major operating variables for optimizing the process performance have been identified. The process appears to be an effective dry magnetic method for fine coal desulfurization.