GB2129134A - Catalytic combustible-gas detectors - Google Patents

Abstract

A catalytic combustible-gas detector element comprises a heatable wire filament (10; preferably a helical platinum wire) embedded in a pellet formed overall of an oxidation catalyst (preferably palladium) and a porous, non-catalytic, inert, particulate carrier therefor (preferably 75 wt% alumina, 25 wt% zeolite), wherein the pellet has a laminated, onion-like structure, and consists of a multiplicity (preferably 6 to 20) of concentric layers in which layers (11, 13, 15, 17; preferably in pairs) of a carrier+catalyst admixture, each in the form of a porous cohesive mass of individual carrier material particles having catalyst mixed therewith, alternate with layers of catalyst (12, 14, 16). <IMAGE>

Description

SPECIFICATION Combustible-gas detectors This invention relates to combustible-gas detectors, and concerns more particularly combustible gas detectors of the kind in which a heatable wire filament constituting the detector element exhibits a change in resistance occasioned by the change in its temperature which occurs due to the oxidation of a combustible-gas passing over it, the resistance change being utilized to provide an indication of the concentration of the combustible gas.

Whilst it is possible to use as the detector element a naked wire filament, it is nowadays more common to use as the element a wire filament which is embedded in a pellet of ceramic material, so providing a more rugged structure, and generally the pellet is coated with, or there is included within the mix from which the pellet is made, an oxidation catalyst which reduces the temperature at which oxidation of the combustible gas takes place. A pellet made using such a mixture (of catalyst and carrier therefor) is described in the Complete Specification of our British Letters Patent No: 1,387,412 (1/5304/V), A difficulty which has been experienced with catalytic detectors of the pellettal type is that in some circumstances changes in the electrical characteristics of the detector occur in service.These changes are believed to be due to non-volatile residues deposited on the surface of the detector element, which residues tend to poison the catalyst and/or obstruct the normal flow of gas to the element's surface, so reducing the sensitivity of the device. One such poisonous residue is lead (derived from the burning of leaded petroleum spirit vapour), while another is the class of silicones (siliconcontaining compounds analogous to the carbon-containing compounds or organic life), and this latter class is becoming a significant problem as the utilisation of silicones increases.One classic use of combustible-gas detectors occurs in coal mines, where it is required to detect and measure the level of methane (fire-damp) in the atmosphere; unfortunately, the machinery used adjacent the detector may well employ silicone rubber seals and gaskets, and silicone-based lubricating oils, and all of these inevitably release silicone vapours into the atmosphere so that the ambient environment is loaded with material poisonous to the detector catalyst.

Many attempts have already been made to produce detectors which are unaffected by the more obvious catalyst poisons. For example: in the Complete Specification of our British Letters Patent No.

1,549,640 (1/5958/V) we have described how the detector pellet may be given an outer coating of a non-catalytic porous material (thus, alumina or a zeolite); in the Complete Specification of our British Letters Patent No. 1,554,831(1/6142/V) we have described giving the pellet an outer porous layer of a mixture of a zeolite and kaolin; in the Complete Specification of our British Letters Patent No.

1,556,339 (1/6076/V) we have described making the pellet of a homogeneous mixture of catalyst and zeolite (optionaliy with alumina); and in the Specification of our British Patent Application No.

2,096,321 A (1/6530/V) we have described constructing the pellet in onion-like form, of alternating layers of carrier and catalyst. All these pellets have had improved resistance to poisoning, and yet even so have succumbed sooner than considered desirable.

It is the purpose of the present invention to allow the formation of a detector pellet having even greater resistance to catalyst poisoning, especially by silicones, and the invention seeks to achieve this by building the pellet as a multilayered structure rather like an onion, layers of catalyst alternating with layers of carrier+catalyst admixture.

In one aspect, therefore, this invention provides a combustible-gas detector element comprising a heatable wire filament embedded in a pellet formed overall of an oxidation catalyst and a porous, particulate, non-catalytic, inert carrier therefor, wherein the pellet has a laminated, onion-like structure, and consists of a multiplicity of concentric layers in which layers of a carrier+catalyst admixture, each in the form of a porous cohesive mass of individual carrier material particles having catalyst admixed therewith, alternate with layers of catalyst.

