Multi-scale fluidization of ultrafine powders in a fast-bed-riser/conical-dipleg CFB loop

Abstract

The multi-scale methodology has been used as a conceptual tool in devising an apparatus for continuous processing of ultra-fine particles in the fluidized state. The apparatus combines a fast-bed riser with a conical dipleg into a circulating fluidized bed (CFB) loop. The micro-scale discrete ultra-fine particles gather together into meso-scale agglomerates on account of inter-particle forces, in both the fast-bed riser and the conical dipleg. In particular, in the latter, the fine particles are sufficiently entrapped because of reduced gas velocity in the top region, into the top-to-bottom and center-to-periphery mixing currents of the solids, to form agglomerates before being recycled to the bottom of the fast-bed riser. The fast-bed riser, because of its high and relatively uniform gas velocity, serves to adjust the size, especially the size distribution of the agglomerates, that is, further capture of discrete micro-scale discrete particles for the smaller agglomerates to make them larger, and attrition of over-size agglomerates to make them smaller. Uniform agglomerates of a given average size ensures stable fluidization, minimizing the danger of deposition of macro-scale lumps of agglomerates to lead to blockage as well as defluidization elsewhere in the loop. The above concept is shown experimentally operable using 4 kinds of ultra-fine particles: calcium carbonate, titanium dioxide, silicon dioxide and saponite.