Method and apparatus for producing nitrogen-lean syngas from nitrogen-containing natural gas

Abstract

A method and apparatus for producing lean nitrogen syngas (20) from natural gas (1) containing nitrogen and carbon dioxide, wherein water and carbon dioxide are separated from the natural gas in a first temperature-switching adsorption unit (T1), and then nitrogen (7) is separated from the natural gas in a cryogenic gas separator (N), thereby producing lean nitrogen natural gas (6) free of water and carbon dioxide. The natural gas is then sent to a thermochemical conversion (K) to obtain crude syngas (16) containing hydrogen, carbon monoxide, water, and carbon dioxide. The lean nitrogen syngas (20) is obtained from the crude syngas by separating water and carbon dioxide at least in a second temperature-switching adsorption unit (T2). The method is characterized in that at least a portion of the lean nitrogen natural gas (6) free of water and carbon dioxide is used as regeneration gas (9, 10) during regeneration in the first temperature-switching adsorption unit (T1) and / or the second temperature-switching adsorption unit (T2) before being regenerated by the thermochemical conversion (K).

Description

Technical Field

[0001] This invention relates to a method for producing nitrogen-lean syngas from natural gas containing nitrogen and carbon dioxide. The method involves separating water and carbon dioxide from the natural gas in a first temperature-switching adsorption unit, followed by separating nitrogen from the natural gas in a cryogenic gas separator, thereby producing nitrogen-lean natural gas free of water and carbon dioxide. The natural gas is then sent for thermochemical conversion to obtain crude syngas containing hydrogen, carbon monoxide, water, and carbon dioxide. At least water and carbon dioxide are separated in a second temperature-switching adsorption unit to obtain nitrogen-lean syngas from the crude syngas.

[0002] The present invention further relates to an apparatus for carrying out the method according to the invention. Prior Technology

[0003] In the usage of this case, if the proportion of a certain component in a gas mixture is no more than 0.1 ppmv, the gas mixture is considered to be free of that component. If the proportion of a certain component in a gas mixture is greater than 0.5 mol%, it is said to contain that component. If the proportion of a certain component is between 0.5 mol% and 1 ppmv, the gas mixture is said to be deficient in that component.

[0004] Syngas refers to a gas mixture containing at least hydrogen and carbon monoxide that can be used to synthesize various products. The main feedstock for syngas is natural gas, which is produced through thermochemical conversion of natural gas using methods such as autothermal reforming (ATR), partial oxidation (POX), steam reforming (SMR), or a combination of two or more methods known in the prior art for many years. Natural gas typically contains between 1 mol% and 10 mol% nitrogen and carbon dioxide. After treatments such as mercury separation, desulfurization, heating, and compression, it forms a natural gas feedstock. Then, with the addition of water and / or carbon dioxide, it is converted into crude syngas. Crude syngas contains more carbon dioxide and water, as well as other undesirable substances, in addition to carbon monoxide and hydrogen. To obtain nitrogen-lean syngas, nitrogen can be removed either downstream of the produced crude syngas during thermochemical conversion or upstream from the natural gas used.

[0005] If the nitrogen separation system is located downstream of the thermochemical conversion, the desulfurized and mercury-removed natural gas, along with its entire nitrogen load, will be converted into nitrogen-containing crude syngas. First, most of the carbon dioxide is separated from this crude syngas, for example, by amine washing. Then, water and remaining carbon dioxide residue are removed in a temperature swing adsorption (TWA) unit. The resulting gas mixture mainly consists of hydrogen, carbon monoxide, and nitrogen. A cryogenic gas separator then separates this mixture into crude hydrogen, carbon monoxide, and a nitrogen fraction containing combustible substances. After being used as regeneration gas in the TWA, the crude hydrogen 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 nitrogen-lean syngas.

[0006] In cryogenic gas separators, carbon monoxide and nitrogen are separated by distillation in the column. Because these two substances have very similar boiling points, separation can only be achieved through a high reflux ratio and / or multiple separation stages in the column. Another compressor is required to transport the hydrogen-containing residual gas produced during the crude hydrogen purification process in the DWA back and forth, and to use it for thermochemical conversion to obtain a sufficiently high yield, or to use it as fuel for bottom combustion. The resulting high equipment and energy costs significantly impair the economic efficiency of this process variant.

