Page 55 - Distillation theory

P1: FCH/FFX P2: FCH/FFX QC: VINOD/IYP T1: FCH
0521820928c02 CB644-Petlyuk-v1 June 11, 2004 17:58

2.6 Minimum Reflux Mode: Fractionation Classes 29

The given trajectory belongs to some trajectory bundle bounded by its fixed
−
+
+
points (points N , S r and N of Fig. 2.5a and points S s and N of Fig. 2.5b), the
r r s
separatrixes of the saddle points S and the sides of the concentration triangle.
Knowledge about the regularities of the trajectory bundles arrangement under
the finite reflux provides an opportunity to develop the reliable and fast-acting
algorithm to fulfill design calculations of distillation to determine the required
number of trays for each section.

2.6. Minimum Reflux Mode: Fractionation Classes
Knowledge about the distillation process regularities under minimum reflux is
the background of the distillation theory, and this mode analyzing is the most
important stage of the distillation column design.
As it has been already mentioned above, at minimum reflux a column has in-
finite number of steps (N =∞) (i.e., the trajectory passes through one or more
stationary points of the bundle). In the column, these stationary points will cor-
respond to the so-called zones of constant concentrations that are identical at
adjacent trays.

2.6.1. Binary Distillation
Let’s consider now the binary distillation (Fig. 2.6a,b). Having a set value of pa-
rameter D/F, we start to increase R from0upto ∞ in the infinite column.
In Fig. 2.6a, R 1 = 0, R 3 > R 2 > R 1 . With the increase of R, while maintaining
D/F ratio, points x iD and x iB become remote from point x F , maintaining the
constant concentration area of both sections in the feed cross-section. Such a
mode is called the first class of fractionation. Its specific feature is that the feed
composition and the compositions in the areas of constant concentrations of both
sections, adjoining the feed tray, coincide.
In the case of R = R 3 , point x D coincides with the vertex 1 (x 1D = 1, x 2D = 0).
Such a mode is a boundary one for the first class of fractionation. Under this

a) b)
1 1
y 1 R 3 y 1 R 3
R 2 R 4
R 1 R 5
R 6

x 1 x 1 1
x
0 x B(3) x B(2) B(1) x F x D(1) x D(2) 1 0 x B(3) x F x D(3)
x
D(3)
Figure 2.6. Operating lines for (a) first and (b) third class of fractionation
for given feed x F . R 6 > R 5 > R 4 > R 3 > R 2 > R 1 , splits x D(1) : x B(1) at R 1 ,
x D(2) : x B(2) at R 2 , x D(3) : x B(3) at R 3 ,at R 4 ,at R 5 , and at R 6 .