Catalyst support system for ammoxidation burner

Abstract

A catalyst support system (1) for an ammoxidation burner (2), comprising a catalyst mesh for ammoxidation; a basket (14) connected to the support ring (10) for holding an inert substance and / or a catalyst for removing N2O from the gaseous effluent of the catalyst mesh (9); the basket (14) has a modular structure comprising a plurality of modules, where each module comprises an air-permeable base plate (16) and a support frame (15), where each module is connected to an adjacent module by means of a connection suitable for limited displacement between the modules, where only the outer modules forming the periphery of the basket are connected to the support ring (10).

Description

Technical Field

[0001] The invention belongs to the field of catalyst carrier systems for ammonia oxidation burners. Background Art

[0002] In the process of producing nitric acid, ammonia reacts with an oxygen-containing gas such as air or pure oxygen in a high temperature burner over a catalyst mesh. The catalyst mesh is usually made of a noble metal such as Pt / Rh.

[0003] The product of ammonia oxidation is a gas mixture whose main components are nitric oxide NO and nitrogen dioxide NO2 and contains a small amount of dinitrogen oxide (also known as nitrous oxide) N2 O. The ammonia oxidation gas is cooled in one or more heat exchangers to promote the conversion of NO to NO2, and finally absorbed in water using a suitable absorption column to produce nitric acid.

[0004] In general, the synthesis of nitric acid is demanding in terms of temperature and temperature gradients, so that the equipment is exposed to severe stresses. In particular, the ammonia burner must withstand the high temperatures generated by the oxidation of ammonia, which can exceed 900° C., for example up to 920° C.

[0005] Nitrous oxide N2O is a known pollutant and does not participate in the synthesis of nitric acid. Therefore, the N2O contained in the ammonia gas can be removed directly in the ammonia burner by passing the gas mixture over a catalyst suitable for decomposing the N2O into nitrogen and oxygen. Catalysts suitable for removing N2O before the absorption step are called N2O secondary elimination catalysts.

[0006] Therefore, known ammonia burner designs include a basket disposed below the ammonia oxidation catalyst mesh and adapted to hold the N2O secondary elimination catalyst. The basket may also hold an inert material to evenly distribute the gas.

[0007] An ammonia burner generally comprises a supporting structure which is located on the container of the ammonia burner and comprises a metal ring for supporting the catalyst mesh and a carrier plate for supporting the basket.

[0008] The basket may be realized as a single piece or may comprise a beam-like rectangular piece supported at both ends on the carrier plate.The basket has a gas permeable bottom surface for supporting the catalyst or inert material, the bottom surface usually being in the form of a grid or mesh.

[0009] However, the above basket design has shown some disadvantages, especially under the severe thermal stress caused by hot ammonia oxidation gas. The metal ring and carrier plate are subject to considerable thermal expansion and must be able to withstand the weight and pressure drop of the catalyst mesh and basket containing the abatement catalyst.

[0010] In particular, a one-piece basket has the disadvantage that thermal expansion in the circumferential direction can only be compensated at the outer edge of the basket. The known method of dividing the basket into relatively large segments supported at both ends on rings does not solve this problem.

[0011] Different thermal expansions of components connected to each other may cause deformation or failure.

[0012] In practice, it has been observed that the metal rings supporting the mesh develop cracks and that the baskets develop deformations and / or bulges. Displacements between components of the supporting structure caused by thermal stresses can damage the basket mesh and / or cause ripples to form on the floor surface. Negative consequences can include the formation of gas bypass paths and uneven distribution of catalyst or inert material. Certain areas of the catalyst layer may receive less gas or remain essentially inactive.

[0013] In addition, when the basket containing the N2O secondary elimination catalyst is fixed with a joint welded to the carrier plate, the high temperature cycle generated by the ammonia oxidation reaction may cause the catalyst to move from the outer edge of the basket to the central area of ​​the basket, and in some cases, the basket may break and cause the catalyst to be discharged from the basket.

[0014] Thus, there is a need for a catalyst support system for an ammonia oxidizing burner that is not susceptible to the structural integrity issues discussed above.

[0015] US2018 / 305210 discloses a catalyst support system for an ammonia oxidation burner. Summary of the invention

[0016] The object of the present invention is to overcome the above-mentioned disadvantages of the prior art. The object of the present invention is to provide a novel catalyst support system that can better withstand the thermal stresses in an ammonia burner. The present invention solves the problem of how to improve the design of the basket used to support the N2O secondary catalyst and / or inert material downstream of the ammonia oxidation catalyst net in order to avoid or reduce the above-mentioned disadvantages.

