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(a) Assume a pressure at the top of 314 psia, with 316 psia at the bottom and 315 psia at
the feed location. Using the Flash model, run a bubble-point temperature calculation of the
assumed distillate in lbmol/h of 26 C1, 9 C2, 24.6 C3, and 0.3 nC4 at 314 psia for both 0% (total
condenser case) and 100% vaporized (partial condenser case). For a total condenser, the
distillate temperature is found to be -100oF, which is infeasible. For a partial condenser, the
distillate temperature is 73.1oF, which is possible, but very close to the cooling water temperature
of 65oF. A bubble point calculation on the assumed bottoms in lbmol/h of 0.4 C3, 16.7 nC4, 11
nC5, and 12 nC6, at 316 psia is 301.9oF. The saturation temperature of steam at 315 psia is
421oF. Therefore, the steam is adequate, but may have to be throttled to a lower pressure to
avoid film boiling in the reboiler.
(b) When the feed is flashed at 315 psia to 66 mol% vapor, the temperature = 183.3oF.
The computed K-values and relative volatilities (relative to the HK, nC4) are:
K-value α referred
0.208 Analysis (continued) Exercise 10.30 (continued) (c) The recovery of the LK, C3, to the distillate = 24.6/25 = 0.984. The recovery of the HK, nC4,
to the distillate = 0.3/17 = 0.01765. Using the Shortcut (FUG) model of Chemcad, the following
results are obtained:
Minimum equilibrium stages = 10.5
Minimum reflux ratio = 0.87
Actual reflux ratio for 1.3 times minimum = 1.131
Number of equilibrium stages = 23.5
Feed stage from top = 11.5
Therefore, use, say, 10 theoretical plates in rectifying section plus a partial condenser, and
12 theoretical plates in the stripping section plus a partial reboiler.
(d) For the rigorous TOWER model of chemcad, first use the following specifications for the
case of a reflux ratio = 1.3 times minimum:
Top pressure = 314 psia
No condenser pressure drop. Column pressure drop = 2 psi
Therefore, bottom pressure = 316 psia
Number of stages = 24 (includes partial condenser and partial reboiler)
Feed stage = 12 from the top
At the top, reflux ratio = 1.131
At the bottom, bottoms flow rate = 40.1 lbmol/h
Estimated distillate rate = 59.9 lbmol/h (actual rate)
Estimated reflux rate = 59.9(1.131) = 67.75 lbmol/h (actual rate)
Estimated temperature of stage 1 = 73oF
Estimated temperature of bottom stage = 302oF
Estimated temperature of stage 2 = 83oF
The TOWER model converged in 4 iterations, with the following results, including the
temperatures and concentrations at each stage, as requested in Part (e):
Condenser duty = 417,600 Btu/h
Reboiler duty = 482,500 Btu/h
Top temperature = 72.9oF compared to assumed value of 73oF
Bottom temperature = 302oF compared to assumed value of 302oF
Stage 2 temperature = 105.2oF compared to assumed value of 83oF
Note that the desired split of the two key components is achieved with:
0.2747 lbmol/h of nC4 (HK) in the distillate compared to a specification of 0.3 lbmol/h
0.3747 lbmol/h of C3 (LK)...
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This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.
- Spring '11
- The Land