PINNING IN A RECTANGULAR MOVING BED REACTOR WITH GAS CROSS-FLOW

In the petroleum and process industries, renewed interest is being directed towards more efficient ways for contacting granular solid and gas. Within the last decade one novel way, now finding acceptance, is the formation of granular solid into a descending bed through which gas is then passed transversely, a cross-flow moving bed. This arrangement is advantageous in that a process such as catalyst regeneration (say) can be effected without removal of the solid from the process and thereby permits continuous operation. In practice, however, the physical interaction of gas and solid is known to cause adverse phenomena and these are addressed here. Experimental studies have been carried out in some four narrow sized granular materials of mean diameters from 1.6 to 7.0 mm. In beds where the distance separating the two parallel gas screens was 0.1 m, FOUR distinct modes by which the bed ceased to descend, termed pinning, were identified by altering the gas flow. Each was characterized by a particular behaviour of the particles adjacent to the gas screens. At each screen, the particles either moved as a block or displaced relative to each other. Displacement, termed particle drag, when adjacent to the gas inlet screen, was found to promote cavity formation. When adjacent to the gas outlet screen, particle drag was found to cause lower gas throughputs at which the bed ceased to descend. Whereas some granular solids were found to exhibit all four modes, others did not. In large sized beds, where screen separation was 0.3 m, only two of the four modes were identified since particle drag was either present at both gas screens or not. When the separation was 0.5 m, only the former mode was observed. The key to the occurrence of the phenomenon of particle drag is considered to be in the packing developed in the feeding system and its dilation against the surface of the gas screens. © 1990.