WO2012131318A1

WO2012131318A1 - Steam reforming

Info

Publication number: WO2012131318A1
Application number: PCT/GB2012/050493
Authority: WO
Inventors: Peter William Farnell; Martin Fowles; William Maurice Sengelow
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2011-03-28
Filing date: 2012-03-06
Publication date: 2012-10-04
Also published as: GB201105131D0; GB2503152A; GB201316315D0

Abstract

A process for the steam reforming of hydrocarbons in an autothermal or secondary reformer is described comprising (i) non-catalytically partially combusting a feedgas comprising a hydrocarbon with an oxygen-containing gas in the presence of steam to form a partially oxidised hydrocarbon gas mixture at a temperature ≥1200^oC and (ii) passing the partially oxidised hydrocarbon gas mixture through a first particulate layer of steam reforming catalyst, a second particulate layer of steam reforming catalyst and a third particulate layer of steam reforming catalyst, wherein the first layer comprises a catalytically active metal selected from platinum, palladium, iridium, ruthenium or rhodium supported on a zirconia, and the second and third layers comprise nickel on a refractory support selected from alumina, calcium aluminate, magnesium aluminate, titania, zirconia, magnesia or mixtures thereof, wherein the second layer has a voidage ≥0.5 and a higher equivalent passage diameter than the third layer.

Description

Steam Reforming

This invention relates to a process and apparatus for the catalytic steam reforming of a partially combusted hydrocarbon feedstock for the preparation of synthesis gas.

Steam reforming is widely practised and is used to produce hydrogen streams and synthesis gas comprising hydrogen and carbon oxides for a number of processes such as ammonia and methanol synthesis and the Fischer-Tropsch process. In order to obtain a synthesis gas more suited to Fischer-Tropsch or other downstream processes, a primary or pre-reformed gas containing methane is typically subjected to secondary or autothermal reforming in an adiabatic refractory lined reformer by partially combusting the primary-reformed or pre-reformed gas in a combustion zone using a suitable oxidant, e.g. air, oxygen or oxygen-enriched air using burner apparatus mounted usually near the top of the reformer. The process of partially combusting the primary-reformed or pre-reformed gas produces a partially oxidised gas mixture. The refractory lining typically contains multiple layers and is designed to protect the pressure vessel from the hot gases in the reformer and to provide insulation to minimise heat losses. One or more layers of low thermal conductivity materials containing alumina may be used to provide the thermal insulation. The hot face material is typically made from dense high purity alpha alumina bricks that are chemically inert to the hot gases and resist physical attrition due the high gas velocities employed. These blocks are cemented in place to prevent the hot process gas contacting the more reactive insulating layers beneath.

The partial combustion reactions are exothermic and the partial oxidation increases the temperature of the partially oxidised gas to between 1200 and 1500°C. The partially oxidised reformed gas is then passed from the combustion zone through a bed of a steam reforming catalyst disposed below the burner apparatus, to bring the gas composition towards equilibrium. Heat for the endothermic steam reforming reaction is supplied by the hot, partially oxidised reformed gas. As the partially oxidised reformed gas contacts the steam reforming catalyst it is cooled adiabatically by the endothermic steam reforming reaction to temperatures in the range 900-1 100°C.

Typically the steam reforming catalyst in the secondary or autothermal reformer is a nickel catalyst supported on an alpha alumina or a magnesium-, or calcium-aluminate support.

WO2006/126018 discloses a process for the steam reforming of hydrocarbons in an autothermal or secondary reformer, wherein in order to prevent volatilisation of the catalyst support, the steam reforming catalyst bed comprises a first layer and a second layer, wherein the oxidic support for the first layer is a zirconia. Preferably the catalytic activity of the first layer is higher than the second layer so that the endothermic steam reforming reactions act to reduce the gas temperature quickly and so prevent volatilisation of the catalyst. This may be achieved by using a more catalytically active metal in the first layer such as platinum, palladium, iridium, ruthenium or rhodium.

