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).
TRAY TOWERS
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.
7 7. Bubble cap trays are used only when liquid level must be maintained at low turndown ratio; they can be designed for lower pressure drop than either sieve or valve trays.
8 8. Weir heights are 50 mm (2 in.), weir lengths are about 75% of trays diameter, and liquid rate a maximum of about 1.2 m3/min-m of weir (8 gpm/in. of weir); multi-pass arrangements are used at higher liquid rates.
PACKED TOWERS
1 1. Structured and random packings are suitable for packed towers less than 0.9 m (3 ft) when low pressure drop is required.
2 2. Replacing trays with packing allows greater throughput and separation in existing tower shells.
3 3. For gas rates of 14.2 m3/min (500 ft3/min), use 25.4-mm (1-in.) packing; for 56.6m3/min (2000 ft3/min) or more use 50-mm (2-in.) packing.
4 4. Ratio of tower diameter/packing diameter should be > 15/1.
5 5. Because of deformability, plastic packing is limited to 3–4 m (10–15 ft) and metal packing to 6.0–7.6 m (20–25 ft) unsupported depth.
6 6. Liquid distributors are required every 5–10 tower diameters with pall rings and at least every 6.5 m (20 ft) for other types of dumped packing.
7 7. Number of liquid distributions should be >32–55/m2 (3–5/ft2) in towers greater than 0.9 m (3 ft) diameter and more numerous in smaller columns.
8 8. Packed towers should operate near 70% of the flooding rate (evaluated from Sherwood and Lobo correlation).
9 9. Height Equivalent to a Theoretical Stage (HETS) for vapor–liquid contacting is 0.4–0.56 m (1.3–1.8 ft) for 25-mm (1-in.) pall rings and 0.76–0.9 m (2.5–3.0 ft) for 50-mm (2-in.) pall rings.
10 10.Generalized pressure dropsDesign pressure drops (cm of H2O/m of packing)Design pressure drops (in. of H2O/ft of packing)Absorbers and Regenerators (non-foaming systems)2.1–3.30.25–0.40Absorbers and Regenerators0.8–2.10.10–0.25Atmospheric/Pressure Stills and Fractionators3.3–6.70.40–0.80Vacuum Stills and Fractionators0.8–3.30.10–0.40Maximum value8.331.0
DRIVERS AND POWER RECOVERY EQUIPMENT
1 1. Efficiency is greater for larger machines. Motors, 85–95%; steam turbines, 42–78%; gas engines and turbines, 28–38%.
2 2. For under 74.6 kW (100 hp), electric motors are used almost exclusively. They are made for up to 14,900 kW (20,000 hp).
3 3. Induction motors are most popular. Synchronous motors are made for speeds as low as 150 rpm and are thus suited, for example, for low-speed reciprocating compressors, but are not made smaller than 50 hp. A variety of enclosures are available, from weather-proof to explosion-proof.
4 4. Steam turbines are competitive above 76.6 kW (100 hp). They are speed-controllable. They are frequency used as spares in case of power failure.
5 5. Combustion engines and turbines are restricted to mobile and remote locations.
6 6. Gas expanders for power recovery may be justified at capacities of several hundred hp; otherwise any pressure reduction in a process is done with throttling valves.
7 7. The following useful definitions are given:
DRYING OF SOLIDS
1 1. Drying times range from a few seconds in spray dryers to 1 h or less in rotary dryers and up to several hours or even several days in tunnel shelf or belt dryers.
2 2. Continuous tray and belt dryers for granular material of natural size or pelleted to 3–15 mm have drying in the range of 10–200 min.
3 3. Rotary cylindrical dryers operate with superficial air velocities of 1.52–3.05 m/s (5–10 ft/s), sometimes up to 10.67 m/s (35 ft/s) when the material is coarse. Residence times are 5–90 min. Holdup of solid is 7–8%. An 85% free cross section is taken for design purposes. In countercurrent flow, the exit gas is 10–20°C above the solid; in parallel flow, the temperature of the exit solid is 100°C. Rotation speeds of about 4 rpm are used, but the product of rpm and diameter in feet is typically between 15 and 25.
4 4. Drum dryers for pastes and slurries operate with contact times of 3–12 s, and produce flakes 1–3 mm thick with evaporation rates of 15–30 kg/m2-h. Diameters are in the range of 1.5–5.0 ft; and rotation rate is 2–10 rpm. The greatest evaporative capacity is of the order of 1360.7 kg/h (3000 lb/h) in commercial units.
5 5. Pneumatic conveying dryers normally take particles 1–3 mm diameter but up to 10 mm when the moisture is mostly on the surface. Air velocities are 10–30 m/s. Single-pass residence times are 0.5–3.0 s, but with normal recycling the average residence time is brought up to 60 s. Units in use range from 0.2 m in diameter by 1 m long to 0.3 m in diameter by 38 m long. Air requirement is several SCFM per lb of dry product/h.
6 6.
