Method for arranging a packing in a burner and burner basket for a burner

A method for disposing a bed comprising particles in a burner through which a gas can flow, more particularly in a burner basket of an ammonia oxidation burner, where the particles are disposed such that the bed has a greater flow resistance in an edge region of the burner than in an inner region of the burner. Further, a burner basket for a burner may have a bed comprising particles, wherein the particles are disposed such that the bed has a greater flow resistance in an edge region of the burner basket than in an inner region of the burner basket.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Entry of International Patent Application Serial Number PCT/EP2015/080315, filed Dec. 17, 2015, which claims priority to German Patent Application No. DE 10 2014 226 791.9 filed Dec. 22, 2014, the entire contents of both of which are incorporated herein by reference.

FIELD

The present disclosure generally relates to burners, ammonia oxidation burners, burner baskets, and methods for arranging beds consisting of particles in burners and in burner baskets of ammonia oxidation burners.

BACKGROUND

Ammonia oxidation burners are used in the synthesis of nitric acid, for example. In that case ammonia (NH3) and oxygen (O2) are reacted catalytically to form nitrogen monoxide (NO) and water (H2O) in the ammonia oxidation burner. The NO obtained is then used further for preparing nitric acid.

Catalysts used in ammonia oxidation burners are customarily platinum/rhodium gauzes, which are placed on a bed consisting of particles. The particles generally are designed as packing elements made of stoneware, glass, porcelain or stainless steel, and are introduced into a burner basket arranged within the burner. The burner basket customarily has a gas-permeable bottom plate, allowing the NH3introduced into the burner to flow through the burner basket and through the bed.

The oxidation of ammonia in the burner requires an operating temperature of about 890° C. at a pressure of about 10 bar. Owing to the high temperature, in operation of the ammonia oxidation burner, the burner basket undergoes expansion. An observation here is that the bottom plate of the burner basket expands with a delay in comparison to the side walls of the burner basket. On repeated start-up and run-down of the ammonia oxidation burner, these differences in expansion characteristics between burner basket side walls and bottom plate result in development of cavities and cracks in the bed in the edge region of the burner basket. This destruction of the bed structure reduces the flow resistance for the permeating stream of NH3, and the catalyst gauzes are no longer supported uniformly by the bed. These phenomena give rise to a loss of combustion efficiency and to ammonia slip.

DETAILED DESCRIPTION

One example object of the present disclosure is to raise the combustion efficiency and reduce the ammonia slip.

The object is achieved by a method for arranging a bed consisting of particles in a burner through which a gas can flow, more particularly in a burner basket of an ammonia oxidation burner, the particles being arranged such that the bed has a greater flow resistance in an edge region of the burner than in an inner region of the burner.

Proposed further for achieving the object is a burner basket for a burner, more particularly for an ammonia oxidation burner, having a bed consisting of particles, the particles being arranged such that the bed has a greater flow resistance in one edge region of the burner basket than in an inner region of the burner basket.

The arrangement of the particles is selected such that the bed has a greater flow resistance relative to the interior in the burner edge region that is susceptible to cavitation and/or gapping. The permeating gas is guided increasingly through the inner region of the reactor, thereby reducing the permeating gas loading on cavities present in the edge region, consequently reducing the development and expansion of cavities and gaps in the edge region. The destruction of the bulk-materials structure because of the difference in expansion characteristics between side walls and bottom plate, as a result of temperature fluctuations, is limited, and so there is an increase in combustion efficiency and reduction in ammonia slip.

The bed preferably has a greater bulk density in the edge region than in the inner region. As a result of the greater edge region bulk density, i.e., the greater mass of particles per unit volume in the edge region, the free space needed for the permeation of the gas between the particles in the edge region is reduced and the flow resistance in the edge region is increased. The higher bulk density contributes to restricting the freedom of movement of the particles in the edge region, thereby reducing the formation of cavities and/or gaps because of the expansion of the burner basket. In the inner region the bulk density is preferably set such that it is lower than in the edge region.

In one advantageous refinement, the bed comprises small particles and large particles, the small particles having a smaller diameter than the large particles. Through the use of particles of different diameters it is possible to adjust the flow resistance of the bed. It is possible to form regions which have essentially small particles, in order to set a high flow resistance, and regions which have essentially large particles, in order to set a low flow resistance. Furthermore, the large particles and small particles can be mixed in order to adjust the flow resistance.

In this context it has emerged as being particularly advantageous if the small particles have a diameter in the range from 1 mm to 10 mm, preferably in the range from 2 mm to 5 mm. A bed of small particles with diameters in the stated range has a high flow resistance.