The detector element of the invention employs a heatable wire filament the electrical resistance of which changes with temperature, such a temperature change occurring when the combustible gas being detected is burnt (oxidized) in contact with the filament. The filament should accordingly be of a material that exhibits the desired characteristics, and that is in particular relatively chemically inert to the combustible gas, its oxidation products, and any likely contaminants. Suitable filament materials (and filament structures) are well known in the art; the preferred one is platinum or one of its alloys, for example a platinum/zirconia alloy as described in the Complete Specification of our British Patent No.

1,567,736 O/61 74)V), the filament taking the form of a helical winding.

The oxidation catalyst employed with the detector element may be any of those catalysts or mixtures of catalysts used or suggested for use for this purpose. Preferably, however, it is palladium or platinum.

The detector element of the invention uses a particulate non-catalytic inert material as the carrier for the oxidation catalyst, and each carrier+catalyst layer is in the form of a porous cohesive mass of individual particles.

The carrier material may be any of those carrier materials or mixtures thereof used or suggested for use for this purpose. Preferably, however, it is alumina (aluminium oxide), advantageously mixed with a proportion -- about 25% by weight, say - of a acid-stable high silica alumina-silicate (a zeolite such as H-mordenite, for example).

The carrier material is itself particulate in nature, and within each layer takes the form of a porous cohesive mass of individual particles having catalyst admixed therewith. It is the particulate nature of the carrier material that is mainly responsible for conferring upon each carrier layer its porous nature, and it is the porosity of the carrier layer that allows the molecules of a gas mixture being tested, and specifically the molecules of any combustible gas within that mixture, to percolate through the pellet to the catalyst distributed therewithin. Preferably the particulate carrier material originates in fine powder for -- particulate sizes of up to 10 micrometres are satisfactory.

The layers containing carrier are not layers of carrier alone but are instead layers of an admixture of carrier and catalyst. The proportion of the carrier and catalyst components may be any of those used or suggested for use in the art from 0.05 to 0.2 wt% catalyst, for example. In a preferred case the proportion is such that the admixture contains about 0.1 wt% catalyst.

The detector element pellet of the invention exists as a laminated, or onion-like, structure. Thus, it consists of a multiplicity of concentric layers, layers of catalyst alternating with layers of carrier+catalyst admixture (the term "concentric" is here used loosely; it is not intended to mean that the layers should be truly spherical, or truly concentric, merely that they should approximate to each in so far as they can).

There may be any number of layers ("laminae"), provided that they are sufficient in number and thickness fuily to cover the detector filament, and provided there is a reasonably minimum number say, two or three - of each kind; the layers may be of any reasonable thickness; and the term "alternating" is also used loosely, and intended to cover sequences of layers other than that in which the two types strictly alternate (so that there may be, for example, two carrier+catalyst layers between two catalyst layers).Nevertheless, preferably: the pellet as a whole has at least 5 layers (of which two are catalyst), and conveniently has from 6 to 20 layers (with more than 20 layers the pellet tends to be so large -- relatively speaking -- that heat loss from its surface distorts the results) slightly more of which are carrier+catalyst than catalyst; the carrier+catalyst layers are from 0.05 to 0.2 mm thick, while the catalyst layers are very much thinner (about 0.01 mm); and the layers do "alternate" so that within the body of the pellet a catalyst layer follows two carrier+catalyst layers.The final layer - that is, the external layer - may be either catalyst or carrier+catalyst; the latter is marginally preferred in most cases, but it depends primarily upon the type of heat radiation compensation that is to be employed. A specific preferred detector pellet has 9 layers-- an inner layer of carrier+catalyst, followed by 1 of catalyst, 2 of carrier+catalyst, 1 of catalyst, 2 of carrier+catalyst, 1 of catalyst, and a final (external) layer carriertcatalyst.

In accordance with the disclosure of our aforementioned Specifications No.1,549,648 and 1,554,831 the pellet may be given a final coating of carrier devoid of catalyst.

