Page 209 - Separation process principles 2

174 Chapter 5 Cascades and Hybrid Systems

Distillate From the values of AE, Ax, SE. and Sx, and the numbers of theo-
retical stages specified in Figure 5.15, the following values of 4 are 1
1 D = 530 Ibmollh computed from (5-48) and (5-50) or read from Figure 5.9: 1 i
I Partial
condenser
Component

From the values in the above two tables, values of (bld) are
Feed 400 psia (2.76 MPa) computed for each component from (5-66). Since an overall bal-
stage throughout ance for each component is given by f = d + b, values of d and b
Ibmollh can then be computed from
C, 160.0 5
C, 370.0
C, 240.0
C4 25.0
5.0
c5 -
F = 800.0
I l l
The following results are obtained:
lbmolih
Partial
reboiler

c1 0.000002 160 0
c2 0.00924 366.6 3.4
Bottoms
c3 86.8 2.7 237.3
Figure 5.15 Specifications for fractionator of Example 5.4.
nC4 937,000 0 25
nC5 Very large 0 5
the column, where the liquid reflux is specified:
Totals 529.3 270.7
Average Flow Rates, Total distillate rate is somewhat less than the 530.0 Ibmolth speci-
lbmolih fied. Values of di and bi can be corrected to force the total to 530.0
Stage or Average
by the method of Lyster et al. [5], which involves finding the posi-
Section Temperature, OF Vapor Liquid
tive root of 0 in the relation
Condenser 12 530 1,000
Enricher 59 1,530 1,000 D=E fi
Feed 105 1,530 1,000 i 1 + 0(bi/di)
Exhauster 135 730 1,000
followed by recalculation of di from
Reboiler 165 730 270

From the column pressure and the estimated temperature
values, K-values are read from Figure 2.8. These values are then
used to estimate absorption and stripping factors for the five sec- and b, from f, - d, . The resulting value of 0 is 0.8973, which gives
tions, with the following results: dCZ = 367, bCz = 3, dC3 = 3, and bc3 = 237, with no changes for
other components.
The separation achieved by distillation is considerably im-
Component
proved over the separation achieved by absorption in Example 5.3.
Although overhead vapor flow rates are approximately the same
(530 lbmolh) in this example and in Example 5.3, a reasonably
sharp split between ethane and propane occurs for distillation be-
cause of the two-section cascade, while the absorber, with only a
one-section cascade, allows appreciable quantities of both ethane
and propane to exit in the overhead vapor and bottoms liquid. Even
if the absorbent rate in Example 5.3 is doubled so that the recovery 1
of propane in the bottoms exit liquid approaches loo%, more than
50% of the ethane also appears in the bottoms.