Bottom-spray fluidised bed

In a top-spray fluidised bed, air is introduced through a uniform air distribution plate. Compared to the particle motion in the bottom-spray fluidised bed, the fluidisation pattern is less controlled. Particular to the top-spray method, the nozzle is positioned above the particle bed, and coating liquid is sprayed coimtercurrently, or down, into the fluidising core particles. Compared to a Wurster fluid bed, it is more... 

The top-spray system has also been used successfully to coat particles as small as 100 pm and attempts to coat smaller particles may lead to the same difficulties as encountered in the bottom-spray system. Figure 16.5 shows a schematic representation of the top-spray fluidised bed configuration. 

The principle and possible applications of the three basic processing options (top-spray, bottom-spray or tangential-spray) of the batch-fluidised bed are discussed below (Jones 1994). Both the top- and bottom-spray methods are characterised by a conical product container and expansion chamber, resulting in a more vigorous fluidisation pattern and a decrease in velocity as the particles move upward in the expansion chamber. 

In the case of the electrostatic fluidised bed, a combination of the electrostatic spray technique and the fluidised bed technique is used. The apparatus consists of an array of electrostatic elements on the bottom of a shallow bed. A small quantity of powder is placed on the elements and fluidised by the normal method. The elements impart a negative charge to the powder with the result that a cloud of charged powder is obtained. An earthed object held in the cloud collects a layer of powder by electrostatic rather than fluidised bed principles. Unfortunately, the density of the cloud varies considerably from top to bottom and is sensibly uniform over only 4 in. This is offset somewhat by the fact that an electrostatic fluidised bed requires only one-tenth of the raw material compared with an equal sized conventional fluidised bed and this helps considerably as far as stock holding of colours is concerned. 

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