METHOD AND SYSTEM FOR PRODUCING A GAS PRODUCT CONTAINING CARBON MONOXIDE

Abstract

The invention relates to a method (100-500) for producing a gas product containing at least carbon monoxide, in which method at least carbon dioxide is subjected to an electrolysis process (10) in order to obtain a raw gas (A) containing at least carbon monoxide and carbon dioxide and the carbon dioxide contained in the raw gas (A) is partially or completely fed back to the electrolysis process (10). According to the invention, the raw gas (A) is partially or completely subjected to a membrane separation process (20) in order to obtain a retentate mixture (B) and a permeate mixture (C), which is enriched in carbon dioxide in comparison with the raw gas (A), and that the retentate mixture (B) is partially or completely subjected to a pressure swing adsorption process (40) in order to obtain the gas product (D), which is enriched in carbon monoxide and depleted of carbon dioxide in comparison with the retentate mixture (B), and a residual mixture (E), which is depleted of carbon monoxide and enriched in carbon dioxide in comparison with the retentate mixture (B). The invention further relates to a corresponding system.

Claims

1. Method (100-500) for producing a gas product (D) containing at least carbon monoxide, in which method at least carbon dioxide is subjected to an electrolysis process (10) in order to obtain a raw gas (A) containing at least carbon monoxide and carbon dioxide, and the carbon dioxide contained in the raw gas (A) is partially or completely fed back to the electrolysis process (10), characterized in that the raw gas (A) is partially or completely subjected to a membrane separation process (20) to obtain a retentate mixture (B) and a permeate mixture (C), which is enriched in carbon dioxide in comparison with the raw gas (A), and that the retentate mixture (B) is partially or completely subjected to an adsorption process (40) to obtain the gas product (D), which is enriched in carbon monoxide and depleted of carbon dioxide in comparison with the retentate mixture (B), and a residual mixture (E), which is depleted of carbon monoxide and enriched in carbon dioxide in comparison with the retentate mixture (B).

2. Method according to claim 1, wherein the permeate mixture (C) and/or the residual mixture (E) are partially or completely fed back to the electrolysis process (10) in the form of one or more recirculation flows (F).

3. Method according to claim 2, wherein the electrolysis process (10) is carried out at a pressure level corresponding to a pressure level at which the raw gas (A) is supplied to the membrane separation process (20), the recirculation flow or flows (F) being compressed to the pressure level of the electrolysis process (10) using one or more compressors (30).

4. Method according to claim 2, wherein the electrolysis process (10) is carried out at a pressure level lower than a pressure level at which the raw gas (A) is supplied to the membrane separation process (20), the raw gas (A) being compressed to the pressure level of the membrane separation process (20) using one or more compressors.

5. Method (100-500) according to claim 2, wherein the raw gas (A) contains hydrogen, and the membrane separation process (20) is carried out in such a way that the retentate mixture (B) is depleted of hydrogen in comparison with the raw gas (A), and the permeate mixture (C) is enriched in hydrogen in comparison with the raw gas (A).

6. Method (100-500) according to claim 5, wherein at least a portion of the hydrogen present in the permeate mixture (C) from the method (100) is discharged.

7. Method (400-500) according to claim 5, wherein at least a portion of the recirculation flow (F) is subjected to a hydrogen removal process (60)in particular, in the form of catalytic and/or non-catalytic oxidationand what remains after the hydrogen removal process (60) is partially or completely fed back to the electrolysis process (10).

8. Method (200) according to claim 1, wherein a first fraction of the permeate mixture (C) and/or of the residual mixture (E) is combined with the raw gas (A) in the form of the recirculation flow or flows (F) and subjected to the membrane separation process (20), and a second fraction of the permeate mixture (C) and/or of the residual mixture (E) is combined with a fresh feed (G) and fed back to the electrolysis process (10).

9. Method (300) according to claim 1, wherein a first fraction of the raw gas (A) is combined with the recirculation flow or flows (F) and fed back to the electrolysis process (10), and a second fraction of the raw gas (A) is subjected to the membrane separation process (20) to obtain the retentate mixture (B) and the permeate mixture (C).

