Urea purification with recovery of ammonia and carbon dioxide

Claim 1

IN A PROCESS FOR THE PRODUCTION OF A LOW BIURET CONTENT UREA SOLUTION WHICH COMPRISES REACTING AMMONIA AND CARBON DIOXIDE IN A UREA AUTOCLAVE AT HIGH TEMPERATURES AND PRESSURES TO FORM A REACTION MIXTURE CONSISTING ESSENTIALLY OF UREA, AMMONIUM CARBAMATE, WATER, AND UNREACTED AMMONIA AND CARBON DIOXIDE, HEATING SAID REACTION MIXTURE AT A MODERATE PRESSURE TO DECOMPOSE A PORTION OF SAID AMMONIUM CARBAMATE, SEPARATING FROM SAID REACTION MIXTURE A FIRST GASEOUS FRACTION RICH IN AMMONIA AND CARBON DIOXIDE AND AN AQUEOUS UREA-RICH LIQUID FRACTION, FURTHER HEATING SAID LIQUID FRACTION AT A MODERATE TEMPERATURE FOR A RELATIVELY SHORT PERIOD OF TIME AT SUBSTANTIALLY ATMOSPHERIC PRESSURE TO DECOMPOSE THE REMAINING PORTION OF AMMONIUM CARBAMATE, SEPARATIN A SECOND GASEOUS FRACTION RICH IN AMMONIA AND CARBON DIOXIDE AND WITHDRAWING A UREA SOLUTION CONTAINING RESIDUAL AMOUNTS OF AMMONIA AND CARBON DIOXIDE, THE IMPROVEMENT WHICH COMPRISES; PASSING SAID UREA SOLUTION INTO A TREATING ZONE MAINTAINED UNDER A MODERATE VACUUM AND VERY RAPIDLY REDUCING THE TEMPERATURE OF SAID SOLUTION IN SAID ZONE TO A TEMPERATURE BETWEEN APPROXIMATELY 70*C. TO 80*C., PASSING A PORTION OF SAID SOLUTION THROUGH A VAPORIZING ZONE TO PROVIDE A STRIPPING MEDIUM COMPRISING A MAJOR PORTION OF WATER VAPOR, STRIPPING FROM SAID UREA SOLUTIION THE RESIDUAL AMOUNTS OF AMMONIA AND CARBON DIOXIDE BY CONTACTING THE LIQUID PORTION OF SAID SOLUTION WITH SAID STRIPPING MEDIUM; AND RECOVERING A UREA SOLUTION SUBSTANTIALLY FREE OF RESIDUAL AMOUNTS OF AMMONIA AND CARBON DIOXIDE: AND RECOVERING AND RECYCLING SAID STRIPPING MEDIUM AND CONTAINED AMMONIA AND CARBON DIOXIDE TO PRODUCE FURTHER QUANTITIES OF UREA.