The manufacturing of nitric acid for use in fertilizers, plastics, dyes, textiles, and other industrial products, has evolved to three industry-standard processes. They all produce acid with 60-68% concentration, and each uses essentially the same nitric acid plant design. The choice between medium- and high-pressure mono and dual-pressure production process often comes down to financial considerations and owner priorities.

Nitric Acid Production Technology

While nitric acid plant technology has made production more efficient and environmental controls more effective, the process remains the same:

1. Ammonia combustion in the presence of hot air and a platinum/rhodium catalyst creates nitric oxide (NO).
2. Oxidation of the NO creates nitrogen dioxide (NO2). The NO is cooled and compressed as it travels to the absorption tower. The cooling process creates nitrogen dioxide and produces heat, which is recycled for use in other plant operations.
3. Absorption of the NO2 in water creates nitric acid (HNO3). This dilute solution dissolves in the remaining water. The reaction also forms small quantities of NO, which combine with ambient oxygen to reform NO2 and continue the reaction.

Modern nitric acid plants are designed to carry out the three chemical processes in a variety of ways, considering initial investment costs, space constraints, feedstock cost, and other factors. The oxidation reaction is more efficient under medium pressure, while the absorption and ultimate production of nitric acid solution works better under high pressure. As a result, three distinct processes have been developed for carrying out the various reactions.  

1. Mono Medium Pressure Process

Using an uncooled radial compressor to deliver air at 4 to 5 bar abs or an axial flow compressor (5 to 6 bar abs) a nitric acid plant can produce up to 700 tons of nitric acid per day (maximum 65 percent concentration) using a single ammonia combustion unit and a single absorption tower. Production can be increased to 1,000 tons per day with the addition of a second absorption tower.

Because the oxidation of nitric oxide into nitrogen dioxide occurs more quickly at lower temperatures, nitric acid plants using mono medium are more efficient than their high-pressure counterparts at this portion of the production process. And because the reaction is exothermic, the mono-medium setup’s ability to maximize energy recovery helps offset operating costs. The compressor can be driven by reusing steam captured during nitric acid production. Or, if steam can be exported for credits, the compressor can be driven by electric motor so all generated steam can be collected for export.

2. Mono High Pressure Process

While medium-pressure nitric acid plants optimize the second critical chemical reaction, raising the pressure to 8 to 12 bars enhances the final step. The higher pressure “forces” the nitrogen dioxide (and dinitrogen tetraoxide, which is also produced in the oxidation phase) gas into water.

Manufacturers seeking a quick capital often take advantage of this nitric acid plant technology, as a single absorption tower is sufficient to produce up to 1,000 tons of nitric acid per day. The higher pressure also means the burner unit, piping, and other equipment can be made smaller so the plant occupies a more compact footprint, making the initial capital investment less and the road to profitability shorter.

3. Dual Pressure Process

As might be surmised, the dual pressure process uses two different pressures during nitric acid production. It uses medium pressure for the compressed air used in ammonia combustion, then compresses to high pressure to optimize absorption. An attempt to capitalize on the best of both mono processes, the dual pressure process can realize up to 1,500 tons per day of nitric acid (up to 70% or higher concentration) with one ammonia combustion and process gas cooler unit installed. Additional units can be added to increase capacity. The lower oxidation pressure also takes less of a toll on the catalyst – a major savings, considering the cost of platinum. Varying the pressure not only extends catalyst life, but also keeps the nitric acid concentration low as it exits the condenser, fostering its absorption into water.

Which is the Best Nitric Acid Production Process?

All three processes have their place in nitric acid plant design, depending on strategic goals and other factors. Initial investment in medium- and high-pressure mono process plants are generally lower than those required for dual-pressure facilities. Variable costs, however, can run higher, requiring slightly larger quantities of ammonia and taking a greater toll on the catalyst. As a result, mono –pressure nitric acid technology is more prevalent in smaller (900 tons per day and less) plants. Most experts consider the dual-pressure process the best available technology, and most new high-capacity nitric acid plants are designed for dual pressure. Because both technologies are incorporated, a higher initial investment is required, but the greater capacity and lower operating costs help recoup the investment more quickly.

Typical Consumption Figures of Modern Nitric Acid Plants

The following chart is the comparison of typical consumption figures for steam turbine driven and inline compressor set nitric acid plants, per ton of nitric acid (100%), with NOx content in the tail gas of less than 50 ppm.

Energy Recovery and Tail Gas Treatment

Whichever technology a nitric acid plant uses, each reaction in the production process releases energy, which can be recovered and used throughout the plant. For instance, the air used to oxidize the ammonia must be filtered, compressed and heated. It is possible to obtain nearly all the energy required for these processes by extracting heat from tail gasses.

Nitric acid plants must treat the NO, N2O, and NO2 gasses produced throughout the manufacturing process. Both are greenhouse gasses and can do great harm to the environment if not properly neutralized. A variety of technologies exist to make these waste gasses less harmful including:

• Heating them in the presence of a catalyst to convert NOx to nitrogen
• Installing additional absorption trays (or a complete additional absorption tower)
• Cooling weak acid within the absorption tower
• Attaching molecular sieves to contact tail gasses to return NO for another pass through the absorber
• Use of liquid alkali, carbonates, or potassium permanganate (wet scrubbers) to pull NO from tail gas to form nitrate salts
  
  

Leading Technology Licensors

• Weatherly, a company of Chematur Engineering Group of Sweden;
• ESPINDESA, a company of Técnicas Reunidas of Spain;
• Borealis of Austria;
• Uhde (now ThyssenKrupp Industrial Solutions) of Germany;
• MECS Technology of USA;
• KBR of USA;
• Technip of France.

Planning to start or expand a nitric acid plant? Phoenix Equipment buys and sells complete nitric acid plants. Take a look at our current nitric acid plant inventory or if you can’t find what you need, contact us today and one of our plant experts can assist you in finding exactly what you need, no matter which nitric acid production process you are looking for.