Urea

Urea is the most important nitrogenous fertilizer contain about 45 – 47% of nitrogen.

Raw Materials

The raw materials required for the manufacture process of urea are carbon di oxide (CO₂) and ammonia (NH₃).

Manufacture Process

The following steps are used for the manufactured of urea---

Through a silver lined special autoclave, liquid carbon di oxide (CO₂) and liquid ammonia (NH₃) are passed. So, carbon di oxide (CO₂) and ammonia (NH₃) are compressed and reacted at about 100 – 200 atm and at 170 – 190 ⁰C to form ammonium carbamate (NH₄COONH₂).

CO₂ + 2NH₃ -------> NH₄COONH₂    ΔH⁰ = -37.4 Kcal

In the next step urea is formed by dehydration in a low-pressure stripping operation.

NH₄COONH₂ -------> NH₂CONH₂ + H₂O    ΔH⁰ = 6.3 Kcal

Process modifications occurs in recycle of unreacted ammonia (NH₃). Undesirable side reaction is---

2NH₂CONH₂ -------> NH₂CONHCONH₂.H₂O

Effect of Temperature

According to Le-chatelier principle an increase in temperature favoured the overall reaction because the rate determining step is endothermic. It has been observed that, at 170 ⁰C, the reaction rate reaches the maximum limit and the reaction rate falls below this temperature.

Effect of Pressure

Increase of pressure has a favourable effect on the yield of urea, so the reaction occurs at high pressure such as 100 – 200 atm.

[Ammonia/Carbon di oxide] Ratio

Used of ammonia in slight excess (at about 10% excess) shift the equilibrium to the product side, so more and more urea is produced.

Phase of Reactant

Carbon di oxide (CO₂) and ammonia (NH₃) are present in liquid state.

Percentage of Conversion

At about 40% urea is produced from ammonium carbamate in this reaction.

Types of the Manufacturing Process

There are three major manufacturing process for the manufactured of urea depending upon the utilization of unreacted ammonia (NH₃) and carbon di oxide (CO₂).

Method – 1

By passing liquid carbon di oxide (CO₂) and liquid ammonia (NH₃) through a silver lined special autoclave in 1:2 molar ratio, ammonium carbamate is formed. Then ammonium carbamate is heated at about 130 – 140 ⁰C and under about 35 atm pressure urea is formed. By this method yield of urea is 40%.

Method – 2

At 160 – 180 ⁰C and at 150 – 200 atm pressure urea can be manufactured by reacting of one part of solid carbon di oxide (CO₂) by weight and two or more parts of ammonia (NH₃) by weight. By this method yield of urea is 65 – 70%.

Method – 3

By this method urea can be manufactured by reacting 3 – 5 moles of liquid ammonia (NH₃) with liquid carbon di oxide (CO₂) at about 400 atm pressure and at about 200 – 210 ⁰C. By this method yield of urea is 75%.

Uses of Urea

Since urea contain about 45 – 47 % of nitrogen, so urea widely used as most important nitrogenous fertilizer. The P^H of the soil does not change by urea, so urea widely used as fertilizer for all types of crops in all kind of soil.

Urea should be applied in combination with earth or sand because urea is highly concentrated.

When the soil contains free water, urea should not be applied.