Manufacture of urea

Abstract

Urea is in many ways the most convenient form for fixed nitrogen. It has the highest nitrogen content available in a solid fertilizer (46 %). It is easy to produce as prills or granules and easily transported in bulk or bags with no explosive hazard. It leaves no salt residue after use on crops. Its specific gravity is 1.335, decomposes on boiling and is fairly soluble in water. The principal raw materials required for this purpose are NH3 & CO2.Two reactions are involved in the manufacture of urea. First, ammonium carbamate is formed under pressure by reaction between CO2 & NH3.
CO2 + 2NH3 NH2COONH4 H= -37.4 Kcal
This highly exothermic reaction is followed by an endothermic decomposition of the ammonium carbamate.
NH2COONH4 NH2CONH2 + H2O H= + 6.3 Kcal
Various processes for the manufacture of urea are:
1) Snamprogetti ammonia stripping process
2) Stamicarbon CO2 stripping process
3) Once through urea process
4) Mitsui Toatsu total recycle urea process
We selected the Snamprogetti ammonia stripping process for the manufacture of urea. In this process ammonia & CO2 are compressed & fed to the reactor. The unconverted carbamate is stripped and recovered from the urea synthesis reactor effluent solution at reactor pressure, condensed to an aqueous solution in a steam producing high pressure condenser & recycled back to the reactor by gravity. Part of the liquid NH3 reactor feed, vapourized in a steam heated exchanger, is used as inert gas to decompose & strip ammonium carbamate in the steam heated high pressure stripper. Energy balance & material balance of the plant is done. The selected capacity of the plant is 4,50,000 tons/year of urea producing 62,500 kg/hr of urea with 98 % purity. Urea reactor & vacuum evaporator are designed. The volume of reactor is calculated & found to be 195 m3. The length & diameter of the reactor are 40 m & 2.5 m respectively. The evaporator used is of climbing-film long- tube type. Snamprogetti ammonia-stripping urea process is selected because it involves a high NH3 to CO2 ratio in the reactor, ensuring the high conversion of carbamate to urea. The highly efficient ammonia stripping operation drastically reduces the recycling of carbamate and the size of equipment in the carbamate decomposition . Snamprogetti technology differs from competitors in being based on the use of excess ammonia to avoid corrosion as well as promote the decomposition of unconverted carbamate into urea.
Uses of Urea:
• About 56 % of Urea manufactured is used in solid fertilizer.
• About 31 % of Urea manufactured is used in liquid fertilizer.
• Urea-formaldehyde resins have large use as a plywood adhesive.
• Melamine-formaldehyde resins are used as dinnerware & for making extra hard surfaces.