Process for urea synthesis

Claim 1

IN A COMPLETE RECYCLE UREA SYNTHESIS PROCESS IN WHICH AMMONIA, CARBON DIOXIDE AND RECYCLE AMMONIUM CARBAMATE SOLUTION ARE REACTED IN AN OVERALL MOLAR RATIO OF AMMONIA TO CARBON DIOXIDE BETWEEN 2:1 TO ABOUT 5:1 AT A PRESSURE BETWEEN 2000 P.S.I.G. TO 6000 P.S.I.G. AND TEMPERATURE BETWEEN 320*F. TO 430*F. WHEREBY BETWEEN ABOUT 40% TO 60% CONVERSION OF AMMONIUM CARBAMATE TO UREA TAKES PLACE, THE RESULTING PROCESS STREAM IS HEATED TO A TEMPERATURE BETWEEN ABOUT 240* TO 300*F. IN A FIRST STAGE AT A REDUCED PRESSURE BETWEEN ABOUT 200 P.S.I.G. TO 400 P.S.I.G. TO DECOMPOSE A MAJOR PORTION OF CONTAINED AMMONIUM CARBAMATE AND GENERATE A FIRST OFF-GAS CONTAINING AMMONIA, CARBON DIOXIDE AND WATER VAPOR, SAID FIRST OFF-GAS IS SEPARATED FROM RESIDUAL LIQUID PROCESS STREAM AND SCRUBBED WITH A FIRST AMMONIUM CARBAMATE SOLUTION AT A TEMPERATURE IN THE RANGE OF 140*F. TO 212*F., WHEREBY A PORTION OF SAID FIRST OFFGAS IS CONDENSED AND ABSORBED INTO SAID FIRST SOLUTION, A PORTION OF SAID FIRST AMMONIUM CARBAMATE SOLUTION IS COMPRESSED TO UREA SYNTHESIS PRESSURE BETWEEN 2000 P.S.I.G. AND 6000 P.S.I.G. AND RECYCLED TO UREA SYNTHESIS AS SAID RECYCLE AMMONIUM CARBAMATE SOLUTION, THE REMAINING UNABSORBED FIRST OFF-GAS IS FURTHER COOLED AND REFLUXED TO A FINAL TEMPERATURE BELOW 120*F. WHEREBY A FINAL FIRST OFF-GAS STREAM IS PRODUCED COMPRISING AMMONIA GAS AT A PRESSURE IN THE RANGE OF 200 P.S.I.G. TO 400 P.S.I.G. AND SUBSTANTIALLY FREE OF CARBON DIOXIDE, THE PRESSURE OF SAID RESIDUAL LIQUID PROCESS STREAM IS LOWERED TO A FINAL REDUCED PRESSURE BETWEEN ABOUT 5 P.S.I.G. TO 50 P.S.I.G., SAID RESIDUAL LIQUID PROCESS STREAM IS HEATED TO A TEMPERATURE BETWEEN ABOUT 170*F. TO 250*F. IN A SECOND STAGE TO DECOMPOSE THE BALANCE OF CONTAINED AMMONIUM CARBAMATE AND GENERATE A SECOND OFF-GAS STREAM CONTAINING AMMONIA, CARBON DIOXIDE AND WATER VAPOR, SAID SECOND OFF-GAS STREAM IS SEPARATED FROM FRINAL RESIDUAL LIQUID STREAM COMPRISING PRODUCT AQUEOUS UREA SOLUTION, SAID SECOND OFF-GAS STREAM IS SCRUBBED WITH A SECOND AMMONIUM CARBAMATE SOLUTION AT A TEMPERATURE IN THE RANGE OF 125*F. TO 145*F., WHEREBY A PORTION OF SAID SECOND OFF-GAS IS CONDENSED AND ABSORBED INTO SAID SECOND SOLUTION, THE REMAINING UNABSORBED SECOND OFF-GAS IS FURTHER COOLED AND REFLUXED TO A FINAL TEMPERATURE BELOW 100*F., WHEREBY A FINAL SECOND OFF-GAS STREAM IS PRODUCED COMPRISING AMMONIA GAS AT A PRESSURE BETWEEN ABOUT 5 P.S.I.G. TO 50 P.S.I.G., AND SAID FINAL FIRST AND SECOND OFF-GAS STREAMS COMPRISING AMMONIA GAS ARE RECYCLED TO UREA SYNTHESIS, THE IMPROVEMENT WHICH COMPRISES (A) PASSING SAID FIRST OFF-GAS STREAM UPWARDLY THROUGH A VERTICAL HEAT EXCHANGE ZONE PRIOR TO SAID SCRUBBING WITH FIRST AMMONIUM CARBAMATE SOLUTION, (B) WITHDRAWING A PORTION OF SAID SECOND AMMONIUM CARBAMATE SOLUTION DERIVED FROM SAID SCUBBING OF SECOND OFF-GAS AND PRESSURIZING SAID WITHDRAWN SECOND AMMONIUM CARBAMATE SOLUTION TO A PRESSURE BETWEEN ABOUT 200 P.S.I.G. TO 400 P.S.I.G., (C) PASSING SAID PRESSURIZED SECOND AMMONIUM CARBAMATE SOLUTION FROM STEP (B) DOWNWARDLY THROUGH SAID VERTICAL HEAT EXCHANGE ZONE AS A THIN LIQUID FILM AND IN EXTENDED SURFACE CONTACT WITH SAID FIRST OFF-GAS STREAM, WHEREBY SAID FIRST OFF-GAS STREAM IS PARTIALLY ABSORBED IN SAID SECOND AMMONIUM CARBAMATE SOLUTION AT A TEMPERATURE BETWEEN ABOUT 230*F. TO 320*F. AND HEAT IS GENERATED, (D) COMBINING THE AMMONIUM CARBAMATE SOLUTION DERIVED FROM STEP (C) WITH SAID FIRST AMMONIUM CARBAMATE SOLUTION, AND (E) PASSING SAID RESIDUAL LIQUID PROCESS STREAM AT A PRESSURE IN THE RANGE OF 5 P.S.I.G. TO 50 P.S.I.G. AND CONTAINING UNDECOMPOSED AMMONIUM CARBAMATE IN HEAT EXCHANGE RELATION WITH SAID FIRST OFF-GAS STREAM AND SECOND AMMONIUM CARBAMATE SOLUTION IN SAID VERTICAL HEAT EXCHANGE ZONE, WHEREBY HEAT IS ABSORBED BY SAID RESIDUAL PROCESS STREAM AND THE BALANCE OF CONTAINED AMMONIUM CARBAMATE IS DECOMPOSED TO GENERATE SAID SECOND OFF-GAS.