Reactive Multistage Separation

When components entering a multistage vapor-liquid separation column are mutually reactive, chemical reactions and phase separation can occur simultaneously in what is generally described as reactive distillation. This phenomenon is found in several operations in the petroleum, chemical, and petrochemical industries. 

Phase separation is controlled by phase equilibrium relations or rate-based mass and heat transfer mechanisms. Chemical reactions are controlled by chemical equilibrium relations or by reaction kinetics. For reactive distillation to have practical applications, both these operations must have favorable rates at the column conditions of temperature and pressure. If, for instance, the chemical reaction is irreversible, it may be advantageous to carry out the reaction and the separation of products in two distinct operations a reactor followed by a distillation column. Situations in which reactive distillation is feasible can result in savings in energy and equipment cost. Examples of such processes include the separation of close-boilers, shifting of equilibrium reactions toward higher yields, and removal of impurities by reactive absorption or stripping. 

In reactive distillation, chemical reactions are assumed to occur mainly in the liquid phase. Hence the liquid holdup on the trays, or the residence time, is an important design factor for these processes. Other column design considerations, such as number of trays, feed and product tray locations, can be of particular importance in reactive distillation columns. Moreover, since chemical reactions can be exothermic or endothermic, intercoolers or heaters may be required to maintain optimum stage temperatures. Column models of reactive distillation must include chemical reaction 

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Earlier chapters use simplified and binary models to analyze in a very informative manner some fundamentals such as the effect of reflux ratio and feed tray location, and to delineate the differences between absorption/stripping and distillation. Following chapters concentrate on specific areas such as complex distillation, with detailed analyses of various features such as pumparounds and side-strippers, and when they should be used. Also discussed are azeotropic, extractive, and three-phase distillation operations, multi-component liquid-liquid and supercritical extraction, and reactive multistage separation. The applications are clearly explained with many practical examples. 

Chapter 7 will consider separations achieved under the bulk flow-force combination of (b). Separation systems utilizing the configurations of (c) are treated in Chapter 8. (There will be occasional examples of two combinations of bulk flow and force directions.) Chapters 6, 7 and 8 will generally employ one separator vessel. Reactive separations will be treated immediately alongside non-reactive separations as often as possible. Different feed introduction modes will be considered as required in all three configurations, (a), (b) and (c). Multistage separation schemes, widely used in the processes of gas absorption, distillation, solvent extraction, etc., are studied in Chapter 8 when only one vessel is used. When multiple devices are used to form a separation cascade, an introductory treatment is provided in Chapter 9. 

We report on a number of on-line chemical procedures which were developed for the study of short-lived fission products and products from heavy-ion interactions. These techniques combine gas-jet recoil-transport systems with I) multistage solvent extraction methods using high-speed centrifuges for rapid phase separation and II) thermochromatographic columns. The formation of volatile species between recoil atoms and reactive gases is another alternative. We have also coupled a gas-jet transport system to a mass separator equipped with a hollow cathode- or a high temperature ion source. Typical applications of these methods for studies of short-lived nuclides are described. 

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