Process for separating and recovering unreacted materials in urea synthesis

Claim 1

In a process for recovering unreacted materials and heat from a urea synthesis effluent including the steps of reacting in a urea synthesis zone carbon dioxide with ammonia under urea synthesis temperature and pressure conditions to obtain said urea synthesis effluent containing urea, an excess of ammonia, unreacted ammonium carbamate and water, subjecting said urea synthesis effluent to at least two ammonium carbamate decomposition stages including at least one high pressure steps at a gauge pressure above 10 kg/cm 2 and a low pressure stage at a gauge pressure below 5 kg/cm 2 to separate from said urea synthesis effluent gaseous mixtures each composed of ammonia, carbon dioxide and water vapor in the respective separation zones of said high pressure and low pressure decomposition stages, at least a portion of the separation zone of the low pressure stage being a rectification zone having a top zone and a bottom zone, contacting each of said gaseous mixtures with an absorbent in absorption zones each of which has substantially the same pressure as that of each of the corresponding separation zones to absorb said gaseous mixtures in said absorbent successively, and recycling the thus-obtained absorbate to the urea synthesis zone, the improvement which comprises cooling the urea synthesis effluent from the separation zone of said high pressure stage to a temperature of 105° - 170° C. by indirect heat exchange in a heat exchange zone with the urea synthesis effluent present in said bottom zone of the rectification zone of said low pressure stage, reducing the pressure of said cooled urea synthesis effluent to that of the low pressure stage, introducing the thus pressure reduced effluent into the top zone of the rectification zone and at the same time heating said urea synthesis effluent present in the bottom zone of said rectification zone by said indirect heat exchange in said heat exchange zone and in an additional heating zone to maintain the temperature of the top zone of the rectification zone at 60° - 120° C. and the temperature of the bottom zone of the rectification zone at 100° - 140° C.