The invention is directed to a process for steam and/or CO2 reforming of a hydrocarbonaceous feedstock to form a reformate, a reactor for performing such process, a process for preparing synthesis gas and a process for preparing hydrocarbons from such synthesis gas by a Fischer-Tropsch process.

In EP-A-168892 an endothermic steam reforming reaction is described, which reaction is carried out in a fixed bed situated in at least one pipe in which a temperature of between 800 and 950 ⁰ C is maintained by routing at least part of the hot product gas from a partial oxidation reaction along the pipe(s) . According to this publication the combined partial oxidation and endothermic production of synthesis gas result in a better yield of synthesis gas, an increased hydrogen to carbon monoxide ratio, a lower usage of oxygen per πW of synthesis gas product obtained and a lower capital cost of the plant for the production of CO and ^-containing gas mixtures (as compared to partial oxidation) . A reactor and process for performing a steam reforming reaction is described in DE-A-3345088. This publication describes a reactor vessel for performing a steam reforming reaction starting from a natural gas feedstock. The vessel consisted of a tube sheet from which a plurality of tubes filled with a suitable catalyst extended into the vessel. The required heat of reaction is provided by passing the hot effluent of a partial oxidation reaction of natural gas at the exterior

of the reactor tubes in the vessel. Such steam reformer reactors are also referred to as so-called convective steam reformer reactors.

A disadvantage of the known reactor vessel designs is that fouling may occur at the exterior surface of the reactor tubes. This fouling will result in a less favourable heat exchange between hot gas and the catalyst bed and in time result in a less efficient operation. Short run times will result due to frequent shutdowns in order to remove the deposits. Fouling is especially a problem when the hot effluent of a partial oxidation reaction is used. This effluent is especially suited for providing the required heat on the one hand due to its high temperatures. However the soot present in this effluent will cause the above fouling problems and for this reason no commercial applications directed to the combined process of steam reforming and partial oxidation such as described in DE-A-3345088 has been developed according to our knowledge at the time of filing. WO 01/37982 discloses a reformer tube of the so- called double-tube configuration of a steam reformer reactor. The double-tube configuration consists of a reactor tube provided with a catalyst bed in which bed an inner return tube is provided for passage of the reactants being discharged from said catalyst bed. The inner tube as disclosed in WO 01/37982 has a non-circular cross-section. As a result of the use of an inner tube of non-circular cross-section, the inner tube is more capable of local deflection than a tube of circular cross-section, thereby decreasing the crushing forces to which the catalyst units are subjected.

The configuration of WO 01/37982 is limited to the specific design of the inner conduit. Differences in

thermal expansion coefficients of the catalyst, conduits and reactor tube lead to friction forces occurring between the hot inner conduits and second conduit and/or reactor wall and the catalyst. The friction forces may result in fracturing or other damaging of the catalyst, inner return tubes and reactor tube.

WO 00/26137 discloses a method for producing hydrogen-containing gas streams by steam reforming of a hydrocarbon feedstock. Use is made of a steam reforming catalyst, which steam reforming catalyst is supported on a mesh or mesh-like material, or the catalyst is in the form of a mesh. The steam reforming catalyst of WO 00/26137 is contained in a tube and is heated from the outside. An object of the present invention is to provide a process for preparing a mixture comprising carbon monoxide and hydrogen by steam and/or CO2 reforming of a hydrocarbonaceous feedstock, which makes use of a hot gaseous medium, which may cause fouling, as for example the effluent of a partial oxidation, as the heating medium. In addition, a catalyst arrangement is used which shown minimum damages to the catalyst arrangements due to temperature changes. Summary of the invention This object is achieved with the following process. A process for steam and/or CO ₂ reforming of a hydrocarbonaceous feedstock to form a reformate, wherein the hydrocarbonaceous feedstock, steam and/or CO2 are contacted with a porous fixed catalyst arrangement comprising a catalytically active component for reforming and one or more metal structures, wherein the porous fixed catalyst arrangement is contained in a reactor conduit and wherein one or more inner passageways are

provided in the fixed arrangement, in which process a hot gaseous medium is passed through the inner passageways to provide heat for the reforming reaction by indirect heat exchange. It is an advantage of the process according to the invention that a convective steam and/or CO2 reforming process can be conducted by making use of a gaseous mixture, which inherently can cause fouling. By passing the gaseous medium through said passageways a minimum velocity can be realised at which a self-cleaning effect is achieved. The risk that the catalyst, inner passageways and reactor conduits are damaged induced by differences in thermal expansion of the separate components combined with the reduced mechanical strength of the inner conduits at high temperatures is reduced by using a porous fixed arrangement of catalyst that comprises one or more metal structures.

