The system nitric acid / water (HNO3/H2O) displays a maximum azeotrope at about 68 wt% and at a boiling temperature of 120°C at ambient pressure. In order to manufacture high-concentrated nitric acid, which is needed in many technical processes, it is thus necessary to overcome the azeotrope point by using high-concentrated sulfuric acid.

Rectification processes are used for the pre concentration of nitric acid (HNO3) from below 55wt%, whereby wateris distoilled to the head section and nitric acid to the bottom, up to 67 wt%. These columns are operated with minimum reflux ratio for energy saving reasons so either tray columns or glass packaging columns (DURAPACK) are used.

Sulfuric Acid addition for Azeotrope Change
Diagram 2 shows the impact of different sulfuric acid concentrations to the azeotrope point. Azeotrope disappears beyond a sulfuric acid concentration of 50 wt%, yet for technical rectification, an acid concentration of about 70 wt% is to be targeted.

Flow sheet of Nitric Acid Concentrationing
The flow sheet shows the acid feed points, and also the bleach column that may be needed, which stripps free NOx with air from the concentrated nitric acid. The contaminated air is lead to the NOx absorption.
Within the nitric acid concentration unit, sulfuric acid extracts water from the bottom of the column and must again be re-concentrated.

Air/NOx Absorption HNO3 concentrated, bleached H2SO4 dilute, to sulfuric acid concentration

overall process of Nitric Acid Concentration
The flow chart shows the interaction of different process steps ranging from pre concentration to high-concentration with bleach line, sulfuric acid concentration, and NOx absorption. QVF has researched this process with regard to energy saving potentials, and has applied for a patent, now pending (USA 10/296, 297).

Low energy consumption is achieved by:

• Indirect heating of the high-concentrated nitric acid by employing a horizontal evaporator.
• Supplementary and indirect heating performed by intermediate heaters and energy integration.
• Optimized inflow conditions due to preliminary mixing of the feed flows.
  

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Nitric acid column