Inline concentration measurements for the optimization of nitric acid production in various process steps

1 Nitric acid synthesis

Nitric acid has been technically produced by the Ostwald process since 1908. This involves the catalytic oxidation of ammonia. The mixture of ammonia and air is passed rapidly through hot platinum-rhodium nets (catalyst) which are located in the contact furnace. This produces nitrogen monoxide at 800 °C, which is then cooled back down to below 50 °C. In the oxidation tower, it oxidizes with excess oxygen to form nitrogen dioxide and then reacts with water in an absorption tower to form around 60% nitric acid. Nitric acid can be concentrated up to 68 % by distillation, which corresponds to the azeotrope with a maximum boiling point (122 °C). Higher concentrations up to 90 % can be achieved by dehydration with magnesium nitrate Mg(NO3)2. With the density sensor L-Dens 7400 and the sound velocity sensor L-Sonic 5100, the different nitric acid concentrations in various process steps can be precisely measured and monitored. This ensures highest quality and optimizes production costs.

2 Application solution

2.1 Absorption tower

In this step of the Ostwald process, both components, the NO2 and the N2O4 flow into the absorption column. There, finely distributed water trickles down from above and comes into intimate contact with the two gases flowing in from below. Additionally, oxygen often flows into the trickle column from above so that a gas phase reaction can also take place to a lesser extent without the gas mixture having to dissolve in the water first. By this further reaction the conversion of nitric acid in the whole process can be further increased. The HNO3 leaves the absorption tower with max. 60 % and is measured with the process density sensor L-Dens 7400 or the sound velocity sensor L-Sonic 5500.

2.2 Distillation column

The mixture evaporates in the distillation column and condenses due to the different boiling points of the components. This process step can be repeated until the desired concentration is reached. A maximum nitric acid concentration of 68 % can be reached. Anton Paar process sensors measure the concentration precisely, thus guaranteeing the optimal product quality.

2.3 Dehydration tower

The final step is for highly concentrated nitric acid. In this process step, the water-absorbing property of magnesium nitrate is utilized. The nitric acid is further concentrated with magnesium nitrate up to 90 %. Even for this high concentration Anton Paar can offer a perfectly suitable density sensor made of tantalum.

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