NITRIC ACID Production Process

NITRIC ACID Production Process

Properties of NITRIC ACID

1. Appearance Colorless to yellowish liquid
2. Odor Suffocating, acrid
3. Solubility Infinitely soluble
4. Molecular weight 63.013
5. Boiling point 86°C
6. Viscosity 1.62cp
7. Density (60% conc.)

• 0°C 1.3931Kg/m³
• 25°C 1.36 Kg/m³
• 100°C 1.2547 Kg/m³

08. Melting point -42°C
09. Specific gravity 1.502
10. Critical temperature 520°K
11. Critical pressure 68.9 bar
12. Critical volume 145 cm³/mol
13. Diffusivity in the water 2.9X10-5 cm²/s
14. Vapor Density (Air=1) 2-3 Kg/m³
15. Vapor Pressure (mm Hg) 48 @ 20°C (68 °F)
16. Specific heat (20°C) 0.64 cal/g

Classification of Nitric Acid Production Processes:

1. Ammonia oxidation process (Ostwald’s process)
2. NaNO3 +H2SO4 process (Chile Salt Peter process)
3. N2 fixation from air (Wisconsin process)
4. Nitrogen fixation by nuclear fission fragments
OSTWALD PROCESS

Reactions involved in the Ostwald’s process

Main reactions
1.       Oxidation of NH3 to NO

        NH₃+5/4O₂ →NO+3/4H₂O        ∆H= -54Kcal

2.       Oxidation of NO to NO2

        2NO+O_{2  →}  2 NO₂                  ∆H= -27.2Kcal
3.       Absorption of NO2 in water

        2NO₂+H₂O  →  HNO₃ +HNO₂
4.       Concentration of HNO3 

Side reactions 

       NH₃+3/4O_{2  →}  1/2N₂+3/2H₂O      ∆H =-75.7Kcal

       NH_{3            →}     1/2 N₂ +3/2H₂

       NH₃+O₂         →   1/2N₂ O+3/2H₂O

       NH₃+3/2NO   →   5/4N₂ +3/2H₂O    ∆H= -107.9Kcal

Nitric acid production flow sheet (Ostwald’s process)

Process Description:

Raw materials required for manufacture of nitric acid

a. Anhydrous Ammonia
b. Filtered air
c. Platinum –Rhodium  catalyst
d. Water

Anhydrous Ammonia 

In 1909 Fritz Haber established the conditions under which nitrogen, N2 (g), and hydrogen, H2 (g), would combine at the conditions:
        Medium temperature (~500 degC)
        Very high pressure (~250 atmospheres, ~351kPa)
        A catalyst (a porous iron catalyst prepared by reducing magnetite, Fe3O4).

Osmium is a much better catalyst for the reaction but is very expensive. This process produces an ammonia, NH3 (g), yield of approximately 10-20%. The Haber synthesis was developed into an industrial process by Carl Bosch. The reaction between nitrogen gas and hydrogen gas to produce ammonia gas is exothermic, releasing 92.4kJ/mol of energy at 298°K (25degC).

Platinum-Rhodium catalyst

Properties of platinum

•  Atomic number:- 78
•  Atomic weight:-195.09g./mol
•  Density:-21.45gm/cm3
•  Melting point:-1769°C
•  Boiling point:- 3827°C
•  Thermal conductivity :-73 Watt/meter/°c
•  Tensile strength:- 14kg/mm2
•  Isotopes:-6
•  Electrical resistivity:- 9.85 micro hg.cm at°C

Chemical properties
 It has the third highest density behind osmium and iridium
 Platinum is unaffected by air and water but will dissolve in hot aqua regia, in hot concentrated phosphoric and Sulphuric acid in the molten alkali
 It is as resistant as gold to corrosion and tarnishing. Indeed, platinum will not oxidize in air no matter how strongly it is heated.

Properties of Rhodium
 Rhodium is silver white metal
 Melting point:-1966°C
 Boiling point:-4500°C
 Density:- 12.41 gm/cm3

Special properties
 High electrical and Heat conductivity. That means heat and electricity pass through rhodium easily
 Chemical properties
 Is relatively in active metal. It is not attacked by the strong acids. When heated in air, it is combined slowly with O2.

Main components involved in the process are:

•  Kobe air compressor
•  Secondary air compressor – 1and 2
•  Instrument air compressor-A,B
•  Air drying unit
•  Air receiver
•  Silica gel for dry air
•  Turbine
•  Catalytic converter
•  Air-heater
•  Oil separator
•  Ammonia evaporator
•  Ammonia super heater
•  Air – ammonia mixer
•  Mixed gas filters -1 and 2
•  Waste heat boiler (W.H.B)
•  Deaerator
•  Tail gas heater -1,2 and 3
•  Boiler feed water (B.F.W)
•  Start acid up tank
•  Absorption tower
•  Bleaching tower
•  Product acid cooler
•  Storage tank

Air Compressor and Turbine:

Air from atmosphere is suck at ambient temperature (room temperature) into the compressor. The compression is done in three stage driven by electric motor and turbine which is in turn run by tail gases .The air first passes through 1st stage at room temperature and leaves at temperature of 130°C and pressure of 2.02kg/cm2.This is then cooled in the inter cooler to 55°C by using raw water as cooling medium. Then air enter the second stage where it is compressed to 3.5kg/cm2 and temperature of 118°C and after the second stage is cooled to 70°C in the inter cooler then it enters the third stage where it is compressed to 4.5kg/cm2 abs and outlet temperature of 143°C. 40% of the energy required for carrying out compression operation is supplied by tail gas turbine. These tail gases are generally taken from absorption tower which leave at 19°C.it is heated up to 260°C by the series of heat exchangers. Major amount of air which is called as a primary air is sent to Air-heater.

