Reactor for the generation of hydrogen

Abstract

The invention relates to a reactor (2) for generating hydrogen and at least one other product from at least one reactant, the reactor comprising a tubular reactor vessel (4) which contains a catalyst (6) in the form of a ceramic bed. Improved corrosion resistance against a variety of media and thus an increased service life of the reactor (2) is achieved by forming the reactor vessel (4) from silicon-infiltrated silicon carbide (SiSiC).

Description

[0001] Description

[0002] Reactor for producing hydrogen

[0003] The invention relates to a reactor for producing hydrogen and at least one further product from at least one reactant.

[0004] In many areas of chemical process engineering, reactors, catalysts, heat exchangers, and evaporators are used to split or synthesize substances. The basic principle of such plants is based on the spatial separation of two material streams and the transfer of thermal energy through the reactor structure. High temperatures, possibly elevated pressures, and the presence of a catalyst are often required to activate these processes. Two examples of such chemical processes are steam reforming and ammonia dehydration to produce hydrogen (H2) from hydrocarbons such as methane (CH4) and ammonia (NH3).

[0005] In ammonia cracking, ammonia is evaporated, heated, and split into hydrogen and nitrogen over a nickel catalyst in a reactor (cracker) at temperatures of up to 950 °C and a pressure of more than 1 bar. To activate and maintain the endothermic process, thermal energy is supplied by electrical heating or a burner. A particular challenge is the corrosive attack on the corrosion resistance of the reactor materials used by the starting media (hydrocarbons, steam, ammonia) and the reaction products (hydrogen, carbon, nitrogen). The lack of corrosion resistance of metal-based systems (stainless steels and nickel-based alloys) under ammonia, hydrogen, and nitrogen atmospheres represents the major limitation to achieving high conversion rates.If the heat input is not achieved by electrical heating, heating with a burner results in the additional requirement of resistance to oxidising atmospheres.

[0006] The main requirement is to achieve a matched heat transfer performance with the highest possible efficiency and the lowest possible flow resistance or pressure loss. Further essential requirements for the design and properties of a reactor arise from a structural and process-related perspective. High efficiency is promoted by a high thermal conductivity of the reactor material and a large internal heat exchanger surface (heat transfer) with the smallest possible external surface (heat loss through radiation and convection), as well as the requirement for the lowest possible heat loss through the escaping reaction products.

[0007] Plants for ammonia cracking currently only exist in the capacity range < 100 kg H2 / h and are constructed similarly to steam reformers.

[0008] Metal-based plant concepts for steam reforming are usually operated with beds of mostly extruded ceramic profiles (pellets) that are coated or vapor-deposited with the catalyst material.

[0009] The invention is based on the object of improving the corrosion resistance of a reactor for producing hydrogen and thus ensuring an increased service life.

[0010] The object is achieved according to the invention by a reactor for producing hydrogen and at least one further product from at least one reactant, comprising a tubular reactor vessel in which a catalyst is contained in the form of a bed, wherein the reactor vessel is made of silicon-infiltrated silicon carbide (SiSiC). An improvement in corrosion resistance against a large number of media (in particular against the influence of water vapor, ammonia, hydrogen and nitrogen) and thus an increased service life of the reactor is achieved in that the reactor vessel, thus the main component of the reactor, is made of a ceramic material. In order to ensure material separation of the heat and material flows, the ceramic material is gas-tight.

[0011] In the case of ammonia cracking at high temperatures, for example, there is significant corrosion of the metallic reactor materials caused by the hydrogen and nitrogen atmosphere. The use of corrosion- and high-temperature-resistant ceramic materials solves this problem, enables high conversion rates (>99.8%), and offers the potential for significantly increasing component service life. With regard to chemical resistance for ammonia cracking, the ceramic tube vessel achieves high corrosion resistance against the ammonia medium being converted and against the reaction products, particularly hydrogen and nitrogen, in the temperature range of 300–950 °C and pressures of 1 to 40 bar.

[0012] According to the invention, the ceramic material from which the reactor vessel is made is silicon-infiltrated silicon carbide (SiSiC). This material is characterized by a number of advantages with regard to its mechanical, thermal, and chemical properties. It meets the strength requirements in the temperature range from room temperature to 1100 °C due to the high steady-state and transient thermal gradients. In addition, it exhibits high oxidation and corrosion resistance, high thermal conductivity, and high resistance to thermal shock. A further advantage is the rapid heat-up behavior due to the thermal-mechanical material properties of SiSiC and thus a short switch-on delay (this enables use in cyclically operated systems). The low weight when using SiSiC also enables its use for non-stationary applications.

