Mixing and circulation in fluidized beds of flour

Abstract

The mixing and bulk circulation characteristics of a fluidized bed of wheat flour having a relatively high aspect ratio were determined with a 6 in. diam. column fitted with a slowly revolving paddle just above the gas distributor. In this system the bulk of the solids moved down the column in stick-slip flow which was readily observed because of the appearance of relatively stable horizontal cracks in the bed. At lower gas velocities the behavior of the system was characterized by zone fluidization which led to erratic and unreproducible mixing performance. Operation at greater than twice the minimum fluidization velocity overcame this problem and provided consistent mixing. An increase in gas velocity increased the bulk circulation rate and reduced the batch mixing time. The solids next to the wall of the column seemed to move at the same rate as the core of the bed, at least at lower gas velocities. The downward velocity of these solids was directly proportional to gas velocity and to bed height and a complex function of flour moisture content. The velocity also depended on vertical position but not on the speed of the rotating paddle. Adding an anti-agglomerant promoted smooth fluidization and reduced the minimum fluidization velocity, and for the same relative gas velocity it increased the bulk circulation rate and reduced the batch mixing time.