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Nitric Acid Concentration (NAC)

Nitric acid forms an azeotrope at about 69% HNO₃ boiling at 120.5° C under atmospheric pressure. The concentration of nitric acid by simple distillation can only reach the conditions of the azeotrope. By adding sulfuric acid to the nitric acid/water mix, the boiling behaviour can be altered so that more highly concentrated nitric acid can be distilled. This feature is demonstrated in Figure 3 of U.S. Patent 4,966,276, which shows the ternary diagram for the system H₂SO₄, HNO₃ and H₂O and includes the vapor composition in equilibrium with the boiling mixed acid.

"Conventional" Nitric Acid Technology
In the first installations using sulfuric acid as a dehydrating agent, which were built in the early 1920's, strong sulfuric acid and weak nitric acid were blended in suitable proportion and fed as a mixed acid to the top of a packed column. The energy required to boil off the nitric acid was provided by injecting live steam into the lower part of the column where the steam was absorbed by the acid. Strong nitric acid was obtained by condensing the overheads. The spent sulfuric acid of about 68% H₂SO₄, having absorbed the water originally present in the weak nitric acid and the live steam, was commonly sent to a fertilizer plant.

The process economics were later improved by feeding nitric acid and strong sulfuric acid separately into the column. In this way, the heat of dilution is available to assist the concentration process. Further improvement in economy is possible by avoiding live steam injection, using instead a reboiler as the source of energy input. This reboiler provides the stripping action required to push nitric acid to the top of the column without dilution of the sulfuric acid. The use of reboilers became the industry standard when tantalum became available as a material of construction.

As a final step in the gradual refinement of nitric acid concentration, sulfuric acid reconcentration and recycle has become an integral part of the process. Reconcentration of sulfuric acid avoids the problem of spent acid disposal.

For environmental and economic reasons, modern nitric acid concentration plants are designed to operate with reconcentrated sulfuric acid of about 85% H₂SO₄, the reason being that acid vapor losses in the sulfuric acid concentrator increase exponentially with sulfuric acid strength. The spent sulfuric acid concentration must always be above 68% H₂SO₄ in order to break the azeotrope.

Key Features and Benefits of the NORAM Process
The NORAM nitric acid concentration process differs from conventional technology in a number of ways. In the NORAM process:
•  A much larger volume of sulfuric acid is circulated
•  The strong sulfuric acid has an unusually low concentration of less than 75% H₂SO₄ and this concentration changes
   by a small percentage as the acid circulates through process. This feature is shared by NORAM's adiabatic nitrobenzene process.
•  The energy required for nitric acid concentration is supplied mostly by vaporizing weak nitric acid before it enters the
   concentration column.

Because of the high sulfuric acid circulation rate, this acid has a relatively large capacity to absorb energy. Use of sulfuric acid of low concentration eliminates the need to cool the strong acid leaving the sulfuric acid concentrator. This is done in conventional processes to reduce the risk of thermal shock (at 50mm Hg, 75% H₂SO₄ boils at 110° C, while 85% H₂SO₄ boils at 149° C). Rejection of heat from the acid loop requires resupply of an equivalent amount of energy elsewhere in the loop, a feature which accounts for the poor energy efficiency of conventional processes.

Finally, by vaporizing weak nitric acid, it is feasible to substantially reduce the requirement for expensive tantalum heat transfer area. Also, the process can be designed to use low pressure steam only.

Depending on schemes used in energy recovery, the process requires about 1 ton of steam per ton of nitric acid, whereas the conventional process requires about 1.8 ton. Capital costs are lower because tantalum requirements are only about 25% of those in a conventional plant.

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