Page 228 - Distillation theory

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202 Distillation Trajectories in Infinite Complex Columns and Complexes

We now discuss the general algorithm of calculation of minimum reflux mode
for the column with several side withdrawals located above and below feed cross-
section at sharp separation in each section and at the best separation between
products.
Minimum reflux mode is determined by the conditions of joining of trajectories
of two sections adjacent to the feed cross-section. Therefore, the interconnected
parameters (L/V) min and (V/L) min are determined initially for these two sections.
r s
Compositions in points x and x are calculated preliminarily for these sections

D B
at set requirements to compositions of all the products at the conditions of sharp
or quasisharp separation in each section. Minimum reflux mode is calculated in
the same way as for the simple column that separates initial raw materials into
products of compositions x and x . Liquid and vapor flow rates for the other

B
D
sections are calculated at the obtained values of (L/V) min and (V/L) min with the
r
s
help of material balance equations (strictly speaking, with the help of equations
of material and thermal balance).
Trajectories of each section can be calculated at the composition of the pseudo-
product of this section. Part of section trajectory between the points of its product
and pseudoproduct is fictitious as is seen at Fig. 6.14 (segment x D1 − x ).

D1
Minimum reflux mode for the columns with side withdrawals of flows was
discussed in the works (Sugie & Benjamin, 1970; Kohler, Kuen, & Blass, 1994;
Rooks, Malone, & Doherty, 1996; Bausa, Watzdorf, & Marquardt, 1997).
6.8.2. The Columns with Side Strippings
We now turn to the columns with side strippings (for the columns with side recti-
fiers, the calculation of minimum reflux mode is carried out the same way).
Minimum reflux mode for the columns with side strippings at separation of
three-component mixtures was investigated in the works (Glinos & Malone,
1985; Fidkowski & Krolikowski, 1987), and at separation of multicomponent
mixtures using Underwood equations system, this mode was investigated in the
work (Carlberg & Westerberg, 1989a). Splits without distributed components in
each two-section column entering distillation complex were examined in these
works.
We shortly examine the general algorithm for multicomponent nonideal mix-
tures.
Figure 6.15 shows the simple example of separation of a four-component mix-
tureintofourpurecomponentsinacolumnwithsidestrippings.Asforthecolumns
with side withdrawals of the products, the calculation of minimum reflux mode
should be started with determination of the conditions of joining of trajectories
of two sections adjacent to feed cross-section. For section r 1 , the pseudoproduct
equals the sum of top and two side products. The minimum reflux mode for the
first two-section column is calculated the same way it is done for the correspond-
ing simple column with split 1,2,3 : 4 (indirect split). In a more general case, when
the bottom product contains more than one product component, the intermediate
split will be in this column.