Hydrogen and nitrogen recovery from ammonia purge gas

Patent metadata

Field	Value
Publication number	US-10040691-B2
Application number	US-201514937285-A
Country	US
Kind code	B2
Filing date	Nov 10, 2015
Priority date	Apr 7, 2010
Publication date	Aug 7, 2018
Grant date	Aug 7, 2018

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.

First claim

The invention claimed is:

1. 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 treated to remove ammonia and directed as an ammonia-free purge gas at loop pressure to a recovery unit for recovering at least part of its nitrogen and hydrogen content,
the process comprising the following steps:
cooling said ammonia-free purge gas feed at loop pressure to a cryogenic temperature, and partial liquefaction of methane and argon is achieved;
separating at least one gaseous stream containing hydrogen and nitrogen 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 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 a recovery gaseous stream containing hydrogen and nitrogen 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 at loop pressure obtained at said separating step, 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.

2. The process according to claim 1 , wherein a liquid stream obtained by separation at said separating step 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.

3. The process according to claim 1 , wherein said separating step 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.

4. The process according to claim 1 , wherein the flow rate of said purge is such that the gas circulating in the ammonia synthesis loop has a concentration of argon and methane, not greater than 10 mol %, 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.

5. The process according to claim 1 , 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.

6. The process according to claim 1 , wherein an argon-rich gaseous stream is also separated and exported from said recovery unit.

7. The process according to claim 1 , 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.

8. The process according to claim 5 , wherein the one or more liquid streams is expanded by work-expanding the stream and mechanical energy/work is recovered by depressurizing through a turbine, expander, or valve.

9. The process according to claim 1 , 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.

Assignees

• Casale Sa

Inventors

• Ostuni Raffaele
• Filippi Ermanno
• Skinner Geoffrey Frederick

Classifications

• F25J3/0285

  of argon · CPC title

• F25J3/0219

  Refinery gas, cracking gas, coke oven gas, gaseous mixtures containing aliphatic unsaturated CnHm or gaseous mixtures of undefined nature · CPC title

• F25J2270/12

  External refrigeration with liquid vaporising loop · CPC title

• F25J3/062

  Refinery gas, cracking gas, coke oven gas, gaseous mixtures containing aliphatic unsaturated CnHm or gaseous mixtures of undefined nature · CPC title

• F25J2245/02

  Recycle of a stream in general, e.g. a by-pass stream · CPC title