The fluidization phenomena of gas-solids systems depend very much on the types of powders employed. Geldart (1973) was the first to classify the behavior of solids fluidized by gases into four clearly recognizable groups (Groups A, B, C, and D) characterized by the density difference between the particles and the fluidizing medium and by the mean particle size. Geldart's classification has since become the standard to demarcate the types of gas fluidization. In developing his powder classification, Geldart employd fluidization data obtained only at ambient temperature and pressure and from beds fluidized only with air. Unfortunately, industrial applications of fluidized bed technology invariably are at elevated temperature and pressure and with fluidizing gas other than air.

Geldart classification of powders does not apply at elevated temperature and pressure. There are ample evidences reported in the literature indicating that normally Group B powders at ambient conditions, such as polymer particles, can behave like a Group A powder under polymerization conditions at elevated temperature and pressure with substantial emulsion-phase expansion, relatively small bubbles, smooth fluidization, and reduced gas bypassing (Grace, 1986; Burdett et al., 2001).

The original Geldart's classificatin is modified and re-interpreted by plotting the density ratio of particle and fluid against the Archimedes number. The new parameters allow powders with different properties fluidized at different temperatures and pressures with gases of different properties to be plotted in the same graph. The selection of these two parameters for plotting is not arbitrary, however. The experimental and theoretical development, and the approach for future experimental verification of the group boundaries will be discussed.