This is perhaps an idea you remember from high school, but never quite understood. The phase rule corresponds to determining how many independent variables we can fix in a process before all the other variables become dependent variables. In a reflux drum, we can fix the temperature and composition of the liquid in the drum. The temperature and composition are called independent variables. The pressure in the drum could now be calculated from the chart in Fig. 5.4. The pressure is a dependent variable. The phase rule for the reflux drum system states that we can select any two variables arbitrarily (temperature, pressure, or composition), but then the remaining variable is fixed.

A simple distillation tower, like that shown in Fig. 5.2, also must obey its own phase rule. Here, because the distillation tower is a more complex system than the reflux drum, there are three independent variables that must be specified. The operator can choose from a large number of variables, but must select no more than three from the following list:

• Tower pressure

• Reflux rate, or reflux ratio

• Reboiler duty

• Tower-top temperature

• Tower-bottom temperature

• Overhead product rate

• Bottoms product rate

• Overhead product composition

• Bottoms product composition