The economy of the technical production of urea has been substantially improved through the recycling of unconverted reactants at synthesis pressure. Separation of these reactants from the urea synthesis solution is effected by stripping with carbon dioxide gas. The high pressure of the gases stripped off allows condensation at elevated temperature (150-170oC). At this high thermal level all of the condensation heat developed can be recovered in the form of steam. The urea-containing solution left after the stripping is processed in a low pressure recirculation stage, in which small quantities of ammonia and carbon dioxide remaining in the solution are expelled, to be dissolved in water and recycled to the synthesis section. The urea solution left after the expansion is subjected to evaporation and granulation (e.g. prilling).
To achieve optimum operation of the synthesis, knowledge on the points of the corrosion behaviour of construction materials, reaction heat development reaction rates, heat- and mass-transfer quantities, the flow behaviour of liquid and gases, and above all, the interaction of liquid and gas phases in contact with each other is of great importance. By the side of chemical, physical and technological aspects, the phase equilibria playing a role in the synthesis of urea are dealt with. An analysis of these equilibria on the basis of the phase theory of the system NH3-CO2 is given. It is concluded that there is no reason for assuming azeotropic phenomena to occur.
P J C Kaasenbrood and Dr H A G Chermin, UKF, Fertilizer Research and Development, Netherlands
48 pages, 15 figures, 15 references.