Page 17 - Chemical Process Equipment - Selection and Design
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xiv RULES OF THUMB: SUMMARY
are 0.1-0.3%. Blowdown of 2.5-3.0% of the circulation is DISTILLATION AND GAS ABSORPTION
necessary to prevent excessive salt buildup.
1. Distillation usually is the most economical method of separating
liquids, superior to extraction, adsorption, crystallization, or
others.
CRYSTALLIZATION FROM SOLUTION 2. For ideal mixtures, relative volatility is the ratio of vapor
1. Complete recovery of dissolved solids is obtainable by pressures n12 = P2/P,.
evaporation, but only to the eutectic composition by chilling. 3. Tower operating pressure is determined most often by the
Recovery by melt crystallization also is limited by the eutectic temperature of the available condensing medium, 100-120°F if
composition. cooling water; or by the maximum allowable reboiler
2. Growth rates and ultimate sizes of crystals are controlled by temperature, 150 psig steam, 366°F.
limiting the extent of supersaturation at any time. 4. Sequencing of columns for separating multicomponent mix-
3. The ratio S = C/C,, of prevailing concentration to saturation tures: (a) perform the easiest separation first, that is, the one
concentration is kept near the range of 1.02-1.05. least demanding of trays and reflux, and leave the most difficult
4. In crystallization by chilling, the temperature of the solution is to the last; (b) when neither relative volatility nor feed
kept at most 1-2°F below the saturation temperature at the concentration vary widely, remove the components one by one
prevailing concentration. as overhead products; (c) when the adjacent ordered
5. Growth rates of crystals under satisfactory conditions are in the components in the feed vary widely in relative volatility,
range of 0.1-0.8 mm/hr. The growth rates are approximately the sequence the splits in the order of decreasing volatility; (d)
same in all directions. when the concentrations in the feed vary widely but the relative
6. Growth rates are influenced greatly by the presence of impurities volatilities do not, remove the components in the order of
and of certain specific additives that vary from case to case. decreasing concentration in the feed.
5. Economically optimum reflux ratio is about 1.2 times the
minimum reflux ratio R,.
6. The economically optimum number of trays is near twice the
DISINTEGRATION minimum value N,,,.
1. Percentages of material greater than 50% of the maximum size 7. The minimum number of trays is found with the Fenske-
are about 50% from rolls, 15% from tumbling mills, and 5% Underwood equation
from closed circuit ball mills.
2. Closed circuit grinding employs external size classification and
return of oversize for regrinding. The rules of pneumatic
conveying are applied to design of air classifiers. Closed circuit is 8. Minimum reflux for binary or pseudobinary mixtures is given by
most common with ball and roller mills. the following when separation is esentially complete (xD = 1)
3. Jaw crushers take lumps of several feet in diameter down to 4 in. and D/F is the ratio of overhead product and feed rates:
Stroke rates are 100-300/min. The average feed is subjected to
8-10 strokes before it becomes small enough to escape. R,D/F = l/(n - l), when feed is at the bubblepoint,
Gyratory crushers are suited to slabby feeds and make a more (R, + 1)D/F = n/(a - l), when feed is at the dewpoint.
rounded product.
4. Roll crushers are made either smooth or with teeth. A 24in. 9. A safety factor of 10% of the number of trays calculated by the
toothed roll can accept lumps 14in. dia. Smooth rolls effect best means is advisable.
reduction ratios up to about 4. Speeds are 50-900 rpm. Capacity 10. Reflux pumps are made at least 25% oversize.
is about 25% of the maximum corresponding to a continuous 11. For reasons of accessibility, tray spacings are made 20-24 in.
ribbon of material passing through the rolls. 12. Peak efficiency of trays is at values of the vapor factor
5. Hammer mills beat the material until it is small enough to pass F, = u 6 in the range 1.0-1.2 (ft/sec) m. This range of
through the screen at the bottom of the casing. Reduction ratios F, establishes the diameter of the tower. Roughly, linear
of 40 are feasible. Large units operate at 900 rpm, smaller ones velocities are 2ft/sec at moderate pressures and 6ft/sec in
up to 16,000rpm. For fibrous materials the screen is provided vacuum.
with cutting edges. 13. The optimum value of the Kremser-Brown absorption factor
6. Rod mills are capable of taking feed as large as 50mm and A = K(V/L) is in the range 1.25-2.0.
reducing it to 300 mesh, but normally the product range is 8-65 14. Pressure drop per tray is of the order of 3 in. of water or 0.1 psi.
mesh. Rods are 25-150mm dia. Ratio of rod length to mill 15. Tray efficiencies for distillation of light hydrocarbons and
diameter is about 1.5. About 45% of the mill volume is occupied aqueous solutions are 60-90%; for gas absorption and
by rods. Rotation is at 50-6570 of critical. stripping, 10-20%.
7. Ball mills are better suited than rod mills to fine grinding. The 16. Sieve trays have holes 0.25-0.50 in. dia, hole area being 10% of
charge is of equal weights of 1.5, 2, and 3 in. balls for the finest the active cross section.
grinding. Volume occupied by the balls is 50% of the mill 17. Valve trays have holes 1.5in. dia each provided with a liftable
volume. Rotation speed is 7040% of critical. Ball mills have a cap, 12-14 caps/sqft of active cross section. Valve trays usually
length to diameter ratio in the range 1-1.5. Tube mills have a are cheaper than sieve trays.
ratio of 4-5 and are capable of very fine grinding. Pebble mills 18. Bubblecap trays are used only when a liquid level must be
have ceramic grinding elements, used when contamination with maintained at low turndown ratio; they can be designed for
metal is to be avoided. lower pressure drop than either sieve or valve trays.
8. Roller mills employ cylindrical or tapered surfaces that roll along 19. Weir heights are 2in., weir lengths about 75% of tray diameter,
flatter surfaces and crush nipped particles. Products of 20-200 liquid rate a maximum of about 8gpm/in. of weir; multipass
mesh are made. arrangements are used at high liquid rates.