In the simple cascade of Fig. 12.12, whose performance was illustrated in Fig. 12.14, it is impossible to obtain high recovery of desired component because of losses in the tails streams leaving every stage. Desired component in these streams can be recovered by recycling these

streams to a lower stage in the cascade. Figure 12.13 illustrates the simplest type of recycle cascade, in which the tails stream from stage і +1 is recycled to become part of the feed to stage і from which stage / + 1 received part of its feed. This recycle flow scheme is by far the most common in countercurrent separation processes. It is approached, for example, in a bubble-plate distillation column and is used in a battery of series-connected solvent extraction mixer-settlers or in the gaseous diffusion cascade of Fig. 12.2. More complex recycle flow schemes will be treated in Sec. 14.