Unformatted text preview: lue is close to 0.9. An
easier approach is to use the batch distillation program in Chemcad, which follows the same
procedure. If the Wilson equation is used, with the builtin Wilson binary interaction parameters
of 151.089 for acetoneethanol and 292.508 for ethanolacetone, to predict the vaporliquid
equilibrium, the following results are obtained for a single operation step that is stopped when xW
= 0.1. As indicated, a reflux ratio of 2.2 gives the desired result.
R=L/D
y D avg
6.5
0.971
4
0.955
3
0.930
2.5
0.911
2.35
0.905
2.2
0.90
Assume the initial charge is 100 kmol. An acetone material balance gives a total distillate of 50
kmol. At R = 2.2, the total vapor generated in the still = 3.2(50) = 160 kmol/100 kmol of initial
charge.
From the above table in Part (b) for the constant distillate composition case, it is seen that the
reflux ratio starts at a value of about 2 and increases up to approximately 7 when the mole
fraction of acetone in the residue has dropped to the desired value of 0.1. Therefore, the energy
requirement for the constant distillate composition case is greater than for the constant reflux
case because the reflux ratio is almost always greater than 2.2 and thus more vapor generation is
needed. One possible operating policy that might be more efficient is to start with constant
distillate composition, but switch to constant reflux ratio when the value becomes 2.2. Analysis: (b) (continued) Exercise 13.13 (continued) Exercise 13.13 (continued)
Analysis: (c) (continued)
It is noted that the determination of the optimal operating policy for batch distillation has
received considerable attention in the literature. An early article, "The timeoptimal problem in
binary batch distillation" by I. Coward, Chem. Eng. Sci., 22, 503516 (1967), considers the case
of batch rectification of a binary mixture at constant boilup rate. The optimal variation of reflux
ratio is determined so as to minimize the operating time for achieving a distillate of a specified
cumulative composition. The results are compared to the constant reflux ratio and constant
distillate composition cases, with the finding for a few examples that only a few per cent of time
is saved. A more recent article, Accurate Determination of Optimal Reflux Policies for the
Maximum Distillate Problem in Batch Distillation", by J. S. Logsdon and L. T. Biegler, Ind.
Eng. Chem. Res., 32, 692700 (1993) considers the case of optimizing the reflux ratio policy to
obtain the maximum amount of distillate of a desired composition for a specified distillation
time. Exercise 13.14
Subject: Batch rectification of a mixture of ethanol and water at constant distillate composition
and constant molar vapor boilup rate.
Given: Initial charge of 2,000 gallons of 70 wt% ethanol and 30 wt% water having a specific
gravity of 0.871. Operation at 1 atm to produce a distillate product of 85 mol% ethanol and a
residual waste water of 3 wt% ethanol in 24  4 = 20 hours of rectification. Vaporliquid
equilibrium data f...
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 Spring '11
 Levicky
 The Land

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