FR2472415A1 - CATALYST PROTECTED AGAINST OVERHEATING - Google Patents

Abstract

A. DEVICE FOR THE REDUCTION OF TEMPERATURE POINTS INSIDE A CATALYST WITH METAL STRUCTURE ENHANCER. B. CHARACTERIZED BY AT LEAST ONE THERMAL TUBE 1 FOR HEAT TRANSFER BETWEEN A HEATING ZONE AND A COOLING ZONE, PROVIDED WITH EXTERNAL COOLING SURFACES, PLACED AT THE CENTER OF THE CATALYST BED 2 SURROUNDED BY A ENCLOSURE 3. C. LAYOUT APPLICABLE TO THE PROTECTION OF THE CATALYST AGAINST TEMPERATURE POINTS, IN PARTICULAR IN THE EXHAUST SYSTEM OF AN ENGINE.

Description

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  The invention relates to a device for

  reduction of temperature peaks inside a catalytic

  it can be destroyed by these points.

  The application of metal elements as structural reinforcers for catalysts for the purification of car exhaust gases is already known. Thus, there is a series of documents (US-PS 3,920,583, DE-OS 2,302,746, DE-OS 2,450,664) describing a hot-rolled stainless steel catalyst matrix consisting of a

  extended metal support, o thick steel plates

  determined range are arranged alternately

smooth and corrugated plate.

  In addition, matrices of

  support for catalysts, consisting of superimposed layers

  made of high temperature resistant hot-rolled stainless steel, the layers being a smooth wire mesh, in which spacers larger in diameter than the fabric are incorporated by weaving, in parallel directions, in the direction of the channels desired flow, or layers of smooth wire cloth or corrugated plate

  being alternated with layers of smooth or wavy cloth.

  Layers can be stacked in

  boot, or wound into a cylindrical, oval, rectangular spiral

gular or polygonal.

  The corrugated layer may have different

  conformations. It is preferable that it has a sinu

  soldier, or the form of an involuntary, or a form

rectangular or square.

  The advantages of metal elements as structural reinforcers for catalysts for the purification of car exhaust gases, as compared with ceramic-based boosters, such as cordierite or mullite, are characterized inter alia by the low

weight and economy of place.

  As a result of the good thermal conductivity

  metal substrates and low heat capacity.

  However, there are still advantages to starting up

  catalysts for exhaust gases. This property is partly

  importantly, because in the cold running regime of an engine, most polluting elements are emitted, and a rapid implementation of catalytic reactions

decreases these emissions.

  It has also been proposed to surround the exhaust catalysts based on metal structure reinforcers by an insulating sheath. This device prevents the release of heat to the outside, and thus leads to a faster heating of the catalyst, ensuring

  this makes the catalytic reaction start more quickly.

  By insulating the outer surfaces of the catalyst, the inside of the catalyst is ensured in

  normal conditions, a uniform distribution of temperature.

  Normal conditions mean an internal combustion engine speed where the air / fuel ratio is adjusted to the optimum operating range of a catalyst

implemented.

  Although this type of regime is the most common, it can appear, even in the case of modern internal combustion engines, defective functions, including

  the action influences the activity of an exhaust gas catalyst.

  which can even destroy the catalyst itself. For example, it is possible for the automatic cold-starting device, after heating of the engine, to no longer be returned to the state of the normal mixing formation, but its setting

  be maintained for a highly enriched air / fuel mixture.

  At this time, the catalyst receives, for normal exhaust gas temperatures, the heat produced by the reaction of unused, unburned fuel, which leads to overheating.

catalyst.

  If the temperature exceeds the transformation point of the aluminum oxide and aluminum oxide used for example as a support for the noble metal phase, the catalyst is then brought, depending on the degree of conversion, to a loss of corresponding activity. In the case of complete conversion of aluminum oxide to aluminum oxide,

  catalytic activity is reduced to such an extent that

  he can no longer meet the legal requirements. In case of extreme air / fuel mixture admission, as is the case for example in the case of ignition failures, fusions may occur in the catalyst. A catalyst damaged in this way not only loses its effectiveness, but also increases, as a result of melting, or other incidents, the flow resistance inside the exhaust system of a internal combustion engine. He is coming

  loss of capacity and increased fuel consumption.

  The invention relates to a device for

  reduce temperature peaks inside a catalytic

  with reinforcing steel structure, for purification

  industrial exhaust gas or car exhaust.

  It avoids temperatures of an undesirable level, which could lead to deactivation or destruction

catalyst.

  The device of the invention is characterized

  at least one heat pipe arranged in the center of the

  catalyst, with external cooling surfaces.

  The catalyst system according to the invention is constituted by a thermal tube, around which is disposed for example a monolithic metal support catalyst. The monolithic metal supported catalysts are most often constituted by smooth or corrugated metal plates, even folded, made of high temperature resistant and temperature resistant stainless alloys, the plates being stacked or spirally wound with flow channels. longitudinal formed inside. The inner layers of these metal monoliths must be in intimate contact with

  the tube, to allow a satisfactory transfer of chaleigr.

