Bayonet-tube exchanger reformer

Abstract

A shell-and tube apparatus (1) for a reforming process of a hydrocarbon feedstock, said apparatus comprising a plurality of bayonet tubes (2) where the reforming process is performed, wherein each bayonet tube (2) includes an outer tube (3), a first inner tube (4) and a second inner tube (5) in a coaxial arrangement; the reforming process is performed in an outer annular passage (11) between the outer tube (3) and the first inner tube (4); the reformed gas is collected in a central channel (8) delimited by the inside of the second inner tube (5); an annular interspace (9) between the first inner tube and the second inner tube provides a thermal insulation between the reforming gas in the outer annular passage and the product gas in the central channel.

Description

[0001] Bayonet-tube exchanger reformer

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention is in the field of multi-tubular reformers for the production of reformed gas. The invention particularly concerns an exchanger reformer with bayonet-type tubes.

[0005] Prior art

[0006] The industrial production of several gas or chemical products of interest, such as hydrogen, methanol or ammonia, typically starts with the production of a hydrogen-containing synthesis gas (“syngas"). The term syngas denotes a gaseous mixture comprising carbon monoxide (CO), hydrogen and minor amounts of carbon dioxide (CO2) and methane (CPU).

[0007] Syngas is obtained by treating a hydrocarbon-containing feedstock, typically natural gas and steam, in a reforming section which may include an exchangerreformer. Natural gas reforming is a highly endothermic reaction which requires a heat input. In a heat exchanger reformer, said heat input is provided by indirect heat transfer from a hot medium, typically a hot gas.

[0008] A common type of exchanger reformer includes a plurality of bayonet type tubes filled with catalyst and internally traversed by the reagent gas. Said bayonet tubes are attached to the pressure vessel of the reformer by means of one or more plates that can be sealed at the top and / or at the bottom of said pressure vessel. The heating medium is fed to the shell side of the reformer, so that the tubes are externally heated by said medium.

[0009] A challenge of the exchanger reformers known in the art is to reduce the corrosion due to metal dusting. Metal dusting is an aggressive type of corrosion that may arise when a hot gas with high carbon content is cooled below certain critical temperature values. Particularly, in the reforming of natural gas, the product gas (syngas) includes a relevant amount of CO and therefore of carbon. Acrossbayonet tube walls, the reacted gas cools down by transferring heat to the reactant gas, resulting in a high risk of metal dusting and subsequent significant mechanical problems including potential catastrophic corrosion.

[0010] More specifically, a conventional bayonet tube includes basically an outer tube and an inner tube arranged coaxially. The reforming reaction takes place in the annular space between the outer tube and the inner tube, and the reacted gas is collected in the central channel delimited by the inner tube. A drawback of this configuration is that the reacted gas in the inner tube, containing significant amounts of CO, is cooled by the gas reacting in the annular passage around the inner tube. This leads to increased risk of metal dusting.

[0011] To avoid corrosion-related problems, conventional exchanger reformers include portions made of metal dusting resistant materials, typically materials with a high content of nickel also named Inconel-based alloys; additionally, a special aluminizing process can be applied to provide higher resistance to metal dusting process. However, said Inconel-based alloys and the aluminizing process are expensive and their cost significantly impacts the overall cost of the exchanger reformer.

[0012] US 2020 / 406212 discloses a chemical reactor comprising reformer tubes. EP 1138 630 discloses a reforming apparatus and a seal for use in the apparatus. US 2021 / 001297 discloses a bayonet catalytic reactor. US 9,227,844 B2 discloses a heat exchange reformer with double-tubes.

[0013] Summary of the invention

[0014] The invention aims to overcome the above drawbacks of the prior art. In particular, the present invention addresses the problem of how to make an exchanger reformer with bayonet-type tubes less sensitive to metal dusting, without the need to use expensive materials and / or without implementing special treatments such as aluminizing process.

[0015] The invention solves this problem with an apparatus according to the claims. The invention proposes a novel design of the bayonet tubes to reduce the heattransfer between the feedstock and the reacted gas.