[0007] Because the boiling temperature difference between nitrogen and methane is much greater than that between nitrogen and carbon monoxide, it is relatively easy to separate nitrogen from nitrogen-containing natural gas by distillation in a cryogenic gas separator. In particular, the separator used can operate in an energy-efficient manner without reflux. Therefore, an alternative process variant is envisioned: instead of separating nitrogen from crude syngas, nitrogen is separated from the natural gas used in the cryogenic process, and a similarly nitrogen-lean crude syngas is produced from the nitrogen-lean natural gas feedstock obtained in this manner through thermochemical conversion. Although this process variant still requires a carbon dioxide removal unit and a TWA for separating water and carbon dioxide to process the crude syngas into nitrogen-lean syngas, it eliminates the need for a complex cryogenic syngas separator, a DWA for purifying crude hydrogen, and a return compressor for the residual gas from the hydrogen-containing DWA. However, a disadvantage is that at least one additional TWA and possibly a carbon dioxide removal unit must be installed upstream of the cryogenic natural gas separator to prevent water and carbon dioxide from entering the cryogenic natural gas separator with the natural gas, freezing there, and causing blockage. In particular, when carbon dioxide is removed by methods based on water-containing detergents (such as amine washing), water saturation occurs. Therefore, even if the natural gas used is water-free, a water separation unit (TWA) is required.

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

[0009] Because the adsorbent material has a limited capacity to absorb water and carbon dioxide, the airflow to the adsorber must be interrupted after a certain period of time before the water or carbon dioxide content in the exhaust gas exceeds the limit. While the gas to be treated is transferred to another adsorber in the TWA where the adsorbent material still has absorption capacity, the adsorber loaded with water and carbon dioxide is regenerated. For this purpose, the adsorber is purged with a regeneration gas having a second temperature higher than the first temperature at which the gas to be treated enters the adsorber. The adsorption capacity of the adsorbent material decreases as the temperature increases; therefore, the separated substances are desorbed and discharged from the adsorber with the regeneration gas. If the aforementioned crude hydrogen is unavailable, nitrogen is typically used as the regeneration gas; however, the adsorbent material does not adsorb nitrogen, or the adsorption effect is extremely poor. Since, in addition to water and carbon dioxide, environmentally harmful substances such as methane and carbon monoxide are also separated in small amounts from natural gas and crude syngas and enter the regeneration gas during adsorber regeneration, the loaded nitrogen cannot be released into the atmosphere untreated. Therefore, in order to comply with existing emission limits, the nitrogen is disposed of by catalytic combustion or by auxiliary combustion in a flare or process furnace.

[0010] Since both the first and second TWAs require regeneration, the demand for regenerated gas is particularly high in general syngas production. According to prior art, low-pressure nitrogen is used as the regenerated gas, and its supply and disposal are significant cost factors, negatively impacting the economic efficiency of syngas production. Summary of the Invention

[0011] The object of the present invention is to provide a similar method and an apparatus for implementing the method, both of which can overcome the aforementioned disadvantages of the prior art.

[0012] In terms of method, the solution of the present invention to achieve this objective is that at least a portion of the water- and carbon dioxide-free nitrogen-poor natural gas is used as a regeneration gas during regeneration in the first temperature-switching adsorption device and / or the second temperature-switching adsorption device before being thermochemically converted.

[0013] Nitrogen-poor natural gas, free of water and carbon dioxide, is suitable as a regeneration gas because it is mainly composed of methane. TWA used for the separation of water and / or carbon dioxide does not adsorb or only adsorbs very small amounts of methane, especially at the temperatures present in the adsorber during regeneration.

[0014] It is well known that adsorbents used for separating water and carbon dioxide can catalytically decompose unsaturated hydrocarbons, especially at higher temperatures, resulting in undesirable carbon deposition. Although nitrogen-poor natural gas containing no water and carbon dioxide actually does not contain unsaturated hydrocarbons, and the risk of decomposition of saturated hydrocarbons is quite low, it is recommended that the temperature for TWA regeneration according to the present invention be limited to between 170°C and 230°C, preferably between 170°C and 200°C.

[0015] If all nitrogen- and carbon dioxide-containing natural gas intended for syngas production is used to produce lean nitrogen natural gas free of water and carbon dioxide, the amount of this natural gas is usually sufficient to meet the regeneration gas requirements of the first and second TWAs. Thus, there is no need to provide another regeneration gas, such as nitrogen. Excess lean nitrogen natural gas free of water and carbon dioxide, which is not required for TWA regeneration, can be directly sent to thermochemical conversion. However, it is also possible to produce only the necessary lean nitrogen natural gas free of water and carbon dioxide from the nitrogen-containing natural gas as regeneration gas. Preferably, after necessary separation of mercury and / or sulfur components, the portion of the nitrogen- and carbon dioxide-containing natural gas that is not intended for water and / or carbon dioxide separation is directly sent to thermochemical conversion.

