Recovery of residual ammonia and carbon dioxide in the synthesis of urea

Claim 1

IN A PROCESS FOR UREA SYNTHESIS WHICH COMPRISES REACTING AMMONIA, CARBON DIOXIDE AND RECYCLE AQUEOUS AMMONIUM CARBAMATE SOLUTION IN AN OVERALL AMMONIA-CARBON DIOXIDE MOLAR RATIO BETWEEN 2 TO 1 AND 5 TO 1 AND AT UREA SYNTHESIS PRESSURE BETWEEN 2000 P.S.I.G. TO 6000 P.S.I.G. AND TEMPERATURE BETWEEN 320*F. TO 430*F., REDUCING THE PRESSURE OF THE RESULTING UREA-CONTAINING PROCESS STREAM TO THE RANGE OF 200 P.S.I.G. TO 400 P.S.I.G., HEATING THE PROCESS STREAM AT A TEMPERATURE BETWEEN 240*F. TO 300*F. WHEREBY A MAJOR PORTION OF THE CONTAINED AMMONIUM CARBAMATE IS DECOMPOSED, SEPARATING A FIRST OFFGAS COMPRISING AMMONIA, CARBON DIOXIDE AND WATER VAPOR, FURTHER REDUCING THE PRESSURE OF THE RESIDUAL UREACONTAINING PROCESS STREAM TO THE RANGE OF 5 P.S.I.G. TO 50 P.S.I.G., FURTHER HEATING THE RESIDUAL PROCESS STREAM AT A TEMPERATURE BETWEEN 150*F. TO 230*F. TO DECOMPOSE THE BALANCE OF CONTAINED AMMONIUM CARBAMATE, SEPARATING A SECOND OFF-GAS COMPRISING AMMONIA, CARBON DIOXIDE AND WATER VAPOR FROM THE FINAL RESIDUAL PROCESS STREAS, SAID FINAL RESIDUAL PROCESS STREAM COMPRISING PRODUCT AQUEOUS UREA SOLUTION, THE IMPROVEMENT WHICH COMPRISES SCRUBBING SAID SECOND OFF-GAS WITH COLD AQUEOUS AMMONIAAMMONIUM CARBAMATE SOLUTION WHEREBY SAID SECOND OFFGAS IS COOLED TO A TEMPERATURE BETWEEN 125*F. TO 140* F. AND A PORTION THEREOF IS CONDENSED AND ABSORBED INTO SAID SOLUTION, SAID SOLUTION BEING HEATED FROM AN INITIAL TEMPERATURE BETWEEN 110*F. TO 140*F. TO A FINAL TEMPERATURE BETWEEN 115*F. TO 145*F. BY SAID SCRUBBING STEP, FURTHER COOLING AND REFLUXING THE RESIDUAL GAS PORTION OF SAID SECOND OFF-GAS TO A FINAL TEMPERATURE BELOW 100*F. WHEREBY SUBSTANTIALLY ALL CARBON DIOXIDE IS REMOVED FROM SAID SECOND OFF-GAS, RECYCLING THE FINAL RESIDUAL SECOND OFF-GAS COMPRISING AMMONIA AT 5 P.S.I.G. TO 50 P.S.I.G. AND FREE OF CARBON DIOXIDE TO UREA SYNTHESIS, COOLING AND RECYCLING A PORTION OF SAID AQUEOUS AMMONIAAMMONIUM CARBAMATE SOLUTION TO SAID SCRUBBING OF SECOND OFF-GAS, COMPRESSING THE BALANCE OF SAID SOLUTION TO SAID INTERMEDIATE PRESSURE OF FIRST CARBAMATE DECOMPOSITION IN THE RANGE OF 200 P.S.I.G. TO 400 P.S.I.G., COMBINING SAID COMPRESSED SOLUTION WITH SAID FIRST OFF-GAS, SCRUBBING THE RESULTING HOT MIXED GAS-LIQUID STREAM WITH WARM AQUEOUS AMMONIA-AMMONIUM CARBAMATE SOLUTION WHEREBY THE RESIDUAL GAS PHASE OF SAID MIXED STREAM IS COOLED TO A TEMPERATURE BETWEEN 150*F. TO 220*F. AND A PORTION OF SAID MIXED STREM IS CONDENSED AND ABSORBED INTO SAID SOLUTION AND SAID SOLUTION IS HEATED, FURTHER COOLING AND REFLUXING THE RESIDUAL GAS PHASE TO A FINAL TEMPERATURE BETWEEN 110*F. TO 120*F. WHEREBY SUBSTANTIALLY ALL CARBON DIOXIDE IS REMOVED FROM SAID GAS PHASE, RECYCLING THE FINAL RESIDUAL GAS PHASE COMPRISING AMMONIA AT 200 P.S.I.G. TO 400 P.S.I.G. AND FREE OF CARBON DIOXIDE TO UREA SYNTHESIS, COOLING AND RECYCLING A PORTION OF SAID WARM AQUEOUS AMMONIA-AMMONIUM CARBAMATE SOLUTION TO SAID SCRUBBING OF HOT MIXED GAS-LIQUID STREAM, AND COMPRESSING AND RECYCLING THE BALANCE OF SAID SOLUTION TO UREA SYNTHESIS AS SAID RECYCLE AQUEOUS AMMONIUM CARBAMATE SOLUTION.