278 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES
a setting point of 49°C, the pure p-nitrotoluene (s.p. 51°C) remaining in the crys-
tallizer is melted out, dried and flaked. The mother liquor is distilled, and the
oil collected during the heating up process is put back into the crystallizer.
Distillation of the mother liquor. 10,000 kg of p- nitrotoluene mother liquor
are sucked into a still of 10 m^3 capacity with a column of 40 plates, of 8 m height
and 1 m diameter. Distillation conditions are the same as in the distillation ofcrude nitrotoluene.
The first fraction (ca. 1500 kg) consists of crude o- nitrotoluene (70-80% o-
and 20-30% m- isomer). The distillate has a setting point of ca. - 15°C. It drops
to -20°C and at this temperature the collection of a second fraction (ca. 4000 kg)
starts. The setting point continues to fall to ca. -30°C and then rises to +5°C.
The distillate contains ca. 50% meta-, the rest being mainly ortho- isomer.
When the setting point of still contents reaches 40°C the third fraction is dis-
tilled off without the column. This is crude p- nitrotoluene (4500 kg). It is purified
by crystallization as above.
Pure m- nitrotoluene is isolated by further distillation.Continuous vacuum distillation of mononitrotolueneThe following modem continuous distillation unit is described by Vorontsov
[20].
A neutralized, washed and dried crude mixture of nitrotoluenes passes from
container (2) into the rectifying column (4), where o- nitrotoluene is distilled off.
The column is 12 m high and 0.6 m diameter and is provided with boiler (3)
still-head (5) and condenser (6). Before entering the column, the crude material
passes through the lower part of the still-head, the expansion vessel (20) and
a flowmeter which regulates the rate of supply of the liquid.
Steam supplied for heating the column (through a coil) has a pressure of 18-
20 atm. It is fed through the boiler (3) and regulating valve (22). A constant level
in the boiler is maintained by means of regulator (25). The vapours are partially
condensed in the still-head (5), cooled by water flowing from (1). The same water
is used to cool the condensers of both columns. The flow of water is regulated by
means of valves (29) and (27).
The condensed distillate (pure o- nitrotoluene) flows partly back to the column
but mostly down through (19) and the pump (14) transfers it into a storage tank.
The vapours passing from the still-head (also o-nitrotoluene) are condensed in (6).
From here they pass through (18) and pump (13) into a storage tank. If the con-
densed distillate is not of sufficient purity, it is sent back to the container (2) and
subjected to a second distillation.
A mixture of m- and p- nitrotoluene which leaves the lower part of the column
passes through the pump (15) into a second column (8). A part of it returns to column
(4) (to avoid a fall of the level there in it). If, due to any disturbance of the process,
the product flowing from the lower part of the column contains o- nitrotoluene,
it is fed to (2) through pump (15) and rectified again.