[0016] During the adsorber regeneration process, desorbed substances, primarily water and carbon dioxide, are loaded onto the anhydrous and carbon dioxide-free nitrogen-lean natural gas used as the regeneration gas in the thermochemical converter (TWA). It is advantageous to send the loaded regeneration gas along with the desorbed substances from the adsorber regeneration process to the thermochemical converter without altering its chemical composition. For this purpose, either the adsorber regeneration is performed slowly, allowing only small amounts of water and carbon dioxide to enter the thermochemical converter via the regeneration gas path, where their impact on the syngas composition is negligible, or, since water and carbon dioxide are required as reactants or temperature regulators in the thermochemical converter and are fed into the converter anyway, an alternative process variant is proposed to control the amount of water and carbon dioxide entering the converter to ensure that the amount always meets process requirements, regardless of the amount of regeneration gas loaded.

[0017] If the nitrogen-lean natural gas, free of water and carbon dioxide, is to be used for the regeneration of the first and second temperature-switched adsorption (TWA) units, the portion of the nitrogen-lean natural gas designated as regeneration gas is preferably divided into a first and a second stream. The first stream is specifically supplied to the first TWA as regeneration gas, and the second stream is specifically supplied to the second TWA as regeneration gas. To minimize flow loss, it is reasonable to supply each TWA only the minimum amount of regeneration gas required.

[0018] However, it is also possible to provide all of the nitrogen-lean natural gas, which is free of water and carbon dioxide and is designated as the regeneration gas, to regenerate each of the two TWAs. The natural gas is pre-loaded with the desorbed material in one of the two TWAs and then used for the regeneration of the adsorber in the other TWA.

[0019] For economic and technical reasons, carbon dioxide separation via TWA is only meaningful when the carbon dioxide content of the gas mixture to be treated does not exceed a maximum value typically of 1 mol%. If the carbon dioxide content of the nitrogen- and carbon dioxide-containing natural gas exceeds a level that can be reasonably separated in TWA, one technical solution according to the method of the present invention proposes to reduce the carbon dioxide content of the nitrogen- and carbon dioxide-containing natural gas upstream of the TWA to below the aforementioned maximum value. For this purpose, it is preferable to perform acid gas scrubbing, such as amine scrubbing, on the nitrogen- and carbon dioxide-containing natural gas. The carbon dioxide separated here can either be released into the atmosphere or sent for material recovery. The separated carbon dioxide is preferably used for downstream thermochemical conversion.

[0020] The method according to the invention is largely independent of the type of thermochemical conversion. For example, nitrogen-poor natural gas containing no water and carbon dioxide can be converted into crude syngas through autothermal reforming, partial oxidation, steam reforming, or a combination of at least two of these methods.

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

[0022] In terms of the apparatus, the solution of the present invention to achieve the aforementioned objective is as follows: the cryogenic gas separator is connected to the thermochemical converter via the first and second temperature-switching adsorption devices, so that at least a portion of the water- and carbon dioxide-free nitrogen-poor natural gas can be used as regeneration gas during the regeneration of the first and / or second temperature-switching adsorption devices before it is converted in the thermochemical converter.

[0023] The connections between the first and second TWAs and the thermochemical converter are preferably designed such that a portion of the nitrogen-lean natural gas used as regeneration gas, free of water and carbon dioxide, can be fed into the thermochemical converter along with the substances desorbed during adsorber regeneration. Specifically, this connection does not include devices for separating water and / or carbon dioxide from the regeneration gas used.

[0024] In a preferred embodiment of the apparatus according to the invention, the cryogenic gas separator is connected to both the first and second TWAs, such that a first portion of the anhydrous and carbon dioxide-free nitrogen-lean natural gas designated as regeneration gas is available as regeneration gas only in the first TWA, and a second portion is available as regeneration gas only in the second TWA. This connection reasonably includes a distributor by which the mass flow rate of the anhydrous and carbon dioxide-free nitrogen-lean natural gas can be adjusted according to the current regeneration gas requirements of the two TWAs.

