Nitric Acid Concentration (NAC)
Nitric acid forms an azeotrope at about 68% HNO3 boiling and 120° C at 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 relative volatilities of the nitric acid and water in the mixture are 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.
History of NAC Technology
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 limited to 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 remain above about 68% H2SO4 to fully break the azeotrope and avoid excessive loss of nitric acid to the spent sulfuric acid.
Key Features and Benefits
Pre-heating of weak nitric acid feed reduces the consumption of medium to high pressure steam, and shifts heat exchanger surface area from tantalum to less expensive materials like zirconium and various stainless steel alloys specifically developed for nitric acid service.
NORAM's extensive knowledge of glass-lined piping systems ensures a safe and robust piping design with minimal maintenance costs.
For specialized applications where organics-laden nitric acid streams are recycled to the NAC, NORAM draws on its vast nitration experience to ensure safe designs in which organic compounds are unable to accumulate and form unstable mixtures.