Processes and plants for the production of purified urea solution are described. In a described urea production process, urea is produced in a synthesis section without a high pressure stripper and the urea solution is subjected to purification after the recovery section, to give purified urea solut
Processes and plants for the production of purified urea solution are described. In a described urea production process, urea is produced in a synthesis section without a high pressure stripper and the urea solution is subjected to purification after the recovery section, to give purified urea solution and off-gas. The purification comprises e.g. steam stripping.
대표청구항▼
1. A process for the production of a purified urea solution that is preferably suitable for use in NOx abatement, in particular for use as diesel exhaust fluid (DEF) or dilution to DEF, the process comprising: A) reacting CO2 and NH3 under urea synthesis conditions in a urea synthesis reactor operat
1. A process for the production of a purified urea solution that is preferably suitable for use in NOx abatement, in particular for use as diesel exhaust fluid (DEF) or dilution to DEF, the process comprising: A) reacting CO2 and NH3 under urea synthesis conditions in a urea synthesis reactor operating at high pressure, to give a urea synthesis stream containing urea, NH3, CO2 and an amount of carbamate,B) expanding the urea synthesis stream in a recovery section thereby reducing the pressure, wherein the urea synthesis stream that is expanded comprises at least 90 wt. % of said amount of carbamate, and heating at least part of the expanded urea synthesis stream in one or more dissociation units at medium and/or low pressure, to give an aqueous urea stream and a recovery section vapour containing NH3 and CO2, C) subjecting in a purification section at least part of the aqueous urea stream to purification, wherein the purification is stripping to remove ammonia and/or wherein the purification yields a urea solution with an alkalinity as NH3 of less than 0.2 wt. % when at 32.5 wt. % urea, giving a purified urea solution and a purification section off-gas containing water and ammonia,D) optionally diluting at least part of the purified urea solution and/or the aqueous urea stream to be purified with water to obtain a target concentration of urea,E) wherein the purification section off-gas is condensed to give purification section condensate and said purification section condensate is recycled to said urea synthesis reactor. 2. A process according to claim 1, wherein the purification section condensate is supplied by liquid flow to the urea synthesis reactor, and wherein the purification is low pressure steam stripping and involves contacting the aqueous urea solution in counter current flow with steam at a pressure of less than 3 bar. 3. A process according to claim 1, comprising: a) condensing the recovery section vapour to give a liquid carbamate recycle stream in a recovery section condenser, wherein said condenser operates at lower pressure than said urea synthesis reactor,b) supplying the carbamate recycle stream to the urea synthesis reactor, preferably using a pump,c) and supplying said purification section condensate to said recovery section condenser. 4. A process according to claim 3, further comprising a step of supplying non-condensed gases from the recovery section condenser, optionally through an additional condenser, to a scrubbing step with scrubbing liquid in a scrubber giving a scrubbed gas and a spent scrubbing liquid, wherein the process further comprises supplying a second off-gas from the purification section condenser to said scrubber, and preferably supplying part of said spent scrubbing liquid to said recovery section condenser, optionally through said additional condenser. 5. A process according to claim 1, wherein step B comprises B1) expanding the urea synthesis stream to medium pressure, optionally gas/liquid separation, and heating at least part of the urea synthesis stream at medium pressure to give a MP recovery section vapour and a MP aqueous urea solution, andB2) expanding the MP aqueous urea solution to low pressure, and heating the expanded aqueous urea solution at low pressure to give an LP recovery section vapour and an LP aqueous urea solution. 6. A process according to claim 1, wherein at least 85% of the fresh CO2 feed is added directly into the reactor. 7. A process according to claim 1, wherein said step C comprises expanding said aqueous urea stream, preferably the second aqueous urea solution, to the operating pressure of the purification section, wherein the purification section operating pressure is preferably in the range of 0.010 to 1.5 bar absolute, and wherein step C comprises steam stripping the expanded aqueous urea stream, wherein the steam stripping involves direct injection of steam in the expanded aqueous urea stream. 8. A process according to claim 7, wherein the steam stripping is carried out at 0.010 to 1.5 bar absolute in a stripping column with a cyclone separator at the top of the column, a liquid distribution tray and a packed bed, wherein gas is separated from the expanded aqueous urea solution stream in the cyclone separator, the gas exits through an outlet at the top, liquid flows down from the cyclone separator on the liquid distribution tray, and the liquid distribution tray distributes the liquid over the packed bed. 9. A process according to claim 1, wherein no passivation air is used and/or wherein the CO2 feed contains less than 0.05 wt. % oxygen. 10. A plant for the production of a purified urea solution which urea solution is preferably suitable for use in NOx abatement systems, in particular for use as diesel exhaust fluid (DEF) or dilution to DEF, the plant comprising: A) a urea synthesis section comprising a high pressure urea synthesis reactor having an inlet for CO2 and an inlet for NH3, wherein CO2 and NH3 are reacted under urea synthesis conditions to form a urea synthesis stream comprising urea, water, and ammonium carbamate,B) a recovery section wherein the urea synthesis stream is heated at reduced pressure, giving an aqueous urea stream and a recovery section vapour comprising CO2 and NH3, and wherein said recovery section vapour is condensed to give carbamate and ammonia, preferably with separate condensers for ammonia and for carbamate,C) a purification section wherein the aqueous urea stream is stripped in a purification section treatment unit to remove ammonia, giving a purified urea stream and a purification section off-gas,D) optionally a urea dilution section, wherein the purified urea stream and/or the aqueous urea stream is diluted with water,wherein the plant comprises a recycle conduit for carbamate from the recovery section to the urea synthesis section and a separate second recycle conduit for ammonia from the recovery section to the urea synthesis section,wherein the purification section further comprises a purification section condenser for condensing the purification section off-gas to purification section condensate, and wherein the plant comprises a liquid flow connection for purification section condensate from said purification section condenser to said urea synthesis reactor. 11. A plant according to claim 10, wherein the purification section treatment unit comprises a low pressure steam stripper receiving the aqueous urea stream, wherein the steam stripper has an inlet for steam, and wherein the purification section comprises an ejector for withdrawing gas from the purification section. 12. A plant according to claim 11, wherein the steam stripper comprises a cyclone separator at the top, a liquid distribution tray, and a packed bed, configured such that in operation a two phase fluid of expanded aqueous urea stream enters the cyclone separator, gas leaves at the top, liquid flows down from the cyclone separator on the liquid distribution tray, and the liquid distribution tray distributes the liquid over the packed bed. 13. A plant according to claim 10, wherein the recovery section further comprises a recovery section condenser for the recovery section vapour, wherein the plant comprises a liquid flow line from the purification section condenser to said recovery section condenser, and a liquid flow line from said recovery section condenser to the urea synthesis reactor. 14. A plant according to claim 10, wherein the urea synthesis reactor comprises a ferritic-austenitic duplex stainless steel. 15. A plant according to claim 10, wherein the plant does not include a waste water treatment section.
※ AI-Helper는 부적절한 답변을 할 수 있습니다.