Azeotropic distillation minimum reflux

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). 

Levy SG, Van Dongen DB and Doherty MF (1985) Design and Synthesis of Homogeneous Azeotropic Distillation 2. Minimum Reflux Calculations for Non-ideal and Azeotropic Columns, Ind Eng Chem Fund, 24 463. 

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... 

An example of azeotropic distillation is the use of benzene to permit the complete separation of ethanol and water, which forms a minimum-boiling azeotrope with 95.6 weight percent alcohol. The alcohol-water mixture with about 95 percent alcohol is fed to the azeotropic distillation column with a benzene-rich stream added at the top. The bottom product is nearly pure alcohol, and the overhead vapor is a ternary azeotrope. The overhead vapor is condensed and separated into two phases. The organic layer is refluxed, and the water layer is sent to a benzene recovery column. All the benzene and some alcohol is taken overhead and sent back to the first column, and the bottoms stream is distilled in a third column to give pure water and some of the binary azeotrope. 

The concept of minimum reflux is more complex in azeotropic distillation, because of the high non-ideal behaviour and distillation boundaries. For the special case of ternary distillation, the analysis may be simplified. It is useful to mention that the minimum reflux is linked with the concept of distillation pinch. This represents a zone of constant phase composition, so that the driving force becomes very small. Consequently, the number of necessary stages for separation goes to infinite. Similarly, there is a minimum reboil rate. In this respect, three classes of limiting separations may be distinguished (Stichlmair and Fair, 1999). Figures 9.36 to 9.38 present concentration profiles obtained by simulation with an ideal system benzene-toluene-ethyl-benzene. 

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]. 

If reflux cannot be cut back (e.g., in an unrefluxed stripper, in azeotropic distillation, or when the packed section above the feed is close to its minimum wetting limit), boilup will need to be raised to compensate for the excess subcooling. Vapor and liquid traffic below the feed and reboiler duty will rise and effectively lower the column feed capacity. Premature flooding may result. If the lower capacity or higher reboiler duty cannot be tolerated, feed preheating (Fig. 12.5a)... 

Levy, S.G., D.B.Van Dongen, and MJ. Doherty, Design and synthesis of homo geneous azeotropic distillations. 2. Minimum reflux calculations for nonideal and azeotropic columns. Industrial Engineering Chemistry, Fundamentals, 1985, 24(4) 463 474. 

Levy, S.G. and M.F. Doherty, Design and synthesis of homogeneous azeotropic distillations. 4. Minimum reflux calculations for multiple feed columns. Industrial and Engineering Chemistry Fundamentals, 2002, 25(2) 269 279. 

The synthesis of optimum sequences for the multicomponent azeotropic mixture is the issue of the distillation theory. Geometric theory of distillation overcomes the principal part of this problem - the determination of possible splits for each potential distillation column that may be included into the synthesized sequence. The best feasible sequences selection is carried out on the basis of the criteria of a minimum number of columns, as well as minimum liquid and vapor flows, under the minimum reflux mode. 

Kiva, V. N., Timofeev, V. S., Vizhesinghe, A. D. M. C., Chyue Vu Tam (1983). The Separation of Binary Azeotropic Mixtures with a Low-Boiling Entrainer. In The Theses of 5th Distillation Conference in USSR. Severodonezk (Rus.). Knapp, J. R, Doherty, M. F. (1994). Minimum Entrainer Flows for Extractive Distillation A Bifurcation Theoretic Approach. AlChE J., 40,243-68. Koehler, J., Aguirre, R, Blass, E. (1991). Minimum Reflux Calculations for Nonideal Mixtures Using the Reversible Distillation Model. Chem. Eng. Set, 46,3007-21. 

The approximate calculation method of minimum reflux mode (Koehler, Aguirre, Blass, 1991) - the method of the smallest angle, which holds good for mixtures with any component numbers and for any sphts, including frequently found at azeotropic mixtures separation cases of tangential pinch, is based on the calculation of reversible distillation trajectories for the given product compositions. 

The development of distillation traj ectory bundles theory at finite reflux showed that the task of minimum reflux mode calculation for nonideal zeotropic and azeotropic mixtures can be solved in another statement at set composition xf and thermal state q of feeding, it is necessary to determine minimum reflux number i min for the set product compositions xd and xb of sharp separation and set permissible concentrations of admixtures in the products. 

Castillo, F. G. L., Towler, G. P. (1998). Influence of Multicomponent Mass Transfer on Homogeneous Azeotropic Distillation. Chem. Eng. Sci., 53,963-76. Chien, H. H. Y. (1978). A Rigorous Method for Calculating Minimum Reflux Rates in Distillation. AIChEJ., 24,606-13. 

Levy, S. G., Van Dongen, D. B., Doherty, M. F. (1985). Design and Synthesis of Homogenous Azeotropic Distillation. 2. Minimum reflux Calculations for Nonideal and Azeotropic Columns. Ind. Eng. Chem. Fundam., 24, 463-74. 

Petlyuk, F. B., Danilov, R. Yu. (1998). Calculations of Distillation Trajectories at Minimum Reflux for Ternary Azeotropic Mixtures. Theor. Found. Chem. Eng., 32, 548-59. 

Petlyuk, F. B., Vinogradova, E. L, Serafimov, L. A. (1984). Possible Compositions of Products of Ternary Azeotropic Mixture Distillation at Minimum Reflux. Theor Found. Chem. Eng., 18,87-94. 

This develops the general algorithm of calculation of minimum reflux mode for the columns with two feed inputs at distillation of nonideal zeotropic and azeotropic mixtures with any number of components. The same way as for the columns with one feed, the coordinates of stationary points of three-section trajectory bundles are defined at the beginning at different values of the parameter (L/V)r. Besides that, for the intermediate section proper values of the system of distillation differential equations are determined for both stationary points from the values of phase equihbrium coefficients. From these proper values, one finds which of the stationary points is the saddle one Sm, and states the direction of proper vectors for the saddle point. The directions of the proper vectors obtain linear equations describing linearized boundary elements of the working trajectory bundle of the intermediate section. We note that, for sharp separation in the top and bottom sections, there is no necessity to determine the proper vectors of stationary points in order to obtain linear equations describing boundary elements of their trajectory bundles, because to obtain these linear equations it is sufficient to have... 

Figwe 6.10. Joining of the stripping, intermediate, and rectifying section trajectories of extractive distillation of the acetone(l)-water(2)-methanol(3) azeotropic mixture at minimum reflux (bottom feed is the control one) xf+e, total composition of inital feed F and entrainer E,... 

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

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