The use of capacitance probes in gas fluidised beds

Abstract

There has been a trend recently towards using capacitance probe circuits to investigate events occurring in freely bubbling fluidised beds. The development of a capacitance probe circuit and the geometry of a mobile probe head to give minimum disturbance to passing bubbles is described. The calibration of events on the probe trace with respect to cine film records of bubbles passing the probe head is shown to be essential if reliance is to be placed on the probe results. Results of such a calibration using a two-dimensional fluidised bed are described together with the application of this calibration to three-dimensional situations. The limitations of this procedure are emphasised together with the necessity for always calibrating the probe in circumstances closely similar to those in which it is to be used. It is suggested that interpretation of capacitance probe results in terms of bubble sizes, bubble voidages or cumulative numbers of bubbles passing a given level should be treated with some reserve unless it is known that careful calibration has been employed.