Page 182 - Separation process engineering

and minimum reflux. In total reflux, all of the overhead vapor is returned to the column as reflux, and all
of the underflow liquid is returned as boilup. Thus distillate and bottoms flow rates are zero. At steady
state the feed rate must also be zero. Total reflux is used for starting up columns, for keeping a column
operating when another part of the plant is shut down, and for testing column efficiency.

The analysis of total reflux is simple. Since all of the vapor is refluxed, L = V and L/V = 1.0. Also, =
and / = 1.0. Thus both operating lines become the y = x line. This is illustrated in Figure 4-25B. Total
reflux represents the maximum separation that can be obtained with a given number of stages but zero
throughput. Total reflux also gives the minimum number of stages required for a given separation.
Although simple, total reflux can cause safety problems. Leakage near the top of the column can cause
concentration of high boilers with a corresponding increase in temperature. This can result in
polymerization, fires, or explosions (Kister, 1990). Thus, the temperature at the top of the column should
be monitored, and an alarm should sound if this temperature becomes too high.

Figure 4-25. Total reflux; (A) column, (B) McCabe-Thiele diagram

Minimum reflux, (L/D) min , is defined as the external reflux ratio at which the desired separation could just
be obtained with an infinite number of stages. This is obviously not a real condition, but it is a useful

hypothetical construct. To have an infinite number of stages, the operating and equilibrium lines must
touch. In general, this can happen either at the feed or at a point tangent to the equilibrium curve. These
two points are illustrated in Figures 4-26A and 4-26B. The point where the operating line touches the
equilibrium curve is called the pinch point. At the pinch point the concentrations of liquid and vapor do
not change from stage to stage. This is illustrated in Figure 4-26C for a pinch at the feed stage. If the
reflux ratio is increased slightly, then the desired separation can be achieved with a finite number of
stages.
Figure 4-26. Minimum reflux; (A) pinch at feed stage, (B) tangent pinch, (C) concentration profile
for L/D ~ (L/D) min