Splits semisharp

Because sharp separation is not always feasible for azeotropic mixtures, we also consider the best semisharp splits, when one of the products contains a smaller number of components than the feeding and when the possible product point of the second product is the farthest from the product point of the first product in the concentration space. 

Figure 3.2. P roduct points and distillation trajectories under infinite refiux for different number of trays (a) semisharp split, (b) sharp direct split, and (c) split with distributed component. Ideal mixture K > Ki> K/),xd( ),xd(1),xd(3), xb(i), xb(2),(3). product points for different number of trays, xp = const, D/F= const short segments with arrows, conjugated tie-lines hquid-vapor (distillation trajectories under infinite reflux) thick solid lines, lines product composition for different number of trays.

For the sake of briefness, we call the first of these conditions the term of connectedness. It has general nature - it can be applied to mixtures with any number of components and azeotropes. Moreover, the term of connectedness embraces not only sharp splits, when the product points lie in the boundary elements of the concentration simplex, but also the semisharp and nonsharp splits, when the product points lie in the boundary elements of the distillation region. 

Feasible sharp reversible distillation split of ideal mixtures can be presented as follows 1, 2,... (n - 1) 2,3... n. Therefore, at the reversible distillation, components 2, 3,... (n - 1) are distributed among the top and the bottom products. At nonsharp and semisharp reversible distillation, both products contain all the components or one of the products does not contain the lightest or the heaviest component. At nonsharp reversible distillation, product points lie in the same straight line as at sharp distillation but at some distance from the hyperfaces of the concentration simplex. 

The fifth rule sharp splits with a preliminary recycle for change of feed composition are more preferable than semisharp splits without preliminary recycle. Figure 8.22 shows two possible separation flowsheets of mixture acetone(l)-benzene (2)-chloroform(3) into pure components in the case of the bottom product point... 

However, sharp extractive distillation is not expedient because bottom product Xb is a binary mixture with azeotrope 13. Split of extractive semisharp distillation 2 1,2,3 (xg ) is preferable. Indirect split 2,3 1, at which the top product point... 

Figure 8.26. Phase equilibria map (a) and sequence for distillation of the acetic acid(l)-water (2)-formic acid(3) mixture with two pressures (pi andP2) in three columns with best semisharp splits.

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See also in sourсe #XX -- [ ]

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