Page 299 - Separation process engineering

(8-4)

(8-5)

Note that this result does not depend on the details of the distillation system.

The bottom operating equation for column 1 is Eq. (8-2). The top operating line is a bit different. The
easiest mass balance to write uses the mass balance envelope shown in Figure 8-4A. Then the top
operating equation is

(8-6)

This is somewhat unusual, because it includes a bottoms concentration leaving the stripping column
(column 2). The reflux for this top operating line is liquid of composition x w,β . The McCabe-Thiele

diagram for this system is shown in Figure 8-4B and is almost identical to Figure 8-3B.
Analysis of stripping column 2 is straightforward. It is easiest to use Figure 8-2A and develop the
operating equation for n-butanol. This bottom operating equation is

(8-7)
The feed to this column is the saturated liquid reflux of composition x B,α . This is a vertical feed line. Then

the overhead vapor y B,1,col2 is found on the operating line at x B,α (Figure 8-4C).

The two dashed lines in Figures 8-4B and C show the route of the overhead vapor streams as they are
condensed to saturated liquids (made into x values), and then sent to the liquid-liquid separator. The
lever-arm rule [see Eq. (2-26) and Figure 2-10] can be applied to the liquid-liquid separator (see
Example 8-1).

Note that each column operates in a region where either water (column 1) or n-butanol (column 2) is more
volatile. McCabe-Thiele diagrams are easiest to use if plotted for the more volatile component. Thus, we
plotted water mole fractions for column 1 and n-butanol mole fractions for column 2. It is also possible to
do the calculations on a single diagram (Separation Process Engineering, 2nd ed., p. 228), which results
in one of the calculations looking upside down.

The operation of distillation columns separating heterogeneous azeotropes can be quite erratic (Kovach
and Seider, 1987). Small shifts in the aqueous reflux rate can cause a number of trays to shift from
operation in the homogeneous region to a heterogeneous region. This erratic switching will cause large
variations in the product purity. These columns need very careful control of the reflux stream flow rate to
operate properly.
Several modifications of the basic arrangement shown in Figure 8-4A can be used. If the feed

composition is less than x butanol,α , then column 2 should be a complete column and column 1 would be just
a stripping column. The liquids may be subcooled so that the liquid-liquid separator operates below the