Unformatted text preview: l/s
Estimated reflux rate = 23 mol/s (corresponding to a small reflux ratio of 0.1)
Estimated temperatures: Stage 1, 67oC Stage 6, 80oC Stage 2, 70oC Analysis: Exercise 11.13 (continued) Column 1 (continued) Exercise 11.13 (continued)
Analysis: Column 2 (continued) A converged result was obtained in 21 iterations, with a small reflux ratio of 0.16 or 36.9
mol/s. The material balance was in agreement with the above table. Although the feed location
could probably be improved, no pinch region is evident, as shown in the liquid-phase
composition profile below. Also included are the column profiles for temperature, liquid flow,
and vapor flow.
Column 2 was sized with Chemcad for sieve trays, with:
24-inch tray spacing
12% downcomer area
10% hole area
80% of flooding
The resulting column inside diameter was determined to be 9 feet. The condenser duty was
computed to be 9.38 MW, with a reboiler duty of 9.30 MW. For the 4 theoretical plates, the total
pressure drop was computed to be 3.9kPa. For a final design, the computations should be
repeated taking into account tray pressure drop and tray efficiency. Exercise 11.13 (continued)
Analysis: Column 2 (continued) Exercise 11.14
Subject: Separation of a mixture of ethanol and benzene by pressure-swing distillation.
Given: Feed of 100 mol/s of 20 mol% ethanol and 80 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 K-values.
Find: Feasible design using pressure-swing distillation in the manner of Example 11.5.
Analysis: Use the two-column 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 kPa at the reboiler. The distillate product is the near azeotrope at 101.3 kPa
and the bottoms product is 99 mol% benzene. Using the Chemcad program with the UNIFAC
method, the azeotrope compositions in mol% ethanol are 35.7 at 26 kPa, and 45.2 at 101.3 kPa.
A major factor in the design of a pressure-swing distillation system is the recycle-to-feed
ratio, D2/F in Fig. 11.23(b), which is related to compositions as follows:
Let x be the mole fraction of ethanol in any stream. Referring to Fig. 11.23(b), an overall system
material balance for total flows gives:
F = B1 + B2
An overall system balance on ethanol gives: x F F = x B1 B1 + x B2 B2
A total material balance around Column 2 gives:
D1 = D2 + B 2
An ethanol balance around Column 2 gives: x D1 D1 = x D2 D2 + x B2 B2
Substitute Eq. (1) into (2) to eliminate B1. After rearrangement,
x B1 − x B2
F = B2
x B1 − x F
Substitute Eq. (3) into (4) to eliminate D1. After rearrangement,
x D1 − x B2
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