Page 230 - Fluid mechanics, heat transfer, and mass transfer

208 TWO-PHASE FLOW SYSTEMS

. How does particle attrition occur in fluidized beds? Will & No. Conveying powders that are susceptible to oxi-
it take place throughout the bed uniformly? dation and that form explosive mixtures with air need
& Attrition occurs mainly from high-velocity impact to be conveyed by gases other than air, for example,
within the bed. inert gases such as nitrogen.
& Within a fluid bed reactor, attrition predominantly . What are the important reasons for adopting pneumatic
occurs in the vicinity of the fluid distribution grid and conveyance for the transport of solids?
within the cyclone recovery system, since in both of & Enclosed, safe, and environmentally attractive meth-
these locations gas streams involve high levels of od of transport suitable for a wide variety of products,
turbulence and are capable of entraining and accel- including those with bacteria-prone, toxic, or explo-
erating particles to their local high velocities, which sive properties. Dust-free transport.
can be over 100 m/s. & Simple systems requiring a prime mover, a feeding
& Some degree of attrition, more erosive than fractur- device, a conveying pipeline, and a cleaning or
ing in character involving substantially lower veloc- disengaging device.
ities than those at the grid and cyclone inlet areas, & Flexibility in pipeline layout. Can be transported
also takes place within the bed as bulk solid streams vertically (over 300 m in a single lift) or horizontally
flow into random bubble voids, down cyclone dip (up to 3 km).
legs, and in standpipe transfer lines between reactor & Ability to distribute product to a number of different
and regenerator of a fluid catalytic cracking process.
areas within a plant and/or pick up material from
. In what way knowledge of attrition processes within several different locations.
fluidized beds is useful? & Low maintenance and manpower costs.
& Understanding the high attrition areas within the bed
& Multiple Uses: A single pipeline can be used for a
will be helpful in improving the design of distribution
variety of products.
grids and cyclones.
& Ease of automation and control.
& Better recognition and control of particle size
. What are the disadvantages of pneumatic conveying?
analysis.
& High power consumption.
& Preparation of catalysts with higher levels of resis-
& Abrasion and wear of equipment.
tance to attrition, which is sometimes measured in
terms of attrition index. & Incorrect design can lead to attrition or degradation
of the bulk solid particles.
& Requires high skills in designing, operation, and
maintenance.
7.1.2.3 Gas–Solids Transport
& Pneumatic conveying systems are limited in overall
conveying distance and conveying capacity, com-
7.1.2.3.1 Pneumatic Conveyance
pared to hydraulic transport.
. What is pneumatic conveyance? & Cohesive or sticky materials are often difficult to
convey pneumatically. Moist substances that are wet
& Transport of solid particles with air as the carrier is
enough to stick to the walls of the pipeline usually
called pneumatic transport. As velocity of air is
cannot be handled successfully. Materials with high
increased beyond velocities at which fluidization
oil or fat contents can also cause severe buildup in
takes place, the entrained particles get carried away
pipelines such that conveying is not practical, al-
by the air. Fluidization process gets transformed
though this can sometimes be overcome with tem-
into pneumatic transport. The velocities involved
perature control or flexible pipelines.
will be such to support particulates in the fluid
overcoming gravitational pull on the particles plus . Give examples of applications of pneumatic conveyors
overcoming frictional losses in the conduit as well in industry?
as maintaining bulk movement of the fluid and & Cement transport from grinders to silos and silos to
solids. loading points.
& When a fluid other than air is used, the process might & Conveying finely ground powders such as flours,
be termed as gas–solids transport, hydraulic trans- pigments, and catalyst powders.
port, slurry transport, and so on. & Unloading trucks, railcars, and barges, transferring
. “Pneumatic conveying implies use of air for conveying materials to and from storage vessels, injecting
solids.” Is it always true? Under what circumstances a solids into reactors and combustion chambers, and
gas other than air is used? collecting fugitive dust by vacuum.