Method of removing carbon dioxide from ammonia

Claim 1

IN A PROCESS FOR RECOVERING SUBSTANTIALLY CARBON DIOXIDE-FREE AMMONIA FROM AN AQUEOUS UREA SOLUTION OBTAINED BY REACTING AMMONIA AND CARBON DIOXIDE IN A UREA SYNTHESIS AUTOCLAVE AT UREA FORMING TEMPERATURES AND PRESSURES IN THE PRESENCE OF AN EXCESS OF AMMONIA OVER STOICHIOMETRICAL REQUIREMENTS THE STEPS COMPRISING: (A) PASSING SAID AQUEOUS UREA SOLUTION INCLUDING UNREACTED AND ABSORBED AMMONIA AND CARBON DIOXIDE AND INTERMEDIATE REACTION PRODUCTS THROUGH A DECOMPRESSION ZONE; (B) HEATING SAID SOLUTION IN SAID DECOMPRESSION ZONE TO A TEMPERATURE SUFFICIENT TO EVOLVE AN OFF-GAS MIXTURE COMPRISED OF A MAJOR PORTION OF AMMONIA AND A MINOR PORTION OF CARBON DIOXIDE AND WATER VAPOR, A PART OF THE AMMONIA AND CARBON DIOXIDE BEING FORMED BY DECOMPOSING AT LEAST A PORTION OF SAID INTERMEDIATE REACTION PRODUCTS; (C) PASSING SAID OFF-GAS MIXTURE UPWARDLY THROUGH AT LEAST TWO SUCCESSIVE ABSORPTION ZONES IN COUNTERCURRENT CONTACT WITH AN ABSORBING MEDIUM COMPRISED OF AN AQUEOUS SOLUTION OF AMMONIUM CARBONATE AND AMMONIA WHEREBY SUSBTANTIALLY ALL OF THE CARBON DIOXIDE IS ABSORBED IN SAID MEDIUM AND SUBSTANTIALLY ALL OF THE WATER VAPOR IN SAID OFF-GAS IS CONDENSED, SAID CONTACTS BEING CARRIED OUT AT A PRESSURE OF FROM AMOUNT 5.5 TO 15 KG./CM.2 AND MAINTAINING THE TEMPERATURE AT EACH STAGE BETWEEN APPROXIMATELY THE BOILING POINT OF SAID ABSORBING MEDIUM AND THE CRYSTALLIZATION TEMPERATURE OF AMMONIUM CARBONATES AND CARBAMATE WITHIN SAID ABSORBING MEDIUM; AND (D) REMOVING SUBSTANTIALLY CARBON DIOXDE-FREE AMMONIA AS AN OVERHEAD GAS FROM THE FINAL ABSORPTION ZONE.