2 Manufacture of Ammonia , Nitric Acid and Calcium Ammonium Nitrate

8/3/2019 2 Manufacture of Ammonia , Nitric Acid and Calcium Ammonium Nitrate

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Ammonia synthesis Ammonia is synthesised from hydrogen (from natural gas)

and nitrogen (from the air).

Natural gas contains some sulfurous compounds whichdamage the catalysts used in this process. These areremoved by reacting them with zinc oxide,

ZnO + H2S ZnS + H2O


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The methane from the natural gas is then converted

to hydrogen gas by the following reactions:CH4 + H2O 3H2 + CO

CH4 + 2H2O 4H2 + CO2

CO + H2O H2 + CO2

CO + H2O CO2 + H2

Water, carbon monoxide and carbon dioxide (all of which poison the iron catalyst

used in the ammonia synthesis) are removed.

The carbon monoxide is converted to carbon dioxide which is removed and later

used in the synthesis of urea:


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Air is mixed in with the gas stream in a ratio ofhydrogen:nitrogen ratio of 3:1.

The nitrogen and hydrogen are then reacted at hightemperature and pressure using an iron catalyst to form

ammonia:The reaction

N2 + 3H2 2NH3

AMMONIA


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Manufacture of Nitric Acid.

There are three stages in the production of nitric acid

i. The oxidation of ammonia,

ii. The oxidation of nitrogen monoxideiii. The absorption of nitrogen dioxide in water.


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Step 1 Oxidation of Ammonia

Oxidation of ammonia occurs in a catalytic reactor where The mixture ofammonia and air reacts to form nitrogen monoxide, on a Pt/Rhcatalyst.

i. The catalyst gauzes are initially heated using a flaming bar, whichuses hydrogen as the fuelto raise the temp to about 800c.

ii. Ammonia/air mixture containing about 10% ammonia is passed overthe catalyst, the reaction is very rapid and exothermic. Thus nofurther external heating of the catalyst is necessary.

iii. Pressure should be maintained at 3 - 4 atm and the temperature at800 C and 900 C.

iv. These conditions help prevent any side reactions and approximately95% conversion of ammonia occurs.


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The main reaction

Pt/Rh catalyst4NH3(g) + 5O2(g) 4NO(g) + 6H2O H = -903 kJ

T= 890C, P =3-4 atm

Parrarel reactions

4NH3 + 3O2 2N2 + 6H2O

4NH3 + 4O2 2N2O + 6H O


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THE CATALYST The catalyst typically consists of several woven or knitted gauzes

formed from wire containing about 90% platinum alloyed withrhodium for greater strength and sometimes containing

palladium. Air pollution and contamination from the ammonia as well as

poor ammonia-air mixing and poor gas distribution across thecatalyst can poison the catalyst and may reduce the yield by 10%.

Some of the platinum and rhodium vaporises during the reaction

process and in most cases a platinum recovery system is installedbelow the catalyst. In this system a palladium alloy, known as agetter or catchment, allows a 60 to 80% recovery of the totalcatalyst losses.


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Step 2 Oxidation of Nitrogen Monoxide

The nitrogen monoxide produced is passed through a heat exchanger to the condenserwhere nitrogen monoxide is converted to nitrogen dioxide.

The second reaction is the oxidation of nitrogen monoxide to nitrogen dioxide ,secondary air is added to the gas mixture obtained from the ammonia oxidation to supplythe oxygen required for oxidation

Oxygen content of the leaving stream should have content between 2 and 4% by volume

to ensure maximum oxidation. The reaction

This reaction is also very exothermic releasing a lot of energy which is used to producesteam and/or to preheat the waste gas (tail gas). The heated waste gas is discharged to theatmosphere through a gas turbine for energy recovery.

The combustion gas after this heat transfer for energy recovery has a temperature of 100to 200C, depending on the process and it is then further cooled with water.

2NO(g) + O2(g) 2NO2(g) H = -115 kJ


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Step 3 Absorption of Nitrogen Dioxide into Water

The final reaction is the absorption of nitrogen dioxide in water in Thisis carried out in an absorption tower, which has trays at different levels The absorber is operated with a counter-current flow of water

Deionised water is brought in at the top of the tower and cascadesdown through the trays as the gas is drawn upwards.

The absorption of the nitrogen dioxide and its reaction to nitric acid

and nitric oxide take place simultaneously in the gaseous and liquidphases according to equations below.

To maximize the yield of nitric acid, the temperature is kept low (50 C)and the pressure high (9-10 atm) in the absorption tower.

The absorption of nitrogen dioxide in water forming nitric acid is99.9% efficient. Therefore the yield of nitric acid from ammonia is 95 x0.999 = 94.9%.

3NO2(g) + H2O 2HNO3(aq) + NO(g) H = -16 kJ


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Some of the uses of nitric acid


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Manufacture of Calcium Ammonium Nitrate (CAN)

There are two stages in the production of calcium ammonium nitrate Formation of Ammonium Nitrate

Granulation

FEEDSTOCK

Ammonia,

Nitric acid

Limestone/dolomite

Reaction

Gaseous ammonia and 60% w/w nitric acid react to produce an aqueoussolution of ammonium nitrate (80%).

NH3(g) + HNO3(aq) NH4NO3(aq) H = -92 kJ mol-1


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Concentration of the Aqueous Ammonium Nitrate

The ammonium nitrate solution is also heated by steamin a heat exchanger which is operating at reduced

pressure. This concentrates the ammonium nitratesolution by evaporating the water and a 96%-99%ammonium nitrate solution is formed.

F ti f G l f CAN (C l i A i


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Formation of Granules of CAN (Calcium Ammonium

Nitrate)

To form granules of calcium ammonium nitrate, the hotammonium nitrate solution is mixed with groundlimestone/dolomite at 180C, in a granulator.

The granules formed have a moisture level of 3%, they aredried to a 0.2-0.3% moisture in a rotary dryer. This is doneto prevent the final product from caking or stickingtogether.

The granules size are reduced to the range of2-4 mm. by

crushing and screening the granules to remove theoversized and undersized product.

The granules are then cooled and coated with oil and chinaclay because the uncoated granules are hygroscopic.

2 Manufacture of Ammonia , Nitric Acid and Calcium Ammonium Nitrate

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