In a further aspect the invention relates to a reactor for performing a steam and/or CO2 reforming process to form a reformate comprising: a reactor inlet for hydrocarbonaceous feedstock and steam, a reactor inlet for hot gaseous medium, a reactor outlet for a gaseous product comprising the reformate, and a reactor conduit containing a porous fixed catalyst arrangement comprising a catalytically active component for reforming and one or more metal structures, which reactor conduit has an inlet which is fluidly connected to the reactor inlet for hydrocarbonaceous feedstock and steam and has an outlet which is fluidly connected with the reactor outlet for the gaseous product, wherein one or more inner passageways are provided in the porous

fixed catalyst arrangement, which passageways are fluidly connected to the reactor inlet for hot gaseous medium to allow passage of hot gaseous mixture.

In still a further aspect the invention relates to a process for the preparation of synthesis gas from a hydrocarbonaceous feedstock comprising the following steps:

(a) partial oxidation, auto-thermal reforming and/or steam reforming of part of the hydrocarbonaceous feedstock to obtain a first effluent comprising hydrogen and carbon monoxide; and

(b) catalytic steam and/or CO ₂ reforming of the remainder of the hydrocarbonaceous feedstock to form a reformate according to a process as hereinabove defined, wherein part or all of the reformate formed in step (b) and the effluent of step (a) form the hot gaseous medium and flow through the inner passageways.

In yet a further aspect the invention relates to a process for producing hydrocarbons comprising the following steps: i) producing synthesis gas according to the process as hereinabove defined; ii) supplying the synthesis gas to a Fischer-Tropsch synthesis step to form a hydrocarbonaceous Fischer- Tropsch product; and iii) upgrading the hydrocarbonaceous Fischer-Tropsch product to obtain a hydrocarbon comprising stream in one or more hydroconversion steps using the hydrogen obtained by the process as hereinabove defined wherein the hydrogen is recovered from the part of the reformate formed in step (b) that is not used to form the hot gaseous medium.

Figure 1 shows an embodiment of the convective steam reformer reactor according to the invention.

Figure 2 illustrates a second embodiment of the convective steam reformer reactor according to the invention.

Figure 3 to 6 each show an embodiment of a porous fixed catalyst arrangement in a reactor conduit that may be used in the process of the invention.

In the process according to the invention a reformate is formed by convective steam and/or CO2 reforming of a hydrocarbonaceous feedstock.

Steam and/or CO2 reforming of a hydrocarbonaceous feedstock are endothermic processes. In convective steam and/or CO2 reforming, the heat required for the reaction is supplied by indirect heat exchange with a hot (gaseous) medium. In the process according to the invention, a hot gaseous medium flows through inner passageways in the porous fixed catalyst arrangement. The porous fixed catalyst arrangement is contained in a reactor conduit, which may be the reactor vessel as such (see figure 1.) or one or more reactor conduits within a reactor vessel (see figure 2.)

Applicants found that by passing the hot gas medium through one or more inner passageways in the porous fixed catalyst arrangement gas velocity conditions are achieved wherein soot does not adhere or at least adheres significantly less to the heat exchanging surface of the convective steam reformer reactor as compared to the prior art designs. Preferably the inner passageways are designed such that the gas will flow with a certain velocity wherein the gas has a so-called self cleaning capacity. The gas velocity at design capacity is preferably above 10 m/s and more preferably above 30 m/s.

A maximum gas velocity is preferably below 100 m/s and more preferably below 60 m/s. A further advantage is that if any fouling does occur, such deposits could easily be removed in a shut down by state of the art methods for cleaning the interior of passageways.

Preferably, the one or more inner passageways are aligned co-axially with the reactor conduit.

Typically, the inner passageways are metal conduits, which are temperature resistant and facilitate the transfer of heat. Consequently, inner passageways are sensitive to thermally induced expansion. In addition, the hot gaseous medium may contact the inner passageways at temperatures up to 1500 ⁰ C, resulting in a reduced mechanical strength of the conduit. The risk of damaging the inner passageways, reactor conduits and catalyst induced by differences in thermal expansion coefficients of the separate components combined with the reduced mechanical strength of the inner passageways at high temperature is reduced by using a porous fixed catalyst arrangement.

