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 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 1-52, thus reducing inert steam in the flow rate trough the equipments of the front-end.

Claims

1. A process for producing ammonia from an ammonia synthesis gas obtained from a hydrocarbon-containing feedstock, the process comprising the steps of: pre-reforming said feedstock; primary reforming all of said feedstock with steam to partially reform said feedstock, wherein said primary reforming is operated with a steam-to-carbon ratio lower than 2; secondary reforming a partially reformed gas stream delivered from said primary reforming step with an oxidant stream, wherein the ammonia synthesis gas produced by said secondary reforming is directly subject to a medium-temperature shift in a medium-temperature shift reactor over a copper-based catalyst at a temperature between 200 and 350 C. to convert CO into CO.sub.2, obtaining a first shifted gas; purification of the first shifted gas involving at least a further shift conversion of CO to CO.sub.2, obtaining a second CO.sub.2 containing shifted gas and subsequent removal of carbon dioxide from the second shifted gas, obtaining a purified ammonia synthesis gas, and reacting the purified ammonia synthesis gas to form ammonia, wherein the ammonia synthesis gas comprises hydrogen (H.sub.2) and nitrogen (N.sub.2) in a suitable ratio of about 3:1, and wherein the overall feedstock required for producing the ammonia synthesis gas according to the process is fed to the pre-reforming step, and said pre-reforming step of said feedstock is carried out before the step of primary reforming.

2. The process according to claim 1, wherein said steam-to-carbon ratio is 1.5 to 2.

3. The process according to claim 1, wherein said oxidant stream is any of air, O.sub.2-enriched air or substantially pure oxygen.

4. The process according to claim 1, wherein said medium-temperature shift is carried out in a substantially isothermal condition, by removing heat with a cooling medium.

5. The process according to claim 1, further comprising the step of removing unreacted methane from the synthesis gas by cryogenic separation of methane from the synthesis gas, or by means of a treatment step of adsorption.

6. The process according to claim 1, wherein said steam-to-carbon ratio is 1.5 to 1.7.

7. The process according to claim 1, wherein the unreacted methane is removed from the synthesis gas by pressure swing adsorption.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a block diagram of the front-end of an ammonia synthesis plant, according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(2) Referring to FIG. 1, a hydrocarbon feed 10, preferably a desulphurized methane flow, and steam 11 are pre-heated in a heat exchanger 26 and reacted in a primary reformer 12, and optionally pre-reformed in a pre-reformer 12a.

(3) The feed of natural gas 10 and steam 11 is such that the primary reformer 12 is operated with steam-to-carbon ratio lower than 2, as stated in the above disclosure of the invention. For example, feed 11 provides 1.5 moles of steam for each mole of methane in the hydrocarbon feed 10.

(4) A partially reformed gas stream 13, delivered by primary reformer 12 is further treated in a secondary reformer 14. Oxidant is supplied with a stream 15 that may provide excess air, enriched air or substantially pure oxygen, preferably with a purity >95%, according to various embodiments of the invention.

(5) The gas stream 16 from the secondary reformer 14, usually at a temperature around 1000 C., is then cooled in a heat exchanger 17 to 220-320 C. (stream 18), and sent to a medium-temperature shift (MTS) reactor 19.

(6) The MTS reactor 19 is an isothermal catalytic reactor, comprising a copper-based catalytic bed and a plate heat exchanger immersed in the catalytic bed. The inlet and outlet of a cooling medium are shown as 30, 31.

(7) Downstream the MTS reactor 19, the syngas 20 can be further treated in an optional low-temperature shift (LTS) reactor 21, to maximize the conversion of the carbon monoxide into CO.sub.2.

(8) The syngas is then further cooled in a heat exchanger 22 and the cooled syngas stream 23 is sent to treatment steps generally denoted by block 24 and including CO.sub.2 removal, methanation and optionally cryogenic purification or removal of excess methane by a PSA process. Said cryogenic purification or PSA process may serve to remove unreacted methane in the stream 23, caused by low SC ratio in the primary reformer 12. Nitrogen (stream 32) may be added when necessary to reach the H/N ratio suitable for synthesis of ammonia, in particular when the oxidizer feed 15 is highly enriched air or pure oxygen, i.e. nitrogen in the stream 23 is low. Then the syngas is compressed and sent to an ammonia synthesis loop.

(9) Preferably, all the natural gas feed is supplied to the primary reformer; in another embodiment of the invention (not shown), a portion of the natural gas feedstock may be directed to the secondary reformer.

EXAMPLES

(10) A conventional ammonia plant rated at 1700 MTD (metric tons per day) of ammonia is revamped according to the following embodiments of the invention: A) reduction of SC ratio in the primary reformer to about 1.5 and installation of a pre-refomer such as 12a in FIG. 1; B) same as A with further step of providing excess air to the secondary reformer; C) same as A with further step of providing enriched air to the secondary reformer; D) same as A with further step of providing pure (>95%) oxygen to the secondary reformer.

(11) The production rate can be increased to 2150 MTD (+26%) in case A; 2200 MTD (+29%) in case B; 2500 MTD (+47%) in case C and 2700 MTD (+59%) in case D. The specific energy consumption (Gcal per MTD), including energy consumed for air separation and production of oxygen for air enrichment (case C) or pure oxygen feed (case D), is reduced by about 0.1 Gcal/MTD in case A; about 0.2 Gcal/MTD in case B and about 0.5 Gcal/MTD in cases C and D.