Page 305 - Chemical process engineering design and economics

284 Chapter 6

r 7.83 (4) Y /2
D = | ————— | = 3.157 ft (0.9677 m)
I 3.142 )
According to step five in Table 6.8, round off the diameter to 3.5 ft (1.07 m).

Finally, from Equation 6.7.5, the length of the drum is

L = 2 (3.5) = 7.0 ft (2.13m)

Vertical Gas-Liquid Separators

There are several design procedures reported in the literature - not all of them are
in agreement. A schematic diagram of a vertical gas-liquid separator is shown in
Figure 6.4. Gas-liquid separators may be designed for horizontal or vertical opera-
tion, but Younger [11] found that for seven separators in use, with L/D varying
from 1.7 to 3.6, all were installed vertically. This is consistent with the rule given
by Branan [49] that if L/D > 5, a horizontal separator should be used. Equations
for sizing vertical gas-liquid separators are summarized in Table 6.9, and a calcu-
lation procedure is outlined in Table 6.10. The volume of the dished heads is not
included in the calculation procedure. As for sizing knockout drums, first calculate
the drum diameter by solving Equations 6.9.1 to 6.9.4.
Next calculate the droplet settling length. This is the length from the center
line of the inlet nozzle to the bottom of the mist eliminator. Scheiman [72] rec-
ommends that the settling length should be to 0.75 D or a minimum of 12 in
(0.305 m) whereas Gerunda [4] specifies a length equal to the diameter or a mini-
mum of 3 ft (0.914 m). Gerunda's recommendation is used in Figure 6.4.
Also, to prevent flooding the inlet nozzle, Scheiman allows a minimum of 6
in (0.152 m) from the bottom of the nozzle to the liquid surface or a minimum of
12 in (0.305 m) from the center line of the nozzle to the liquid surface. Branan
[49] recommends using 12 in (0.305 m) plus % of the inlet nozzle outside diameter
or 18 in (0.4570 m) minimum. Gerunda specifies a length equal to 0.5 D or 2 ft
(0.610 m) minimum, which is used in Figure 6.4.
Now calculate the liquid height. The separator is also sized as an accumula-
tor to dampen variations in the liquid flow rate by allowing sufficient liquid resi-
dence time or surge time in the separator. Scheiman [72] recommends a surge time
in the range of 2 to 5 min, whereas Younger [11] recommends 3 to 5 min. In Ta-
ble 6.9, 3 to 5 min is selected. There is a minimum liquid height required to pre-
vent a vortex from forming. The design of the separator will have to include a vor-
tex breaker. The minimum liquid level should cover the vortex breaker plus an
additional liquid height. Experiments conducted by Patterson [16] showed that the

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