A test of the equilibrium conversion of CO2 to urea was carried out using a batch autoclave. Some results different from those reported by Frèjacques were obtained, as was expected from the latest data on commercial urea plants. The equilibrium conversion reached its maximum value at about 195°C by 5–15% higher than that reported by Frèjacques. On the contrary, at temperatures above 200°C the value was lower, and falled rapidly with the raise of temperature. This phenomenon may be due to the mechanism of the equilibrium of urea synthesis, and not to the experimental method, as our sampling was made directly from the liquid phase. The equilibrium conversion, x, may be expressed with respect to NH3/CO2, a, H2O/CO2, b, and temperature, t, as (Remark: Graphics omitted.) Therefore, a limited conversion exists in the conventional urea process.

In the preceding paper (This Bulletin, 45, 1339 (1972)) it was reported that the existence of the maximum equilibrium conversion was not due to the experimental method. For this phenomenon, we have presented the following interpretation. While NH4CO2NH2 is converted to urea and water, it dissociates into NH3 and CO2 in the solution (not in the gas phase), and the degree of the dissociation increases exponentially as the temperature becomes higher, while the conversion to urea gradually increases. Therefore, at a certain temperature, the equilibrium conversion reaches its maximum value. Although it is nearly impossible to prove analytically the dissociation of NH4CO2NH2 in the solution, we are sure of it from the following facts: 1) The dissociation of NH4-CO2NH2 is a great endothermic reaction, even under the conditions of urea synthesis. 2) The value of NH3/CO2which gives the minimum equilibrium pressure at a usual temperature is not always equal to 2.0, but varies with the temperature. 3) The effect of H2O/CO2 on the equilibrium pressure varies with the temperature; that is, when the temperature is lower than about 200°C, an increase in H2O/CO2 reduces the equilibrium pressure, but when the temperature is higher than 200°C, an increase in H2O/CO2 makes the equilibrium pressure higher.

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