[0016] Each of the bayonet tubes is configured as follows: the bayonet tube includes an outer tube, a first inner tube and a second inner tube. Said outer tube, first inner tube and second inner tube are coaxially arranged and the second inner tube is innermost relative to the first inner tube. Accordingly, an outer annular passage is delimited between the outer tube and the first inner tube; an inner annular interspace is delimited between the first inner tube and the second inner tube; a central channel is delimited by the inside of the second inner tube.

[0017] The reforming reaction occurs in the outer annular passage which, during operation, is filled with a reforming catalyst. The central channel collects the reacted gas and is connected to the gas outlet.

[0018] The bayonet tube has a first end connected to the feedstock inlet and to the gas outlet and a second end opposite to said first end. The second end of the tube is configured to direct the reacted gas effluent from the outer annular passage into the central channel while preventing gas passage into / from the inner annular interspace. Accordingly, the second end of the bayonet tube is suitably arranged to prevent passage of the reacted gas into the inner annular interspace and to prevent a gas passage from said inner annular interspace into any of the outer annular interspace or the central channel.

[0019] The inner annular interspace provides a thermal insulation between the outer annular passage and the central channel, thus reducing the heat transfer from the reformed raw syngas to the reacting gas flowing within the annular passage. In other terms, the invention provides a double-wall separation between the passage of the reacting gas and the passage of the reacted gas, reducing the undesired heat transfer between them.

[0020] Accordingly, the risk of cooling the reformed raw syngas flowing in the central passage below the metal dusting temperature is reduced, thus reducing the risk of metal dusting of at least the second inner tube. Advantageously, the use of expensive materials resistant to metal dusting such as alloys with high content ofNickel (Ni > 50%) and Chromium (Cr > 20%) is not necessary for avoiding metal dusting problems.

[0021] The prevention of gas passage into or from the inner annular interspace avoids that reformed gas enters said inner annular interspace, or that fresh gas bypasses the outer annular passage, according to different embodiments.

[0022] In some embodiments, the configuration of the second end of bayonet tubes prevents the reacted gas from entering the inner annular interspace instead of the central channel. In some embodiments, however, the inner annular interspace may be connected to the feed gas inlet, and said interspace may be filled by the feed gas during operation. In such case, the pressure drop through the annular interspace would be generally lower than the pressure drop through the outer annular passage filled with catalyst; accordingly, said configuration of the second end of the tube prevents the feed from completely or partially bypassing the catalyst in the outer annular passage.

[0023] Description of the invention

[0024] The apparatus of the invention is a shell-and-tube apparatus for reforming of a hydrocarbon-containing feed gas (hydrocarbon feedstock). The apparatus provides a suitable residence time at a suitable temperature, in contact with a suitable catalyst, to said hydrocarbon feedstock that is reformed to produce a reformed raw syngas containing hydrogen, carbon monoxide and traces of other compounds as carbon dioxide and water.

[0025] The apparatus of the invention has bayonet tubes with the inventive structure including an outer tube, a first inner tube and a second inner tube. A conventional exchanger-reformer includes bayonet tubes typically including an outer tube and only one inner tube. The presence of a second inner tube and an annular interspace between the two inner tubes, in accordance with the invention, provides an increased thermal insulation that results in less cooling of the reformed syngas flowing within the central passage internal to the second inner tube.Each tube has a first end connected to gas inlet / outlet, and a second end (distal end) opposite to the first end. Said second end of the bayonet tube is configured to prevent gas passage into / from the annular interspace. According to various embodiments, this configuration may partially or totally seal the annular interspace at the second end of the bayonet tube. This configuration prevents undesired gas passage which may be undesired flow of reacted gas into the annular interspace (instead of the central channel), or undesired flow of feed gas through the annular interspace bypassing the catalyst-containing outer passage. Said configuration may include closing the end of the inner annular interspace or providing the end of the inner annular interspace with flow prevention means such as labyrinths or packings that substantially prevent the gas flow.

[0026] The reacted gas, effluent from the outer annular passage, is therefore collected in the central channel. In embodiments where the inner annular interspace is connected with the feed inlet, so that the fresh gas acts as thermal insulation medium, the feed gas is prevented from bypassing the catalyst-containing outer annular passage flowing in the inner annular space. Preferably said flow prevention means are configured so that a bypass, if any, is not greater than 1% of the volumetric flow rate of the feed gas.