[0017] Objects of the present invention include avoiding catalyst bypass, accommodating thermal expansion, better utilizing secondary catalyst and reducing inactive catalyst, and simplifying the installation process.

[0018] The object is achieved with a catalyst support system according to claim 1. The basket for retaining the N2O secondary catalyst and / or the inert material has a modular structure, wherein each module comprises a gas permeable bottom plate and a support frame, each module being connected to an adjacent module by a connector adapted to allow a limited displacement between the modules, the outer modules forming the periphery of the basket being connected to the support ring.

[0019] In a highly preferred embodiment, the module comprises an interior centrally located in the basket and a plurality of modules arranged to form one or more concentric annular arrays.

[0020] The above-described modular construction of the basket has many advantages.

[0021] The main advantage is better adaptation to thermal expansion. In contrast to the case of a one-piece basket, thermal expansion is distributed between the modules. Arranging the modules in multiple concentric annular groups is particularly effective in adapting to radial and circumferential expansion. The modular basket has an increased ability to follow thermal expansion and adapt to the temperature profile of the ammoxidation reactor.

[0022] The external modules are preferably connected to the support ring by means of radial joints, i.e. joints suitable for coping with radial stresses but leaving the modules free for limited displacement relative to the ring in other directions. Thus, the external modules can move with the support ring to accommodate thermal expansion. Possible relative displacements between the external modules and adjacent modules are absorbed by the corresponding connectors. Related advantages are reduced displacement of the N2O catalyst and avoidance or reduction of gas bypass paths.

[0023] The connections between the modules prevent uncontrolled displacements, thereby securing the modules properly in place; yet at the same time, thermal elongation can be accommodated, thereby making the entire modular structure of the basket adaptable to thermal stresses.

[0024] A further advantage is that the amount of catalyst used for N2O decomposition can be reduced and the gas flow distribution within the basket is optimized. Providing radially rigid joints only on the outside of the basket simplifies the installation process of the basket above the support ring.

[0025] Finally, the applicant has also found that the integration of the catalyst support system of the present invention into an ammonia burner is very effective in avoiding the occurrence of excessive thermal stress, and therefore, is expected to extend the service life of the ammonia burner.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a cross-sectional view of a catalyst support system according to an embodiment of the present invention.

[0028] Figure 2 is a schematic diagram of a basket of a catalyst support system according to an embodiment of the present invention.

[0029] Figure 3 yes Figure 1 Schematic diagram of a portion of the basket of the system.

[0030] Figure 4 yes Figure 3 Exploded view of the connections between the modules in the basket.

[0031] Figure 5yes Figure 4 Cross-sectional view of the joint of the connecting piece.

[0032] Figure 6 yes Figure 1 Schematic diagram of a portion of the upper ring of the system.

[0033] Figure 7 yes Figure 1 Detail of the mid-radial joint.

[0034] Figure 1 A catalyst support system 1 for an ammonia oxidation burner 2 is shown.

[0035] The catalyst support system 1 comprises a support structure 3 which comprises an outer wall 4 facing the wall of the ammonia burner 2 , an inner wall 5 facing the reaction zone of the ammonia burner, a lower joint 6 and an upper joint 7 .

[0036] The lower joint 6 is welded to the outer wall 4 and connects the load-bearing structure 3 with the ammonia burner 2. The upper joint 7 is welded between the outer wall 4 and the inner wall 5 and has a curved profile that can accommodate thermal stresses. In particular, the curved upper joint 7 forms corrugations that provide some radial elasticity to the system.

[0037] In some embodiments, the lower joint 6 is optional and the outer wall 4 can be welded directly to the container of the burner 2 .

[0038] The catalyst support system 1 further comprises an upper ring 8 and a bottom ring 10. The upper ring 8 is welded to the inner wall 5 and is configured to support a catalyst mesh 9 for ammonia oxidation. The bottom ring 10 is located below the upper ring 8.

[0039] The upper ring 8 and the bottom ring 10 are connected by means of a sealing wall 11 which is welded to the lower edge of the upper ring 8 and also to the upper edge of the bottom ring 10 .

[0040] The catalyst mesh 9 can be held in place above the upper ring 8 by means of suitable counterweights 27 .

[0041] The bottom ring 10 supports a basket 14. The basket 14 may contain an inert material and / or a catalyst for removing N2O from the gaseous effluent of the catalyst mesh 9. When provided, the inert material serves the purpose of flow distribution, i.e., to evenly distribute the gas flow at the outlet of the basket.

