Process and apparatus for producing a low-nitrogen synthesis gas from a nitrogen-containing natural gas

Abstract

The invention relates to a method and an installation for producing a low-nitrogen synthesis gas (20) from a nitrogen- and carbon dioxide-containing natural gas (1), wherein water and carbon dioxide are separated from the nitrogen- and carbon dioxide-containing natural gas in a first temperature swing adsorption installation (T1), followed by separation of nitrogen (7) in a cryogenic gas separator (N), so that a low-nitrogen, water- and carbon dioxide-free natural gas (6) is produced, which is then fed to a thermochemical conversion (K) in order to obtain a synthesis crude gas (16) comprising hydrogen, carbon monoxide, water and carbon dioxide, from which the low-nitrogen synthesis gas (20) is obtained by at least separating water and carbon dioxide from the synthesis crude gas comprising hydrogen, carbon monoxide, water and carbon dioxide in a second temperature swing adsorption installation (T1). Characterized in that at least a part (8) of the low-nitrogen, water- and carbon dioxide-free natural gas (6) is used as a regeneration gas (9, 10) for the regeneration of the first temperature swing adsorption installation (T1) and / or the second temperature swing adsorption installation (T2) before the thermochemical conversion (K) of the low-nitrogen, water- and carbon dioxide-free natural gas (6).

Description

[0001] This invention relates to a method for preparing low-nitrogen syngas from nitrogen- and carbon dioxide-containing natural gas, wherein water and carbon dioxide are separated from the nitrogen- and carbon dioxide-containing natural gas in a first temperature-switching adsorption unit, and nitrogen is subsequently separated in a cryogenic gas separator, thereby producing low-nitrogen, anhydrous, and carbon dioxide-free natural gas, which is then fed into a thermochemical conversion process to obtain a crude syngas comprising hydrogen, carbon monoxide, water, and carbon dioxide, wherein the low-nitrogen syngas is obtained from the crude syngas comprising hydrogen, carbon monoxide, water, and carbon dioxide, at least by separating water and carbon dioxide in a second temperature-switching adsorption unit.

[0002] Furthermore, the present invention relates to an apparatus for performing the method.

[0003] In the language used here, a gas mixture is considered to be free of a component if its fraction in the gas mixture is no more than 0.1 ppmv. Conversely, a gas mixture is said to contain a component if its fraction in the gas mixture is greater than 0.5 mol%. A gas mixture is considered to lack a component if its fraction in the gas mixture is between 0.5 mol% and 1 ppmv.

[0004] Syngas, also known as syngas, is a mixture of gases containing at least hydrogen and carbon monoxide that can be used to synthesize various products. These gas mixtures are primarily produced from natural gas, which is thermochemically converted through autothermal reforming (ATR), partial oxidation (POX), steam reforming (SMR), or a combination of two or more methods known in the art over many years. Carbon dioxide-containing natural gas, typically with a nitrogen content between 1 mol% and 10 mol%, is processed into a natural gas feed, for example, through mercury separation, desulfurization, heating, and compression. This feed is then converted into syngas crude by introducing water and / or carbon dioxide, which contains significantly more carbon dioxide and water in addition to carbon monoxide and hydrogen, as well as other substances not desired in syngas. To obtain low-nitrogen syngas, nitrogen can be removed from the prepared syngas crude downstream of the thermochemical conversion process, or it can be removed upstream from the feed natural gas.

[0005] If nitrogen separation occurs downstream of the thermochemical conversion, the desulfurized and mercury-removed natural gas is converted into nitrogen-containing syngas with its full nitrogen load. Most of the carbon dioxide is first separated from this syngas in an amine scrubber, for example, and then water and remaining carbon dioxide residue are removed in a temperature swing adsorption (TWA) unit. The resulting gas mixture, essentially composed of hydrogen, carbon monoxide, and nitrogen, is then separated into crude hydrogen, carbon monoxide, and a nitrogen fraction containing combustible substances using a cryogenic gas separator. After the crude hydrogen is used as regeneration gas in the TWA, it is purified to pure hydrogen in a pressure swing adsorption (DWA) unit, at least a portion of which is mixed with carbon monoxide from the cryogenic gas separator to form a low-NOx syngas.