In general, the catalyst bed according to the present invention is a relatively large catalyst bed, preferably situated in an elongated, cylindrical reactor. The inner passageway are relatively small pipes (1-5 inches, preferably 2.3 inch diameter), the pipes arranged parallel with the central reactor axis. Suitably the catalyst bed has a diameter of 1 to 6 m, preferably 2.4 m. The length of the catalyst bed is suitably 2 to 25 m, preferably 4 to 15 m. In the reforming process of the invention the catalyst is in the form of a porous fixed catalyst arrangement that suitably comprises one or more metal structures and a catalytically active component. The

metal structures are suitably in the form of arrangement comprising (thin) metal wire, metal foam and/or metal foil, preferably (thin) metal wire. These arrangements show good flexibility in at least two directions. Metal wire, especially thin metal wire, is preferred, especially in view of its flexibility in three directions. The metal wire arrangements may be relatively small (e.g. spheres or cubicles of a few cm^) or large (e.g. in the form of woven or non-woven mats of dimensions of a few meter in two directions and up till 10 or 20 cm in the third direction) . It is observed that the flexibility of the catalyst arrangement in general is due to two properties: the high void ratio and the use of thin materials (e.g. metal wires or metal foils) . In general, the thickness of metal wires etc. is less than 1 mm, preferably less than 0.2 mm, more preferably less than 0.05 mm. The thickness of the metal foils is of the same dimensions. The porous fixed catalyst arrangement does not comprise any fixed catalyst beds, e.g. ceramic spheres or extrudates comprising catalytically active metals or metal compounds. It is observed that the porosity of such fixed bed usually is between 0.30 or 0.35 (for spheres) and 0.60 or 0.65 (for (larger) extrudates) (N.B. the porosity is defined as the open space between the particles or arrangements, the internal porosity of e.g. ceramic carriers (pore volume) is not taken into account) .

The catalytically active component may be supported on the one or more metal structures. Alternatively, the catalytically active component is supported on particles that are contained in the one or more metal structures. An example hereof is metal wire mesh based ^λ bags' (metal wire arrangement) containing particles wherein the

particles cannot pass through the interstices of the metal wire arrangement. These ^λ bags' can be stacked in the reactor conduit. Yet another example is metal wire mesh screens positioned at different positions along the axis of the reactor conduit supporting catalyst particles, which particles cannot pass through the interstices of the wire mesh, preventing such particles to accumulate at a lower level in the reactor conduit. The particles are preferably ceramic particles supporting the catalytically active component. Suitable ceramic particles are particles from e.g. alumina, silica, titania, zirconia and mixtures thereof.

The metal structures are preferably arrangements of metal wire, metal foam and/or metal foil. The hydrodynamic diameters of the metal structures are preferably greater than l/6 ^th of the minimal distance between two adjacent walls in the reactor conduit containing the porous fixed catalyst arrangement.

The one or more metal structures may comprise one or more monolithic metal structures, which are shaped such that they allow the provision of the inner passageways. Examples hereof are monoliths of metal foam, metal wire mats or other metal wire monoliths, which comprise openings, which when stacked appropriately allow the provision of inner passageways.

Suitable metal foil arrangements may have a flat, corrugated or wavy structure. The metal foil structures may be applied as randomly stacked structures in the porous fixed catalyst arrangement or alternatively as roles of sheets inserted co-axially in the reactor conduit. The metal foil arrangements can also be attached at one end to a wall extending radially into the reactor

conduit, optionally attached with the other end to an adjacent wall.

Suitable metal wire arrangements for catalyst support are known in the art. The metal wires may be randomly dispersed in the metal wire arrangement, but preferably are ordered mesh-like structures as a wire mesh, felt or gauze or the like, or structures obtained by knitting or weaving techniques. Examples of ordered structures are mats, optionally stacked, or three-dimensional structures. In a preferred embodiment the metal wire arrangement is a knitted or woven elastic stocking positioned around the inner passageways as an annular sleeve.

Any high temperature-resistant metal may be used for the metal structures. Examples of suitable metals are alloys containing Fe, Cr and Al, as for example FeCrAlloy ^® .

