Production of Ammonia Make-Up Syngas with Cryogenic Purification

Abstract

A process and a related equipment for making ammonia make-up synthesis gas are disclosed, where: a hydrocarbon feedstock is reformed obtaining a raw ammonia make-up syngas stream; said raw syngas is purified in a cryogenic purification section refrigerated by a nitrogen-rich stream produced in an air separation unit; the nitrogen-rich stream at output of said cryogenic section is further used for adjusting the hydrogen/nitrogen ratio of the purified make-up syngas; an oxygen-rich stream is also produced in said air separation unit and is fed to the reforming section.

1 . A process for making ammonia make-up synthesis gas, comprising the steps of:
 reforming a hydrocarbon feedstock, followed by steps of shift, CO 2  removal and methanation, to obtain a raw ammonia make-up syngas stream comprising hydrogen and nitrogen;   treating said raw syngas in a cryogenic purification section obtaining a purified syngas stream;   feeding a liquid nitrogen-rich stream at a cryogenic temperature to said cryogenic purification section; and   providing an indirect heat exchange between the syngas and said liquid nitrogen-rich stream in the cryogenic section, said liquid nitrogen-rich stream being at least partly evaporated to provide refrigeration of said cryogenic section.   
     
     
         2 . A process according to  claim 1 , where said liquid nitrogen-rich stream, after at least a partial evaporation through the cryogenic section, is recovered at an output of said cryogenic section, and mixed with the purified syngas to provide at least a portion of the nitrogen required to adjust the hydrogen/nitrogen ratio of the ammonia make-up syngas. 
     
     
         3 . A process according to  claim 1 , further comprising the step of treating an air stream in an air separation unit, obtaining said liquid nitrogen-rich stream and an oxygen-rich stream. 
     
     
         4 . A process according to  claim 3 , where said air separation unit provides said liquid nitrogen-rich stream, and a second nitrogen-rich stream at ambient temperature and in a gaseous state, and where the amount of nitrogen required to adjust the HN ratio of the ammonia make-up syngas is provided partly by the evaporated liquid nitrogen-rich stream recovered at the output of the cryogenic section and partly by said nitrogen-rich stream at ambient temperature. 
     
     
         5 . A process according to  claim 3 , where said oxygen-rich stream is used as further oxidant in the reforming process, by injection of said oxygen-rich stream into a secondary reformer of the reforming section. 
     
     
         6 . A process according to  claim 1 , wherein:
 said raw syngas is cooled down to a cryogenic temperature in a main heat exchanger of the cryogenic section, obtaining a cooled raw syngas;   said cooled raw syngas is fed to a contacting device where a liquid fraction containing impurities is obtained by cryogenic liquefaction and separated from the syngas;   a purified syngas is recovered from said contacting device and is further cooled and purified in a condenser which is refrigerated by at least partial evaporation of said liquid nitrogen-rich stream;   a further purified syngas is taken at the output of said condenser and re-heated in said main heat exchanger, by heat exchange with the incoming raw syngas and with evaporated nitrogen stream taken from said condenser.   
     
     
         7 . A process according to  claim 6 , wherein said liquid fraction containing impurities is further used as a refrigerating medium for the main heat exchanger of the cryogenic section. 
     
     
         8 . A process according to  claim 1 , wherein said liquid nitrogen-rich stream and/or a second nitrogen-rich stream at ambient temperature are substantially pure nitrogen. 
     
     
         9 . An equipment for producing ammonia make-up synthesis gas comprising:
 a front-end section comprising a reforming section adapted to reform a hydrocarbon feedstock and to produce a raw ammonia syngas stream;   a cryogenic purification section treating the raw syngas produced in the front-end;   means feeding a liquid nitrogen-rich stream at a cryogenic temperature to said cryogenic purification section, for use as a heat exchange medium to refrigerate said cryogenic purification section; and   at least one indirect heat exchanger between the syngas and said liquid nitrogen-rich stream in the cryogenic section, said liquid nitrogen-rich stream being at least partially evaporated in said heat exchanger(s) to provide refrigeration of said cryogenic section.   
     
     
         10 . The equipment according to  claim 9 , further comprising means for recovering the evaporated nitrogen-rich stream at an output of the cryogenic purification section, and for mixing said nitrogen-rich stream with purified syngas, to provide at least a portion of nitrogen required for adjusting the hydrogen/nitrogen ratio of the ammonia make-up syngas. 
     
     
         11 . The equipment according to  claim 9 , comprising an air separation unit delivering said liquid nitrogen-rich stream and a second stream of nitrogen at ambient temperature for HN ratio adjustment, and additionally delivering an oxygen-rich stream which is fed as oxidizer to the reforming section. 
     
     
         12 . An equipment according to  claim 11 , the front-end comprising a primary reformer, a secondary reformer, and equipments for shift, CO 2  removal and methanation, said oxygen-rich stream being fed to the secondary reformer of the reforming section. 
     
     
         13 . The equipment according to  claim 9 , the cryogenic section comprising:
 a contacting device such as a cryogenic condenser column;   a condenser receiving a partially-purified syngas obtained in the contacting device, said condenser being refrigerated by the liquid nitrogen-rich stream;   a main heat exchanger where the incoming raw syngas is cooled by heat exchange with one or more of the following: the nitrogen stream evaporated in said condenser, the purified syngas, a bottom effluent of said contacting device.   
     
     
         14 . A method for revamping the front-end of an ammonia plant, said front-end section comprising a reforming section with at least a primary reformer and a secondary reformer for converting a hydrocarbon feedstock into ammonia raw make-up syngas, the method comprising at least the steps of:
 installing an air separation unit in parallel to said front-end;   providing a cryogenic section for treatment of the raw syngas, if not present in the original plant;   providing means feeding a liquid nitrogen-rich stream produced in said air separation unit to said cryogenic section, for use as refrigerating medium;   providing a line feeding oxygen-rich stream produced in said air separation unit to the secondary reformer, in order to increase the capability of said reforming section.