Field of the invention

The invention relates to a reactor for steam reforming of hydrocarbon-containing gas to afford synthesis gas, comprising

- a reactor shell

- feed pipes for hydrocarbon-containing gas and for steam,

- discharge pipes for synthesis gas and for flue gas,

- means for providing the heat energy required for steam reforming

- means for accommodating the reaction zone for the steam reforming reactions, wherein a catalyst active for the steam reforming reactions is arranged in the reaction zone, and wherein the means is designed and arranged relative to the abovementioned means such that the required heat energy for the steam reforming reactions may be transferred to this means by indirect heat transfer.

The invention further relates to a method for steam reforming. Prior art

Hydrocarbons may be catalytically reacted with steam to afford synthesis gas, i.e. mix- tures of hydrogen (H ₂ ) and carbon monoxide (CO). As is explained in Ullmann ^' s Encyclopedia of Industrial Chemistry, Sixth Edition, 1998 Electronic Release, keyword "Gas Production" so-called steam reforming is the most commonly employed method of producing synthesis gas which may then be converted into further important commodity chemicals such as methanol or ammonia. While different hydrocarbons, such as for example naphtha, liquid gas or refinery gases may be converted, it is steam reforming of methane-containing natural gas that dominates.

Steam reforming of natural gas is highly endothermic. It is often performed in a reformer furnace in which numerous catalyst-containing reformer tubes in which the steam reforming reaction takes place are arranged in parallel. The outer walls of the reactor and its ceiling and floor are faced or lined with a plurality of layers of refractory material which withstands temperatures of up to 1200°C. The reformer tubes are usually fired with burners mounted on the top or bottom or on the side walls of the reformer furnace and directly heat the interspace between the reformer tubes. Heat transfer to the reformer tubes is effected by heat radiation and convective heat transfer from the hot flue gases.

After pre-heating by heat exchangers or fired heaters to about 500°C the hydrocarbon- steam mixture enters the reformer tubes after end-heating to about 500°C to 700°C and is therein converted into carbon monoxide and hydrogen over the reforming catalyst. Nickel-based reforming catalysts are widespread. However, the product gas comprises not only carbon monoxide and hydrogen but also carbon dioxide, unconverted methane and water vapour.

Steam reforming of natural gas is notable for its high energy requirements. The prior art therefore already contains proposals which aim to minimize external energy requirements through optimized process design, for example through energy recovery. For instance Higman demonstrated a so-called HCT reformer tube with internal heat exchange in the EUROGAS-90 conference, Trondheim, June 1990, also disclosed at http://www.higman.de/gasification/papers/eurogas.pdf (retrieved 27.09.201 1 ). This com- prises an outer catalyst-filled and externally heated reformer tube where the input gas traverses the catalyst bed from top to bottom. Inside the catalyst bed are two coiled double helix heat exchanger tubes made of a suitable material through which the partially reformed gas flows after leaving the catalyst bed, thus transferring a portion of its sensible heat to the steam reforming process taking place over the catalyst. Calculations and operating trials have shown that for a typical entry temperature of 550°C into the catalyst bed and for a typical exit temperature of 860°C out of the catalyst bed up to 20% of the energy needed for steam cracking may be recycled to the steam reforming by internal heat exchange. Furthermore, capital expenditure savings of up to 15% may be made since the convection sector in the reformer furnace may be made smaller and fewer reformer tubes are required. However, so-called "metal dusting" corrosion, ex- plained hereinebelow, becomes more markedly apparent in these heat exchanger tubes since longer sections of the heat exchanger tubes are subjected to the temperature range relevant for metal dusting corrosion.

In many synthesis gas generation plants at higher gas temperatures, in particular in the range from 820°C down to 520°C, corrosion problems occur on the metallic materials of construction used in the gas generation plants themselves and in the heat exchangers arranged downstream thereof when a certain C0 ₂ /CO/H ₂ 0 ratio and an elevated carbon activity in the synthesis gas is reached. This applies to both ferritic and austenitic steels. This removal of material known as "metal dusting" results in erosion/destruction of the material and there are only limited options for resisting this corrosion by means of material composition.

Description of the invention

It is accordingly an object of the present invention to specify a reactor which exhibits fur- ther improved properties in terms of energy recovery by internal heat exchange and where the hazard of metal dusting corrosion is reduced.

