Page 386 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

There are other column arrangements possible for simple distillation. Some of these are illustrated in
Figure 12.2. Figure 12.2(a) illustrates distillation with a side stream. It must be understood that in a
typical distillation column, a side stream does not contain a pure component; it contains a mixture. In
certain petroleum refining operations, side streams are common because a mixture is the desired product.

Sometimes a side stream is withdrawn because it contains a maximum concentration of a third component
—for example, in the purification of argon from air.

Figure 12.2 Other Distillation Column Arrangements for Simple Distillation

Figure 12.2(a) should not be confused with Figure 12.1(d), because the presence of the vertical baffle in
the latter makes that column behave differently. Figure 12.2(b) represents a stripping column (sometimes

called a reboiled absorber), and Figure 12.2(c) represents a rectifying column. A stripping column is
used when the light components are very dilute and when they need not be purified. A rectifying column is
used when the heavy components are very dilute and need not be purified. The performance of the
stripping and rectifying columns can be understood from their McCabe-Thiele representations, shown in
Figures 12.2(d) and 12.2(e), respectively.

12.3.3 Azeotropic Distillation

Distillation involving azeotropes does not conform to the guidelines discussed in Section 12.3.2. This is
best illustrated by examining the thermodynamic behavior of a binary homogeneous azeotrope, illustrated