GB2063092A - Catalyst protected against overheating - Google Patents

Abstract

An arrangement for dissipating temperature peaks in a metal supported catalyst (2), eg an I.C. engine exhaust catalyst, which comprises at least one heat tube (1) centrally arranged in the catalyst with cooling surfaces (4), eg ribs, situated outside the catalyst. <IMAGE>

Description

SPECIFICATION Catalyst protected against overheating This invention relates to a catalyst protected against overheating. More particularly an arrangement is provided for dissipating temperature peaks in a catalyst which could otherwise result in the destruction of the catalyst.

The use of metal elements as a structural reinforcement for catalysts for cleaning exhaust gases from motor vehicles is already known. Thus, numerous patent specifications (US-PS No. 3,920,583, DE-OS No. 2302746 and DE-OS No. 24 50 664) describe a catalyst matrix of non-scaling steel consisting of an expanded, metallic support in which steel plates having a certain thickness are made into flat and corrugated plates and arranged alternately in layers.

Other support matrices for catalysts which have already been described consist of layers of hightemperatu re-resistant, non-scaling steel arranged one on top of the other, the layers consisting of a flat screen cloth into which or onto which spacers having a larger cross-section than the screen cloth are woven at parallel intervals in the direction of the required flow channels, or layers of flat screen cloth or corrugated sheet metal alternating with layers of flat or corrugated screen cloth.

The layers may be arranged one above the other to form a pack or may be wound to form a cylindrical, oval, rectangular or polygonal spiral.

The corrugated layer may assume different shapes. Advantageously, it assumes a sinusoidal form or the form of an evolvent or rectangular or square form.

The advantages of metal elements as a structural reinforcement for catalysts for cleaning exhaust gases from motor vehicles over reinforcing elements based on ceramic materials, such as cordierite or mullite, lie inter alia in their lighter weight and in the space which they save.

The high thermal conductivity of metallic substrates combined with their lower thermal capacity also afford advantages in the activation behaviour of vehicle emission control catalysts. This property is particularly important because most pollutants are emitted in the cold running phase of an engine and a rapid onset of the catalyst reaction prevents their emission.

It has also been proposed to arrange an insulating jacket around vehicle emission control catalysts based on metal structural reinforcements. This arrangement prevents heat from being given off to the outside and, hence, enables the catalyst to heat up more quickly, thus allowing the catalytic reaction to begin more quickly.

Insulation of the surfaces of the catalyst provides for a more uniform distribution of temperature within the catalyst under normal conditions.

Normal conditions are understood to be those operational states of an internal combustion engine in which the air/fuel ratio fits into the optimal working range of the catalyst used.

Although these operational states are the rule, malfunctions can also occur in modern internal combustion engines, affecting the activity of an emission control catalyst and even resulting in destruction of the catalyst itself. Thus, it is quite possible for example for automatic chokes not to return to normal mixture formation after the engine has reached its normal operating temperature, but instead to return to their setting for a very rich air/fuel mixture. Now, in the catalyst, the heat produced by the reaction of unused, unburntfuel is added to the normal exhaust gas temperature, resulting in overheating of the catalyst.If the temperature exceeds the point at which they-aluminium oxide used for example as support for the noble metal phase is converted into a a-aluminium oxide, the catalyst undergoes a corresponding loss of activity, depending on the degree of conversion. In the event of complete conversion of the y-aluminium oxide into a-aluminium oxide, catalytic activity is reduced to such an extent that the catalyst no longer satisfies legal requirements. In the event of extreme exposure to air/fuel mixture, as is also the case for example in the event of ignition failures, melting may even occur in the catalyst. The catalyst destroyed in this way is not only ineffectual, it also increases flow resistance in the exhaust gas system of an internal combustion engine as a result of the melting or other destructive phenomena.This in turn results in a loss of power and in higher fuel consumption.

The present invention relates to an arrangement for dissipating temperature peaks in a catalyst having a metallic structural reinforcement for cleaning industrial and vehicle exhaust gases. It enables undesirably high temperatures, which could result in deactivation or destruction of the catalyst, to be avoided.

The present invention relates to a catalyst comprising a metallic structural reinforcement, the catalyst being provided with an arrangement for dissipating temperature peaks comprising least one heat tube with external cooling surfaces arranged centrally in the catalyst.

Accordingly, the catalyst system according to the invention consists of a heat tube around which for example a monolithic metal supported catalyst is arranged. Monolithic metal supported catalysts are usually made up of flat and corrugated or folded layers of non-scaling, temperature-resistant alloys, the layers being arranged one on top of the other to form a pack or being wound to form a spiral with flow passages extending through longitudinally in either case. The inner layers of this metal monolith are intended to be in intimate contact with the heat tube to provide for an adequate transfer of heat.

However, the heat tube may also be arranged in the middle of a loose bed of metal supported catalysts.