The abovementioned object is achieved by a reactor according to Claim 1. Reactor according to the invention: Reactor for steam reforming of hydrocarbon-containing gas to afford synthesis gas, comprising

a) a reactor shell,

b) feed pipes for hydrocarbon-containing gas and for steam,

c) discharge pipes for synthesis gas and for flue gas,

d) means for providing the heat energy required for steam reforming,

e) means for accommodating the reaction zone for the steam reforming reactions, wherein a catalyst active for the steam reforming reactions is arranged in the reaction zone, and wherein the means is designed and arranged relative to the means listed un- der d) such that the required heat energy for the steam reforming reactions may be transferred to the means by indirect heat transfer from the means listed under d), f) characterized in that the reactor comprises a further means which is suitable for accommodating the reaction zone for the water gas shift reaction in the synthesis gas and in which catalyst active therefor is arranged, wherein this means is designed and arranged such that indirect heat exchange may take place between the reaction zones for the steam reforming reactions and the water gas shift reaction.

The reactor according to the invention makes it possible to lower the carbon monoxide content in the synthesis gas by utilizing the water gas shift reaction, to thus increase the hydrogen content and to utilize the thus liberated enthalpy of reaction for steam reforming.

The water gas shift reaction (synonymous with CO shift reaction or CO conversion) de scribes the reaction

CO + H ₂ O = CO ₂ + H ₂ which, progressing from left to right, liberates an enthalpy of reaction of 41 kJ/mol. Establishing an appropriately high steam content in the input gas makes it possible to influ- ence the direction of the reaction. The progress of the water gas shift reaction is promoted by a catalyst active therefor. The reactor is designed such that heat exchange is possible between the reaction zones for the endothermic reforming reactions and the exothermic water gas shift reaction. This improves the energy efficiency of the reactor.

Simultaneously the carbon monoxide content and thus, as per the reaction,

2 CO = C + C0 ₂ the carbon activity in the synthesis gas and consequently also the hazard posed to plant parts by metal dusting corrosion is reduced.

Steam reforming reactors are often used for processes for obtaining hydrogen. For this application the reactor according to the invention provides the additional advantage that the water gas shift reactor arranged downstream of the steam reforming reactor and often used in these processes experiences a reduced load.

Preferred embodiments of the invention

One preferred embodiment of the invention is characterized in that a tubular reactor is concerned, wherein said reactor comprises as the means for providing the heat energy required for steam reforming at least one burner which generates heat radiation and hot flue gas by combustion of a hydrocarbon-containing gas and/or combustible gas with oxygen-containing gas via its flame and where the means for accommodating the reaction zone for the steam reforming reactions are reformer tubes having the following features:

(a) an outer tube whose interior is at least partly provided as a reaction zone for steam reforming, wherein at one end this tube is fitted with openings for feeding of the input gas and for discharge of the synthesis gas and at the opposite end is closed and wherein said tube contains a dumped bed of a catalyst active for steam reforming,

(b) at least one inner tube as the means for accommodating the reaction zone for the water gas shift reaction which is arranged inside the outer tube and the dumped catalyst bed present therein, wherein a catalyst active for the water gas reaction is arranged in this inner tube. This embodiment has the advantage that it is based on the long-proven concept of tubular reactors for the steam reforming process. The embedding of the inner tube, in which the reaction zone for the water gas shift reaction is disposed, in the fixed bed of the re- forming catalyst allows very good heat exchange between the reaction zones.

A further preferred embodiment of the invention is characterized in that the catalyst active for the water gas shift reaction is arranged as a coating on the inner wall of the inner tube. This has the advantage that a large part of the tube interior remains free for gas flow.

A further preferred embodiment of the invention is characterized in that the catalyst active for the water gas shift reaction is present in the inner tube deposited on moulded articles, such as pellets or honeycombed or foamed bodies. This has the advantage that a large contact area between the catalyst and the gas can be made available.

A further preferred embodiment of the invention is characterized in that the inner tube is helical at least along part of its length. This has the advantage that the contact area and thus the heat exchange area of the inner tube can be enlarged with the bed of the re- forming catalyst.

