A phase model for the gas–liquid equilibria in the ammonia–carbon dioxide–water–urea system in chemical equilibrium at urea synthesis conditions. I. Theory

A ternary phase model of the reciprocal salt-pair type (square composition plane) is presented to describe the gas–liquid equilibria of the ammonia–carbon dioxide–water–urea system in chemical equilibrium at urea synthesis conditions. The components of the model are CO2, H2O and (2NH3). Urea is represented by the equation:. and is therefore located at the corner of the composition square diagonally opposed to water. Characteristic of this phase model (at constant temperature) is the occurrence of minimum bubble- and dew-points in planes located parallel to the (2NH3)-CO2 diagonal. The point of tangency of the bubble- and dew-point lines formed by these points forms the ternary azeotropic point. Intercepting the azeotropic point is a plane parallel to the urea–water diagonal; at the azeotropic point the bubble-point line in this plane reaches a maximum pressure value. The ternary azeotrope is of the saddle type.