Fp. Mccandless, THE ROLE OF RECYCLE IN COUNTERCURRENT RECYCLE DISTILLATION CASCADES .1. CONSTANT REFLUX, IDEAL, AND SQUARED-OFF CASCADES, Separation science and technology, 30(18), 1995, pp. 3405-3439
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
.