In the case of high-volume catalyst units, it is possible to provide several heat tubes.

Heat tubes are containers provided with a vacuum-tight seal, for example tubes which are provided on their inside with a capillary structure for example grooves. After they have been completely evacuated, they are filled with a small quantity of liquid, the heat carrier, to such an extent that the capillary structure is just saturated. The liquid of the capillary structure is in equilibrium with its vapour in the available space. If one side of the tube is heated (heating zone) and the other side cooled (cooling zone), the liquid evaporates, taking up the heat of evaporation. The vapour flows into the cooling zone where it condenses, giving off the heat of condensation. The condensate flows back th rough the capillary structure into the heating zone. The driving force behind the return of the condensate is the capillary force.The high transferable heat output of a heat tube is attributable to the intense heat of evaporation of a liquid during the change of phase. The temperature difference between the heating zone and the cooling zone is also minimal in view of the high heat output transmitted, because the changes of phase take place at uniform temperature.

The heat removed from the catalyst by the heat tube is given off to the surrounding atmosphere through cooling ribs or through fin-like cooling plates which are welded or soldered onto the heat tube. The surface of the cooling zone has to be commensurate with the expected heat flow. Suitable materials are any materials which show high thermal conductivity and which are capable of entering into an intimate union with the constituent material of the heat tube.

The elements caesium, potassium, sodium and lithium are suitable heat carriers in the field of emission control catalysis, i.e. at relatively high temperatures. The metals potassium and sodium are preferably used. Sodium is particularly suitable for the purposes of the invention because in the case of sodium the transport of heat begins at around 600 C.

In order optimally to utilize its activation behaviour, the catalyst may be insulated without any danger of the temperature prevailing inside the catalyst in normal operation being excessively increased as a result. The system thus designed provides for a more uniform distribution of temperature and hence for better utilisation of the catalyst surfaces. By avoiding excessively high temperatures inside the catalyst, its service life is increased and the system as a whole is made more reliable.

Particularly suitable heat tube materials for the purposes of the invention are heat-resistant, nonscaling, metals and their alloys which may be used in the corrosive atmosphere of vehicle exhaust gases. Examples of alloys such as these are Kanthal D, Fecralloy, Aluchrom W, Nirosta, Hastelloy and other high-temperature-resistant steels.

Particularly suitable materials for the expanded support surrounding the heat tube are metal plates and screen cloths consisting of an alloy of iron, chromium, aluminium and optically, cerium or yttrium and the usual accompanying elements or other heat-resistant non-scaling materials.

Any standard high-temperature insulating materials, such as asbestos, glass wool and mineral wool, may be used for insulating the catalyst and the heat tube. A material commercially available under the name of "Microtherm" has proved to be particularly suitable for high-temperature insulations.

The catalysts of the arrangement according to the invention may be directly coated with noble metal and/or base metal by standard methods, for example by electroplating or impregnation. However, they may also be processed to form a support matrix which is coated with a standard catalyst support material and which may be impregnated with solutions of active catalyst metals. The individual layers are surface-coated with a catalysis-promoting support material, generally a large-surface metal oxide, such as active Awl203. The elements cerium, zirconium, iron, nickel, rare earths or a combination thereof may also be incorporated in this metal oxide as catalysis-promoting additives. The intermediate layer of support material is then provided with the actual catalyst by standard methods.

By means of the arrangement according to the invention, the temperature prevailing within a catalyst can always be kept optimal. Temperatures exceeding the normally permitted limit are quickly and completely dissipated without the catalyst being deactivated or destroyed.

An embodiment of the invention is described in more detail in the following with reference to the accompanying drawings, wherein: Figure 1 shows two view of an arrangement according to the invention in which reference (1) denotes a heat tube used for heat transfer, reference (2) a metallic catalyst reinforcement arranged around the heat tube, reference (3) denotes thermal insulation and reference (4) denotes cooling ribs for dissipating the heat to the surrounding air.

Figure 2 shows the core of the metal matrix on a larger scale.

Figure 3 is a diagram illustrating the principle on which a heat tube functions. Energy is delivered from a heat source (1) to an evaporation zone (2). A transport medium situated therein is evaporated and delivered as a stream (3) of vapour to a condensation zone via an insulating zone (4) surrounding a vapour zone (5). In the condensation zone, the transport medium is condensed and heat is given off to the surrounding atmosphere (7). Condensed heat transport medium (8) is returned to the evaporation zone via capillary structure (9) of the vessel (heat tube)(10). The insulation zone is insulated by insulation (11).

Claims (2)

1. A catalyst comprising a metallic structural reinforcement, the catalyst being provided with an arrangement for dissipating temperature peaks com.

prising at least one heat tube with external cooling surfaces arranged centrally in the catalyst.

2. A catalyst substantially as described with particular reference to the accompanying drawings.