[0042] The first group of cooling coils 25 is disposed between the inner wall 5 and the outer wall 4 of the catalyst support system 1. A cooling medium such as water or steam passes through the cooling coils 25 to remove heat generated by the ammonia oxidation reaction from the catalyst support system 1.

[0043] A second set of cooling coils 26 is located below the basket 14. The second set of cooling coils 26 removes heat from the gaseous effluent of the basket 14 and the removed heat can be used for steam generation or steam superheating purposes. The second set of cooling coils 26 can also provide a structural function; in particular, can be used to support at least part of the weight of the basket 14 to increase the mechanical stability of the overall carrier system 1.

[0044] The basket 14 has a modular structure including a central module and a plurality of modules arranged to form one or more concentrically arranged annular arrays. Each module has a permeable bottom plate 16 (preferably in the form of a grid and a support frame 15). Figure 1 ). In other words, each module forms part of the supporting structure and part of the air-permeable bottom plate of the entire basket 14.

[0045] Figure 2 The basic design principle of a modular basket 14 is shown. The basket 14 comprises a central module 20 and modules 21, 21a arranged to form one or more annular arrangements 17, 18. The outer modules 21a form an outer annular group 18, while the modules 21 form one or more annular groups 17.

[0046] Each module is connected to adjacent modules along radial connection channels 23 and circumferential connection channels 23a. The term circumferential here means connection channels along lines perpendicular to the median radius of the modules.

[0047] The central module 20 can be circular or polygonal. If the central module is polygonal, the central module 20 preferably has many sides, such as at least 10 or 12 sides, to approximate a circle. Each module 21 is preferably bounded by straight sides and preferably has a generally trapezoidal shape to approximate the shape of a portion of a ring.

[0048] The shape and size of the modules can vary depending on their position within the basket. Figure 2 In the embodiment of the present invention, the inner module has a shape close to a triangle, while the outer module 21a is close to a rectangle.

[0049] Each module 21 , 21 a has a perforated bottom of a grid or mesh or the like with holes 33 , so that each module provides a portion of the air-permeable bottom 16 of the basket 14 .

[0050] The connecting passages 23 and 23a are arranged to allow relative displacement between adjacent modules, thus acting as expansion joints to compensate for thermal expansion. The connecting passages 23 and 23a extend beyond the boundary line between modules that are close to each other. The boundary line is preferably a straight line.

[0051] The modules 21a of the outermost array 18 are connected to the base ring 10 by radial joints 22 and are configured to prevent radial displacement. Figure 7A preferred embodiment of such a joint 22 is shown.

[0052] Figure 3 A portion of a basket 14 extending over 90° is shown according to an embodiment, wherein the modules 21, 21a form two concentric annular arrays 17 and an outer array 18. The figure shows radial connecting channels 23 and circumferential connecting channels 23a.

[0053] Figure 4 A preferred embodiment of the connecting channel 23 and 23a is shown. The connecting channel 23 or 23a comprises a segment 24 with an overall C-shaped or U-shaped cross section, facing downwards and arranged to surround the side walls of two adjacent modules 21 or 21a. The segment 24 is fixed in place by one or more latches 36.

[0054] The connecting channel also comprises a cover 35 at the intersection of channels 23 and 23a to avoid the passage of catalyst particles through the spaces between the modules. Such passage of catalyst particles is undesirable because it would hinder the freedom of displacement between the modules for compensating thermal stresses.

[0055] Figure 5 A more detailed arrangement of the fixing pin 36 is shown. The pin 36 is inserted into a hole in the side wall 151 , 152 of the module 21 , 21 a and has a bent end 361 located on the top surface 241 of the segment 24 .

[0056] Figure 6 A portion of an upper ring 8 in a preferred embodiment is shown. The upper ring 8 comprises notches 28 periodically arranged along the ring 8. The notches 28 are configured to compensate for thermal expansion.

[0057] Figure 7 A preferred embodiment of a radial joint 22 for the connection between the support frame 15 and the bottom ring 10 of the outer module 21a is shown. The joint 22 comprises a pin 220 passing through a hole in the support frame 15 and a hole in the bottom ring 10. The pin 220 is welded to the bottom ring 10 at the weld joint 13. The pin 220 has a head 37 of suitable shape to prevent the basket from escaping from the bottom ring 10.

