If the fluid is a liquid, increase in velocity causes the quiescent bed to rock and break, allowing individual particles to move randomly in all directions.
FLUIDIZATION
The
movement of fluids through a fixed bed was described in chapter 2. If the fluid
velocity is low, the same situation is found when fluid is driven upward
through a loose particulate bed. At higher velocities, however, frictional drag
causes the particles to move into a more expanded packing, which offers less
resistance to flow. At some critical velocity, the particles are just touching
and the pressure drop across the bed just balances its weight. This is the
point of incipient fluidization, and beyond it true fluidization occurs, the
bed acquiring the properties of surface leveling and flow.
If
the fluid is a liquid, increase in velocity causes the quiescent bed to rock
and break, allowing individual particles to move randomly in all directions.
Increase in velocity causes progressive thinning of this system. In
fluidization by gases, the behavior of the bed is quite different. Although
much of the gas passes between individual particles in the manner already
described, the remainder passes in the form of bubbles so that the bed looks
like a boiling liquid. Bubbles rise through the bed, producing an extensive
wake from which material is continually lost and gained, and breaking at the
surface distributes powder widely. This is an effective mixing mechanism, and
any nonhomogeneity in the bed is quickly destroyed. Rates of heat and mass
transfer in the bed are therefore high.
Bubble
size and movement vary in different systems. In general, both decrease as the
particle size decreases. As the size decreases and the powder becomes more
cohesive, fluidization becomes more difficult. Eventually, bub-bles do not form
and very fine powders cannot be fluidized in this way.
The
final stage of fluidization occurs at very high velocities when in both liquid
and gaseous systems, the particles become entrained in the fluid and are
carried along with it. These conditions are used to convey particulate solids
from one place to another.
TH 2019 - 2023 pharmacy180.com; Developed by Therithal info.