Urea synthesis

Claim 1

IN A PROCESS FOR SYNTHESIZING UREA WHICH COMPRISES REACTING AMMONIA AND CARBON DIOXIDE AT A TEMPERATURE IN THE RANGE OF 340 TO 390*F. AND AT A PRESSURE IN THE RANGE OF 2700 TO 4000 P.S.I.G. IN THE PRESENCE OF RECYCLED AMMLNIUM CARBAMATE, WITHDRAWING FROM THE SYNTHESIS ZONE A REACTION MIXTURE CONSISTING ESSENTIALLY OF UREA, WATER, AMMONIUM CARBAMATE AND EXCESS AMMONIA, REMOVING THE MAJOR PORTION OF SAID EXCESS AMMONIA FROM SAID REACTION MIXTURE BY DISTILLATION, DECOMPSOING AMMONIUM CARBAMATE IN THE REMAING MIXTURE AT A PRESSURE IN THE RANGE OF 90 TO 175 P.S.I.G. TO SEPARATE THEREFROM A FIRST GASEOUS FRACTION RICH IN AMMONIA AND CARBON DIOXIDE AND LEAVE AN AQUEOUS UREA-RICH LIQUID FRACTION CONTAINING AMMONIA CARBAMATE AND FURTHER DECOMPOSING AMMONIUM CARBAMATE, IN SAID UREA-RICH LIQUID FRACTION AT A PRESSURE IN THE RANGE OF 0 TO 15 P.S.I.G. TO SEPARATE THEREFROM A SECOND GASEOUS FRACTION RICH IN AMMONIA AND CARBON DIOXIDE AND LEAVE AN AQUEOUS SOLUTION OF UREA SUBSTANTIALLY FREE OF AMMONIUM CARBAMATE, THE IMPROVEMENT WHICH COMPRISES REFLUXING AMMONIA DURING THE DISTILLATION OF SAID REACTION MIXTURE SO THAT SAID MAJOR PORTION OF SAID EXCESS AMMONIA IS REMOVED SUBSTANTIALLY FREE OF WATER AND CARBON DIOXIDE, LIQUEFY ING THE BULK OF THE REMOVED AMMONIA, FORMING FROM SAID SECOND GASEOUS FRACTION AN AMMONIACAL AMMONIUM CARBAMATE SOLUTION CONTAINING THE MINIMUM QUANTITY OF WATER TO PREVENT PRECIAPATATION OF AMMONIUM CARBAMATE COMBINING SAID AMMONIUM CARBAMATE SOLUTION, SAID FIRST GASEOUS FRACTION AND THE LIQUEFIED AMMONIA REMOVED FROM SAID REACTION MIXTURE WITH FRESH FEED LIQUID AMMONIA TO PROVIDE A RECYCLE FEED COMPRISING A SLURRY OF AMMONIUM CARBAMATE IN LIQUID AMMONIA, AND INTRODUCING SAID RECYCLE FEED AND FRESH FEED CARBON DIOXIDE AS A GROSS FEED INTO SAID SYNETHESIS ZONE FOR THE AFORESAID REACTION OF AMMONIA AND CARBON DIOXIDE, SAID GROSS FEED HAVING A MOLAR RATIO OF AMMONIA TO CARBON DIOXIDE IN THE RANGE OF 6.5:1 TO 9.0:1 AND A MOLAR RATIO OF WATER TO CARBON DIOXIDE NOT EXCEEDING 0.25:1.