Urea is the most popular concentrated solid straight nitrogen fertilizer. Its prills or granules are white in colour and free flowing. About 4 million twines is now produced annually in Pakistan. Urea is readily soluble in water. It contains 46% Nitrogen in amide (NH-12) form which is changed to ammonium (NH3+) in the soil. Urea sometimes contains small quantities of biuret, a toxic impurity but commercial supplies are ensured at a level not toxic. Poor agronomic practices may cause loss to the atmosphere therefore it requires high degree of understanding for efficient use. Because of its high water solubility, it is well suited for use in solution fertilizers or foliar sprays.
The manufacturing process consists of the following steps.
Preparation of NH3 and CO2
Chemical reaction between NH3 and CO2
Distillation and processing to end product.
PRODUCTION OF NH3 HABER’S PROCESS
Nitrogen is obtained from air by burning any combustible material which utilizes O2 leaving N2 or by liquefaction of air H2 gas is obtained by cracking of natural gas or by heating the natural gas with steam in presence of Ni as catalyst.
CO2 is obtained as a by product which is used in the manufacturing of urea N2 and H2 gases obtained from above methods are mixed in the ratio 1:3 heated to optimal temperature 450-500 C° and pressure 200 atm in the presence of catalyst.
When coal is heated in a close retort to red hot (1000C°) in the absence of air or O2, it produces coal gas, coal tar and coke as major fractions. Ammonium salts such as (NH4)2CO3,(NH4)2S, (NH4)HS, NH4CN, (NH)2SO4 and NH4Cl present in ammoincal liquor are obtained as byproducts which on heating or treatment with lime produce NH3 gas.
(1) PRODUCTION OF CARBON DIOXIDE
Carbon dioxide is obtained from natural gas. The natural gas after being purified and compressed is subjected to combustion in the presence of nickel pellets as catalyst. The out coming gases contain carbon dioxide, hydrogen and nitrogen from air. The CO2 gas is absorbed in ammonium hydroxide solution to form ammonium bicarbonate.
From this ammonium bicarbonate solution, blowing steam though the solution can regenerate CO2 gas.
The remaining gases containing Hydrogen and Nitrogen are used for the production of ammonia.
(2) REACTION BETWEEN NH3 and CO2
NH3 and CO2 obtained are reacted with each other in reaction tower as under:
(3) PROCESSING TO END PRODUCT
According to solve process one part of compressed CO2 is reacted with two parts of liquid NH3 at 180 C° and 200 atm. pressure. The reaction between liquid NH3 and CO2 gas results in a liquid mass known as ammonium carbamate. The conversion ratio increases with temperature. The ammonium carbamate formed is passed on Co the distillation tower made of steel where it undergoes dehydration forming urea and water.
The unreactal NH3, CO2 and H20 vapours are sent back to the reaction chamber. The solution containing molten urea is concentrated to molten mass.