A. Kayode Coker - Petroleum Refining Design and Applications Handbook

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.

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.

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.

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 is near twice the minimum value Nm.

7 7. The minimum number of trays is found with the Fenske–Underwood equation:

8 8. Minimum reflux for binary or pseudobinary mixtures is given by the following when separation is essentially complete (xD ≌ 1) and D/F is the ratio of overhead products to feed rate:

9 9. A safety factor of 10% of the number of trays calculated by the best means is advisable.

10 10. Reflux pumps are made at least 10% oversize.

11 11. The optimum value of the Kremser—Brown absorption factor A = (L/VK) is in the range 1.25–2.0.

12 12. Reflux drums usually are horizontal, with a liquid holdup of 5 min half-full. A takeoff pot for a second liquid phase, such as water in hydrocarbon systems, is sized for a linear velocity of that phase of 0.15 m/s (0.5 ft/s) minimum diameter of 406.4 mm (16 in.).

13 13. For towers about 914 mm (3 ft) diameter, add 1219 mm (4 ft) at the top for vapor disengagement and 1829 mm (6 ft) at the bottom for liquid level and reboiler return.

14 14. Limit the tower height to about 53 m (175 ft) maximum because of wind load and foundation considerations. An additional criterion is that L/D be less than 30 (20 < L/D < 30 often will require special design).

1 1. For reasons of accessibility, tray spacings are made 0.5–0.6 m (20–24 in.).

2 2. Peak efficiency of trays is at values of the vapor factor Fs = µ(ρv)0.5 in the range of 1.2–1.5 m/s (kg/m3)0.5 [1–1.2 ft/s (lb/ft3)0.5]. This range of Fs establishes the diameter of tower. Roughly, linear velocities are 0.6 m/s (2 ft/s) at moderate pressures and 1.8 m/s (6 ft/s) in vacuum.

3 3. Pressure drop per tray is of the order of 747 Pa (3 in. water) or 689.5 Pa (0.1 psi).

4 4. Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60–90%; for gas absorption and stripping, 10–20%.

5 5. Sieve trays have holes of 6–7 mm (0.25–0.50 in.) diameter, hole area being 10% of the active cross section.

6 6. Valve trays have holes of 38 mm (1.5 in.) diameter, each provided with a liftable cap, with 130–150 caps per square meter (12–14 caps per square feet) of active cross section. Valve trays are usually cheaper than sieve trays.