The large particles preferably have a diameter in the range from 5 mm to 50 mm, preferably in the range from 10 mm to 40 mm, more preferably in the range from 20 mm to 30 mm. A bed of large particles having diameters in the stated range has a low flow resistance.

The ratio of the diameter of the small particles to the diameter of the large particles is preferably in the range from 1/50 to 1, more preferably in the range from 1/50 to 1/25.

In one preferred refinement, more small particles than large particles are arranged in the edge region and/or more large particles than small particles are arranged in the inner region. The preponderance of small particles in the edge region raises the bulk density and the flow resistance in the edge region. The preponderance of large particles in the inner region reduces the flow resistance in the inner region. With particular preference, small particles are arranged substantially in the edge region and/or large particles are arranged substantially in the inner region, so producing a maximum flow resistance in the edge region and/or a minimum flow resistance in the inner region.

According to an alternative refinement, more small particles than large particles are arranged in the edge region and in the inner region two layers are arranged, the lower layer having more small particles than large particles and the upper layer having more large particles than small particles. An arrangement of this kind for the particles allows the stability of the bed structure to be improved further.

In a further alternative refinement, in the edge region a mixture of small particles and large particles is arranged, and so the flow resistance in the edge region is set through the mixing ratio of the small particles and large particles. The mixing ratio can be set by means of a mixing apparatus to which small particles and large particles are supplied separately. The number of large particles and small particles in the mixture is preferably substantially the same.

Preference is given to the arrangement in the edge region of mutually superposed layers of large particles and of small particles. Large particles and small particles may be introduced in alternation into the edge region.

The width of the edge region preferably has a value in the range from 1% to 6% of the diameter of the burner and/or of the diameter of the burner basket. The width of the edge region is advantageously in the range from 5 cm to 30 cm, preferably in the range from 10 cm to 20 cm.

According to one advantageous refinement, a gas-permeable separation material to which the bed is applied is arranged on a bottom plate of the burner or of the burner basket. The gas-permeable separation material prevents the particles of the bed slipping through any openings in the bottom plate of the burner basket and/or through any gaps between the bottom plate and the side wall of the burner basket.

In one preferred refinement, a separating device is introduced into the burner or the burner basket and separates the edge region from the inner region. As a result of the separating device it is possible to prevent unwanted mixing of the edge region particles with the inner region particles. The separating device is preferably introduced into the burner or the burner basket before the particles of the bed are arranged in the edge region and/or in the inner region. The separating device may be removed from the burner or the burner basket after the bed has been introduced.

It has proven preferable for a gas-permeable separation material, more particularly a mesh, to be introduced between the edge region and the inner region. As a result of the gas-permeable separation material, mixing of the particles introduced into the edge region and the inner region can be prevented. The gas-permeable separation material is arranged preferably in the burner, more particularly in the burner basket, before the bed is introduced. With particular preference the design of the separation material is such that it is not permeable for small particles and large particles. The gas-permeable separation material can be introduced loose into the burner, more particularly into the burner basket, or anchored, for example to the bottom plate of the burner basket and/or to a separation material lying on the bottom plate. It is preferred, furthermore, for the gas-permeable separation material to be elastic, and so able to deform when the particles move. The gas-permeable separation material which is introduced between the edge region and the inner region may be formed from the same material as the gas-permeable separation material which is arranged on the bottom plate. The separation material separating the edge region and the inner region may remain in the burner during operation of the burner.

It is advantageous if the edge region has a rectangular, more particularly square, cross section. As a result of the rectangular, more particularly square, cross section, the stability of the structure of the bed in the edge region can be increased and the formation of depressions, cavities and/or gaps can be counteracted.

In an alternative refinement, the edge region has a trapezoidal cross section. A trapezoidal edge region is an advantage in those burners and/or burner baskets which are conical in design. The trapezium may be formed with an upward or downward taper.

The particles of the bed may be designed as packing elements, for example as Raschig rings, Pall rings, Berl, Interlox or Torus saddles and/or Interpack bodies. The material of the packing elements is preferably stoneware, porcelain, glass or stainless steel. Alternatively or additionally, the particles of the bed may have a catalyst. The bed accordingly may form a secondary catalyst, which increases the efficiency of the catalytic reaction. For example, the particles may be designed as packing elements which are impregnated with a catalyst, or as particles formed from a catalyst-comprising material. It is possible to use a mixture of particles formed of a non-catalyst-comprising material and particles formed of a catalyst-comprising material.

The advantageous features described above can be used in the case both of the method of the invention and of the burner basket of the invention, alone or in combination.

Further details, features and advantages of the invention are apparent from the drawings, and also from the description below of preferred embodiments with reference to the drawings. These drawings illustrate merely exemplary embodiments of the invention, which do not restrict the inventive concept.