Volatilisation of the dense alpha-alumina bricks used to line the adiabatic reformer, can occur when the refractory lining in the vessel is new or when the dense alpha alumina bricks have been replaced or repaired. This is a particular problem in the burner region and combustion zone of the reformer where the gas temperatures are hottest. Volatilisation occurs until the bricks and the cement used to hold the blocks together have aged in-situ. We have found surprisingly that, where volatilised alumina is formed, it does not condense on the first zirconia- supported catalyst layer, despite the fact that it is cooled by the endothermic steam reforming reactions, and is deposited instead in the second layer. In extreme cases, this can increase the pressure drop though the bed of steam reforming catalyst causing the plant to reduce throughput, and/or result in the non-uniform flow of gas through the catalyst bed resulting in the apparent poor performance of the catalyst. Therefore we have devised an arrangement that overcomes these problems.

Accordingly, the invention provides a process for the steam reforming of hydrocarbons in an autothermal or secondary reformer comprising (i) non-catalytically partially combusting a feedgas comprising a hydrocarbon with an oxygen-containing gas in the presence of steam to form a partially oxidised hydrocarbon gas mixture at a temperature >1200°C and (ii) passing the partially oxidised hydrocarbon gas mixture through a first particulate layer of steam reforming catalyst, a second particulate layer of steam reforming catalyst and a third particulate layer of steam reforming catalyst, wherein the first layer comprises a catalytically active metal selected from platinum, palladium, iridium, ruthenium or rhodium supported on a zirconia, and the second and third layers comprise nickel on a refractory support selected from alumina, calcium aluminate, magnesium aluminate, titania, zirconia, magnesia or mixtures thereof, wherein the second layer has a voidage > 0.5 and a higher equivalent passage diameter than the third layer.

The invention further provides a bed of steam reforming catalyst disposed below burner apparatus in an autothermal or secondary reformer, wherein the bed comprises a first particulate layer, a second particulate layer and a third particulate layer, wherein the first layer comprises a catalytically active metal selected from platinum, palladium, iridium, ruthenium or rhodium supported on a zirconia, and the second and third layers comprise nickel on a refractory support selected from alumina, calcium aluminate, magnesium aluminate, titania, zirconia, magnesia or mixtures thereof, and wherein the second layer has a voidage > 0.5 and a higher equivalent passage diameter than the third layer. In the process, the feed gas may comprise a desulphurised hydrocarbon feedstock, such as methane or a natural gas, which may further comprise recycled gas streams from downstream processes comprising one or more of methanol, hydrocarbons, carbon oxides or hydrogen, or may be a pre-reformed or primary reformed gas stream comprising methane, steam, hydrogen and carbon oxides. The latter are preferable as it may be desirable to ensure that the feed to the secondary reformer or autothermal reformer contains no hydrocarbons higher than methane and also contains a significant amount of hydrogen, as these factors reduce the risk of carbon or soot formation above or on the steam reforming catalyst.

The oxygen containing gas may be substantially pure oxygen, air or an oxygen enriched air. The oxygen:carbon molar ratio may be in the range 0.4 to 0.7:1 . Steam is preferably present at a steam:carbon ratio between 0.5 and 3.5. Steam may be provided by adding it directly to the combustion zone or by mixing, either with the feedgas or the oxygen-containing gas.

Alternatively, if the feedgas is a pre- or primary-reformed gas, no additional steam may be necessary. Furthermore, if the feedgas contains hydrogen, its combustion with oxygen will generate steam under the reaction conditions. The hydrocarbon in the feedgas is non- catalytically partially combusted by the oxygen in the oxygen-containing gas in the combustion zone of a secondary or autothermal reformer. The combustion zone is generally formed beneath a burner disposed near the top of the reformer to which the feed gas and oxygen- containing gas are fed.

The partially oxidised hydrocarbon/steam mixture then passes from the combustion zone to the surface of the first layer of steam reforming catalyst at a temperature >1200°C and preferably in the range 1200-1500°C. When the partially oxidised feedgas/steam mixture, e.g. a partially combusted pre- or primary reformed gas, contacts the steam reforming catalyst, the steam reforming reaction, which is endothermic, cools the gas and the surface of the catalyst. The surface temperature of the catalyst may therefore, depending upon the conditions and activity of the catalyst, be in the range about 900-1400°C. In particular, we have found alumina or metal aluminate volatilisation to occur when the catalyst temperature is above about 1 150°C, particularly about 1200°C. Thus, preferably the first layer reduces the catalyst temperature below about 1200°C, more preferably below about 1 100°C.

