Page 83 - Wastewater Solids Incineration Systems

52 Wastewater Solids Incineration Systems

FIGURE 5.3 Pressure drop versus gas velocity for a bed of uniformly sized sand particles
(Shirai, 1958).

2.4.2 Terminal Gas Velocity U
t
The terminal velocity or free-fall velocity U of a particle in a fluid bed of uniform size
t
is the velocity at which entrainment or carry-over of the particle occurs. It is greatly
dependent not only on the characteristic of the gas but also on the size, density, and
physical shape of the particle (spherical, irregular, rough, or smooth particle surface).
It could be determined by measuring the bed pressure drop with gas flow rate
(Figure 5.3). It could also be estimated by correlations established from fluid
mechanics by Kunii and Levenspiel (1969):
1 ⁄
U [4gd ( )/3 C ] 2 (5.2)
t p s g g d
where C is an experimentally determined drag coefficient.
d
Depending on the size and density of the particle and also on the characteristic
of the fluidizing gas, U could range from 10 U to 100 U . When fluidization is
t mf mf
applied to incineration, the bed support material has a wide size distribution. It is
composed of fresh sand, eroded sand of smaller size, and fine ash particles. The ter-
minal velocity of the smallest particle in a poly-disperse or mixed-particle assembly
limits the operational range of velocity. The terminal velocity of the smallest particle
may be just equal to, or even less than, the minimum fluidization velocity of the
largest particle. In such a case, the carry-over of fines will take place while the largest
particles are fluidized or kept in a fixed bed condition. Particles that fall between the