HYDROGEN

Field of the invention

The invention is directed to a process to prepare a mixture of carbon monoxide and hydrogen by means of a reforming reaction of a hydrocarbon feedstock. Background of the invention

In EP-A-168892 an endothermic steam reforming reaction is described, which is carried out in a fixed bed situated in at least one pipe in which a temperature of between 800 and 950 °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 H2/CO ratio, a lower usage of oxygen per m^ 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.

WO-A-03036166 discloses a process to make a mixture of carbon monoxide and hydrogen in a convective steam reforming reactor vessel wherein the required heat is the effluent of a partial oxidation reactor.

EP-A-983964 describes a convective steam reforming reactor vessel, wherein the vessel is provided with a plurality of reactor tubes containing a catalyst bed. Around the reactor tubes an annular sleeve is provided to transport a hot effluent of an auto thermal reformer (ATR) . By indirect heat exchange between this hot effluent and the reactants passing through the catalyst bed the steam reforming reaction can take place. WO-A-0137982 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 double-tube configuration is described in more detail in US-A-4690690 according to WO-A-0137982. The inner tube as disclosed in WO-A-0137982 has a non-circular cross- section. WO-A-8801983 discloses a convective steam reforming reactor vessel wherein the hot gas, which is used to heat the reactor tubes, is obtained by burning heating gas in a lower part of the vessel.

US-A-3980440 discloses a steam-hydrocarbon reformer comprising a plurality of parallel reactor tubes. The reactor tubes comprised of an outer tube and an associated central coaxial inner tube. The catalyst was present as an annular ring fixed to the inner tube. The

required heat for the steam reforming reaction is provided by circulating heating fluid around the external surface of the outer tube.

A disadvantage of the known reactor vessel design is that fouling may occur at the exterior surface of the reactor tubes of the above described convective steam reformer assemblies. 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 time of filing. In addition, the known designs, using packed, fixed bed results in the formation of fines due to movements relating to expansion movements due to temperature differences. The object of the present invention is to provide a process for preparing a mixture of carbon monoxide and hydrogen, which does not have the above-described disadvantages. Summary of the invention This object is achieved with the following process.

Process to prepare a mixture of carbon monoxide and hydrogen by means of a steam and/or carbon dioxide reforming reaction of a hydrocarbon feedstock, wherein

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the hydrocarbon feedstock is provided together with steam and/or carbon dioxide to one end of a reactor conduit, wherein the wall of the reactor conduit comprises a shaped reformer catalyst and wherein the required heat for the reforming reaction is provided by a second conduit placed co-axial to and within the reactor conduit through which a hot gas flows.

The invention is also directed to a steam reformer reactor conduit comprising of a shaped reformer catalyst as the reactor conduit and a second conduit placed co¬ axial to and within the reactor conduit.

The invention is also directed to a steam reformer reactor vessel comprising a plurality of parallel- arranged reactor conduits as described above. An inlet for hydrocarbon feed and steam fluidly connected to the space between the shaped catalyst and the second conduit. An inlet for hot gas fluidly connected to the second conduit and an outlet for the product.

The invention is also directed to a shaped reformer catalyst in the form of a reactor conduit or alternatively to a shaped reformer catalyst capable of being used as building blocks to make a reactor tube.

It is observed that the process of the present invention uses a shaped reformer catalyst which is relatively large with respect to the diameter of the reactor. In general the length of the shaped reformer catalyst is at least the same of the diameter of the reactor conduit (or in the case that the reactor conduit is not cylindrical, the largest diameter present in a conduit section) , preferably at least two times the

(largest) diameter, more preferably at least five times. At most the shaped reformer catalyst is about the same size of the length of reactor conduit, or up till half

the length of the reactor conduit. A suitable size of the shaped reformer catalyst is at least 25 cm, preferably at least 1 m, at most 10 m, preferably at most 5 m. The process of the present invention does not relate to reactor conduits filled with relatively small structures catalyst packings, e.g. reactor conduits filled with large amounts of (randomly packed) honeycombs etc. These relatively small catalyst packings have a length less than the (largest) diameter of the reactor conduit, preferably less than half of the diameter. Brief description of the drawings

Figure 1 is a top view of a plurality of shaped catalyst reactor conduits.

Figure 2 is a side view of two shaped reactor conduits.

Figure 3 is a simplified illustration of a reactor vessel comprising the shaped catalyst reactor conduits.

Figures 4, 5, 6 and 7 illustrate a variation of reactor vessels comprising the shaped catalyst reactor conduits.

Detailed description of the invention

Applicants found that by passing the hot gas through such an inner second conduit gas velocity conditions are achieved wherein soot which may be present in such a gas will not adhere or at least adheres significantly less to the inner surface of the second conduit as compared to the fooling which will take place in the state of the art designs .