[0006] In cryogenic gas separators, carbon monoxide and nitrogen are separated in a rectified manner within the column. Because the boiling temperatures of these two substances are very similar, separation can only be achieved in columns with high reflux ratios and / or many separation stages. An additional compressor is required to recover the hydrogen-containing residual gas generated during the purification of crude hydrogen in the DWA and to use it for thermochemical conversion to achieve sufficiently high yields, or to burn it as fuel. The resulting high equipment and energy costs are clearly detrimental to the economic viability of this method variant.

[0007] Because the boiling temperature difference between nitrogen and methane is significantly greater than that between nitrogen and carbon monoxide, it is relatively easy to separate nitrogen from nitrogen-containing natural gas in a rectified manner within a cryogenic gas separator. In particular, the separator used can operate energy-efficiently without reflux. Therefore, an alternative method variant is configured to separate nitrogen not only from the syngas but also from the feed natural gas in a cryogenic process, and to prepare equally low-nitrogen syngas from the resulting low-nitrogen feed gas via thermochemical conversion. While this method variant requires units for carbon dioxide removal and a TWA for water and carbon dioxide separation, it eliminates the need for a complex cryogenic syngas separator, a DWA for cleaning crude hydrogen, and a recirculation compressor for the residual gas from the hydrogen-containing DWA. However, a disadvantage is the need to arrange at least one additional TWA upstream of the cryogenic natural gas separator, and in some cases, units for carbon dioxide removal, to prevent water and carbon dioxide from entering the cryogenic natural gas separator with the natural gas, where they may freeze and cause blockage. If carbon dioxide is removed using water-based detergents (such as amine scrubbers), water saturation can occur, necessitating a TWA (water separation system) even if the feed gas is anhydrous.

[0008] Temperature-switched adsorption (TWA) devices for removing water and carbon dioxide from gas streams have been well-known to those skilled in the art for many years. The gas stream to be treated is fed into the TWA at a first temperature, where it flows through one of several adsorbents, each filled with adsorbent material that adsorbs and retains the water and carbon dioxide contained in the gas stream, while allowing other substances such as methane, hydrogen, or carbon monoxide to pass through substantially unimpeded. Thus, the gas stream exits the adsorber with water and carbon dioxide concentrations significantly below 1 ppmv.

[0009] Because the adsorbent material has a limited capacity to absorb water and carbon dioxide, the gas flow to the adsorber must be interrupted after a certain period of time; otherwise, the water or carbon dioxide content in the outflowing gas will exceed the limit. While the gas to be treated is being transferred to the adsorber containing still-absorbent adsorbent material from the TWA, the adsorber loaded with water and carbon dioxide is regenerated. For this purpose, the adsorber is flushed with a regeneration gas having a second temperature higher than the first temperature at which the gas to be treated was introduced into the adsorber. The adsorption capacity of the adsorbent material decreases with increasing temperature, causing the separated substances to desorb and be removed from the adsorber with the regeneration gas. If crude hydrogen is unavailable, nitrogen is typically used as the regeneration gas, but nitrogen is not adsorbed by the adsorbent material or its adsorption effect is very poor. Because, in addition to water and carbon dioxide, small amounts of environmentally harmful substances such as methane and carbon monoxide are also separated from natural gas and syngas crude gas, and these harmful substances are transferred to the regeneration gas during adsorber regeneration, nitrogen loaded with harmful substances cannot be released into the atmosphere untreated. Therefore, in order to comply with existing emission limits, nitrogen is disposed of through catalytic post-combustion or by means of auxiliary combustion in a flare or process furnace.

[0010] Since both the first and second TWAs must undergo regeneration, the demand for regenerated gas is particularly high when preparing syngas using conventional methods. According to existing technology, low-pressure nitrogen is introduced as the regenerated gas. However, the supply and treatment of this low-pressure nitrogen constitute a significant cost factor, negatively impacting the economics of syngas production.