[0058] The gap 38 between the head 37 of the latch 220 and the basket 14 allows the module 21a, and then the entire basket 14, to be displaced axially (vertically) relative to the base ring 10. The provision of the gap 38 is particularly effective in accommodating axial thermal expansion. DETAILED DESCRIPTION

[0059] The present invention discloses a catalyst carrier system for an ammonia oxidation burner, comprising:

[0060] A supporting structure comprising a first supporting ring supporting a catalyst mesh for ammonia oxidation and a second supporting ring located below the first supporting ring;

[0061] a basket connected to the second support ring and adapted to contain an inert material and / or a catalyst for removing N2O from the gaseous effluent of the catalyst mesh;

[0062] Wherein the basket has a modular structure comprising a plurality of modules, wherein each module comprises a breathable bottom plate and a support frame, wherein each module is connected to an adjacent module by a connector adapted to allow limited displacement between the modules, wherein only outer modules forming the periphery of the basket are connected to the support ring.

[0063] The support structure generally includes a first support ring for the catalyst mesh and a second support ring for the modular basket. The second ring is located below the first ring; therefore, the first ring can be referred to as the upper ring and the second ring can be referred to as the bottom ring. In vertical direction, the second ring is located below the first ring.

[0064] The structure may include an outer wall, an inner wall and an upper joint welded between the outer wall and the inner wall, the first ring being welded to the inner wall. The bearing structure may also include a sealing wall configured to seal the side surface between the first ring and the second ring in an airtight manner. The sealing wall may be welded to the first ring and the second ring.

[0065] In general, the modular basket has a breathable bottom surface of a mesh or net or the like and a support structure. The purpose of the support structure is to support the breathable surface. In the modular construction of the present invention, each module provides a portion of the breathable bottom surface and a portion of the associated support structure.

[0066] Each outer module of the basket may be connected to the second support ring with a radial joint configured to prevent radial displacement of the module relative to the ring. Preferably, the radial joint allows limited displacement of the module in the vertical direction. More preferably, the radial joint also allows a small rotation of the module relative to the support ring.

[0067] In a preferred embodiment, the radial joint comprises at least one fixing pin for each module, wherein the fixing pin is movably inserted into a hole of the frame of the basket module and welded to the supporting ring.

[0068] In a highly preferred embodiment, the system includes a set of cooling tubes located below the basket to provide additional support means for the basket modules. Preferably, the only support means for the basket are the cooling tubes and radial joints between the outer modules of the basket and the second ring. Thus, the weight of the basket modules is substantially supported by the cooling tubes below; the securing of the outer modules to the support ring provides radial support; and the connections between the modules secure the modules in place, so that the overall configuration of the basket is maintained while allowing relative movement to compensate for thermal expansion. The outer modules may be supported partly by the cooling tubes below and partly by the second support ring.

[0069] In a preferred embodiment, the connection between modules comprises a segment having an overall C-shaped or U-shaped cross-section, which is arranged downwardly above the side walls of adjacent modules so that the C-shaped or U-shaped segment surrounds the side walls of the two modules. The segment is fixed in place by a latch inserted through a channel in the side wall.

[0070] The parts of the connector between the modules are preferably gas permeable, for example, they have suitable gas passages. Preferably, the gas passages are circular holes.

[0071] The modules are arranged to form a two-dimensional array. In a highly preferred embodiment, the modules are arranged in one or more concentric annular groups or arrays. For example, the outer modules form an outer ring, and the additional modules form one or more additional annular groups. The modules may have a shape similar to the shape of a portion of a ring. The preferred shape is generally trapezoidal. A single inner module may form the center of the basket.

[0072] In an embodiment of the modules arranged in annular groups, the connections between the modules preferably comprise radial joints and circumferential joints, each joint being configured to allow limited displacement between the connected modules.

[0073] The upper ring is preferably a circular crown and may include notches periodically arranged along the circular crown and configured to compensate for thermal expansion. Preferably, the notches are aligned in a radial direction starting from a point located in the center of the upper ring.

[0074] The system may include a first set of cooling tubes disposed between the outer wall and the inner wall.

[0075] The first set of cooling tubes may be attached to the outer or inner wall of the catalyst support system and may be used to prevent overheating of the pressure vessel of the ammonia burner. A cooling medium such as water or steam may be passed through the cooling tubes to remove heat from the support structure and the N2O waste gas exiting the basket.

[0076] The catalyst mesh may be held in place over the upper ring using one or more weights.

[0077] In a preferred embodiment, the upper engagement portion has a curved profile. The outer wall and the inner wall are preferably parallel, so that the load-bearing structure acts as a radially elastic support.

