Page 316 - Distillation theory

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290 Synthesis of Separation Flowsheets

and 2-3 are entirely filled up with possible top product points at distillation of the
4 4 4
mentioned ternary mixtures ([1-2] ≡ Reg D , [1-3] ≡ Reg , [2-3] ≡ Reg D ). Thus,
D
1,2 1,3 2,3
the whole contour of face 1-2-3 is filled up with possible top product points (i.e.,
4
belong Reg bound,D ). Therefore, we can state that the whole face 1-2-3 is also filled
1,2,3
(3)
up with possible top product points ({1-2-3} ≡ Reg ).
D
For face 2-3-4, it is sufficient to determine possible bottom product composi-
1 1
tion segment Reg B ∈ Reg bound,B at edge 2-3 at distillation of mixture 1,2,3 (this
2,3 2,3,4 1 1
segment joins vertex 2), possible bottom product segment Reg B ∈ Reg bound,B at
2,4 2,3,4
edge 2-4 at distillation of mixture 1,2,4 (the whole edge is entirely filled up with
possible bottom product compositions) and possible bottom product segment
1 1
Reg B ∈ Reg bound,B at edge 3-4 at distillation of mixture 1,3,4 (this segment joins
3,4 2,3,4
vertex 4). If we join with a straight line, the ends of possible bottom product seg-
1
ments at edge 2-3 and at edge 3-4, we obtain at face 2-3-4 the contour Reg bound,B
of possible bottom product region (Fig. 5.27b). 2,3,4
Therefore, we obtained the complete set of possible product points in the ver-
(1) (2)
texes Reg (1, 3 and 4), at the edges Reg (edge 1-3, the segment of edge 1-2,
D,B D,B
(3)
edge 2-4), and at the faces Reg of tetrahedron (face 1-2-3 and the region at
D,B
face 2-3-4).
If the feed composition is set, it is possible to determine feasible sharp split in
the column with one feeding. We accept that the composition of the mixture being
separated is equimolar.
We examine complementary boundary elements of concentration tetrahedron
that contain possible product regions. Such boundary elements are vertex 1 and
face 2-3-4 (direct split), edge 1-3 and edge 2-4 (intermediate split), and face 1-2-3
and vertex 4 (indirect split).
Atthedirectsplit1:2,3,4pointx B (0,333;0,333;0,333)islocatedoutsidepossible
bottom product region (see Fig. 5.27b). Therefore, such a split is feasible only if
the feed composition is changed with the recycle of component 2, component 4, or
their mixture. If the equimolar mixture of components 2 and 4 is used as a recycle,
then the rate of recycle flow will be close to minimum. Intermediate 1,3 : 2,4 and
indirect 1,2,3 : 4 split will be possible at any feed composition.
The following splits with a distributed component are also feasible: 1,2 : 2,3,4
and 1,2,3 : 2,4. The question of selection of preferable splits is discussed in the
section 8.5.1.

8.4.6.2. Example 1: Extractive Distillation
We now examine the possible sharp splits in a column with two feedings. At the
t(2)
beginning, we find the segments Reg e of the trajectory tear-off from the edges
t(3)
of the concentration tetrahedron and regions Reg e of tear-off from its faces in
the intermediate section.