[0011] The object of the present invention is to provide a general type of method and an apparatus for performing the method, which makes it possible to overcome the disadvantages described in the prior art.

[0012] According to the invention, this objective is achieved in terms of method, namely, using at least a portion of the low-nitrogen, anhydrous, and carbon dioxide-free natural gas as regeneration gas during the regeneration of the first and / or second temperature-switching adsorption units before the thermochemical conversion.

[0013] Low-NOx, anhydrous, and carbon dioxide-free natural gas is suitable as a regeneration gas because it is primarily composed of methane, which is not adsorbed or is only adsorbed in very small amounts at the temperatures during the regeneration of the adsorber in TWAs used for water and / or carbon dioxide separation.

[0014] It is known that adsorbent materials capable of separating water and carbon dioxide can catalyze the decomposition of unsaturated hydrocarbons, particularly at elevated temperatures, leading to unwanted carbon deposition. Although low-NOx, anhydrous, and carbon dioxide-free natural gas practically contains no unsaturated hydrocarbons, and the risk of decomposing saturated hydrocarbons is also quite low, the present invention proposes that the regeneration temperature of TWA according to the invention be limited to between 170°C and 230°C, preferably between 170°C and 200°C.

[0015] If low-NOx, anhydrous, and carbon dioxide-free natural gas is produced from the total amount of nitrogen- and carbon dioxide-containing natural gas supplied for syngas production, the amount of low-NOx, anhydrous, and carbon dioxide-free natural gas is generally sufficient to meet the regeneration gas requirements of the first and second TWAs. Therefore, there is no need to supply another regeneration gas, such as nitrogen. Excess low-NOx, anhydrous, and carbon dioxide-free natural gas produced that is not needed for TWA regeneration can be directly fed into thermochemical conversion. However, it is also feasible to produce only the amount of low-NOx, anhydrous, and carbon dioxide-free natural gas actually needed as a regeneration gas from the nitrogen-containing natural gas. Preferably, after separating mercury and / or sulfur components from the nitrogen- and carbon dioxide-containing natural gas, which may be necessary, the portion of the nitrogen- and carbon dioxide-containing natural gas not supplied for water and carbon dioxide separation is directly fed into thermochemical conversion.

[0016] Therefore, either the adsorber regeneration is performed so slowly that only small amounts of water and carbon dioxide enter the thermochemical converter via the regeneration gas path, where the water and carbon dioxide have only a negligible effect on the syngas composition, or, since water and carbon dioxide are required as reactants or temperature regulators during thermochemical conversion, and are fed into the thermochemical conversion process anyway, an alternative method variant incorporates a control device that ensures the required amount of water and carbon dioxide always enters the converter, regardless of the regeneration gas loading.

[0017] If the low-NOx, anhydrous, and carbon dioxide-free natural gas is to be used for regeneration in both the first and second temperature-switched adsorption (TWA), then the portion of the low-NOx, anhydrous, and carbon dioxide-free natural gas designated as regeneration gas is preferably divided into a first split and a second split. The first split is fed only into the first TWA as regeneration gas, and the second split is fed only into the second TWA as regeneration gas. To minimize flow loss, it is reasonable to supply only the minimum required amount of regeneration gas to each TWA.

[0018] Alternatively, it is feasible to supply a total amount of low-NOx, anhydrous, and carbon dioxide-free natural gas, determined as the regeneration gas, for regeneration in one of the two TWAs, wherein the low-NOx, anhydrous, and carbon dioxide-free natural gas is preloaded with desorbed material in one of the two TWAs, and then used for adsorber regeneration in the other TWA.

[0019] For economic and technical reasons, carbon dioxide separation using a TWA (Total Vapor Scrubber) is only meaningful when the carbon dioxide content of the gas mixture to be processed does not exceed a maximum value typically of 1 mol%. If the nitrogen- and carbon dioxide-containing natural gas contains more carbon dioxide than can be reasonably separated in a TWA, one embodiment of the method according to the invention is configured to reduce the carbon dioxide content of the nitrogen- and carbon dioxide-containing natural gas to below the maximum value upstream of the TWA. Preferably, this is done by subjecting the nitrogen- and carbon dioxide-containing natural gas to an acid gas scrubber, such as an amine scrubber. The separated carbon dioxide can be released into the atmosphere or sent to a material recovery unit. Preferably, the separated carbon dioxide is used for thermochemical conversion downstream.