[0013] Preferably, the catalyst bed is also made of a ceramic material, in particular silicon-infiltrated silicon carbide (SiSiC), and coated with a catalytic material. If ammonia decomposition takes place in the reactor, the catalytic coating is nickel, for example. For other chemical reactions, however, the coating material is different, depending on the requirements. The advantages of silicon-infiltrated silicon carbide already listed with regard to the reactor vessel can be transferred to the catalyst bed. Since nickel (Ni) is a preferred and economically viable catalyst material for the production of hydrogen (H2) by decomposition of, for example, ammonia (NH3), the ceramic material must be coatable or wettable with nickel. Silicon-infiltrated silicon carbide also exhibits this property.

[0014] The particle size and shape of the catalyst depend on the respective flow channel. The most important geometric factor with regard to the activity of the catalytically coated bed is the ratio of the geometric surface area of ​​the particles to their volume. To achieve a high reactor surface area, the smallest possible particle size is desirable. However, this is counteracted by the requirement for the lowest possible pressure drop and flow resistance, which preferably requires a large particle size.

[0015] Cylindrical structures with one or more longitudinal bores and grooves on the outer surfaces to increase the surface area are advantageously used as catalyst particles. For smaller flow cross-sections, simple geometric structures such as spheres and solid cylinders are also suitable. The particle size depends on the respective flow channel. For example, depending on the size of the flow channel, particles with a diameter of 1 to 20 mm and a length-to-diameter ratio of 0.5 to 1.5 are provided. The ratio of reactor diameter to particle diameter is usually 3 to 10.

[0016] According to a first preferred embodiment

[0017] - the tubular reactor vessel is constructed from a cylindrical tube,

[0018] - the tube has at one end a feed opening for the at least one reactant and at the other end a discharge opening for the at least one product, and

[0019] - the catalyst is located in a central region of the tube and is held in position by means of a flow-through fixing element. This represents a simple design of the reactor. A cylindrical tube is understood here to be an essentially straight tube without significant bends. The fixing element is porous so that the reactant stream and / or the product stream can flow through it. The fixing element is preferably also made of SiSiC (silicon-infiltrated silicon carbide) or another ceramic material, as it is located in the region of the highest process temperatures. The pores of the fixing element are smaller than the diameter of the particles in the catalyst bed. If the tube is aligned vertically, only a single fixing element is required, which supports the catalyst bed from below. For example,If the tube is arranged horizontally, two fixing elements may be required on either side of the catalyst bed.

[0020] Advantageously, the fixing element is attached to the cylindrical tube via a positive and / or non-positive connection. Such a connection is suitable for use at high temperatures, such as those encountered during reactor operation. In principle, a material-fit connection is also suitable. When using a standard tube geometry, a positive connection is particularly advantageous from a design point of view. In its simplest version, this can be realized with a preferably ceramic support rod that holds the fixing element in position. In alternative embodiments, the fixing element can be held by a pin attached to the tube, a shoulder, or a conical inner tube geometry.

[0021] According to a second preferred embodiment, the tubular reactor vessel is formed from a U-shaped tube with an apex, the tube is directed upwards with both ends and has a feed opening for the reactant at a first end and a discharge opening for the products at a second end, and the catalyst is located in the region of the apex of the tube. In this embodiment, the fixing element is omitted since the catalyst bed is held in the lower region, in the region of the apex of the tube, by the shape of the curved tube and the gravitational forces.

[0022] According to a third preferred embodiment, the tubular reactor vessel is formed from a W-shaped or multiple W-shaped tube, the tube having two or more lower vertices, the tube being directed upwards with both its ends and having a feed opening for the reactant at a first end and a discharge opening for the products at a second end, and the catalyst being located in the region of the lower vertices of the tube. At least two catalyst beds are provided. In this embodiment, no fixing element for the catalyst beds is required. In addition, the W-shaped design of the ceramic tube offers the advantage of a longer path for the reactants in the reactor and a longer residence time.

