GB2027609A

GB2027609A - Catalytic waste heat recovery

Info

Publication number: GB2027609A
Application number: GB7926204A
Authority: GB
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 1978-08-02
Filing date: 1979-07-27
Publication date: 1980-02-27

Abstract

Heat energy is recovered from waste or effluent gases such as exhaust gases which contain combustible vapour or droplets of combustible liquid or both by bringing the gas into contact with an oxidation catalyst to oxidise the vapour or the liquid or both and simultaneously or subsequently removing in a heat exchanger at least a part of the heat originally present in the gas and of the heat produced by the catalytic oxidation of the combustible vapour or liquid or both.

Description

SPECIFICATION Catalytic waste heat recovery This invention relates to the recovery of energy from gases. In particular, the invention is concerned with the recovery of heat energy from waste or effluent gases such as exhaust gases, which contain combustible vapour or droplets of combustible liquid or both.

Energy is frequently lost when waste or effluent gases carry heat away from the process in which they are formed and some of this heat, may be recovered by use of a heat exchanger. The efficiency of a heat exchanger is however, impaired if droplets of liquid carried with the gases coalesce on the cooler surfaces of the heat exchanger and/or vapours carried with the gasses condense thereon thereby tending to reduce the thermal coupling between the gases and the surface(s) of the heat exchanger.

An object of the present invention is to provide methods and means for the recovery of energy from waste or effluent gases which may contain combustible vapour cr droplets of combustible liquid or both.

According to a first aspect of the present invention, a method of recovering energy frorn a gas which contains combustible vapour or droplets of combustible liquid or both comprises bringing the gas into contact with an oxidation catalyst to catalytically oxidise the vapour or the liquid or both and simultaneously or subsequently removing in a heat exchanger at least a part of theheat originally present in the gas and of the heat produced by the catalytic oxidation of the combustible vapour or liquid or both.

It is, of course, desirable when carrying out the method of the invention that the temperature of the waste or effluent gas when it reaches the catalyst should be not lower than "light off" temperature, that is, not lower than the minimum temperature at which catalytic oxidation of the combustible liquid and/or vapour will be initiated.

In carrying out the method just described, it is convenient for the oxidation catalyst to be carried by the heat exchanger plates, or other heat exchanger material such as ceramic blocks, tubes and the like.

According to a second aspect of the invention, an apparatus for carrying out the method of the first aspect of the invention comprises a heat exchanger including an oxidation catalyst carried by the heat exchanger material of the heat exchanger. Suitable catalysts include one or more of the platinum group metals other than osmium, namely, platinum, rhodium, ruthenium, palladium and iridium. The catalyst may also be in the form of an alloy, for example, an alloy of the platinum group metals with each other or alloys of the platinum group metals with other metals.

Preferably a high surface area intermediate layer is interposed between the catalyst layer and the heat exchanger material and, conveniently, at least 10% by weight of the catalyst consists of platinum group metal.

The intermediate layer may comprise one or more refractory metal oxides such as beryllia, magnesia or silica or combinations of metal oxides such as boria-alumina or silica-alumina and the heat exchange material may be metallic or ceramic.

The heat exchanger may take a number of forms such as those outlined in the examples below but the form of the heat exchanger is not limited by these examples. Further, the invention can be applied to heat exchangers where the heat exchanger material is rotated relative to ducts for feeding the heated gas and gas to be heated therethrough and vice versa The heat exchanger is the means by which energy in the form of heat is transferred from the waste or effluent gases to a second fluid, i.e. the coolant.

Example 1.

A catalysed "heat-wheel" (that is, a wheel including a plurality of passages extending therethrough and in the general direction of the axis thereof with an oxidation catalyst applied to the surfaces of the passages) is slowly rotated with a stream of waste or effluent gas passing through one portion of the wheel and stream of coolant fluid passing through a different portion.

During rotation, a given portion of the wheel will pass through the stream of the waste or effluent gas, and energy, in the form of heat, is transferred from the gas thereby causing the temperature of that portion to rise. The so heated portion of the wheel will, during continued rotation, move out of the flow of the gas into the stream of the coolant which being at a -low temperature receives energy, in the form of heat from this heated portion, the temperature of which is, therefore, lowered. The wheel continues to rotate and the portion passes through the stream of waste or effluent gas again and the cycle is repeated.

Example 2.

A heat exchanger in the form of a "heat band" follows the same principle as the heat wheel. A belt or band of material, which may be of knitmesh construction is passed around two pulleys in a continuous loop. The material is "catalysed", that is it has a catalyst covering part of the surfaces of the material. In this particular case the catalyst is an oxidation catalyst. A given section of the loop passes through a stream of waste or effluent gas and as energy is transferred as heat from the gas the temperature of the given section rises. As the loop is pulled round the two pulleys the said section will move out of the flow of the gas into the flow of the coolant. As energy is transferred from the said section of the loop to the coolant, the temperature of the said section is lowered.The loop continues round the pulleys and the section passes through the waste or effluent gas again and the cycle is repeated.

