PROCESS OF RECOVERING UNREACTED MATERIALS AND HEAT IN UREASYNTHESIS

摘要

1505445 Urea synthesis MITSUI TOATSU CHEMICALS Inc 1 Dec 1976 [5 Dec 1975] 50152/76 Heading C2C Urea is synthesized in a process comprising reacting carbon dioxide and ammonia to produce a urea-synthesis effluent, treating the ureasynthesis effluent in a plurality of pressure decomposition stages wherein the pressure is stepwise reduced thereby to decompose the ammonium carbamate therein into ammonia and carbon dioxide in each stage; separating a mixed gas of ammonia and carbon dioxide from the urea-synthesis effluent at each decomposition stage; contacting in turn the mixed gas separated in each decomposition stage with an absorbent under substantially the same pressure as the decomposition pressure in the stage concerned to absorb the mixed gas in the absorbent in stages; recycling the final resulting absorbate containing ammonium carbamate to the ureasynthesis zone; concentrating the aqueous urea solution discharged from the final decomposition stage and still containing a small amount of ammonium carbamate thereby to obtain a concentrated aqueous urea solution; condensing water vapour generated by the said concentration to obtain a condensate containing relatively small amounts of ammonia and carbon dioxide, and stripping the condensate to separate and recover the ammonia and carbon dioxide therefrom, and which includes absorbing the off-gas from a high-pressure decomposition stage in an absorbate in a high-pressure absorption zone, the absorbate being obtained by absorbing in an absorbent the off-gas from at least one medium-pressure decomposition stage, recovering the heat of absorption liberated by the absorption by an indirect heat exchange in the form of steam having a pressure of 2-4 kg./cm.2, contacting the steam with the condensate containing relatively small amounts of ammonia and carbon dioxide in a stripping zone to strip off substantially all of the ammonia and carbon dioxide from the condensate, and introducing the discharged steam containing the ammonia and carbon dioxide into the rectification zone of a low-pressure decomposition stage to directly heat the urea-synthesis effluent, thereby recovering the mixed gas separated from the urea-synthesis effluent in the lowpressure decomposition stage together with the ammonia and carbon dioxide contained in the steam.