Distillation and Gas Absorption Design Rules of Thumb:

1. For ideal mixtures, relative volatility is the ratio of vapour pressures α 12 = p2 / p1

2. Tower operating pressure is determined most often by the temperature of the available condensing medium, 100―120 °F if cooling water; or by the maximum allowable reboiler temperature, 150 psig steam, 366˚F.
3. Sequencing of columns for separating multi-component 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 product; 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
4. Economically optimum reflux ratio is about 1.2 times the minimum reflux ration Rm

5. A safety factor of 10% of the number of trays calculated by the best means is advisable.
6. Reflux pumps are made at least 25% oversize.

7. For reasons of accessibility, tray spacings are made 20-24 in.

8. Peak efficiency of trays is at values of the vapour factor FS  = U PV    in the range 1.0-1.2 ( ft / sec) (lb / cuft)^0.5. 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.

9. Pressure drop per tray is of the order of 3 in. of water or 0.1 psi.

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

11. Sieve trays have holes 0.25―0.50 in. dia, whole area being 10% of the active cross section.
12. Valve trays have holes 1.5 in. dia each provided with a table cap, 12-14 caps/sqft of active cross section.

13. Bubblecap trays are used only when a liquid level must be maintained at turndown pressure drop than either sieve or valve trays.
14. Weir heights are 2in., weir lengths about 75% of tray diameter, liquid rate a maximum of about 8 rates.