Method for controlling the ammonia and CO.sub.2 ratios in a urea manufacturing process

Abstract

A process for urea manufacture is described wherein the molar ratio of unreacted NH.sub.3 to CO.sub.2 (including NH.sub.3 and CO.sub.2 in the form of ammonium carbamate) in the carbamate reaction liquid phase is between 2:1 and 3:1, and the same ratio in the liquid phase in the urea autoclave is maintained between 3:1 and 6:1, control of the process being achieved by analysis of the gas phase and maintaining the NH.sub.3 to CO.sub. 2 molar ratio therein between about 1:1 and 6:1, preferably between about 1.5:1 and 3.5:1, in both the carbamate reactor and the urea autoclave.

Description

Claims

1. In continuous processes for producing urea which comprise (1) reacting NH.sub.3 and CO.sub.2 at a pressure of 100-140 atm. and a temperature of 155.degree.-170.degree. C to form initially an ammonium carbamate solution (2) subsequently converting said solution at a pressure of 100-140 atm. and a temperature of 170.degree.-190.degree. C into a urea solution which contains ammonium carbamate and/or dissolved NH.sub.3 and CO.sub.2, (3) thereafter stripping the said urea solution with a member selected from the group consisting of NH.sub.3, CO.sub.2 and mixtures thereof, whereby a large part of the unreacted NH.sub.3 and CO.sub.2 and the additional NH.sub.3 and CO.sub.2 from the decomposition of the unconverted ammonium carbamate present in said solution is removed therefrom in the gaseous state, (4) feeding the stripped NH.sub.3 and CO.sub.2 to the ammonium carbamate-formation zone or the urea-formation zone, (5) expanding the resulting urea solution to a low pressure to effect further removal of still dissolved NH.sub.3 and CO.sub. 2, (6) removing the urea solution freed of dissolved NH.sub.3 and CO.sub.2 for further processing,

the improvement wherein there is maintained (a) in the ammonium carbamate-formation zone a value of from 2:1 to 3:1 for the NH.sub.3 /CO.sub.2 molar ratio of the NH.sub.3 and CO.sub.2 bonded otherwise than in the form of urea and biuret, and (b) in the urea-formation zone, a value of from 3:1 to 6:1 for the NH.sub.3 /CO.sub.2 molar ratio of the NH.sub.3 and CO.sub.2 present in the liquid phase as such and as NH.sub.3 and CO.sub.2 bonded otherwise than in the form of urea and biuret, by continual sensing and analysis of the molar NH.sub.3 /CO.sub.2 gas ratio in one of the gas phases that are in contact with the liquid phases in said respective zones, and controlling the supply of NH.sub.3 and/or CO.sub.2 so as to maintain the thus-sensed said gas ratio within the range of 1:1 to 6:1, whereby the said NH.sub.3 /CO.sub.2 ratios in the respective said liquid phases, and the desired temperatures in the said respective zones are thereby maintained.

2. A process according to claim 1 in which the molar ratio of NH.sub.3 to CO.sub.2 in the said gas phases is kept within the range from 1.5:1 to 3.5:1.