All commercial processes of production of urea are based on the exothermic reaction of ammonia and carbon dioxide. As a result of this reaction, ammonia carbonate is formed. Which gets converted into urea after dehydration.
The reaction of ammonia and carbon dioxide takes place in an autoclave at high pressure and high temperature. Low pressure is required for the disintegration of ammonia carbonate. This pressure depends on temperature and is called dissociation pressure. The working pressure of the autoclave at a particular temperature is higher than the dissociation pressure at a particular temperature. The working pressure and temperature are different in different types of urea fertilizer production. Normal pressure is between 175 to 400 atm. and temperature is between 170 to 210 °C.
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Chemical Reaction for Urea Production
2NH3 + CO2 ⇌NH4COONH2 + HEAT + 381 {Kcal}/{gm, mol}
NH4 COONH2 ⇌NH2CONH2 + H2O - 7.1 {Kcal}/{gm, mol}
Partial Properties
i. Chemical Formula - NH2CONH2
ii. Melting point - 172.4 °C
iii. Boiling point - Decompose
iv. Colur - White
v. State - Solid state
vi. Specific Gravity - 1.333 at 26°C
vii. Molecular weight - 60.05
viii. Solubility - Fairly soluble in water
ix. Absorbs moisture quickly in humid environments.
x. Urea has a tendency to crack, hence the moisture content in it should be less than 1%.
xi. When applied to soil, urea is quickly hydrolysed to form ammonia.
Urea Composition
The percentage of moisture in urea by wt. is 1.0 max, and the total nitrogen content in it is 46.6 by wt., and it also contains 1.5% max biuret. And it also contains carbon dioxide.
Side Reaction
2NH4 COONH2 ⇌NH2CONHCONH2 + NH3
Manufacturing Process
The various sections of a urea plant are-
I. Reactor
II. Carbamate
III. Unreacted NH3 and CO2 की Recycle
The major difference in the various commercial methods of urea production is in the material section. That is, there is a difference in the method of treating the off gases, urea technologies have been divided into three parts based on the degree of recycling of these gases.
I. Once through
II. Partial through
III. Total Recycle
NH3 and CO2 are compressed separately and sent to the autoclave at high pressure. The temperature in it is kept between 170 to 180°C and the pressure is 180 atm. The reaction that takes place in this is the first. Since the reaction taking place in the autoclave is exothermic (heat is released). To remove this heat, water is cooked around the autoclave, due to which the heat is converted into steam and this steam is used in the process. The autoclave is made of alloy steel and metal steel.
Due to the corrosive nature of the reaction of carbon dioxide and ammonia, it has a titanium lining inside it. Here the reaction gets completed in 1.5 to 2.8 hrs. As a result, urea, ammonia, carbamate, H2O inert NH3 and CO2 are recycled. Since this reaction is exothermic, hence the steam produced in the first part of the plant, the autoclave, is used here. The pressure of the flash drum is 1 atm. Off gas is produced from the dissociation of urea, which is recycled again. The mixture of urea and carbamate is sent to the flash drum vacuum evaporator at a low temperature.
From here we get 99% molten urea which is sent to the piping tower by a pump. It is sprayed from the top in the prilling tower and cooling air is flowed from the bottom which gets mixed with the molten urea in a counter-current way. The prilling tower is made in such a way that the droplets get sufficient time to solidify. The granules obtained from the prilling tower are sent to the bagging section. At high temperatures, the molten urea gets converted into biuret which is an unwanted product and is harmful for cattle and solids. To avoid its formation the urea is sprayed almost at its melting point.
Handling, Storage, and Packing of Urea
Urea pirls flow freely. Hence there is no problem in its handling and storage. Since coke formation starts in it due to high humidity. Hence for storage urea psyllo are air-conditioned and are packed in plastic bags.
Uses of Urea
I. Urea is mainly used by farmers for farming.
II. It is used in industries for making plastics, adhesives, coatings etc.