Page 183 - Separation process engineering

For binary systems the minimum reflux ratio is easily determined. The top operating line is drawn to a
pinch point as in Figure 4-26A or 4-26B. Then (L/V) min is equal to the slope of this top operating line

(which cannot be used for an actual column, since an infinite number of stages are needed), and

(4-55)

Note that the minimum reflux ratio depends on x , z, and q and can depend on x . The calculation of
D
B
minimum reflux may be more complex when there are two feeds or a sidestream. This is explored in the
homework problems.

The minimum reflux ratio is commonly used in specifying operating conditions. For example, we may
specify the reflux ratio as L/D = 1.2(L/D) min . Minimum reflux would use the minimum amount of reflux

liquid and hence the minimum amount of heat in the reboiler, but the maximum (infinite) number of stages
and a maximum (infinite) diameter for a given separation. Obviously, the best operating conditions lies
somewhere between minimum and total reflux. As a rule of thumb, the optimum external reflux ratio is
between 1.05 and 1.25 times (L/D) min . (See Chapter 11 for more details.)

A maximum / and hence a minimum boilup ratio /B can also be defined. The pinch points will look
the same as in Figure 4-26A or 4-26B. Problem 4.C12 looks at this situation further.

4.11 Efficiencies

Up until now we have always assumed that the stages are equilibrium stages. Stages that are very close to
equilibrium can be constructed, but they are only used for special purposes, such as determining
equilibrium concentrations. To compare the performance of an actual stage to an equilibrium stage, we