18.08.2015   NUCLEAR CHEMICAL ENGINEERING

Minimum Reflux Ratio

At total reflux, the difference in composition between corresponding streams on adjacent stages is a maximum. As the reflux ratio is decreased, the difference in composition decreases, and reaches zero at minimum reflux. A condition for minimum reflux thus is

37+i =Уі

From (12.20),

_ (a — 1)у,<1 -Уд

Уї *1+1 у. + a(! — у.)

(Ni+i _ <УР ~Уі)Уі+ eQ -37)]

V P /min (<*“IMl-37)

In terms of the tails composition x,+I, this is

(N>*i УРІСШІ*! + 1 — xi+1)-axi+1

P / min (a l)*l+l0 */+l)

Several special forms of Eq. (12.78) will be useful. When У( < 1,

(NuA v УР — У і a

P ) min 37 a f

But this difference in composition is already given by the material-balance equation (12.62), so that

This equation is applicable to the portions of a heavy-water separation plant or 235 U plant near the feed point. In a close-separation cascade, in which о — 1 < 1,

These equations all show that the minimum reflux ratio increases as the composition departs more from product or tails composition. In isotope separation cascades in which q is close to unity, the minimum reflux ratio is enormous. For example, at the feed point of a plant
(12.82)

Yet, as the product end of this cascade is approached, the minimum reflux ratio approaches zero.