(74e) The Transport Disengaging Height in Fluidised Beds for Fine and Coarse Particles Measured by Positron Particle Tracking

In fluidised bed applications, bubbles bursting at the surface of the bed will project particles into the freeboard, the space between the bed surface and the gas off-take. Depending on their terminal velocities and the gas velocity, solids are carried up the column to various heights. The larger particles fall back and the smallest are evacuated with the off gas through the off-take tube; consequently the solids loading (commonly expressed as kg solids/m³ gas) will decline with height. The region within which the solids loading falls is called the Transport Disengaging Height (TDH) and this term TDH is applied to two related but different circumstances: coarse particles having a terminal velocity larger than the superficial gas velocity are nevertheless flung upwards by the bursting bubbles at the bed surface and then fall back. Above the height they reach, only fines i.e. particles with terminal velocities smaller than the gas velocity are found. The concentration (or hold-up) of fines decreases with height and eventually reaches a constant value. The height which the coarse particles reach is called the splash height or TDH(C). The height above which the fines hold-up changes little or not at all will be called TDH(F). It is clear that the definition of a particle as coarse or fine will depend upon its terminal velocity and the superficial gas velocity in the freeboard.

The TDH was determined experimentally from positron emission particle tracking in bubbling fluidised beds of 8.6 and 16 cm I.D. respectively. The bulk bed material was sand of 120 and 250 µm. 18F-tracer particles were used. These tracers included sand (120, 250, 375, 580 µm), -alumina of 150 µm and coal of 920 µm. The static bed height was between 30 and 40 cm. The operating velocity varied from 0.05 to 0.5 m/s. The detection camera covered approx. 0.6 x 0.6 m² and started detecting the tracer from about 20 cm above the distributor level. Collected data determine the particle velocity when ejected by a bubble, the particle profile in the freeboard and the maximum height reached (TDH) for particles with a terminal velocity in excess of the operating gas velocity in the freeboard. Illustration of particle tracking profiles will be given, clearly showing that the velocity of ejection exceeds the bubble rise velocity, as calculated from common equations..

The experimental results will thereafter be used (i) to assess the accuracy of existing empirical correlations for the TDH, (ii) to develop a fundamental-principle based empirical equation, and (iii) to develop a model that will predict the trajectories of ejected particles from a balance of forces.

To illustrate the use of both empirical equations and model approach, TDH predictions are made for various superficial gas velocities and/or diameters of the fluidised bed. At low superficial gas velocities, the TDH is determined by the ejection of coarse particles, at higher velocities entrained fines fix the TDH.

*Corresponding author: Professor Jan Baejens; e-mail: J.Baejens@bham.ac.uk