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Nitric Acid Plant Technology

Date Published:   10/04/2017

Modern nitric acid plants are designed mostly according to three nitric acid processes: mono medium pressure, mono high pressure and dual pressure. Medium pressure at about 4 – 6 bar will be preferred where a high ammonia efficiency is decisive for the plant economics. High pressure of more than 8 – 12 bar is an advantage with regard to the absorption performance. The dual pressure process offers both of these advantages as this process combine a medium pressure combustion with the use of an absorption at higher pressure.

Nitric acid plants are an important source of nitrous oxide (N2O). The concentration of N2O in the tail gas of nitric acid plants ranges between 800 – 2,000 ppm. Some countries have already limited the emissions of N2O to the atmosphere while many other countries will introduce restrictions in the future. Thus, emission reduction at nitric acid plants is also exercising the minds of suppliers of technology to nitric acid plants.

 

Mono Medium Pressure Process

In this process, the air required for burning the ammonia is supplied by an uncooled air compressor. The compressor set can be designed either as an inline train configuration or preferably as a bull gear type with an integrated tail gas turbine. The operation pressure is governed by the maximum final pressure obtainable in an uncooled compressor, i.e. 4 – 5 bar abs. in the case of radial compressor and 5 – 6 bar abs. with axial flow compressors. Plant capacities of up to 500 mtpd of nitric acid (100%) can be realized using a single ammonia combustion unit and one absorption tower. Due to the process pressure, higher capacities of up to 1,000 mtpd are feasible if a second absorption tower is used. The medium pressure process is the process of choice when maximum recovery of energy is required. The air compressor is usually driven by a tail gas expansion turbine and a steam turbine, the steam being generated within the plant. If the credit for exported steam is high then the compressor train can be driven by a high voltage synchronous or asynchronous electric motor rather than a steam turbine so that all the steam generated can be exported.

With this plant type, it is possible to produce one type of nitric acid with a max. concentration of 65% or two types of acid with different concentrations, e.g. 60% and 65%, while the NOx content in the tail gas can be reduced by absorption to less than 500 ppm. The NOx cotent has to be further reduced to the required value by selective catalytic reduction using a non-noble-metal catalyst and ammonia as the reducing agent. The catalyst gauze only need to be changed once every 6 months due to the low burner load.

The medium-pressure process is characteristic of a high overall nitrogen yield of about 95.0 % or 95.2% in conjunction with the tail gas treatment process, a low platinum consumption and a high steam export rate.

 

Mono High Pressure Process

In the high pressure process, a radial multistage compressor with an intercooler section is used to compress the process air to a final pressure of 8 – 12 bar abs. Preferably, the bull gear type with integrated tail gas turbine is selected but alternatively an inline machine can be used. The compressor may be driven by a steam turbine or an electric motor. Due to the higher pressure, all equipment and piping can be of a smaller size and only one absorption tower is required. The arrangement of all equipment is very compact so that the building for the machine set and the burner unit can be kept small, but this does not pose a problem for maintenance work. This type of plant is always recommended when a quick capital return is desirable.

Plant capacities between 100 mtpd (100%) and 1,000 mtpd of nitric acid can be realized. The achieved acid concentrations of up to 67% are slightly higher than with the medium pressure process. Two or more product streams with different concentrations are likewise possible.

The NOx concentration in the tail gas can be less than 200 ppm by absorption alone. If lower NOx is required, an additional catalytic tail gas treatment unit can be easily integrated. The nitrogen yield attained by the high pressure process is in the order of 94.5%.

For capacity below 100 mtpd, a low capital investment cost may be much more preferable to an optimum recovery of energy. In such a case, a simplified process, which does not include tail gas and steam turbines, can be used. The heating of the tail gas downstream of the absorption section is not necessary. Special design concepts for the process gas cooler unit, condenser and bleacher reduce the cost even further. The operating pressure required by such a process depends on the maximum permissible NOx content in the tail gas. If the specified NOx limit is below 200 ppm, a pressure of at least 7 bar abs. must be considered, depending on the cooling tower water temperature. Lower NOx values of less than 50 ppm can be achieved, if required, by integrating a catalytic tail gas treatment process.