Secondary Air Compressor-1, 2:

The secondary air which is supplied by secondary air compressor-1, 2 used is in the bleaching tower

Air-Heater:

The air from the compressor enters the air-heater at 143°C and there it is further heated to 208°C by using high pressure steam and leaves at 208°C.

Ammonia Evaporator:

The liquid ammonia stored in ammonia bullets is sent to the evaporator at 10-12kg/cm2 through tube side and 21°C temperature, where it is vaporized by the chill water coming from the absorption tower passing through shell side. The ammonia leaves the evaporator at 19°C.

Ammonia Super Heater:

The ammonia which enters the super heater is further heated to 80°C by using low pressure steam. Here the shell side flow is ammonia and the tube side flow is low pressure steam.

Ammonia-Air Mixture:

Ammonia enters the mixer at 80°C and air at 208°C and mixing takes place and heat is exchanged between them and leaves at 180°C. proportion of Ammonia and Air is10-10.7.

Mixed Gas Filter 1, 2:

Mixed gas filters consists of “SS –candle” as filter medium. The air –ammonia mixer enters this filter in order to remove the impurities present in the mixture. Presence of the impurities in the mixture may corrode the catalyst surface.

Waste Heat Boiler:

At the entrance of the waste heat boiler the mixture may be around 180°C. Here the hydrogen flame is used to raise the temperature. This waste heat boiler consists of platinum-rhodium catalyst for the reaction to start. First it consists of supporting bars on which the nichrome mesh is placed, above which the palladium catchment gauge is placed and finally the platinum (95%)-rhodium (5%) catalyst is placed. The reaction is carried at 850°C. At the bottom part of the waste heat boiler which is in the form of cylinder, consists of tubes in which the NOX gases flow and on shell side boiler feed water is supplied for cooling the NOX gases. The heat that which is produced by the NOX gases is gained by the boiler feed water and high pressure steam is generated. This high pressure steam is sent to steam generation station where it is split into low pressure, medium pressure and high pressure steam. They are utilized in some parts of the plant. Here the rich gases is sent into the tail gas heat exchanger III and the lean gases is sent into tail gas heat exchanger-II

Tail Gas Heat Exchanger-II:

Tail gas heater –II is a shell and tube heat-exchanger in which the nitrous gas is passed through shell side which enters at 320°C and leaves at 280°C and tail gas (coming from T.G.H-I) is passed through tube side which enters at 160°C and leaves at 250°C. The tail gases are sent to catalytic converter and the nitrous gases are sent to boiler feed water.

Boiler Feed Water:

Here de-mineralized water which is de-aerated by steam and sent through tube side is used for cooling the nitrous gas passing through shell side. The inlet temperature of the nitrous gas is 280°C and outlet 180°C. The outlet steam is sent into waste heat boiler. The nitrous gas is further sent to T.G.H-I.

Tail Gas Heater-I:

Tail gas heater –I is a shell and tube heat-exchanger in which the nitrous gas is passed through shell side which enters at 180°C and leaves at 155°C and tail gas (coming from Tail gas pre-heater) is passed through tube side which enters at 45°C and leaves at 160°C. The tail gases are sent to catalytic converter through T.G.H-II and the nitrous gases are sent to condenser.

Cooler Condenser:

The nitrous gas from T.G.H-I is sent to the shell side of the cooler condenser and the cold water from the VAM unit is circulated on the tube side. The nitrous gas enters the cooler condenser at 155°C and leaves at 56°C.The cooled water is recycled and the nitrous gas is sent the adsorption tower.

Absorption Tower:

This tower consists of 69 trays in which 64 trays are absorption trays and the 5 trays are oxidation trays. The nitrous gas are first sent to oxidation trays there nitrous gas converted into NO2 and then it is sent into the absorption trays in which the De-mineralized is sprayed from top and the NO2 gets converted into HNO3 (nitric acid). Then it is finally sent to bleaching tower. The tail gases are sent to tail gas pre-heater

Tail Gas Pre-Heater:

The tail gas from the absorption tower is sent to the tail gas pre-heater in which the tail gases are heated by using low pressure steam. The tail gas enters the tail gas pre-heater at 19°C and leaves at 45°C. The outlet tail gas is sent to new tail gas heater-III and also to T.G.H-I.

New Tail Gas Heater-III:

The rich gases from the waste heat boiler are circulated on shell side of the new tail gas heater-III. This enters at 320°C and leaves at 160°C. The tail gas from the tail gas pre-heater is circulated on tube side. This enters at 45°C and leaves at 285°C. Finally the tail gas from the tail gas per-heater-III is used to run the turbine.

Bleaching Tower:

The secondary air from the secondary air compressor is supplied to the bleaching tower to remove the color of the nitric acid.

Product Acid Cooler:

The nitric acid thus obtained is cooled by using the cooling water in the product acid cooler. Then it is finally sent to the storage tank-A, B

Catalytic Convertor:

Tail gases from the T.G.H-II are sent to catalytic converter and the vapor ammonia is also fed from NH3 super heater.

NO + NO2 +2NH3→ 2N2 + 3H2O

Here tail gases and ammonia reacts with each other and forms nitrogen and water vapour. This can then be safely disposed to atmosphere

NITRIC ACID PRODUCTION FROM CHILE SALT PETER  PROCESS:

NaNO₃ + H₂SO₄    →    NaHSO₄ + HNO₃