Page 151 - Chemical Process Equipment - Selection and Design

6.11. FLUIDIZATION OF BEDS OF PARTICLES WITH GASES 123

particles appear to interlock to form a skin around the bubble and
thus prevent free particles from raining through those spaces.
Bubble sizes become large at high rates of flow and may eventually
reach the diameter of the vessel, at which time slugging and severe
entrainment will occur.
Onset of fluidEation commonly is detected by noting a break in
the plot of flow against pressure drop. For a range beyond the
minimum fluidizing velocity, the pressure drop remains constant
and equal to the weight of the bed but the bed level rises gradually
and bubbles arle generated at an increasing rate. Not in all cases,
however, is the fluidization behavior entirely smooth. Figure 6.lO(a)
compares “nonmal” with a case of “abnormal” behavior. Among
the reasons for abnormality are aggregation of particles because of
stickiness or attractive forces between small particles and
interlocking of rough surfaces. It is even possible for bubbling to
occur before the onset of fluidization by formation of channels in
the bed.

I
F
CHARACTE RlSTICS 0 FLU DlZATlO N Figure 6.11. Six regimes of fluidization identified with increasing gas
superficial velocity (Grace, 2982).
Six different regimes of fluidization are identified in Figure 6.11 and
its legend. Particulate fluidization, class (b) of the figure, is
desirable for most processing since it affords intimate contacting of Velocity Appearance and
phases. Fluidization depends primarily on the sizes and densities of Range Regime Principal Features
the particles, but also on their roughness and the temperature,
pressure, and humidity of the gas. Especially small particles are (a) Os u<umf fixed particles are quiescent; gas flows
subject to electrostatic and interparticle forces. bed through interstices
Four main classes characterized by diameters and differences in (b) umf 5 u < umb particulate bed expands smoothly in a homoge-
densities of the phases are identified in Figure 6.12 and its legend. fluid- neous manner; top surface is well
Groups A and1 €3 are most frequently encountered; the boundary ization defined; some small-scale particle
between them is defined by the equation given in the legend. Group motion; little tendency for particles to
aggregate; very little fluctuation
A particles are relatively small, 30-150 ym dia, with densities below (c) om,, r u < urn, bubbling void regions form near the distributor,
1.5 g/cc. Their bed behavior is ‘‘abnormal” in that the bed expands fluid- grow mostly by coalescence, and rise
appreciably before bubbling sets in, and the minimum bubbling ization to the surface; top surface is well
velocity always is greater than the minimum fluidization velocity. defined with bubbles breaking
The bubbles disengage quickly. Cracking catalysts that have been through periodically; irregular pres-
studied extensively for their fluidization behavior are in this class. sure fluctuations of appreciable
Group I? materials have dp = 150-500 pm and are 1.5-4.0 g/mL. amplitude
The bed expansion is small, and minimum bubbling and fluidization (d) urn, 5 u < uk slugging voids fill most of the column cross
velocities are inearly the same. The bubbles also disengage rapidly. regime section; top surface rises and col-
lapses with reasonably regular fre-
Coarse sand and glass beads that have been favorite study materials quency; large and regular pressure
fall in this group. Group C comprises small cohesive particles whose fluctuations
behavior is influenced by electrostatic and van der Waals forces. (e) u, 5 u < utr turbulent small voids and particle clusters danto
Their beds are difficult to fluidize and subject to channelling. Group regime and fro; top surface difficult to
D particles are large, 1 mm or more, such as lead shot and grains. distinguish; small-amplitude pressure
They do not fluidize well and are usually handled in spouted beds, fluctuations only
such as Figure 9.13(f). (f) utrs u fast no upper surface to bed; particles are
Among the properties of particles most conducive to smooth fluid- transported out the top and rnus be
fluidization are the following: ization replaced by adding solids at or near
the bottom; clusters or strands of
particles move downward, mostly
1. rounded and smooth shape, near the wall, while gas, containing
2. in the range of 50-500 ym diameter, widely dispersed particles, moves
3. a broad spectrum of particle sizes, with ratios of largest to upward; at fixed solid feed rate,
smallest sizes in the range of 10 to 25, increasingly dilute as u is increased
4. enough toughness to resist attrition.
SIZING EQUIPMENT
Such tailoring of properties is feasible for many catalyst-camer
formulations, but drying processes, for instance, may be restricted Various aspects of the hydrodynamics of gas-solid fluidization have
by other considerations. Fluidization of difficult materials can be been studied extensively with conclusions that afford guidance to
maintained by mechanical or ultrasonic vibration of the vessel, or the interpretation and extension of pilot plant data. Some of the
pulsation of the supply of the fluid, or mechanical agitation of the leading results bearing on the sizing of vessels will be discussed
contents of the vessel, or by addition of fluidization aids such as fine here. Heat transfer performance is covered in Chapter 17. Example
foreign solids. 6.16 applies to some of the cited data.