Nitric Acid Concentration with Sulfuric Acid

Process Description


The system nitric acid / water (HNO3/H2O) displays a maximum azeotrope at about 68 wt% and a boiling temperature of 120°C at ambient pressure. In order to manufacture high-concentrated nitric acid, it is 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 55 wt%, where water is distilled 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

The diagrams below show 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 recommended.


Flow Sheet of Nitric Acid Concentrationing

The flow sheet shows the acid feedpoints, and also the bleach column that may be needed, which strips 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.


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.


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.

Nitric Acid Concentration Column

Nitric Acid Concentration Column

Bleaching Process

bleaching process

Nitric acid bleaching column with clear nitric acid in lower column part

Nitric acid bleaching column with clear nitric acid in lower column part

Process Systems Questionnaire

Request More Information

NOx Waste Gas Treatment


Purifying vent gas, capturing hazardous gas, producing nitric acid

Nitrogen oxides, also called nitrous gases or NOx-gases, are generated by many processes during the reaction and concentration of nitric acid or the combustion of fossil energy sources. NOx-gases are harmful to health but also a valuable source for the production of nitric acid.

Gases with a low content of NOx that are generated during a combustion process are mostly converted with ammonia via catalyzed processes into nitrogen and water.

In the case of higher NOx concentrations, the gases are mostly guided through aqueous absorption columns where the aim can be to remove or scrub the NOx from the waste gases and/or to produce nitric acid. Scrubbers are custom designed and built by De Dietrich Process Systems as per your needs. If oxygen is not present and the scrubber is operated with an aqueous non-oxidizing media such as water, the NO of the NOx will pass the scrubber while other NOx are dissolved as nitrous and nitric acid or nitrites and nitrate under caustic conditions. When adding oxygen the NO will be oxidized in the gas phase to NO2 to become nitric acid and is dissolved in water which is the common way to produce nitric acid. When adding oxidizing agents to the liquid phase as hydrogen peroxide or potassium permanganate nitrites are oxidized to nitrates improving the scrubbing process. These semi-batch operated absorption systems are commonly provided by DDPS when periodically operated processes as the dissolving process of metals with nitric acid generate the NOx-waste stream.

With oxygen (as is present in air) the conversion of NOx into nitric acid can be accomplished in a quantitative manner advantageously at a higher pressure, which is basically also the common method for the syntheses of nitric acid based on the ammonia oxidation. DDPS typically supplies such continuously operated NOx-Absorption towers along with nitric acid concentration plants to reduce NOx-emissions below regulated values and to improve the efficency of the overall nitric acid concentration process.

DDPS can provide solutions including:

  • Laboratory and pilot scale atmospheric NOx-Scrubbers mostly operated semi-batch wise to clean waste gases
  • Pilot and production scale atmospheric NOx-Absorption units operated semi-batch wise or continuously to capture nitrous gases
  • Production scale pressurized NOx-Absorption towers to continuously produce nitric acid

Atmospheric NOx-Scrubber


Process Flow Chart of QVF NOx-Pressure absorption to produce nitric acid


Process to convert NOx into Nitric Acid with air

The NOx gases coming from the extractive rectification column to produce high concentrated nitric acid are mixed with air and pressurized by a liquid ring compressor into the bottom of a stainless steel absorption column whereas water is introduced from the top of the column. The oxidation of NOx is quite exothermic so it is necessary to remove the heat as good as possible to increase the conversion rate and concentration of nitric acid. Since this heat removal is crucial for the conversion rate, De Dietrich Process Systems has developed column trays with integrated cooling loops. The generated nitric acid leaving the column at the bottom has a concentration of about 50-65 wt%. The vent gas leaving the column at the top meets the specification of the EPA or even lower.

Heat removal in the column is a significant issue due to the exothermal reaction; to solve this problem, DDPS installs specially designed cooled trays.


Process Flow Chart of QVF NOx-Pressure absorption to produce nitric acid


Case Study

One of the largest NOx pressure absorption columns that De Dietrich Process Systems has built weighs 41 metric tons, has a height of 115 ft and a diameter of 75 inches.

DDPS delivers pressure absorption plants in accordance with valid standards, such as:

  • AD2000 / PED
  • ASME
  • US Stamp
  • China Stamp

Reference plants can be found around the world.

Fill out our Reglassing Questionnaire

Request More Information

Column Internals Questionnaire

Request a Quote

Want pricing? Have Questions?
We are here to help.