Azeotropic distillation minimum reflux ratio

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

If the mixture is a ternary one, the conditions of distillation may be determined with the aid of a graph on triangular coordinates [72 — 74, 78], The minimum reflux ratio in the continuous distillation of ternary and quaternary ideal and azeotropic mixtures can be calculated with a method evolved by Kohrt [172]. 

Tanake S. and J. Yamada, Graphical calculation method for minimum reflux ratio in azeotropic distillation, J. Chem. Eng. Japan, 5, 20-26 (1972). 

An important topic in azeotropic distillation regards the purity. This can be managed by means of both entrainer and reflux ratios. For a given purity there is a minimum entrainer ratio that can be determined from a RCM plot. In... 

The occurence of azeotropes, such as the one in the ethyl acetate-benzene system above, results in difficulties in separations by distillation. For example, suppose it is desired to produce a distillate containing 98 mol % ethyl, acetate and a bottoms product containing 98 mol % benzene from a feed that contains 40 mol % ethyl acetate and 60 mol % benzene. Rather than consider detailed distillation calculations at various reflux ratios, we will consider only the case of total reflux, q — oo, which you should remember results in the minimum number of stages to accomplish a given separation. Also, if the desired separation cannot be made at total reflux, it will not be possible to accomplish the separation at any lower reflux ratio. 

After condensation in a total condenser and separation of the two liquid layers, the water layer is withdrawn as product and the diisopropyl ether layer is returned as reflux. We operate at an external reflux ratio that is 2.0 times the minimum external reflux ratio. Operation is at 1.0 atm. Find the minimum external reflux ratio, the actual L/D, the distillate flow rate and mole frac water, the bottoms flow rate and mole frac water, and the number of equilibrium stages required (number the top stage as number 1). Use an expanded McCabe-Thiele diagram to determine the number of stages. Data for the azeotropic conposition fProblem 8.D81 can be used to find the mole fracs of water in the two layers in the separator and the relative volatility of water with respect to ether at low water concentrations. The weight fractions have to be converted to mole fracs first. 

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