Page 177 - Separation process engineering

be valid by the McCabe-Thiele diagram. We could also calculate and and check that the slope
of the bottom operating line is correct.
F. Generalize. The method of inserting the overall mass balance to simplify the intersection of the y =

x line and middle operating line to derive Eq. (4-45) can be used in other cases. The method for
calculating L′/V′ can also be generalized to other situations. That is, we can calculate D (or B),
find flow rate in section above (or below), and use feed conditions to find flow rates in the desired
section. Since we stepped off stages from the bottom up, the fractional stage is calculated from the
difference in y values (that is, vertical distances) in Eq. (4-49). Industrial problems use lower
reflux ratios and have more stages. A relatively large reflux ratio is used in this example to keep
the graph simple.

4.9 Other Distillation Column Situations

A variety of modifications of the basic columns are often used. In this section we will briefly consider the
unique aspects of several of these. CMO will be assumed. Detailed examples will not be given but will
be left to serve as homework problems.

4.9.1 Partial Condensers
A partial condenser condenses only part of the overhead stream and returns this as reflux. This distillate
product is removed as vapor as shown in Figure 4-20. If a vapor distillate is desired, then a partial

condenser will be very convenient. The partial condenser acts as one equilibrium contact.
Figure 4-20. Partial condenser; (A) balance envelope, (B) top operating line

If a mass balance is done on the more volatile component using the mass balance envelope shown in
Figure 4-20, we obtain

Vy = Lx + Dy D

Removing D and solving for y, we obtain the operating equation

(4-50)

This is essentially the same as the equation for a top operating line with a total condenser except that y D
has replaced x . The top operating line will intersect the y = x line at y = x= y . The top operating line is
D
D
shown in Figure 4-20. The major difference between this case and that for a total condenser is that the
partial condenser serves as the first equilibrium contact.