Belt conveyors are for high capacity and long distances (a mile or more, but only several hundred feet in a plant), up inclines of 30° maximum. A 609.6-mm (24 in.) wide belt can carry 84.95 m3/h (3000 ft3/h) at a speed of 0.508 m/s (100 ft/min), but speeds up to 3.048 m/s (600 ft/min) are suited to some materials. Power consumption is relatively low.3 3. Bucket elevators are suited to vertical transport of sticky and abrasive materials. With 508 × 508-mm (20 × 20-in.) buckets, capacity can reach 28.3 m3/h (1000 ft3/h) at a speed of 0.508 m/s (100 ft/min), but speeds up to 1.524 m/s (300 ft/min) are used.
4 4. Drag-type conveyors (Redler) are suited to short distances in any direction and are completely enclosed. Units range in size from 19.4 × 10−4 to 122.6 × 10−4 m2 (3–19 in.2) and may travel from 0.15 m/s (30 ft/min) (fly ash) to 1.27 m/s (250 ft/min) (grains). Power requirements are high.
5 5. Pneumatic conveyors are for high capacity, short distance (122 m (400 ft)) transport simultaneously from several sources to several destinations. Either vacuum or low pressure 0.4–0.8 barg (6–12 psig) is used with a range of air velocities from 10.7 to 36.6 m/s (35–120 ft/s); depending on the material and pressure and air requirements, 0.03–0.2 m3/m3 (1–7 ft3/ft3) of solid is transferred.
COOLING TOWERS
1 1. Water in contact with air under adiabatic conditions eventually cools to the wet bulb temperature.
2 2. In commercial units, 90% of saturation of the air is feasible.
3 3. Relative cooling tower size is sensitive to the difference between the exit and the wet bulb temperatures:∆T, °F51525Relative volume2.41.00.55
4 4. Tower fill is of a highly open structure so as to minimize pressure drop, which is in standard practice a maximum of 497.6 Pa (2 in. of water).
5 5. Water circulation rate is 48.9–195.7 L/min m2 (1–4 gpm/ft2) and air rate is 6344–8784 kg/h m2 (1300–1800 lb/h ft2) or 1.52–2.03 m/s (300–400 ft/min).
6 6. Chimney-assisted natural draft towers are hyperboloidally shaped because they have greater strength for a given thickness; a tower 76.2 m (250 ft) high has concrete walls 127–152.4 mm (5–6 in.) thick. The enlarge cross section at the top aids in dispersion of exit humid air into the atmosphere.
7 7. Countercurrent-induced draft towers are the most common in process industries. They are able to cool water within 2°F of the wet bulb.
8 8. Evaporation losses are 1% of the circulation for every 10°F of cooling range. Windage or drift losses of mechanical draft towers are 0.1–0.3% Blowdown of 2.5–3.0% of the circulation is necessary to prevent excessive salt buildup.
CRYSTALLIZATION FROM SOLUTION
1 1. Complete recovery of dissolved solids is obtainable by evaporation, but only to the eutectic composition by chilling. Recovery by melt crystallization also is limited by the eutectic composition.
2 2. Growth rates and ultimate sizes of crystals are controlled by limiting the extent of supersaturation at any time.
3 3. The ratio S = C/Csat of prevailing concentration to saturation concentration is kept near the range 1.02–1.05.
4 4. In crystallization by chilling, the temperature of the solution is kept almost 1–2°F below the saturation temperature at the prevailing concentration.
5 5. Growth rates of crystals under satisfactory conditions are in the range of 0.1–0.8 mm/h. The growth rates are approximately the same in all directions.
6 6. Growth rates are influenced greatly by the presence of impurities and of certain specific additives, which vary from case to case.
DISINTEGRATION
1 1. Percentages of material greater than 50% of the maximum size are about 50% from rolls, 15% from tumbling mills, and 5% from closed-circuit ball mills.
2 2. Closed-circuit grinding employs external size classification and return of oversize for regrinding. The rules of pneumatic conveying are applied to the design of air classifiers. Closed circuit is most common with ball and roller mills.
3 3. Jaw crushers take lumps of several feet in diameter to 102 mm (4 in.). Stroke rates are 100–300/min. The average feed is subjected to 8–10 strokes before it becomes small enough to escape. Gyratory crushers are suited to slabby feeds and makes a more rounded product.
4 4. Roll crushers are made either smooth or with teeth. A 610-mm (24-in.) toothed roll can accept lumps of 356 mm (14 in.) diameter. Smooth rolls affect reduction ratios up to about 4. Speeds are 50–90 rpm. Capacity is about 25% of the maximum, corresponding to a continuous ribbon of material passing through the rolls.
5 5. Hammer mills beat the material until it is small enough to pass through the screen at the bottom of the casing. Reduction ratios of 40 are feasible. Large units operate at 900 rpm, smaller ones up to 16,000 rpm. For fibrous materials the screen is provided with cutting edges.
6 6. Rod mills are capable of taking feed as large as 50 mm and reducing it to 300 mesh, but normally the product range is 8–65 mesh. Rods are 25–150 mm in diameter. The ratio of rod length to mill diameter is about 1.5. About 45% of the mill volume is occupied by rods. Rotation is at 50–65% of critical.
7 7. Ball mills are better suited than rod mills to fine grinding. The charge is of equal weights of 1.5-, 2-, and 3-in. balls for the finest grinding. The volume occupied by the balls is 50% of the mill volume. Rotation speed is 70–80% of critical. Ball mills have a length-to-diameter ratio in the range 1–1.5. Tube mills have a ratio of 4–5 and are capable of very find grinding. Pebble mills have ceramic grinding elements, used when contamination with metal is to be avoided.
8 8. Roller mills employ cylindrical or tapered surfaces that roll along flatter surfaces and crush nipped particles. Products of 20–200 mesh are made.
TOWERS
1 1. Distillation usually is the most economical method of separating liquids, superior to extraction, absorption, crystallization, or others.
2 2. For ideal mixtures, relative volatility is the ratio of vapor pressure, α12 = P2/P1.
3 3. Tower operating pressure is most often determined by the temperature of the available condensing medium, 38–50°C (100–120°F) if cooling water, or by the maximum allowable reboiler temperature, 10.34 barg (150 psig) steam, 186°C (366° F) to avoid chemical decomposition/degradation.
4 4. Sequencing of columns for separating multicomponent mixtures:a. Perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last.b. When neither relative volatility nor feed concentration vary widely, remove the components one by one as overhead products.c. When the adjacent ordered components in the feed vary widely in relative volatility, sequence the splits in the order of decreasing volatility.d. And when the concentrations in the feed vary widely but the relative volatilities do not, remove the components in the order of decreasing concentration in the feed.
5 5. The economically optimum reflux ratio is about 1.2–1.5 times the minimum reflux ratio Rm.
6 6. The economically optimum number of theoretical trays
