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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 bubblepoint 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 Kvalues and relative volatilities (relative to the HK, nC4) are:
Component
Kvalue α referred
to nC4
Methane
9.250
15.29
Ethane
3.081
5.09
Propane, LK
1.405
2.32
nButane, HK
0.605
1.00
nPentane
0.274
0.453
nHexane
0.126
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:
Partial condenser
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
 Levicky
 The Land

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