[0023] According to a fourth preferred embodiment, the tubular reactor vessel comprises a cylindrical inner tube and a cylindrical outer tube, the inner tube and the outer tube extend vertically, the inner tube is arranged in the outer tube, in particular concentrically, and the diameter of the inner tube is smaller than the diameter of the outer tube, so that a flow channel is formed around the inner tube, the lower end of the inner tube is higher than the lower end of the outer tube, so that an intermediate space is formed in the region of the lower end of the outer tube, the outer tube has a feed opening for the reactant at its upper end and its lower end is closed, the inner tube has a discharge opening for the product at its upper end and its lower end is open, and the intermediate space is filled with the catalyst, wherein the lower end of the inner tube extends into the catalyst.In this way, the at least one reactant and the products are transported in opposite directions. The main advantage of this embodiment is that heat exchange takes place between the inner and outer tubes. The products in the inner tube, which are at a high temperature, give up their heat to the reactants flowing in the opposite direction in the flow channel of the outer tube. These are thereby heated, so that the amount of heat that has to be supplied from the outside is lower. A high level of efficiency is achieved by integrating the heat exchanger and reactor and by reducing heat losses due to the small external surface area.

[0024] To achieve the temperature level required for the reaction to proceed in the reactor, the tubular reactor vessel is preferably arranged in a heating device. The tubular reactor vessel can be completely enclosed by the heating device, or it may be sufficient if only the region of the tube containing the catalyst and the chemical reaction is surrounded by the heating device. The heating device can be an electric heater, contain a burner, or be based on another heating principle.

[0025] Embodiments of the invention are explained with reference to a drawing. The individual figures schematically show:

[0026] FIG 1 shows a first embodiment of a catalytic reactor, FIG 2 shows a second embodiment of a catalytic reactor, and

[0027] FIG 3 a third embodiment of a catalytic reactor.

[0028] The same reference symbols have the same meaning in the figures.

[0029] FIG 1 shows a first catalytic reactor 2, i.e. a reactor for the catalytic conversion of at least one reactant, in this case ammonia NH3, into hydrogen H2 and nitrogen N2. The reactor 2 comprises a tubular reactor vessel 4 in which a catalyst 6 is contained in the form of a bed. The reactor vessel 4 is made of a ceramic material, in this case silicon-infiltrated silicon carbide (SiSiC). The bed 6 of catalytically coated particles is preferably also made of ceramic material due to its higher corrosion resistance. Commercially available catalyst supports (beds) are based, for example, on aluminum oxide (Al2O2) or steatite (magnesium silicate). In the exemplary embodiment shown, however, the catalyst 6 is also made of silicon-infiltrated silicon carbide (SiSiC). The catalyst particles are also coated with a catalytic coating made of nickel.

[0030] In the embodiment shown, the tubular reactor vessel 4 is formed from a straight, cylindrical tube 3. The tube comprises connecting flanges 8 at one or both ends. The tube 3 is aligned vertically and has a feed opening 10 for the ammonia stream NH3 at the upper end and a discharge opening 12 for discharging the product gases hydrogen H2 and nitrogen N2 at the lower end. The catalyst 6 is located in a central region of the tube 4 and is held in position by means of a flow-through fixing element 14. The fixing element 14 is in particular porous or designed as a honeycomb body and allows the product gases to flow through it.

[0031] The process gas flows through the tube in the longitudinal direction and is heated in the inlet area, close to the feed opening 10, by an externally supplied heat flow Q to the temperature level required for the subsequent cracking reaction of 700 to 950 °C (for high gas mass flows and high required conversion rates / purities even up to 1000 °C). The required temperature level depends on the required degree of purity of the process gases after cracking and on the gas mass flow. Partial cracking already takes place from a temperature of 600 °C. The technical advantage of the SiSiC tubular reactor over a metallic solution, however, is in particular its corrosion resistance under the described atmospheres and at high temperatures. The conversion rate, i.e. the degree of conversion of ammonia NH3 into hydrogen H2 and nitrogen N2, is determined as a function of the gas temperature, the catalyst parameters and the gas mass flow.

[0032] The reactor according to FIG 2 differs from the first embodiment in the shape of the reactor vessel 4 . The ceramic tube 16 is based on a U-shaped tube geometry of the SiSiC tube . This design variant enables the feed of the reactant gas and the discharge of the product gas from the same side . In this design variant, the flow-through fixing element 14 according to FIG 1 is omitted since the catalytically coated bed is located at the lower apex of the U-tube 16 . In addition, a heating device 18 is shown in which the reactor vessel 4 is integrated, through which the heat for starting and maintaining the reaction is supplied, which is indicated by Q.