The porous fixed catalyst arrangement preferably has a void volume of at least 50% especially more than 65%, more preferably at least 80%, even more preferably at least 85%, and preferably at most 99%, more preferably at most 95%, even more preferably at most 90%. The porous fixed catalyst arrangement has the ability to deform, preferably compress, under influence of the thermal deformation of other features i.e. the metal conduits forming the inner passageways, reactor conduit and/or metal structures with are part of the catalyst.

The catalytically active component may be any component suitable for steam and/or CO2 reforming known in the art. The catalytically active component is preferably a group VIII metal, more preferably a metal selected from Pt, Ni, Ir, Pd and Co, even more preferably Pt, Ir and Pd, yet even more preferably Pt.

If a low steam to carbon ratio is applied the catalyst is preferably selected from Pt, Ir and Pd, more preferably Pt.

Before applying the catalytically active component on the metal structure it may be advantageous to first apply a layer comprising an inorganic oxide on the metal structures, for example by washcoating. The use of such a washcoat is well known in the art. The inorganic oxide may be a refractory oxide as e.g. zirconia, alumina or magnesium oxide.

Examples of suitable catalyst arrangements are described in e.g. WO-A-0026137.

As feedstock any suitable hydrocarbonaceous feedstock may be used, preferably a gaseous hydrocarbon stream. The gaseous hydrocarbon stream preferably comprises mainly, i.e. more than 90 v/v%, especially more than 94 v/v%, Ci_4 hydrocarbons, more preferably comprises at least

60 v/v% methane, more preferably at least 75 v/v%, even more preferably 90 v/v% . Natural gas or associated gas are particularly preferred as feedstock. Preferably any sulphur in the feedstock is removed.

Suitable process conditions for steam and/or CO2 reforming are known in the art.

The hot gaseous medium may flow through the inner passageways co-currently or counter currently to the flow of the reactants in the catalyst arrangement, preferably counter currently.

The hot gaseous medium is preferably supplied to the inner passageways at a pressure in the range of from 30 to 75 bar (absolute) , more preferably in the range of from 35 to 70 bar (absolute) , even more preferably in the range of from 40 to 65 bar (absolute) .

The hot gaseous medium is preferably passed through the inner passageways at a velocity in the range of from 10 to 100 m/s, more preferably in the range of from 30 to 60 m/s to obtain the before mentioned self-cleaning effect.

The hot gaseous medium preferably has a temperature in the range of from 500 to 1500 ⁰ C, more preferably in the range of from 800 to 1400 ⁰ C.

The hot gaseous medium is preferably a mixture comprising carbon monoxide and hydrogen, such as the effluent of a (catalytic) partial oxidation of a hydrocarbonaceous feed.

The invention is also directed to a process for the preparation of synthesis gas from a hydrocarbonaceous feedstock comprising the following steps:

(a) partial oxidation, auto-thermal reforming and/or steam reforming of part of the hydrocarbonaceous feedstock to obtain a first effluent comprising hydrogen and carbon monoxide; and (b) catalytic steam and/or CO2 reforming of the remainder of the hydrocarbonaceous feedstock to form a reformate according to a process as hereinabove defined, wherein part or all of the reformate formed in step (b) and the effluent of step (a) form the hot gaseous medium and flow through the inner passageways.

To step (a) is fed from 10 to 90 wt%, more preferably from 50 to 80 wt%, of the total hydrocarbonaceous feedstock to steps (a) and (b) .

Step (a) may be a partial oxidation, auto-thermal reforming and/or steam reforming process. Preferably, step (a) is a partial oxidation process.

In step (a) the partial oxidation of the hydrocarbonaceous feedstock may be performed according to

well known principles as for example described for the Shell Gasification Process in the Oil and Gas Journal, September 6, 1971, pp 85-90. Publications describing examples of partial oxidation processes are EP-A-291111, WO 97/22547, WO 96/39354 and WO 96/03345. In such processes the feedstock is contacted with an oxygen- containing gas under partial oxidation conditions, preferably in the absence of a catalyst.

The oxygen-containing gas may be air (containing about 21 v/v% of oxygen) and preferably oxygen enriched air, suitably containing up to 100 v/v% of oxygen, preferably containing at least 60 v/v% oxygen, more preferably at least 80 v/v%, more preferably at least 98 v/v% of oxygen. Oxygen enriched air may be produced via cryogenic techniques, but is preferably produced by a membrane based process, e.g. the process as described in WO 93/06041.