10. Method (500) according to claim 1, wherein the membrane separation process (20) comprises at least two membrane separation steps (21, 22), the permeate mixture (C) comprising permeate fractions (C1, C2), each formed in the at least two membrane separation steps (21, 22).

11. Method (100-500) according to claim 1, wherein the permeate mixture (C) and the residual mixture (E) are each formed at a pressure level of 1 to 10 bar.

12. Method (100-500) according to claim 1, wherein the gas product (D) formed is carbon monoxide or a carbon monoxide-rich gas mixture, the gas product (D) containing 90 to 100% carbon monoxide.

13. Method (100-500) according to claim 1, in which the gas product (D) formed is synthesis gas, wherein the gas product (D) contains in total 90 to 100% carbon monoxide and hydrogen, and wherein a ratio of hydrogen to carbon monoxide in the gas product is 1 to 4 and/or the gas product has a stoichiometric number of 0.8 to 2.1.

14. Method (100-500) according to claim 1, wherein the electrolysis process (10) in the form of a high-temperature electrolysis process using one or more solid oxide electrolysis cells and/or a low-temperature co-electrolysis process is carried out on a liquid electrolyte.

15. System for producing a gas product (D) containing at least carbon monoxide, comprising an electrolysis unit configured to subject at least one carbon dioxide to an electrolysis process (10) to obtain a raw gas (A) containing at least carbon monoxide and carbon dioxide, and comprising means configured to partially or completely feed back the carbon dioxide contained in the raw gas (A) to the electrolysis process (10), characterized by means configured to partially or completely subject the raw gas (A) to a membrane separation process (20) to obtain a retentate mixture (B) and a permeate mixture (D), which is enriched in carbon dioxide in comparison with the raw gas (A), and means configured to partially or completely subject the retentate mixture (B) to a pressure swing adsorption process (40) to obtain the gas product (D), which is enriched in carbon monoxide and depleted of carbon dioxide in comparison with the retentate mixture (B), and a residual mixture (E), which is depleted of carbon monoxide and enriched in carbon dioxide in comparison with the retentate mixture (B).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] FIG. 1 illustrates a method according to an embodiment of the invention.

[0055] FIG. 2 illustrates a method according to an embodiment of the invention.

[0056] FIG. 3 illustrates a method according to an embodiment of the invention.

[0057] FIG. 4 illustrates a method according to an embodiment of the invention.

[0058] FIG. 5 illustrates a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0059] In the figures, method steps, technical units, apparatuses, and the like, which correspond to one another in terms of function and/or design or structure, are denoted by identical reference symbols and are not explained repeatedly, for the sake of clarity. Even though methods according to embodiments of the invention are illustrated in the drawings and will be explained in more detail below, the corresponding explanations apply similarly to systems configured according to the embodiments of the invention. As a result, where method steps are explained hereafter, these explanations apply similarly to system parts.

[0060] FIG. 1 schematically illustrates a method according to an embodiment of the invention and is denoted overall by 100.

[0061] An electrolysis process 10 is provided as an essential method step of the method 100, which can be carried out, in particular, in the form of a high-temperature electrolysis process using one or more solid oxide electrolysis cells and/or a low-temperature co-electrolysis process on an aqueous electrolyte, as was explained at the outset. It is also possible to use mixed forms of such electrolysis techniques within the scope of the present invention. In particular, the electrolysis process 10 may be carried out using one or more electrolysis cells, groups of electrolytic cells, and the like. A feed in the form of a material flow H supplied to the electrolysis process 10 is explained below. It comprises at least carbon dioxide, which is partially converted into carbon monoxide in the electrolysis process 10. In this way, using the electrolysis process 10, a raw gas A is obtained, having a composition that depends on the feeds supplied to the electrolysis process 10 and the electrolysis conditions.

[0062] The raw gas A contains hydrogen, carbon monoxide, and carbon dioxide. The carbon monoxide present in the raw gas A is one of the target products of the method 100. The carbon dioxide present in the raw gas A is the carbon dioxide that was supplied to the electrolysis process 10, but was not converted there. As already explained above, a corresponding raw gas A also contains fractions of hydrogen that should not be disregarded, since a formation of hydrogen in the electrolysis process 10 may not be completely avoided or is desired. As likewise explained above, in this embodiment, the present invention is aimed, in particular, at ensuring that such hydrogen does not pass into a carbon monoxide-rich gas product D of the method 100.

