Method for preparing methanol synthesis gas

Abstract

A process for producing methanol synthesis gas combining in parallel two-step reforming and steam reforming of a feed containing carbon dioxide.

Description

[0001] The present application relates to the production of methanol synthesis gas. More particularly, the present invention combines two-step reforming in parallel with steam reforming.

[0002] The production of synthesis gas (eg for methanol synthesis) from a natural gas feed is usually carried out by steam reforming or a combination of steam reforming and oxygen reforming.

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

[0004]

[0005] And the oxygen conversion (given for methane):

[0006]

[0007] Similar reactions occur for other hydrocarbons. Any conversion is usually accompanied by a water-gas shift reaction:

[0008]

[0009] The sequential implementation of steam reforming (also called SMR) and oxygen reforming (also called autothermal reforming or ATR for short) is known as two-step reforming.

[0010] Further details of steam reforming and two-step reforming can be found in the literature.

[0011] The product gas from the two-step reforming comprises hydrogen, carbon monoxide and carbon dioxide, and other components typically including methane and steam.

[0012] The methanol synthesis gas preferably has a composition corresponding to a so-called module (M=(H2-CO2) / (CO+CO2)) of 1.90-2.20 or more preferably slightly above 2, for example 2.00-2.10.

[0013] The two-step conversion produces the preferred stoichiometric moduli described above.

[0014] In the chemical industry, there is an increasing desire to reduce carbon dioxide emissions and / or to utilize carbon dioxide as feed or part of the feed in the production of chemical products.

[0015] In existing methanol plants or newly built methanol plants that use or are designed with two-step conversion in the preparation of methanol synthesis gas, since the two-step conversion produces a stoichiometrically correct synthesis gas for methanol production, the addition of carbon dioxide to the conversion process is limited or impossible, and therefore the use of carbon dioxide as at least a portion of the conversion feed is not selected.

[0016] We have found a method for producing methanol synthesis gas from carbon dioxide by combining a two-step reforming step with a steam reforming step arranged in parallel to the two-step reforming step, wherein carbon dioxide is used as part of the reforming feed in the parallel steam reforming step.

[0017] Therefore, in one aspect, the present invention provides a method for preparing methanol synthesis gas, comprising the following steps:

[0018] (a) providing a first feed comprising hydrocarbons and steam;

[0019] (b) providing a second feed comprising hydrocarbons, steam and carbon dioxide;

[0020] (c) converting the first feed into a first synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide with a modulus M of 2.05 + / - 0.2 in a two-step steam reforming process, wherein the modulus M is (H2-CO2) / (CO+CO2);

[0021] (d) converting the second feed steam into a second synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide in parallel with the two-step conversion process in step (c); and

[0022] (e) Using a combination of the first synthesis gas and the second synthesis gas as a feed for methanol synthesis.

[0023] The carbon dioxide content in the second feed is preferably controlled so as to obtain a modulus M in the second synthesis gas discharged from the steam reforming of between 1.6 and 2.6, preferably between 1.85 and 2.35.

[0024] As an example, for a natural gas containing only methane, a natural gas / CO2 ratio of 3 as feed to the steam reformer will result in a syngas leaving the steam reformer of M = 2, as shown below, with two reforming equations occurring for methane:

[0025]

[0026] It is further preferred that the modulus M of the methanol synthesis gas obtained by the process according to the invention is from 1.6 to 2.6, preferably between 1.85 and 2.35.

[0027] In one embodiment of the invention, the methanol synthesis gas is converted to a methanol product in a downstream step.

[0028] The steam reforming in step (d) may be carried out in a conventional tubular reformer, a tubular reformer followed by an adiabatic reformer wherein carbon dioxide is added before and / or after the tubular reformer, a bayonet type reformer, a countercurrent reformer or a tubular reformer wherein carbon dioxide is added to the reformer tubes after the feed begins to contact the reforming catalyst.

[0029] Suitable reforming catalyst compositions for use in steam reforming depend on the amount of carbon dioxide added to the feed and the type of reformer used in the steam reforming. In most cases, a nickel catalyst is sufficient, but some types of steam reformers may require a precious metal catalyst due to the high carbon dioxide content in the feed.

[0030] The carbon dioxide for use in the process according to the invention can advantageously be obtained from flue gases or waste gases, for example flue gases originating from a burner in a steam reformer.

[0031] Depending on the feed composition, it may be preferred to desulfurize the first feed upstream of the two-step reforming and / or to desulfurize the second feed upstream of the steam reforming.

[0032] The advantages of the method according to the invention are inter alia reduced energy consumption and reduced carbon dioxide footprint.

[0033] As already mentioned above, the invention can furthermore be used to increase the capacity of existing methanol plants based on two-step reforming in the production of methanol synthesis gas or of other methanol plants in which the reforming stage produces a synthesis gas with a stoichiometry M close to about 2.

[0034] Therefore, a further aspect of the invention is a method for converting a methanol synthesis gas plant comprising:

[0035] adding a steam reformer in parallel with the existing two-step reforming section;

[0036] connecting the steam reforming section to a source of carbon dioxide; and

[0037] The outlet pipeline of the parallel steam reforming section is connected to the outlet pipeline of the existing two-step reforming section, or the outlet pipeline of the parallel steam reforming section is connected to the existing methanol reactor in the methanol synthesis unit.

Claims

1. A method for preparing methanol synthesis gas, comprising the following steps: (a) providing a first feed comprising hydrocarbons and steam; (b) providing a second feed comprising hydrocarbons, steam and carbon dioxide, wherein the molecular CO2 / hydrocarbon ratio is between 0.1 and 0.6; (c) converting the first feed into a first synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide and having a modulus M of 2.05+ / -0.2 in a two-step conversion process, wherein M is (H2-CO2) / (CO+CO2); (d) converting the second feed steam into a second synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide in parallel with the two-step conversion process in step (c); and (e) using a combination of the first synthesis gas and the second synthesis gas as a feed for methanol synthesis.

2. The process of claim 1, wherein in the second feed provided in step (b), the molecular CO2 / hydrocarbon ratio is between 0.25 and 0.

4.

3. The process of claim 1, comprising the additional step of desulfurizing the first feed upstream of the two-step conversion process.

4. The process as claimed in any one of claims 1 to 3, comprising the further step of desulfurizing the second feed upstream of the tubular steam reforming.

5. The method of any one of claims 1 to 3, wherein the modulus M of the second synthesis gas is from 1.6 to 2.

6.

6. The method of claim 5, wherein the modulus M of the second syngas is between 1.85 and 2.

35.

7. The method according to any one of claims 1 to 3, wherein the modulus M of the methanol synthesis gas is from 1.9 to 2.

2.

8. The process of any one of claims 1 to 3, wherein the carbon dioxide in the second feed is obtained from flue gas and / or waste gas and / or is input to the plant.

9. The process of any one of claims 1 to 3, wherein the steam reforming in step (d) is carried out in the following reformers: a standard tubular reformer; a tubular reformer followed by an adiabatic reformer, wherein carbon dioxide is added before and / or after the tubular reformer; a bayonet reformer; a convective reformer or a tubular reformer, wherein carbon dioxide is added to the reformer tubes after the feed begins to contact the reforming catalyst.

10. The process of any one of claims 1 to 3, wherein the methanol synthesis gas is converted into a methanol product in a further step.