Method for the preparation of ammonia synthesis gas

Abstract

Method for the preparation of ammonia synthesis gas based on a combination of autothermal reforming and electrolysis of water.

Claims

1. Method for the preparation of ammonia synthesis gas comprising the steps of: (a) providing a hydrocarbon feed stock; (b) preparing a separate hydrogen stream and a separate oxygen stream by electrolysis of water; (c) providing process air for use in autothermal reforming by enriching atmospheric air with the oxygen stream from step (b); (d) autothermal reforming at least a part of the hydrocarbon feed stock with the oxygen enriched process air to a process gas stream comprising hydrogen, nitrogen, carbon monoxide and carbon dioxide; (e) treating the process gas stream obtained in the autothermal reforming step (d) in one or more water gas shift reactions; (f) removing the carbon dioxide from the water gas shift treated process gas stream; (g) purifying the process gas stream from step (f); and (h) obtaining the ammonia synthesis gas, wherein at least a part of the hydrogen stream obtained from the electrolysis of water in step (b) is added to the process gas stream after step (g) in an amount to provide a molar ratio of the hydrogen to the nitrogen of 2.7-3.3 in the ammonia synthesis gas, and wherein the electrolysis of water in step (b) is performed at an increased pressure relative to the pressure of the process gas stream.

2. The method of claim 1, comprising the further step of steam reforming a part of the hydrocarbon feed stock in indirect heat transfer relationship with the process stream leaving the autothermal reforming step (d) to obtain a heat exchange steam reformed gas stream, and mixing the heat exchange steam reformed process gas stream with autothermal reformed process gas stream upstream step (e).

3. The method of claim 1, comprising the further step of heat exchange steam reforming the hydrocarbon feed stock in indirect heat transfer relationship with the process stream leaving the autothermal reforming step (d) and passing the heat exchanged steam reformed hydrocarbon feed stock to step (d).

4. The method of claim 1, wherein the process air contains between 22 and 45 mole% oxygen.

5. The method according to claim 1, wherein the purifying of the process gas stream in step (f) is performed by methanation.

6. The method according to claim 1, wherein the purifying of the process gas stream in step (f) is performed cryogenic.

7. The method according to claim 1, wherein the electrolysis of water is powered by renewable energy.

Description

(1) A specific embodiment of the invention is disclosed in more detail by reference to the drawings, in which

(2) FIG. 1 is a simplified flow diagram of the method according to the invention for the preparation of ammonia synthesis gas utilizing water electrolysis combined with ATR and heat exchange steam reforming.

(3) In the simplified block diagram as shown in FIG. 1, the method is performed by autothermal reforming a stream 2 of a hydrocarbon feedstock, e.g. natural gas (NG) admixed with a stream of steam 4 in autothermal reformer (ATR) 6 and heat exchange reformer (HTER) 8. A part of stream 2 is bypassed ATR 6 in stream 3b and introduced into HTER 8.

(4) ATR 6 is operated with oxygen enriched air. Oxygen for the enrichment of air is produced by water electrolysis in water electrolyzer (WE) 10 and a stream of oxygen 7 produced in WE 10, is admixed into air 5 in amount to produce process air 9 with an oxygen content of 37 mole %. In ATR 6 a part of natural gas 4 is introduced in stream 3a and autothermal reformed by the known autothermal reforming process. Hot autothermal reformed effluent 11 being withdrawn from ATR 6 at a temperature of about 1000° C. is passed through HTER 8 in indirect heat exchange with the natural gas provided to HTER 8 in line 3b and provides heat for the steam reforming reactions in HTER 8. The total amount of steam reformed and autothermal reformed process gas stream 13 containing hydrogen, nitrogen, carbon monoxide and carbon dioxide is passed to water gas shift (WGS) unit 12 and most of the carbon oxides in stream 13 is converted by the known WGS reactions to carbon dioxide.

(5) Carbon dioxide formed by the WGS reactions and by the steam reforming reactions is removed from the water gas shift treated process gas stream 15 in gas-liquid contactor 14 by absorption in N-methyldiethanolamine (MDEA), as known in the art.

(6) The final purification of the process gas stream 17 withdrawn from gas-liquid contactor 14 is performed by methanation of residual amounts of carbon monoxide in methanator 16 by reaction of carbon monoxide to methane.

(7) The nitrogen/hydrogen molar ratio in the purified process gas stream 19 from methanator 16 is adjusted to about 3 by introducing the appropriate amount of hydrogen formed in WE 10 through stream 21 into stream 19.

(8) The thus prepared ammonia synthesis gas is the passed through line 23 to a make up gas compressor (not shown) and fed into the ammonia synthesis loop (not shown).