[0020] The method according to the invention is essentially independent of the type of thermochemical conversion. Therefore, low-nitrogen, anhydrous, and carbon dioxide-free natural gas can be converted into syngas, for example, by autothermal reforming, partial oxidation, steam reforming, or a combination of at least two of these methods.

[0021] Furthermore, the present invention relates to an apparatus for preparing low-nitrogen syngas from nitrogen- and carbon dioxide-containing natural gas, the apparatus comprising: a first temperature-switching adsorption device for separating water and carbon dioxide from the nitrogen- and carbon dioxide-containing natural gas, and for obtaining nitrogen-containing, anhydrous, and carbon dioxide-free natural gas; a cryogenic gas separator, which enables the separation of nitrogen from the nitrogen-containing, anhydrous, and carbon dioxide-containing natural gas to obtain low-nitrogen, water, and carbon dioxide-containing natural gas; a thermochemical converter for converting the low-nitrogen, anhydrous, and carbon dioxide-free natural gas into syngas crude gas comprising hydrogen, carbon monoxide, water, and carbon dioxide; and a second temperature-switching adsorption device, which enables the separation of water and carbon dioxide from the syngas crude gas to obtain low-nitrogen syngas.

[0022] According to the invention, the proposed objective is thus achieved in terms of equipment, namely, the cryogenic gas separator is connected to the thermochemical converter via a first temperature-switching adsorption device and a second temperature-switching adsorption device such that at least a portion of the low-NOx, anhydrous, and carbon dioxide-free natural gas can be used as regeneration gas during the regeneration of the first temperature-switching adsorption device and / or the second temperature-switching adsorption device before it is converted in the thermochemical converter.

[0023] Preferably, the connection between the first and second TWAs and the thermochemical converter is implemented such that a portion of the low-NOx, anhydrous, and carbon dioxide-free natural gas used as regeneration gas can be fed into the thermochemical converter along with the substances desorbed during adsorber regeneration. Specifically, this connection does not include means for separating water and / or carbon dioxide from the regeneration gas used.

[0024] In a preferred variant of the apparatus according to the invention, the cryogenic gas separator is connected to the first and second TWAs such that a first portion of the low-NOx, anhydrous, and carbon dioxide-free natural gas, determined as regeneration gas, can only be used as regeneration gas in the first TWA, and a second portion can only be used as regeneration gas in the second TWA. Logically, this connection includes a distributor via which the mass flow of the low-NOx, anhydrous, and carbon dioxide-free natural gas can be adjusted according to the current regeneration gas demand of the two TWAs.

[0025] Such a connection is also feasible, in which the cryogenic gas separator and two TWAs are arranged in series, allowing the total amount of low-NOx, anhydrous, and carbon dioxide-free natural gas, identified as regeneration gas, to be fed first into one TWA, followed by the other. In this case, it is preferable that only the last TWA in the flow direction is directly connected to the thermochemical converter, thereby enabling the introduction of low-NOx, anhydrous, and carbon dioxide-free natural gas, used as regeneration gas in both TWAs and loaded with desorbed substances during adsorber regeneration, into the thermochemical converter.

[0026] If the nitrogen- and carbon dioxide-containing natural gas contains more carbon dioxide than can be reasonably separated in a TWA, the apparatus according to the invention is implemented with a device arranged upstream of a first TWA for separating the majority of the carbon dioxide from the natural gas. Preferably, this device is an acid gas scrubber, such as an amine scrubber. Suitablely, the device for separating carbon dioxide is connected to the thermochemical converter in such a way that the carbon dioxide separated from the natural gas can be fed into the thermochemical converter as feed material.

