Page 232 - Distillation theory

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206 Distillation Trajectories in Infinite Complex Columns and Complexes

(Amminudin et al., 2001). In spite of the fact that real zeotropic mixtures with
wide interval of boiling deviate considerably from the conditions at which the
Underwood equation system (Underwood, 1948) is rigorous (the constancy of
relative volatilities of the components and the constancy of molar flows of liquid
and vapor), calculation separation product compositions in the general case get
inside regions for which the joining of section trajectories is feasible. Along with
this, the value of minimum reflux number obtained with the help of Underwood
equations system should not be used. Instead of that, it is necessary to apply the
general algorithm of calculation of minimum reflux mode described in Section 5.6
for splits with one distributed component.

6.9. Distillation Trajectories in Complexes of Heteroazeotropic and
Heteroextractive Distillation
We examine separation of the mixtures, concentration space of which contains
region of existence of two liquid phases and points of heteroazeotropes. It is
considerably easier to separate such mixtures into pure components because one
can use for separation the combination of distillation columns and decanters (i.e.,
heteroazeotropic and heteroextractive complexes). Such complexes are widely
used now for separation of binary azeotropic mixtures (e.g., of ethanol and water)
and of mixtures that form a tangential azeotrope (e.g., acetic acid and water),
adding an entrainer that forms two liquid phases with one or both components of
the separated azeotropic mixture. In a number of cases, the initial mixture itself
containsacomponentthatformstwoliquidphaseswithoneorseveralcomponents
of this mixture. Such a component is an autoentrainer, and it is the easiest to
separate the mixture under consideration with the help of heteroazeotropic or
heteroextractive complex. The example can be the mixture of acetone, butanol,
and water, where butanol is autoentrainer. First, heteroazeotropic distillation of
the mixture of ethanol and water with the help of benzene as an entrainer was
offered in the work (Young, 1902) in the form of a periodical process and then in
the form of a continuous process in the work (Kubierschky, 1915).
General regularities of location of residue curves in the region of existence of
two liquid phases, if change of total composition of liquid at open evaporation is
shown, remain the same as for homogeneous mixtures (Haasze, 1950; Storonkin
& Smirnova, 1963).
Therefore, residue curves intersect the boundary of the region of existence of
two liquid phases without any leaps and twists.
This uses the previously stated general theory of section trajectory bundles
in columns with one and two feeds for analysis of mixtures separation in the
complexes of heteroazeotropic and heteroextractive distillation.
Specific peculiarities of mixtures with two liquid phases are the following ones:
(1) the point of heteroazeotrope x Haz can only be the unstable node N or the
−
saddle S; and (2) the points of equilibrium vapor y for points of liquid x in the
regions of existence of two liquid phases Reg L1 − L2 should lie in the concentration