Burner for a reforming reactor

Abstract

Burner (1) for a reformer (11), the burner including an oxidizer pipe (4), a process gas channel (5) arranged coaxially around the oxidizer pipe, a plurality of gas distribution plates (121, 122) in the process gas channel, each of said plates being perforated with a pattern of holes, wherein said gas distribution plates are arranged so that the holes (131, 132) of consecutive gas distribution plates are staggered in the direction of flow.

Description

[0001] Burner for a reforming reactor

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention relates to a novel design of a burner for a reforming reactor, particularly for a secondary reformer.

[0005] Prior art

[0006] A known design of reforming reactor includes a combustion chamber where a process gas to be reformed is contacted with an oxidizer. The reforming reactor may include a catalytic bed downstream the combustion chamber. The process gas is a combustible gas such as, for example a partially oxidized gas resulting from a previous step of primary reforming. The oxidizer may be air, enriched air or pure oxygen. The mixture of process gas and oxidizer is heated by the combustion process to a temperature suitable for the catalytic reaction. Noticeable examples of such reforming reactors include the secondary reformers used in the reforming of hydrocarbons for the production of hydrogen or ammonia.

[0007] A burner of a reforming reactor is a device designed to introduce said process gas and said oxidizer into the combustion chamber. The burner is usually mounted vertically on top of the pressure vessel of the reformer. A known design of the burner includes coaxial pipes for the oxidizer and the process gas, particularly a central pipe for the oxidizer surrounded by an annular channel for the process gas. The oxidizer pipe is normally equipped with a swirler.

[0008] A known design includes one or more perforated plates in the process gas annular channel, to distribute uniformly the process gas in the channel. A very common design has a side inlet of process gas directed perpendicular to the axis of the burner, which means the process gas flow has to be turned 90° to align with the oxidizer flow, and must be suitably distributed across the annular cross section of the process gas channel. To this purpose, a known design uses a process gas distributor consisting of one or two perforated plates in the channel.

[0009] The task of said process gas distributor is essentially to provide a good redistribution of the process gas at the expense of an acceptable pressure drop. Efforts are directed to distribute uniformly the process gas in the channel because of related advantages in terms of well-controlled combustion process and uniform temperature of the gas at the inlet of the catalytic bed located downstream of the burner. Hence, the distribution of the process gas in the annular channel is crucial for the operation of the burner and of the entire reactor.

[0010] For example, a fundamental issue of a secondary reformer fed with natural gas is the methane slip, that is unconverted methane that escapes the catalytic bed. Said methane slip is highly undesired, being a waste of fuel and possibly affecting the operation of downstream equipment. It has been found that a uniform distribution of temperature in the gas gives a higher conversion of methane and, consequently, reduces the methane slip. As stated above, a uniform temperature of gas requires a good distribution of the process gas, so the design of the burner is of great importance.

[0011] A parameter representative of the quality of the process gas distribution is the temperature spread on the surface of the downstream catalyst. In a state-of-the- art secondary reformer, this spread is around 150 to 200 °C with two perforated plates in the process gas channel inducing a pressure drop of 0.4 - 0.5 bar.

[0012] EP 1 680 355 describes a burner with an oxidizer pipe arranged coaxially within a process gas annular channel. EP 2 811 228 discloses a burner for secondary reformers with two perforated plates arranged to balance the process gas flow. WO 2004 / 112954 discloses a cooled apparatus for secondary reforming and WO 2014 / 111315 discloses an autothermal reforming reactor. Summary of the invention

[0013] The invention derives from the continuous effort to improve the above-described design of a burner. Particularly, the invention aims at solving the problem of how to improve the distribution of the process gas at the outlet of the burner.

[0014] The problem is solved with a burner according to the claims.

[0015] The burner of the invention has a plurality of gas distribution perforated plates arranged in the process gas channel, wherein the holes of consecutive gas distribution plates are not axially aligned. It has been surprisingly found that staggering the holes of the distribution plates has the effect of providing a better distribution of the process gas while pressure drop is kept low.

[0016] Description of the invention

[0017] The invention is directed to burners including an oxidizer pipe and a process gas channel having an annular shape and arranged coaxially around the oxidizer pipe. Two or more gas distribution plates are arranged in the process gas channel and each of said plates is perforated with a suitable pattern of holes to allow passage of the process gas. The preferred number of gas distribution plates is two, however a greater number may be provided in some embodiments.

[0018] The gas distribution plates form a sequence from a first gas distribution plate to a last gas distribution plate in the flow direction of the gas channel. An upstreamdownstream direction is identified by the direction of flow of the process gas. The term consecutive denotes plates arranged next to each other in said direction of flow of the process gas. Any pair of consecutive distribution plates therefore includes an upstream plate and a downstream plate. In most embodiments the burner is mounted vertically and therefore the upstream plate can be termed top plate or upper plate and the downstream plate can be termed bottom plate or lower plate. Said gas distribution plates are arranged so that the holes of consecutive gas distribution plates are not axially aligned. The staggered arrangement of the holes provides that the holes of the downstream plate are at least partially located in the so-called blind region of the upstream plate. Said term blind region denotes the region downstream which is axially aligned with the non-perforated portion of the upstream plate.

