Page 342 - Chemical process engineering design and economics

Separator Design 321

For stripping, the entering liquid and gas concentrations are known. The
fraction stripped and therefore the exit liquid concentration is also known, but the
exit gas concentration is unknown. Therefore point 2 - at the bottom of the col-
umn is fixed, but point 1 - at the top of the column - is not fixed. The maximum
exit and minimum gas flow rate gas concentration is obtained when the operating
line intersects the equilibrium curve, as shown by the dashed line in Figure 6.16.
The intersection is given by Equation 6.21.1S, which is also obtained by the simul-
taneous solution of a component balance and an equilibrium relation.
After finding the minimum flow rate, the optimum or operating flow rate
can be calculated by using the rules-of-thumb from Treybal [29], which are given
by Equations 6.21.3A or 6.21.3S. The 1.5 given in the equations is within the
range of 1.2 to 2.0 for both absorbers and strippers given by McNulty [36].
To minimize channeling of liquid in packed absorbers and strippers require
that the packing be sufficiently small when compared to the column diameter.
Small packing, however, will result in a high pressure drop.Treybal [29] specifies
that the ratio of separator diameter to the packing diameter should be 15/1.
The recovery of the key component is specified to calculate the exit compo-
sition of the gas stream for absorbers or the exit composition of the liquid stream
for strippers from Equation 6.21.2A or 6.21.2S. For both cases, it is assumed that
the operating line intersects the equilibrium curve at one end and not at some in-
termediate point between the ends of the operating line. This is the case for dilute
solutions when both the operating and equilibrium lines are linear. Separation of
dilute solutions occurs frequently when purifying waste streams. Because both the
equilibrium and operating curves are linear for dilute solutions, the equation de-
rived by Kremser [59] can be used to calculate the number of equilibrium stages.
Next calculate the number of actual stages. For tray columns the efficiency,
E 0, is obtained from Equation 6.21.10T. For packed columns the HETS (height
equivalent to a theoretical stage) is given by Equation 6.21.9P, as recommended
by Ulrich [50]. The column height is the sum of the height occupied by packing or
trays, a section above the top tray, room for manholes and handholes, and an addi-
tional section below the bottom tray. The manholes and handholes are required
for inspection and maintenance. The top section de-entrains liquid from gas (phase
separation). For a packed column, Vatavuk and Neveril [60] recommended add-
ing 2 ft (0.610 m) to 3 ft (0.914 m) plus 25% of the column diameter to allow for
gas-liquid separation, handholes, and manholes. Ulrich [50]. Both Henley and
Seader [31] and Valle-Riestra [53] recommend 4 ft (1.22 m) above the top tray.
Valle-Riestra's recommendation is based on a two-foot diameter column. He rec-
ommends adjusting the number for a larger or smaller diameter column, but he did
not give any recommendations for making the adjustment. In Reference 76, 2.0 m
(6.56 ft) is recommended for an ethane column.

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