The minimum fluidization and minimum bubbling velocities of silica sand particles in air-blown micro beds of 120 mm high and with inner diameters of less than 32 mm were investigated to understand the wall effect in micro fluidized beds (MFBs) and the operability of the MFBs. Experimental results demonstrated that both the quoted velocities obviously increase with decreasing the inner diameter of the bed. A specific wall effect determined as the pressure drop per unit volume of particle bed in excess of the predicted pressure drop from the Ergun equation was proposed to quantitatively account for the influence of bed wall friction. By characterizing the fluidization qualities of differently sized particles in different MFBs, the article suggested further an optimal combination of bed diameter and particle size in the range of the static bed heights from 20 to 50 mm for the so-called micro fluidized bed (MFB) reactor devised to perform reaction(s) with minimal suffering from external gas mixing and gas diffusion. (c) 2007 Elsevier B.V. All fights reserved.