Some Solids Residence Time Studies in Tapered Gas Fluidised Beds.

An investigation has been made into the nature and extent of solid mixing in a series of cylindrical and tapered fluidised beds, the operating variables being solid flow rate, fluidising gas rate, bed height and taper of bed profile. Exploratory work was carried out using spherical particles of glass coated with a tracer dye. The main body of the work was, however, performed using an entirely new method which enables the solid outflow from the bed to be analysed continuously, thus providing a great saving in time, and a considerable improvement in accuracy of analysis. An attempt is made to fit the results to a hypothesis which attributes the mixing process in a fluidised bed to a diffusion type mechanism. Treatment of the results by three different methods is undertaken, in order to eliminate any shortcomings inherent in such methods and also to provide a quantitative comparison between them. The diffusional hypothesis is shown to give a good representation of the mixing process for the range of variables studied. Further, a new set of design criteria are suggested, for the equalisation of fluidisation quality throughout deep beds. A comparison is also made between beds of this new design and those designed according to earlier workers