Page 282 - Distillation theory

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256 Trajectories of the Finite Columns and Their Design Calculation

liquid–liquid tie-lines for heteroazeotropes, of component-order regions, and of
residue curves. The most interesting splits in the main distillation column with a
decantor are chosen, taking into consideration this analysis (see Section 6.9).
Thefollowing centralstageofdesigningisdesign calculationofthemaincolumn
with a decantor for a chosen split (i.e., the determination of the necessary number
of trays and sections and of the best conditions of refluxing).
The method “tray by tray” in the direction upward along the column with
posterior calculation of decantor at corresponding organization of iterations is
the fastest and the most reliable method to solve the set task. Such choice of
calculation methods is conditioned by the fact that, for the most interesting splits
discussed in Section 6.9, the column complete with the decantor is an analog
of a simple column working at direct split (see Section 7.3). Besides stability of
such calculation, its additional advantage at heteroazeotropic distillation is in the
possibility to carry out the determination of the necessary number of sections in
the column (one or two) and of the necessary number of phases brought in into
reflux (one or two) in the process of calculation itself “tray by tray.” It is supposed
that the choice of one section and refluxing by one phase are preferable. Therefore,
the calculation is begun, taking into consideration these preferable conditions. If
during the calculation it turns out that at these conditions the required result of
separation cannot be achieved, they are rejected.
We examine the main steps of the algorithm. The given data at heteroazeotropic
and heteroextractive distillation are concentration of impurity components in the
bottom product. This information unambiguously determines the bottom product
composition if one component is impurity one (see Section 6.9 Figs. 7.16e,f and
7.17a,b). If two or more components are impurity ones (Fig. 6.16a÷d), then the
bottom product composition is set in initial approximation, taking into consider-
ation the ratios of phase equilibrium coefficients of impurity components in the
bottom product point. In this case, the bottom product composition is defined
more exactly later at iterations.
For the beginning of calculation by method “tray by tray,” it is necessary to
also determine the estimated composition at the top end of the column y D .In
the cases in Fig. 6.16e,f, this composition is determined by the set concentration
of impurities in the top product and, in the other cases in Figs. 6.16 and 6.17,
this composition is set at the intersection of vapor line with a certain liquid–
liquid tie-line close the heteroazeotropic liquid–liquid tie-line (the distance in the
concentration space from the point of vapor going into the decantor to this tie-
line is set). For heteroextractive distillation, it is necessary to also set the flow
rate of entrainer determining preliminarily its minimum flow rate (see Section
6.6).
Based on these data, the material balance of the column is calculated, the
composition of gross-feeding x F+E , compositions, and flow rates of phases L 1 and
L 2 , and flow rates of vapor and liquid in column sections at refluxing by one phase
are determined.
Then the calculation of the bottom section by method “tray by tray” is car-
ried out until obtaining at some tray the vapor composition corresponding to