#### 9.5.2 Variable Reflux Ratio

The batch distillation column can also be operated with variable reflux ratio to keep x_{D} constant. The operating Eq. (9-26) is still valid. Now the slope will vary, but the intersection with the y=x line will be constant at x_{D}. The McCabe-Thiele diagram for this case is shown in Figure 9-7. This diagram relates x_{W} to x_{D}. Since x_{D} is kept constant, the calculation procedure is somewhat different.

**Figure 9-7. McCabe-Thiele diagram for multistage batch distillation with constant x**_{D} and variable reflux ratio

With x_{D} and the number of stages specified, the initial value of L/V is found by trial and error to give the feed ...