Reactor for catalytic conversion of a fuel

Abstract

A reactor is provided for the catalytic conversion of a mixture of a reactants, such as fuel, into at least one reaction product, the reactor having a reaction chamber containing at least one catalytically active substance for the conversion. The reactor chamber further has a reaction area that at start-up is not yet reactive at ambient temperature, and an ignition area that is already capable of ignition at ambient temperature. Ignition area is in only partial contact with reaction area, and the ignition area is porous and contains one or more noble metals. The thermal contact between the two different areas is such that the heat output dissipated from the ignition area at ambient temperature is less than the heat output generated in this area.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority under 35 U.S.C. .sctn.119 to German Application No. 10132673.4, filed Jul. 5, 2001, which priority application is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention pertains to a reactor for catalytic conversion of a mixture of reactants, as well as to the operation thereof.

[0004] 2. Description of the Related Art

[0005] Reactors for catalytic conversion are normally started in their entirety or have precombustors that introduce the heat convectively. Between the precombustor and the actual reactor, additional metering may be provided to allow combustible mixtures in normal operation to be created only downstream of the precombustor. EP 0 757 968 A1 describes a device for the generation of hydrogen in which the upstream combustor stages are integrated into the reactor such that no intermediate metering of a combustible mixture is possible. This was implemented by a mixture of powder at the input of a bed-type reactor. Even in reactors with integrated precombustors, however, the heating of the entire reactor takes place essentially in a convective manner.

[0006] For cold-starting reactors with catalyst-supporting structures of high heat capacity, a rapid and reliable cold start is highly desirable. This applies in particular to reactors with a plate heat exchanger construction. The start-up must also be reliable under conditions of high humidity and freezing temperatures.

[0007] If a preliminary stage is used for cold-starting, high temperatures are necessary for heat input. If a combustible mixture is fed to the reactor (e.g., a CO oxidizer) even in normal operation, the oxidation of the fuel already takes place in the preliminary stage, at least in part, and results in a preliminary stage running at a high temperature during normal operation. Because of the unselective oxidation taking place at these temperatures, undesired reaction products are also obtained. The high temperatures are detrimental to the service life of the reactor, not only in respect to the high temperatures of the gas generated, which must subsequently be cooled, but also in respect to high material stress and premature catalyst aging. If one wishes to avoid premature oxidation of the fuel, the combustible mixture must be fed in only between the precombustor and the reactor. This makes an additional metering point, including a mixer, necessary and thus leads to a more elaborate and expensive device.

[0008] Accordingly, there remains a need for improved reactors for catalytic conversion of a fuel, particularly reactors with improved cold-start and/or rapid response behavior under conditions of high humidity and freezing temperatures. The present invention addresses some or all of these needs and provides further related advantages.

BRIEF SUMMARY OF THE INVENTION

[0009] In brief, this invention obviates the need for a preliminary stage or a precombustor upstream of the reactor and/or additional metering points for the fuel mixture between precombustor and reactor.

[0010] In one embodiment, a reactor for the catalytic conversion of a mixture of reactants into at least one reaction product is provided, wherein the reactor contains a reactor chamber with at least one inlet opening for the reactants and at least one outlet opening for one or more reaction products, and at least one catalytically active substance for the conversion. The reactor chamber has two different areas, a reaction area that at start-up is not yet reactive at ambient temperature, and an ignition area which is already capable of ignition at ambient temperature, wherein the ignition area is only in partial contact with the reaction area and the ignition area is porous and contains one or more noble metals. The thermal contact between the two different areas is such that the heat output dissipated from the ignition area at ambient temperature is less than the heat output generated in this area.

[0011] In a more specific embodiment, and for optimal starting behavior of the reactor, the ignition area has a high concentration of catalytically active substance or of catalytically active catalyst surface in relation to its macroscopic external surface. This can be achieved, for instance, by greater layer thicknesses of the catalytically active substance of roughly 10-1000 .mu.m, preferably 50-300 .mu.m in this area, or by large particle diameters of the catalytically active substance.

[0012] In further embodiments, a low flow velocity is employed in the ignition area during the starting phase, so that a reduced amount of heat is emitted to the circulating gas. In still another embodiment, the amount of catalyst or catalytically active catalyst surface in the reaction area of the reactor is large in relation to the overall mass or overall heat capacity of the reaction area. Advantageously, the ignition area of the reactor according to the invention is already capable of ignition at temperatures down to -40.degree. C., and typically at temperatures down to -20.degree. C.

[0013] These and other aspects will be evident upon reference to the attached figures and following detailed description.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0014] FIG. 1 is a representative arrangement of reaction area 1 and ignition area 2.

