GB2121180A - Catalytic combustible-gas detectors - Google Patents

Abstract

Catalytic combustible-gas detectors are subject to poisoning or obstruction by certain non-volatile residues - such as lead or silicone vapours, so reducing the sensitivity of the device. A detector pellet having much greater resistance to catalyst poisoning, especially by silicones, incorporates within the pellet a significant proportion of a colloidal silica carrier material (particle size 0.001 to 1.0 ???m). Such a detector comprises a heatable wire filament (10) embedded in a pellet formed overall of an oxidation catalyst and a porous non-catalytic inert carrier, of colloidal silica. Preferably the pellet is in two main parts; an inner part (11) is a mixture of catalyst and carrier, while an outer part (12) is carrier alone. <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 utilised 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 difficulty which has been experienced with catalytic detectors of the 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 (silicon-containing compounds analogous to the carbon-containing compounds of 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 (firedamp) 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 kaoline; in the Complete Specification of our British Letters Patent No. 1,556,339 (1/6076/V) we have described making the pellet of a homogenous mixture of catalyst and zeolite (optionally with alumina); and in the Specification of our copending British Application for Letters Patent No. 81/10625 (1/6530/V) we have described giving the pellet a laminated, onion-like structure-making it of a multiplicity of concentric layers in which layers of carrier alternate with layers of 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 incorporating within the pellet a significant proportion of a colloidal carrier material.

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 non-catalytic inert carrier therefor, wherein the carrier, at least in part, is-or is formed from--colioidal silica.

The detector element of the invention employs a heatable wire filament the electrical resistance of which changes with temperature, such a temperature change occuring 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 material is platinum or one of its alloys, for example a platinum/zirconia alloy as described in the Complete Specification of British Patent No. 1,567,736 (1/6174/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. As explained hereinafter, the catalyst is conveniently employed "admixed" with some of the carrier material together with one or more refractory oxides.

The detector element of the invention uses a non-catalytic inert material with a porous structure as the carrier for the oxidation catalyst, and in accordance with what may be regarded as the inventive feature at least part of the carrier is, or is formed from, colloidal silica.

The term "colloidal" refers to the size of the individual particles making up the material (though large numbers of these may well clump together to form a loose agglomerate), and as such has its normal meaning, which is to indicate particle sizes in the general range 0.001 to 1 micrometre. The commercially-available colloidal silicas useful in this invention tend to have an average single particle size of around 0.04 micrometre.

The colloidal silica carrier may be any silica in colloidal form. Typical such materials are those available from Cabot Corp. under the name Cab 0coil, from Degussa Inc. under the name Aerosil, and from British Drug Houses simply as "colloidal silica". The latter material has an average particle size of about 0.04 micrometre.

The inventive pellet may employ a carrier which is formed wholly of the colloidai silica. On occasion, however, it may be desirable to incorporate-as a binder or additional filter, sayone or more other, possibly non-colloidal, inert carrier material as well. Such an additional carrier material may be present in a total amount of up to about 50 wt% of the combined carrier, though naturally to obtain the full benefits of the colloidal silica care is needed to avoid using too much of the other carrier. Where another carrier is used, then it may be any of those materials or mixtures of materials used or suggested for use for this purpose. It may be alumina (aluminium oxide), employed in fine (up to 10 micrometres) powder form, it may be an acid-stable high silica aluminasilicate (a zeolite such as H. mordenite, for example), or it may be a mixture of the two.

The nature of the physical structure of the pellet of the invention may be any used or suggested for use in the art. It may, therefore, simply be a homogenous mixture of carrier and catalyst, or it may be centrally such a mixture but having a porous outer coating of non-catalytic material. It may, even, be "onion-like"- constructed in alternating layers of either noncatalytic carrier or catalyst. These structures are disclosed in the aforementioned Patent Specifications. Presently, however, it is preferred that the pellet's structure be that of a core, formed of a homogenous mixture of carrier and catalyst, surrounded by a porous outer layer of non-catalytic carrier material. The preparation of such a pellet is described in more detail hereinafter.