The pressures at which the process may be operated are preferably >20 bar abs, more preferably in the range 20 to 80 bar abs. The gas-hourly space velocity in the catalyst layers may be >5000h \ preferably 5000 to 30000h \ more preferably 5000 to 20000h ^~

In the present invention, the first layer catalyst support is a zirconia. Preferred zirconia supports are stabilised zirconias, such as magnesia-, calcia-, lanthana-, yttria- or ceria- stabilised zirconias, which are most preferably in the cubic form. Such zirconias are known and are commercially available. Yttria-stabilised cubic zirconia is most preferred, e.g. a 16% wt yttria-stabilised cubic zirconia. Typically such stabilised zirconias have been fired to temperatures above 1200°C. We have found zirconia-containing supports to have lower volatility than supports comprising alumina or magnesium- or calcium-aluminate and so the presence in the first layer of alumina or magnesium- or calcium-aluminate is undesirable.

The catalytically active metal in the first layer of steam reforming catalyst may platinum, palladium, iridium, ruthenium or rhodium, preferably platinum and/or rhodium. These metals have a higher catalytic activity/g and greater stability than the conventional nickel steam reforming catalysts, which has the further advantage or reducing catalyst volatilisation. By increasing the catalytic activity of the first layer, the endothermic steam reforming reactions take place to a greater extent and thereby act to cool the gas stream passing through the bed more rapidly than in the case where the layer of increased activity is absent. Rhodium is particularly preferred as it has a lower vapour pressure than nickel under typical reaction conditions. Suitably active rhodium catalysts comprise 0.01-1.00%, preferably 0.05 to 0.5%, more preferably 0.1 to 0.25% Rh by weight. A particularly preferred first layer catalyst therefore consists of a rhodium impregnated zirconia catalyst, particularly a 0.05 to 0.5%wt rhodium- impregnated stabilised zirconia.

The partially reformed gas mixture passes from the first layer to the second and third layers respectively. The second and third layers comprise a nickel catalyst on a suitable refractory support. The refractory catalyst support for the second layer preferably comprises alumina, calcium aluminate, magnesium aluminate, titania, zirconia or magnesia or mixtures thereof. More preferably, the second layer catalyst and third layer catalyst comprise nickel on alumina, magnesium aluminate or calcium aluminate.

Steam reforming catalysts may be made using a variety of methods. Impregnation methods are particularly suitable and are well known to those skilled in the art of catalyst manufacture. For example, rhodium may be provided in the first layer steam reforming catalyst by impregnation of the zirconia support with a solution of a suitable rhodium compound, followed by heating in air to convert the compound to rhodium oxide. If desired, the rhodium oxide may then be reduced to elemental form by treatment with a reducing gas such as hydrogen at elevated temperature, although it is generally more convenient to install the catalyst in the unreduced oxidic form and perform the reduction immediately prior to use in-situ by reaction with a reducing gas (hydrogen and/or carbon monoxide). For example, a rhodium catalyst may be prepared by impregnating a stabilised cubic zirconia with an aqueous solution of rhodium nitrate, if necessary separating the impregnated material from the solution, drying and calcining to 400-500°C. The rhodium oxide, Rh₂0₃, is subsequently reduced in-situ. In a preferred embodiment, the rhodium is provided on the support as a so-called "eggshell" catalyst in which the rhodium is concentrated in the surface layers of the catalyst support rather than being distributed evenly throughout the support. This provides a more efficient use of the rhodium, which is relatively expensive, compared to e.g. nickel.

Nickel catalysts may similarly be formed by impregnation of the refractory oxide support with nickel acetate or nickel nitrate, followed by calcination, or by precipitation of nickel compounds by combining a solution of a nickel salt such as nickel nitrate with a base, followed by washing, drying, and calcining. Again prior to use the nickel may be reduced with a reducing gas stream.

Whereas impregnation of soluble salts is a particularly suitable method for preparing the catalysts, other techniques whereby sols or suspensions of catalytic metals are applied to the support material may also be used.

The first, second and third layer reforming catalysts are particulate, i.e. in the form of shaped units such as pellets, rings or extrudates, which may be lobed or fluted and/or which may contain one or more through holes. Such catalysts are commercially available or may be made using conventional techniques.

In the present invention, the second layer has a voidage > 0.5, preferably > 0.55 and the equivalent passage diameter (D_ep) is higher in the second layer, in which the volatilised alumina condenses, than in the third. In this way the condensation of volatilised alumina will have a reduced effect on the pressure drop of the catalyst bed and is less likely to result in nonuniform flow through the bed. However, it is still desirable that the second layer catalyst activity is sufficient, and this may be achieved by using catalysts with a suitable geometric surface area (GSA).