[0063] In the example shown, the raw gas A contains, for example, approximately 2.5% hydrogen, 34% carbon monoxide, and 63% carbon dioxide. In the example shown, it is formed, for example, in an amount of 478 standard cubic meters per hour and supplied completely to a membrane separation process 20. The raw gas A is present at a pressure of approximately 20 bar, for example. In the example shown, the electrolysis process 10 is carried out, for example, at a temperature of 30 C. The temperatures used in a corresponding LT electrolysis 10 are, for example, in a range of approximately 20 to 80 C. So as to achieve good electrolysis efficiency, it is necessary to use an excess of carbon dioxide in the electrolysis process 10. Complete conversion is therefore not possible, and non-converted carbon dioxide is found in the raw gas A.

[0064] In the membrane separation process 20, the raw gas A is processed to obtain a retentate mixture B that is enriched in carbon monoxide and depleted of carbon dioxide and hydrogen in comparison with the raw gas A, and a permeate mixture C that is depleted of carbon monoxide and enriched in carbon dioxide and hydrogen in comparison with the raw gas A. As mentioned, the use of the membrane separation process makes it possible to obtain a substantially hydrogen-free and carbon monoxide-rich product in a subsequent adsorption process, denoted here by 40. This will be explained below. While the removal of hydrogen essentially influences the purity of the carbon monoxide product D, the reduced carbon dioxide content in the retentate C leads to a marked reduction in the adsorber material, and thus to cost savings, since less carbon dioxide has to be adsorbed.

[0065] So as to set the temperature in the electrolysis process 10 and the membrane separation process 20, a heat exchange can be carried out upstream and/or downstream of the electrolysis process 10. A so-called feed-effluent heat exchanger can also be used, in which, for example, the material flow H to the electrolysis process 10 is heated, and the raw gas A is cooled for this purposefor example, in a counter-flow. This is not illustrated in FIG. 1. A water separation process is also not illustrated, within the scope of which water vapor present in the raw gas A can be condensed out and, if necessary, fed back to the electrolysis process 10. After such a water separation process, renewed heatingtypically by approximately 5 to 20 C.can also be carried out upstream of the membrane separation process 20 so that the temperature level of the raw gas A is above the dew point.

[0066] So as to reduce possible oxygen fractions in the gas product D, a catalytic deoxo reactor can also be installed in the flow of the raw gas A so as to remove oxygen. By selecting suitable catalysts, hydrogen oxidizes to water starting at 70 C., for example, and carbon monoxide oxidizes to carbon dioxide starting at 150 C.

[0067] The retentate mixture B, which is formed as a retentate of the membrane separation process 20, contains, for example, approximately 0.2% hydrogen, 70% carbon monoxide, and 30% carbon dioxide in the example shown. In the example shown, it is formed, for example, in an amount of 202 standard cubic meters per hour. A membrane surface in the membrane separation process 20 is preferably designed in such a way that an accordingly low fraction of hydrogen is present in the retentate mixture B.

[0068] The permeate mixture C, which is formed as a permeate of the membrane separation process 20, is present, for example, at a pressure level of approximately 1.2 bar in the example shown. In the example shown, it is formed, for example, in an amount of 277 standard cubic meters per hour and has a hydrogen content of approximately 4%, a carbon monoxide content of approximately 7%, and a carbon dioxide content of approximately 88%.

[0069] In the example shown, a purge flow denoted by H.sub.2 in the example shown, e.g., in an amount of approximately 20 standard cubic meters per hour, is separated from the permeate mixture C and is discharged from the method 100. In this way, an accumulation of hydrogen in a cycle formed in the method 100 by the recirculation of corresponding gas mixtures can be avoided. In other words, a portion of the hydrogen present in the permeate mixture C is discharged from the process here, wherein, by simply branching off and eliminating a portion of the permeate mixture C, the remaining components thereof are also removed in corresponding proportions. A portion of the permeate mixture C remaining after the separation is, as will also be explained below, fed back to the electrolysis process 10.

