Page 229 - Fluid mechanics, heat transfer, and mass transfer

TWO-PHASE FLOW 207

FIGURE 7.13 Fluid catalytic cracking unit with a two-stage regenerator (UOP).

fluidized mechanically by imposing vibration to . What are the characteristics of catalysts used in fluid
throw particles vertically upward cyclically. It can beds?
be accomplished by vibrating thegas distributor plate & Diameters range from 30 to 150 mm.
or by a combination of vibration and gas pulsation. & Density is about 1.5 g/mL.
Bed mixing will be more vigorous. Lateral mixing
& Bed expansion occurs to an appreciable extent before
will reduce channeling of different size particles.
fluidization sets in.
& Vibrofluidization enables the bed to operate with
& Minimum bubbling velocity is greater than minimum
either no gas upward velocity or reduced gas flow.
fluidization velocity.
Entrainment can also be greatly reduced compared to
& Rapid disengagement of bubbles takes place.
unaided fluidization.
. What are the types of particles that are not suitable for
& The technique is used commercially in drying and
fluid bed operations?
other applications.
& Cohesive particles and particles larger than 1 mm do
& Chemical reaction applications are possible.
not fluidize well and are processed in other ways.
. What are the desirable characteristics of particles for
& What are the effects of particle attrition in fluidized
smooth fluidization?
beds?
& Rounded, smooth shape (sand and glass beads).
& Reduction in uniformity in fluidization of the parti-
& Enough toughness (abrasion) to resist attrition.
cles within the bed.
& Size range: 50–500 mm.
& Increased loads on cyclones and loss of cyclone
& Ratio of largest to smallest sizes: 10–25.
efficiency.
& Cohesive and large (>1 mm) particles are not good
& Loss of valuable catalyst requiring increased fresh
for fluidization.
makeup.