Fertilizers and Manufacture of Urea

 

This is the notes for fertilizers and manufacture of urea (NH2CONH2).

Fertilizers: 

A fertilizer is a substance which posses the essential nutrients for the healthy growth of plants and for the better yield. They are added to the soil at the time of growing of the plants.

A good fertilizer should posses following characteristics: 

•  The essential components of the fertilizers should be present in a large percentage and in a form which can be easily utilized by the plants.
• They should be reasonably soluble in water.
• They should be stable in the soil for quite sometime.
•  They should be cheap.
•  They should be non-toxic and safe to handle.

The fertilizers are of two types:

1. Organic fertilizers: The organic fertilizers are prepared from the dead and rotten/decayed plants and animal bodies and their wastes. In these fertilizers, nearly all the nutrient elements (N, K, Ca, Zn, etc.) needed for the plants are available. 
2. Chemical or synthetic fertilizers: These are manures manufactured in factories. There are large no. of chemical fertilizers which contain Nitrogen, Phosphorus and Potassium.

• Nitrogenous fertilizers: They are nitrogen supplying compounds which help in rapid growth of the plants and impart dark green color to the leaves. For e.g. Urea(NH2CONH2), Ammonium sulphate(NH4)2SO4, Calcium ammonium nitrate(Ca(NO3 )2NH4NO3), ammonium nitrate(NH4NO3), etc.
•  Phosphorus fertilizers: They help in the development of root, flowering, seed formation and ripening. For e.g. Ammonium phosphate= (NH4 )3 PO4 ,Calcium phosphate= Ca3 (PO4 )2 , etc.
•  Potash Fertilizers: Potassium helps in the development of healthy root system, quickens the ripening of cereals and offers resistant against various diseases and frost. It is particularly for potato, tobacco, cotton, wheat, etc. For e.g. Potassium chloride= KCl, Potassium sulphate= K2 SO4 Potassium nitrate-= KNO3 , etc.

Simple and mixed fertilizers

• Fertilizers containing only one essential element (primary nutrient) are called simple fertilizers. For e.g. Urea, K2 SO4 , KCl, etc.
•  Fertilizers containing more than one essential elements are called mixed fertilizers. For e.g. CAN= Ca(NO3 )2NH4NO3 , KNO3, (NH4 )3 PO4 , etc.

Fertilizers as pollutants

Manufacture of Urea

Raw materials: 

• Ammonia (NH3 ), manufactured by Haber’s process
• Carbon dioxide (CO2 ), produced by the decomposition of limestone (CaCO3 ) 

Principle: Liquid ammonia and carbon dioxide is allowed to react in a reactor (autoclave) at high temperature (1800C-2100C) and high pressure (150 atm) whereby ammonium carbamate is formed which upon decomposition form urea solution. The urea solution is evaporated up to dryness to make granules of urea. The chemical reaction takes place as follows: 

• Formation of ammonium carbamate: Ammonia and carbon dioxide react under high temperature (180-2100 C) and high pressure (150 atm) to form ammonium carbamate. This reaction is exothermic, fast and almost goes to completion.

                  2NH3 + CO2 ↔ NH2COONH4 + heat

                                       ammonium carbamate

• Decomposition of ammonium carbamate: Ammonium carbamate is decomposed to urea. The reaction is endothermic, slow and does not go to completion. 

           NH2COONH4 + heat ↔ NH2CONH2 + H2O 

In the manufacture of urea, there may be some unreacted ammonia, carbon dioxide and ammonium carbamate. Unreacted ammonia and carbon dioxide are recirculated. Ammonium carbamate is removed by reducing the pressure (application of Le Chatelier’s principle).

Hope this will help you a lot.

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