Page 331 - Distillation theory

P1: JPJ/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c08 CB644-Petlyuk-v1 June 11, 2004 20:20

8.5 Multicomponent Azeotropic Mixtures: Automatic Sequencing and Selection 305

8.5.2.2. Example 2

For the second example, separation of mixture water(1)-methanol(2)-acetic
acid(3)-aceton(4)-pyridin(5), both possible splits in the first column with one feed-
ing without distributed component 2,4 : 1,3,5 and 1,2,4 : 3,5, are not expedient
because one of the products is binary mixture with azeotrope.
Split with distributed component for the first column with one feeding (1)
1,2,4,5 : 1,3 and three splits for the first column with two feedings and two-
component entrainer; (2) 4 : 1,2,3,5 (entrainer 1,3); (3) 2 : 1,3,4,5 (entrainer 3,5);
and (4) 4 : 1,2,3,5 (entrainer 3,5) are preferable splits in the first column.
We turn to the following columns. For split 1,2,4,5 : 1,3 in the first column,
top product is mixture 1,2,4,5, which can be separated in the second column with
one feeding at split 1,2,4 : 5 because, in face 1-2-4 of the concentration simplex
5
1-2-4-5, there is a possible top product region Reg D joining side 2-4 (Fig. 8.13b),
1,2,4
1,2,4
+
and vertex 5 is a stable node N ≡ Reg B . Therefore, for the part of possible
5
feed composition x F(2) = x D(1) obtained from the previous column, the recycle of
component 2 will be necessary. For further separation of the top product of second
column (mixture 1,2,4), it is necessary to use in the third column autoextractive
distillation with entrainer − component 1, top product – component 4, and bottom
product − zeotropic mixture 1,2 because, in edge 1-4 of the concentration triangle
2 4,2,1
t
1-2-4, there is tear-off region Reg = Reg ord (Fig. 8.13b). Component 1 is the top
e
1,4
product at separation of the bottom product of the first column (mixture 1,3), and
it simultaneously plays the role of the bottom product at separation of the bottom
product of the third column (mixture 1,2). That unites two columns of binary
distillation into one complex column with two feed flows and one side product –
component 1. Therefore, we got a sequence in Fig. 8.24a.
We turn to the following splits in the first column. Mixtures 1,2,3,5 and 1,3,4,5
(bottom products at extractive distillation in first column at splits 2 ÷ 4) can be
separatedonlyinthecolumnwithonefeedingatsplitswithdistributedcomponent.
For separation of mixture 1,2,3,5 in the second column (sequences in Fig. 8.24b,
d), split 1,2,5 : 1,3 is possible (in face 1-2-5, there is a possible top product region
3
Reg D [Fig. 8.13c] and, in edge 1-3, there is a possible bottom product segment
1,2,5
2,5
Reg B [Fig. 8.12]), and the top product of this column, containing binary azeotrope
1,3
15, can be separated by means of distillation in the simple third column with
5 1,2
preliminary recycle of component 2 at split 1,2 : 5 (there are Reg D and Reg B [Fig.
8.13c]). 1,2 5
For separation of mixture 1,3,4,5 in the second column (sequence in Fig. 8.24c),
split 1,4,5 : 1,3 is possible (the whole face 1-4-5 is possible top product region
3 4,5
Reg D [Fig. 8.13f] and, in edge 1-3, there is possible bottom product segment Reg B
1,4,5 1,3
[Fig. 8.12]), and the top product of this column, containing binary azeotrope 15,