PREPRATION OF NITRIC ACID

METHOD OF PRODUCTION

• Ammonia Oxidation process

Accounts for 90% production process is devided into four fundamental steps

• oxidation of NH3 to NO
• oxidation of NO to NO2
• absorption of NO2 in water
• concentration of HNO3

CHEMICAL REACTION

• Oxidation of ammonia

        NH3  +  5/4  O2  =  NO +  3/4 H2O                   H= - 54.0 Kcal

        NO2  +  O2     =    2NO2                                     H= - 27.2 Kcal

• Nitrous oxide oxidation and absorption

        2NO  +  O2  =  2NO2                                          H= - 27.2 Kcal

        3NO2 + H2O  = 2HNO3 +NO                            H= - 32.2 Kcal

RAW MATERIALS

• NH3 from synthetic ammonia process
• Filtered air
• Platinum-rhodium make-up catalyst

PROCESS DESCRIPTION

Compressed air is mixed with anhydrous ammonia;fed to a shell and tube convertor designed so that  the preheater and a steam heat recovery boiler superheater are within the same reactor shell. The convertor section consist of 10-30 sheets of Pt-Rh alloy in the form of 60-80 mesh wire gage packed in layers inside the tube.Gas passes downward with a velocity desigend to  give a contact time of about .000025 sec in the catalyst zone at 800 celcious.

Product gas from reactor ,containing 10-12%NO,are sent through heat recovery units,a quench unit for rapid cooling to remove a large fraction of the product heat ,and into the oxidizer-absorber system.Air is added to convert NO to NO2 at the more favorable low temperature (40-50* C) enviornment of the absorption system.

The euipment in the absorption train may be series of packed or sieve tray vertical towers or a series of horizontal cascade absorbers .The product from this water absorption system is 57-60% HNO3 which can be sold.

CONCENTRATION OF NITRIC ACID TO 95% CAN BE DONE BY SEVERA PROCESS

• Concentration byH2SO4

Rectification by 93% H2SO4 in silicon-iron or stoneware towers produces concentrated nitric acid and 70% H2SO4 solution which can be reevaporated to 93% or used elsewhere inn plant.

• Concentration by Mg(NO3)2

Devloped in 1958 by Hercules Powder Company.Magnisium nitrate solution containing 70-75% Mg(NO3)2 is fed to a dehydrating tray tower4 along with the dilute  HNO3 from the absorption system.The salt solution acts as extractive ditillation agent,removing water at 100*C or higher,thus allowing rectification without azeotropic formation.The dilute Mg(NO3)2 solution is reconcentrated by evaporation.

Advantage of Mg(NO3)2 process are:

1. Opersting cost about half of those encountered with H2SO4 concentration
2. Capital requirments are only 70%  as great
3. Acid quality  and yeild is improved