Page 18 - Chemical Process Equipment - Selection and Design
P. 18
RULES OF THUMB: SUMMARY XV
20. Packings of random and structured character are suited An 85% free cross section is taken for design purposes. In
especially to towers under 3 ft dia and where low pressure drop countercurrent flow, the exit gas is 10-20°C above the solid; in
is desirable. With proper initial distribution and periodic parallel flow, the temperature of the exit solid is 100°C. Rotation
redistribulion, volumetric efficiencies can be made greater than speeds of about 4rpm are used, but the product of rpm and
those of tiray towers. Packed internals are used as replacements diameter in feet is typically between 15 and 25.
for achieving greater throughput or separation in existing tower 4. Drum dryers for pastes and slurries operate with contact times of
shells. 3-12 sec, produce flakes 1-3 mm thick with evaporation rates of
21 For gas rates of 500 cfm, use 1 in. packing; for gas rates of 15-30 kg/m2 hr. Diameters are 1.5-5.0 ft; the rotation rate is
*
2000 cfm or more, use 2 in. 2-10rpm. The greatest evaporative capacity is of the order of
22. The ratio of diameters of tower and packing should be at least 3000 lb/hr in commercial units.
15. 5. Pneumatic conveying dryers normally take particles 1-3 mm dia
23. Because of deformability, plastic packing is limited to a 10-15 ft but up to 10 mm when the moisture is mostly on the surface. Air
depth unsupported, metal to 20-25 ft. velocities are 10-30 m/sec. Single pass residence times are
Liquid redistributors are needed every 5-10 tower diameters 0.5-3.0 sec but with normal recycling the average residence time
with pall rings but at least every 20ft. The number of liquid is brought up to 60 sec. Units in use range from 0.2 m dia by 1 m
streams should be 3-5/sqft in towers larger than 3 ft dia (some high to 0.3m dia by 38m long. Air requirement is several
experts say 9-12/sqft), and more numerous in smaller towers. SCFM/lb of dry product/hr.
25. Height equivalent to a theoretical plate (HETP) for Fluidized bed dryers work best on particles of a few tenths of a
vapor-liquid conlacting is 1.3-1.8ft for 1 in. pall rings, mm dia, but up to 4 mm dia have been processed. Gas velocities
2.5-3.0 f: for 2 in. pall rings. of twice the minimum fluidization velocity are a safe
26. Packed towers should operate near 70% of the flooding rate prescription. In continuous operation, drying times of 1-2 min
given by the correlation of Sherwood, Lobo, et al. are enough, but batch drying of some pharmaceutical products
27- Reflux drums usually are horizontal, with a liquid holdup of 5 employs drying times of 2-3 hr.
min half full. A takeoff pot for a second liquid phase, such as Spray dryers: Surface moisture is removed in about 5 sec, and
water in hydrocarbon systems, is slzed for a linear velocity of most drying is completed in less than 60 sec. Parallel flow of air
that phase of 0.5 ft/sec. minimum diameter of 16 in. and stock is most common. Atomizing nozzles have openings
For towers about 3ft dia, add 4ft at the top for vapor 0.012-0.15 in. and operate at pressures of 300-4000 psi.
disengagement and 6ft at the bottom for liquid level and Atomizing spray wheels rotate at speeds to 20,000rpm with
reboiler return. peripheral speeds of 250-600 ft/sec. With nozzles, the length to
29. Limit the tower height to about 175 ft max because of wind load diameter ratio of the dryer is 4-5; with spray wheels, the ratio is
and foundation considerations. An additional criterion is that 0.5-1.0. For the final design, the experts say, pilot tests in a unit
L/D be less than 30. of 2 m dia should be made.
RIVERS AND POWER RECOVERY EQUIPMENT
EVAPORATORS
1. Efficiency IS greater for larger machines. Motors are 85-95%;
steam turbines an: 42-78%; gas engines and turbines are 1. Long tube vertical evaporators with either natural or forced
28-38%. circulation are most popular. Tubes are 19-63mm dia and
2. For under 100HP, electric motors are used almost exclusively. 12-30 ft long.
They are made for up to 20,000 HP. 2. In forced circulation, linear velocities in the tubes are
3. Induction motors are most popular. Synchronous motors are 15-20 ft/sec.
made for speeds as low as 150rpm and are thus suited for 3. Elevation of boiling point by dissolved solids results in
example for low speed reciprocating compressors, but are not differences of 3-10°F between solution and saturated vapor.
made smaller than 50 MP. A variety of enclosures is available, 4. When the boiling point rise is appreciable, the economic number
from weather-proof to explosion-proof. of effects in series with forward feed is 4-5.
4. Steam turbines are competitive above 100HP. They are speed 5. When the boiling point rise is small, minimum cost is obtained
controllable. Frequently they are employed as spares in case of with 8-10 effects in series.
power failure. 6. In backward feed the more concentrated solution is heated with
5. combustion engines and turbines are restricted to mobile and the highest temperature steam so that heating surface is
remote locations. lessened, but the solution must be pumped between stages.
5. Gas expanders for power recovery may be justified at capacities 7. The steam economy of an N-stage battery is approximately
of several lhundred HP; otherwise any needed pressure reduction 0.8N lb evaporation/lb of outside steam.
in process is effected with throttling valves. 8. Interstage steam pressures can be boosted with steam jet
compressors of 20-30% efficiency or with mechanical compres-
RYING OF SOLIDS sors of 70-75% efficiency.
1. Drying times range from a few seconds in spray dryers to 1 hr or EXTRACTION, LIQUID-LIQUID
less in rotary dryers, and up to several hours or even several days
in tunnel shelf or belt dryers. 1. The dispersed phase should be the one that has the higher
2. Continuous tray and belt dryers for granular material of natural volumetric rate except in equipment subject to backmixing
size or pellleted to 3-15 mm have drying times in the range of where it should be the one with the smaller volumetric rate. It
10-200 mnn. should be the phase that wets the material of construction less
3. Rotary cylindrical dryers operate with superficial air velocities of well. Since the holdup of continuous phase usually is greater,
S-10 ft/sec, sometimes up to 35 ft/sec when the material is that phase should be made up of the less expensive or less
coarse. Residence times are 5-90 min. Holdup of solid is 7-8%. hazardous material.