Urea, also known as carbamide, is an end product of protein metabolism in humans and mammals and is also a major organic nitrogen compound excreted by animal bodies. Urea appears as white granular or needle-like and prismatic crystals when it is pure whereas it appears light red or yellow when including iron or other heavy metal. Besides, urea is tasteless, orderless, easily soluble in water, ethanol and benzene, insoluble in ether and chloroform. At 20°C, 105 kg of urea can be dissolved in 100 kg of water during which thermal energy from the surrounding will be absorbed. Dissolved in water, it is neither acidic nor alkaline. The nitrogen content of pure urea is 46.65% while the nitrogen content of agricultural urea is 42%-46%, containing a small amount of biuret.
Urea is mainly used as chemical fertilizer. In addition, it is also used industrially as a raw material for the manufacture of urea-formaldehyde esters, polyurethane and melamine-formaldehyde resins. Moreover, it is widely used in medicine, explosives, tanning, flotation agents, pigments and dewaxing of petroleum products. When heated to 200℃, urea turns to cyanuric acid. The derivatives of cyanuric acid, such as trichloroisocyanuric acid, sodium dichloroisocyanate, tris(2-hydroxyethyl) isocyanuric acid, tris(3,5-di-tert-butyl-4-hydroxybenzyl) isocyanate, triglycidyl ether of isocyanuric acid, cyanuric acid triglycidylamine complex, etc. have many important applications. The first two are new high-grade disinfectants and bleaching agents, and the total production capacity of trichloroisocyanuric acid worldwide is more than 80,000t.
4.The method for urea synthesis
Method I
Synthesize ammonium carbamate with carbon dioxide and ammonia under high temperature and pressure, then crystallize, separate and dry after decomposition and absorption transformation.
Method II
Mix purified ammonia and carbon dioxide at a molar ratio of 2.8 to 4.5 into the synthesis reactor. The pressure in the tower is kept at 13.8 to 24.6 MPa and the temperature is kept at 180 to 200 ℃. The reaction in the synthesis reactor lasts 25 to 40 min to get the urea solution containing excess ammonia and ammonium carbamate. Then, ammonia and ammonium carbamate are removed from the solution
by reducing the temperature and the pressure. Subsequently, the solution is evaporated to increase the concentration to over 99.5% and passed then into a prilling tower to obtain the urea.
Method III
Urea is the end product of protein metabolism in mammals. React ammonia with carbon dioxide to form carbamate, then dehydrate to carbamate to gain urea.
Industrial production method
Production method: Liquid ammonia and carbon dioxide are industrially used as raw materials to synthesize urea directly under high temperature and pressure conditions.
5.Application methods:
Urea application methods include basal fertilization, top-dress application, seed fertilization and foliage top-dressing. The basal fertilization and top-dress application should be carried out 4 to 5 days before the application of ammonium nitrogen fertilizers.
Urea, when used as base fertilizer for paddy fields, should be spread 5 to 7 days before irrigation, followed by immediate plowing. To avoid the leach of urea, irrigation should not be carried out right after basal fertilization. When top dressing, irrigate the field first to form a thin layer of water and then plow the field right after fertilizing, followed by no irrigation for 2 to 3 days. In basal fertilization and top dressing, the application method of urea and various ammonium nitrogen fertilizers are similar.
Urea is not used as fertilizer directly because the increasing pH caused by fertilizers containing urea or ammonia will lead to protein denaturation, which prevents seed germination process and seedling growth and even deprives the seed of its germination ability. This is especially harmful to small-sized seeds. But if urea mixed with dry and fine soil lies deep under a thin layer of soil where the seed is buried, the urea will not exert an adverse effect on the seed. Moreover, applying urea to the soil 2.5 cm away from the seed also does no harm to it.
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