In the various figures, identical parts are always provided with the same reference numerals, and are therefore in general only identified or mentioned once in each case as well. The drawings are schematic representations which serve to illustrate fundamental relationships. The representations are not true to scale and nor do they correctly reproduce the size relationships described.

FIG. 1shows a burner basket1of a burner10formed as an ammonia oxidation burner, by means of which ammonia and oxygen are reacted catalytically to give nitrogen monoxide and water. The burner basket1has a substantially conical shape and, in operation of the burner10, it is arranged in the interior of the burner10, and so can be traversed by flows of ammonia and oxygen. The burner basket1is formed from a gas-permeable bottom plate3and side walls2. In the case of the present exemplary embodiment, the gas-permeable bottom plate3and the side walls2are fixed independently of one another in the burner and are not directly joined to one another. Accordingly there is a gap between the gas-permeable bottom plate3and the side walls2. Arranged above the bottom plate3is a gas-permeable separation material4, which allows the passage of ammonia and oxygen and at the same time prevents particles falling through the gap between bottom plate3and side walls2or through the bottom plate3.

Situated within the burner basket1is a bed5of particles which are in the form of packing elements8,9. In the figures, the packing elements8,9are shown for simplification as substantially spherical particles, although particles of any predetermined form—as Raschig rings, Pall rings, Berl, Interlox or Torus saddles and/or Interpack bodies, for example, may constitute these elements, in deviation from the representation in the figures. The material of the packing elements is preferably stoneware, porcelain, glass or stainless steel. Arranged above the bed5, not shown in the figures, may be a catalyst gauze, such as a platinum/rhodium catalyst gauze, for example. The particles may optionally have a catalyst material, and so the catalytic activity is enhanced.

In order to increase the combustion efficiency and to reduce the ammonia slip, the particles8,9are arranged in such a way that the bed5has a greater flow resistance in an edge region6of the burner basket1than in an inner region7of the burner basket1. As a consequence of the increased flow resistance in the edge region6, the mixture of ammonia and oxygen is guided to an increased extent through the inner region6of the burner basket1. The bed5has a greater bulk density in the edge region6than in the inner region7. The higher bulk density in the edge region6contributes to restricting the freedom of movement of the particles8in the edge region6, thereby reducing the formation of cavities and/or gaps because of thermally induced expansions of the bottom plate3and/or of the side walls2.

As is also apparent from the representation inFIG. 1, the bed5comprises small particles8and large particles9, the small particles8being smaller in form than the large particles9. The diameter of the small particles8is in the range from 1 mm to 10 mm and is smaller than the diameter of the large particles9, which is in the range from 5 mm to 50 mm.

Substantially small particles8are arranged in the edge region6of the burner basket1, while substantially large particles are arranged in the inner region7. Accordingly in the edge region6there is a preponderance of small particles and in the inner region7there is a preponderance of large particles. The edge region6has a width which is between 1% and 6% of the diameter of the burner.

FIG. 2shows a second exemplary embodiment of a burner basket1of the invention. Fundamentally, the burner basket1has a construction similar to that of the burner basket of the first exemplary embodiment, and so what was said there is also valid for the second exemplary embodiment. In contrast to the burner basket1of the first exemplary embodiment, the burner basket1according toFIG. 2additionally has a gas-permeable separation material11which is arranged between the edge region6and the inner region7. The gas-permeable separation material11is designed as an elastic mesh which is able to deform on expansion of the burner basket1, as a result of the heating thereof, thereby removing a risk of damage to the gas-permeable separation material11as a result of the movement of the particles8,9.

FIG. 3shows a third exemplary embodiment of a burner basket1according to the invention. The burner basket1of the third exemplary embodiment corresponds to the burner basket1of the first exemplary embodiment, with the difference that the arrangement of the particles in the inner region7of the burner basket1is different. According toFIG. 3, there are two layers arranged in the inner region7, with the lower layer having more small particles8than large particles9and the upper layer having more large particles9than small particles8. In the edge region6there are more small particles8than large particles9arranged. As a result, the stability of the bed5is improved relative to the bed5of the first exemplary embodiment.

The representation inFIG. 4shows a fourth exemplary embodiment of a burner basket1of the invention. In comparison to the preceding exemplary embodiments, the basic form of the burner basket1according toFIG. 4is cylindrical. The side walls2are arranged substantially at a right angle to the bottom plate3. Moreover, the side walls2are joined directly to the bottom plate3.

Since the side walls2run substantially vertically, the edge region6has a rectangular, more particularly square, cross section. Arranged in the edge region6is a mixture of small particles8and large particles9. The particles8,9of the bed5are arranged in layers in the edge region6, each layer having essentially small particles8or large particles9.