[0019] The staggered arrangement provides interruptions to the preferential paths for the process gas and forces a uniform distribution of the process gas in the annular channel. The applicant has found that this effect of redistribution is surprisingly achieved with a limited pressure drop. In a secondary reformer, the above- mentioned temperature spread can be reduced to about 70 °C with a pressure drop of about 0.3 bar.

[0020] The finding of the present invention is in contrast with the conventional belief that alignment of the holes of consecutive distribution plates would facilitate the gas flow. The applicant has found that staggering of holes forces a redistribution of the flow obtaining a best result in term of uniformity and redirection of the flow from tangential to axial. The optimal effect is achieved with a proper distance between the gas distribution plates, as explained below.

[0021] The holes of the gas distribution plates are preferably circular holes, although gas passages of a different shape are possible. Preferably, each individual gas distribution plate has all holes of the same size, whereas different gas distribution plates may have holes of the same size or of a different size. In case of circular holes, the size is given conventionally by the diameter.

[0022] The applicant has also found that the staggered arrangement of holes is particularly effective in improving the distribution of the process gas if the distance between consecutive gas distribution plates is not greater than a maximum distance. Said maximum distance is in relation with the size of the holes and can be determined as a multiple of a characterizing size of the holes, such as diameter of circular holes. In a preferred embodiment, the distribution plates have circular holes of a given diameter, all the holes of the plates having the same diameter, and the distance between consecutive plates is 8 times said diameter. An even more preferred distance is 3 to 5 times said diameter or 3.5 to 4.5 times said diameter.

[0023] The distance between two consecutive plates can be taken between the outlet surface of the plate upstream and the inlet surface of the plate downstream.

[0024] Alternatively, the distance between consecutive plates can be in relation to the outer diameter of the plates. In that case, preferably, said distance is not greater than 20% of the diameter.

[0025] Two consecutive gas distribution plates can be described as first gas distribution plate (upstream) and second gas distribution plate (downstream). The projection of the holes of the plate upstream may fall partially or entirely in a non-perforated portion of the plate downstream the pattern of holes.

[0026] When viewed in a plane perpendicular to the axis of the oxidizer pipe and annular gas channel, the holes of the first gas distribution plate may have a partial overlap or no overlap with the holes of the second gas distribution plate. In an embodiment, still viewed in said plane, the holes of the first gas distribution plate are tangent to the holes of the second gas distribution plate.

[0027] References to the axis of the oxidizer pipe denotes the main axis which, in most applications, is a vertical axis.

[0028] In a preferred embodiment, for each pair of consecutive first plate and second plate, the holes of the first plate are distributed according to a triangular pitch and the holes of the second plate are in position corresponding to the centres of the triangles defined by the pattern of holes of the first plate.

[0029] In an interesting embodiment, each gas distribution plate is made in a single piece. More preferably, the gas distribution plates are connected to each other by stiffening ribs to form a self-supporting assembly. In other embodiments, the gas distribution plates may have a modular structure.

[0030] The number of gas distribution plates in the process gas channel may vary. In a highly preferred embodiment said number is two, which means there is only one pair of consecutive plates. If more than two plates are provided, different consecutive pairs can be identified, for example in an arrangement of three gas distribution plates one pair is formed by the first and second plate and another pair is formed by the second and third plate.

[0031] In embodiments with two distribution plates, the pressure drop of the process gas passing through upstream gas distribution plate is 25% to 75%, preferably 50%, of the entire pressure drop of the process gas through the two plates.

[0032] The invention also relates to a reactor for reforming a gas feed comprising a combustion chamber and a burner. The reactor is preferably a secondary reformer. The term of secondary reformer denotes a reactor adapted for secondary reforming of a process gas produced after a primary reforming of a hydrocarbon feedstock and steam. An interesting application is production of hydrogen or of a mixture of hydrogen and nitrogen as ammonia make-up gas. In most applications, the burner is installed above the combustion chamber of the reactor so that the oxidizer pipe and the process gas annular channel are arranged vertically.

[0033] Description of the drawings

[0034] The invention is now elucidated with the help of the figures where:

[0035] Fig. 1 is a sketch of a secondary reformer according to an embodiment of the invention.

[0036] Fig. 2 illustrates the process gas distribution plates of the burner of Fig. 1 . Fig. 3 illustrates, in an embodiment, the arrangement of holes of the process gas distribution plates.