 

Dual Pressure Process

The dual pressure process is developed to accommodate even more stringent environmental pollution control requirements. The process air is compressed to a final pressure of 4 – 6 bar abs. The NOx gas from the ammonia combustion unit is cooled in a heat exchange train, producing steam and preheating tail gas, and then compressed to 10 – 12 bar abs. in the NOx compressor. The final pressure is selected so as to ensure that the absorption section is optimized for the specified NOx content of the tail gas and that the compressors, driven by a steam turbine, can be operated using only the steam generated in the process gas cooler unit while ensuring that some excess steam will always be available in order to guarantee steady operating conditions at all times. Alternatively the compressor set can be driven by either a high voltage asynchronous or synchronous motor and the steam generated can be exported. The dual pressure process combines in an economical way the advantages of the low pressure in the combustion section and the high pressure in the absorption section. Plant capacities of up to 1,500 mtpd of nitric acid (100%) can be achieved in a signle-train configuration. The machine set can be designed either as an inline-shaft set or optically as a bull gear type unit with integrated air / NOx compression and tail gas expander stages. The inline machine concept is favorable in the case of high plant capacities in excess of 1,100 mtpd up to 2,200 mtpd or if an increased energy export is required. For plant capacities in excess of 1,500 mtpd the ammonia combustion and process gas cooler unit needs only to be duplicated.

The nitrogen yield in a plant of this type is more than 96% with a NOx content in the untreated tails gas of less than 150 ppm (vol.) being possible. The NOx content may be further reduced to less than 50 ppm, if required, by selective catalytic reduction using non-noble-metal catalyst and ammonia as the reducing agent. Acid concentration of more than 68% can be achieved. Two or more product streams with different concentrations are also possible. In addition, external streams of weak nitric acid (various concentrations) can be processed if required. Due to the low burner load, the catalyst gauze can remain in the burner for an operating period of 6 – 8 months or longer before being partially or completely replaced.

 

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.

 

Plant Type Medium Pressure Process High Pressure Process Dual Pressure Process
Operating Pressure (abs.) 5.8 bar 10.0 bar 4.6 / 12.0 bar
Ammonia 284 kg 286 kg 282 kg
Electricity 9.0 KWH 13.0 KWH 8.5 KWH
Platinum, Primary Losses 0.15 g 0.26 g 0.13 g
Platinum losses after recovery 0.04 g 0.08 g 0.03 g
Cooling water (Δt = 10 k), including water for steam turbine condenser 100 t 130 t 105 t
Process Water 0.3 t 0.3 t 0.3 t
LP heating steam, 8 bar, saturated 0.05 t 0.20 t 0.05 t
HP excess steam, 40 bar, 450 ºC 0.76 t 0.55 t 0.65 t

 

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.

 

Cost and Cost Effectiveness for Emission Control

Nowadays a big challenge to all nitric acid plants is emission control. The following table identifies the costs and cost effectiveness calculations included in EPA’s ACT document, which was developed for three model plants that cover most of the range of U.S. nitric acid plants.

As shown, cost effectiveness ($/ton of NO, removed) varies from 576-5297/ton for extended absorption, to 5507-5715/ton for NSCR, to $231-$305/ton for SCR. In all cases, cost effectiveness improves with plant size. NSCR is considerably less cost effective thin extended absorption largely because of NSCR’s higher utility costs and lack of any nitric acid recovery credit, which exists in the case of extended absorption.

Phoenix Equipment sells second hand nitric acid plants that are immediately available for purchase and relocation. Buying a used plant can save you significant capital and drastically shorten the time required to build a plant. Here are some nitric acid plants we currently have for sale:

If you are interested in learning about any of the above plants, please contact Edward Zhang (Tel: 732-520-2187; email: edz@phxequip.com).

Modern nitric acid plants are designed mostly according to three nitric acid processes: mono medium pressure, mono high pressure and dual pressure. Medium pressure at about 4 – 6 bar will be preferred where a high ammonia efficiency is decisive for the plant economics. High pressure of more than 8 – 12 bar is an advantage with regard to the absorption performance. The dual pressure process offers both of these advantages as this process combine a medium pressure combustion with the use of an absorption at higher pressure.

Nitric acid plants are an important source of nitrous oxide (N2O). The concentration of N2O in the tail gas of nitric acid plants ranges between 800 – 2,000 ppm. Some countries have already limited the emissions of N2O to the atmosphere while many other countries will introduce restrictions in the future. Thus, emission reduction at nitric acid plants is also exercising the minds of suppliers of technology to nitric acid plants.

References
Plant Subcategory Nitric Acid Plants
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