DE19539648C2 - Reactor for selective CO oxidation in H¶2¶-rich gas - Google Patents

Description

The invention relates to a reactor for selective oxidation of carbon monoxide in H₂-rich gas according to the preamble of Claim 1.

For the operation of fuel cells with polymer electrolyte membranes, in short PEM fuel cells, for mobile applications is said to be hydrogen, for example, from methanol through water steam reforming or generated by partial oxidation. In both processes, carbon monoxide is by-produced Amounts that are not tolerated by a PEM fuel cell can be. The reformer product gas must therefore be cleaned. In addition to selective separation, a possible solution is of the hydrogen over a palladium-silver membrane or one Methanation of carbon monoxide on a suitable catalyst the selective oxidation of carbon monoxide on suitable Oxidation catalysts known.

CO oxidation is a highly exothermic reaction that takes place in a Hydrogen atmosphere selectively only in a narrow temperature interval takes place. With known catalysts on platinum This range is approximately between 80 ° and 400 ° Celsius. At temperatures below 80 ° C the activity of the Low CO oxidation catalyst. At temperatures above 400 ° C is also oxidized with the hydrogen, that is, the reaction can no longer be carried out selectively. At CO input concentrations of 1-3% as at the output of Compact reformers for mobile fuel cell applications can occur, but are adiabatic temperature increases up to 200 K possible.  

Most of the exothermic ones are found in conventional tubular reactors Reaction takes place in a very narrow area of the reactor, the Location and expansion is dependent on the load. From WO 93/19005 A1 for example, a two-stage bulk reactor for selective Oxidation of carbon monoxide in an H₂-rich and oxygen containing gas known. There is one in each of the two stages Cooling coil provided. The heat dissipation is only in sufficient area of the cooling coil. In other Areas of the reactor, however, have no heat dissipation reaching. At gas inlet temperatures of around 100 ° C therefore, temperature peaks occur in the reactor that exceed 400 ° C lie and therefore do not allow a selective reaction. As a result of that the reaction takes place in a limited range also low catalyst utilization.

A generic reactor in plate construction is from the JP 07-126001 A known. This is done by each other stacked dividing plates alternately reaction and tempering spaces formed. The supplied are in the reaction rooms Educts evaporated one after the other in the direction of flow, reformed and then cleaned. Because the gas composition is based on changed the way through the reaction chamber, the catalyst for example, not evenly in the gas cleaning stage claimed. Rather, the catalyst at the entrance of the Gas cleaning stage heavily loaded, while the catalyst in rear area due to the decrease in CO concentration is fully exploited.

It is the object of the invention to provide an isothermal reactor selective oxidation of carbon monoxide in a hydrogen atmosphere to create that is compact and with the CO portion even in dynamic operation to values below 50 ppm can be reduced.

The object is achieved by the features of the kenn drawing part of the main claim solved.  

Due to the plate construction and the application of the catalyst the heat dissipation is greatly reduced on the partition to the refrigerator improves and thereby prevents impermissible temperature peaks. With With the help of the additional catalyst carrier foils can additionally the catalytically active surface can be enlarged.

The introduction of porous intermediate layers has the advantage that the gas supply is more uniform over the length of the barrel and thus almost the entire catalyst material with high Efficiency is used.

The introduction of support structures serves for mechanical Support and to create defined reaction spaces. Soon this can result in a more even flow distribution and who achieves an improvement in heat and mass transport the. Through the catalyst coating of these support structures in The reaction spaces can eventually be the effective catalyst surface can be further increased.

An optimization of the ratio of geometric area that is decisive for heat transfer, for effective Kataly sator surface is a variation of the amount of catalyst in Depending on the reactor run length reached. The through The porous interlayer can also be made non-permeable Function of the reactor run length vary, creating a targeted Dosing of the gas flows can be achieved. An areal Distribution of the gas streams can also be achieved by that the feed spaces are higher than the reaction spaces Show pressure level.

Further advantages and configurations come from the sub claims and the description. The invention is described below with reference to a drawing, wherein

Fig. 1 shows the basic structure of a reactor according to the prior art, in section,

Fig. 2 shows another embodiment with a zusätz union catalyst carrier film in the reaction chamber,

Embodiments of Fig. 3a-c according to the invention a reactor with porous intermediate layers, and

Fig. 4a-b show further embodiments of the invention with porous intermediate layers, and separate supply of reformate and oxygen FIG.