Apart from the provision of alternating layers of carrier+catalyst and catalyst, the former each being a porous cohesive mass of individual particles, the detector element pellet of the invention may be made by what is generaily one of any of the standard methods for making detector element pellets. in particular, it may be made by the dipping of the filament into a slurry of carrier+catalyst (or a solution/slurry of catalyst) followed by a curing (or conditioning) heat treatment, this being repeated as many times as is appropriate.Thus, for instance, to produce each carrier+catalyst layer the filament (or pellet so far) is dipped into an organic slurry of alumina in a mixture of acetone and methanol containing some catalyst (ammonim chloropalladite, say), dried, and then heated for a few seconds to about 1 0000C (conveniently using the detector filament as a heating element) to cure the carrier+catalyst slurry layer so formed. The slurry can with advantage also contain various additives, for example: a methacrylate binder; aluminum nitrate (which is converted to crystalline aluminum oxide, further binding the carrier together); calcium nitrate; thorium nitrate (which is converted to the refractory thorium oxide, improving the stability of the carrier); and a zeolite such as H-mordenite.To produce each desired catalyst layer the pellet so far is dipped into an aqueous acid solution of a suitable salt of the chosen catalyst, and then "conditioned" by exposing it to air containing a high concentration of some suitable combustible vapour/gas (which it catalytically oxidises at about 6000C). The catalyst solution can be ammonium chloropalladite in 1/3M aqueous HNO3, preferably containing some thorium nitrate, and the subsequent conditioning can be effected by exposing the dried pellet to a stream of 30 to 40 vol?/o methane or ligroin vapour in air, allowing the pellet temperature to reach 1000 C, for several minutes.

The invention extends, of course, to a detector element of the invention whenever made by such a process.

It is not entirely clear why the onion-skin layered detector element of the present invention containing catalyst within the "carrier" layers, should be so much better - so much more resistant to poisoning by silicone vapours -- than earlier elements (as, for example, those of our aforementioned Specifications Nos. 1,549,640, 1,556,339 or even 2,096,321 A). Presumably the inert carrier acts as a filter, preventing the relatively large poison molecules from reaching the catalyst disposed further within the pellet, and it seems reasonable to suppose that the layering of the pellet into successive layers of catalyst and carrier serves further to reduce the amount of catalyst that is poisoned.Moreover, as regard the earlier onion-like pellet of Specification No. 2,096,231 A the present pellets are significantly superior, even allowing for the extra life resulting from the simple increase in catalyst content. The reasons for this are not presently understood -- but whatever the explanation, it remains a fact that tests (discussed in more detail hereinafter) have shown that the elements of the invention show a remarkably small sensitivity drop even after a considerable length of time under test, which is surely proof of the surprising efficiency of the inventive pellet construction.

The detector pellet of the invention will usually be employed in a bridge circuit of the type disclosed in our aforementioned Specifications, and no more need be said about that here, except to point out that the invention extends, of course, to any apparatus for the detecting of combustible gases when employing an inventive detector element pellet.

An embodiment of the invention is now described, though only by way of illustration, with reference to the accompanying drawings in which: Figure 1 shows an axial cross-section (in diagrammatic, not-to-scale, form) of a detector element pellet of the invention; Figure 2 shows a cross-axial cross-section of the same pellet (taken on the line Il-Il in Figure 1); Figure 3 is an axial cross-section of a conventional holder for a detector element (with a side-byside compensating element), as used in the Test described hereinafter; Figure 4 is a circuit diagram showing the manner in which the detector element is connected up in use (and in the Test described hereinafter); and Figure 5 is a graph representing the Test Results obtained using the Test described hereinafter.

As is more or less self-evident from Figures 1 and 2, the inventive detector element pellet there depicted comprises a helical coil of wire (10) with a multilayer coating made up (from the inside) of: a single carrier+catalyst layer (11); a catalyst layer (the heavy line, 12); a double carrier+catalyst layer (13); a second single catalyst layer (14); a second double carrier+catalyst layer (15); a third single catalyst layer (16); and a final (external) carrier+catalyst layer (17) - nine layers in all. The pellet structure is onion-like, layers of carrier+catalyst "alternating" with layers of catalyst, each layer being roughly concentric with the pellet as a whole.

The following Example is now given, though also only by way of illustration, to show details of one embodiment of the invention.

Example: Preparation of an inventive Detector Element Pellet.

A) Preparation of the carrier+catalyst slurry A first organic slurry of alumina carrier combined with organic binder was made by mixing the following ingredients: Washed Alumina powder ( < 10 micrometre) 1600 g Methacrylate binder

DIAKON acrylic polymer 86 g Acetone 1000 ml Methanol 1000 ml 2250 ml di-Acetone alcohol 275 ml diButyl phthalate 43 ml Calcium nitrate

Calcium nitrate (anhydrous) 80.5 g Acetone 400 ml .....

Methanol 400 ml Aluminium nitrate

Aluminium nitrate (hydrate) 28.16 7 Acetone 80 ml ......

