Fluidization and soot filtration in a rotating fluidized bed
Department of Chemical Engineering, Chemistry and Environmental Science
Doctor of Philosophy
Stevens, John G.
Loney, Norman W.
Rotating Fluidized Bed (RFB)
A horizontal rotating fluidized bed (RFB) has been investigated for the removal of soot from diesel engine exhaust. This requires necessitating an understanding of the nature of fluidization, particle mixing and filtration in the RFB.
In an RFB, particles are subject to a centrifugal force which can be much larger than the force of gravity. Therefore the Geldart classification for conventional fluidized beds must be modified for rotating fluidized beds. A theoretical analysis shows that group A particles (minimum bubbling velocity Umb > minimum fluidization velocity Umf can shift to group B Umb = Umf and group C particles (agglomerating) can shift to group A under a centrifugal force. Therefore, certain group C particles can be fluidized in rotating fluidized beds. However, for very high "g", such particles shift to group D (spouting) and cannot be fluidized. This was verified experimentally by successfully fluidizing 7 µm alumina particles in the RFB which behave as group C in a conventional fluidized bed. Thus an important and unique feature of RFB technology is that it enables the use of very fine bed particles in a fluidization mode.
Since the RFB works as a shallow bed, the distributor plays an especially important role in its fluidization behavior. The pressure drop in an RFB was measured using slotted, perforated and sintered metal cylindrical gas distributors as a function of rotating speed, gas velocity and bed thickness with both polydisperse alumina particles and nearly monodisperse glass beads. The measured pressure drop for the different distributors depends strongly on the distributor design. A theoretical model available in the literature is used to calculate the minimum fluidization velocity and the pressure drop as a function of rotating speed, mass loading and gas velocity which are then compared to the experimental results.
Particle motion in an RFB was studied by observing the mixing of two layers of particles of different color and different density. Experiments show that bubbles are responsible for particle motion and mixing for layers of the same material. When a layer of denser particles is placed on the distributor, the mixing behavior is similar to that observed for layers of the same material. However, when a layer of less dense particles is placed on the distributor, mixing is dominated by differences in density and occurs before bubbles are visible.
A horizontal rotating fluidized bed filter (RFBF) charged with either polydisperse alumina granules or nearly monodisperse glass beads, was used to capture soot from diesel engine exhaust. The mass average filtration efficiency, calculated on the basis of the total mass of soot that was captured in the bed, was found to be up to 90% at the start of filtration at steady state flow conditions. Time and equipment limitations did not permit obtaining data with a build up of soot in the bed or the exploration of the effect of varying engine load conditions. A critical factor in obtaining high filtration efficiency is the use of fine sized bed particles (high surface area/volume), which is enabled by the use of an RFB. The filtration efficiency increased with increasing gas flow rate as the bed passes from the packed bed to the fluidized bed mode. The filtration efficiency also varied as a function of agglomerated soot size, showing a minimum for soot particles in the 0.3 to 0.6 µm range. This is a consequence of particles larger than 0.6 gm being removed mainly by inertial impaction and interception and smaller particles mainly by diffusion. Distributor plugging and fines generation due to attrition of the bed media were identified as critical issues and need to be addressed for realizing improvements in this area.
njit-etd2000-028 (172 pages ~ 9,744 KB pdf)
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Created October 12, 2001