Page 352 - Chemical process engineering design and economics

Separator Design 331

efficiency for steam stripping varies from 25 to 40%. We expect that the efficiency
for air stripping is about the same. If we use the average of 25 and 40, according
to Equation 6.21.8T, the number of actual stages, N A = 3.727 / 0.325 = 11.47. If
we use the same safety factor of 20 % as given in Table 6.30 for fractionators, the
number of trays are 14.
From Equation 6.21.7T, the liquid height at the bottom of the column,
6
Ixl0 gal 1 day 1 h 5 min 1 ft 3 4

1 day 24 h 60 min 1 7.481 gal n (4.0) 2 ft 2
= 36.93 ft (11.26m)

which is unreasonable.

If we use the second equation for L s,

L s = 0.06 (14) + 2.0 = 2.840 m (9.318 ft)

which is somewhat on the high side when compared to the other rules of thumb.

From Table 6.25, the tray spacing is 1.5 ft (0.4572 m). Thus, from Equation
6.21.7T, the column height,

Z =14 (1.5) + 3.0 + 0.25 (4) + 9.318 = 34.31 ft (10.58 m)

After rounding off, the column height is 34.5 ft (10.5 m). Because of the un-
certainty of the column efficiency and other properties, estimates of column di-
ameter and height are usually complemented with testing.

Fractionator Sizing

Occasionally separating multicomponent solutions requires designing a sequence
of fractionators. Henley and Seader [31] discuss some aspects of this problem.
Once the sequence has been established, then estimate the size of each fractiona-
tor. Table 6.27 lists the equations for a short cut method for calculating the height
and diameter of fractionators and Table 6.28 outlines the calculation procedure.
Like rotary drum filtration, absorbers, and strippers, discussed earlier, the final
design may require testing to support the calculations.
As for absorbers and strippers, the height of a fractionator is the sum of the
height occupied by trays or packing plus the heights of the top and bottom sections

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