Synthesis of urea and melamine

Claim 1

IN THE PROCESS OF MELAMINE SYNTHESIS FROM UREA WHEREIN AMMONIA, CARBONDIOXIDE AND COMPRSSED RECYCLE OFF-GAS DERIVED FROM AMMONIUMCARBAMATE DECOMPOSITION ARE REACTED AT A PRESSURE IN THE RANGE OF 2000 P.S.I.G. TO 6000 P.S.I.G. TO FORM UREA, THE PRESSURE OF THE RESULTING UREA SYNTHESIS STREAM IS REDUCED TO THE RANGE OF 1200 P.S.I.G. TO 1700 P.S.I.G. WHEREBY A PORTION OF THE UNCONVERTED AMONIUM CARBAMATE IN THE STREAM IS DECOMPOSED TO EVOLVE A FIRST OFF-GASS CONTAINING AMMONIA AND CARBON DIOXIDE, THE RESIDUAL UREA SYNTHESIS STREAM IS HEATED AT A PRESSURE IN THE RANGE OF 600 P.S.I.G. TO 850 P.S.I.G. TO DECOMPOSE ADDITIONAL AMMONIUM CARBAMATE AND PRODUCE A SECOND OFF-GAS, THE RESIDUAL UREA SYNTHESIS STREAM IS HEATED AT A PRESSURE IN THE RANGE OF 275 P.S.I.G. TO 500 P.S.I.G. TO DECOMPOSE ADDITIONAL AMMONIUM CARBAMATE AND PRODUCE A THIRD OFF-GAS, THE PRESSURE OF THE RESIDUAL UREA SYNTHESIS STREAM IS REDUCED TO THE RANGE OF 5 P.S.I.G. TO 50 P.S.I.G. WHEREBY THE BAANCE OF RESIDUAL AMMONIUM CARBAMATE IS DECOMPOSED AND A FOURTH OFF-GAS IS EVOLVED, SAID FOURTH OFF-GAS IS ABSORBED IN RECYCLING AQUEOUS AMMONIUM CARBAMATE SOLUTION, A PORTION OF SAID SOLUTION IS COMBINED WITH SAID RESIDUAL UREA SYNTHESIS STREAM AT A PRESSURE IN THE RANGE OF 275 P.S.I.G. TO 500 P.S.I.G., SAID THIRD OFF-GAS IS COMPRESSED TO THE PRESSURE OF SECOND OFF-GAS AND COMBINED THEREWITH, THE COMBINED OFF-GAS STREAM IS COMPRESSED TO THE PRESSURE OF SAID FIRST OFF-GAS AND COMBINED THEREWITH, THE FINAL COMBINED OFF-GAS STREAM IS COMPRESSED AND RECYCLED TO UREA SYNTHESIS AS SAID COMPRESSED RECYCLE OFF-GAS, THE RESIDUAL UREA SYNTHESIS STREAM REMAINING AFTER SEPARATION OF SAID PARTIAL OFF-GAS STREAMS IS PROCESSED TO CONVERT ITS UREA CONTENT TO SUSTANTIALLY ANHYDROUS UREA, AND SAID UREA IS HEATED TO A TEMPERATURE OF 250*C. TO 600*C. AT A PRESSURE OF 1200 P.S.I.G. TO 15,000 P.S.I.G. TOGETHER WITH ADDED AMMONIA TO FORM MELAMINE VAPOR TOGETHER WITH AN OFF-GAS CONTAINING AMMONIA AND CARBON DIOXIDE, THE MELAMINE SYNTHESIS STREAM IS COOLED TO CONDENSE SOLID MELAMINE, AND THE SOLID MELAMINE IS SEPARATED FROM THE MELAMINE OFF-GAS AT REDUCED PRESSURE, THE IMPROVEMENT WHICH COMPRISES SEPARATING A FIRST PARTIAL MELAMINE OFF-GAS AT A PRESSURE IN THE RANGE OF 1200 P.S.I.G. TO 1700 P.S.I.G., SEPARATING A SECOND PARTIAL MELAMINE OFF-GAS AT A PRESSURE IN THE RANGE OF 600 P.S.I.G. TO 850 P.S.I.G. SEPARATING A RESIDUAL MELAMINE OFF-GAS AT A PRESSURE IN THE RANGE OF 5 P.S.I.G. TO 50 P.S.I.G., COMBINING SAID FIRST PARTIAL MELAMINE OFF-GAS WITH FIRST UREA SYNTHESIS OFF-GAS, COMBINING SAIDSECOND PARTIAL MELAMINE OFF-GAS WITH SAID SECOND UREA SYNTHESIS OFF-GAS, COMBINING SAID RESIDUAL MELAMINE OFF-GAS WITH SAID FOURTH UREA SYNTHESIS OFF-GAS, AND COMPRESSING THE COMBINED STREAMS FOR RECYCLE TO UREA SYNTHESIS.