A. Kayode Coker - Petroleum Refining Design and Applications Handbook

4 4. Spray towers as tall as 6–12 m (20–40 ft) cannot be depended on to function as more than a single stage.

5 5. Packed towers are employed when 5–10 stages suffice. Pall rings 25–38 mm (1–1.5 in.) in size are best. Dispersed-phase loadings should not exceed 10.2m3/min-m2 (25 gal./min-ft2), and HETS of 1.5–3.0 m (5–10 ft) may be realized. The dispersed phase must be redistributed every 1.5–2.1 m (5–7 ft). Packed towers are not satisfactory when the surface tension is more than 10 dyne/cm.

6 6. Sieve tray towers have holes of only 3–8 mm diameter. Velocities through the holes are kept below 0.24 m/s (0.8 ft/s) to avoid formation of small drops. Re-dispersion of either phase at each tray can be designed for. Tray spacings are 152–600 mm (6–24 in). Tray efficiencies are in the range of 20–30%.

7 7. Pulse packed and sieve tray towers may operate at frequencies of 90 cycles/min and amplitudes of 6–25 mm. In large-diameter tower, HETS of about 1 m has been observed. Surface tensions as high as 30–40 dyn/cm have no adverse effect.

8 8. Reciprocating tray towers can have holes of 150 mm (9/16 in.) diameter, 50–60% open area, stroke length 190 mm (0.75 in.), 100–150 strokes/min, and plate spacing normally 50 mm (2 in.) but in the range of 25.0–150 mm (1–6 in.). In a 760-mm (30-in.) diameter tower, HETS is 500–650 mm (20–25 in.) and throughput is 13.7 m3/min-m2 (2000 gal./h-ft2). Power requirements are much less than those of pulsed towers.

9 9. Rotating disk contractors or other rotary agitated towers realize HETS in the range of 0.1–0.5 m (0.33–1.64 ft). The especially efficient Kuhni with perforated disks of 40% free cross section has HETS of 0.2 m (0.66 ft) and a capacity of 50 m3 /m2-h (164 ft3/ft2-h).

1 1. Process are classified by their rate of cake buildup in a laboratory vacuum leaf filter: rapid, 0.1–10.0 cm/s; medium, 0.1–10.0 cm/min; and slow, 0.1–10.0 cm/h.

2 2. Continuous filtration should not be attempted if 1/8 in. cake thickness cannot be formed in less than 5 min.

3 3. Rapid filtering is accomplished with belts, top feed drums, or pusher centrifuges.

4 4. Medium rate filtering is accomplished with vacuum drums or disks or peeler centrifuges.

5 5. Slow-filtering slurries are handled in pressure filters or sedimenting centrifuges.

6 6. Clarification with negligible cake buildup is accomplished with cartridges, precoat drums, or sand filters.

7 7. Laboratory tests are advisable when the filtering surface is expected to be more than a few square meters, when cake washing is critical, when cake drying may be a problem, and when precoating may be needed.

8 8. For finely ground ores and minerals, rotary drum filtration rates may be 15,000 lb/day-ft2 at 20 rev/h and 18–25 in. Hg vacuum.

9 9. Coarse solids and crystals may be filtered at rates of 6000 lb/day-ft2 at 20 rev/h and 2–6 in. Hg vacuum.

1 1. Properties of particles that are conducive to smooth fluidization include rounded or smooth shape, enough toughness to resist attrition, sizes in the range of 50–500 µm diameter, and a spectrum of sizes with ratio of largest to smallest in the range of 10–25.

2 2. Cracking catalysts are members of a broad class characterized by diameters of 30–150 µm, density of 1.5 g/ml or so, and appreciable expansion of the bed before fluidization sets in, minimum bubbling velocity greater than minimum fluidizing velocity, and rapid disengagement of bubbles.

3 3. The other extreme of smoothly fluidizing particles are typified by coarse sand and glass beads, both of which have been the subject of much laboratory investigation. Their sizes are in the range of 150–500 µm, densities 1.5–4.0 g/ml, have small bed expansion and about the same magnitudes of minimum bubbling and minimum fluidizing velocities, and they also have rapidly disengaging bubbles.

