(54f) Minimum Fluidization and Bubbling Properties - the Effects of Fines Content and Pressure

Gas-solids fluidized beds have wide industrial applications as they offer rapid solids mixing, relatively uniform temperature, good solids-gas contacting and heat and mass transfer.  In the fluidization processes, generally once the velocity of the gas exceeds the minimum velocity necessary to suspend the solids, Umf, any excess flow passes through the bed in the form of gas bubbles or voids.  Geldart’s Group A powders (Geldart, 1986) are an exception in that they show an appreciable bubble-free fluidization range at gas velocities between the minimum fluidization velocity and the velocity at which bubbles start to appear, the minimum bubbling velocity. There is interest in knowing the bed densities, rhomf and rhomb, corresponding to the two fluidization points.  Wells(2001) gas bypassing model suggested that the emulsion phase density in deep beds isn't constant but rather it increases from a value equal to rhomb at the bed surface to a value approaching rhomf near the bed bottom and that this increase in the emulsion phase density eventually leads to gas bypassing. Our earlier gas bypassing studies in deep beds of FCC catalyst particles showed that increasing the system pressure and/or addition of fines eliminated gas bypassing or reduced it significantly.  were conducted in a 15-cm-diameter column to determine how the two factors affect the the minimum fluidization and bubbling densities of FCC catalyst particles. System pressures of up to 200 kPag and fines contents of up to 12% <44 microns were used. There was a significant decrease of the minimum bubbling density  with fines content. For the pressure range tested there was only a slight decrease in the minimum bubbling density. There was no significant influence of pressure or fines content on the minimum fluidization density. The results are used to explain the effect of pressure and fines on gas bypassing in deep beds of FCC catalyt particles.