[0025] The following connection method is also feasible: a cryogenic gas separator is connected in series with two TWAs, so that all the anhydrous and carbon dioxide-free nitrogen-lean natural gas designated as regeneration gas can be fed into one TWA first, and then into the other TWA. In this case, a reasonable approach is to directly connect only the last TWA in the flow direction to the thermochemical converter, so that the anhydrous and carbon dioxide-free nitrogen-lean natural gas used as regeneration gas in both TWAs and loaded with desorbed substances during adsorber regeneration can be introduced into the thermochemical converter.

[0026] If the carbon dioxide content in the nitrogen- and carbon dioxide-containing natural gas exceeds a level that can be reasonably separated in a thermochemical reactor (TWA), the apparatus according to the invention is designed to be installed upstream of the first TWA for separating most of the carbon dioxide from the natural gas. This apparatus is preferably an acid gas scrubber, such as an amine scrubber. This carbon dioxide separation device is rationally connected to the thermochemical converter so that the carbon dioxide separated from the natural gas can be fed into the thermochemical converter as feed.

[0027] A variant of the apparatus according to the invention provides a bypass line through which a portion of nitrogen- and carbon dioxide-containing natural gas, which is not required to generate regeneration gas for the two TWAs, can be directly fed into the thermochemical converter after mercury and / or sulfur components are separated as appropriate, bypassing the first TWA, the cryogenic gas separator, and a carbon dioxide separation device as appropriate.

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

[0029] The present invention will now be explained in more detail with reference to the embodiments illustrated in FIG1.

[0030] Figure 1 illustrates a variant of the invention in which nitrogen-lean syngas (Oxogas) is produced from natural gas containing nitrogen and carbon dioxide. Implementation

[0031] Natural gas containing nitrogen and carbon dioxide is fed into purification unit R via pipeline 1, where substances such as mercury are separated in the first purification step. The treated natural gas 2 is then sent to acid gas scrubbing W1, such as amine scrubbing, to remove most of the contained carbon dioxide 3. With the carbon dioxide content reduced, the natural gas flows into the first TWA T1 via pipeline 4, where water and remaining carbon dioxide are separated, producing nitrogen-containing natural gas 5 free of water and carbon dioxide. This natural gas is then separated in a cryogenic gas separator N into nitrogen-lean natural gas 6 free of water and carbon dioxide and a nitrogen-rich combustible gas fraction 7. After being pressurized in compressor P1, the nitrogen-lean natural gas 8 free of water and carbon dioxide is divided into a first fraction 9 and a second fraction 10. The first fraction 9 is used as regeneration gas for the adsorber regeneration in the first TWA T1, while the second fraction 10 is sent downstream to the second TWA T2 for gas drying for the same purpose. Two regenerated gas streams, 11 and 12, containing desorbed water and desorbed carbon dioxide, are returned and fed into the thermochemical converter K via pipeline 13 as natural gas feed. In the thermochemical converter K, the natural gas feed 13 reacts with steam 14, carbon dioxide 15, and possibly oxygen 22 to produce crude syngas 16 containing hydrogen, carbon monoxide, water, and carbon dioxide. This crude syngas is cooled in cooling unit G and then sent via pipeline 17 to further acid gas scrubbing W2 to separate carbon dioxide 18. This acid gas scrubbing can also be performed as amine scrubbing. In the second TWA T2, the crude syngas 19, with reduced carbon dioxide content, is cleaned of water and residual carbon dioxide to obtain nitrogen-free syngas 20, primarily composed of hydrogen and carbon monoxide, as the product. The carbon dioxide 18 separated from the cooled crude syngas 17 is mixed with carbon dioxide 3 separated from natural gas 2 and input carbon dioxide 21 to increase carbon monoxide production, and then returned to the thermochemical converter K via the second compressor P2 and pipeline 15 as feed.

