We have studied in Section 8.1 how a number of double-entry separation stages can be connected together in a countercurrent fashion. We have also learnt that such an arrangement achieves a far greater extent of separation in separation processes such as distillation, extraction, absorption and adsorption compared to what can be achieved in a single stage. Such multistage configurations are generally identified as “cascades,” and in the case of configurations of an overall countercurrent flow as “countercurrent cascades.” Such countercurrent cascades can have significant variations, for example ideal cascades, squared off cascades, tapered cascades, etc. Further, there can be novel countercurrent cascades with single-entry separation stages. In addition, there can be other types of connections between a number of single stages, resulting in, for example, crosscurrent cascades, two-dimensional cascades, etc.

We will briefly focus in Section 9.1 of this chapter on many such configurations of cascades, beginning with countercurrent cascades. Special attention will be paid to single-entry separation stages in, for example, the configuration of an ideal cascade. These cascades are generally employed to enhance substantially the separation achieved in a single stage for a binary mixture. Such cascades can also reduce the amount of energy or mass-separating agent needed for the separation of a binary mixture. In the separation of a multicomponent mixture by distillation, a number of such cascades may be joined together in particular ways – Section 9.2 will provide a brief introduction to this topic. Section 9.3 briefly considers cascades for multicomponent mixture separation involving other separation processes.