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Difficult Separations

As mentioned before, RD can be used as an efficient separator to achieve difficult quantitative separations. The difference in the reactivity of the components can be exploited advantageously. A comprehensive review of separations through selective reactions has been published, which covers several systems that fall under this category [110]. The use of reversible chemical reactions to enhance isomer distillation can be traced back to the 1970s. The separation of a mixture of isomers such as m-xylene and p-xylene through selective transalkylation of m-xylene has been reported and is still under investigation. Gau suggested transmetallization with organome-tallic compounds for this purpose [50]. RD of pyridine mixtures with organic acids [87] and cresol isomers with amines [101] has also been explored. Similarly, the separation of cyclohexene and cyclohexane through either esterification or hydration may be conveniently performed in an RD column. This separation is of prime importance in a new route for manufacturing cyclohexene by partial hydrogenation of benzene.

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