Page 206 - Separation process principles 2

5.4 Multicomponent Vapor-Liquid Cascades 171

As an example, consider Figure 5.12a, where n-hexane
(H) is separated from n-octane by a series of three flashes at
Absorbent oil 1 atm (pressure drop and pump needs are ignored). The feed
To = 90°F
> to the first flash stage is an equimolar bubble-point liquid at
'0, a flow rate of 100 lbmolk. A bubble-point temperature
Ibmol/h
n-Butane (C4) 0.05 calculation yields 192.3"F. If the vapor rate leaving stage 1 is
n-Pentane (C5) 0.78 400 psia (2.76 MPa) set equal to the amount of n-hexane in the feed to stage 1, the
164.17
Oil - throughout calculated equilibrium exit phases are as shown. The vapor
Lo = 165.00
V1 is enriched to a hexane mole fraction of 0.690. The
Feed gas heating requirement is 751,000 Btuh. Equilibrium vapor
T, = 105OF
from stage 1 is condensed to bubble-point liquid with a
Rich oil cooling duty of 734,000 Btuh. Repeated flash calculations
Ibmollh
Methane (C1) 160.0 for stages 2 and 3 give the results shown. For each stage, the
Ethane (C2) 370.0 leaving molar vapor rate is set equal to the moles of hexane
Propane (C3) 240.0 in the feed to the stage. The purity of n-hexane is increased
n-Butane (C4) 25.0
n-Pentane (C5) - from 50 mol% in the feed to 86.6 mol% in the final con-
5.0
v, = 800.0 densed vapor product, but the recovery of hexane is
only 27.7(0.866)/50 or 48%. Total heating requirement is
Figure 5.11 Specifications for absorber of Example 5.3.
1,614,000 Btu/h and liquid products total 72.3 lbmolk.
In comparing feed and liquid products from two contigu-
The above results indicate that approximately 20% of the gas is ous stages, we note that liquid from the later stage and the
absorbed. Less than 0.1 % of the absorbent oil is stripped.
feed to the earlier stage are both leaner in hexane, the more
volatile species, than the feed to the later stage. Thus, if in-
termediate streams are recycled, intermediate recovery of
Two-Section Cascades
hexane is improved. This processing scheme is depicted in
A single-stage flash distillation produces a vapor that is Figure 5.12b, where again the molar fraction vaporized in
somewhat richer in the lower-boiling constituents than the each stage equals the mole fraction of hexane in the com-
feed. Further enrichment can be achieved by a series of flash bined feeds to the stage. The mole fraction of hexane in the
distillations in which the vapor from each stage is con- final condensed vapor product is 0.853, just slightly less
densed, then reflashed. In principle, any desired product pu- than that achieved by successive flashes without recycle.
rity can be obtained by a multistage flash technique, pro- However, the use of recycle increases recovery of hexane
vided a suitable volatility difference exists and a suitable from 48% to 61.6%. As shown in Figure 5.12b, increased
number of stages is employed. In practice, however, the re- recovery of hexane is accompanied by approximately 28%
covery of product is small, heating and cooling requirements increased heating and cooling requirements. If the same
are high, and relatively large quantities of various liquid degree of heating and cooling is used for the no-recycle
products are produced. scheme in Figure 5.12a as in Figure 5.12b, the final hexane

6 MBH V3 = 36.1 Q = 493 MBH
j'~~ 0.853&
=
>
170.4"F
164.8"F
L, = 63.9 Q = 904 MBH . Figure 5.12 Successive flashes
for recovering hexane from
octane: (a) no recycle; (b) with
recycle. Flow rates in lbmollh.
MBH = 1,000 Btu/h.