(2) the main nitrator ;
TNT MANUFACTURE 369(3) one or more supplementary nitrators;
(4) a vessel for extraction of the waste acid (detoluator);
(5) two separators.
A schematic diagram of such a nitration unit with a washing column and a separator
is shown in Fig. 87.
FIG. 87. Flow-sheet of continuous nitration of toluene according to J. Meissner ([20]).The acid and toluene are introduced from tanks (1) to the nitrator (2), in which
the main part of the nitration proceeds. From here the contents pass to another
nitrator (3), and if necessary to still another where the reaction is completed. The
nitrators are arranged in series, each of them being operated co-currently. The
reaction mixture passes from nitrator (3) to separator (d), where the separation of
the nitro compound from the waste acid takes place. The nitrotoluene then flows to
a system of washing tanks ((6) is the first of these). The waste acid passes to vessel (5),
also continuously fed with toluene. By stirring the spent acid with a very small
quantity of toluene, the former is freed from any nitro compounds present and
at the same time the nitric acid still remaining in the spent acid (1-3%) reacts with
toluene to yield nitrotoluene. The purified acid along with toluene is transferred
by pump (8) to separator (9), where they are separated. The toluene, still containing
some nitrotoluene, is introduced to the main nitrator (2), in a continuous way, thus
realizing the counter-current principle and the spent acid, now containing only
about 0.3% of HNO 3 and not more than 0.5% of aromatic compounds, goes
direct to concentration, previous denitration being unnecessary here.