The reactor in filter press design, identified overall by 1 in FIG. 1, can be constructed from a multiplicity of individual cells 2 , an individual cell 2 in the simplest case being formed by two partition walls 3 arranged essentially parallel to one another. The spaces between the partitions 3 are used alternately as reaction space 4 and as cooling space 5 . Here, at least the surfaces of the partition walls 3 facing the reaction chamber 4 are covered with catalyst material 6 . This ensures optimal heat dissipation.

The reactor 1 is used to remove carbon monoxide CO by means of selective oxidation from an H₂-rich gas. A preferred embodiment of this is the CO removal from a reformate, which is produced by steam reforming from methanol in a reformer, not shown. This reformate, consisting for example of hydrogen with a CO content of 1-3 vol%, is preferably used for the mobile use of PEM fuel cells in vehicles. In the selective oxidation, the reformate is additionally supplied with oxygen, for example in the form of ambient air, in which case the carbon monoxide CO is oxidized to carbon dioxide CO₂ by the oxygen O₂. This reaction is carried out on a suitable oxidation catalyst 6 , for example platinum and / or ruthenium on a suitable support, such as Al₂O₃ or a zeolite in powder form. Here, the reformate is supplied with air or oxygen in a CO to O ratio of approximately 1: 1 to 1: 4. In order to prevent a predominant reaction between the oxygen and the hydrogen, the reaction should not take place at temperatures above 400 ° C. Below about 80 ° C, on the other hand, the activity of the catalyst 6 for the CO oxidation is low. Therefore, the reaction should take place depending on the catalyst 6 used in a predetermined temperature interval, here approximately between 80 ° and 400 ° C.

Since it is a highly exothermic reaction, the reactor must be cooled to maintain a predetermined temperature level. For this purpose, the cooling rooms 5 are flowed through by a suitable heat transfer medium. For better flow management, the surfaces of the partition walls 3 facing the cooling chamber 5 can be structured. This can be done, for example, during production by stamping or rolling. For example, a two-phase mixture of water and / or methanol can be used as the heat transfer medium, the vapor content being 10 to 90 percent by mass. As a result, heat can be supplied during the cold start by condensation of the gas phase and the heat of reaction can be removed during operation by evaporating the liquid phase. The temperature level of the reactor 1 can be adjusted via the composition of the methanol / water mixture. As a further embodiment, however, it is also possible to design the cooling space according to the so-called heat pipe principle.

In the reaction and / or cooling rooms 4 , 5 , additional support structures, for example in the form of nets 7 or structured foils, can be introduced. These are used primarily to support the individual partitions 3 and to create defined geometric arrangements. In addition, the nets 7 , which consist for example of stainless steel, copper, aluminum, plastic or graphite, but also lead to a more uniform flow distribution and an improvement in the heat and mass transfer. However, additional structures can also be provided for this purpose. To further increase the effective catalyst surface area, the nets 7 or the additional structures in the reaction space 4 can also be covered with catalyst material 6 . The catalyst material 6 can be applied to the dividing walls 3 and the nets 6 by spraying, dipping, film casting or screen printing and can be fixed and stabilized using conventional methods, for example drying, calcining or heat treatment.

In order to ensure sufficient mixing of the reformate air flow, the inlet area of the reaction spaces 4 can be designed as a mixing area. No catalyst material 6 is applied in this mixing area, so that only the gas stream is mixed at the distributor structures.

To form a complete reactor, the stack of dividing walls 3 and nets 7 is in each case delimited by an end plate (not shown), optionally introduced into a housing (not shown) and provided with corresponding feed and discharge lines. Seals made of plastic, metal, graphite or a composite material are inserted between the individual plates. Alternatively, the individual plates can also be connected gas-tight by welding. The end plates are made of stainless steel, aluminum, ceramic and / or a composite material. For thermal insulation, an insulating plate made of plastic is additionally installed between the individual cells 2 and the end plates. The reaction gases and the cooling medium can be distributed by structures integrated in the end plates and / or by means of separate distributor plates made of plastic or metal. In order to improve the flow guidance, the surfaces of the partition walls 3 facing the reaction spaces 4 can also be structured. Materials such as stainless steel, aluminum, copper, graphite, Kevlar or alternative old lightweight materials can be used for the partitions between the reaction and cooling rooms 4 , 5 . The partitions within the reaction spaces 4 can consist of stainless steel, aluminum, copper, graphite or graphite paper, Kevlar or of porous plastic membranes.

