Continuous Distillation: In a typical distillation column, feed enters the distillation tower at the boiling point. The feed gets in contact with the liquid reflux traveling down the rectification section. The vapors leave the column and are condensed in a total condenser. Liquid is withdrawn and a part of it is used as reflux. The residue from the bottom of the column is withdrawn and is reboiled to strip off the more-volatile product. The section above the feed plate is called rectification section where more-volatile component is fractionated. The section below the feed plate is called stripping section where less-volatile component is fractionated. Heating steam is provided to strip more-volatile component.

The McCabe-Thiele method is used to calculate the number of stages. Typically, the composition of the distillate and residue are available. If the amount and composition of streams are given in mass basis, then we can obtain the corresponding values in molar basis as follows:

And the mole fraction of feed is given as

Where X_F is mass fraction of component A (more-volatile) in the feed, F' is mass flow rate of feed, x_F is

mole fraction of A in the feed, and F is molar flow rate of feed. Molecular weight of the feed can be computed as follows:

We can also obtain these values for distillate and residue as shown in the following example.

Example 6.7: Five thousand pounds per hour of a solution containing A and B are continuously rectified. Feed contains 50 % A and 50 % B on mass basis. The feed enters the column at 80 °F and is to be continuously rectified at 1 atm pressure. The distillate collected contains 95 % A and 5 % B on mass basis. The residue collected contains 1 % A and 99 % B on mass basis. The distillate is totally condensed to liquid. The reflux from the condenser is returned at the bubble point. For each of the streams (feed, distillate, and residue), determine molecular weight, composition on molar basis, and molar flow rate. The following data applies:

M_A = 32.04, M_B = 18.02

Solution: X_F = 0.5, F' = 5000 lb/h, X_D = 0.95, X_W = 0.01.

Molar flow rate of feed can be found using Equation 6.20

Feed composition on molar basis can be found using Equation 6.21

Feed contains 36 % A and 64 % B on molar basis.

Molecular weight of feed can be found using Equation 6.22

The molecular weight of residue can be calculated using Equation 6.23

And residue composition on molar basis can be calculated using Equation 6.24

The molecular weight of distillate can be found using Equation 6.25

And distillate composition on molar basis can be found using Equation 6.26

Distillate collected can be found using Equation 6.27

Residue collected can be found from mass balance equation as follows: