Separation Process Principles- 2n - Seader & Henley - Solutions Manual

The minimum number of stages can be stepped between

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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 built-in Wilson binary interaction parameters of 151.089 for acetone-ethanol and 292.508 for ethanol-acetone, to predict the vapor-liquid 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 time-optimal problem in binary batch distillation" by I. Coward, Chem. Eng. Sci., 22, 503-516 (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, 692-700 (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. Vapor-liquid equilibrium data f...
View Full Document

This document was uploaded on 02/24/2014 for the course CBE 2124 at NYU Poly.

Ask a homework question - tutors are online