Page 261 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

262 Chapter 10 Absorption and stripping

The same operating line equation holds for the stripper column, but due to the column top and
bottom concentration values, the equilibrium line lies below the operating line in absorbers and above
it in case of strippers. This can be seen from the typical cases shown in Fig. 10.3A and B. Subscripts 1
and 2 in the figure denote the bottom and the top position of the tower.
Since the equilibrium curve frequently concave upward, (L/G) min is usually calculated based on
equilibrium at the column bottom, where both liquid and gas have the highest concentration of solute.
However, the equilibrium curve can be shaped, such that, the pinch point exists not at the end but
somewhere in the middle, as shown in Fig. 10.3C for an absorber. Then, the operating line shown in the
figure represents (L/G) min . Such a situation may be a result of heat effects that alter the equilibrium
conditions within the column.

(A) y 2 x 2 (B)
Operating line (min.
liq. flow) Equilibrium
y 1 y 2 line Optg.
Operating max line
line (actual (min.
liq. flow) gas flow)
y 2
Equilibrium Optg.
line line (actual
y 2 gas flow)
y 1
x 2 x 1 y 1 x 1 x 1 x 2

(C)

y 1
Operating P
lines

y 2
Equilibrium
line

x 2 x 1 x *
1
FIGURE 10.3
Equilibrium curve, operating line, and minimum (L/G) operating line for (A) absorber, (B) stripper,
(C) absorber with pinch point P between y 1 and y 2 .

For the cases of very soluble gases or operation under vacuum,L min may not be sufficient to wet the
entire packing surface, leading to poor liquid distribution. In such cases, it is desirable to recirculate
the liquid over the packing at the expense of a reduced concentration driving force and operate the