Among the elements known in nature, nitrogen (N), phosphorus (P) and potassium (K) are the most effective in plant growth and therefore some compounds containing these elements are used as chemical fertilizers in agriculture. Urea with the chemical formula NH2CONH2 is the most popular nitrogen fertilizer and due to the fact that it is suitable for most plants, especially all cereals, it does not leave any salt residue in the soil and does not change the concentration of soil salts. Of the total urea produced in the world, more than 70% of it is used for agriculture and the rest is used in the production of melamine powder, use as animal feed, medical purposes and so on.
Urea is obtained from the reaction between ammonia liquid and carbon dioxide gas at a pressure of 143 kg / cm2 and a temperature of about 170-180 ° C during the following equilibrium reactions:
2NH3+CO2 NH2 COONH4
NH2 COONH4 NH2 CONH2 + H20
The first reaction is calorific and rapid, during which ammonia and carbon dioxide are converted to liquid ammonium carbamate (NH2COONH4) and the second two reactions, which are slow and endothermic, ammonium carbamate to urea (NH2CONH2) and water. Therefore, in this unit, it is necessary to provide the conditions for performing the above two reactions in terms of pressure and temperature.
The feed of urea unit includes ammonia and carbon dioxide which is supplied from ammonia unit. Liquid ammonia is compressed by the relevant pumps to a pressure of about 160 kg / cm2 and carbon dioxide is compressed by a compressor to a pressure of about 146 kg / cm2, and then these two streams enter the urea synthesis section. This section includes all devices that operate at the above pressure and temperature.
In a high pressure carbamate maker, ammonium carbamate is formed in the presence of ammonia and carbon dioxide under optimal reaction conditions. As mentioned above, this reaction is very fast and exothermic and the heat generated by the reaction is used to produce low pressure steam (4.5 kg / cm2). This steam is used in different parts of the unit