DE19627289A1 - Catalyst assembly for e.g. exhaust gas purification - Google Patents

Abstract

A new catalyst assembly for gas purification has several catalyst substrates, arranged in series with intermediate zones, or in parallel. Preferably, three or more catalyst substrates are included. The length Ikat of individual catalyst blocks is determined by a given relationship, which links the mean flow velocity across the channel cross section, the hydrodynamic diameter of the channel, the density of the gas mixture and the dynamic viscosity of the gas mixture. Individual catalyst block lengths lie between 0.2 and 10 cm. The spacing between substrates, i.e. the length Izone of the intermediate zones is expressed by: 0.01 dzone <= Izone <= 60 dzone, in which dzone is the hydrodynamic diameter of the intermediate zone.

Description

The invention relates to a device according to the preamble of the saying 1.

Those that occur in thermal and catalytic exhaust gas purification processes chemical transformations can be in

• - oxidation reactions
• - reduction reactions
• - decomposition reactions

divide.

Those processes are termed catalytic exhaust gas purification understood, in which the contaminating gaseous and vaporous substances (Pollutants) through chemical reaction on the surface of a solid auxiliary (Catalyst) can be converted into harmless substances.

The majority of technical catalysts therefore consist of highly porous materials materials that provide a large surface area for the chemical reaction len. Due to the fact that the reactants (pollutants and oxidation or reducing agent) in a different physical state than the catalyst  physical transport processes in addition to the chemical reaction instead of.

The catalytic effect is due to the existence of areas of the surface of the festival attributed to their electronic and / or geometric properties chemisorption of the reactants (e.g. pollutants and oxygen) enable [1].

These active centers can both on the outer (geometric) surface as well as on the inner surface (porosity).

The sub-steps of heterogeneously catalyzed reactions are [5, 6]:

• 1. Transport of the reactants through the gas flow to the boundary layer.
• 2. Diffusion of the reactants through the boundary layer to the outer catalyst surface.
• 3. Diffusion of the reactants through the pores to the inner catalyst surface area.
• 4. Adsorption on the surface of the catalyst.
• 5. Chemical reaction.
• 6. Desorption of the products from the catalyst surface.
• 7. Diffusion of the products through the pores to the outer surface of the catalyst tors.
• 8. Diffusion of the products through the boundary layer.
• 9. Transport of the products in the gas flow.

By shifting the charge or changing the atomic distances in the chemi sorbed molecules becomes the energy required for the reaction to proceed reduced level. The reaction products desorb from the active sites and leave the catalyst through diffusive and convective transport. To this In this way, the active centers are available for the conversion of additional molecules [1, 2].

In addition to the mass transfer, heat is transferred between the gas space and catalyst instead. This allows the catalyst to be preheated  Gas flow preheated to the necessary operating temperature or by the exothermic oxidation / combustion reaction warmed up catalyst surface be cooled [2].

The rate of heterogeneously catalyzed reactions thus becomes macroscopic of the conditions of mass and heat transport between the gas space and Catalyst (flow rate, concentration, geometry, temperature) be influences [3, 4]. Microscopically determine the number, arrangement and accessibility of the active centers sales of reactants (pore radius distribution, dispersi degree).

For example, in the catalytic aftertreatment of exhaust gases from sta stationary and transient internal combustion engines in engines based on the Otto principle Three-way catalytic converters and, for engines based on the diesel principle, oxidation catalytic converters analyzers used. These catalysts consist of one with channels through drawn ceramic or metal support on which the wash coat and the catalytic active coating are applied. Previously used catalysts are used rarely made in two parts.

The disadvantage here is that the one or two-part design of the catalyst the catalyst blocks have a relatively large heat capacity with good thermal conductivity own what is only a slow heating ("light") to that for the chemical reactions required temperature results. During the on heating period there is no or insufficient conversion of pollutants. The up previous catalyst blocks are too large to quickly reach operating temperature [7].

The catalytic degradation of the pollutants on the catalyst surface are the Transport to and adsorption of pollutants on the catalyst surface and the desorption and transport of the products are superimposed on the surface. It is known [8] that the pollutant conversion on the catalyst predominantly during of the thermodynamic inflow of the flow takes place in the first millimeters. After that there is hardly any further pollutant turnover in the channels. At first milli Meters of the catalyst prevail turbulent flow states, which the material and Promote heat transfer. The flow continues through the channels  a laminar flow profile due to the wall friction. A transport of the Pollutants from the core of the flow to the catalytically active channel wall area takes place only to a limited extent due to the thermodynamic conditions (Formation of a laminar flow, Reynolds number less than 100). It comes to Formation of a concentration profile over the channel cross-section. The Konzen trationsprofil is characterized in that the concentration of pollutants from the middle of the canal decreases towards the canal wall. Pollutant molecules that are not are in the immediate vicinity of the catalyst surface are no longer abge builds.

