- Homogeneous Azeotropic Distillation
The most general definition of homogeneous azeotropic
distillation is the separation of any single liquid-phase mixture containing one
or more azeotropes into the desired pure component or azeotropic products by
continuous distillation. Thus, in addition to azeotropic mixtures which require
the addition of a miscible separating agent in order to be separated,
homogeneous azeotropic distillation also includes self-entrained mixtures that
can be separated without the addition of a separating agent.
The first step in the synthesis of a homogeneous azeotropic
distillation sequence is to determine the separation objective. Sometimes it is
desirable to recover all of the constituents in the mixture as pure components
other times it is sufficient to recover only some of the pure components as
product. In our example, we would like to recover the cyclohexane product at 90%
purity and recycle the separating agent back to the initial separating column
for further use.
The second step is to sketch the residue curve map for the
mixture to be separated. The residue curve map allows one to determine whether
the goal can be reached and if so how to reach it, or the goal needs to be
redefined.
Distillation boundaries for continuous distillation are
approximated by simple distillation boundaries. It is a good approximation for
mixtures with nearly simple distillation boundaries. For a separation to be
feasible by distillation, the simple distillation boundary should not be
crossed, i.e. the distillate and bottom composition should lie in the same
distillation region. A more detail calculation method involving the composition
will be discuss in the later section.
In the most common situation, a separating agent is added to
separate a minimum boiling binary azeotrope into its two constituent pure
components by homogeneous azeotropic distillation. Michael F. D. and Jeffrey P.
K. presented seven of the most favorable residue curve maps for this task. Of
the seven, the map representing extractive distillation is by far the most
common and the most important. Its corresponding residue curve and column
sequences are shown in Fig. 2 below.
- Extractive distillation
Extractive distillation is defined as distillation in the
present of a miscible, high boiling, relatively nonvolatile component, the
solvent, that forms no azeotropes with the other components in the mixture. It
is widely used in the chemical and petrochemical industries for separating
azeotropic, close-boiling, and others low relative volatility mixture.
Extractive distillation works because the solvent is
specially chosen to interact differently with the components of the original
mixture, thereby altering their relative volatilities. Because these
interactions occur predominantly in the liquid phase , the solvent is
continuously added near the top of the extractive distillation column so that an
appreciable amount is present in the liquid phase on all of the trays below. The
mixture to be separated is added through second feed point further down the
column. In the extractive column, the component having the greater volatility,
not necessarily the component having the lowest boiling point, is taken overhead
as a relatively pure distillate. The other component leaves with the solvent via
the column bottoms. The solvent is separated from the remaining components in a
second distillation column and then recycled back to the first column.
Fig. 2 Extractive distillation with a
heavy solvent
which introduce no new azeotrope for a minimum boiling
azeotrope. In some case, B can come off the top of the first column.
There are several industrial application for homogeneous
azeotropic distillation listed in the Encyclopedia of Separation Technology
by Michael F. D., Jeffrey P. K.
Extractive distillations can be divided into three
categories, each correspond to the different residue curve maps, the minimum
boiling azeotropes, maximum boiling azeotropes and the nonazeotrope mixtures.
Since our benzene-cyclohexane mixture to be separated is of the second type of
mixture, i.e. the minimum boiling azeotrope, we will focus our attention on
column sequencing this type of azeotropic separation method in the following
section.
As in azeotropic distillation, design of extractive
distillation system will also requires significant preliminary work including:
- Choosing the solvent
- Developing or finding necessary data, such as azeotropic condition or
residue curve
- Preliminary screening
- Computer simulation
- Small scale testing
For our example, we will consider the first four steps.
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