[0070] In the example shown, the retentate mixture B is subjected to the aforementioned pressure swing adsorption 40, by means of which a gas product D that is enriched in carbon monoxide and depleted of carbon dioxide in comparison with the retentate mixture B, and a residual mixture E that is depleted of carbon monoxide and enriched in carbon dioxide in comparison with the retentate mixture B, are formed.

[0071] The gas product D represents a typical product of the method 100, which, in the example shown, is formed, for example, in an amount of 100 standard cubic meters per hour, having a hydrogen content of approximately 0.3%, a carbon monoxide content of approximately 99.7%, and a carbon dioxide content of approximately 100 ppm. In the example shown, the residual mixture E is formed, for example, in an amount of 101 standard cubic meters per hour, having a hydrogen content of approximately 400 ppm, a carbon monoxide content of approximately 40%, and a carbon dioxide content of approximately 60%. The residual mixture E is also fed back to the electrolysis process 10 in the example shown.

[0072] In the example shown, a portion of the permeate mixture C remaining after the fraction denoted by H.sub.2 has been branched off and the residual mixture E are combined before being fed back to the electrolysis process 10 to obtain a collection mixture forming a recirculation flow F, and are appropriately pressurized using a compressor 30. The extent of pressurization depends on the electrolysis conditions during the electrolysis process 10. As mentioned, a pressure of approximately 20 bar can be used during the electrolysis process 10, so that the mixture is pressurized to a corresponding pressure level. In the example shown, the collection mixture or the recirculation flow F is formed, for example, in an amount of approximately 358 standard cubic meters per hour, having a hydrogen content of approximately 3%, a carbon monoxide content of approximately 17%, and a carbon dioxide content of approximately 80%.

[0073] As an alternative to the illustration in this and the subsequent figures, the electrolysis process 10 can also be carried out at a pressure level lower than the inlet pressure of the membrane separation process 20. In this case, a corresponding raw gas compressor is used to compress the raw gas A. In such a case, it is possible to dispense with the compressor 30, and to supply the collection mixture or the recirculation flow F to the electrolysis process 10 at an appropriate lower pressure level. This variant is usually associated with higher compressor costs, since a larger gas flow has to be compressed.

[0074] Before being fed back to the electrolysis process 10, the collection mixture or the recirculation flow F is combined with a gaseous fresh feed G, which, in the example shown, is provided, for example, in an amount of 119 standard cubic meters per hour. The fresh feed G has a carbon dioxide content of approximately 99.9975%, for example. A material flow H, which is formed using the collection mixture F and the fresh feed G, is supplied to the electrolysis process 10. This results in an amount of approximately 477 standard cubic meters per hour for the material flow, having a hydrogen content of approximately 2%, a carbon monoxide content of approximately 13%, and a carbon dioxide content of approximately 85%. However, other fresh supplies G having typical purities can also be used. In particular, impurities of hydrogen, carbon monoxide, and water are typically not harmful in a feed of an LT electrolysis process, and can be tolerated. Other impurities such as saturated hydrocarbons, nitrogen, argon, and oxygen can also be tolerated in a feed within certain limits. In the case of HT electrolysis, water could be removed from the feed if the gas product to be produced is carbon monoxide.

[0075] FIG. 2 schematically illustrates a method according to another embodiment of the invention, which is denoted overall by 200.

[0076] The method 200 illustrated in FIG. 2 differs, in particular, from the method 100 illustrated in FIG. 1 in that a portion of the collection mixture, as illustrated here in the form of a material flow K, is fed back in the form of the recirculation flow F to the membrane separation process 20, and not to the electrolysis process 10. In other words, a first fraction of the collection mixture is combined with the raw gas A here and subjected to the membrane separation process 20, while a second fraction of the collection mixture is combined with a fresh feed G and fed back to the electrolysis process 10.

[0077] The fraction of carbon monoxide in the material flow H supplied to the electrolysis process 10 can be reduced by an appropriate partial recirculation. Depending on the particular design of the electrolysis process 10, such a reduction may be advantageous for the performance and/or the service life of the devices used herein. Since the membrane used in the membrane separation process 20 preferably selectively separates hydrogen and carbon dioxide from carbon monoxide, a partial recirculation has little influence on the downstream pressure swing adsorption process 40, provided that the membrane surface is adapted accordingly.

