Nitric acid dual pressure process


Ammonia evaporation

Liquid Ammonia is received from the Battery Limits and sent to the evaporator E101. The heat for evaporation is brought by hot cooling water coming from the Absorption column C201and HP condenser E202.

A continuous extraction of liquid ammonia is made from the bottom of the Ammonia Evaporator in order to control the water content in the liquid phase. The extraction is sent to the Auxiliary Evaporator E103 where the mixing of ammonia and water is evaporated via a coil fed with steam.

After evaporation the Ammonia gas is superheated in the Ammonia Superheater E102, and then sent to the Ammonia Filter F103.

Air compression

The atmospheric air used for the process is first filtered in the Air Compressor Inlet Filter F101 and then sent to the Air Compressor K102. The Air Compressor Air Filter is fitted with a heater used in winter conditions.

At the outlet of the Air Compressor, the air is divided in primary air, to be sent to the reactor R101 and in secondary air, directed to the Bleaching Column C202 via the Ammonia Superheated E102. The primary air is filtered in the Process Air Filter F102.

Ammonia – Air Ratio Control

The ammonia leaving the Ammonia filter is introduced in the primary air and mixed by the Ammonia-Air Mixer M101. The quantity of Ammonia is controlled in function of the primary air flow. The leading parameter is the primary air flow rate.

Ammonia oxidation

The Ammonia-air mixture at about 210 °C and 520 kPa is introduced to the reactor R101. The top of reactor is fitted with specific devices allowing a perfect distribution of the mixing above the catalyst gauzes.

On the Platinum gauzes, the ammonia and air react to form the NO gas, generating a big amount of heat which increases the gas temperature to about 890 °C.

Heat Recovery system

The heat generated by reactions of ammonia in air and NOx gas oxidation is recovered in two ways:

  • Production of HP steam at 4.2-4.5 MPa and about 400°C. The pressure level and superheating will depend upon the choice of turbine and Customer requirement,
  • Re-heating of tail gas before it enters in the Expander K202 where major part of power necessary for the two compressors is generated.

After the catalyst gauzes, the NOx gas crosses the following equipments:

  • The superheater E104, in which the saturated steam is heated till 400°C,
  • The Waste Heat Boiler E105,
  • The Process Gas Cooler E106,
  • The Economiser E109,
  • The LP Cooler Condenser E110.

Weak Acid formation

After the Economiser E109, the NO gas is sent to the LP Cooler Condenser E110, cooled by cooling water, where part of the acid is condensed. The liquid / gas mixing flows to the Weak Acid Separator S101.

The condensed acid is pumped to the Absorption Column C201 by the Weak Acid Pump P101AB.

The process gas leaving the Weak Acid Separator S101 is mixed with the secondary air coming from the Bleaching column C202. The mixing is then directed to the NO Compressor K201 suction.

Nitrous Gas compression and heat recovery

At the outlet of the NO Compressor K201, the HP NO Gas is at a pressure of about 1.160 MPa.

The heat recovery of the NO HP gas is insured by re-heating of the tail gas coming from the Absorption Column C201, via the First NO HP Gas Cooler E201.

At the outlet of E201, the NO gas is then sent the HP Cooler Condenser E202, where part of acid is condensed and sent to the bottom of the Absorption Column C201.

The NO HP gas leaving the HP Cooler Condenser E202 is directed to the lower part of the Column C201.

NOx absorption and Nitric Acid production

The Absorption Column C201 is composed of perforated trays where NO gas is put in contact with liquid flowing from the upper tray.

The heat generated by the formation of Nitric Acid is released by coils submerged into the emulsion on the tray.

The number of trays, the surfaces of coils, and the distance between the trays are optimized according to kinetics of reactions in the Absorption Column, thermodynamic and transport properties of the streams. The tools for optimisation allow also the optimization of the overall pressure drop of the absorber.