A further preferred embodiment of the invention is characterized by a cavity arranged inside the outer tube at the sealed tube end and separated from the dumped catalyst bed by a gas-permeable separating apparatus, wherein the entry end of at least one heat exchanger tube projects into the cavity and wherein the cavity is in fluid connection with the dumped catalyst bed and the heat exchanger tube. This measure ensures that aspiration of catalyst support material from the reforming zone into the inner tube is avoided. A further preferred embodiment of the invention is characterized in that the means for providing the heat energy required for steam reforming and the means for accommodat- ing the reaction zone for the steam reforming reaction and the water gas shift reactions are in each case microchannels arranged in parallel to one another such that heat exchange between them is possible via the channel walls. It is advantageous when in each case a multiplicity of channels are apposed to one another in parallel and thus grouped together to form a tier or level, for example a tier channelling a heating medium, for example a flue gas, or fitted with a heating means, for example electrical heating bars, followed by a tier for steam reforming and then a tier for the water gas shift reaction. These channel tiers, to the extent that manufacturing technology allows, have the smallest possible distance between them so that heat exchange between them is possible.

These channels should preferably have a hydraulic diameter of 1 to 50 mm and particularly preferably of 2 to 25 mm, wherein the hydraulic diameter is the product of the cross sectional area multiplied by a factor of 4 divided by the cross sectional circumference. The use of such microstructured components allows for a particularly compact construc- tion of the reactors.

In a particular embodiment the channels are arranged in a block made of suitable material which may be produced by a 3-D printing process, for example laser sintering.

A further preferred embodiment of the invention is characterized in that the catalyst pro- vided for the water gas shift reaction is selected from the elements Al, Ce, Zr, Fe, Cr, Zn, and/or Cu and is present in metallic and/or oxidic form. These elements are known to be active catalysts for this reaction. A catalyst having the composition CeZrOx is particularly suitable. The invention also relates to a process for steam reforming of hydrocarbon-containing gas to afford synthesis gas, which employs the reactor according to the invention and which comprises the following steps:

- provision of the hydrocarbon-containing gas and steam as input gases in a state suitable for the process,

- introduction of the input gases into a reactor according to the invention which is in a state ready for operation, - catalytic steam reforming of the input gas to afford a synthesis gas comprising carbon oxides, hydrogen and water vapour,

- catalytic establishment of the water gas equilibrium in the synthesis gas by heat exchange between the reaction zones of the steam reforming and the establishment of the water gas equilibrium,

- discharging of the synthesis gas from the steam reforming reactor for further processing outside the process.

A further preferred embodiment of the process according to the invention is character- ized in that the quantitative ratio of steam and hydrocarbon-containing gas in the input gas, i.e. in the gas that is supplied to the reactor, is adjusted such that upon establishment of the water gas equilibrium the hydrogen proportion in the synthesis gas is increased and the carbon monoxide proportion reduced. The low carbon monoxide proportion, and thus also carbon proportion, in the gas reduces the hazard to the reactor posed by metal dusting corrosion. This execution of the water gas shift reaction also liberates heat which can be utilized for the steam reforming reactions.

Working and numerical examples

Developments, advantages and possible applications of the invention are also apparent from the following description of working and numerical examples and the drawings. All described and/or depicted features on their own or in any desired combination form the subject matter of the invention, irrespective of the way in which they are combined in the claims or the way in which said claims refer back to one another. In the figures:

Fig. 1 shows a cross section of a reactor according to the invention with a tubular reactor construction,

Fig. 2 shows a longitudinal section of a reactor according to the invention with a microstructure construction, Fig. 3 shows a cross section of a reactor according to the invention with a micro- structure construction.

