Distillation - Principles of Chemical Separations with Environmental Applications - page 90

where D total is calculated from the Rayleigh equation. The distillate flowrate is based on
the maximum vapor, or flooding, velocity for which a column is designed. A mass balance
around the condenser gives

V max

D max =

D ) ,

(4.12)

1 + ( L /

where D max is the distillate flow corresponding to flooding velocity. Columns are usually
operated at a fraction of the maximum flowrate [2].

Example 4.1: batch distillation

Problem:
A single-staged batch distillation process is to be used to remove highly volatile
organics from water. 1000 kgmol of feed with x F = 1VOCs is charged into a batch

still. The water that is removed from the still can be no greater than x B , f =

0 .

005
VOCs. If the relative volatility is 4.3 and does not change appreciably, what quantities
of fluid are collected in the bottoms and distillate and what is the VOC composition
of the distillate stream?
Solution:
Unknowns are D , B , x D ; solve three equations:

0 .

F = 1000 = D + B (4.2)

Fx F = 0 . 1(1000) = Dx D + 0 . 005 B (4.3)

ln

3 ln 0

ln 0

B
1000

1
3

. 005(0 . 9) . 9

= + . (4.7)

. 0 . 1(0 . 995) 0 . 995

Solving (4.7) B = 354 kgmol;

then (4.2) D 646 kgmol;

and (4.3) x D = 0.154.

=

4.4 Continuous distillation

Most distillation processes are continuous and separate mixtures in which all the feed
components are relatively volatile. The feed to a distillation column is usually a liquid.
Foravapor feed, the column must be cooled to allow both liquid and vapor flows within
the column. This approach would not normally be needed in environmental applications.
A distillation column is designed with the feed stream entering somewhere close to the
middle where the temperature is above that of the more volatile species and below that of
the less volatile ones. A column consists of a number of discrete stages each at a different
temperature such that temperature increases from the top of the column to the bottom. The