Voidage (ε) is a dimensionless number related to the interstitial "empty" volume between and within the catalyst particles present in the layers. Voidage is described, for example, in The Catalyst Handbook, 2^nd Edition, Manson Publishing, 1989, pages 101-5. The shape of the catalyst particle can be used to determine the voidage of the catalyst layer - generally the more eccentric the shape, the greater the voidage. Thus higher voidages may be achieved using particles with an aspect ratio, i.e. a length/diameter value, other than 1.0. Solid cylinders generally have a low voidage, typically <0.4. Voidage may also be increased by providing the catalyst particles with one or more through holes, preferably 1 to 10 through holes. Whereas rings with adequate strength can provide increased voidage, their GSA may not be suitable. Accordingly preferred catalysts have 2-10 through holes. Voidage may also be increased by providing the catalyst particles with lobes or flutes. Particularly preferred catalyst particles are 4 to 10 holed cylindrical pellets, optionally with lobes or flutes arranged around their periphery, and which may be domed or flat ended.

The second layer may have the same or different voidage to the first or third layers. Preferably the voidage in the second layer will be the same or higher than that in the third layer. The voidage of the second layer may also be the same or higher than that of the first layer. A high voidage of the second layer allows the volatilised alumina to be captured without adversely increasing the pressure drop through the bed.

The equivalent passage diameter, or hydraulic diameter, D_ep, is given in the Chemical Engineer's Handbook, 5^th Edition, Table 5-1 1 as 4 x R_H, where R_H is the area of the fluid stream cross-section/wetted perimeter. In a particulate catalyst bed D_ep (in metres) may be expressed as 4 x voidage/geometric surface area in m²/m³, i.e. D_ep = 4e/GSA.

The Geometric Surface Area (GSA) may be calculated from the dimensions of the catalyst particles using known methods. For example, GSA may be calculated for ring or multi-hole cylindrical catalysts using the expression, GSA = S.(1-e) V, where S is the surface area of the catalyst, ε is the voidage and V is the volume of the catalyst particle.

5 = n±.(D_o+(N_h |))+0.5.n.(D_o ²-(N_h.D|²)) and V = 0.25.7i.L(D_o ²-(N_h.Dⁱ²) where L is the length of the catalyst particle, D₀ is the outside diameter, D, is the inside diameter, and N_h is the number of holes.

The second layer has a higher equivalent passage diameter than third layer. It may also have a higher equivalent passage diameter than the first layer. This may be achieved by using larger particles of the same voidage (as a result of their lower GSA) or by increasing the voidage of the catalyst particles in the second layer as described above.

The equivalent passage diameter of the second layer is preferably > 6mm, more preferably > 7mm, most preferably > 8mm, especially > 9mm. A suitable upper limit for the equivalent passage diameter in the second layer may be 15mm. The equivalent passage diameter in the first or third layers is preferably smaller by 1 or more millimetres, i.e. the difference between the D_ep of the second and first or third layers is preferably > 1 mm, with a minimum value of the D_ep in the first and third layers preferably of 2 mm.

The second layer therefore preferably has a combination of sufficient volume to capture the volatilised alumina without adversely affecting the pressure drop or flow distribution and sufficient GSA to catalyse the reaction to further reduce gas temperature thereby promoting complete condensation of the volatilised alumina in the second layer and at the same time contributing towards the approach to equilibrium for the reformed gas. This is achieved by selecting catalyst particles with the appropriate voidage and equivalent passage diameter. The geometric surface area of the second layer is preferably >150m²/m³, more preferably

>200m²/m³, up to about 350m²/m³. The GSA of the third layer is preferably higher than that of the second layer so that the reacting gasses may be brought towards equilibrium in the autothermal reformer. The geometric surface area of the first layer may also be higher than that of the second layer for the same reason. The GSA of the first layer and the third layer is preferably >300m²/m³, more preferably >400m²/m³, with an upper limit preferably about 650m²/m³. Preferably, there is a difference in GSA between the first or third layers and the second layer of at least 50 m²/m³.