  The thermal tube may, however, be placed in the middle of a fluidized bed of metal supported catalyst. In the case of large volume catalyst units, provision can be made for

several heat pipes.

  The heat pipes are vacuum-tight enclosures, for example tubes, the inside of which

  is coated with a capillary structure, for example grooves.

  After evacuation, they are filled with a small amount of heat-carrying liquid, so that the capillary structure is saturated. The liquid of the capillary structure is in equilibrium with its vapor phase, inside the space

  available. If one side of the tube is heated (heating zone)

  the other cooled (cooling zone), then the liquid evaporates by absorbing the heat of evaporation. Steam flows into the cooling zone and condenses into it,

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  by releasing the heat of condensation. The condensate flows into the capillary structure, again, into the heating zone. The propulsive force required for condensate reflux is the capillary force. The high caloric transfer capacity of a heat pipe is heat-based

  high evaporation of a liquid during a phase change.

  The temperature difference between the heating and cooling zones remains minimal even in case of high heat transfer capacities, since the phase changes are effected.

at constant temperature.

  The heat withdrawn from the catalyst by the

  tube, is discharged to the outside atmosphere,

  cooling fins or cooling plates

  in the form of fins, welded to the heat pipe.

  The surface of the cooling zone must comply with

  adapted to the expected heat flow. All materials having a good thermal conductivity and allowing an intimate connection with the material can be used

thermal tube.

  As a vehicle of heat in the field of the catalysis of exhaust gases, that is to say to

  relatively high temperatures, one can choose the elements.

  cesium, potassium, sodium and lithium. We will take

  preferably potassium and sodium. Sodium is especially suitable-

  for the application according to the invention, because with sodium, the heat transfer starts at about 6000 C.

  For optimal exploitation of the

  At start-up, the catalyst can be isolated without fear of raising the temperature above the allowable limit, inside the catalyst, under normal conditions. The system thus constituted allows a uniform distribution of the temperature and

  thus a better exploitation of the catalyst surfaces.

  By preventing overheating of the catalyst, we obtain

  higher lifetimes, and the whole system gains in endurance.

  As a material for heat pipes

  according to the invention, particular metals

  firstly resistant to heat and temperature oxidation

  high temperature, or their alloys, which can be implemented in the corrosive atmosphere of the exhaust gas. Examples of such alloys are Kanthal D, Fecralloy,

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  Aluchron W, Nirosta, Hastelloy and other

high temperatures. -

  For the extended support surrounding the thermal tube, special metal plates and sheets consisting of alloys based on iron, chromium, aluminum and possibly cerium and ythrium and the usual trace elements of the alloys should be used. , or other high-temperature, stainless-resistant materials

at these temperatures.

  For the isolation of the catalyst, that is

  Thermal tube can be used to make use of all usual high-temperature insulating materials, such as asbestos, glass wool or mineral wool. A material

  marketed under the trademark "Microtherm" was particularly

  Proudly suited to this type of application.

  The catalysts of the device according to the invention can be filled according to the usual methods, for example by galvanizing or by direct impregnation, with noble metals or not. They can also be converted into a support matrix coated with standard catalyst support materials which can be impregnated with an active solution of metal catalyst. For this purpose, the individual layers are coated on the surface with a support material promoting catalysis, most often a metal oxide with a high specific surface area, such as active Al 203. As additives promoting catalysis, the cerium, zirconium, iron, nickel, rare earth elements, or a combination of these elements, may also be incorporated in this metal oxide. The intermediate layer of support material is then provided, according to

  the current processes, the catalyst itself.

  With the device according to the invention, it is possible to maintain the temperature inside a

  catalyst, always optimally. Temperatures exceeded

  the accepted level are promptly and completely reduced,

  in this way, the destruction or destruction of the cat-

  lyst. The invention will be better understood with the aid of an embodiment described below, and represented in the accompanying drawings, in which:

  4o - Figure 1 is two views from above of the

  positive according to the invention, comprising the heat pipe 1 used

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  for heat transfer, the metal catalyst booster 2 surrounding the heat pipe, the thermal insulation 3 and the cooling fins 4 for the removal of heat towards the surrounding atmosphere; FIG. 2 is an enlarged representation of the core of the metal matrix;

  FIG. 3 is a schematic diagram of

  of a heat pipe. The energy is brought from a

  heat source 1 to the vaporization zone.

  The caloric carrier medium therein is evaporated and is fed as a vapor stream 3 through the isolation zone 4 into the vapor chamber 5 of the condensation zone. The vehicle medium

  condensed caloric 8 is brought back into the

  poration, through the 9. capillary structure of the container (thermal tube) 10. The isolation zone is isolated by means of

insulation 11.

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Claims (1)

CLAIM   Device for the reduction of temperature peaks inside a catalyst with metal structure booster, characterized by at least one heat transfer tube (1) between a heating zone and a cooling zone, provided with surfaces external cooling, arranged in the center of the bed of   catalyst (2) surrounded by an envelope (3).