Petroleum Refining Design and Applications Handbook. A. Kayode Coker

Чтение книги онлайн.

      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.

      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

      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:

      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. СКАЧАТЬ