Page 281 - Distillation theory

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7.6 Determination of Necessary Tray Numbers 255

3. Calculationofnecessarytraynumbersinthesectionsofthefirsttwo-section
column at the specified set of excess reflux coefficients.
4. Choice of the following parameters for the first two-section column: (1)
excess coefficient of reflux, (2) the way of stripping, and (3) duty on
pumparound.
5. Repetition of items 2 ÷ 4 for the rest of two-section columns.

7.6. Determination of Necessary Tray Numbers at Heteroazeotropic
and Heteroextractive Distillation
For homogeneous mixtures, all steps of conceptual design calculation from the
choice of the split until determination of all main design and mode parameters can
be carried out almost without any participation of the designer. For heterogeneous
mixtures, the situation is more complicated. The structure of the field of equilib-
rium coefficients liquid–liquid (regions of the existence of two liquid phases) is
superimposed on the structure of the field of equilibrium coefficients liquid–vapor
(distillation regions, component-order regions). This leads to a great variety of
possible conditions of separation at heteroazeotropic and heteroextractive distil-
lation of various mixtures. Various configurations of the columns (one, two, or
three sections), and various sequences of columns and decantors, are possible.
It is necessary to solve the questions of optimal arrangement and of optimal
sequenceforeachparticularmixturebymeansofconductionofcalculationinvesti-
gations. At this stage, there is a necessity for participation of a specialist possessing
software for calculation of necessary tray numbers. This software should guaran-
tee the determination of necessary tray numbers in sections, the determination
of the best conditions of refluxing (reflux with one or two phases at their optimal
correlation) and the determination of the best number of sections.
Questions of optimal designing of heteroazeotropic distillation units were dis-
cussed in the works (Bril et al., 1974; Bril et al., 1975; Bril et al., 1977; Bril et al.,
1985; Ryan & Doherty, 1989; Pham & Doherty, 1990a; Pham & Doherty, 1990b;
Pham & Doherty, 1990c).
The preliminary stage of design – collection of experimental data on phase
equlibrium liquid–vapour (VLE) and liquid–liquid (LLE) for binary and ternary
constituents of the mixture under separation and creation of adequate model on
this basis – is of great importance for heteroazeotropic mixtures.
Works of a number of investigators have shown that usage of data only on
binary VLE does not create an adequate model that would satisfactorily describe
the conditions at heteroazeotropic or heteroextractive distillation. The same can
be said of the usage of data only on ternary LLE.
To adequately describe three-phase equilibrium liquid–liquid–vapor (VLLE),
with the help of such models of solution as NRTL and UNIQUAC, it is neces-
sary to find parameters of these models by means of simultaneous processing of
experimental data on binary VLE and ternary LLE.
The following preliminary stage of designing is analysis of the structure of
concentration space (i.e., the analysis of location in concentration simplex of node
and saddle stationary points) of regions of the existence of two liquid phases, of