In addition, the average pollutant concentration varies in the individual channels len. The reason for this is the uneven exposure of the ducts to exhaust gas, deactivation due to aging and the different duct temperatures. He If there was a concentration equalization between the channels, the unrefined could Exhaust gas from ineffective channels can be treated in catalytically active channels.

The object of the invention is to provide a catalyst with increased mass transfer Pollutants to the catalytically active layer, a faster start of the catalyze sators and a concentration and temperature equalization between the Channels.

This object is achieved by a catalyst with the features of claim 1 solved.

The advantages of the segmented catalyst are:

• - faster start
  The multi-part structure of the catalyst results in small catalyst blocks with low heat capacity. The first catalyst block, through which the hot engine exhausts, is also heated first and reaches the temperature required for the chemical reactions much faster.
• - Uniformity of the concentration and temperature profile over the cross section of the catalyst
  In the zones located between the catalyst blocks, in which internals can also be arranged, the concentration and temperature are equalized over the flow cross-section. By equalizing the concentration in the intermediate zone, pollutant molecules that have not been broken down in inactive channels can be broken down in the following.
• - Inlet flows with optimal pollutant transport to the catalytically active layer
  By flowing onto the next catalyst block, conditions arise in this during the thermodynamic inflow of the flow, which promote the transport of the pollutant molecules to the catalyst surface from the core of the flow.
• - Combination of different supports and coatings
  The combination of different catalyst supports offers advantages in terms of thermal stability and pressure loss.
  The coating of the individual catalyst blocks depends on the exhaust gas composition. The cheapest and simplest option is to apply the same coating to the catalyst supports. Depending on the raw gas composition and the requirements for the clean gas, it makes sense to apply different coatings to the different catalyst elements.
• - enlargement of the inflow area
  One advantage of dividing the catalyst into several successive blocks is the increase in the total inflow area, which is composed of the inflow surfaces of the individual blocks. Compared to a one-part catalytic converter with an identical diameter, a four-part catalytic converter has four times the inflow area.
• - Reduction of the diameter of the catalyst block
  In return, while maintaining the inflow area, it is possible to reduce the diameter of a four-part catalyst to half the diameter of a one-part. The reduction in the installation dimensions is advantageous, for. B. in the arrangement of the catalyst in the exhaust strand of vehicles.
• - enlargement of the cell diameter (reduction of pressure loss)
  The enlargement of the inflow area and the increased effectiveness of the multi-part construction make it possible to enlarge the cell diameter and thus to reduce the pressure loss.
• - Low loss of activity due to catalyst poisoning
  The thermal and / or chemical aging only covers part of the catalytic converter, specifically the catalytic converter block through which the hot exhaust gas first flows. The chemical and thermal deactivation decreases continuously towards the rear and to the side [9]. With one-piece catalytic converters, the areas in which the conversion takes place are very susceptible to deactivation. In the case of multi-part catalysts, the deactivation only affects the first catalyst block and therefore has only a minor influence.

The advantages are offset by an overall higher pressure loss, which is due to Ver Larger cell diameter can be partially compensated.

example 1

Fig. 1 shows an axial section through a multi-part exhaust gas catalytic converter, as it could be used for exhaust gas aftertreatment on an internal combustion engine. The catalytic converter is composed of four ceramic catalyst blocks ( 1-4 ), which are arranged in series. The length I _{Kat of} the catalyst blocks is 2 cm, that of the intermediate _zones I _zone 2 cm with a diameter D of 10 cm. The cell density is 400 cpsi (cells per square inch, 1 sq.in. approx. 6.54 cm²). The catalyst carrier is coated with platinum and rhodium in a ratio of 5: 1 with a loading of 50 g / ft³. The precious metal coating is supplemented by promoters such as cerium oxide. The catalyst corresponds to a catalyst sold by Engelhard Technologie, as used in small cars.

The catalyst blocks are embedded in a metal housing. Between Metallge The housing and catalytic converter are covered with an insulating layer that supports the ceramic support protects.

Example 2

A segmentally arranged catalyst according to Example 1 can also be composed of different supports with different coatings. In the catalyst shown in FIG. 1, the two first ceramic catalyst carriers ( 1 , 2 ) are replaced by two metallic ones, which is advantageous for the thermal stability. The two metallic supports are coated with palladium and rhodium, mass ratio 5: 1, loading 50 g / ft³. The two ceramic supports have a tri-metal coating of platinum, palladium and rhodium, mass ratio 1: 14: 1, loading 105 g / ft³.

Example 3

A sensible improvement of Example 1 is a modular design ( Fig. 2), in which the individual catalyst block, the intermediate zone or the functional group, consisting of the catalyst block and intermediate zone, are each embedded in their own Ge. The multi-part catalyst is then assembled from individual modules and connected to one another via flange connections.

The combination of different monoliths and coatings is due to the Mo dulbau very easily possible.

