Urea Hydrolysis and Distillation of Its Solution

Abstract

In the total recycle urea process, distillation under high pressure and temperature is required for recovering highly concentrated solution. In this case there are two problems: one is urea hydrolysis and the other is control of amount of H₂0 in the overhead gas of the distillation column. About the former, rate of the hydrolysis was measured and it was found that the reaction is second order and the rate constant becomes about twice as the temperature rises by 10°C. A simple equation was derived for estimating degree of hydrolysis within a short time. About the latter, the reduction of amount of H₂0 was achieved by flash of the autoclave effluent at the top of the column and successive distillation. Composition and temperature were constant through the middle stages of the column and the concentration of CO₂ was higher than that at any other part. This phenomenon is due to the reduction of NH₃/CO₂ in the solution by the absorption of CO₂ gas from the bottom, of which partial pressure is relatively higher than that any other part. Therefore more than some stages are not needed. Based on these results a new total recycle urea process was developed.