[0037] Figs. 1 -2 illustrate a burner 1 mounted on top of a secondary reformer 11 . Said secondary reformer has a combustion chamber 2 and a catalytic bed 3 downstream the combustion chamber. The size of the burner 1 relative to the reformer is exaggerated in the figure.

[0038] The burner 1 includes a pipe 4 for an oxidizer 6 and an annular channel 5 for a combustible process gas 7. The annular channel 5 is coaxial around the pipe 4. The oxidizer 6 may be air, enriched air or oxygen. The process gas 7 may be the effluent of a primary reformer or of a gas-heated reformer upstream the secondary reformer.

[0039] The item 8 denotes a swirler in the oxidizer pipe 4. The main axis 9 of the burner 1 is also illustrated.

[0040] It can be seen that the process gas 7 enters the burner 1 tangentially and must be redirected according to the axis 9.

[0041] Fig. 1 illustrates the burner tip 10. Here, the process gas 7 meets the oxidizer 6 and a combustion flame is formed. The flame heats the mixture to a suitable temperature for the catalytic reforming reaction in the bed 3.

[0042] In the process gas channel 5, a gas distributor 12 is provided. Said gas distributor provides redirection and uniform distribution of the process gas 7 to improve mixing with the oxidizer 6 at the outlet of the burner. Said gas distributor 12 is an assembly including a first perforated plate 121 and a second perforated plate 122. Said perforated plates 121 , 122 are connected by ribs 14 to form the assembly of the gas distributor 12 (Fig. 2).

[0043] The gas distribution plates 121 , 122 have pattern of holes 131 , 132. As seen in Fig. 3, the holes 131 , 132 are staggered so that they are not axially aligned. The holes 132 of the downstream plate 122 are located in the so-called blind region of the upstream plate 121. This feature can be appreciated in Fig. 3 which is a top view in a plane perpendicular to the axis 9. Said Fig. 3 illustrates an embodiment wherein the holes do not overlap. The projections of the holes of one plate, in the axial direction of flow, fall entirely in non-perforated regions of the other plate. In other embodiment the holes 131 , 132 may partially overlap.

[0044] Fig. 2 illustrates also the axial distance d between the plates. Said distance d is taken in the direction of the axis 9 between adjoining surfaces of the plates 121 , 122. The optimum distance d can be determined as a multiple of the size of the holes; in preferred embodiments said distance is 3.5 to 4.5 the diameter of the holes.

Claims

CLAIMS1 ) Burner (1 ) for a reformer, the burner including: an oxidizer pipe (4), a process gas channel (5) having an annular shape and arranged coaxially around the oxidizer pipe (4), a plurality of gas distribution plates (121 , 122) arranged in the process gas channel (5), each of said plates (121 , 122) being perforated with a pattern of holes (131 , 132) to allow gas passage through the plate, wherein said gas distribution plates (121 , 122) are arranged so that the holes of consecutive gas distribution plates are not axially aligned.2) Burner according to claim 1 wherein the holes of the gas distribution plates (121 , 122) are circular holes.3) Burner according to claim 2 wherein each individual gas distribution plate has all holes of the same diameter, whereas different gas distribution plates have holes of the same diameter or of a different diameter.4) Burner according to claim 3, wherein the gas distribution plates (121 , 122) have circular holes (131 , 132) all of the same diameter and the axial distance between consecutive plates is not greater than 8 times the diameter of said holes.5) Burner according to claim 4, said distance being 3 to 5 times the diameter of the holes, preferably 3.5 to 4.5 times said diameter.6) Burner according to any of the previous claims, wherein the axial distance between consecutive gas distribution plates (121 , 122) is not greater than7) Burner according to any of the previous claims, wherein for each pair of consecutive gas distribution plates (121 , 122), the projections of the holes (131 ) of the distribution plate upstream (121 ) fall entirely in non-perforated portions of the distribution plate downstream (122).8) Burner according to any of the previous claims wherein each gas distribution plate is made in a single piece and the gas distribution plates are connected to each other by stiffening ribs to form a self-supporting gas distribution assembly (12).9) Burner according to any of the previous claims wherein the number of gas distribution plates is two, including an upstream gas distribution plate and a downstream gas distribution plate.10) Burner according to claim 9 wherein the pressure drop of the process gas passing through the upstream gas distribution plate is 25% to 75%, preferably 50%, of the entire pressure drop of the process gas through the two plates.11 ) Reactor (11 ) for reforming a gas feed comprising a combustion chamber and a burner (1 ) according to any of the previous claims.12) Reactor according to claim 11 wherein the burner (1 ) is installed above the combustion chamber (2) so that the oxidizer pipe (4) and the process gas annular channel (5) are arranged vertically.13) A reactor according to claim 12, said reactor being adapted for secondary reforming of a process gas produced after primary reforming with steam of a hydrocarbon feedstock.