[0033] According to FIG 3, the reactor 2 consists of an outer tube 18 with a closed bottom and an inner tube 20 which is guided concentrically in the outer tube 18. The inner tube 20 does not reach as far as the bottom of the outer tube 18, so that in the bottom region between the inner tube 20 and the outer tube 18 a gap is created which defines an intermediate space 22. The bottom region of the outer tube 18 is filled with the ceramic, catalytically coated bed 6. The inner tube 20 projects into this catalytic bed 6. The reactant gas flowing into a flow channel 24 in the region of the annular gap between the outer tube 18 and the inner tube 20 flows in the direction of the outer tube bottom and flows through the catalytically coated bed 6, within which the splitting to hydrogen H2 and nitrogen N2 takes place.The resulting product gas flows through the free space 22 into the inner tube and, counter to the flow direction of the incoming ammonia NH3, towards the discharge opening of the inner tube 20 and, releasing heat to the incoming ammonia NH3 via the wall of the inner tube 20.

[0034] In all three figures, the heat flows supplied from the outside to the reactor vessel 4, which are generated by a heating device (18 in FIG. 2), are designated by Q (FIG. 1 and FIG. 3 without representation of the heating device). These heat flows are generated, for example, in a furnace chamber, either by heating with electrical resistance heating elements or by gas flames. The furnace is a conventional high-temperature furnace, such as is used, for example, for ceramic firing or heat treatment, and into which the ceramic reactor tube, including the inlet and outlet openings, is integrated through the furnace wall(s).

Claims

Patent claims 1. Reactor (2) for producing hydrogen and at least one further product from at least one reactant, comprising a tubular reactor vessel (4) in which a catalyst (6) is contained in the form of a bed, wherein the reactor vessel (4) is formed from silicon-infiltrated silicon carbide (SiSiC).

2. Reactor (2) according to claim 1, wherein the catalyst bed is made of another ceramic material, in particular silicon-infiltrated silicon carbide (SiSiC), and is provided with a catalytic material is coated.

3. Reactor (2) according to claim 1 or 2, wherein - the tubular reactor vessel (4) is constructed from a cylindrical tube (3), - the tube (3) has at one end a feed opening (10) for the at least one reactant and at the other end a discharge opening (12) for the at least one product, and - the catalyst (6) is located in a central region of the tube (3) and is held in position by means of a flow-through fixing element (14).

4. Reactor (2) according to claim 3, wherein the fixing element (14) is attached to the cylindrical tube (3) via a positive and / or non-positive connection.

5. Reactor (2) according to one of claims 1 or 2, wherein the tubular reactor vessel (4) is formed from a U-shaped tube (16) with an apex, the tube (16) is directed upwards with its two ends and has a feed opening (10) for the reactant at a first end and a discharge opening (12) for the products at a second end, and the catalyst (6) is located in the region of the apex of the tube (16).

6. Reactor (2) according to one of claims 1 or 2, wherein the tubular reactor vessel (4) is formed from a W-shaped or a multiple W-shaped tube, wherein the tube has two or more lower vertices, the tube is directed with its two ends upwards and has a feed opening (10) for the reactant at a first end and a discharge opening (12) for the products at a second end, and the catalyst (6) is located in the region of the lower vertices of the tube.

7. Reactor according to one of claims 1 or 2, wherein the tubular reactor vessel (4) comprises a cylindrical inner tube (20) and a cylindrical outer tube (18), the inner tube (20) and the outer tube (18) extend vertically, the inner tube (20) is arranged in the outer tube (18), in particular concentrically, and the diameter of the inner tube (20) is smaller than the diameter of the outer tube (18), so that a flow channel is formed around the inner tube (20), the lower end of the inner tube (20) is higher than the lower end of the outer tube (18), so that an intermediate space (22) is formed in the region of the lower end of the outer tube (18), the outer tube (18) has a feed opening (10) for the reactant at its upper end and its lower end is closed, the inner tube (20) has a discharge opening (12) for the product at its upper end and its lower End is open, and the intermediate space (22) is filled with the catalyst (6), wherein the lower end of the inner tube (20) extends into the catalyst (6).

8. Reactor (2) according to one of the preceding claims, wherein the tubular reactor vessel (4) is arranged in a heating device (18).

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