Page 231 - Fluid mechanics, heat transfer, and mass transfer

TWO-PHASE FLOW 209
TABLE 7.2 Materials Transported by Pneumatic Conveyance

ABS powder Acetyl salicylic acid Adipic acid Activated earth Alumina
Ammonium sulfate Ammonium nitrate Granulated cattle feed Anthracite Cement
Bagasse fines Cellulose acetate Charcoal granules Chalk Clays
Ground coffee Coke dust Bleaching powder Cotton seeds Copra
Calcium chloride Limestone Magnesia Flour Bran
Milk powder Glass fibers Nitrocellulose Sawdust Soot
Graphite powder Gypsum Paper mill wood waste Peanut shells Starch
Phthalic anhydride Plaster Polyethylene granules Granulated sugar Sulfur
Polystyrene beads Polyester fibers PVC powder and granules Sodium sulfate Talc
Terephthalic acid Titanium dioxide Soap flakes Silica gel Yeast
Sodium carbonate Zinc oxide Wood shavings Tobacco Fly ash

. Give a list of some materials conveyed by pneumatic & Minimum conveying velocity is a function of flow
conveyance. rate.
& Table 7.2 provides a list of materials that are con- & Relative humidity of conveying air should be more
veyed by pneumatic conveyance. than 70%, to avoid electrostatic effects.
. What are the important guidelines with respect to . What are the characteristics of particulates that are
selection, specification, and design of pneumatic con- handled by pneumatic conveying?
veyance systems? & Pneumatic conveying can be used for particles rang-
& During the project stage, current conveying rate ing from fine powders to pellets with bulk densities of
3
3
requirements and plant layout along with plans for 16–3200 kg/m (1–200 lb/ft ).
future expansions or potential rate enhancements due & Among the important particle characteristics are
to improvements in process technologies need to be particle size and size distribution, shape, density,
considered. hardness, and friability.
& Minimum and maximum conveying rates should be ➢ Particle size, distribution, and shape are known to
carefully defined. be among the most significant variables affecting
& A reasonable horizontal conveying length (15–20 pneumatic conveying. For example, uniformly
pipe diameters), with minimum flexible hose length, sized round and smooth particles are easier to
should be allowed before the first bend to allow the convey than angular, rough ones having a wide
bulk material to accelerate. size distribution.
& Placing bends/elbows back to back in the line should ➢ Another rule of thumb is that, to prevent mechan-
be avoided. The number of bends should be kept to ical plugging of conveying lines, particularly
minimum. when conveying materials containing large parti-
cles, the pipe diameter must be at least three to five
& Pipe diameter should be increased at the end of the
times the maximum particle dimension.
line to prevent excessive velocity and reentrainment
of the particulates. ➢ Particle density affects the minimum conveying
velocity and pressure drop required for transport.
& Proper venting of the rotary air lock and feeder in a
positive pressure system is critical for reliable & Bulk properties that are important include bulk den-
operation. Gas leakage at feeder must be consid- sity and compressibility, permeability, cohesive
ered in design calculations and adequately strength, segregation tendency, explosibility, toxici-
compensated. ty, reactivity, and electrostatic effects.
& The gas flow control system for multiproduct and ➢ The compressibility and permeability of a bulk
multidestination systems must ensure that the oper- solid determine how readily the material will
ating point is maintained within the stable operating deaerate.
zone. ➢ The direct effect of the cohesiveness of a bulk solid
& It must be noted that product damage and wear at pipe on pneumatic conveying can be buildup in the
bends depend on the nature of the material. lines. Flow stoppages, or erratic flow through
equipment upstream of a conveying line, such as
& A proper purge control sequence need to be de-
in a bin, feeder, or chute, can be detrimental to
signed and tested to avoid product degradation or
achieving the desired transfer rates.
blockage.