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Separator Design 283

Example 6.2 Sizing a Compressor Knock-Out Drum______________

A gas stream having the composition given in Table 6.2.1 flows into a compressor
suction. Size the knockout drum to prevent liquid from entering the compressor.
The gas enters the drum at 105 °F (40.6 °C) and 150 psig (10.3 bar).

Table 6.2.1 Gas Composition_________________________

Gas Flow
Rate 3
Ibmol/h

2312.8
H 2
277.5
CH 4
C 2H 6 246.7
C 3H 8 185.0
i-Butane 61.7

a) To covert to kgmol/h multiply by 0.4536.

Follow the calculation procedure outlined in Table 6.8. Assume that the
drum will have a mist eliminator. From Equation 6.7.2, k v = 0.35 ft/s (0.107 m/s).
The effect of the mist eliminator is to increase the maximum allowable velocity
and therefore to reduce the drum diameter. The densities obtained from ASPEN
3
3
[57] are: p v = 0.2493 lb/ft 3 (3.99 kg/m ) and p L = 33.19 lb/ft 3 (532 kg/m ). The
3
3
5
volumetric flow rate, also obtained from ASPEN, is 1.134xl0 ft /h (3210 m /h).
From Equation 6.7.1, the maximum-allowable gas velocity,
( 33.19-0.2493 Y /2
v v = 0.35 I ————————— I = 4.023 ft/s (1.23 m/s)
I 0.2493 J
From Equation 6.7.3, the cross-sectional area,

5 3
1
1.134xl0 ft /h 1 1
= 7.830 ft 2
1 3600 s/h 4.023 fl/s
The drum diameter from Equation 6.7.4 is

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