Fluidization characterisitcs of cohesive powder in a pilot plant scale vibrated fluidized bed dryer at low vibration frequencies

Abstract

Mechanical vibration is often applied in industrial scale fluidized bed dryers in food and pharmaceutical processes to overcome operational problems due to cohesiveness. However, the understanding of the process is still insufficient regarding detailed modelling, apparatus design, up-scaling and process optimization. Almost all of the experimental research in the field of vibrating fluidized beds is conducted on a lab-scale. Within the current project a pilot plant scale unit with a cross section of 250 x 500 mm2 and a total height of 3 m was built. The influence of process parameters, such as gas velocity, vibration intensity, powder moisture content and bed mass, on the fluidization characteristics of whole milk powder was investigated. In order to characterize the lower limit of fluidization from an operational point of view, the velocity of full fluidization (uff) was introduced. It is defined as the lowest superficial gas velocity at which the entire bed is fully fluidized. It could be observed that an increase in powder moisture content results in significant increase of utr. Introduction of mechanical vibration into the bed resulted in the reduction of Uff, expansion of the bed and reduction of bubble volume fraction. This expansion of the suspension phase explains reported increase in heat and mass transfer in vibrated fluidized beds.