[0032] 1: Pipeline 2: Natural Gas 3: Carbon dioxide 4: Pipelines 5: Nitrogen-containing natural gas that contains no water or carbon dioxide 6: Nitrogen-poor natural gas containing no water or carbon dioxide 7: Nitrogen-rich combustible gas portion 8: Nitrogen-poor natural gas containing no water or carbon dioxide 9: First Diversion 10: Second Diversion 11: Regenerated airflow 12: Regenerated airflow 13: Pipeline / Natural Gas Feed 14: Steam 15: Carbon Dioxide / Pipeline 16: Crude Syngas 17: Pipeline / Cooled crude syngas 18: Carbon dioxide 19: Crude syngas with reduced carbon dioxide content 20: Syngas 21: Input carbon dioxide 22: Oxygen G: Cooling device K: Thermochemical converter N: Cryogenic gas separator P1: Compressor P2: Second compressor R: Purification device T1: First TWA T2: Second TWA W1: Acidic gas scrubbing W2: Further acid gas scrubbing

Claims

1. A method for producing nitrogen-lean syngas (20) from natural gas (1) containing nitrogen and carbon dioxide, wherein, Water and carbon dioxide are separated from the natural gas in a first temperature-switching adsorption unit (T1), and then nitrogen (7) is separated from the natural gas in a cryogenic gas separator (N), thereby producing nitrogen-lean natural gas (6) that is free of water and carbon dioxide. The natural gas is then sent to a thermochemical conversion (K) to obtain crude syngas (16) containing hydrogen, carbon monoxide, water and carbon dioxide. The nitrogen-lean syngas (20) is obtained from the crude syngas by separating water and carbon dioxide at least in a second temperature-switching adsorption unit (T2). The nitrogen-lean natural gas (6) free of water and carbon dioxide is characterized in that at least a portion of the nitrogen-lean natural gas (6) is used as regeneration gas (9, 10) during regeneration in the first temperature-switching adsorption unit (T1) and / or the second temperature-switching adsorption unit (T2) before being regenerated by the thermochemical conversion (K).

2. The method as claimed in claim 1, characterized in that a portion of the water- and carbon dioxide-free nitrogen-poor natural gas used as regeneration gas (11, 12) is sent to the thermochemical conversion (K) together with the substances desorbed during the regeneration of the adsorber.

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

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

5. The method as described in any one of claims 1 or 2, characterized in that the thermochemical conversion (K) is carried out as autothermal reforming or partial oxidation or steam reforming or as a combination of at least two of these methods.

6. An apparatus for producing lean nitrogen syngas (20) from natural gas (1) containing nitrogen and carbon dioxide, comprising: A first temperature-switching adsorption unit (T1) is used to separate water and carbon dioxide from the natural gas (1) containing nitrogen and carbon dioxide to obtain nitrogen-containing natural gas (5) free of water and carbon dioxide; a cryogenic gas separator (N) is used to obtain nitrogen-lean natural gas (6) free of water and carbon dioxide from the nitrogen-containing natural gas (5) free of water and carbon dioxide by separating nitrogen (7); a thermochemical converter (K) is used to convert the nitrogen-lean natural gas (6) free of water and carbon dioxide into crude syngas (16) containing hydrogen, carbon monoxide, water and carbon dioxide; and a second temperature-switching adsorption unit (T1) is used to separate water and carbon dioxide from the natural gas (1) containing nitrogen (7); The attached device (T2) is used to separate water and carbon dioxide from the crude syngas (16) to obtain the nitrogen-poor syngas (20), characterized in that the low-temperature 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 of the water- and carbon dioxide-free nitrogen-poor natural gas (6) can be used as regeneration gas (9, 10) during the regeneration of the first temperature-switching adsorption device (T1) and / or the second temperature-switching adsorption device (T2) before it is converted in the thermochemical converter (K).

7. The apparatus as claimed in claim 6, characterized in that the first temperature-switching adsorption device (T1) and the second temperature-switching adsorption device (T2) are both connected to the thermochemical converter (K), such that a portion of the nitrogen-poor natural gas containing no water and carbon dioxide used as regeneration gas (11, 12) can be fed into the thermochemical converter (K) together with the substances desorbed during the regeneration of the adsorber.

8. The apparatus as claimed in any one of claims 6 or 7, 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 nitrogen-poor natural gas, which is designated as the regeneration gas and is free of water and carbon dioxide, can be used as the regeneration gas only in the first temperature-switching adsorption device (T1), and the second portion (10) can be used as the regeneration gas only in the second temperature-switching adsorption device (T2).

9. The apparatus as claimed in any one of claims 6 or 7, characterized in that the apparatus includes a device (W1) for separating carbon dioxide disposed upstream of the first temperature-switching adsorption device (T1), by means of which carbon dioxide (3) present in the natural gas (1) containing nitrogen and carbon dioxide can be separated.

10. The apparatus as claimed in any one of claims 6 or 7, characterized in that the thermochemical converter (K) is 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.

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