Distillation - Principles of Chemical Separations with Environmental Applications - page 98

Lower (stripping) section

The portion of the column that lies below the feed plate is called the stripping section. The
function of this section is to obtain a nearly pure bottoms stream, B ,ofthe less volatile
component of the feed. As with the rectifying section, we can analyze in terms of a mass
balance to obtain an operating line relating the vapor-phase mole fraction to the liquid-
phase mole fraction exiting a stage. Again, it is assumed that the molar flowrates B , L and
V are constant and that constant molar overflow applies. The overall mass balance around
the bottom of the column, then, becomes:

L = B + VB = L − V , (4.21)

and the component mass balance of the more volatile species around any stage m is:

x B B = x m L − ( L − B ) y m + 1 . (4.22)

Combining these two equations and solving for the vapor mole fraction in terms of process
variables gives:

L B x B

L

y m + 1 =

B x m − B ,

(4.23)

L − −

or

−

L
V x m −

x B ( L V )

y m + 1 =

.

(4.24)

V

Similarly, with a single point and the L V ratio, a line describing mass balance in the

stripping-section of the column can be drawn. To obtain the point, let us look at what
happens when the liquid exiting the bottom of the column is at its bottoms concentration,
x B . Substituting x B into the stripping-section mass balance, we see that y

/

= x B or the

stripping line intersects the diagonal at ( x B , x B )atthe bottom of the column. Hence, if

L and V are known in addition to the bottoms concentration of the more volatile species,
a stripping line can be drawn. As before, it is important to note that since there is more
liquid than vapor in the stripping section (liquid removed in bottoms), the slope of this
operating line is GREATER than 1 .
Stripping columns also exist, just like the enriching, or rectifying, column in the pre-
vious example. The purpose of a stripping column would be to purify the least volatile
component; an enriching column would purify the most volatile component.

Macroscopic energy balance

In distillation, the separating agent is energy. Any complete analysis of a distillation system
would include energy balances. For a distillation column with both stripping and enriching
sections, the energy balance must take into account the enthalpies of all entering and
exiting streams and the energy requirements, or duties, of the condenser and the reboiler.