Methanol 80 ml This slurry was then further admixed with thorium nitrate, H.mordenite, alumina and ammonium chloropalladite to make the final, ready-for-use, carrier+catalyst slurry, as follows: First alumina slurry 5 ml Thorium nitrate (as a saturated 50 : 50 vol. methanol+acetone solution) 1 ml H. mordenite 0.3 g Alumina powder (10 micrometre) 0.3 g Saturated ammonium chloropalladite in 50 : 50 vol. acetone+methanol 1 ml B) Preparation of the Catalyst Solution 12.8 g ammonium chloropalladite was dissolved in sufficient 1/3M aqueous nitric acid, the whole being made up to 50 mls with more acid. 65 g thorium nitrate were dissolved in 40 ml water. Equal volumes of the two solutions were then combined to give the desired catalyst solution.

C) Formation of the Detector Element Pellet A suitable platinum wire (about 0.5 mm diameter) coiled into helical form (about 1 5 turns per mm) was pelletised in accordance with the invention in the following manner.

The coil was first dipped into the carrier+catalyst coating slurry (see (A) above), then removed. An electric current was passed through the coil sufficient to raise its temperature to about 1 0000 C, and the coil was maintained at that temperature (in air) for 2 seconds, whereupon it was allowed to cool to room temperature. It then bore a coating about 0.1 mm thick of "fused" but porous aluminium oxide homogenously admixed with palladium catalyst, this coating constituting the required porous cohesive mass of individual particles.

The thus-coated coil was then dipped into the catalyst solution (see (B) above), removed, and heated to about 600"C for 2 seconds (again, by using the coil as its own heating element) so as to decompose the catalyst. It was then placed for "conditioning" in a mixture of air and ligroin vapour, heated (by passing a current through the coil) to about 5000--6000C, left for a few seconds (during which time the catalytic reaction raised the temperature to about 1000 C), and removed and allowed to cool; it then bore a very thin "palladium" layer partly diffused into the underlying layer of alumina+"palladium".

The carrier+catalyst-coating and catalyst-coating procedures were then repeated to give the pellet in all a sequence of 9 coatings comprising carrier+catalyst (inner), catalyst, carrier+catalyst carrier+catalyst, catalyst, carrier+catalyst, carrier+catalyst, catalyst and carrier+catalyst (outer). The finished pellet was about 1.4 mm in diameter.

The Test A) The Apparatus The inventive detector element pellet made according to the above-described procedure was then subjected to an accelerated poison test (described hereinafter), being compared both with a Prior Art pellet of the simple type consisting of a similar helical platinum wire pelletised in alumina and having an outer layer of palladium mixed with thorium oxide (and commercially available from English Electric Valve Co. Limited as the VQ3), and with the more complex Prior Art pellet of the onion-like type described in our aforementioned Specification No. 2,096,321 A. In each case the element was used in a bridge together with a similar, but inactive, compensating element, the two elements being mounted side-by-side in a standard holder of the type shown in Figure 3 of the accompanying drawings.In this standard holder the detector element (30) and the compensating element (31) are each mounted within a holder (32, 33 respectively), secured within bores (34) in the thickened side wall (35) of a pipe (36) through which flows the gas being used in the Test. Each bore 34 communicates with the interior of the pipe 35 but is separated therefrom by a wire gauze filter (37) lining that part of the pipe interior surface.

The conductive terminals (as 38) of each element are in use connected into a bridge circuit as shown in Figure 4.

In the circuit of Figure 4 the detector element 30 is included in one arm of a balanced bridge arrangement consisting of resistors (41,42) of equal value and the compensating element 31. Across the bridge is connected a voltmeter (43), calibrated to indicate combustible gas concentrations. The meter may be set at zero by the adjustment of the slider on a potentiometer (44). Terminals (as 45) allows the bridge to be connected to a source of power (not shown) providing both the heating current for the detector element filaments and the voltage of the bridge.

Except for the nature of the detector element 30, the arrangement is, in fact, as known per se.

In operation the detector element 30 and the compensating element 31 are exposed to a normal atmosphere, and the slider/potentiometer 44 is adjusted to give a zero reading on the meter 43. The two elements are then exposed to the test atmosphere which it is required to monitor. The large "poison" molecules in the atmosphere tend to remain or or in the outer porous carrier+catalyst layers 1 5 and 1 7, whilst any of the smaller combustible gas molecules not there oxidised tend to diffuse through the highly porous structure to the inner catalyst layers 12 and 14, to oxidise in the normal way.

