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This underutilized technique can provide an economic way to handle homogeneous binary azeotropes.
Recycling of process solvents is receiving a good deal of attention lately as engineers and chemists strive to reduce longterm costs and minimize waste. Distillation frequently is considered for handling these recycle streams because it is a familiar and robust unit operation, and because the required equipment already may exist on-site. The presence of an azeotrope, however, severely limits the use of standard distillation.
A minimum-boiling azeotrope restricts a distillation column's ability to completely separate the components of a binary mixture by limiting the composition of the overheads stream. The separation is particularly challenging if the azeotrope is homogeneous - that is, if the azeotrope does not phase split on condensation. In many cases, phase splitting does not occur even with significant subcooling; so, decantation cannot be used to facilitate the process. The separation sometimes can be improved by adding an agent that alters the relative volatility of the feed components (1,2), but this complicates the process by introducing a third component.
A simple change in pressure also can alter relative volatility; in some cases, this results in a significant change in the azeotropic composition and allows the recovery of feed components without adding a separating agent. In this article, we will review distillation processes that take advantage of this pressure sensitivity. We will only consider minimum-boiling azeotropes because most azeotropes encountered in solvent recycling applications fall into this category. Depending upon whether one or both components need to be isolated, pressure-sensitive distillation (PSD) can involve either one or two columns. The dual-column process sometimes is called pressure-swing distillation.
Now, if a homogeneous azeotrope is encountered in the course of developing or modifying a manufacturing process, to simplify recycling efforts first consider substituting alternative materials that do not form such an azeotrope. When this is not possible, several commercially available unit operations may be feasible for breaking the azeotrope. In addition to PSD, these include extractive and azeotropic distillations (1,2), liquid/liquid extraction (2,3), adsorption (4), pervaporation using membranes (5), and the addition of a salt to the mixture (6). The nondistillation techniques usually are used in combination with distillation. Of course, no single option is best for all applications.
Well-known commercially...