Process for the preparation of a urea-comprising aqueous stream

摘要

The invention relates to process for the preparation of a urea-comprising aqueous stream, that is suitable for use in a unit for the reduction of NOx in combustion engine exhaust gases, wherein the urea-comprising aqueous stream is separated directly from or after a recovery section in a urea production process and is thereafter diluted with water until the urea-comprising stream comprises 30-35 wt % urea.

主权项

1. A process for the preparation of a urea-comprising aqueous stream that is suitable for use in a unit for the reduction of NOx in combustion engine exhaust gases according to DIN V70070, wherein the process comprises:
(a) forming an aqueous urea-comprising process stream at a synthesis pressure in a urea synthesis section of a urea production plant; (b) passing the aqueous urea-comprising process stream to at least one recovery section of the urea production plant and subjecting the aqueous urea-comprising process stream in the at least one recovery section to a recovery pressure which is reduced as compared to the synthesis pressure in the urea synthesis section so as to cause dissociation of ammonium carbamate in the aqueous urea-comprising process stream to free ammonia and carbon dioxide; (c) removing the free ammonia and carbon dioxide as gas from the aqueous urea-comprising process stream to form a primary urea product stream of the urea production plant; (d) separating a first urea-comprising aqueous stream from the aqueous urea-comprising process stream directly after the at least one recovery section in the urea production plant at a point upstream of any evaporation or crystallization section in the urea production plant, and thereafter (e) diluting the separated first urea-comprising aqueous stream with water to obtain a second urea-comprising stream which comprises an amount between 31.8-33.2 wt. % of urea and which further has a content of alkalinity as NH3 of max. 0.2% w/w, of biuret of max. 0.3% w/w, of aldehydes of max. 5 mg/kg, of insolubles of max 20 mg/kg, of phosphate of max. 0.5 mg/kg, of copper of max. 0.2 mg/kg, of zinc of max. 0.2 mg/kg, of chromium of max. 0.2 mg/kg, of nickel of max. 0.2 mg/kg, of aluminum of max. 0.5 mg/kg, of magnesium of max. 0.5 mg/kg, of sodium of max 0.5 mg/kg and of potassium of max. 0.5 mg/kg according to the quality requirements for reduction of NOx in combustion engine exhaust gases according to DIN V70070; and (f) removing the second urea-comprising stream from the urea production plant as an aqueous secondary urea product stream suitable for use in a unit for the reduction of NOx in combustion engine exhaust gases.