Page 19 - Chemical Process Equipment - Selection and Design
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XVi RULES OF THUMB: SUMMARY
2. There are no known commercial applications of reflux to attrition, sizes in the range 50-500pm dia, a spectrum of sizes
extraction processes, although the theory is favorable (Treybal). with ratio of largest to smallest in the range of 10-25.
3. Mixer-settler arrangements are limited to at most five stages. 2. Cracking catalysts are members of a broad class characterized by
Mixing is accomplished with rotating impellers or circulating diameters of 30-150 pm, density of 1.5 g/mL or so, appreciable
pumps. Settlers are designed on the assumption that droplet expansion of the bed before fluidization sets in, minimum
sizes are about 150pm dia. In open vessels, residence times of bubbling velocity greater than minimum fluidizing velocity, and
30-60 min or superficial velocities of 0.5-1.5 ft/min are provided rapid disengagement of bubbles.
in settlers. Extraction stage efficiencies commonly are taken as 3. The other extreme of smoothly fluidizing particles is typified by
80%. coarse sand and glass beads both of which have been the subject
4. Spray towers even 20-40 ft high cannot be depended on to of much laboratory investigation. Their sizes are in the range
function as more than a single stage. 150-500 pm, densities 1.5-4.0 g/mL, small bed expansion, about
5. Packed towers are employed when 5-10 stages suffice. Pall rings the same magnitudes of minimum bubbling and minimum
of 1-1.5in. size are best. Dispersed phase loadings should not fluidizing velocities, and also have rapidly disengaging bubbles.
exceed 25 gal/(min) (sqft). HETS of 5-10 ft may be realizable. 4. Cohesive particles and large particles of 1 mm or more do not
The dispersed phase must be redistributed every 5-7 ft. Packed fluidize well and usually are processed in other ways.
towers are not satisfactory when the surface tension is more than 5. Rough correlations have been made of minimum fluidization
10 dyn/cm. velocity, minimum bubbling velocity, bed expansion, bed level
6. Sieve tray towers have holes of only 3-8mm dia. Velocities fluctuation, and disengaging height. Experts recommend,
through the holes are kept below 0.8 ft/sec to avoid formation of however, that any real design be based on pilot plant work.
small drops. Redispersion of either phase at each tray can be 6. Practical operations are conducted at two or more multiples of
designed for. Tray spacings are 6-24in. Tray efficiencies are in the minimum fluidizing velocity. In reactors, the entrained
the range of 20-30%. material is recovered with cyclones and returned to process. In
7. Pulsed packed and sieve tray towers may operate at frequencies dryers, the fine particles dry most quickly so the entrained
of 90 cycles/min and amplitudes of 6-25 mm. In large diameter material need not be recycled.
towers, HETS of about 1 m has been observed. Surface tensions
as high as 30-40 dyn/cm have no adverse effect. HEAT EXCHANGERS
8. Reciprocating tray towers can have holes 9/16in. dia, 50-60% 1. Take true countercurrent flow in a shell-and-tube exchanger as
open area, stroke length 0.75 in., 100-150 strokes/min, plate
a basis.
spacing normally 2in. but in the range 1-6in. In a 30in. dia 2. Standard tubes are 3/4 in. OD, 1 in. triangular spacing, 16 ft
tower, HETS is 20-25 in. and throughput is 2000 gal/(hr)(sqft).
Power requirements are much less than of pulsed towers. long; a shell 1 ft dia accommodates 100 sqft; 2ft dia, 400 sqft,
3 ft dia, 1100 sqft.
9. Rotating disk contactors or other rotary agitated towers realize 3. Tube side is for corrosive, fouling, scaling, and high pressure
HETS in the range 0.1-0.5m. The especially efficient Kuhni fluids.
with perforated disks of 40% free cross section has HETS 0.2 m 4. Shell side is for viscous and condensing fluids.
and a capacity of 50 m3/m2 hr.
5. Pressure drops are 1.5psi for boiling and 3-9psi for other
services.
FILTRATION
6. Minimum temperature approach is 20°F with normal coolants,
1. Processes are classified by their rate of cake buildup in a 10°F or less with refrigerants.
laboratory vacuum leaf filter: rapid, 0.1-10.0 cm/sec; medium, 7. Water inlet temperature is 90°F, maximum outlet 120°F.
0.1-10.0 cm/min; slow, 0.1-10.0 cm/hr. 8. Heat transfer coefficients for estimating purposes,
2. Continuous filtration should not be attempted if 1/8in. cake Btu/(hr)(sqft)("F): water to liquid, 150; condensers, 150; liquid
thickness cannot be formed in less than 5 min. to liquid, 50; liquid to gas, 5; gas to gas, 5; reboiler, 200. Max
3. Rapid filtering is accomplished with belts, top feed drums, or flux in reboilers, 10,000 Btu/(hr)(sqft).
pusher-type centrifuges. 9. Double-pipe exchanger is competitive at duties requiring
4. Medium rate filtering is accomplished with vacuum drums or 100-200 sqft.
disks or peeler-type centrifuges. 10. Compact (plate and fin) exchangers have 350sqft/cuft, and
5. Slow filtering slurries are handled in pressure filters or about 4 times the heat transfer per cuft of shell-and-tube units.
sedimenting centrifuges. 11. Plate and frame exchangers are suited to high sanitation
6. Clarification with negligible cake buildup is accomplished with services, and are 2540% cheaper in stainless construction than
cartridges, precoat drums, or sand filters. shell-and-tube units.
7. Laboratory tests are advisable when the filtering surface is 12. Air coolers: Tubes are 0.75-1.00in. OD, total finned surface
expected to be more than a few square meters, when cake 15-20 sqft/sqft bare surface, U = 80-100 Btu/(hr)(sqft bare
washing is critical, when cake drying may be a problem, or when surface)("F), fan power input 2-5 HP/(MBtu/hr), approach
precoating may be needed. 50°F or more.
8. For finely ground ores and minerals, rotary drum filtration rates W. Fired heaters: radiant rate, 12,000 Btu/(hr)(sqft); convection
may be 1500 lb/(day)(sqft), at 20rev/hr and 18-25in. Hg rate, 4000; cold oil tube velocity, 6 ft/sec; approx equal transfers
vacuum. of heat in the two sections; thermal efficiency 70-75%; flue gas
9. Coarse solids and crystals may be filtered at rates of 6000 temperature 250-350°F above feed inlet; stack gas temperature
lb/(day)(sqft) at 20 rev/hr, 2-6 in. Hg vacuum. 650-950°F.
INSULATION
FLUIDIZATION OF PARTICLES WITH GASES
1. Up to 650"F, 85% magnesia is most used.
1. Properties of particles that are conducive to smooth fluidization 2. Up to 1600-1900"F, a mixture of asbestos and diatomaceous
include: rounded or smooth shape, enough toughness to resist earth is used.