Page 302 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

304 Chapter 11 Distillation

The design reflux for a column is required to lie between the limiting conditions of total and
minimum reflux. It has already been shown that the minimum reflux ratio R min for a separation
problem is fixed.

1.0

Rectification Operating
line at minimum
reflux
0.8

q-line
0.6 slope = q
q–1
Stripping Operating
line at
minimum
y reflux
0.4

0.2

0.0
0.0 0.2 0.4 0.6 0.8 1.0
x
B x F x D
x
FIGURE 11.11
McCabeeThiele construction for finding minimum reflux ratio.

A third limiting condition of interest involves the degree of separation. As a perfect split (x D ¼ 1
and x B ¼ 1) is approached for a reflux ratio at or greater than R min , the number of stages required near
the top and near the bottom of the column increases rapidly and without limit until pinch points occur
at x D ¼ 1 and x B ¼ 0. Hence, a prefect separation of a binary mixture that does not form an azeotrope
requires infinite number of stages in both sections of the column. However, the value of R is finite for a
perfect separation. For example, if the feed is saturated liquid, the minimum reflux for a perfect binary
separation is
1
(11.8)
R min ¼
x F ða 1Þ
where a is the relative volatility at the feed condition.
Hot and subcooled reflux
The McCabeeThiele construction considers the reflux stream to be generated by totally
condensing the overhead vapour, i.e., the reflux stream is at the bubble temperature corresponding to
the top tray pressure.