[0015] FIGS. 2a and 2b are further representative arrangements of reaction area 1 and ignition area 2.

[0016] FIG. 3 illustrates the dependency of the heat output generated at and dissipated from the catalyst on catalyst temperature.

DETAILED DESCRIPTION OF THE INVENTION

[0017] As noted above, this invention eliminates the need for a preliminary stage or a precombustor upstream of the reactor and/or additional metering points for the fuel mixture between precombustor and reactor. In the practice of this invention, a reactor for the catalytic conversion of a mixture of reactants into at least one reaction product is provided. The reactor contains a reactor chamber with at least one inlet opening for the reactants and at least one outlet opening for one or more reaction products. The reactor chamber also contains at least one catalytically active substance for the conversion.

[0018] Referring to FIG. 1, the reactor chamber has two different areas, reaction area 1 that is not yet reactive at ambient temperature, and ignition area 2 which is already capable of ignition at ambient temperature, wherein ignition area 2 is only in partial contact with reaction area 1 and ignition area 2 is porous and contains one or more noble metals. The thermal contact between the two different areas is such that the heat output dissipated from the ignition area at ambient temperature is less than the heat output generated in this area.

[0019] Starting of the reactor is initially accomplished by rapid catalytic heating (ignition) of the ignition area, which is optimized for this purpose. In this context, ignition means that the reaction rate at the starting temperature is sufficiently high that the removed heat output is not enough to keep the area in the range of the starting temperature. The reaction rate then also increases exponentially with the temperature and increases the heat output. By solid-state thermal conduction from this thermally poorly coupled ignition area 2, heat is then conducted by way of existing thermal bridges into reaction area 1. Additionally, reaction area 1 is gradually heated up by means of catalytic oxidation by the catalyst located therein. Only at considerably higher temperatures, roughly temperatures around 200-300.degree. C., when material transport limits the reaction rate, does a stationary state arise. Ignition area 2 can optionally be electrically heated or ignited at the start.

[0020] The resulting equilibrium temperatures are highly dependent on the thermal coupling between the reaction area and ignition area. Referring to FIG. 3, curve 1 shows a typical temperature-dependency of the reaction rate, which first increases exponentially and is then limited by material transport at higher temperatures. On the other hand, curve 2 shows that with good thermal coupling to the environment, the stationary temperature of the catalyst is only slightly higher than the ambient temperature (i.e., even a slight temperature increase results in greater energy emission). A cold start is therefore not possible, due to the very good thermal coupling. In contrast, if heat transfer to the environment is poor, as shown in curve 3, then a stationary operating point arises only at a temperature that is considerably elevated with respect to the environment. That means that ignition results. (The heat transfer coefficients labeled .beta..sub.1 and .beta..sub.2 in FIG. 3 follow the relation: .beta..sub.2>.beta..sub.1.) In a representative embodiment, and again referring to FIG. 1, ignition area 2 comprises a porous noble metal-containing particle with a particle size or particle size distribution between roughly 10 and 1000 .mu.m, and typically between roughly 50 and 300 .mu.m. The noble metal-containing particle, which typically contains platinum or palladium, may be on a substrate. It is well know to one skilled in this field that there are a number of substrate materials suitable for catalysts, including ceramics, carbon, metal, plastic, and the like. For example, porous solids, on the surface of which catalytically active material can be deposited, are particularly suited. Ceramic materials such Al.sub.2O.sub.3, zeolites, SiO.sub.2, ZrO.sub.2, CeO.sub.2 and/or mixtures thereof are also used as substrate materials, with Al.sub.2O.sub.3 having been found to be particularly suitable. The porosity of the catalyst particle has the effect that the reaction can run in the inner areas of the catalyst particles both in the starting phase and in normal operation of a reactor.

[0021] As shown in FIG. 1, the noble metal-containing porous particle is only in partial contact with reaction area 1, these contacts to reaction area 1 representing thermal bridges; otherwise, ignition area 2 is weakly coupled thermally to reaction area 1. The microscopic catalyst surface in the particle (also called internal surface O.sub.1), the reaction rate at a given temperature (r(T)) and the reaction enthalpy (H) determine, among other things, the heat generated at start-up (P.sub.generated) at ambient temperature according to the following equation:

P.sub.generated=O.sub.1.times.r(T).times.H

[0022] The number and extent of the contact points, the diameter and the associated external surface (O.sub.a) of the particle, and the coefficient of thermal conductivity from the particle into the surrounding phase and the heat transfer coefficient from the solid into the gaseous phase, both of which are taken into account by the coefficient .beta., determine among other things the drawn-off heat output P.sub.drawn-off according to the following equation:

P.sub.drawn-off=O.sup.a.times..beta..times.(T.sub.catalyst structure-T.sub.ambient)

[0023] Both P.sub.generated and P.sub.drawn-off can be matched to one another such that the heat output dissipated from the ignition area at ambient temperature is less than the heat output generated in this area, which causes the reaction to start.

