Unformatted text preview: the partial reboiler)
Feed stage = 12 from the top
Reflux ratio = 2
Bottoms mole flow rate = 1200 lbmol/h
Estimated distillate rate = 200 lbmol/h
Estimated reflux rate = 400 lbmol/h
Estimated temperatures: Stage 1, 200oF Stage 16, 300oF Stage 2, 220oF
A converged result, which showed a slightly better separation than specified for the FUG
method, was obtained in 17 iterations: However, as shown in the composition profile on the next
page, a pinch region (where no composition changes occur) of approximately 3 stages is evident
just above the feed stage. Accordingly, the calculations with the Tower model were repeated by
removing 3 stages above the feed, resulting in the following new input:
Number of stages = 13 (includes the total condenser and the partial reboiler)
Feed stage = 9 Exercise 11.12 (continued)
Analysis: Ordinary Distillation Column (continued)
A converged result was obtained in 17 iterations, with little change in the separation. The
composition profile for this run appears on the next page. No pinch is evident.
The ordinary distillation column for separating methanol from ethanol was sized with Chemcad
for sieve trays, with:
24inch tray spacing
12% downcomer area
10% hole area
1/4inch holes
80% of flooding
The resulting column inside diameter was determined to be 5.5 feet. The condenser duty was
computed to be 8.53 million Btu/h, with a reboiler duty of 11.70 million Btu/h. For a final
design, the computations should be repeated taking into account tray pressure drop and tray
efficiency.
The final material balance for the ordinary distillation column was as follows:
Component
nHeptane
Toluene
Phenol
Total:
Temp., oF lbmol/h:
Feed Distillate
1.8
1.8
198.8
197.0
1199.4
1.2
1400.0
200.0
323.3
251.0 Bottoms
0.00
1.8
1198.2
1200.0
378.2 Because the flow rate of phenol in the bottoms is only 1198.2 lbmol/h and 1200.0 is needed for
the extractive distillation column, the makeup phenol rate is 1200.0  1198.2 = 1.8 lbmol/h. Exercise 11.12 (continued)
Analysis: Ordinary Distillation Column (continued) Exercise 11.12 (continued)
Analysis: Ordinary Distillation Column (continued) Exercise 11.13
Subject: Separation of a mixture of ethanol and benzene by pressureswing distillation.
Given: Feed of 100 mol/s of 55 mol% ethanol and 45 mol% benzene at the bubble point at
101.3 kPa. Desired products are 99 mol% ethanol and 99 mol% benzene.
Assumptions: UNIFAC method for estimating Kvalues.
Find: Feasible design using pressureswing distillation in the manner of Example 11.5 and Fig.
11.23(b).
Analysis: Use the twocolumn system of Fig. 11.23(b). Ethanol and benzene form a minimumboiling azeotrope. Use the same column pressures as in Example 11.5. Therefore, in Column 1,
the pressures are 26 kPa at the condenser outlet, 30 kPa at the top tray, and 40 kPa at the reboiler.
The distillate product is the near azeotrope at 30 kPa and the bottoms product is 99 mol%
ethanol. In Column 2, the pressures are 106kPa before the condenser, 101.3 kPa after the
condenser, and 120...
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 Spring '11
 Levicky
 The Land

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