16.3 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.

16.3.1 Raw Materials

The raw materials for the manufacturing of urea are:

(i) Ammonia $(\mathrm{N}{\mathrm{H}}_3)$$(\mathrm{NH_3})$ (ii) Carbon dioxide $\mathrm{C}{\mathrm{O}}_2$$\mathrm{CO_2}$

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) is passed over iron catalyst at $450^{\circ}C$ and 200 atm pressure.

16.3.2 Process

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 carbamate.

(ii) Urea formation:

When ammonium carbamate is evaporated with the help of steam, it dehydrates to form urea.