Geltard suggested a four-group classification within which the range bed behaviour under ambient conditions can be categorized on the basis of the particle and fluid density difference and the average particle diameter. Gelatrd’s categories are referred to as groups, A, B, C and D. Group A powders are typically made up of materials with a small mean size and low particle density (less than 1. 4 gcm-3). Beds of powders in this group undergo considerable expansion before bubbling commences. Also, when the supply of gas is abruptly cut off, the bed collapses slowly, typically at rate of 0.
3 to 0. 6 cms-1, this being similar to the superficial velocity of the gas in the dense phase. The circulation of the solids, on a large scale is visible when such powders are fluidised. Also, the bubbles in these beds appear to have a maximum size. This suggests the bed expansion and mean density would vary in linear fashion with fluidising gas velocity. The maximum stable diameter of the bubbles appears as a consequence of the shearing force created due to the downward movement of solids relative to the bubble.
This force creates a circulation within the bubbles, the velocity of which attains the rise velocity of bubbles. When these bubbles coalesce the solids in the wake are pulled up, tearing the bubble, and the mechanism restricts the maximum stable size. FCC catalysts are representative of these solids. Group B contains the most commonly encountered powders and have an average size of 40 to 500 ? m and densities ranging from 1400 to 4000 kg/m3. These powders fluidise easily and in contrast to group A, have bubbles occurring naturally at, or only slightly above minimum fluidisation velocity.
The bed expansion is small and the bed collapses very rapidly when the gas supply is suddenly cut off. These particles are generally of a sand-like nature. There is no evidence of a maximum bubble size. At comparable condition i. e. , at the same distance from the distributor, bubble size seems to be independent of particle size. Group C includes powders that are in any way cohesive. The powders are generally very fine with a mean diameter less than 30 ? m and low density. The beds of such particles are extremely difficult to fluidise. The powder lifts as a plug in small diameter tubes or channels badly.
This occurs because the inter particle forces are noticeably greater than those which the fluid can exert on a particle. Face powders, flour and starch are typical examples of these solids. Group D powders are confined to large and/or very dense particles. All but the largest bubbles rise more slowly than the interstitial fluidising gas, so that the gas merely flows into the base of the bubble, and out of the top. These fluids behave erratically, giving large, exploding bubbles or severe channelling, and the gas velocity in the dense phase is high.
If gas is admitted only through a centrally positioned hole, group D powders can be made to spout. Drying grains and peas roasting coffee beans and gasifying coals are such solids, and they are usually processed in shallow beds or spouting mode. Geldarts classification is clear and easy to use and is displayed in figure (1) for air fluidisation at ambient conditions and for uo less than about 10 umf. For any solid of linear density ? s and mean particle size dp, this graph shows the type of fluidisation that would be expected.