In a preferred embodiment, the bed of steam reforming catalyst comprises a first layer comprising 0.05 to 0.5% wt Rh on a stabilised zirconia, in the form of multi-holed cylindrical pellets, which may be lobed or fluted, with a voidage > 0.5, over a second layer comprising 5 - 20% wt Ni on alumina in the form of rings or multi-holed cylindrical pellets having voidage > 0.5 and a D_ep > 6mm, over a third layer comprising a 5 - 20% wt Ni on alumina in the form of multi- holed pellets, which may be lobed or fluted, with a voidage > 0.5 and D_ep > 1 mm smaller than that of the second layer.

The thickness of the bed of the steam reforming catalyst will depend upon the feed rate, the activity of the catalytically active metals, the conditions under which it is operated and whether the feedgas is a hydrocarbon/steam mixture or a pre- or primary-reformed gas. The thickness of the bed of steam reforming catalyst may be in the range 1-10 metres, preferably 2-5 metres with the first and second layers each preferably comprising between 3 and 25%, most preferably 5 and 15% of the thickness of the bed.

If desired, a layer of zirconia balls, pellets or perforate tiles may be placed on top of the first layer of reforming catalyst to protect the surface of the steam reforming catalyst from irregularities in the combusting gas flow. A benefit of providing this layer is to prevent disturbance of the surface of the particulate catalyst bed.

It will be understood by those skilled in the art that it may be useful to graduate the activity of the steam reforming catalyst through the bed. Therefore the third layer may comprise two or more successive layers of a steam reforming catalyst, the fourth or further layers having a higher catalytic activity than that preceding it.

The invention will now be further described by reference to the figures in which;

Figure 1 is depiction of an autothermal reformer containing a catalyst bed according to one embodiment of the invention. In Figure 1 and autothermal reformer vessel 1 comprises an upper section comprising a burner tube 10 to which an oxygen-containing gas may be fed and an inlet 12 to which a feedgas comprising a hydrocarbon may be fed. The hydrocarbon-containing feedgas passes from the inlet and though a perforate diffuser 14 to a lower combustion zone 16 to which the burner tube 10 extends. The walls of the vessel are lined with multi-layer alumina-containing refractory liner 18. The oxygen-containing gas from the burner tube mixes with the hydrocarbon- containing feedgas and partially combusts in the combustion zone 16. The hot, partially oxidised gas then passes through a layer of perforate refractory oxide target tiles 20 and a catalyst bed 22, 24, 26 disposed beneath the combustion zone 16. The catalyst bed comprises three layers 22, 24, and 26. The first layer 22 comprises a catalytically active metal selected from platinum, palladium, iridium, ruthenium or rhodium supported on a zirconia, and the second 24 and third 26 layers comprise nickel on a refractory support selected from the group consisting of alumina, calcium aluminate, magnesium aluminate, titania, zirconia, magnesia or mixtures thereof. The second layer has a voidage >0.5 and a higher equivalent passage diameter than the third layer. The partially oxidised gas is steam reformed by the steam reforming catalysts 22, 24, 26 and the resulting gas mixture brought towards equilibrium as it passes down through the bed. The catalyst bed is supported within the vessel by refractory oxide units 28, such as alumina spheres, which allow the reformed gas to pass from the catalyst bed to a perforate end-member 30 and thence from the vessel via a reformed gas outlet 32.

The alumina volatilised from refractory liner 18 passes though the tiles 20 (which are at the same temperature as the hot gasses in the combustion zone), and surprisingly also through the first layer 22 despite the cooling effect of the endothermic stream reforming reactions. The volatilised alumina condenses in the second layer. Because the second layer has a voidage >0.5 and a higher equivalent passage diameter, the deposited alumina does not negatively affect the pressure drop through the bed compared to the case where the second layer is absent.

In a specific embodiment an autothermal reformer was loaded with a catalyst bed comprising a 0.1-0.3 m thick layer of perforate zirconia tiles and lumps on top of a 0.1-0.2 m thick first layer of a Rh/Zr0₂ steam reforming catalyst (KATALCO_JM™ 89-6GQ) on a 0.2-0.3m thick second layer of a large diameter 4-holed cylindrical Ni/Al₂0₃ steam reforming catalyst (KATALCO_JM 23- 8E) on a 1-2 m thick third layer of a smaller diameter 4-hole quadralobe Ni/Al₂0₃ steam reforming catalyst (KATALCO_JM 28-4Q).

