8.7. Azeotropic Distillation with Added Solvent

When a homogeneous azeotrope is formed or the mixture is very close boiling, the procedures shown in Section 8.2 cannot be used. However, the engineer can add a solvent (or entrainer) that forms a binary or ternary azeotrope and use this to separate the mixture. The trick is to pick a solvent that forms an azeotrope that is either heterogeneous (then the procedures of Section 8.2 are useful) or easy to separate by other means such as extraction with a water wash. Since there are now three components, it is possible to have one or more binary azeotropes or a ternary azeotrope. The flowsheet depends upon the equilibrium behavior of the system, which can be investigated with distillation curves and residue curves (Section 8.5). A few typical examples will be illustrated here.

Figure 8-16 shows a simplified flowsheet (extensive heat exchange is not shown) for the separation of butadiene from butylenes using liquid ammonia as the entrainer (Poffenberger et al., 1946). Note the use of the intermediate reboiler in the azeotropic distillation column to minimize polymerization. At 40°C the azeotrope is homogeneous. The ammonia can be recovered by cooling, since at temperatures below 20°C two liquid phases are formed. The colder the operation of the settler the purer the two liquid phases. At the −40°C used in commercial plants during World War II, the ammonia phase contained about 7 wt % butylene. This ammonia is recycled to the azeotropic column either as reflux or on stage 30. The top phase is fed to the stripping column and contains about 5 wt % ammonia. The azeotrope produced in the stripping column is recycled to the separator. This example illustrates the following general points: (1) The azeotrope formed is often cooled to obtain two phases and/or to optimize the operation for the liquid-liquid settler. (2) Streams obtained from a settler are seldom pure and have to be further purified. This is illustrated by the stripping column in Figure 8-16. (3) Product (butylene) can often be recovered from solvent (NH₃) in a stripping column instead of a complete distillation column because the azeotrope is recycled.