[0027] The inner annular interspace can be filled with a gas acting as a thermal insulation medium. Preferably, as mentioned above, said annular interspace is in a fluid communication with the feed gas inlet, so that the feed gas fills the annular interspace during operation and acts as thermal insulation medium. The feed gas typically does not contain relevant amounts of carbon monoxide. Thus, advantageously, it can be easily used as thermal insulation medium without creating conditions favourable for metal dusting.

[0028] In other embodiments, a different medium and / or a suitable layer of a thermal insulation material may be provided in the annular interspace.

[0029] When the feed gas fills the annular interspace, it is preferably introduced in the annular interspace from a top end of the bayonet tube, while the other end is closed by a welded cap or other sealing systems such as flanges, labyrinths, orpackings.

[0030] The inner annular interspace provides thermal insulation and no reaction occurs in said interspace. Particularly, no reforming of the feed gas occurs in said inner annular interspace.

[0031] According to a preferred embodiment, the feed gas flows in the annular passage in counter current with respect to the reformed gas flowing in the central passage so that the feed inlet is at the same side of the outlet of the reformed gas.

[0032] Preferably, the apparatus of the invention is vertically arranged so that the feed gas flows downstream in the annular passage during the reforming reaction.

[0033] A preferred application of the apparatus of the invention concerns the reforming of natural gas. The involved reforming reaction is strongly endothermic and the heat required by the reforming reaction can be supplied by a hot gas flowing in the shell side of the apparatus. Said hot gas is preferably a flue gas produced for example during a combustion process. In certain embodiments, the heat required by said reforming reaction may be provided by a gas, such as a CO-containing gas, that may potentially induce metal dusting to the external surface of the outer tube. In such case, either a proper material or a proper treatment (for instance an aluminizing process) shall be applied to the external surface of the outer tube.

[0034] In an embodiment, the shell side of the apparatus is in communication with a line transporting a hot gas, preferably a flue gas, so that said hot gas or flue gas is used as heating medium. Said gas flows in the shell side of the apparatus and heats the feed gas in the outer annular passage of the bayonet tubes.

[0035] The reformed gas flowing in the central passage is CO-rich, containing for example 5% to 30% by weight of CO, and has a high temperature preferably higher than 700 °C. In such conditions, corrosion by metal dusting may occur. The apparatus of the invention reduces this undesired corrosion phenomena, compared to the equipment of the prior art, thanks to the presence of said annular interspace that reduces the heat transfer from the reacting gas to the reformed gas, and thus reduces the risk that the reformed gas cools down below the metaldusting temperature.

[0036] The first inner tube and the second inner tube may be supported according to various embodiments. In a first embodiment, the first inner tube and the second inner tube are fixed laterally to the outer tube; in a second embodiment, the first inner tube and the second inner tube are fixed, in correspondence of their bottom end, to the outer tube; in a third embodiment the inner tube lays on the end cap of the bayonet tube without being fixed laterally to the outer tube. Preferably, the first inner tube and the second inner tube are not fixed to the outer tube in correspondence of their upper end.

[0037] The embodiment with the inner tubes laying on the end cap of the bayonet tube provides some relevant advantages.

[0038] A first advantage is that it allows filling the annular interspace with feed gas and not with the reacted gas that could lead to metal dusting.

[0039] A second advantage is that the first inner tube has a temperature close to the temperature the outer tube, sue to the annular interspace that separates the first inner tube from the reacted raw syngas. Thus, the differences in thermal expansion between the outer tube and the first inner tube are reduced, leading to a reduced relative movement between said tubes.

[0040] Said relative movement between outer tube and inner tube may be further reduced thanks by fixing the first inner tube to the outer tube in correspondence to the bottom of the outer tube. This is particularly advantageous in a vertically arranged apparatus where the catalyst in the bottom part of the tube is subject to higher pressure and is more constrained by the above column of catalyst, and thus the differential movement of the enclosing walls is more likely to cause a damage.

[0041] The reduced relative movement at the bottom of the tube provides a further significant advantage related to a limited increase of pressure drops caused by possible damage and pulverization of the catalyst.Description of the figures

[0042] Fig. 1 is a schematic sectional view of a heat exchanger reformer according to an embodiment of the invention.

