Method of recovering unreacted materials and heat in urea synthesis
AssigneeMitsui Toatsu Chemicals, Inc.
InventorsShigeru Inoue, Katsumi Kagechika
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Abstract
Urea synthesis effluent obtained by reacting carbon dioxide with ammonia is subjected to a plurality of decomposition stages, e.g., three stages, to decompose and recover the unreacted ammonium carbamate with stepwise reduction of pressure, and the off-gas is absorbed in an absorbent in each stage and is recovered. The aqueous urea solution from the final decomposition stage is concentrated, water vapor generated upon said concentration is condensed, and the condensate containing small amounts of ammonia and carbon dioxide is stripped with steam recovered from the high pressure absorption zone. The steam discharged from the stripping step is introduced into the final decomposition stage to directly heat the urea synthesis effluent.
Claims14
In a process for recovering unreacted materials and heat from a urea synthesis which includes the steps of:
- reacting carbon dioxide with ammonia under urea synthesis conditions to produce a urea synthesis effluent containing urea, unreacted ammonium carbamate, an excess of ammonia and water;
- treating said urea synthesis effluent in a plurality of pressure decomposition stages wherein the pressure is stepwise reduced thereby to decompose said ammonium carbamate into ammonia and carbon dioxide in each stage;
- separating a mixed gas of ammonia and carbon dioxide from said 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 said stage to absorb said mixed gas in said absorbent in stages;
- recycling the final resulting absorbate containing ammonium carbamate to the urea synthesis 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 vapor generated upon said concentration to obtain a condensate containing small amounts of ammonia and carbon dioxide;
- and stripping said condensate to separate and recover said ammonia and carbon dioxide therefrom, the improvement which comprises absorbing the off-gas from a high pressure decomposition stage in an absorbate in a high pressure absorption zone, said 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 upon said absorption by an indirect heat exchange in the form of steam having a pressure of 2-4 kg/cm.sup.2 (gauge), contacting said steam with said condensate containing small amounts of ammonia and carbon dioxide in a stripping zone to strip off substantially all of the ammonia and carbon dioxide from said 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 said mixed gas separated from said urea synthesis effluent in said low pressure decomposition stage together with the ammonia and carbon dioxide contained in said steam.
Dependencies are inferred from the claim text.