Page 236 - Distillation theory

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210 Distillation Trajectories in Infinite Complex Columns and Complexes

x B → S s → x f ⇐⇓ x f −1 → qS m → y D ≈ N Haz ⇔ x L2
R
Reg B Reg t s Reg sh,R Reg att Reg w,int Reg vap Reg bound
sep,s
L1−L2
Comparison of the calculated trajectories of a two-section column (Fig. 6.16b)
with those of one-section column (Fig. 6.16c) shows a sharp difference between
them.Trajectoryoftheone-sectioncolumnliescompletelyinthedistillationregion
∞
Reg containing isopropanol. This trajectory does not have points of sharp twist
1
that are quasizones of constant concentrations:
x B → S s → x f → y D ≈ N Haz ⇔ x L2
Reg Reg t Reg sh,R Reg Reg bound
B s sep,s vap L1−L2
We note that, in some cases, complex of heteroazeotropic distillation can be
used even if the ternary azeotrope–unstable node is located outside the region of
existence of two liquid phases (i.e., it is a homoazeotrope). In such cases, compo-
sition of the mixture in decanter can be shifted into the region of existence of two
liquid phases by adding entrainer into the decanter.
We examined above mixtures with ternary heteroazeotrope. Figure 6.16d shows
calculated trajectory for the mixture with binary heteroazeotrope–unstable node.
The example is separation of the mixture acetic acid(2)-water(3) with butyl ac-
etate(1) as an entrainer (Othmer, 1978; Bril et al., 1985). Calculated trajectory
in homogeneous region from the point of bottom product x B (acetic acid) passes
t
along the side acetic acid(2)-water(3), then tears off from it in point S s = x , in-
s
tersects the boundary of the region of existence of two liquid phases Reg bound ,
L1−L2
and passes through this region to the point of heteroazeotrope y D ≈ x Haz (vapor
compositions at trays in this region Reg L1−L2 lie at vapor line Reg vap ). We note
that the bottom section trajectory tear-off from the side acetic acid(2)-water(3)
is feasible only in the segment α 13 − 3 because the structural condition of bot-
tom section trajectory tear-off is valid only in region Reg 132 (absent at the side
ord
component–butyl acetate(1) becomes the lightest component).
Figure 6.16e shows separate usage of a distillation column and a decanter at
the bottom product when binary heteroazeotrope is saddle. The example can be
separation of the mixture butanol(1)-acetone(2)-water(3) (Pucci, Mihitenho, &
Asselineau, 1986). Sections trajectories do not differ from trajectories at separa-
tion of homogeneous mixture of the same type. Figure 6.16f shows joint usage
of the distillation column and decanter for the same mixture. The decanter is in-
stalled at the side product. Water is withdrawn from the decanter, and the organic
phase is returned into the column. The bottom product of the column is butanol.
Heteroazeotropic distillation cannot be used for separation of some types of
mixtures having the region of existence of two liquid phases Reg L1−L2 , but het-
eroextractive distillation can be applied.

6.9.2. Heteroextractive Distillation

At heteroextractive distillation, one of the phases in the decanter plays the role
of entrainer, and the section above the input of the mixture being separated plays