The reaction or cooling spaces 4 , 5 of the individual cells 2 can be flowed through in parallel and / or in series. The media in the reaction and cooling rooms 4 , 5 can be carried out in cocurrent or in countercurrent, but all of the exemplary embodiments shown are designed according to the direct current principle.

Fig. 2 shows a second embodiment, wherein the same parts are identified by the same reference numerals. In the difference from FIG. 1, a catalyst carrier film 8 , which is arranged essentially parallel to the partition walls 3 and is coated on both sides with catalyst material 6, is additionally provided in the reaction spaces 4 . This can further increase the catalytically active surface. It is also possible to coat the catalyst carrier film 8 with another catalyst material 6 that operates in a different temperature range. As a result, the effective temperature range of the reactor 1 can be increased.

An embodiment of the arrangements according to FIGS. 1 and 2 according to the invention is shown in FIG. 3, the same parts again being identified by the same reference numerals. According to FIG. 3, additional porous intermediate layers 9 , which are arranged essentially parallel to the partition walls 3 , are provided in the reaction spaces 4 . The porous intermediate layers 9 can be formed both by a layer made of a porous material and by an impermeable film or wall with a multiplicity of small holes 10 . By the intermediate layers 9, the reaction chambers 4 in relation to the main flow direction in a plurality of parallel sub-spaces 4 are divided 4a.

A part of the sub-rooms 4 , 4 a is sealed gas-tight on the outflow side and therefore serves as a supply space 4 a. The other subspaces are sealed gas-tight on the inflow side and in this case form the actual reaction spaces 4 . Per single cell 2 is in each case at least one feed compartment 4a and a reaction chamber 4 is provided, wherein the gas exchange between the supply chambers 4 a and the reaction chambers 4 via the porous intermediate layers. 9

In the exemplary embodiment according to FIG. 3 a, an individual cell 2 consists of a feed space 4 a and a reaction space 4 . About the supply chamber 4 a reactor 1 is fed, which consists of a mixture of starting material reformate air flowing then evenly distributed over the porous intermediate layer 9 in the reaction space. 4 The CO portion in the reformate reacts as completely as possible on the catalyst 6 in the reaction space 4 with the formation of carbon dioxide, so that the purest possible hydrogen gas, ie with a CO portion below 50 ppm, leaves the reactor 1 via the reaction space 4 .

A slightly modified arrangement is shown in FIGS. 3b and 3c. Here, two porous intermediate layers 9 are provided for each individual cell 2 . In this case, the space is closed between the two intermediate layers 9 on the downstream side and serves as a supply chamber 4 a, while the other two partial spaces between the intermediate layers 9 and are each closed opposite partition walls 3 in the formation of reaction chambers 4 on the upstream side. In the calculations according Anord FIGS. 3a and b only the inner walls and / or networks 7 are coated in the reaction spaces 4 with Catalyst 6, so that the selective oxidation takes place solely in the reaction spaces. Referring to FIG. 3c and the inner walls and / or networks 7 may be coated with a catalyst 6 in addition in the feed chamber 4, so that a part of the starting material is reacted already here. In this case, the inlet area of the feed channels 4 a can again be designed as a mixing area, that is to say it cannot be coated with catalyst material 6 .

In contrast to the arrangements described above, according to FIG. 4, the reformate and the oxygen are not mixed before the reactor 1 , but are fed separately. Here, the oxygen or the air over the Zuführräume 4 a is supplied, while the reformate is fed directly into the reaction chambers. 4 In contrast to the arrangements according to FIG. 3, in this case the reaction spaces 4 on the upstream side are not sealed gas-tight. As already described in the previous exemplary embodiment, two variants are again conceivable here. According to Fig. 4a two intermediate layers 9 are provided per single cell 2, so that each single cell 2 in each case comprises a central supply chamber 4 a and two adjacent on opposite sides of the reaction spaces 4. The second variant is shown in Fig. 4b, which here again only one intermediate layer 9, and thus, only one supply chamber 4 a and a reaction chamber 4 are provided. Since there is no reformate in the feed chamber 4 a, but only oxygen or air, coating with catalyst material in the feed chamber 4 a is not expedient.

