The Role Of Recycle In Countercurrent Recycle Distillation Cascades .1. Constant Reflux, Ideal, And Squared-Off Cascades

In most textbooks concerned with countercurrent multistage separations, minimum reflux ratio for continuous distillation is usually defined only in terms of a graphical construction on a McCabe-Thiele diagram: it is the recycle ratio (liquid flow rate) associated with the operating line that touches the equilibrium curve at the feed point. However, it is easily shown that minimum recycle ratio depends on local a and composition, as well as product compositions, and thus, it is a stage-composition phenomenon. As a result, for a specified separation, each ideal stage in a continuous distillation cascade has a specific minimum recycle ratio associated with it. For constant ct, the minimum recycle ratio increases as the stage compositions depart more from product (distillate or bottoms) compositions. As a result, the textbooks only consider the maximum minimum recycle ratio. This paper presents the results of some theoretical calculations which illustrate how minimum recycle ratio varies with stage cc, stage and product compositions, and presents an example of distillation cascade behavior when minimum recycle ratio is approached at a composition other than the feed point. An example is also presented which shows how the separation is effected when the reflux ratio is reduced below the design value in a distillation column containing a fixed number of ideal stages. A brief comparison is also made between constant reflux, ideal, and squared-off cascades in terms of number of stages, total interstage flow, and relative energy requirements for the different designs to illustrate and emphasize the consequences of the stagewise behavior of minimum recycle ratio.