vogel_practical_ochem_3.pdf

94 PRACTICAL ORGANIC CHEMISTRY [II.
(compare previous paragraph). The flask is then heated with a small
flame, protected from draughts so as to ensure a uniform heating ; an air
bath (Fig. //, 5, 3) gives better results. The initial heating must not be
hurried as owing to the considerable extra condensation which occurs
whilst the column is warming up, the latter may easily choke with liquid.
Once distillation has commenced, the size of the flame is adjusted so that
the liquid passes over at the rate of one drop every two or three seconds.
Under these conditions fairly efficient fractionation should be obtained.
When the low boiling point fraction has passed over, distillation should
cease. The heating is then slowly increased, and a sharp rise in boiling
point should occur as the second fraction commences to distil; it is
assumed, of course, that the fractionating system is capable of effecting
a sharp fractionation of the components of the mixture. If the set-up is
inefficient, a relatively large intermediate fraction may be obtained. It
is desired to emphasise the fact that the distillation must be conducted
slowly ; no time is usually saved by distilling rapidly since a second
fractionation will then be necessary.
11,17. IMPROVED APPARATUS FOR FRACTIONAL
DISTILLATION
A detailed discussion of the theory underlying modern developments
in fractional distillation is outside the scope of this volume,* but a brief
description of the terms used in discussing fractionating columns and the
chief desiderata of efficient columns will be given. The capacity of a
column is a measure of the quantity of vapour and liquid which can be
passed counter-current to each other in a column without causing it to
choke or flood. The efficiency of a column is the separating power of a
definite length of the column; it is measured by comparing the performance
of the column with that calculated for a theoretically perfect plate
column under similar conditions. A theoretical plate is defined as a
section of a distilling column of such length that the vapour comes to
equilibrium with the liquid, i.e., the vapour leaving the " plate " has the
same composition as the vapour entering it and the vapour ascending
from the " plate " is in equilibrium with the hquid descending from it.
The number of theoretical plates cannot be determined from the dimensions
of the fractionating column ; it is computed from the separation
effected by distillation of a hquid mixture {e.g., benzene and toluene ;
benzene and carbon tetrachloride ; benzene and dichloroethane ; nheptane
and methylci/cZohexane), the vapour and hquid compositions of
which are accurately known.f An ordinary one cm. tube one metre long
might be equivalent to only one theoretical plate, whilst the same tube
filled with a suitable packing can give the equivalent of 20 or more
theoretical plates. A column with 12 theoretical plates is satisfactory
for the practical separation of a mixture of benzene and toluene. The
effectiveness of a column depends upon the height as well as upon the
* See, for example, A. Weissberger (Editor), Technique of Organic Chemistry, Volume IV.
Distillation, 1951 (Interscience).
t For experimental details, see, for example, Daniels, Matthews and Williams, Expert*
mental Physical Chemistry, 4th Edition, 1949, p. 95 (McGraw-Hill).