According to an advantageous embodiment of the arrangement, the air can be supplied to the Zuführräumen a 4 compared to the reaction chambers 4 with a positive pressure. As a result, the air flows over a wide area from the feed spaces 4 a into the reaction spaces 4 . Here the reaction takes place evenly over the entire length of the reactor, so that the efficiency of the catalyst is increased. For more even dosing of the educt, it is also possible not to keep the porosity constant over the entire intermediate layer, but to vary it, for example, depending on the length of the barrel. In the case of porous layers, the permeability can be changed; in the case of perforated films, the hole spacing and / or the hole diameter can be chosen differently.

In addition to the previously described coating of surfaces with suitable catalyst material 6 , it is of course also possible to introduce the catalyst material in the form of pellets or any fillings.

Another significant advantage of this reactor 1 is the possibility of optimizing the ratio of the geometric area between the reaction and cooling rooms 4 , 5 , which is decisive for the heat transfer, and the effective catalyst surface. The active catalyst surface can namely be changed via the layer thickness or the platinum content in the catalyst powder. In addition, it is possible to alternately provide areas with and without catalyst coating 6 , the occupancy being changeable by the choice of the areas and the spacing of the individual coatings. Any combination of the measures described above makes it possible to influence the reaction in the reactor 1 in a very targeted manner. The temperature at the catalyst 6 can be kept in the optimum range by a suitable choice of the operating temperature and by an isothermal operation of the reactor 1 . This makes it possible to increase the selectivity of the reaction and to reduce the excess air required. In addition, a larger proportion of the catalyst material 6 can be included in the actual reaction.

Several of the reactors 1 described above can also be connected in series in order to achieve a multi-stage process control, if appropriate also with reactors of conventional design. Fresh air is generally metered in between the individual stages. With the help of such multi-stage arrangements, it is possible to reduce the CO content in the hydrogen gas to values below 40 ppm even with changing load requirements.

Claims (7)

1. Reactor for the selective oxidation of carbon monoxide in hydrogen-rich gas, which is constructed by stacking partitions in a filter press design, with two adjacent partitions flowed through by hydrogen-rich gas and oxygen or air and a catalyst for the selective oxidation of the carbon monoxide-containing reaction space or a through which a cooling medium cooling space is formed, characterized in that the catalyst material (6) at least at the reaction chamber is applied (4) facing surfaces of the partition walls (3) and in that the reaction chamber (4) through a porous intermediate layer (9) is separated as a feed space ( 4 a) for the oxygen or for the air and, if appropriate, for the hydrogen-rich gas, the partial space which ends blindly on the outflow side. 2. Reactor according to claim 1, characterized in that the carbon monoxide (CO) containing hydrogen-rich gas directly into the reaction space ( 4 ) and the air or oxygen (O₂) via the feed space ( 4 a) and the porous intermediate layer ( 9 ) in the reaction space ( 4 ) can be fed. 3. Reactor according to claim 1, characterized in that the reaction chamber ( 4 ) ends blindly on the upstream side and that air or oxygen (O₂) and the carbon monoxide (CO) containing hydrogen-rich gas via the feed chamber ( 4 a) and the porous intermediate layer ( 9 ) can be fed into the reaction space ( 4 ). 4. Reactor according to claim 2 or 3, characterized in that the reaction space is divided by a further permeable intermediate layer ( 9 ) into three subspaces ( 4 , 4 a), the central subspace as the feed space ( 4 a) and the two outer subspaces are designed as reaction spaces ( 4 ). 5. Reactor according to claim 1, characterized in that in the reaction, feed and / or cooling rooms ( 4 , 4 a, 5 ) support structures ( 7 ) are introduced. 6. Reactor according to claim 1, characterized in that in addition on the surfaces of the permeable intermediate layers ( 9 ) and / or on the support structures ( 7 ) and / or on additional structures to improve the mass and heat transfer catalyst material ( 6 ) is applied . 7. Reactor according to claim 1, characterized in that the amount of catalyst and / or the permeability of the permeable intermediate layer ( 9 ) varies depending on the length of the reactor ( 1 ).