Page 230 - Distillation theory

P1: FCH/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c06 CB644-Petlyuk-v1 June 11, 2004 20:17

204 Distillation Trajectories in Infinite Complex Columns and Complexes

x , the concentration of this component is smaller than in point S r1 ). This avoids
D1
repeated mixing (remixing) of flows that takes place while using the sequence of
the simple columns. As for the columns with side withdrawals of products, such
joining of sections maintains the best separation at set expenditures of energy
or the smallest expenditures of energy at set requirements to the quality of the
separation products. The difference between vapor and liquid flows of section r 1
is the feed of the second two-section column consisting of sections r 2 and s 2 (in
contrast to that for a simple column the liquid part of the feed is negative). Figure
6.15c shows trajectories of sections r 2 and s 2 in the mode of minimum reflux (part
of the trajectory from point x to tear-off point S r2 is fictitious, there is no leap
D2
of concentrations in the feed cross-section).
In the example under consideration, in the second column as in the first one
there is indirect split 1,2 : 3, but if product of section s 2 contains more than one
product component, then there is an intermediate split.
The last two-section column containing sections r 3 and s 3 is calculated in the
same way as the second one. The described algorithm is a general one: it embraces
columns with any number of side strippings and with any number of product
components in each product.

6.8.3. The Petlyuk Columns
The main difference between the minimum reflux mode calculation algorithms for
Petlyuk columns and those for columns with side sections is the necessity to take
into account the availability of distributed components. The complex in Fig. 6.13d
is an exception. Therefore, this complex can be calculated the same way as the
columns with side strippings, beginning with the first two-sections column, whose
feed is the mixture being separated and whose top and bottom pseudoproducts
are the feeds of the second and third columns correspondingly, and then passing
to two other two-section columns. The mode in one of these columns is the control
one, and the second column works at a reflux bigger than minimum. As well as for
the columns with side sections, for Petlyuk columns, the smallest expenditures of
energy for separation are achieved at the joining of section trajectories of the first
and following columns in cross-sections S r1 and S s1 without remixing of flows.
The availability of distributed components is of considerable importance for
other types of Petlyuk columns. The minimum reflux mode calculation for such
columns was examined for three-component mixtures in the work (Fidkowski &
Krolikowski, 1986) and for multicomponent mixtures with several distributed
components on the basis of the Underwood equation system in the work
(Carlberg & Westerberg, 1989b) and also in a number of other works (Cerda &
Westerberg, 1981; Glinos & Malone, 1988; Nikolaides & Malone, 1988; Chris-
tiansen & Scogestad, 1997). The availability of distributed components, first, leads
to the necessity to use for the calculation of minimum reflux mode the correspond-
ing algorithm, and, second, it creates an additional degree of freedom of designing
in the corresponding two-section columns of the complex. For example, for the
complex at Fig. 6.12c the additional degree of freedom is the ratio between the