In an alternative specific embodiment, an autothermal reformer was loaded with a catalyst bed comprising a layer of perforate zirconia tiles on top of a 0.1-0.3 m thick first layer of a Rh/Zr0₂ steam reforming catalyst (KATALCO_JM™ 89-6GQ or KATALCO_JM™ 89-6EQ ) on a 0.2-0.3m thick second layer of a large diameter 4-holed cylindrical Ni/Al₂0₃ steam reforming catalyst (KATALCOJM 23-8E) on a 2-3 m thick third layer of a smaller diameter 4-holed cylindrical Ni/Al₂0₃ steam reforming catalyst (KATALCOJM 28-4GQ).

In both cases, the catalysts are commercially available from Johnson Matthey Catalysts.

Claims

1. A process for the steam reforming of hydrocarbons in an autothermal or secondary reformer comprising (i) non-catalytically partially combusting a feedgas comprising a hydrocarbon with an oxygen-containing gas in the presence of steam to form a partially oxidised hydrocarbon gas mixture at a temperature >1200°C and (ii) passing the partially oxidised hydrocarbon gas mixture through a first particulate layer of steam reforming catalyst, a second particulate layer of steam reforming catalyst and a third particulate layer of steam reforming catalyst, wherein the first layer comprises a catalytically active metal selected from platinum, palladium, iridium, ruthenium or rhodium supported on a zirconia, and the second and third layers comprise nickel on a refractory support selected from alumina, calcium aluminate, magnesium aluminate, titania, zirconia, magnesia or mixtures thereof, wherein the second layer has a voidage > 0.5 and a higher equivalent passage diameter than the third layer.

2. A process according to claim 1 wherein the catalytically active metal in the first layer of steam reforming catalyst comprises rhodium and/or platinum, preferably rhodium.

3. A process according to claim 1 or claim 2 wherein the refractory support for the second layer is selected from the group consisting of alumina, calcium-aluminate, magnesium- aluminate or mixtures thereof.

4. A process according to any one of claims 1 to 3 wherein the refractory support for the third layer is selected from the group consisting of alumina, calcium-aluminate, magnesium-aluminate or mixtures thereof.

5. A process according to any one of claims 1 to 4 wherein the equivalent passage

diameter of the second layer is > 6mm, preferably > 7mm, more preferably > 8mm, especially > 9mm.

6. A process according to any one of claims 1 to 5 wherein the second layer has a

voidage > 0.55.

7. A process according to any one of claims 1 to 6 wherein the third layer has a geometric surface area higher than that of the second layer.

8. A process according to any one of claims 1 to 7 wherein the third layer has a geometric surface area > 300m²/m³, preferably > 400m²/m³.

9. A process according to any one of claims 1 to 8 wherein a layer of zirconia balls, pellets or tiles is placed on top of the first layer of reforming catalyst.

10. A process according to any one of claims 1 to 9 wherein the third layer comprises two or more successive layers of a steam reforming catalyst, the fourth or further layers having a higher catalytic activity than that preceding it.

1 1. A bed of steam reforming catalyst disposed below burner apparatus in an autothermal or secondary reformer, wherein the bed comprises a first particulate layer, a second particulate layer and a third particulate layer, each layer comprising a catalytically active metal and an oxidic support wherein the first layer comprises a catalytically active metal selected from platinum, palladium, iridium, ruthenium or rhodium supported on a zirconia, the second and third layers comprise nickel on a refractory support selected from alumina, calcium aluminate, magnesium aluminate, titania, zirconia, magnesia or mixtures thereof, and wherein the second layer has a voidage >0.5 and a higher equivalent passage diameter than the third layer.

12. A catalyst bed according to claim 1 1 wherein the first layer comprises rhodium on a zirconia and the second and third layers comprises nickel on an alumina or magnesium-aluminate or calcium-aluminate support.

13. A catalyst bed according to claim 1 1 or claim 12 wherein the first and second layers are each between 5 and 25% of the thickness of the bed.

14. A catalyst bed according to any one of claims 1 1 to 13 wherein an inert layer of zirconia balls, pellets or tiles is placed on top of the first layer of reforming catalyst.

15. A catalyst bed according to any one of claims 1 1 to 14 wherein the third layer

comprises two or more successive layers of a steam reforming catalyst, the fourth or further layers having a higher catalytic activity than that preceding it.