[0078] The upper ring may be manufactured according to any known process or technique, but is preferably obtained by forging.

[0079] According to an interesting application of the present invention, a catalyst for eliminating / decomposing N2O and an inert material are filled in the basket, and the inert material can be used to redistribute the flow in the basket to achieve a uniform flow distribution at the basket outlet.

[0080] Preferably, the upper joint provided for connecting the outer wall and the inner wall of the catalyst support system has a curved profile. The curved profile is particularly effective in accommodating thermal expansion of the catalyst support system at high temperatures.

[0081] The ammonia burner may include a reactor vessel and a catalyst support system as provided herein. The reactor vessel may include a reactor wall, and the catalyst support system may be attached to the reactor wall using one or more welds.

[0082] Preferably, the outer and inner walls of the catalyst support system are parallel so that the support structure acts as a radial elastic support. By the term elastic support, applicants indicate that the catalyst support system can deform in an elastic state under thermal strain and there is no plastic deformation in the structure when thermal stress is removed from the structure.

[0083] The above-mentioned catalyst support system may include a catalyst mesh for ammonia oxidation. Another aspect of the present invention is an ammonia oxidation burner, which includes a catalyst mesh for ammonia oxidation and a support system for the catalyst mesh according to the claim. Preferably, the burner is a vertical burner, more preferably, the burner is a vertical cylindrical burner.

Claims

1. A catalyst support system (1) for an ammonia oxidation burner (2), comprising: A supporting structure (3) comprising a first supporting ring (8) supporting a catalyst mesh (9) for ammonia oxidation and a second supporting ring (10) located below the first ring; a basket (14) connected to the second support ring (10) and suitable for containing an inert substance and / or a catalyst for removing N2O from the gaseous effluent of the catalyst mesh (9); The basket (14) has a modular structure comprising a plurality of modules, wherein each module comprises a breathable bottom plate (16) and a support frame (15), wherein each module is connected to an adjacent module by a connector, wherein the connector is suitable for allowing limited displacement between modules, wherein only the outer modules (21a) forming the periphery of the basket are connected to the support ring.

2. The system according to claim 1, wherein: Each of the outer modules (21a) of the basket is connected to the second ring (10) by a radial joint (22), which is configured to prevent radial displacement of the module relative to the ring (10) and to allow limited displacement of the module in the vertical direction, preferably also allowing rotation.

3. The system of claim 1 or 2, comprising a set of cooling tubes located below the basket, wherein: The modules of the basket rest on the cooling tubes and the only support means of the basket are the cooling tubes and the radial joints between the outer modules of the basket and the second ring (10).

4. A system according to any one of the preceding claims, wherein: The modules include an inner module (20) located in the center of the basket and modules (21, 21a) arranged to form one or more concentric annular arrays (17, 18).

5. The system according to claim 4, wherein: Each module is connected to adjacent modules via radial joints (23) and circumferential joints (23a), wherein the radial joints and the circumferential joints are configured to allow limited displacement between the connected modules.

6. A system according to any one of the preceding claims, wherein: The connection between the basket modules comprises a segment (24) having an overall C-shaped or U-shaped cross-section, which is arranged downwardly above the side walls of two adjacent basket modules so that the segment surrounds the side walls of the modules and is fixed in place by a pin (36) inserted through a hole in the side wall.

7. System according to any of the preceding claims, comprising a sealing wall (11) arranged to seal the side surface between the first support ring (8) and the second support ring (10) in a gas-tight manner.

8. A system according to any one of the preceding claims, wherein: The bearing structure (3) comprises an outer wall (4), an inner wall (5), and an upper joint (7) welded between the outer wall (4) and the inner wall (5); The first support ring (8) is welded to the inner wall (5).

9. The system according to claim 7 and 8, wherein: The sealing wall (11) is welded to the first support ring (8) and the second support ring (10).

10. The system according to claim 8 or 9, wherein: The upper joint (7) has a curved profile, and the outer wall (4) and the inner wall (5) are parallel, so that the bearing structure (3) acts as a radial elastic support.

11. The system (1) according to any one of claims 8 to 10, comprising a set of cooling tubes (25) arranged between the outer wall (4) and the inner wall (5).

12. System (1) according to any one of the preceding claims, wherein: The catalyst mesh (9) is held above the first support ring (8) by one or more weights (27).

13. An ammonia oxidation burner comprising a catalyst mesh (9) for ammonia oxidation and a support system for the catalyst mesh according to any one of the preceding claims.

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