Page 197 - Distillation theory

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

6.1 Introduction 171

D
x D

F1
x F1

D 1
x D1 Figure 6.1. A complex column with two feeds and one side product.
Control volume (dotted line) for obtaining material balance equations
j
for the intermediate section.
L,x
V,y j
j+1
j+1
F
2
x
F2

B
x B

For the section between points of feed input F 2 and product output D 1 , the
equation of material balance is as follows:

Vy j+1 = Lx j + Dx D + D 1 x D1 − F 1 x F1 (6.1)
Let’s designate the flow rate of the pseudoproduct:

D = D + D 1 − F 1 (6.2)
and the composition of the pseudoproduct:

x = (Dx D + D 1 x D1 − F 1 x F1 )/(D + D 1 − F 1 ) (6.3)
D
In contrast to the product point, the pseudoproduct point can be located not
only inside or at the boundary of the concentration simplex, but also outside it.
The latter case refers to columns of extractive distillation with two feeds, which
leads to new regularities of location of trajectory bundles and their stationary
points, that differ from regularities of location of top and bottom section trajec-
tories. Therefore, we pay a lot of attention in this chapter to trajectory bundles of
intermediate sections in extractive distillation columns.
As far as distillation complexes with flows branching are concerned, their main
peculiarity is that they can be described as sets of several two-section columns
interacting with one another.
Pseudoproduct points of intermediate sections of these complexes are always
located inside or at the boundary of the concentration simplex. Therefore, the