Analysis of the possibility of implementing the principle

Theoretical Foundations of Chemical Engineering, Vol. 39, No. 2, 2005, pp. 105–109. Translated from Teoreticheskie Osnovy Khimicheskoi Tekhnologii, Vol. 39, No. 2, 2005, pp. 115–119. Original Russian Text Copyright © 2005 by Solokhin, Nazanskii, Timofeev.

Analysis of the Possibility of Implementing the Principle of Redistribution of Concentration Fields for Reactive Distillation Processes
A.V. Solokhin, S. L. Nazanskii, and V. S. Timofeev
Lomonosov State Academy of Fine Chemical Technology, pr. Vernadskogo 86, Moscow, 117571 Russia E-mail: nazanski@mitht.ru
Received September 16, 2004

Abstract—The evolution of the phase portrait for the dynamic system of open evaporation combined with a chemical reaction for a fixed liquid amount was studied as dependent on the rate constants ofthe reaction. A three-component azeotropic mixture whose vapor–liquid equilibrium diagram is characterized by two distillation regions was considered. The possibility of the open evaporation trajectory to transfer through the separatrix between the distillation regions was shown, as well as the possibility of implementing the principle of redistribution of the concentration fields due to chemicalreactions occurring in reactive distillation columns.

From the theory of recycling systems, one knows that, for any reversible reaction (regardless of its rate and equilibrium constant), full conversion can theoretically be achieved with a recycling complex comprising a reactor and a separation unit provided that the separation unit yields pure reaction products and forms the return flux of theset composition [1–3]. Thus, the ability of a recycling flowsheet to reach the set conversion are, above all, limited by the efficiency of the separation unit [3]. In cases where fractional distillation is employed, the ability of the separation unit are dictated by the configuration of the liquid–vapor phase diagram and the separation efficiency of the distillation columns. However, even on theassumption of an infinite separation efficiency of distillation columns, the isolation of pure reaction products (full exhaust ion of the reagents) is often unachievable for azeotropic mixtures unless special (extractive, azeotropic, and others) distillation techniques are employed. This owes to the fact that, for azeotropic mixtures, the overall concentration simplex is divided into closed concentrationcells in which the distillation process develops [4]. Thus, the development of the distillation process in any two-section column is limited; i.e., the possible compositions of the product flows depend on the phase-separation region in which the feed composition falls. The limitations imposed on the separation of azeotropic mixtures into pure components or fractions with set compositions (theselimitations are associated with the structural features of the equilibrium phase diagram) can be overcome by the redistribution of the concentration fields between the phase-separation regions caused by a chemical reaction [2, 5]. The underlying idea of this principle is the use of a chemical reaction to transfer the feed compositions entering the column from one distillation region to another.

Inrecycling flowsheets, the redistribution of feed compositions between the distillation regions can occur due to the operation of a chemical reactor, and the utility of the redistribution of concentration fields is quite evident [5]. In reactive distillation columns, however [6], the redistribution of concentration fields should be implemented due to the operation of the reaction section, whichcombines chemical transformation and stripping. Therefore, it remains an open question whether the product compositions of a reactive distillation column can fall in different distillation regions. It is known that qualitative analysis of fractional distillation can be accomplished on the basis of distillation curves, which are open-evaporation trajectories [4]. In the case of reactive distillation,...