Answer. Manufacture of Urea
Urea is nitrogenous fertilizer. It consists of 46.6% nitrogen. It is white crystalline compound, highly soluble in water. It is used for the manufacturing of important chemicals, but its major (about 90%) use is as a fertilizer.
The raw materials for the manufacturing of urea are:
(i) Ammonia (NH3) (ii) Carbon dioxide (CO2)
Ammonia is prepared by the Haber's process. One volume of nitrogen (from air) and three volumes of hydrogen (obtained by passing methane and steam over heated nickel catalyst) are passed over iron catalyst at 450°C and 200 atm pressure.
450°C
N2(g) + 3H2(g) 2NH2(g)
200atm
Manufacturing of urea involves three stages:
(i) Reaction of ammonia and carbon dioxide; Carbon dioxide is passed through liquid ammonia under high pressure to form ammonium carbonate
2NH3 + CO2 ——— > NH2COONH4
Ammonium carbonate
(ii) Urea formation; When ammonium carbonate is evaporated with the help of steam, it dehydrates to form urea.
NH2COONH4 ———>NH2CONH2 + H2O
(Urea)
(iii) Granulation of urea: At this stage, liquid urea is evaporated to form granules. When liquid urea is sprayed from top of a tower under pressure and a hot current of air is introduced from the base, it evaporates to form granules. This is stored to be marketed.
(Unreacted NH3, CO2 & H2O)
Flow sheet diagram of manufacturing of urea