Preferably the second conduit is 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 conduits. Examples of cleaning methods are pigging and hydrojetting. Further advantages of the above reactor and of its preferred embodiments will be described below.

Applicants have found that by using a shaped catalyst spaced away from the inner second conduit less friction will occur as compared to a situation wherein the annular space between an inner conduit and an outer conduit would have been filed with state of the art catalyst particles, e.g. catalyst extrudates. If the space had been filled with catalyst particles friction forces would occur between the hot inner second conduit and the catalyst. This is the case especially at start-up and shut down situations in which situations the expansion of the inner metal tube and an outer metal tube would not always be the same. By providing a spaced away catalyst from the hot second conduit no such friction forces will occur making the process less sensitive to failure, especially at start-up and shut down situations. This is an important improvement, as a relatively small amount of catalyst fines may result in a considerable improved pressure drop or even in a full blockage. The open space not only reduces catalyst damage at transient conditions (e.g. start-up, close down), but also (additionally) mitigates the pressure drops increases (or even blockages) . In further addition, part of any fines may also be transported through the open space.

Preferably the hot gas in the inner tube and the hydrocarbon feedstock and its reactants flow counter-

current with respect to each other. Preferably the hot gas is the effluent of a partial oxidation reaction, an autothermal reformer reaction or a steam methane reformer. In order to reduce the high temperatures such a gas may have it is preferred to mix the effluent of the reforming reaction with the hot gas before it passes the second and inner conduit. In this context the reforming reaction takes place in the reactor conduit.

Figure 1 illustrates a cross-sectional top view of an arrangement of several reactor conduits (1) . Reactor conduits (1) have a hexagonal form such that they can be arranged in an economic manner without any gaps between the conduits. This will result in a considerable saving in reactor size when compared to the state of the art convective steam reformer reactors. Also shown are the inner second conduits (2) for passage of hot gas. Reactor conduits (1) are spaced away from the second and inner conduit (2) for passage of feed and reactants through passageway (3) . The form of the shaped catalyst may also be different, for example circular or rectangular. In the embodiment of Figure 1 the shaped catalyst itself provides the reactor conduit (1) . The reactor conduit may then be composed of one catalytically active conduit element covering the full lengths of the reactor. The reactor conduit may also be composed of a multitude of catalytically active elements stacked on top of each other, thus creating one continuous conduit. No additional, optionally metal, conduit is present at the exterior of the shaped catalyst. This is advantageous because there is no need for such an extra conduit.

Figure 2 a possible variation of the arrangement shown in Figure 1. For simplicity only two parallel arranged reactor conduits (Ia, Ib) are shown from the

side. The steam reforming reaction takes place by feeding the reactants via inlet (4) to a first reactor conduit (Ia), feeding the intermediate product via opening (5) to the second conduit (Ib) to further reform the hydrocarbons, which are discharged via opening (6) . This arrangement results in a very compact reactor layout because the height of the reactor vessel may be significantly reduced. Also shown are the second conduits (2a, 2b) for transport of a hot gas. Figure 3 illustrates an embodiment wherein the reactor conduit (6) is provided with an additional outer metal wall (7) . This outer wall (7) provides mechanical strength to the shaped catalyst (6) such that they can be placed in a conventional multi-tubular reactor vessel. Figure 3 also shows the second conduit (8), an annular space (9), a outlet (11) for the effluent of the steam reforming reaction, an inlet (12) for the hot gas and an outlet (13) for the heat exchanged gas. Also shown are tube sheets (14), (15) and (16) . The outlet (11) is preferably fluidly connected to the inlet (12) as described above via common header (17) .

Figure 3 also shows reactor vessel (18), provided with inlet (19) for hot gas, inlet (20) fluidly connected to inlet (10) for hydrocarbon feed and outlet (21) for the synthesis gas product. Figure 3 only shows only one of many conduits (6), which are present in such a reactor.

The shaped steam reforming catalyst may suitably comprise of a shaped type support material and a quantity of catalytically active metal. The shaped type of support material may be alumina, zirconia, calcium or alumina. Shaping of such refractory materials may be performed by suitably extrudation or moulding. Suitable metals are

Group VIII metals, more preferably Pt, Ni, Pd, Ir, Rh and Co. These metals may be added to the support material by well known techniques such as impregnations followed by calcination. The shaped steam reforming catalyst may in another embodiment be a porous fixed catalyst arrangement, comprising a catalytically active component and one or more metal structures. The porous fixed catalyst arrangement typically has a free void volume of at least 50%, preferably at least 80%, more preferably at least 90%, but at most 95%. Free void volume, herein, is defined as the non-occupied volume in the porous fixed catalyst arrangement divided by the total volume of the porous fixed catalyst arrangement. Such a shaped steam reforming catalyst is preferably applied in combination with an outer wall as illustrated in Figure 3 as outer wall (7) . Because of the porous nature of the porous fixed catalyst arrangement, the porous fixed catalyst arrangement can deform, preferably compress, under influence of the thermal deformation of the outer wall (7) .

