Exploiting Boundary Curvature

Exploiting Boundary Curvature A second approach to boundary crossing exploits boundary curvature to produce compositions in different distillation regions. When distillation boundaries exhibit extreme curvature, it may be possible to design a column such that the distillate and bottoms compositions are on the same residue curve in one distillation region, while the feed composition (which is not... 

Shifting Boundaries, Exploiting Boundary Curvature, Reaching Saddle Compositions, and Solvent-Enhanced Separations... 

FIG. 13-67 Separation of methanol-methyl acetate by exploitation of distillation boundary curvature. 

FIG. 13-68 Separation of nitric acid-water system with sulfuric acid in a two-column sequence exploiting extreme boundary curvature. 

As an example. Van Dongen (Ph.D. Thesis, University of Massachusetts, 1983) considered the separation of a methanol-methyl acetate mixture, which forms a homogeneous azeotrope, using n-hexane as an entrainer. The distillation boundaries for this system (Fig. 13-87a) are somewhat curved. A separation sequence that exploits this boundary curvature is shown in Fig. 13-87b. Recycled methanol—methyl acetate binary azeotrope and methanol-methyl acetate—hexane ternary azeotrope are added to the original feed FO to produce a net feed com-... 

Exploitation of boundary curvature for breaking azeotropes is very similar to exploiting pressure sensitivity from a mass-balance point of view, and suffers from the same disadvantages. Separation schemes have large recycle flows, and in the case of minimum-boiling azeotropes, the recycle streams are distillates. However, in the case of maximum-boiling azeotropes, these recycles are underflows and... 

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