[0024] Representative examples of suitable reaction chambers having reaction and ignition areas include the layers produced by powdered metallurgy according to EP 0 906 890 A1 (incorporated herein by reference), in which a powder mixture (reaction area) is added to a platinum-containing catalyst (ignition area) on an Al.sub.2O.sub.3 substrate during manufacture of discs. The reaction area is formed of a macroscopic, metal-containing porous substrate structure, which can also be provided with an additional catalyst insofar as a catalytically active material (such as copper or dendritic copper) is not used in the first instance. This substrate structure is preferably a net-like matrix, which can be obtained by mixing the catalyst powder with a metal powder and pressing the mixture. In the pressing process, the metal powder forms a net-like matrix structure (reaction area), in which the catalyst particles are "built in" (ignition area). Particularly suited as a starting material for the metallic matrix are dendritic copper powders, which can readily be pressed or sintered into a network even with a relatively low mass proportion of the copper to the total mass of the layer, have a large surface area and are themselves catalytically active. When dendritic copper powder is used, for example, a stabilizing, linking and heat distributing network in the micron range is obtained.

[0025] In other embodiments, and as illustrated in FIGS. 2a and 2b, ignition area 2 is formed by a macroscopic, catalyst-containing porous substrate structure lying adjacent to reaction area 1. This may be, for example, a catalyst loaded net structure, nonwoven fabric or foam (ignition area 2) inserted between two heat exchanger plates 3 coated with catalyst (reaction area 1).

[0026] In general, reaction area 1 and ignition area 2 are present in the reactor in a spatially mixed configuration or continuously arranged. Since educts intended for reaction area 1 can react prematurely in normal operation, ignition area 2 may be located downstream of reaction area 1. The heating of reaction area 1 then takes place contrary to the direction of flow, and possibly by way of solid-state heat conduction from area 2 and, in some cases, by catalytic self-heating beginning to occur in reaction area 1. Thus, the reaction front moves forward contrary to the direction of reactant flow. During normal operation, ignition areas 2 do not create any problems, since the oxygen for oxidation is consumed upstream in reaction area 1.

[0027] Such reactors are useful over a wide range of applications, including (but not limited to) use as a catalytic burner, a catalytically heated heat exchanger, for partial oxidation, autothermal reformation, selective CO oxidation or in conjunction with a fuel cell.

[0028] From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

Claims

1. A reactor for the catalytic conversion of a mixture of reactants into at least one reaction product, the reactor having a reactor chamber with at least one inlet opening for the reactants and at least one outlet opening for the one or more reaction products, and at least one catalytically active substance for the conversion of the mixture of reactants into the at least one reaction product, the reactor chamber comprising a reaction area that at start-up is not yet reactive at ambient temperature and an ignition area which is already capable of ignition at ambient temperature, wherein the ignition area is only in partial contact with the reaction area and the ignition area is porous and contains one or more noble metals, wherein the thermal contact between the reaction area and the ignition area is such that the heat output dissipated from the ignition area at ambient temperature is less than the heat output generated in the ignition area.

2. The reactor of claim 1 wherein the ignition area is ignitable at temperatures down to -20.degree. C.

3. The reactor of claim 1 wherein the ignition area has a high concentration of catalytically active substance in relation to its macroscopic external surface.

4. The reactor of claim 1 wherein the ignition area is arranged essentially downstream of the reaction area.

5. The reactor of claim 1 wherein the catalytically active substance of the ignition area has a particle size or particle size distribution ranging from 10 and 1000 .mu.m.

6. The reactor of claim 1 wherein the ignition area is applied in a layer thickness of 10-1000 .mu.m.

7. The reactor of claim 1 wherein the ignition area is electrically heatable or ignitable.

8. A method for the catalytic conversion of a mixture of reactants into at least one reaction product, comprising introducing the mixture of reactants into the at least one inlet opening input of the reactor of claim 1.

9. The method of claim 8 wherein the conversion comprises partial oxidation of the mixture of reactants.

10. The method of claim 8 wherein the conversion comprises autothermal reformation of the mixture of reactants.

11. The method of claim 8 wherein the conversion comprises selective CO oxidation.

12. The method of claim 8 wherein reactor is a catalytic burner.

13. The method of claim 8 wherein the reactor is a catalytically heated heat exchanger.

14. The method of claim 8 wherein the reactor is used in conjunction with a fuel cell.