Three-component mixtures reversible distillation

Diagrams of Three-Component Mixture Reversible Distillation... 

A convenient notation for classifying mixtures employed in liquid-liquid extraction is C/, where C is the number of components and the number of partially miscible pairs. Mixtures 3/1, 3/2, and 3/3 are called Type I, Type II, and Type III by some authors. A typical 3/1 three-component mixture with only one partially miscible pair is furfural-ethylene glycol-water, as shown in Fig. 3.10, where the partially miscible pair is furfural-water. In practice, furfural is used as a solvent to remove the solute, ethylene g yco, from water the furfural-rich phase is called the extract, and the water-rich phase the raffinate. Nomenclature for extraction, leaching, absorption, and adsorption always poses a problem because, unlike distillation, concentrations are expressed in many different ways mole, volume, or mass fractions mass or mole ratios and special solvent-free designations. In this chapter, we will use V to represent the extract phase and L the raffinate phase, and y and x to represent solute concentration in these phases, respectively. The use of V and L does not imply that the extract phase in extraction is conceptually analogous to the vapor phase in distillation indeed the reverse is more correct for many purposes. 

The location of trajectory bundles and possible product composition segments at reversible distillation of three-component mixtures determines the location of trajectory bundles, and of possible product composition regions of multicomponent mixtures and the locations of trajectory bundles of real adiabatic columns. 

Diagrams of reversible distillation of various types of three-component mixtures can be obtained in various ways with the help of the model of phase equilibrium describing these types of mixtures. It is possible to calculate the trajectory consequently for each chosen product point using Eq. (4.6) (4.13) and increasing step... 

It was shown (Petlyuk, 1986) that the diagram of reversible distillation of any three-component mixture can be forecasted by scanning only the sides of the concentration triangle, defining at each point the values of phase equilibrium coefficients of all the components and using Eqs. (4.19) and (4.20). The latter way defines trajectory tear-off segments Reg or Reg(. y and possible product segments Reg or Reg The node points iV v of the trajectory bundles are determined hypothetically on the basis of the data on the location of azeotropes points and a-points. 

Diagrams of Extractive Reversible Distillation for Three-Component Mixtures... 

Let s examine a column of sharp reversible distillation with two feedings (Fig. 4.18a) for separation of a three-component mixture (Petlyuk Danilov, 1999). For an intermediate section of such a column, Eq. (4.6) is as follows (F2, upper feeding) ... 

To understand the structure of section trajectory bundles for multicomponent mixtures and their evolution with the increase of reflux number, let s examine first three-component mixtures, basing on the regularities of distillation trajectory tear-off at finite reflux and the regularities of location of reversible distillation trajectories. 

Let s note, that distillation trajectory of three-component mixture for the set product point is located between trajectory at infinite reflux (ie., at L/V = 1, and reversible distillation trajectory) (Kiva, 1976 Petlyuk Serafimov, 1983 Wahn-schafft et al, 1992 Castillo, Thong, Towler, 1998). 

Locations of reversible distillation trajectories depends on position of pseudoproduct point (i.e., on compositions and on flow rates of feeds and of separation products, as is seen from Eq. [6.3]). Difference from the top and bottom sections appears, when the pseudoproduct point of the intermediate section is located outside the concentration simplex (i.e., if concentrations of some components x j)i obtained from Eq. [6.3], are smaller than zero or bigger than one), which in particular takes place, if concentration of admixture components in separation products are small components (i.e., at sharp separation in the whole column). The location of reversible distillation trajectories of the intermediate sections at x j i < 0 or x, > 1 differs in principle from location of ones for top and bottom sections, as is seen from Fig. 6.3 for ideal three-component mixture (Ki > K2 > K3) and from Fig. 6.4 for ideal four-component mixture (Ki > K2 > K3 > K4). 

Later, these columns were independently rediscovered (Petlyuk, Platonov, Slavinskii, 1965 Platonov, Petlyuk, Zhvanetskiy, 1970) on the basis of theoretical analysis of thermodynamically reversible distillation because this distillation complex by its configuration coincides with the sequence of thermodynamically reversible distillation of three-component mixture (see Chapter 4), but in contrast to this sequence it contains regular adiabatic columns. The peculiarities of Petlyuk columns for multicomponent mixtures are (1) total number of sections is n(n - 1) instead of 2(n - 1) in regular separation sequences (2) it is sufficient to have one reboiler and one condenser (3) the lightest and the heaviest components are the key components in each two-section constituent of the complex and (4) n components of a set purity are products. 

Belk (7) describes a plate-to-plate calculation for hypothetical, two and three-component liquid-phase reversible reactions, carried out continuously in a single distillation column. He disregarded deviations of the liquid mixture from ideality, assumed adiabatic operation of the column and 100% plate efficiency. Heat of reaction was assumed to be negligible and all parameters... 

The scheme of the reversible process is shown in Fig. 2.10a. Figure 2.10b illustrates a trajectory of the reversible distillation for three-component ideal mixture. 

The analysis of the thermodynamically reversible process of distillation for multicomponent azeotropic mixtures was made considerably later. Restrictions at sharp reversible distillation were revealed (Petlyuk, 1978), and trajectory bundles at sharp and nonsharp reversible distillation of three-component azeotropic mixtures were investigated (Petlyuk, Serafimov, Avet yan, Vinogradova, 1981a, 1981b). 

Restrictions at nonsharp reversible distillation of three-component azeotropic mixtures were studied by Poellmann and Blass (1994). 

Let s illustrate the location of trajectory bundles of reversible distillation by the example of three-component acetone(l)-benzene(2)-chloroform(3) mixture with one binary saddle azeotrope with a maximum boiling temperature (Fig. 4.7)... 

Locations of trajectories bundles Regr. of node points of these bundles Nrev, and of possible product segments Reg Wd Regf can be shown in diagrams of three-component azeotropic mixtures sharp reversible distillation for various types of such mixtures (Fig. 4.11). 

The diagrams of reversible distillation were constructed for some types of three-component azeotropic mixtures. It is interesting that some types of mixtures with one binary azeotrope and with two distillation regions [types 3 and 5 according to classification (Gurikov, 1958)] permit sharp separation into component and binary zeotropic mixture at some feed compositions. The mixture acetone(l)-benzene(2)-chloroform(3) is an example of such mixture. 

Determination of possible composition regions Reg Reg of top and bottom products at reversible distillation of all three-component constituents of initial mixture. 

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