[0027] A variant of the device according to the invention is provided with a bypass line through which a portion of the nitrogen- and carbon dioxide-containing natural gas that is not required for the preparation of regeneration gas for the two TWAs can be directly fed into the thermochemical converter after possible separation of mercury and / or sulfur components, bypassing the first TWA, the cryogenic gas separator, and, if necessary, the apparatus for separating carbon dioxide.

[0028] Furthermore, the present invention proposes that the thermochemical converter be implemented as an autothermal reformer or a partial oxidation reactor or a steam reformer, or as a combination of at least two of these devices.

[0029] The following is based on Figure 1 The invention is explained in more detail by the embodiments illustrated schematically.

[0030] Figure 1 A variant of the invention is shown in which low-NOx oxidizing gas is prepared from natural gas containing nitrogen and carbon dioxide.

[0031] Nitrogen- and carbon dioxide-containing natural gas is fed into purification unit R via pipeline 1, where substances such as mercury are separated in a first purification step. The treated natural gas 2 is then fed into an acid gas scrubber W1 to remove the majority of the contained carbon dioxide 3; this acid gas scrubber is, for example, an amine scrubber. The natural gas, with reduced carbon dioxide content, flows into the first TWA T1 via pipeline 4, where water and residual carbon dioxide are separated, and nitrogen-containing, anhydrous, and carbon dioxide-free natural gas 5 is prepared. This nitrogen-containing, anhydrous, and carbon dioxide-free natural gas is then separated in a cryogenic gas separator N into low-nitrogen, anhydrous, and carbon dioxide-free natural gas 6 and a nitrogen-rich fuel gas fraction 7. After being pressurized in compressor P1, the low-nitrogen, anhydrous, and carbon dioxide-free natural gas 8 is divided into a first fraction 9 and a second fraction 10, where the first fraction 9 is used as regeneration gas for the adsorber regeneration in the first TWA T1, while the second fraction 10 is fed to the second TWA T2, located further downstream, for gas drying, for the same purpose. Two regenerated gas streams 11 and 12, containing desorbed water and carbon dioxide, are guided back and fed into the thermochemical converter K via line 13 as natural gas feed. In the thermochemical converter K, the natural gas feed, along with steam 14 and carbon dioxide and, where appropriate, oxygen 22, is converted into a synthesis crude gas 16 comprising hydrogen, carbon monoxide, water, and carbon dioxide. After cooling in a cooling unit G, this synthesis crude gas is guided via line 17 to another acid gas scrubber W2 for separating carbon dioxide 18. This scrubber can also be implemented as an amine scrubber. In the second TWA T2, water and carbon dioxide residues are removed from the carbon dioxide-reduced synthesis crude gas 19, yielding a nitrogen-free synthesis gas 20 consisting primarily of hydrogen and carbon monoxide as the product. The carbon dioxide 18 separated from the cooled synthesis crude gas 17, after the addition of carbon dioxide 3 separated from natural gas 2 and introduced carbon dioxide 21 to increase carbon monoxide production, is guided back to the thermochemical converter K via the second compressor P2 and line 15 as feed.

Claims

1. A method for preparing low-nitrogen syngas (20) from nitrogen- and carbon dioxide-containing natural gas, wherein water and carbon dioxide are separated from the nitrogen- and carbon dioxide-containing natural gas in a first temperature-switching adsorption unit (T1), followed by separation of nitrogen (7) in a cryogenic gas separator (N), thereby producing low-nitrogen, anhydrous, and carbon dioxide-free natural gas (6), which is then fed into a thermochemical conversion process to obtain a syngas crude gas (16) comprising hydrogen, carbon monoxide, water, and carbon dioxide, wherein the low-nitrogen syngas (20) is obtained at least by separating water and carbon dioxide from the syngas crude gas comprising hydrogen, carbon monoxide, water, and carbon dioxide in a second temperature-switching adsorption unit (T2), wherein, Before the thermochemical conversion of the low-nitrogen, anhydrous, and carbon dioxide-free natural gas (6), at least a portion (8) of the low-nitrogen, anhydrous, and carbon dioxide-free natural gas is used as regeneration gas during the regeneration of the first temperature-switching adsorption device (T1) and the second temperature-switching adsorption device (T2), wherein the portion of the low-nitrogen, anhydrous, and carbon dioxide-free natural gas used as regeneration gas is fed into the thermochemical conversion together with the substances desorbed during the regeneration of the first temperature-switching adsorption device and the second temperature-switching adsorption device.