Contacting the feedstock with the oxygen-containing gas in step (a) is preferably performed in a burner placed in a reactor. To adjust the H2/CO ratio in the first effluent obtained in the partial oxidation reaction in step (a) , carbon dioxide and/or steam may be introduced into the feed to step (a) . Preferably up to 15% volume based on the amount of effluent, preferably up to 8% volume, more preferably up to 4% volume, of either carbon dioxide or steam is added to the feed. As a suitable steam source, water produced in a downstream hydrocarbon synthesis step may be used.

The first effluent of the partial oxidation reaction in step (a) preferably has a temperature of between 1100 and 1500 ⁰ C and an H2/CO molar ratio of from 1.5 up to 3 preferably from 1.6 up to 2.3.

It is an advantage of the invention that due to the ability of the porous fixed catalyst arrangement to deform in response to thermally induced deformation as described hereinbefore the first effluent may be applied to step (b) without the need for an intermediate cooling step.

A further advantage is that the H2/CO molar ratio of the hot gaseous medium makes it suitable for various applications as will be discussed here below. Step (b) is the reforming process of the invention.

The steam to carbon (as hydrocarbon and CO) molar ratio in the feed to step (b) is preferably from 0 up to 2.5 and more preferably below 1 and most preferably from 0.5 up to 0.9. Preferably, up to 60 wt% of the reformate formed in step (b) and the effluent of step (a) form the hot gaseous medium and flow through the inner passageway, preferably 5 to 55 wt% .

The remainder of the reformate, i.e. reformate not used to form the hot gaseous medium may be combined with the hot gaseous medium that is discharged from the inner passageways.

Alternatively the effluent of step (a) and the remainder of the reformate may be obtained as separate streams.

The reforming process yields two product streams, the remainder of the reformate and the hot gaseous medium.

This is advantageous when the hot gaseous medium is the product of a partial oxidation reaction, which has a different hydrogen to carbon monoxide molar ratio than the H2/CO molar ratio of the steam reformer product. This allows mixing the two streams later on to a specified and desired H2/CO ratio. It further allows for the recovery

of hydrogen from the stream containing the most hydrogen with a high efficiency. Hydrogen may preferably be separated from such a stream by for example membrane separation followed by a pressure swing absorber step. This is advantageous if the synthesis gas mixture and hydrogen gases are both required in downstream chemical synthesis processes, for example a Fischer-Tropsch process, wherein synthesis gas is required for the Fischer-Tropsch synthesis reaction and hydrogen is needed for the various hydroisomerisation/hydrocracking and hydrodewaxing units which convert the Fischer-Tropsch synthesis product to for example middle distillates and base oils.

In a preferred embodiment the remainder of the reformate is fed to step (a) .

An advantage of mixing the reformate not used to form the hot gaseous medium of step (b) with the feed to step (a) or more preferably directly into the reactor of step (a) is that any methane or higher gaseous hydrocarbon, which may still be present in the reformate, is then further converted to hydrogen and carbon monoxide. This is especially advantageous when the steam reforming step (b) is performed on a feed having a steam to carbon ratio of less than 1, especially between 0.5 and 0.9. Operating the process with a lower steam to carbon ratio in the feed to step (b) is advantageous because the resulting synthesis gas product will then also contain less steam and because smaller reactor equipment may be applied. A disadvantage of operating step (b) at a low steam to carbon ratio is that more unconverted methane will be present in the reformate. By routing the reformate not used to form the hot gaseous medium to step (a) this disadvantage is overcome.

Optionally the hot gaseous medium discharged from the inner passageways is subjected to an autothermal reformer step (c) to obtain a second reformate. Step (c) is performed at the elevated temperatures of step (a) in order to convert the effluent of step (a) into a mixture having a H2/CO molar ratio closer to the desired thermal equilibrium H2/CO molar ratio values valid for said operating temperatures.

The synthesis gas as obtained by the above process, i.e. the hot gaseous medium discharged from the inner passageways, optionally after auto-thermal reforming, may advantageously be used as feedstock for a Fischer- Tropsch synthesis process, methanol synthesis process, a di-methyl ether synthesis process, an acetic acid synthesis process, ammonia synthesis process or to other processes which use a synthesis gas mixture as feed, such as for example processes involving carbonylation and hydroformylation reactions.

