Method for producing synthesis gas

Abstract

A method for preparing synthesis gas by combining water electrolysis, tubular steam reforming and autothermal reforming of hydrocarbon feedstock in parallel.

Description

[0001] The present application relates to the production of synthesis gas. More particularly, the present invention combines in parallel electrolysis of water, tubular steam reforming and autothermal reforming, and optionally additional heat exchange reforming of a hydrocarbon feedstock in the production of synthesis gas comprising hydrogen and carbon oxides.

[0002] The production of synthesis gas, for example for methanol synthesis, is usually carried out by steam reforming hydrocarbons.

[0003] The main reactions of steam reforming are (given for methane):

[0004] CH4 + H2O ⇆ 3H2 + CO

[0005] Similar reactions occur for other hydrocarbons. Steam reforming is usually accompanied by the water-gas shift reaction:

[0006] CO + H2O ⇆ CO2 + H2

[0007] Steam reforming is usually carried out by combining a tubular reformer (also known as a steam methane reformer, SMR) and autothermal reforming (ATR), also known as primary and secondary reforming or two-step reforming. Alternatively, the synthesis gas can be produced with a separate SMR or a separate ATR.

[0008] The main elements of the ATR reactor are the burner, the combustion chamber and the catalyst bed contained in a refractory lined pressure shell. In the ATR reactor, the hydrocarbon feed is partially oxidized or burned by substoichiometric oxygen, and then the partially burned hydrocarbon feed stream is steam reformed in a fixed bed of steam reforming catalyst. Due to the high temperature, steam reforming also occurs to a certain extent in the combustion chamber.

[0009] In two-step reforming, the steam methane reformer (SMR) must be large and requires a lot of heat to drive the endothermic steam reforming reaction. Therefore, it is desirable that the size and load of the steam reformer can be reduced. In addition, the ATR in the two-step reforming concept requires oxygen. Today, this is usually produced in a cryogenic air separation unit (ASU). The size and cost of this ASU are large. It would be desirable if oxygen could be produced by other means.

[0010] The SMR produces hydrogen in excess of that required for a suitable methanol syngas composition, while the ATR produces smaller amounts of hydrogen.

[0011] We have found that when tubular steam reforming and autothermal reforming are combined in parallel with electrolysis of water and / or steam, the oxygen from the electrolysis can be used for the ATR and the expensive ASU becomes redundant in producing syngas.

[0012] At the same time, the hydrogen from the electrolysis contributes the hydrogen required for methanol production, which will allow the addition of additional carbon dioxide to the process, thereby still producing the required modulus for methanol synthesis.

[0013] Therefore, the present invention provides a method for preparing synthesis gas, comprising the following steps:

[0014] (a) providing a hydrocarbon feedstock;

[0015] (b) producing a separate hydrogen-containing stream and a separate oxygen-containing stream by electrolysis of water and / or steam;

[0016] (c) steam reforming the first portion of the hydrocarbon feedstock from step (a) in a tubular steam reformer into a tubular steam reformed gas comprising hydrogen, carbon monoxide and carbon dioxide;

[0017] (d) autothermal reforming the second portion of the hydrocarbon feedstock in an autothermal reformer with at least part of the oxygen-containing stream obtained in step (b) to form an autothermal reformate gas stream comprising hydrogen, carbon monoxide and carbon dioxide;

[0018] (e) combining the steam reformed gas from step (c) with the autothermal reformed gas from step (d);

[0019] (f) adding at least part of the separate hydrogen-containing stream from step (b) to the hydrocarbon feedstock from step (a) and / or to the steam reformed gas stream from step (c) and / or to the autothermal reformed gas stream from step (d) and / or to the combined steam reformed gas and autothermal reformed gas stream from step (e); and

[0020] (g) Extraction of synthesis gas.

[0021] Tubular Steam Reforming In tubular steam reforming CO2 is produced in the flue gases from the burner. In one embodiment of the invention, the CO2 in the flue gases is recovered and fed to the tubular steam reforming process.

[0022] Thus, when added CO 2 is utilized in methanol production, the CO 2 footprint of the process according to the invention is advantageously reduced.

