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 operati ng 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 minimu m recycle ratio associated with it. For constant ct, the minimum recyc le 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 re sults of some theoretical calculations which illustrate how minimum re cycle ratio varies with stage cc, stage and product compositions, and presents an example of distillation cascade behavior when minimum recy cle 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 distillat ion column containing a fixed number of ideal stages. A brief comparis on is also made between constant reflux, ideal, and squared-off cascad es in terms of number of stages, total interstage flow, and relative e nergy requirements for the different designs to illustrate and emphasi ze the consequences of the stagewise behavior of minimum recycle ratio .