The NO gas is introduced under the first tray of the absorption Column, while water is introduced on the upper trays.

The water flowing becomes acidic and acid concentration increases all along the column when liquid stream flows to the bottom. In the same time the NO contained in the gas decreases when the gas stream flows from the bottom tray to the upper ones.

At the lower outlet of the column, the Nitric Acid, with a concentration of 60 - 65% (see options) is sent to the Bleaching Column C202.

On the top of the Absorption column, the two last trays receive the water necessary for the production of Nitric Acid. The last tray receives pure demineralised water, while the tray just before receives process condensate coming from the Ammonium Nitrate plant. This process condensate is acidified under pH control before introduction in the Absorber, in order to be sure that there is no free Ammonia.

For an optimized Ammonia consumption, the Tail Gas leaving the Absorption Column contains about 150-200 ppm of NOx. For an optimized CAPEX, the Absorption Column may be designed for NOx content about 1000 ppm.

Bleaching of Nitric Acid

The Nitric Acid leaving the Absorber contains a certain amount of NOx dissolved occurring a red colour of the acid.

In order to eliminate the dissolved NOx, the acid is strip in the Bleaching Column C202 by hot secondary air coming from the Air Compressor K102 via the Ammonia Superheater E102.

At the outlet of the Bleaching Column, the Nitric Acid is sent to Battery Limits.

Tail Gas heating and expansion turbine

At the top outlet of the Absorption Column C201, the tail gas is re-heated by passing successively through the First NO HP Cooler E201 and the Process Gas Cooler E106.

At the outlet of E106, the gas temperature is about 440°C. The tail gas is then directed to the DeN2O reactor R201 and DeNOx Reactor R202. The content of NOx and N2O after reactor R202 shall fit with local pollution control requirements.

The tail gas goes to the Expansion Turbine K202, at a pressure of about 1.0 MPa.

Steam and condensate system

Note: in this typical case heat recovery from the reactions is used for production of 4.2 MPa steam, superheated at about 400°C. The levels of pressure and temperature can be amended depending upon local conditions of the site and Customer requirement.

Demineralised water from Battery Limits is introduced in the Deaerator V301. The Deaerator receives the demineralised water, the steam turbine condensate and other steam condensates generated in the plant. Elimination of CO2 and oxygen is achieved by introducing LP steam.

The Boiler Feed Water (BFW) leaves the Deaerator at a temperature of about 105°C and receives the final treatment from the BFW Treatment Unit G301.

The Boiling Feed Water is pumped by the BFW pump P302 AB and sent to the Economiser E109 and finally arrives in the Steam Drum V302.

A steam production systems is foreseen:

  • One evaporator with forced circulation (Heat Recovery First Boiler E105 and Steam Boiler Circulation pump P303 AB),
  • One Steam Superheater E104.

Cooling water

Cooling Water network as shown is typical. Final Cooling Water distribution must be confirmed according to the cooling water distribution at site.


Process options

Electrical power production

In the shown typical flow sheet, the excess of HP steam is exported outside Battery Limits.

The flow sheet can be adapted for production of electrical power with turbo-generator instead steam export.

Limited absorption

For reasons of CAPEX optimization, the number of trays and height of the absorption column can be significantly reduced, if a slight increase of ammonia consumption is accepted (more NOx to DeNOx reactor).

The deep absorption is however profitable when later revamping for capacity increase can be anticipated.

Nitric Acid Concentration

Optionally, the Nitric Acid concentration of 68% can be achieved with slight process modifications.


Main figures
Acid Quality (%HNO3) 62-64% (68% optional)
NH3 consumption 282 kg NH3/t + ~2 kg/t for DeNOx
Pt losses: gross 140 – 150 mg/t
Net with recovery gauzes ~40 mg/t
Typical continuous run 330 days/annum
Steam Export 550 – 600 kg/t
N2O / NOx emissions EFMA-BAT (compliance with local requirement)