Hydrogen and Nitrogen Recovery from Ammonia Purge Gas

Abstract

An ammonia plant is disclosed, where ammonia purge gas ( 20 ), is sent to a cryogenic recovery unit, said recovery unit comprising means of cooling ( 102, 202, 302, 402, 502 ) and a high-pressure phase separator ( 103, 203, 303, 403, 503 ) operating at loop pressure; inside said unit the purge gas ( 20 ) is cooled to a cryogenic temperature, and a partial liquefaction of methane and argon is achieved; the high-pressure phase separator separates the cooled stream into a gaseous stream and a bottom liquid; the gaseous stream is reheated in a passage of a heat exchanger; the unit is then capable to export a gaseous stream ( 123, 223, 323, 423, 523 ) containing nitrogen and hydrogen at loop pressure, that can be reintroduced at the suction side of the circulator ( 4 ) of the loop.

1 . An ammonia plant for synthesis of ammonia from a make-up gas containing hydrogen and nitrogen, the plant comprising:
 a synthesis loop operating at a loop pressure, said loop comprising at least a circulator ( 4 ) and a synthesis converter;   a purge line, extracting a purge gas stream at loop pressure from said synthesis loop; said purge gas stream containing hydrogen and nitrogen, plus methane and/or argon;   a recovery unit disposed to recover a part of hydrogen and nitrogen content of a purge gas feed directed to said recovery unit;   wherein said recovery unit comprises at least one heat exchanger to cool down said purge gas feed, and a high-pressure separation section comprising at least one high-pressure phase separator operating at said loop pressure, said heat exchanger(s) and separation section being arranged in such a way that:   a) a purge gas feed entering said recovery unit is cooled in at least a passage of a heat exchanger of the recovery unit, down to a cryogenic temperature suitable to obtain at least a partial liquefaction of methane and argon in the one or more downstream phase separator(s);   b) a liquefied part of the purge gas and a recovery gaseous stream containing hydrogen and nitrogen at loop pressure are separated in said high-pressure separation section, and   c) said recovery gaseous stream at loop pressure is reheated in at least a passage of a heat exchanger of the recovery unit, the so obtained reheated gaseous stream being exported from said recovery unit at said loop pressure, and returned to the synthesis loop at said loop pressure,   wherein said recovery unit comprises a plurality of phase separators arranged in a cascade, said plurality comprising at least one first high pressure separator forming said high pressure separation section and at least a second separator operating at a pressure substantially lower than said loop pressure, the second and any subsequent separator receiving a liquid outlet from a previous separator of said cascade; the recovery unit being able to export said recovery gaseous stream containing hydrogen and nitrogen substantially at loop pressure, and at least another gaseous stream containing hydrogen and nitrogen at a lower pressure.   
     
     
         2 . The ammonia plant according to  claim 1 , said high-pressure separation section comprising a plurality of high pressure phase separators operating at loop pressure and at different temperatures, the second and any subsequent separator of said high pressure separation section receiving a gaseous phase separated in the previous separator and cooled in a heat exchanger of the recovery section; and at least a further separator operating at a pressure substantially lower than said loop pressure, the recovery unit thus being able to export said recovery gaseous stream containing hydrogen and nitrogen substantially at loop pressure, and at least another gaseous stream containing hydrogen and nitrogen at a lower pressure. 
     
     
         3 . The ammonia plant according to  claim 1 , said recovery unit comprising at least one expansion device, arranged to expand a liquid stream obtained in said phase separators to provide at least part of net refrigeration of said recovery unit, said expansion device being a let-down turbine or an expander or a common valve. 
     
     
         4 . The ammonia plant according to  claim 1 , the recovery unit comprising a distillation column, arranged to receive expanded bottom stream from the lowest pressure separator of a cascade of phase separators in the recovery unit, said distillation column providing an argon-rich overhead vapour, said argon-rich vapour being reheated in a passage of a heat exchanger of said recovery unit, and exported from said recovery unit. 
     