In Fig. 1 the reactor 1 comprises firstly the cuboid refractorily lined reactor shell 2 also referred to as a reformer furnace. Numerous reformer tubes 3 are suspended in parallel rows in the reactor 1 . Fig. 1 depicts only one tube by way of example. Installed in the ceilling of the reactor shell 2 are burners 4 for heating the reformer tubes 3. The flames 5 from the burners 4 are oriented vertically downward. The flue gas 6 from the burners 4 is discharged at the bottom of the reactor shell 2. The burners 4 are operated with oxy- gen-containing gas 7, for example air, and with combustible gas 16. "Combustible gas" is to be understood as meaning the residual gas obtained from the synthesis gas after removal of the hydrogen proportion (not shown). The reformer tube 3 is depicted with distorted dimensions for clarity. In reality the tubes are much more slender, with a length between 6 and 14 m and an internal diameter of 4 to 6 inches. A mixture of hydrocar- bon-containing gas 8 and steam 9 forms the input gas 10 introduced into the reformer tube 3. The input gas 10 traverses the catalyst fixed bed 1 1 and, transformed into synthesis gas 12, leaves it through a gas-permeable separating apparatus 13 in the cavity 14 at the bottom of the reformer tube 3. The synthesis gas 12 is discharged from the cavity 14 through the inner tube 15 embedded in the catalyst fixed bed. In this example the tube 15 is helical in order to achieve a greater heat exchange area with the catalyst fixed bed 1 1 . Installed in tube 15 is catalyst active for the water gas shift reaction (not shown). The steam excess in the input gas 10 may be adjusted such that upon establishment of the water gas shift equilibrium during traversal of the tube 15 the hydrogen content in the synthesis gas 12 is increased and the carbon monoxide content reduced. At the top of the reformer tube 3 conduit 15 emerges and conducts the synthesis gas 12 into a collection conduit (not shown) which sends the synthesis gas for further processing known per se.

Fig. 2 and Fig. 3 respectively show a longitudinal section (Fig. 2) and a cross section (Fig. 3) of a reactor 17 according to the invention with a microstructure construction, also known as a microreactor, wherein both the process gas and the heating gas are passed through microchannels arranged in bundles such that mutual heat exchange can occur between them via their outer surfaces. The channels in which the heating gas flows, in which the reaction zone for the steam reforming reactions is located and in which the reaction zone for the water gas shift reaction is located are respectively grouped to- gether to form tiers of channels arranged in parallel to one another. Here, the channels are depicted directly apposed to one another but it is also possible, contingent on manufacturing technology, for the channels to have a certain distance between them. Flue gas 6 flows through the channels in channel tiers 18 and 18' and the reaction zone for the steam reforming reactions is located in each of the channel tiers 19 and 19', the in- put gas 10 being introduced into the channels of tiers 19 and 19'. The synthesis gas 12 formed during traversal of these channels is subsequently passed via a redirection zone 20 into the channels of tier 21 in which the reaction zone for the water gas shift reaction is located.

Numerical example:

A comparison of the following calculated numerical examples shows the advantage of the invention in terms of energy-saving and increased hydrogen proportion in the produced synthesis gas.

Case 1 , prior art, without catalyst for the water gas shift reaction:

An input gas is formed from a stream of a hydrocarbon-containing gas (8) and steam (9), and treated in tubular reactor (1 ), such that the produced synthesis gas (12) leaves the reactor with a temperature of 650°C, a hydrogen content of 46 mol% and a carbon mon- oxide content of 8.2 mol%. Upon traversing the inner tube 15 the synthesis gas transfers 339 kJ/m ³ of heat to the catalyst for the reforming reactions (1 1 ). 2.64 mol of hydrogen are generated per mole of hydrocarbon-containing gas (8).

Case 2, inventive, with catalyst for the water gas shift reaction:

Input gas having the same composition as in case 1 is likewise treated in the tubular reactor (1 ) such that the produced synthesis gas (12) leaves the reactor with a tempera- ture of 650°C. Due to the action of the water gas shift catalyst arranged in the inner tube (15) the synthesis gas (12) leaving the reactor now comprises 48 mol% of hydrogen and 5.9 mol% of carbon monoxide. The amount of heat transferred from the inner tube (15) to the catalyst for the reforming reactions (1 1 ) is 376 kJ/m ³ and thus 1 1 % more than in case 1 . In this case 2, 2.66 mol of hydrogen, and thus 0.6% more than in case 1 , are generated per mole of hydrocarbon-containing gas (8).

Industrial applicability

The invention proposes reactors for generating synthesis gas by steam reforming which make an increased energy yield possible and have a reduced metal dusting corrosion risk. The invention is thus advantageously industrially applicable.

List of reference numerals

1 reactor

2 reactor shell

3 outer reformer tube

4 burner

5 flames from burner

6 flue gas

7 oxygen-containing gas

8 hydrocarbon-containing gas

9 steam

10 input gas

1 1 catalyst for reforming reactions

12 synthesis gas

13 gas-permeable separating apparatus

14 cavity

15 inner tube in which catalyst for the water gas shift reaction is arranged combustible gas

reactor with a microstructure construction channel tier for flue gas

channel tier for steam reforming reactions channel tier for water gas shift reaction