Page 46 - Distillation theory

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Basic Concepts of Distillation

2.1. Purpose and Process Essence of Distillation
Distillation is the oldest and the most universal process of chemical technology
and other branches of industry incorporating separation of mixtures.
Practically, all natural substances and substances produced in the chemical
reactors are mixtures that do not have the properties required for using them
in techniques and for household needs. These mixtures should be separated into
components or groups of components.
Distillation has substantial advantages over other processes applied in order to
separate a mixture, such as extraction, crystallization, semipermeable membranes,
etc. As a rule, it is the most cost-effective process, so it may be used for mixtures
with very diverse properties. This process is based on the fact that the composi-
tion of the boiling liquid and that of the vapor over it differ. Thus, if the boiling
temperature is low (e.g., air separation), it is necessary to use low-temperature
refrigerants and conduct the process at a higher pressure. If it is high (e.g., in
separation of heavy oil fractions or metals), high-temperature heat carriers or fire
preheating have to be used and the process is run under vacuum.
If the composition of the boiling liquid and that of the vapor over it are quite
close (e.g., xylene isomers or isotope separation), there is substantial energy con-
sumption, which results in high capital costs.
It is impossible to conduct the distillation process in the case of azeotropic
composition (i.e., if the composition of the boiling liquid and that of the vapor are
identical).
If the detrimental chemical reactions take place at the boiling temperature of
mixture(i.e.,themixtureisthermolabile),itisalsoimpossibletorunthedistillation
process.
So if we use the basic theory of distillation and the methods that follow from
it, the cost of separation of the mixtures of substances with close boiling tempera-
tures can be decreased, and the problems of azeotropic and thermolabile mixtures
separation may also be substantially overcome. On the whole, in this book, special
attention will be paid to the problems mentioned above.

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