By using the QVF nitric acid concentration
process, it is possible to concentrate nitric
acid up to 99.5%.
The nitric acid concentration process is illustrated
in figure 1 overleaf.
Due to the existence of an azeotrope between
nitric acid and water, when the feed concentration
is less than 68% HNO3 it is necessary to use an
extractive distillation process. By adding sulphuric
acid to the nitric acid/water mixture, the equilibrium
curve is displaced to the point of eliminating
Weak nitric acid (40% to 70%) and concentrated
sulphric acid (85% to 98%) are fed to the top
of a packed tower. Steam is injected into the
bottom of the column. The nitric acid is stripped
out and delivered as overhead product at a concentration
of 98.5% to 99.5%, while weak sulphuric acid is
discharged from the base of the column at 65%
The nitric acid vapour is condensed and bleached
free of NOx by air in a short bleaching column.
The NOx passes thorugh a guard condenser to an
The stripping steam can be generated from a reboiler
at the base of the column. This will give energy
savings where the weak sulphuric acid is recycled
through a sulphuric acid concentration plant (see
The production rate of concentrated sulphuric
acid can be up to 100 tonnes per day in a single
The nitric acid concentration process is adaptable
to a wide range of feed flowrates and specifications.
Plant performance also varies according to the
concentration of sulphuric acid available.
It is usual to minimise the energy consumption
by preheating the feed streams by interchange
with the products.
A typical specification using 85% H2SO4 is as
Nitric Acid Product: 2,100 kg/hr @ 99% w/w
Sulphuric Acid Feed: 9,250 kg/hr @ 85% w/w
Consumption of steam tonne/tonne of nitric acid(100%):
. Using similar technology, it is possible to
recover sulphuric and nitric acids from nitration
waste acids. Nitration waste acid arises from
organic nitration processes and consists of a
mixture of nitric acid, sulphuric acid, water
and organic contaminants.
. The dilute sulphuric acid stream can be reconcentrated
using a QVF sulphuric acid concentration system.
. NOx treatment
The denitration, nitric acid and sulphuric acid
concentration processes with NOx treatment plants
are often integrated into a single system.