FIG. 5shows a fifth exemplary embodiment of a burner basket1. The burner basket1corresponds essentially to the burner basket1shown inFIG. 3, with the difference that a separation material11in the form of a separation mesh is introduced in order to separate the large particles9from the small particles8. The separation material11is arranged between a lower layer, which consists of small particles8, and an upper layer, which consists of large particles9.

FIG. 6shows a sixth exemplary embodiment of a burner basket1, which corresponds essentially to the burner basket1shown inFIG. 4. For the separation of the large particles9from the small particles8, a plurality of separation materials11in the form of separation meshes are introduced into the burner basket1. The separation meshes are arranged substantially horizontally, and separate a layer consisting of large particles9from the bordering layers, which comprise large particles9and small particles8.

The representation inFIG. 7shows a seventh exemplary embodiment of a burner basket1, which has a bed5having in the edge region6an increased flow resistance relative to the inner region7. For this purpose, a mixture of small particles8and large particles9is introduced in the burner basket1, there being fewer large particles9per unit volume arranged in the edge region6than in the inner region7, so producing a mixture of higher bulk density in the edge region6. The number of small particles8per unit volume is greater in the edge region6than in the inner region7.

The small particles8are formed of a catalyst material, while the large particles9consist of ceramic. The large particles9are designed as Raschig rings. The size selected for the Raschig rings is such that the small particles8are able to penetrate the cylindrical cavity formed by the Rashig rings. This brings with it the advantage that the small particles8are held by the large particles9in the form of Raschig rings in the edge region6, thereby reducing the risk of the blowing of the small particles8from the edge region6in the direction of the inner region7. Arranged between the edge region6and the inner region7there are, additionally, separation meshes11made from a gas-permeable material, so making it more difficult for unwanted migration of the small particles8from the edge region6into the inner region7to take place.

A first refinement of the method of the invention for arranging a bed5in a burner10through which a flow of gas may pass will be elucidated below with reference to the representations inFIG. 8.

As shown inFIG. 8a, a gas-permeable separation material4, for example in the form of a mesh, is first of all arranged on the bottom plate3. The separation material4may be arranged in such a way that it protrudes beyond the bottom plate3at the sides and bears against the side walls2.

In a further step, which is shown inFIG. 8b, a separating device12is introduced into the burner basket1. The separating device has at least one separating wall which separates the inner region7from the outer region6of the burner basket1. The separating device12may be designed, for example, in the manner of a cylindrical pipe.

When the separating device12has been introduced into the burner10, the bed5is introduced into the burner basket1of the burner10. As is apparent fromFIG. 8c, the particles8,9of the bed5are arranged in this case such that the bed5has a greater flow resistance in the edge region6of the burner basket1than in an inner region7of the burner basket1. The edge region6is filled with more small particles8than large particles9. In the inner region7there are more large particles9than small particles8introduced.

After the introduction of the bed5into the burner basket1, the separating device12is removed from the burner basket1. The particles8,9fill the space vacated by the separating device12, and an arrangement is produced as shown inFIG. 1.

Lastly it is possible for a catalyst gauze to be placed onto the bed5.

A further refinement of the method of the invention is described below with reference to the representation inFIG. 9.

As shown inFIG. 9a, a gas-permeable separation material4, in the form of a mesh, for example, is first of all arranged on the bottom plate3. The separation material4may be arranged in such a way that it protrudes laterally beyond the bottom plate3and bears against the side walls2.

As shown inFIG. 9b, in a subsequent method step, at least one gas-permeable separation material11is arranged in the region between the edge region6and the inner region7of the burner basket1. The gas-permeable separation material11is preferably joined to the gas-permeable separation material4lying on the bottom plate3.

When the gas-permeable separation material11has been introduced into the burner basket1, the bed5is introduced into the burner basket1of the burner10. As is apparent fromFIG. 9c, the particles8,9of the bed5are arranged in this case such that the bed5has a greater flow resistance in the edge region6of the burner basket1than in an inner region7of the burner basket1. In the edge region6, there are more small particles8introduced than large particles9. In the inner region7, there are more large particles9introduced than small particles8.

Lastly a catalyst gauze can be placed onto the bed5.

With the above-described method for arranging a bed5, consisting of particles8,9, in a burner10through which a flow of gas may pass, more particularly in a burner basket1of an ammonia oxidation burner, the particles8,9are arranged in such a way that the bed5has a greater flow resistance in an edge region6of the burner10, than in an inner region7of the burner10. As a result of this, the combustion efficiency is increased and the ammonia slip is reduced.

LIST OF REFERENCE NUMERALS