4 4. Cohesive particles and large particles of 1 mm or more do not fluidize well and usually are processed in other ways.

5 5. Rough correlations have been made of minimum fluidization velocity, minimum bubbling velocity, bed expansion, bed level fluctuation, and disengaging height. Experts recommend, however, that any real design be based on pilot-plant work.

6 6. Practical operations are conducted at two or more multiples of the minimum fluidizing velocity. In reactors, the entrained material is recovered with cyclones and returned to process. In driers, the fine particles dry most quickly so the entrained material need not be recycled.

1 1. For conservative estimate set F = 0.9 for shell and tube exchangers with no phase changes, q = UAF∆Tlm. When ∆T at exchanger ends differ greatly then check F, reconfigure if F is less than 0.85.

2 2. Take true countercurrent flow in a shell-and-tube exchanger as a basis.

3 3. Standard tubes are 19.0 mm (3/4 in.) outer diameter (OD), 25.4 mm (1 in.) triangular spacing, 4.9 m (16 ft) long.A shell of 300 mm (1 ft) diameter accommodates 9.3 m2 (100 ft2);600 mm (2 ft) diameter accommodates 37.2 m2 (400 ft2);900 mm (3 ft) diameter accommodates 102 m2 (1100 ft2).

4 4. Tube side is for corrosive, fouling, scaling, and high-pressure fluids.

5 5. Shell side is for viscous and condensing fluids.

6 6. Pressure drops are 0.1 bar (1.5 psi) for boiling and 0.2–0.62 bar (3–9 psi) for other services.

7 7. Minimum temperature approach is 10°C (20°F) for fluids and 5°C (10°F) for refrigerants.

8 8. Cooling water inlet temperature is 30°C (90°F), maximum outlet temperature 49°C (120°F).

9 9. Heat-transfer coefficients for estimating purposes, W/m2°C (Btu/h-ft2-°F): water to liquid, 850 (150); condensers, 850 (150); liquid to liquid, 280 (50); liquid to gas, 60 (10); gas to gas, 30 (5); and reboiler 1140 (200). Maximum flux in reboiler is 31.5 kW/m2 (10,000 Btu/h-ft2). When phase changes occur, use a zoned analysis with appropriate coefficient for each zone.

10 10. Double-pipe exchanger is competitive at duties requiring 9.3–18.6 m2 (100–200 ft2).

11 11. Compact (plate and fin) exchangers have 1150 m2/m3 (350 ft2/ft3), and about four times the heat transfer per cut of shell-and-tube units.

12 12. Plate and frame exchangers are suited to high sanitation services and are 25–50% cheaper in stainless steel construction than shell-and-tube units.

13 13. Air coolers: Tubes are 0.75–1.00 in. OD., total finned surface 15–20 ft2/ft2 bare surface, U = 450–570 W/m2°C (80–100 Btu/h-ft2 (bare surface)-°F). Minimum approach temperature = 22°C (40°F). Fan input power = 1.4–3.6 kW/(MJ/h) [2–5 hp/(1000 Btu/h)].

14 14. Fired heaters: radiant rate, 37.6 kW/m2 (12,000 Btu/h-ft2), convection rate, 12.5 kW/m2 (4000 Btu/h-ft2); cold oil tube velocity = 1.8 m/s (6 ft/s); approximately equal heat transfer in the two sections; thermal efficiency, 70–75%; flue gas temperature, 140–195°C (250–350°F) above feed inlet; and stack gas temperature, 345–510°C (650–950°F).

1 1. Up to 345°C (650°F), 85% magnesia is used.

2 2. Up to 870–1040°C (1600–1900°F), a mixture of asbestos and diatomaceous earth is used.

3 3. Ceramic refractories at higher temperatures.

4 4. Cryogenic equipment −130°C (−200°F) employs insulations with fine pores of trapped air, for example, PerliteTM.

5 5. Optimum thickness varies with temperature: 12.7 mm (0.5 in.) at 95°C (200°F), 25.4 mm (1.0 in.) at 200°C (400°F), 32 mm (1.25 in.) at 315°C (600°F).

6 6. Under windy conditions, 12.1 km/h (7.5 miles/h), 10–20% greater thickness of insulation is justified.