Rules Of Thumb : Distillation And Gas Absorption

1. Distillation usually is the most economical method of separating liquids, superior to extraction, adsorption, crystallization,or others.
2. For ideal mixtures, relative volatility is the ratio of vapor pressures a12 1⁄4 P2 =P1 .
3. For a two-component, ideal system, the McCabe-Thiele method offers a good approximation of the number of equilibrium stages.
4. Tower operating pressure is determined most often by the temperature of the available condensing medium, 100–1208F if cooling water; or by the maximum allowable reboiler temperature, 150 psig steam, 3668F.
5. 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) 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.
6. Flashing may be more economical than conventional distillation but is limited by the physical properties of the mixture.
7. Economically optimum reflux ratio is about 1.25 times the minimum reflux ratio Rm.
8. The economically optimum number of trays is nearly twice the minimum value Nm .
9. The minimum number of trays is found with the Fenske–Underwood equation
   Nm 1⁄4 log {[x=(1 À x)]ovhd =[x=(1 À x)]btms }= log a:
10. 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 product and feed rates:
    Rm D=F 1⁄4 1=(a À 1), when feed is at the bubblepoint,
    (Rm þ 1)D=F 1⁄4 a=(a À 1), when feed is at the dewpoint:
11. A safety factor of 10% of the number of trays calculated by the best means is advisable.
12. Reflux pumps are made at least 25% oversize.
13. For reasons of accessibility, tray spacings are made 20–30 in.
14. Peak efficiency of trays is at values pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of the vapor factor
    pffiffiffiffiffi Fs 1⁄4 u rv in the range 1.0–1.2 (ft/sec) lb=cuft. This range of Fs establishes the diameter of the tower. Roughly, linear velocities are 2 ft/sec at moderate pressures and 6 ft/sec in vacuum.
15. The optimum value of the Kremser–Brown absorption factor A 1⁄4 K(V =L) is in the range 1.25–2.0.
16. Pressure drop per tray is of the order of 3 in. of water or 0.1 psi.
17. Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60–90%; for gas absorption and strip- ping, 10–20%.
18. Sieve trays have holes 0.25–0.50 in. dia, hole area being 10% of the active cross section.
19. Valve trays have holes 1.5 in. dia each provided with a liftable cap, 12–14 caps/sqft of active cross section. Valve trays usually are cheaper than sieve trays.
20. Bubblecap trays are used only when a liquid level must be maintained at low turndown ratio; they can be designed for lower pressure drop than either sieve or valve trays.
21. Weir heights are 2 in., weir lengths about 75% of tray diameter, liquid rate a maximum of about 8 gpm/in. of weir; multipass arrangements are used at high liquid rates.
22. Packings of random and structured character are suited especially to towers under 3 ft dia and where low pressure drop is desirable. With proper initial distribution and periodic redistribution, volumetric efficiencies can be made greater than those of tray towers. Packed internals are used as replacements for achieving greater throughput or separation in existing tower shells.
23. For gas rates of 500 cfm, use 1 in. packing; for gas rates of 2000 cfm or more, use 2 in.
24. The ratio of diameters of tower and packing should be at least 15.
25. Because of deformability, plastic packing is limited to a 10–15 ft depth unsupported, metal to 20–25 ft.
26. Liquid redistributors are needed every 5–10 tower diameters with pall rings but at least every 20 ft. The number of liquid streams should be 3–5/sqft in towers larger than 3 ft dia (some experts say 9–12/sqft), and more numerous in smaller towers.
27. Height equivalent to a theoretical plate (HETP) for vapor– liquid contacting is 1.3–1.8 ft for 1 in. pall rings, 2.5–3.0 ft for 2 in. pall rings.
28. Packed towers should operate near 70% of the flooding rate given by the correlation of Sherwood, Lobo, et al.
29. 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 0.5 ft/sec, minimum diameter of 16 in.
30. For towers about 3 ft dia, add 4 ft at the top for vapor disengagement and 6 ft at the bottom for liquid level and reboiler return.
31. Limit the tower height to about 175 ft max because of wind load and foundation considerations. An additional criterion is that L/D be less than 30.

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