Production of Urea

The technical production of urea is based on the reaction of ammonia with carbon dioxide:

The reaction occurs in two steps: ammonium carbamate is formed first, followed by a decomposition step of the carbamate to urea and water. The first reaction is exothermic, and the equilibrium is favored at lower temperatures and higher pressures. Higher operating pressures are also desirable for the separation absorption step that results in a higher carbamate solution concentration. A higher ammonia ratio than stoichiometric is used to compensate for the ammonia that dissolves in the melt.

The reactor temperature ranges between 170–220°C at a pressure of about 200 atmospheres.

The second reaction represents the decomposition of the carbamate. The reaction conditions are 200°C and 30 atmospheres. Decomposition in presence of excess ammonia limits corrosion problems and inhibits the decomposition of the carbamate to ammonia and carbon dioxide. The urea solution leaving the carbamate decomposer is expanded by heating at low pressures and ammonia recycled. The resultant solution is further concentrated to a melt, which is then prilled by passing it through special sprays in an air stream. Figure 1.1 shows the Snamprogetti process for urea production.

Figure 1.1. The Snamprogetti process for producing urea: (1) reactor, (2,3,4) carbonate decomposers, (5,6) crystallizing and prilling.