A useful quantity in describing distillation is the relative volatility ? _AB defined as

(5-21)

This equation for relative volatility indicates the ease or difficulty of achieving separation of A and B (large or small ? values indicate easy separation while a value of 1 means A and B are inseparable). It also provides a means of generating equilibrium data because ? is a fairly constant quantity at a given total pressure.

McCabe-Thiele calculations yield ideal or equilibrium stages. In order to obtain real or actual stages we must use an efficiency. Correlations for overall column performance were developed by Drickamer and Bradford (43) and later by a modification published by O'Connell (44). Figures 5-17 and 5-18 provide these correlations.

Figure 5-17: Overall efficiency; m is K. (Reproduced with permission from reference 44. Copyright 1946 American Institute of Chemical Engineers.)

Figure 5-18: Overall column efficiencies. (Reproduced with permission from reference 45. Copyright 1932 American Chemical Society.)

Both Figures 5-17 and 5-18 were developed for bubble cap trays. However, they also can be used for sieve or valve trays.

More detailed discussions of efficiencies are given in (39 42).