Page 85 - Wastewater Solids Incineration Systems

54 Wastewater Solids Incineration Systems

FIGURE 5.4 Entrainment of solids from different heights above the top surface of a
dense bubbling bed (as shown in Kunii and Levenspiel, 1969, from Zenz and Oth-
mer, 1960).

enough to minimize sand loss but large enough to allow entrainment of ash. Theoret-
ically, the velocity must be greater than the terminal velocity of the ash, but less than
the terminal velocity of the sand.

2.7 Hydrodynamics-Based Groups
In a gas-fluidized bed, the bubbles moving through the dense particulate phase have
a strong influence on the quality of fluidization. Quality of fluidization is not the
same for all solids. Depending on the mean size of the particle and the difference in
density of the gas and the solid, Geldart (1973) has classified powder into four groups
and designated them by the letters A, B, C, and D (Figure 5.5). Of these groups, only
groups A and B are suitable for fluidization. Of these two groups, group A powders
have dense-phase expansion after minimum fluidization but before the commence-
ment of bubbling. Group B powders exhibit bubbling at the minimum fluidization
velocity itself. The fluid bed layer in group B expands less than in group A, but
exhibits higher mixing. Group C is difficult to fluidize because of its extreme electro-
static effect. Group D can create jetlike upward moving dilute phase, referred to as
spoutable. The solid media used in WWTP fluid bed incinerators are of group B.