Synthesis of urea

Claim 1

IN A CYCLIC PROCESS FOR THE SYNTHESIS OF UREA BY HEATING AMMONIA AND CARBON DIOXIDE AT UREA-FORMING TEMPERATURES AND PRESSURES IN A UREA SYNTHESIS ZONE, SUBJECTING THE RESULTING UREA SYNTHESIS EFFLUENT FROM SAID SYNTHESIS ZONE TO DISTILLATION IN A PLURALITY OF DISTILLATION ZONES ARRANGED IN SERIES, THEREBY SEPARATING FROM EACH OTHER AQUEOUS UREA SOLUTION AND UNREACTED SUBSTANCES IN THE FORM OF A GASEOUS MIXTURE OF AMMONIA, CARBON DIOXIDE AND WATER VAPOR, ABSORBING AND CONDENSING SAID GASEOUS MIXTURE IN AN ABSORBENT SELECTED FROM THE GROUP CONSISTING OF WATER AND AQUEOUS UREA SOLUTION, AND RETURNING THE THUS-OBTAINED ABSORBATE TO SAID UREA SYNTHESIS ZONE, THE IMPROVEMENT WHICH COMPRISES: (A) SUBJECTING THE EFFLUENT FROM THE SYNTHESIS ZONE SUCCESSIVELY TO A FIRST PRESSURE DISTILLATION AT A PRESSURE OF FROM 13 TO 40 ATMOSPHERES AND AT 140 TO 155*C., THEN TO A SECOND PRESSURE DISTILLATION AT A PRESSURE OF FROM 10 TO 25 ATMOSPHAERES AT 140 TO 155*C., AND THEN TO A THIRD PRESSURE DISTILLATION AT FROM 1 TO 3 ATMOSPHERES AND 110 TO 130*C., (B) ABSORBING THE RESULTING GASEOUS MIXTURE OF AMMONIA, CARBON DIOXIDE AND WATER VAPOR OBTAINED FROM SAID FIRST DISTILLATION IN AN ABSORBENT SELECTED FROM THE GROUP CONSISTING OF WATER AND AQUEOUS UREA SOLUTION AT A PRESSURE OF FROM 13 TO 40 ATMOSPHERES AND 130 TO 150*C.; AND (C) RECYCLING THE ABSORBATE RESULTING FRON (B) AND HAVING A HIGH HEAT CONTENT INTO HEAT TRANSFER CONTACT WITH THE EFFLUENT DURING SAID THIRD DISTILLATION AND FURTHER INTO THE SYNTHESIS ZONE, THEREBY UTILIZING IN SAID THIRD DISTILLATION ABSORPTION HEAT TRANSFERRED FROM SAID ABSORBATE TO SAID EFFLUENT.