Process for producing ammonia synthesis gas

Abstract

A process for producing ammonia synthesis gas from a hydrocarbon-containing feedstock, with steps of primary reforming, secondary reforming with an oxidant stream, and further treatment of the synthesis gas including shift, removal of carbon dioxide and methanation, wherein the synthesis gas delivered by secondary reforming is subject to a medium-temperature shift (MTS) at a temperature between 200 and 350.degree. C., and primary reforming is operated with a steam-to-carbon ratio lower than 2. A corresponding method for revamping an ammonia plant is disclosed, where an existing HTS reactor is modified to operate at medium temperature, or replaced with a new MTS reactor, and the steam-to-carbon ratio in the primary reformer is lowered to a value in the range 1-5-2, thus reducing inert steam in the flow rate trough the equipments of the front-end.

What is claimed is: 
     
         1 . A method for revamping of an ammonia plant comprising a front-end for production of an ammonia synthesis gas, and a synthesis loop for reaction of said synthesis gas into ammonia, said front-end comprising at least a primary reformer, a secondary reformer, a high-temperature shift HTS reactor, the primary reformer being connected with a hydrocarbon feed and a steam feed,
 said method comprising at least the steps of:   replacing said HTS reactor with a new medium-temperature shift reactor, or modifying said HTS reactor for operation at medium temperature, said medium temperature being 200 to 350° C.;   modifying said hydrocarbon feed and steam feed to the primary reformer, so to obtain operation of said primary reformer with a steam-to-carbon ratio lower than 2; and   adding a pre-reforming section upstream of the primary steam reformer.   
     
     
         2 . The method according to  claim 1 , comprising the step of modifying the hydrocarbon feed and steam feed to the primary reformer, so to obtain operation of said primary reformer with a steam-to-carbon ratio in the range 1.5 to 2. 
     
     
         3 . A method according to  claim 1 , comprising the steps of either:
 keeping the existing vessel of the HTS reactor, replacing the high temperature catalyst with a suitable medium temperature catalyst, such as Cu—Zn catalyst, and providing the vessel with an internal heat exchanger, immersed in the catalyst,   or   installing a new medium-temperature shift reactor replacing the existing HTS reactor, the new reactor having a suitable catalyst for operation at medium temperature, and an internal heat exchanger immersed in said catalyst.   
     
     
         4 . The method according to  claim 1 , further comprising any of the following steps:
 a) the existing air feed to the secondary reformer is modified to provide a larger air input, or b) installing a suitable equipment for enriching the air feed directed to the secondary reformer, or c) installing a suitable source of substantially pure oxygen and feeding said pure oxygen to the secondary reformer.   
     
     
         5 . A method according to  claim 1 , further comprising any of the following steps: installation of equipments for cryogenic separation of excess methane and/or nitrogen in the syngas; installation of equipments suitable for separation of excess nitrogen in the synthesis gas by an adsorption process such as PSA; increasing the purge loop form the synthesis loop, to provide removal residual methane. 
     
     
         6 . The method according to  claim 2 , wherein the operation of said primary reformer has a steam-to-carbon ratio in the range 1.5 to 1.7.