Page 280 - Chemical process engineering design and economics

Compressors, Pumps, and Turbines 259

The skirt diameter is equal to the diameter of the column, and its height var-
ies from 2 to 12 ft (0.61 to 3.66 m). The height of the skirt is determine by mainte-
nance requirements. Maintenance workers need space to repair the bottom section
of a column. Columns could also be supported on a structure. Assume that the
skirt height for the absorber, L Ai, in Figure 5.4.2, is 2.0 m (6.56 ft).
To dampen flow-rate fluctuations, requires liquid holdup at the bottom of the
column. Ludwig [54] recommends 5 to 20 min for the surge time, i.e., the liquid
residence time at the bottom of a column. On the other hand, it is desirable to
minimize the solvent inventory in a process to minimize cost and to minimize the
amount of flammable liquids. Also, if the liquid contains heat-sensitive organic
compounds, it is necessary to reduce the residence time, particularly in strippers,
where the temperature is high. For this problem, select a surge time of 5.0 min to
keep the residence time low for the stated reasons. Therefore, the liquid height in
the absorber,

2.08 m 3 5.0 min 4
LAZ = ————— ———— —————— = 4.323 m(14.2 ft)
2
1 min 1 7i(1.75) m 2
The distance between the liquid level and the gas inlet in the absorber, L A3,
is 1.5 m (4.92 ft), and the distance from the gas inlet to the bottom tray, L A4, is 0.9
m (2.95 ft). From Table 5.4.1, it is seen that there are 26 trays with a spacing of
0.61 m (2.0 ft) Thus, the trays for the absorber will occupy a height of

L A5 = 0.61 (26 - 1) = 15.25 m (50 ft)

The elevation of the pump discharge line,
Z[ = L Ai + LAJ + L A3 + L A4 + L A5

zi = 2.0 + 4.323 + 1.5 + 0.9 + 15.25 = 24.0 m (78.7 ft)

The elevation for the suction side of the pump consists of the sum of the
stripper-skirt height and the liquid level at bottom of the stripper. Again, assume
that the skirt height is 2.0 m (6.56 ft).
To calculate the liquid height, L S2, we again assume 5.0 min for the surge
time.

2.08 m 3 5.0 min 4
L S2 = ———— ————— ———————— = 2.280 m (7.48 ft)
2
1 min 1 7t(2.412) m 2
Therefore, the elevation of the pump suction line,

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