[0078] FIG. 3 schematically illustrates a method according to another embodiment of the invention, which is denoted overall by 300.

[0079] The method 300 illustrated in FIG. 3 differs, in particular, from the methods 100 and 200 explained above and illustrated in FIGS. 1 and 2 in that, here, a portion of the raw gas A, as illustrated in the form of a material flow L, is fed back to the electrolysis process 10 directly, i.e., bypassing the membrane separation process 20. A compressor 50 can be used for this purpose. In other words, a first fraction of the raw gas A is combined with the collection mixture or the recirculation flow F here and is fed back to the electrolysis process 10, and a second fraction of the raw gas A is subjected to the membrane separation process 20 to obtain the retentate mixture B and the permeate mixture C.

[0080] By means of appropriate partial direct recirculation to the electrolysis process, the carbon monoxide content in the electrolysis raw product, and thus the raw gas A, can be increased, in contrast to the method 200 illustrated in FIG. 2. This may have a positive effect on the overall separation sequence of the method 300. Since only the pressure loss of the electrolysis unit in which the electrolysis process 10 is carried out has to be overcome for appropriate recirculation, an inexpensive fan can be used as the compressor 50.

[0081] FIG. 4 schematically illustrates a method according to another embodiment of the invention, which is denoted overall by 400.

[0082] In contrast to the embodiments illustrated in the preceding figures, a hydrogen removal process 60 from the collection flow or recirculation flow F fed back to the electrolysis process 10 is carried out in the method 400 according to FIG. 4. As explained, in the process, partial or complete removal of hydrogen can take place. It is also possible to remove a portion of the carbon monoxide by oxidation to carbon dioxide. By setting the oxidation conditions (in particular, during the catalytic oxidation), first, hydrogen can be at least partially removed by oxidation to water, starting at approximately 70 C., and, at higher temperatures, starting at approximately 150 C., carbon monoxide can also be at least partially removed by oxidation to carbon dioxide. In particular, any remaining oxygen can also be removed using the second oxidation temperature. Thus, the content of carbon monoxide present in the recycle to the electrolysis process 10 can also be set and reduced, if this is advantageous for the life and operability of the electrolysis.

[0083] Such a procedure is, in particular, advantageous in cases in which a particularly pure carbon monoxide product in the form of the gas product D is desired, or a particularly high carbon efficiency of a corresponding process is to be achieved. In this way, an accumulation of hydrogen in the raw gas A can be further avoided. Such a selective removal of hydrogen can take place, for example, by catalytic oxidation, adding the oxygen by-product from the cathode side of the electrolysis process 10. During the catalytic oxidation, water is formed, which can be fed back to the electrolysis process 10 without any problem and can be separated downstream thereof. In addition to such catalytic removal, as an alternative, thermal removal by adding oxygen in a combustion chamber by partial oxidation is also possible. A corresponding thermal reaction can also take place in a gas turbine, for example, so as to achieve better energy efficiency of the method. In principle, the measures illustrated in FIGS. 2 and 3 for methods 200 and 300 can also be used in the method 400 illustrated in FIG. 4, or vice versa. In the method 400, inert components can be discharged in the form of a material flow X (purge). In all methods, as mentioned, the electrolysis process can also be carried out at a low pressure, and a raw gas compressor can also be used upstream of the membrane separation process, instead of the compressor 30.

[0084] FIG. 5 schematically illustrates a method according to another embodiment of the invention, which is denoted overall by 500.

[0085] According to the method 500 illustrated in FIG. 5, it is provided that the membrane separation process 20 comprise at least two membrane separation steps 21, 22, wherein the permeate mixture C comprises permeate fractions C1, C2, each formed in the at least two membrane separation steps 21, 22. In this way, a separation action can be increased, wherein, so as to keep the number of required compressors small, a sequential arrangement, such as is shown in FIG. 5, is particularly advantageous. In principle, it is also possible to use the measures illustrated in FIGS. 2 and 3 for methods 200 and 300 in the method 500 illustrated in FIG. 5, or vice versa.