Example 3.

As in the previous examples a "catalysed" heat exchanger is used in this example to remove heat from a stream of waste or effluent gas. The heat exchanger comprises an assembly of pipes each sealed at both ends and each containing a liquid such as water but often an organic liquid. The said pipes are assembled parallel to each other with one end of each sited in the stream of waste or effluent gas and the other end of each in a stream of coolant. The end of each tube which comes in contact with waste or effluent gas is "catalysed", that is part of the surface of the tube is covered with an oxidation catalyst in a manner described above with or without the intermediate layer. As the waste or effluent gas passes over the ends of the tubes, energy in the form of heat is transferred from the gas to the liquid inside the tubes.As a result the liquid evaporates and the resulting vapour condenses at the other ends of the tubes thereby releasing the latent heat of vaporisation which is then transferred to the coolant. The condensed vapour may be returned to the other ends of the tubes by a number of means, such as by gravity, by a pump or by method described in BP 1,027,719.

The present invention may be used in connection with any form of heat exchanger and one commonly used form of heat exchanger is shown in the accompanying diagrammatic drawing included by way of example.

In the drawing, M is a cylindrical mass of heat exchange material which is rotable about axis X-X in a direction indicated by arrow R. A1/A2 and B 1/B2 are pairs of stationary ducts for leading heated and cooled air to and from the heat exchange material M. Generally, the four ducts Al, A2, B1 and B2 are of sector-shaped cross-section and sealing means not shown are located between the ducts and the mass M to prevent escape and intermingling of the heated gas and gas to be heated.

The mass of heat exchange material may be made from a ceramic material, e.g. of honeycomb or perforate form or metal. Where metal is used a conventional mass comprising alternate plain and corrugated sheets may be used. One particularly useful metallic material is Fecralloy (Registered Trade Mark).

As described previously, the mass of heat exchange material carried a layer of catalyst which may or may not be applied to an intermediate layer of a refractory metal oxide.

If desired, a non-catalysed mass of heat exchange material M may be preceded by a separate mass which is catalysed so that non-catalytic heating takes place within the heat exchanger mass proper. Further, the mass of heat exchanger material may be divided into two or more relatively short cylindrical subsidiary masses having a catalysed mass disposed between adjacent subsidiary masses.

Claims

1. A method of recovering heat energy from a gas which contains combustible vapour or droplets of combustible liquid or both comprises bringing the gas into contact with the oxidation catalyst to catalytically oxidise the vapour or the liquid or both and simultaneously or subsequently removing in a heat exchanger at least a part of the heat originally present in the gas and of the heat produced by the catalytic oxidation of the combustible vapour or liquid or both.

2. A method according to claim 1 wherein the temperature of the gas reaching the catalyst is not lower than the minimum temperature at which catalytic oxidation of the combustible liquid and/or vapour will be initiated.

3. A method according to claim 1 or claim 2 wherein the oxidation catalyst is supported on the heat exchanger material of the heat exchanger.

4. A method according to claim 3 wherein the oxidation catalyst includes one or more of the platinum group metals, platinum, rhodium, ruthenium, palladium and iridium.

5. A method according to claim 4 wherein the oxidation catalyst is an alloy containing at least one of the said platinum group metals and another metal.

6. A method according to claim 5 wherein the oxidation catalyst includes at least 10 wt.% of a platinum group metal.

7. A method according to any preceding claim including a high surface area intermediate layer interposed between a layer of the oxidation catalyst and the heat exchanger material.

8. A method according to claim 7 wherein the intermediate layer comprises one or more of the refractory metal oxides beryllia, magnesia, silica, boria-alumina or silica alumina.

9. A method according to any preceeding claim wherein the heat exchanger material is metallic or ceramic.

10. Apparatus for carrying out the method claimed in any one of claims 1 to 9 comprising a heat exchanger including an oxidation catalyst carried by the heat exchanger material of the heat exchanger.

1 1. Apparatus according to claim 10 wherein the oxidation catalyst is supported on the heat exchanger material of the heat exchanger.

12. Apparatus according to claim 1 1 wherein the oxidation catalyst includes one or more of the platinum group metals, platinum, rhodium, ruthenium, palladium and iridium.

13. Apparatus according to claim 12 wherein the oxidation catalyst is an alloy containing at least one of the said platinum group metals and another metal.

14, Apparatus according to any one of claims 10 to 13 including a high surface area intermediate layer interposed between a layer of the oxidation catalyst and the heat exchanger material.

15. Apparatus according to claim 14 wherein the intermediate layer comprises one or more of the refractory metal oxides beryllia, magnesia, silica, bora-alumina or silica alumina.

16 Apparatus according to any one of claims 10 to 15 wherein the heat exchanger material is metallic or ceramic.

17. Apparatus according to any one of claims 10 to 16 modified in that the oxidation catalyst is supported on an apertured body located upstream (relative to the flow of gas) of the heat exchanger material.