Naturally, no catalytic oxidation occurs on the surface of the compensating element 7. Relatively, therefore, the temperature of the detector element 30 rises, with a consequent change in its resistance, and the reading of the meter 43 then provides a measure of the concentration of the combustible gas in the test atmosphere.

B) The Test Atmosphere Each of the three detector elements was first tested with an atmosphere of 1 vol.% methane in air at room temperature (about 200 C) to establish a basic value for its sensitivity before poisoning. Each was then subjected to an atmosphere of 5 p.p.m. hexamethyldisiloxane (HMDS) in air containing 1 vol.% methane at room temperature (using the apparatus of Figure 3 this Test Atmosphere was driven past the elements at a rate of 500 ml per minute) and the actual value it gave (using the circuit of Figure 4) for the methane content was taken every five minutes and converted into a percentage sensitivity figure, indicating the degree of poisoning, and plotted to give the graphs of Figure 5.Thus, a reading showing an apparent methane content of 0.8 vol.% - 80% of the true value - was converted to an 80% sensitivity value, and so on.

HMDS was chosen as the Test silicone poison because it is convenient and representative of the vapours arising from silicone oils and rubbers. 5 p.p.m. is a much higher amount than would normally arise, but provides an Accelerated Poisoning Test that correlates fairly well with the results obtainable in a real situation.

C) The Results The results of the Test are shown graphically in Figure 5. It will immediately be apparent that though both the detector element of the present invention and the similar onion-like element of our aforementioned Specification No. 2,096,321A performed very well, losing no more than a mere 5% of their original sensitivity in 45 minutes, while the Prior Art VQ3 device lost over 50% in the same time, nevertheless over the much longer Test Period of several hours the element of the present invention was markedly superior, losing only 10% of its initial sensitivity in as long as 300 minutes, compared with just over 40% for the element of our earlier Specification. And after as long as 500 minutes the element of the present invention had stiil only lost 30% of its initial sensitivity. In a real life situation this could mean that the VO3 would need replacing every day. The detector of our earlier Specification could last for several weeks, but the detector of the present invention would last several months.

Claims (15)

1. A combustible-gas detector element comprising a heatable wire filament embedded in a pellet formed overall of an oxidation catalyst and a porous, particulate, non-catalytic, inert carrier therefor, wherein the pellet has a laminated, onion-like structure, and consists of a multiplicity of concentric layers in which layers of a carrier+catalyst admixture, each in the form of a porous cohesive mass of individual carrier material particles having catalyst admixed therewith, alternate with layers of catalyst.

2. An element as claimed in claim 1, wherein the heatable wire filament is made of platinum or one of its alloys.

3. An element as claimed in either of the preceding claims, wherein the oxidation catalyst is palladium or platinum.

4. An element as claimed in any of the preceding claims, wherein the carrier material is alumina (aluminum oxide), and originates in fine powder form.

5. An element as claimed in claim 4, wherein the alumina is mixed with a proportion of an acidstable high silica alumino-silicate.

6. An element as claimed in any of the preceding claims, wherein the proportion of carrier and catalyst components in the carrier+catalyst layers is such that there is from 0.05 to 0.2 wt% catalyst.

7. An element as claimed in claim 6, wherein the carrier+catalyst admixture contains 0.1 wt% catalyst.

8. An element as claimed in any of the preceding claims, wherein there are at least three layers of each kind (carrier+catalyst, and catalyst).

9. An element as claimed in claim 8, wherein there are from 6 to 20 layers in all, slightly more of which are carrier+catalyst than catalyst.

10. An element as claimed in any of the preceding claims, wherein the layers "alternate" so that within the body of the pellet a catalyst layer follows two carrier+catalyst layers.

11. An element as claimed in any of the preceding claims, wherein the carrier+cataiyst layers are from 0.05 to 0.2 mm thick, while the catalyst layers are very much thinner (about 0.01 mm).

12. A detector element as claimed in any of the preceding claims and substantially as described hereinbefore.

13. A process for the preparation of a detector element as claimed in any of the preceding claims, which involves the dipping of the filament into a slurry of carrier+catalyst (or a solution/slurry of catalyst) followed by a curing (or conditioning) heat treatment, this being repeated as many times as is appropriate.

14. A process as claimed in claim 1 3 and substantially as described hereinbefore.

1 5. A detector element of the invention whenever made by a process as claimed in either of claims 13 and 14.

1 6. Apparatus for the detecting of combustible gases, which apparatus employs a detector element as claimed in any of claims 1 to 12 and

1 5.