Fluidization characteristics of cohesive powders in vibrated fluidized bed drying at low vibration frequencies

Mechanical vibration is often applied in industrial scale fluidized bed dryers for food and pharmaceutical powders to overcome operational problems caused by the cohesiveness of the products. However, the understanding of the process regarding detailed modeling, apparatus design, up-scaling and process optimization is still incomplete. Almost all of the experimental research in the field of vibrating fluidized beds is conducted on a lab-scale. Within the current project, experiments are conducted in a pilot plant scale unit with a cross section of 250 x 500 mm(2) and a total height of 3 m. The influence of several process parameters, such as gas velocity, vibration intensity, powder moisture content and bed mass, on the fluidization characteristics of whole milk powder is studied. In order to characterize the lower limit of fluidization from an operational point of view, the velocity of complete fluidization (u(cf)) is used and quantified. This is defined as the lowest superficial gas velocity at which the entire bed is fully fluidized. It could be observed that an increase in moisture content of the powder results in a significant increase of u(cf), The introduction of mechanical vibration into the bed results in the reduction of u(cf), the expansion of the bed and the reduction of the bubble volume fraction. This expansion of the suspension phase explains the increase in heat and mass transfer in vibrated fluidized beds, reported in the literature. Adding the effect of vibration intensity to well established correlations allows for the accurate prediction of gas hold-up and bed porosity of fluidized beds of cohesive whole milk powder under mechanical vibration at low vibration frequencies. (C) 2019 Published by Elsevier B.V.