Studies on fluidization. I—the critical mass velocity

Abstract

This study gives an investigation on the critical mass velocity at which a powdered solid starts to fluidize in an ascending flow of gas. From the measurements on closely sized samples and mixtures of different sizes of carborundum, iron oxide (Fe 3O 4) and coke, using air, argon, carbon dioxide, nitrogen-hydrogen mixtures, town gas and methane as fluidizing gas, the proportionally of the critical mass velocity G 0, with the reciprocal value of the kinematic viscosity μ/ρ of the gas and with the bed density at maximum porosity g̊ bm and the square of the particle diameter d was confirmed in the region Re < 5. By the introduction of the so called effective diameter d e (which is the diameter of the sphere having the same number of particles per unit volume of bed at maximum porosity) and of a generalized shape factor B, which includes the usual porosity term (“Kozeny factor”) it was demonstrated that instead of the porosity and particle diameter, the principal property which determines the resistance to gas flow is the number of particles per unit volume of bed. These considerations resulted in an equation for the critical mass velocity reading ▪ in which when substituting for d the effective diameter d e the generalized shape factor B e equals, even for definitely non-spherical particles like carborundum and ironoxide (see Fig. 2a, b, and c), practically unity. After the introduction of the packing ratio m, being the ratio of bed density at maximum porosity a modified “Carman-Kozeny” equation for the pressure loss in a fixed bed is suggested where the usual porosity term is substituted by a power of the packing ratio.