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Many chemical processes use high strength nitric acid, a typical example being explosives manufacture.

Nitric acid, manufactured by the ammonia oxidantion process, is generally produced at a concertration of about 65%. The nitric acid/water azeotrope at 68% prevents the acid from being concentrated above this level by normal distillation.

With the use of borosilicate glass and other corrosion resistant materials, QVF engineered systems offer a practical solution to nitric acid concentration.

By using the QVF nitric acid concentration process, it is possible to concentrate nitric acid up to 99.5%.

Process Description

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 the azeotrope.

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% to 72%.

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 absorption system.

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 associated processes).

The production rate of concentrated sulphuric acid can be up to 100 tonnes per day in a single stream.

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 follows:
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%): 0.5

Associated Processes

. 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.