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 Liquid is withdrawn and a part of it is used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of the column is withdrawn and is reboiled to strip off the more-volatile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
XF is mass fraction of component A (more-volatile) in the feed, F'
is mass flow rate of feed, xF 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:
MA = 32.04, MB
= 18.02
Solution: XF = 0.5, F'
= 5000 lb/h, XD = 0.95, XW = 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
