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.