Process for production of urea

Claim 1

IN A URES SYNTHESIS PROCESS COMPRISING REACTING AMMONIS AND CARBON DIOXIDE AT ELEVATED PRESSURE TO FORM A PROCESS STREAM CONTAINING UREA, AMMONIUM CARBONATE, RESIDUAL AMMONIA AND WATER, REDUCING THE PRESSURE OF SAID PROCESS STREAM TO THE RANGE OF 215 P,S.I.A. 275 P.S.I.A. SEPARATING SUBSTANTIALLY PURE GASEOUS AMMONIA AT 215 P.S.I.A. TO 265 P.S.I.A. FROM SAID PROCESS STREAM MAINTAINES AT A TEMPERATURE BETWEEN 200* F, TO 250* F., FURTHER REDUCING THE PRESSURE OF THE RESIDUAL PROCESS STREAM TO BETWEEN 20 TO 75 P.S.I.A. HEATING SAID RESIDUAL PROCESS STREAM AT A TEMPERATURE IN THE RANGE OF 200* F, TO 260* F. WHEREBY A PORTION OF SAID AMMONIUM CARBANATE IS DECOMPOSED, SEPARATING A FIRST MIXED AMMONIA-CARBON DIOXIDE GAS STREAM FROM SAID RESIDUAL PROCESS STREAM, FURTHERE HEAATING SAID RESIDUAL PROCESS STREAM AT A TEMPERATURE IN THE RANGE OF 200* F, TO 260* F, WHEREBY THE BALANCE OF SAID AMMONIUM CARBANATE IS DECOMPOSED, AND SEPARATING A SECOND MIXED AMMONIA-CARBON DIOXIDE GAS STREAM FROM THE FINAL RESIDUAL PROCESS STREAM, SAID FINAL RESIDUAL PROCESS STREAM CONSISTING OF PRODUCT AQUEOUS UREA SOLUSTION, THE IMPROVEMENT WHICH COMPRISES HEATING SAID SUBSTANTIALLY PURE GASEOUS AMMONIA TO A TEMPERATURE OF AT LEAST ABOUT 190* F, EXPANDING SAID GASEOUS AMMONIA FROM AN INLET PRESSURE IN THE RANGE OF 215 P.S.I.A. TO 265 P.S.I.A. TO A LOWER OUTLET PRESSURE IN THE RANGE OF 20 P.S.I.A. TO 75 P.S.I.A. UTILIZING THE EXPANSION ENERGY DERIVED THEREFROM TO PRODUCE AS EVACUTING EFFECT ON THE FINAL RESIDUAL PROCESS STREAM, AND ENTRAINING THE SECOND MIXED AMMONIA AND CARBON DIOXIDE GAS STREAM CONTAINED IN THE FINAL RESIDUAL PROCESS STREAM INTO THE SUBSTANTIALLY PURE GASEOUS AMMONIA, THEREBY EFFECTING THE SEPARATION OF SAID SECOND MIXED GAS STREAM FROM SAID FINAL RESIDUAL PROCESS STREAM AT A SUB-ATMOSPHERIC PRESSURE BETWEEN 5 P.S.I.A. TO 15 P.S.I.A. AND COMBINING SAID GASEOUS AMMONIA AND SAID SECOND MIXED AMMONIA-CARBON DIOXIDE GAS STREAM TO FORM A COMBINED OFF-GAS STREAM AT SAID OUTLET PRESSUSRE IN THE RANGE OF 20 P.S.I.A. TO 75 P.S.I.A. AND WHEREBY SAID PRODUCT AQUEOUS UREA SOLUTION IS OBTAINED WITH NEGLIGIBLE BIURET CONTENT AND LESS THAN 0.2% RESIDUAL AMMONIA CONTENT.