Heteroextractive Distillation

Distillation Trajectories in Complexes of Heteroazeotropic and Heteroextractive Distillation... 

We examine separation of the mixtures, concentration space of which contains region of existence of two hquid phases and points of heteroazeotropes. It is considerably easier to separate such mixtures into pure components because one can use for separation the combination of distillation columns and decanters (i.e., heteroazeotropic and heteroextractive complexes). Such complexes are widely used now for separation of binary azeotropic mixtures (e.g., of ethanol and water) and of mixtures that form a tangential azeotrope (e.g., acetic acid and water), adding an entrainer that forms two liquid phases with one or both components of the separated azeotropic mixture. In a number of cases, the initial mixture itself contains a component that forms two liquid phases with one or several components of this mixture. Such a component is an autoentrainer, and it is the easiest to separate the mixture under consideration with the help of heteroazeotropic or heteroextractive complex. The example can be the mixture of acetone, butanol, and water, where butanol is autoentrainer. First, heteroazeotropic distillation of the mixture of ethanol and water with the help of benzene as an entrainer was offered in the work (Young, 1902) in the form of a periodical process and then in the form of a continuous process in the work (Kubierschky, 1915). 

This uses the previously stated general theory of section trajectory bundles in columns with one and two feeds for analysis of mixtures separation in the complexes of heteroazeotropic and heteroextractive distillation. 

We examine the most typical splits and separation sequences for various types of three-component mixtures (Fig. 6.16 shows examples of heteroazeotropic distillation Fig. 6.17 shows examples of heteroextractive distillation). 

Figme 6.17. Trajectories of heteroextractive distillation (a) distillate from azeocolumn to decanter and a stream of the entrainer from decanter and additional entrainer to azeocolumn for separation vinyl acetate(l)-methanol(2)-water(3) mixture (b) distillate from azeocolumn to decanter and a stream of the entrainer from decanter and additional entrainer to azeocolumn for separation chloroform(l)-acetone(2)-water(3) mixture. Regions of two Kquid phases Reg/,i /,2 are shaded. 

Heteroazeotropic distillation cannot be used for separation of some types of mixtures having the region of existence of two liquid phases RegLi-L2, but heteroextractive distillation can be applied. 

At heteroextractive distillation, one of the phases in the decanter plays the role of entrainer, and the section above the input of the mixture being separated plays... 

The above-examined examples of various sequences and splits of three-component mixtures with the help of heteroazeotropic and heteroextractive distillation allow to make a general conclusion about the usage of these distillation complexes not only for three-component mixtures, but also for mixtures with bigger number of components. 

Heteroextractive distillation can be effectively used if at the boundary element of concentration simplex there is a heteroazeotrope-unstable node Xhoz =... 

In all the above-mentioned cases of application of heteroazeotropic or heteroextractive distillation, the second product point of distillation column (of the second product, obtained at that column end, where there is no decanter) should belong to the possible product region at the corresponding boundary element of concentration simplex (of bottom product Reg at Fig. 6.16a d and at Fig. 6.17a,b or of top product Regi) at Fig. 6.16e,f). 

The location of intermediate sections trajectories of columns with two feeds, including those at extractive, heteroazeotropic, and heteroextractive distillation, has fundamental distinctions from that of section trajectories of the simple columns. At sharp extractive or heteroextractive distillation, pseudoproduct point x), of the intermediate section should be located at the continuation of the boundary element, to which components of top product and of entrainer belong. If this condition is valid, the whole trajectory bundle of the intermediate section including trajectory tear-off point x[ from the mentioned boundary element is located in the region Reg where the top product components are more volatile and the entrainer components are less volatile than the rest of components. The trajectory tear-off point of the intermediate section is the stable node x[ = A+). The conditions of intermediate section trajectory tear-off in different points of trajectory tear-off region Reg allow to determine limit modes of extractive distillation for each mixture - the mode of minimum flow rate of the entrainer min, and for the... 

Write the trajectories of heteroazeotropic and heteroextractive distillation indicate the components present and absent in the product points and in trajectory tear-off points. 

Works of a number of investigators have shown that usage of data only on binary VLE does not create an adequate model that would satisfactorily describe the conditions at heteroazeotropic or heteroextractive distillation. The same can be said of the usage of data only on ternary LLE. 

We examine the main steps of the algorithm. The given data at heteroazeotropic and heteroextractive distillation are concentration of impurity components in the bottom product. This information unambiguously determines the bottom product composition if one component is impurity one (see Section 6.9 Figs. 7.16e,f and 7.17a,b). If two or more components are impurity ones (Fig. 6.16a d), then the bottom product composition is set in initial approximation, taking into consideration the ratios of phase equilibrium coefficients of impurity components in the bottom product point. In this case, the bottom product composition is defined more exactly later at iterations. 

For the beginning of calculation by method tray by tray, it is necessary to also determine the estimated composition at the top end of the colunm y. In the cases in Fig. 6.16e,f, this composition is determined by the set concentration of impurities in the top product and, in the other cases in Figs. 6.16 and 6.17, this composition is set at the intersection of vapor line with a certain liquid-liquid tie-line close the heteroazeotropic liquid-liquid tie-line (the distance in the concentration space from the point of vapor going into the decantor to this tieline is set). For heteroextractive distillation, it is necessary to also set the flow rate of entrainer determining preliminarily its minimum flow rate (see Section 6.6). 

Calculation of necessary tray numbers at heteroazeotropic and heteroextractive distillation can be carried out by method tray by tray upward with optimization by number of sections, by number of phases from the decantor for refluxing, and by distribution of trays among the sections. 

Why can one calculate the column of heteroextractive distillation by method tray by tray in the upward direction ... 

Several heuristic rules directed to decrease energy and capital expenditures are used for selection The first rule is for the mixtures having the region of two liquid phases it is necessary to use the most interesting splits at heteroazeotropic and heteroextractive distillation described in the section 8.4.5. Such splits separate, in the cheapest way, the mixture into components. The second rule is to exclude sphts for which one of the products is binary azeotropic mixture, if other splits... 

Some examples of separation of three-component mixtures using curvature of separarix fines are described in Chapter 3. Examples of the application of entrainers forming heteroazeotropes and of columns with decanters for het-eroazeotropic and heteroextractive distillation are given in Chapter 6. 

For heteroazeotropic mixtures, presynthesis also includes the most interesting variants of heteroazeotropic and heteroextractive distillation. 

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