To steps (a) and (b) preferably recycle gases are fed. These recycle gases are obtained in, for example the above exemplified, processes which use the synthesis gas as prepared by the process according to the invention. These recycle gases may comprise C;L_5 hydrocarbons, preferably C]__4 hydrocarbons, more preferably C _] __3 hydrocarbons. These hydrocarbons, or mixtures thereof, are gaseous at temperatures of 5-30 ⁰ C (1 bar), especially at 20 °C (1 bar) . Further, oxygenated compounds, e.g. methanol, dimethylether, acetic acid may be present. The invention is especially directed to a process for producing hydrocarbons comprising the following steps: i) producing synthesis gas according to the process as hereinabove defined;

ii) supplying the synthesis gas to a Fischer-Tropsch synthesis step to form a hydrocarbonaceous Fischer- Tropsch product; and iii) upgrading the hydrocarbonaceous Fischer-Tropsch product to obtain a hydrocarbon comprising stream in one or more hydroconversion steps using the hydrogen obtained by the process as hereinabove defined wherein the hydrogen is recovered from the part of the reformate formed in step (b) that is not used to form the hot gaseous medium.

The obtained hydrocarbons comprising stream is separated into a hydrocarbon product and a gaseous recycle stream. Suitably the hydrocarbon product are those having 5 or more carbon atoms, preferably having 4 or more carbon atoms and more preferably having 3 or more carbon atoms. The gaseous recycle stream may comprise normally gaseous hydrocarbons produced in the synthesis process, nitrogen, unconverted methane and other feedstock hydrocarbons, unconverted carbon monoxide, carbon dioxide, hydrogen and water.

The gaseous recycle stream may be fed to steps (a) and/or (b) . Preferably, the gaseous recycle stream is supplied to the burner of step (a) or directly supplied to the interior of the partial oxidation reactor. Optionally part or all of the carbon dioxide present in such a recycle stream is separated from said recycle stream before being fed to step (a) . Part of the carbon dioxide may suitably be fed to step (a) and/or (b) . The process for producing hydrocarbons may be performed by the well-known Fischer-Tropsch processes, which are for example the Sasol process and the Shell Middle Distillate Process. Examples of suitable catalysts are based on iron and cobalt. Typical reactor

configurations include slurry reactors and tubular reactors. These and other processes are for example described in more detail in EP-A-776959, EP-A-668342, US-A-4943672, US-A-5059299, WO 99/34917 and WO 99/20720. Detailed description of the figures

Figure 1 shows a reactor for steam reforming, according to the invention, comprising a reactor vessel 1, inlet 2 for natural gas and steam, inlet 3 for hot gaseous medium and outlet 4 for reformate. In this embodiment the part of vessel 1 between upper tube sheet 5 and lower tube sheet 6 constitutes the reactor conduit 7 containing the porous fixed catalyst arrangement 8. Inside porous fixed catalyst arrangement 8 a plurality of inner passageways 9 are provided, which start at tube sheet 5 and extend to tube sheet 10 positioned below lower tube sheet 6. Under normal operation, hot gaseous medium flows from inlet 3 through passageways 9 to outlet 12. The space below tube sheet 10 defines space 11 through which the cooled gaseous medium is collected and discharged via reactor outlet 12 from the reactor. Between lower tube sheet 6 and tube sheet 10 space 13 is provided wherein natural gas and steam can be distributed and fed to porous fixed catalyst arrangement 8 via openings 15 in tube sheet 6. Figure 2 shows reactor 20 according to the invention.

Reactor 20 has several parallel positioned reactor conduits 21, each filled with a porous fixed catalyst arrangement 22. Through each catalyst arrangement 22 a passageway 23 for hot gaseous medium is provided parallel to the axis of reactor conduit 21.

Figures 3-6 each show a cross-sectional and longitudinal view of an embodiment of a reactor conduit containing the porous fixed catalyst arrangement. The

porous fixed catalyst arrangement is contained between outer conduit 25 and inner conduit 26, defining inner passageway 27.

In Figure 3, the catalyst arrangement comprises several metal structures 28 each having a diameter greater than l/β ^th of the minimal distance between outer conduit 25 and inner conduit 26.

In Figure 4, the catalyst arrangement comprises stack 29 of metal wire mats. In Figure 5, the catalyst arrangement comprises monolithic metal arrangement 30.

In Figure 6, the catalyst arrangement comprises knitted metal wire elastic stocking 31, which is positioned around conduit 26.