[0023] To further minimize CO2 emissions, a portion of the feedstock may be heat exchange reformed in a heat exchange reformer downstream of the tubular steam reformer, and / or the tubular steam reformed gas may be additionally heat exchange reformed to reduce fuel consumption.

[0024] The electrolysis can be performed by various means known in the art, for example by solid oxide based electrolysis or by alkaline cells or polymer cells (PEM) electrolysis.

[0025] When the electricity used for the electrolysis is (at least partially) generated from a sustainable source, carbon dioxide emissions per unit of product produced by the process are reduced.

[0026] Preferably the electrolysis of water and / or steam is powered solely by renewable energy sources.

[0027] As already described above, all the oxygen from the electrolysis unit is fed to the autothermal reformer in step (d) and an ASU for the preparation of oxygen by separating air is avoided.

[0028] The process according to the invention is preferably used for producing methanol synthesis gas.

[0029] The methanol synthesis gas preferably has a composition corresponding to the so-called modulus (M=(H2-CO2) / (CO+CO2)) of 1.90-2.20 or more preferably slightly above 2 (eg 2.00-2.10).

[0030] Therefore, when the method according to the present invention is used to prepare methanol synthesis gas, the amount of hydrogen added to the feed upstream of the SMR or ATR or to the reformed gas downstream of step (e) can be adjusted so that when the hydrogen is mixed with the synthesis gas produced by the reforming step and optionally mixed with CO2 recovered from the tubular steam reformer, the desired M value between 1.90 and 2.20 or preferably between 2.00 and 2.10 is achieved.

[0031] In general, suitable hydrocarbon feedstocks for use with various embodiments of the present invention include natural gas, methane, LNG, naphtha, or mixtures thereof, either as is or after pre-reformation and / or desulfurization.

[0032] The process according to the invention can also be used to produce synthesis gas for other applications where it is desired to increase the hydrogen concentration in the feed gas and to advantageously produce hydrogen and oxygen by electrolysis.

Claims

1. A method for preparing synthesis gas, the method comprising the following steps: (a) providing a hydrocarbon feedstock; (b) producing a separate hydrogen-containing stream and a separate oxygen-containing stream by electrolysis of water and / or steam; (c) steam reforming the first portion of the hydrocarbon feedstock from step (a) in a tubular steam reformer into a tubular steam reformed gas comprising hydrogen, carbon monoxide and carbon dioxide; (d) autothermal reforming the second portion of the hydrocarbon feedstock in an autothermal reformer with at least part of the oxygen-containing stream obtained in step (b) to form an autothermal reformate gas stream comprising hydrogen, carbon monoxide and carbon dioxide; (e) combining the steam reformed gas from step (c) with the autothermal reformed gas from step (d); (f) adding at least part of the separate hydrogen-containing stream from step (b) to the hydrocarbon feedstock from step (a) and / or to the steam reformed gas stream from step (c) and / or to the autothermal reformed gas stream from step (d) and / or to the combined steam reformed gas and autothermal reformed gas stream from step (e); and (g) Extraction of synthesis gas.

2. The process of claim 1, wherein carbon dioxide in the flue gas from the tubular steam reformer is recovered and fed to the steam reforming in step (c).

3. The method of claim 1 or 2, wherein the electrolysis of water and / or steam in step (b) is provided by electricity from renewable energy sources.

4. A process according to claim 1 or 2, wherein a hydrogen stream is added to the combined tubular steam reformed gas and autothermal reformed gas streams in an amount to provide a modulus M = (H2-CO2) / (CO+CO2) between 1.9 and 2.2 in the synthesis gas extracted in step (g).

5. The process of claim 1 or 2, wherein the modulus M = (H2-CO2) / (CO+CO2) in the synthesis gas extracted in step (g) is in the range of 2 to 2.

1.

6. The process of claim 1 or 2, comprising the further step of heat exchange reforming a portion of the hydrocarbon feedstock from step (a) and / or the tubular steam reforming gas from step (c).

7. The process of claim 1 or 2, wherein the hydrocarbon feedstock comprises natural gas, methane, LNG, naphtha or mixtures thereof, either as is or after pre-reformation and / or desulfurization.

8. The process of claim 1 or 2, wherein the synthesis gas extracted in step (g) is converted into a methanol product in a further step.