DISTILLATION / GAS ABSORTION 

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DISTILLATION AND GAS ABSORPTION A 1
Distillation usually is the most economical method of separating liquids, superior to extraction, adsorption,
crystallization, or others. Exceptions to this rule include: Flash separation
when flash separation is sufficient and Settling (decanting or coalescing) when the mixture has LL immiscibility without addition
of extraction solvent. 2
For ideal mixtures, relative volatility is the ratio of vapor pressures
α12 = P2/ P1. 3
Tower operating pressure is determined most often by the temperature of the available condensing medium,
100120 F if cooling water; or by the maximum allowable reboiler temperature, 150 psig steam, 366 F. 4
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. 5
Economically optimum reflux ratio is about 1.2 times the minimum reflux ratio Rm. 6
The economically optimum number of trays is near twice the minimum value Nm. 7
The minimum number of trays is found with the Fenske Underwood equation
Nm, =
log{[x/(1 x)]ovhd/[x/(1 x)]btms}/log α. 8
Minimum reflux for binary or psuedobinary 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: RmD/F = 1/( α1), when feed is at the bubblepoint; (Rm + 1)D/F
= α/(α1), when feed is at the dewpoint. DISTILLATION AND GAS ABSORPTION
B 1
A safety factor of 10% of the number of trays calculated by the best means is advisable. 2
Reflux pumps are made at least 25% oversize. 3
For reasons of accessibility, tray spacings are made 2024 in. 4
Peak efficiency of trays is at values of the vapor factor Fs = u(ρv)^{0.5} in the range 1.01.2 (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. 5
The optimum value of the KremserBrown absorption factor A = K(V / L) is in the range 1.252.0. 6
Pressure drop per tray is of the order of 3 in. of water or 0.1 psi. 7
Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 6090%; for gas absorption
and stripping, 1020%. 8
Sieve trays have holes 0.250.50 in. dia, hole area being 10% of the active cross section. 9
Valve trays have holes 1.5 in. dia each provided with a liftable cap, 1214 caps/sqft of active cross
section. Valve trays usually are cheaper than sieve trays. 10
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. 11
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. 12
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. DISTILLATION AND GAS ABSORPTION C 1
For gas rates of 500 cfm, use 1 in. packing; for gas rates of 2000 cfm or more, use 2 in. 2
The ratio of diameters of tower and packing should be at least 15. 3
Because of deformability, plastic packing is limited to a 1015 ft depth unsupported, metal to 2025 ft.
4
Liquid redistributors are needed every 510 tower diameters with pall rings but at least every 20 ft.
The number of liquid streams should be 35/sqft in towers larger than 3 ft dia (some experts say 912/sqft), and more numerous
in smaller towers. 5
Height equivalent to a theoretical plate (HETP) for vaporliquid contacting is 1.31.8 ft for 1 in. pall
rings, 2.53.0 ft for 2 in. pall rings. 6
Packed towers should operate near 70% of the flooding rate given by the correlation of Sherwood, Lobo,
et al. 7
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. 8
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. 9
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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