Page 66 - Distillation theory

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Trajectories of Distillation in Infinite Columns

Under Infinite Reflux

3.1. Introduction
Our main purpose is to understand which column sequences can be used in order
to get the necessary products. This task is called the task of sequencing (synthesis).
The sequencing task is being solved in consecutive order beginning with the first
distillation column, where initial n-component mixture comes to. For each column,
it is necessary to determine feasible splits.
As a rule, our task is to separate this mixture in a few distillation columns
into pure components (perhaps, with the addition of subsidiary components –
entrainers, or using, besides distillation, other methods of separation). That is
why we are first interested in the sharp splits in each column, when each product
of the column contains a number of components smaller than the feeding of the
column. The finite number of sharp splits makes determining the sharp splits quite
clear and definite.
The important advantage of mixture separability analysis for each sharp split
consists of the fact that this analysis, as is shown in this and three later chapters,
can be realized with the help of simple formalistic rules without calculation of
distillation. A split is feasible if in the concentration space there is trajectory of
distillation satisfying the distillation equations for each stage and if this trajectory
connectsproductpoints.Thatiswhytodeductconditions(rules)ofseparabilityitis
necessary to study regularities of distillation trajectories location in concentration
space.
Because sharp separation is not always feasible for azeotropic mixtures, we also
consider the best semisharp splits, when one of the products contains a smaller
number of components than the feeding and when the possible product point of
the second product is the farthest from the product point of the first product in
the concentration space.
The investigation of regularities of distillation trajectories’ location in infinite
columns under infinite reflux is directed to the solution of the task of determination
of possible splits.

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