[0043] Fig. 2 is a detail of Fig. 1 showing a scheme of the lower portion of a bayonet tube.

[0044] Fig. 1 illustrates a shell-and tube exchanger-reformer 1 including: a pressure vessel 20; a bundle of bayonet tubes 2; an inlet 21 for a reformable feed gas 6, for example a mixture of natural gas and steam; an outlet 22 for a reformed gas 7 ; an inlet 23 and an outlet 24 for a hot gas G. The bayonet tubes 2 are suspended to a tubesheet 25. The reformer may be fitted with internal baffles or support to prevent the tubes from vibration or improve heat exchange, according to the need.

[0045] The feed gas 6 entering the inlet 21 is distributed in the bayonet tubes 2. The reforming process takes place in an annular portion of the tubes, filled with catalyst. The reformed gas is collected at the bottom of each tube 2 and returns upward through a central portion of each tube, until it reaches a collection chamber in communication with the outlet 22. The tubes 2 are externally heated by the hot gas G traversing the shell side of the reformer 1.

[0046] Each tube 2 has a first end 201 and a second end 202 opposite to said first end.

[0047] Fig. 2 illustrates the structure of the bayonet tubes 2 in greater detail, with reference to the lower portion 2a of a bayonet tube.

[0048] The bayonet tube 2 includes an outer tube 3, a first inner tube 4 and a second inner tube 5. The tubes 3, 4, 5 are coaxially arranged and the second inner tube 5 is internal, e.g. it has a lower diameter, with respect to the first inner tube 4.

[0049] An outer annular passage 11 is delimited between the outer tube 3 and the first inner tube 4. A central channel 8 is delimited by the second inner tube 5. An annular interspace 9 is delimited between the first inner tube 4 and the second inner tube 5.Said annular interspace 9 separates the outer annular passage 11 from the central channel 8. Fig. 2 is not to scale and the width of the annular interspace 9 is exaggerated for illustration purpose.

[0050] A reforming catalyst 10 is packed in the annular passage 11 so that the hydrocarbon feedstock 6 is reformed and a raw syngas 7 is produced.

[0051] The bottom end 202 of the bayonet tube 2 is arranged to direct the reacted gas 7 from the annular passage 11 to the central channel 8, avoiding the reacted gas 7 from entering the interspace 9, or feed gas from bypassing the annular passage 11 flowing thorough the interspace 9. In the shown embodiment, the bottom end 202 is closed by a bottom cover 30 defining a bottom chamber 31. Said bottom chamber 31 is in communication with the annular passage 11 and the central channel 8.

[0052] The raw syngas 7 flows in the central channel 8 through the bottom chamber 31 and upward to the outlet 22.

[0053] The presence of the annular interspace 9 between the tubes 4, 5 provides a thermal insulation and reduces the undesired heat exchange from the reacted gas 7 in the channel 8 to the gas in the annular passage 11. Accordingly, the insulation given by the interspace 9 reduces the cooling of the reacted syngas 7 and the related risk of metal dusting.

[0054] The interspace 9 may contain a gas to act as insulation medium. Preferably the interspace 9 is filled with some of the hydrocarbon feedstock (unreacted gas).

[0055] The annular interspace 9 is preferably closed at its bottom end by a bottom cover 32 preventing the feedstock 6 to bypass the catalyst in the annular passage 11 by flowing in said interspace 9. Preferably said interspace 9 is open at its upper end allowing the hydrocarbon feedstock 6 to enter and stagnate within said interspace 9.

[0056] The presence of said interspace 9 provides an increase in the resistance to heat exchange between the raw syngas 7 flowing through the central channel 8 andthe hydrocarbon feedstock 6 flowing through the annular passage 11. Such an increase of heat transfer resistance results in a significantly reduced cooling of the syngas 7 and thus in a reduced risk of metal dusting.

[0057] The first inner tube 4 and the second inner tube 5 are preferably attached to the bottom of the outer tube 3, leading to a limited thermal expansion difference between the tubes 4, 5 at the bottom, where a damage of the catalyst is more likely to occur due to its movement restrained by the above catalyst.

