The expansion is observed when the gas velocity is higher than minimum fluidization velocity, and is associated to the formation of bubbles in the bed. It is usually expressed as the ratio between expanded bed height and bed height of bed in minimum fluidization velocity for any gas velocity.
Tannous (1994) gave an overview of some influences:
- Regarding the influence of bed geometry, Cranfield and Geldart (1974) found that the expansion of a three-dimensional bed is lower than a bidimensional bed for the same gas velocity;
- Bradshaw and Myers (1963) noted that the bed expansion is independent of the height of the fixed bed. These results were obtained for large spherical and cylindrical particles of average diameter between 420 and 876mm and specific mass between 906 and 1980 kg/m3. Moreover, the authors noted that the bed expansion is independent of the column diameter.
There are certain correlations in the literature that predict the bed expansion at different velocities. These correlations are derived from liquid-solid fluidizations (Richardson and Zaki, 1954), from two phases theory, assuming that excess of gas, regarding the minimum fluidization, crosses the bed in the form of bubbles or by dimensional group theory.