The open space between the second conduit may be between 0,1 mm and several cm, e.g. 5 cm, preferably 1 mm to 3 cm, more preferably 2 to 8 mm. Preferably, the porous fixed catalyst arrangement comprises a catalytically active component that is supported on one or more metal structures comprising arrangements of metal wire, metal foam and/or metal foil forming the wall of the reactor conduit. The catalytically active metal may be applied directly on the metal surface or as part of a coating of a refractory type material on the metal structure. Such a coating may be applied on the metal structure by means of

washcoating. Examples of refractory materials are zirconia, alumina and magnesium oxide. In another embodiment the porous fixed catalyst arrangement comprises a catalytically active component that is supported on one or more porous ceramic structures comprising arrangements of ceramic foam, monolith or ceramic fibres. In such an embodiment, because of the different thermal expansion behaviour, it is important that the catalyst structure is spatially separated from second conduit.

Figure 4 illustrates an embodiment wherein the reactor conduit comprises a porous fixed catalyst arrangement consisting of a fixed bed of catalyst particles (Al) wherein said particles are held spaced away from second conduit (8) by a screen (A2) thus creating and sustaining the annular space (9) . The screen (A2) acts as a porous boundary layer preventing the catalyst from touching the second conduit (8) while still enabling free lateral exchange of the gas between the catalyst bed (Al) and the annular space (9) . The screen

(A2) preferably comprises an open structure, such as, but not limited to woven wire mesh, welded wire mesh, sintered wire mesh, perforated foil, Johnson screen, randomly sintered metal wire, or a foam structure. The screen (A2) preferably is made of metal, although ceramic materials may also be used.

Figure 5 illustrates an embodiment wherein the reactor conduit additionally comprises a second screen (A3), which is situated between the catalyst bed (Al) and the outer wall (7) thus creating a second annular space (A4) . The second screen (A3) acts as a porous boundary layer preventing the catalyst from touching the outer wall (7) while still enabling free lateral exchange of

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the gas between the catalyst bed (Al) and the second annular space (A4) . The second screen (A3) preferably comprises an open structure, such as, but not limited to woven wire mesh, welded wire mesh, sintered wire mesh, perforated foil, Johnson screen, randomly sintered metal wire, or a foam structure. The screen preferably is made of metal, although ceramic materials may also be used.

Figure 6 illustrates an embodiment wherein said first screen (A2) and second screen (A3) may be connected through a bridging structure (A5) thus creating an annular basket. Such annual baskets are stacked on top of each other, thus creating a continuous reactor conduit. The lowest basket is supported onto a support (Aβ) which is integrated with the outer wall (7) . Figure 7 illustrates an embodiment wherein said annular baskets are connected to a continuous structure (A7) suspending from the top of the reactor. This arrangement will create a lower compression force on the baskets that are present in the bottom of the reactor. Such structure may comprise a metal cable or a metal pole.

On the interior wall of the second conduit (2 or 8) which interior wall may be, in at least one embodiment of the present invention, in contact with the effluent of the partial oxidation reaction, carbon may form because part of the carbon monoxide reacts to carbon and carbon dioxide. Also metal dusting corrosion may occur. Furthermore part of the surface may erode resulting eventually in an unacceptable low mechanical integrity of the passageway tubes. These effects are especially significant when the steam to carbon ratio in the hot gas is below 1, more especially below 0.5. Such gas compositions may occur in the above-described

embodiments, especially when the partial oxidation of a gaseous hydrocarbon feed is performed in the absence of added steam and/or when the feed to the steam reforming step has a low steam to carbon ratio. The invention especially relates to one, large reactor comprising a large amount of reactor conduits. In addition one or more distribution compartments (separated from the remaining parts of the reactor by one or more tube sheets) and/or one or more collection compartments (separated from the remaining parts of the reactor by one or more tube sheets) . In addition the present invention relates to a process wherein the process comprises a multitude of reactor conduits suitably situated in a large reactor, preferably using one distribution compartment in fluid connection with to the open ends of the reactor conduits and/or one collection compartment in fluid connection with the other end of the reactor conduit, and optionally another distribution compartment in fluid connection with the second conduit and/or another collection compartment in fluid connection with the outlet of the second conduit.

In order further to minimise the above-described coke formation a preferred material for the passageway is suitably used. The second conduit suitably in the form of a tube, is preferably made from a metal alloy, wherein the metal alloy comprises from 0 and up to 20 wt% and preferably from 0 up to 7 wt% iron. The alloy preferably also contains between 0 and 5 wt% aluminium, preferably from 0 up to 5 wt% silicon, preferably from 20 up to 50 wt% chromium and preferably at least 35 wt% nickel.

