At the same time, the number of methods for studying the structure of phase diagrams is extremely limited, which is connected with the multidimensionality of objects and difficulties in interpreting the results. Special methods (extractive distillation, pressure-swing distillation, and combination of various processes) are used in these cases, which can be evaluated for potential use only by analyzing the complete structure of the phase equilibrium diagram. The saddle-type points in the phase diagram of a multicomponent system generate separatrix manifolds of different dimensions, making it impossible to isolate all components by conventional distillation. Most mixtures are characterized by the presence of azeotropes of different numbers of components and types (nodes, saddles, etc.). Many years of experimental studies of phase equilibria has shown that there are practically no systems with behavior close to ideal. Moreover, multicomponent mixtures have to be separated as a rule. The production of basic organic synthesis products, pharmaceuticals, and polymeric materials and other related industries often involve not only the separation of reaction mixtures in order to isolate the target products, but also the regeneration of solvents used at various production stages.
Possible ways for evolutionary changes in the structure of the VLE diagram with a variation in the pressure are shown, and the Poincare indices of boundary single azeotropes and a double tangential azeotrope are determined. The methodology for determining the structure of the VLE diagram and predicting the presence of an internal azeotrope in the systems under study and in a model system that contains binary azeotropes and a quaternary azeotrope is illustrated. A complete set of parameters that adequately describe the liquid–vapor and liquid–liquid equilibrium is obtained for the following four systems: acetone–chloroform–ethanol–water, acetone–chloroform–propanol-2–water, chloroform–ethanol–cyclohexane–water, and ethyl acetate–methyl ethyl ketone–cyclohexane–propanol-2. A review of the literature published in Russia and abroad on the experimental studies of phase equilibria, methods for analyzing and predicting phase diagram structures, and simulating problems of the vapor–liquid equilibrium (VLE) of quaternary systems containing internal singular points is presented.
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