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The background information and research work described in this dissertation centers about the study of an alternative method, the hot gas recirculation process (HGRP), for the synthesis of urea. For the realization of this process knowledge concerning phase equilibria and corrosion are of fundamental importance. For this reason the bulk of the research work is directly related to these two topics. In the first chapter the fundamentals of urea technology are presented so that the reader can better judge the HGRP in relation to the other urea processes. A sketch of some economic aspects of urea production and the urea market are also presented as general, but also relevant information, and a brief sketch is given of the development of the urea technology and its possible future trends. Originally to determine the optimum reactor conditions for the HGRP over a large range of temperature and composition conditions an extensive study was made of the bubble point pressures of NH3-CO2-H20-urea mixtures at complete equilibrium. Simultaneously, a theoretical phase model was developed which described the gas-liquid equilibrium conditions in a urea reactor as a part of a newly formulated ternary phase model for the NH3-H2O-CO2 system at complete equilibrium in the range of 140 < T < 220°C and 20 < P < 1000 atm.