[0058] A catalyst containing device, permeable to the gas but not to the catalyst, may be provided at the bottom of the outer tube 3. Said device may also be used to fix the inner tubes 4 and 5. Alternatively, the inner tubes 4, 5 can be supported by the bottom cover 30.

Claims

CLAIMS1 ) A shell-and tube apparatus (1 ) for a reforming process of a hydrocarbon- containing feed gas, said apparatus comprising a pressure vessel (20); a plurality of bayonet tubes (2); a shell side delimited by the pressure vessel around the bayonet tubes; an inlet (21 ) for a reformable feed gas (6); an outlet (22) for reformed gas (7);wherein the bayonet tubes (2) are in communication with said feed gas inlet (6) and with said gas outlet (7) and the shell side is connected to an inlet (23) and outlet (24) for a heating medium (G);wherein each bayonet tube (2) includes:an outer tube (3);a first inner tube (4) and a second inner tube (5);said outer tube (3), first inner tube (4) and second inner tube (5) being coaxially arranged, wherein the second inner tube (5) is innermost relative to the first inner tube (4);so that an outer annular passage (11 ) is delimited between the outer tube (3) and the first inner tube (4); an inner annular interspace (9) is delimited between the first inner tube (4) and the second inner tube (5); a central channel (8) is delimited by the inside of the second inner tube (5);the bayonet tube (2) having a first end portion (201 ) where the tube is connected to the feed inlet (21) and to the gas outlet (22), and a second end portion (202) opposite to said first end portion;said outer annular passage (11) is in communication with the feed inlet (21) and during operation is filled with a reforming catalyst (10), so that the reforming reaction occurs in said annular passage (11) traversed by feed gas;said central channel (8) is connected to the gas outlet (22);the second end (202) of the tube (2) is configured to direct the reacted gas (7) effluent from the outer annular passage (11) into the central channel (8);said second end (202) of the tube is further configured to prevent passage of the reacted gas (7) into said inner annular interspace (9) and to prevent a gas passage from said inner annular interspace (9) into the outer annular interspace (11) or into the central channel (8).2) An apparatus according to claim 1 , wherein each bayonet tube (2) includes an end cap (30) arranged at the second end (202) of the bayonet tube and defining an end chamber (31 ) of the tube, wherein said end chamber (31 ) is in fluid communication with the outlet of the outer annular passage (11 ) and with an inlet of the central channel (8), so that reacted gas (7) effluent from the outer annular passage (11) can flow into the end chamber (31) and from the end chamber into the central channel (8), wherein the second end (202) of the tube (2) is configured to prevent gas passage between the annular interspace (9) and the end chamber (31).3) An apparatus according to claim 1 or 2 wherein the outlet end of the inner annular interspace (9) is sealed or is fitted with gas flow prevention means such as labyrinths or packings.4) An apparatus according to any of claims 1 to 3 wherein said inner annular interspace (9) is filled with a thermal insulation medium.5) An apparatus according to any of the previous claims wherein said inner annular interspace (9) is connected to the inlet (21 ) of the feed gas (6), so that during operation said interspace (9) is filled with feed gas.6) An apparatus according to any of the previous claims, said apparatus being vertically arranged and the bayonet tubes (2) being vertical.7) An apparatus according to any of the previous claims wherein the bayonet tubes (2) are configured so that the feed gas (6) flows in the outer annular passage (11) in counter current with respect to the reformed gas (7) flowing in the central channel (8).8) An apparatus according to any of the previous claims wherein the shell side of the apparatus is in communication with a line transporting a hot gas, preferably a flue gas, so that said hot gas or flue gas is the heating medium (G), flowing in the shell side of the apparatus (1) and heating the feed gas in the outer annular passage (11 ) of the bayonet tubes (2).9) An apparatus according to any of the previous claims wherein the first inner tube (4) and second inner tube (5) are fixed laterally or in correspondence of their bottom end to the outer tube (3).10)An apparatus according to any of claims 1 to 8 wherein the inner tubes (4, 5) lay on an end cap (30) of the bayonet tube (2) without being fixed laterally.11)An apparatus according to any of the previous claims wherein the first inner tube (4) and the second inner tube (5) are not fixed to the outer tube (3) in correspondence of their upper end.

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