Page 314 - Distillation theory

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288 Synthesis of Separation Flowsheets

all components at all the edges of the concentration simplex in the points taken
with a certain step.
Sharp split in the column with two feedings is possible, if the conditions for
product compositions are valid: (1) top product point and entrainer point should
belong to the same boundary elements of concentration simplex as the trajectory
j
t j j
tear-off region of the intermediate section (if Reg ∈ C , then x D ∈C and x E ∈
e i i
i
j
C ); (2) bottom product point should belong to the possible composition region
i
x B ∈ Reg B in the concentration simplex, containing all components of feed and of
entrainer C F+E ; (3) top product point should belong to the possible composition
region x D ∈ Reg D in the concentration simplex, containing only the components
of top product and of entrainer C D+E . We note that the coordinates of the bottom
product point depend on the flow rate and composition of the entrainer.

8.4.5. The Most Interesting Splits of Columns with Decanters
The splits in the columns with decanters are the most interesting for synthesis, if
in the concentration simplex there are regions of existence of two liquid phases
Reg L−L and a heteroazeotrope that is the unstable node N − of the concentration
Haz
simplex C n or its boundary element C (k) having a trajectory tear-off region of the
t
t
top or intermediate section Reg or Reg .
r e
In the listed cases, one chooses the split at which a vapor close in composition
to heteroazeotrope y D ≈ N − goes from the column to the decanter, one of the
Haz
liquid phases is taken from the decanter as top product of the complex “column +
decanter” x D = x L2 , bottom product point of the column belongs to the possible
product region of the concentration simplex x B ∈ Reg B , and product points lie in
different distillation regions Reg ∞ and Reg ∞ (see Chapter 6). The second phase
1 2
from the decanter or a mixture of two phases is used as reflux (at heteroazeotropic
distillation) or as entrainer (at heteroextractive distillation, while, besides that, an
additional amount of the entrainer is brought into the column).
The above-discussed algorithms of presynthesis for simple columns, columns
with two feedings, and columns with decanters do not require visualization and
can be executed automatically, which allows them to be used for mixtures with
any number of components.
However, visualization can be useful at the ends of the sequence, when ternary
and binary azeotropic mixtures are obtained. Visual analysis of distillation dia-
grams is desirable at this stage of presynthesis and synthesis for application of
such separation methods as usage of curvatures of distillation regions boundaries,
of different of the pressure in the columns, and of entrainers (see Section 8.5).

8.4.6. Examples of Presynthesis

8.4.6.1. Example 1: Simple Columns

To illustrate the algorithm of presynthesis, we examine two examples of ho-
mogenous azeotropic mixtures. The first example is a four-component mixture