Preferably the nickel content balances the total to 100%.

The tubes and extensions of said tubes are preferably one of the following types: wrought tubes, centrifugal cast tubes or sintered metal tubes.

It has been found beneficial to have at least some aluminium and/or silicon in the metal alloy surface when the concentration of steam in the hot gaseous medium within the second conduit is lower than 50 vol%, preferably lower than 30 vol% and more preferably lower than 15 vol%. Preferably from 1 up to 5 wt% aluminium and/or from 1 up to 5 wt% silicon is present in said metal alloy under such low steam content conditions. The resulting aluminium oxide and/or silicon oxide layers will provide an improved protection against coke formation and erosion when the conditions become more reducing at such low steam concentrations. Examples of suitable metals are Inconel 693 containing according to its producer Special Metals Corp (USA) , typically comprising 60.5 wt% Ni, 29 wt% Cr and 3.1 wt% Al and the Nicrofer 6025H/6025HT alloys 602/602CA as obtainable from Krupp VDM GmbH (DE) .

Preferably the hot gas is the effluent of a partial oxidation reaction. An advantage of such a process is that the hot gas is passed through well defined passages and thus less fouling on the heat exchanging surfaces of said conduits will occur. A further advantage is that the H2/CO molar ratio's of the combined synthesis gas products of the partial oxidation and the steam reforming can be from 1.5 up to 3 and even preferably from 1.9 up to 2.3 making the synthesis gas product suitable for various applications such as Fischer-Tropsch synthesis.

The hydrocarbon feedstock used in the present process is preferably a gaseous hydrocarbon, suitably methane, natural gas, associated gas or a mixture of C]__4

hydrocarbons. Examples of gaseous hydrocarbons are natural gas, refinery gas, associated gas or (coal bed) methane and the like. Preferably natural gas or associated gas is used. Preferably any sulphur in the feedstock is removed. The feed to the steam reforming process may additionally also comprise compounds from other sources that the above described. For example recycle streams from down stream processes, e.g. Fischer- Tropsch off-gas, may be added to the hydrocarbon feed. Preferably also the feed to the partial oxidation or autothermal reformer is as described as above.

The partial oxidation 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-A-9722547, WO-A-9639354 and WO-A-9603345. In such processes the feed is contacted with an oxygen containing gas under partial oxidation conditions preferably in the absence of a catalyst.

The hot gas, preferably the gaseous product of the partial oxidation reaction, preferably has a temperature of between 1100 and 1500 ⁰ C and an H2/CO molar ratio of from 1.5 up to 2.6, preferably from 1.6 up to 2.2. Part or all of the mixture of carbon monoxide and hydrogen, Product Gas A, as obtained in the convective steam reformer may be directly combined with the Product Gas B as obtained in the partial oxidation, authothermal reformer or steam methane reformer. This combined gas, having a lower temperature than the temperature of the effluent of the partial oxidation, can be advantageously used as the hot gas to flow through the second conduit (2, 8) . Alternatively the Product Gas B is used solely as

the hot gas in the second conduit (2, 8) and the effluents of the partial oxidation and steam reforming are obtained as separate streams and optionally combined m any desired ratio. The invention further relates to a reactor comprising one or more of the reactor conduits including the co¬ axial second conduits as described in claim 1, preferably m combination with one or more of the reactor conduit or reactor features as described in any one or more of claims 2 to 10.

The invention will be illustrated with the following example based on model calculations. Example 1

To the top inlet of a shaped catalyst tube having an internal diameter of 40 mm, a length of 14 m, a dry mixture of 84.1 vol% methane, 7.5 vol% carbon dioxide, 7.3 vol% hydrogen having an inlet temperature of 500 ⁰ C was fed.

The required heat of the reaction is provided by a second conduit positioned within the shaped catalyst conduit. Through said second conduit a hot gas flows counter-current to the flow of steam reformer reactants. The hot gas is a mixture of the effluent of a partial oxidation having a temperature of 1350 ⁰ C and the following composition 33.0 vol% CO, 55.8 vol% H2,

2.1 vol% CO2, 0.2 vol% CH ₄ and the effluent of the steam reformer reaction as it is discharged at the lower end of the shaped catalyst tube. The inlet temperature of the thus obtained hot gas is 1150 ⁰ C. The outlet temperature of the hot gas after it has provided the required heat of the reaction is 750 ⁰ C. The composition of the product stream, i.e. the gas exiting the second conduit is 25.2 vol% CO, 51.9 vol% H2, 3.2 vol% CO2, 2.6 vol% CH ₄ .