Example 4

With the segmental arrangement of the catalyst monoliths, the turbulence in the intermediate zones can be increased by various construction elements. Fig. 3 shows basic possibilities of the arrangement. These are e.g. B. turbulence grid, cross-sectional constriction and expansion, various internals, he increase the roughness and static mixer. The diameter of the intermediate zone and catalyst block can vary.

literature

[1] Schlosser, EG: heterogeneous catalysis. Verlag Chemie, Weinheim, 1972, cited in [5]
[2] Butt, JB: Reaction Kinetics and Reactor Design. Prentice-Hall Inc. Engelwood Cliffs, New Jersey, 1980, cited in [5]
[3] Wicke, E: Basics of catalytic afterburning. Chemie Ing. Tech nik 37 (1965), pp. 892-904, cited in [5]
[4] Chancellor, W .; Schedler, Thalhammer, H .: Theoretical and experimental studies in catalytic post-combustion plants. Chemie-Ing.- Technik 59 (1987), pp. 582-585, cited in [5]
[5] Schmidt, T: Fundamentals of catalytic exhaust gas purification. Technical information 82 (1989) 5
[6] Thomas, WJ: The catalytic monolith. Chemistry and Industry, 9 (1987) 5
[7] Faltermeier, G .; Pfalzgraf, B .; Brück, R .; Kruse, C .; Maus, W .; Thursday, A: Catalyst concepts for future exhaust gas legislation using the example of a 1.8 l 5V engine, in: Lenz, HP [ed.]: 17th International Wiener Motoren symposium 25.-26. April 1996, Volume 1, Day 1, Progress-Ber. VDI series 12 no.267, Düsseldorf, VDI-Verlag, 1996
[8] Thursday, A; Degen, A .; Held, W .; Korbel, K .: Fulfillment of the ULEV standard with an electrically heated catalyst, in: Lenz, HP [Ed.]: 16th International Vienna Motor Symposium 4th-5th May 1995, Volume 2, Day 2, Progress-Ber. VDI Row 12 No. 239, Düsseldorf, VDI-Verlag, 1995
[9] Smedler, G .; Jobson, E; Lundgren, S .; Romare, A .; Wirmark, G .; Högberg, E .; Weber, KH: Spatially Resolved Effects of Cesactivation on Field-Aged Auto motive Catalysts. Conference Individual Report: Society of Automotive Engineers (1991), pages 11-25, SAE-No. 910173

Claims (19)

1. A device for cleaning gases, characterized in that the device comprises a plurality of elements which are arranged in series with intermediate intermediate zones or paral lel. 2. Device according to claim 1, characterized in that they have at least three catalyst support elements having. 3. Apparatus according to claim 1 or 2, characterized in that the length I _{Kat of} the individual catalyst blocks between is, where w is the mean flow velocity in the channel cross section, d _channel the hydrodynamic diameter of the channel, ρ the density of the gas mixture and η the dynamic viscosity of the gas mixture. 4. Device according to one of the preceding claims, characterized in that the length of the individual catalyst blocks is between 0.2 cm and 10 cm. 5. Device according to one of the preceding claims, characterized in that the catalyst support elements are spatially separated by intermediate zones whose length I _{zone is} between 0.01 d _zone I _zone 60 d _zone (2), where d _{zone is} the hydrodynamic diameter of the intermediate _zone . 6. Device according to one of the preceding claims, characterized in that the catalyst support elements spatially by Intermediate zones are separated, the length of which is between 0.2 cm and 10 cm.   7. Device according to one of the preceding claims, characterized in that the catalyst carrier elements made of ceramic and / or metallic monoliths exist. 8. Device according to one of the preceding claims, characterized in that the catalyst support monoliths the same or un have different coating. 9. Device according to one of the preceding claims, characterized in that the spatial separation between the cata door support elements is achieved by internals. 10. Device according to one of the preceding claims, characterized in that the gas passed through the device at Combustion of fossil or regenerative energy sources occurs. 11. Device according to one of the preceding claims, characterized in that the gas passed through the device is the exhaust gas of an internal combustion engine. 12. Device according to one of the preceding claims, characterized in that the diameter of the intermediate zone is larger than that of the individual monoliths ( Fig. 3.e). 13. Device according to one of the preceding claims, characterized in that the diameter of the intermediate zone is smaller than that of the individual monoliths ( Fig. 3.d). 14. Device according to one of the preceding claims, characterized in that structural elements are arranged in the intermediate zones ( Fig. 3.a, b, c, d). 15. Device according to one of the preceding claims, characterized in that the structural element in the intermediate zone Turbulence grating or sieve with a mesh size that is between the inlet and 100 times the hydrodynamic channel diameter (cell diameter) of the Catalyst carrier and a wall thickness that is between the input and Is ten times the wall thickness of the catalyst carrier.   16. Device according to one of the preceding claims, characterized in that the diameter of the individual monoliths varies ( Fig. 4). 17. Device according to one of the preceding claims, characterized in that the diameter of the individual intermediate zones varies ( Fig. 3.e). 18. Device according to one of the preceding claims, characterized in that the individual elements different cell density have ten. 19. Device according to one of the preceding claims, characterized in that the length of the individual intermediate zones varies.