Fertilizer

Fertilizer Industry

Three major components are necessary in fertilizers as given by Shreve;

  1. Nitrogen:  Required during early stages of plant growth to promote development of stem & leaves.
  2. Phosphorous: This stimulates early growth and accelerates seeding or fruit formation in the later stages of growth.
  3. Potassium:  Essential to the development of starch in potatoes and grains, the sugar in fruit and vegetables and the fibrous material of plants, an ample supply of potassium in the soil some times prevent diseases and reduce the effect of excess nitrogen.

These materials are constantly added to the soil. The various fertilizers are Ammonium Sulfate, Ammonium Nitrate, Calcium Ammonium Nitrate, Urea, Ammonium Chloride, Super Phosphate, Triple Super Phosphate, Nitro Phosphate, Ammonium Phosphate, etc.


Urea (NH2 CO NH2) or (NH2)2CO

The molecular weight is 60, the melting point is 133 OC, the specific gravity is 1.335 and fairly soluble in water. It’s a granular coated with non-hygroscopic dust similar to phosphate rock. The end users are fertilizers, animal feeds, industrial plastics, adhesives and coatings.

Production of Urea by Ammonium Carbamate Decomposition


Ammonia and CO2 are compressed for reaction at 150 atm. and 180 OC in an autoclave to form Ammonium Carbamate (NH4COONH2). Urea is formed by dehydration in a low pressure stripping operation. Process modification occurs in recycle of un-reacted ammonia and CO2 with the excess percentage of ammonia in the reaction. The following are the chemical reactions;

Main Reactions

  1. CO2 + 2NH3 -----> NH4COONH2
  2. NH4COONH2 -----> NH2CONH2 + H2O

Undesirable Side Reactions

  1. 2NH2CONH2 -----> NH2CONHCONH2.H2O

            Urea                     BIURET

The principle raw materials and utilities for the manufacture of Urea are Ammonia, CO2, Power, Cooling Water and Steam.

Process

NH3 & CO2 are compressed separately and added to the high pressure autoclave which must be water cooled due to highly exothermic nature of the reaction. The average residence time in the autoclave which is a continuous process is about 2 hours. A mixture of Urea, Ammonium Carbamate, Water, Unreacted Ammonia and CO2 are formed. At this stage the liquid is recued to 27 atms. and fed to a flash evaporator followed by condenser to obtain the solution of Unreacted Ammonia, CO2 and Water. This recycled and an aqueous solution of Urea is passed to the flash drum where further decomposition takes place. The off gases from this step can be recycled for making the fertilizer. The 80% Urea solution can be sent to vacuum evaporator to obtain molten urea containing less than 1% water. This molten mass is sprayed into a prilling tower where granules are solidified. At this stage BIURET formation has to be avoided and then the resultant Urea is packed and sold.

Advantages of Urea

  • It replaces highly acidic ammonium salts in places where soil is already highly acidic and lime is scarce.
  • The granules are obtained easily and the polymer called urea formaldehyde is used in the manufacture of plastics.
  • It is used as a cattle feed to the extent of 90% of total requirement.
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