2. The method according to claim 1, characterized in that, The portion (8) of the low-nitrogen, anhydrous and carbon dioxide-free natural gas (6) designated as the regeneration gas is divided into a first portion (9) and a second portion (10), wherein the first portion (9) is used as the regeneration gas only in the first temperature-switching adsorption device (T1), and the second portion (10) is used as the regeneration gas only in the second temperature-switching adsorption device (T2).

3. The method according to claim 1 or 2, characterized in that, Carbon dioxide (3) present in the nitrogen- and carbon dioxide-containing natural gas is separated upstream of the first temperature-switching adsorption device (T1).

4. The method according to claim 1 or 2, characterized in that, The thermochemical conversion is used to perform autothermal reforming or partial oxidation or steam reforming or a combination of at least two of the autothermal reforming, partial oxidation and steam reforming.

5. An apparatus for preparing low-nitrogen syngas (20) from nitrogen- and carbon dioxide-containing natural gas, the apparatus comprising: a first temperature-switching adsorption unit (T1) for separating water and carbon dioxide from the nitrogen- and carbon dioxide-containing natural gas, and for obtaining nitrogen-containing, anhydrous, and carbon dioxide-free natural gas (5); a cryogenic gas separator (N) capable of obtaining low-nitrogen, anhydrous, and carbon dioxide-free natural gas (6) by separating nitrogen (7) from the nitrogen-containing, anhydrous, and carbon dioxide-containing natural gas (5); a thermochemical converter (K) for converting the low-nitrogen, anhydrous, and carbon dioxide-free natural gas (6) into a syngas crude gas (16) comprising hydrogen, carbon monoxide, water, and carbon dioxide; and a second temperature-switching adsorption unit (T2) capable of separating water and carbon dioxide from the syngas crude gas (16) to obtain the low-nitrogen syngas (20), wherein, The cryogenic gas separator (N) is connected to the thermochemical converter (K) via the first temperature-switching adsorption device (T1) and the second temperature-switching adsorption device (T2) such that at least a portion (8) of the low-nitrogen, anhydrous, and carbon dioxide-free natural gas (6) can be used as regeneration gas during the regeneration of the first temperature-switching adsorption device (T1) and the second temperature-switching adsorption device (T2) before the natural gas (6) is converted in the thermochemical converter (K), wherein both the first temperature-switching adsorption device (T1) and the second temperature-switching adsorption device (T2) are connected to the thermochemical converter (K) such that the portion of the low-nitrogen, anhydrous, and carbon dioxide-free natural gas used as regeneration gas can be fed into the thermochemical converter (K) together with the substances desorbed during the regeneration of the first temperature-switching adsorption device and the second temperature-switching adsorption device.

6. The apparatus according to claim 5, characterized in that, The cryogenic gas separator (N) is connected to the first temperature-switching adsorption device (T1) and the second temperature-switching adsorption device (T2) such that the first portion (9) of the low-nitrogen, anhydrous and carbon dioxide-free natural gas, which is determined to be the regeneration gas, can only be used as the regeneration gas in the first temperature-switching adsorption device (T1), and the second portion (10) can only be used as the regeneration gas in the second temperature-switching adsorption device (T2).

7. The apparatus according to claim 5 or 6, characterized in that, The device includes a device (W1) for separating carbon dioxide, arranged upstream of the first temperature-switching adsorption device (T1), capable of separating carbon dioxide (3) contained in the nitrogen- and carbon dioxide-containing natural gas.

8. The apparatus according to claim 5 or 6, characterized in that, The thermochemical converter (K) is implemented as an autothermal reformer, a partial oxidation reactor, or a steam reformer, or as a combination of at least two of the autothermal reformer, the partial oxidation reactor, and the steam reformer.

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