c08s02.pdf - 8.2 Urea 8.2.1 General1-13 Urea[CO(NH2)2 also...

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8.2 Urea 8.2.1 General 1-13 Urea [CO(NH 2 ) 2 ], also known as carbamide or carbonyl diamide, is marketed as a solution or in solid form. Most urea solution produced is used in fertilizer mixtures, with a small amount going to animal feed supplements. Most solids are produced as prills or granules, for use as fertilizer or protein supplement in animal feed, and in plastics manufacturing. Five U. S. plants produce solid urea in crystalline form. About 7.3 million megagrams (Mg) (8 million tons) of urea were produced in the U. S. in 1991. About 85 percent was used in fertilizers (both solid and solution forms), 3 percent in animal feed supplements, and the remaining 12 percent in plastics and other uses. 8.2.2 Process Description 1-2 The process for manufacturing urea involves a combination of up to 7 major unit operations. These operations, illustrated by the flow diagram in Figure 8.2-1, are solution synthesis, solution concentration, solids formation, solids cooling, solids screening, solids coating and bagging, and/or bulk shipping. The combination of processing steps is determined by the desired end products. For example, Figure 8.2-1. Major area manufacturing operations. plants producing urea solution use only the solution formulation and bulk shipping operations. Facilities producing solid urea employ these 2 operations and various combinations of the remaining 5 operations, depending upon the specific end product being produced. In the solution synthesis operation, ammonia (NH 3 ) and carbon dioxide (CO 2 ) are reacted to form ammonium carbamate (NH 2 CO 2 NH 4 ). Typical operating conditions include temperatures from 180 to 200°C (356 to 392°F), pressures from 140 to 250 atmospheres (14,185 to 25,331 kilopascals) NH 3 :CO 2 molar ratios from 3:1 to 4:1, and a retention time of 20 to 30 minutes. The carbamate is then dehydrated to yield 70 to 77 percent aqueous urea solution. These reactions are as follows: (1) 2NH 3 CO 2 NH 2 CO 2 NH 4 7/93 (Reformatted 1/95) Inorganic Chemical Industry 8.2-1
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(2) NH 2 CO 2 NH 4 NH 2 CONH 2 H 2 O The urea solution can be used as an ingredient of nitrogen solution fertilizers, or it can be concentrated further to produce solid urea. The 3 methods of concentrating the urea solution are vacuum concentration, crystallization, and atmospheric evaporation. The method chosen depends upon the level of biuret (NH 2 CONHCONH 2 ) impurity allowable in the end product. Aqueous urea solution begins to decompose at 60°C (140°F) to biuret and ammonia. The most common method of solution concentration is evaporation. The concentration process furnishes urea "melt" for solids formation. Urea solids are produced from the urea melt by 2 basic methods: prilling and granulation. Prilling is a process by which solid particles are produced from molten urea. Molten urea is sprayed from the top of a prill tower. As the droplets fall through a countercurrent air flow, they cool and solidify into nearly spherical particles.
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