     
         5 . The ammonia plant according to  claim 1 , a high-pressure phase separator of the recovery unit being a refluxed mass transfer device or a runback condenser. 
     
     
         6 . The ammonia plant according to  claim 1 , said heat exchanger or heat exchangers being one or more of the following types: plate-fin heat exchanger, or a coil-wound exchanger, or a printed-circuit heat exchanger, or shell and tube exchanger. 
     
     
         7 . The ammonia plant according to  claim 1 , said recovery unit being connected across said circulator, the purge gas being taken either at delivery side of said circulator, or downstream said synthesis converter, and said recovery stream containing hydrogen and nitrogen exported at loop pressure from the recovery unit being reintroduced into the loop at loop pressure. 
     
     
         8 . A process for recovering nitrogen and hydrogen contained in ammonia synthesis loop purge gas, taken from an ammonia synthesis loop and containing hydrogen and nitrogen, plus some methane and/or argon, wherein:
 ammonia purge gas is extracted from an ammonia synthesis loop at a loop pressure;   a purge gas feed is directed to a recovery unit for recovering at least part of its nitrogen and hydrogen content,   
       the process comprising the following steps:
 cooling said purge gas feed to a cryogenic temperature, and partial liquefaction of methane and argon is achieved; 
 separating at least one gaseous stream containing hydrogen and nitrogen is separated from the cooled purge gas feed in a plurality of phase separators arranged in a cascade of said recovery unit, said plurality comprising at least one first high pressure separator forming a high pressure separation section and at least a second separator operating at a pressure substantially lower than said loop pressure, the second and any subsequent separator receiving a liquid outlet from a previous separator of said cascade; the recovery unit being able to export said recovery gaseous stream containing hydrogen and nitrogen substantially at loop pressure, and at least another gaseous stream containing hydrogen and nitrogen at a lower pressure; and 
 reheating said gaseous stream containing hydrogen and nitrogen substantially at loop pressure obtained at step b) is reheated and the so obtained reheated gaseous stream is exported from said recovery unit at loop pressure, and returned to the synthesis loop at loop pressure. 
 
     
     
         9 . The process according to  claim 8 , wherein a liquid stream obtained by separation at said step b) is expanded in a plurality of expansion steps, separating one or more further gaseous streams containing hydrogen and nitrogen at a pressure lower than said loop pressure. 
     
     
         10 . The process according to  claim 8 , wherein said step b) is carried out at a plurality of temperature levels, a gaseous stream being separated from said purge gas feed at a first temperature, said gaseous stream being then cooled and further separated into a gaseous fraction and a liquid fraction at least at another lower temperature. 
     
     
         11 . The process according to  claim 8 , wherein the flow rate of said purge is such that the loop circulating gas has a concentration of inerts, such as CH4 and Ar, not greater than 10 mol %, with substantially no loss of reactant H2, and wherein less than 50% of the nitrogen content of said purge gas is removed and more than 50% of the argon and methane content of said purge gas is removed. 
     
     
         12 . The process according to  claim 8 , wherein one or more liquid streams obtained in the recovery unit is expanded in at least an expansion step, to provide at least part of net refrigeration to said recovery unit. 
     
     
         13 . The process according to  claim 8 , wherein an argon-rich gaseous stream is also separated and exported from said recovery unit. 
     
     
         14 . The process according to  claim 8 , wherein the loop pressure is in the range 50 to 500 bar, and said cryogenic temperature is in the range −230° C. to −130° C. 
     
     
         15 . The process according to  claim 12 , wherein the one or more liquid streams is expanded by work-expanding the stream and recover mechanical energy/work by depressurizing through a turbine or expander, further to a valve. 
     
     
         16 . The process according to  claim 8 , wherein the loop pressure is in the range 80 to 300 bar, and said cryogenic temperature is in the range −230° C. to −130° C.