Page 270 - Distillation theory

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244 Trajectories of the Finite Columns and Their Design Calculation

of section trajectories (see Section 7.3). In contrast to two-section columns, for
columns of extractive distillation, we have an additional degree of freedom of
designing (the entrainer rate) and an additional cross-section of joining of section
trajectories (the cross-section of input of the entrainer).
A number of works paid great attention to the questions of optimal designing
of extractive distillation columns for separation of binary azeotropic mixtures
(Levy & Doherty, 1986; Knight & Doherty, 1989; Knapp & Doherty, 1990;
Knapp & Doherty, 1992; Wahnschafft & Westerberg, 1993; Knapp & Doherty,
1994; Bauer & Stichlmair, 1995; Rooks, Malone, & Doherty, 1996; 1993). The
region of possible mode parameters of extractive distillation process, limited by
minimum rate of the entrainer and by limits of changing of reflux number between
minimum and maximum values, was investigated. Some heuristic rules were in-
troduced for the choice of rate of the entrainer and the reflux number.
The development of the theory of intermediate section trajectory tear-off from
boundary elements of concentration simplex (Petlyuk, 1984; Petlyuk & Danilov,
1999) expanded the application sphere of extractive distillation process to multi-
component mixtures.
This is especially important for the solution of the task of separation flowsheet
synthesis of multicomponent azeotropic mixtures (see Chapter 8) because this,
in many cases, uses autoextractive distillation (i.e., to exclude the application of
entrainers).
The product purity is set (specified) at designing, while the tray numbers in the
three sections of the column n r , n m , and n s , the ratio between the flow rate of the
entrainer, and the flow rate of the initial mixture E/F (it is frequently convenient
to use, instead of this parameter, the ratio between the flow rate of the entrainer
and the flow rate of top product E/D) and the reflux number in top section L/D (it
is frequently convenient to use instead of this parameter the ratio of flow rates of
liquid and vapor in the intermediate section (L/V) m ) are main design parameters
that have to be determined.
The main part of the algorithm is the calculation of section trajectories and the
determination of optimal tray numbers in the sections at set two mode parameters.
This includes in the the same main stages as at the calculation of two-section
columns: the calculation of reversible distillation trajectories of the three sections;
theobtainingoflinearequationsystemsforseparatrixbundlesofthethreesections
and for the manifolds, including the boundary elements of these bundles and point
x F ; the determination of coordinates of possible composition segments in the
feed cross-section and in that of input of the entrainer; the calculation of section
trajectories by method “tray by tray.”
As in the two-section column at reflux larger than minimum, in the column
of extractive distillation at (L/V) m > (L/V) min there are possible composition
m
segments at the trays adjacent to the feed cross-section from above and below
sh
[x f −1 ] and [x f ] sh (Fig. 7.12). Same as for the two-section columns, the coordi-
lin lin
nates of the ends of these segments can be determined from purely geometric
considerations from the known coordinates of the stationary point and point x F
(see Section 7.2).