The detector element pellet of the invention may be made by what is generally 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 and catalyst followed by a curing heat treatment, this being repeated as many times as is appropriate, the finished catalytically-active core then being given an outer coating of carrier only by similar repeated dipping (and curing) in a carrier-only slurry.Thus, for instance, to produce the carrier/catalyst core the filament (or pellet so far) is dipped into a "slurry" of colloidal silica in an aqueous acidic mixture of catalyst (conveniently ammonium chioropalladite in 1/3 M aqueous HNO3, preferably containing some thorium nitrate), acetone and methanol, 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 if desired also contain various additives, for example: a methacrylate binder; aluminium nitrate (which is converted to crystalline aluminium oxide, further binding the carrier together); calcium nitrate; thorium nitrate (which is converted to the refractory thorium oxide, improving the stability of the catalyst); and a zeolite such as H. mordenite.

The outer carrier-only coating is provided by a similar dip coating process, but using a slurry without the acid and catalyst.

Each catalyst/carrier layer may be "conditioned" before the next is applied by exposing it at a high temperature to air containing a high concentration of some suitable combustible vapour/gas (which it catalytically oxidises at about 10000 C). For example, the conditioning can be effected by exposing the dried pellet to a mixture of ligroin vapour in air, at 5000--6000C, allowing the pellet temperature to reach 1 0000C for a few 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 colloidal-silicacarrier detector elements of the invention should be so much better-so much more resistant to poisoning by silicone vapours-than earlier elements. It is presumed, however, that the very large surface area of the colloidal silica results in an enormously increased number of active catalyst sites, and it seems reasonable to suppose that this results in a commensurately large increase in the time before the number of sites poisoned is sufficient significantly to reduce the ability of the pellet to enable detection of the combustible gas.Whatever the explanation, it remains a fact that tests (discussed in more detail hereinafter) have shown that whereas earlier pelletised detector elements can have their sensitivity (to the combustible gas) halved in as little as 45 minutes upon exposure to a typical test silicone poison at 5 p.p.m, the elements of the invention show a sensitivity drop of less than 5% under the same conditionsand indeed can show only such a small drop after several hours- which is surely proof of the surprising efficiency of the inventive pellet.

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-by-side 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 Figures 5A and B are graphs representing various Test Results obtained using the Test described hereinafter.

As is more or less self-evident from the Figures, the inventive detector element pellet there depicted comprises a helical coil of wire (10) with a two-layer coating made up (from the inside) of: a carrier/catalyst portion (11) coated on the outside with a carrier (no catalyst) layer (12).

In what are known as "high power" elements of this type the wire is about 0.002 in (0.05 mm) thick, the helix is 11 to 12 turns long and about 0.025 in (0.625 mm) in diameter, and the pellet itself is about 1.0 to 1.3 mm in diameter, the inner and outer layers being roughly 0.8 to 1.2 mm and 0.05 to 0.10 mm thick respectively.

Such an element will run at about 2 to 2.5 volts, a bridge pair drawing from 300 to 350 milli Amps.

In what are known as "low power" elements of this type the wire is about 0.001 in (0.025 mm) thick, the helix is about 0.015 in (0.375 mm) in diameter, and the pellet itself is about 0.6-0.9 mm in diameter, the inner and outer layers being roughly 0.5 to 0.6 mm and 0.05 to 0.10 mm thick respectively. Such an element runs at about 2 volts, a bridge pair drawing about 150 to 200 milliAmps.

The following Examples are now given, though also only by way of illustration, to show details of two embodiments of the invention.

Example 1: Preparation of an inventive detector element pellet A Preparation of the carrier slurry A first slurry of colloidai silica carrier was made by mixing the following ingredients: Colloidal silica 3.39 H. Mordenite 0.6 g Thorium nitrate solution (as a saturated 50:50 methanol/acetone solution) 2 ml Aluminium nitrate solution (as a 1.37 g/ml density solution in water) 1 ml Water 20 ml B) Preparation of the carrier/catalyst slurry mixture To 5 ml of the carrier slurry of (A) above there were added:: Ammonium chloropailadite solution (12.8 g Ammonium chloropalladite dissolved in N/3 HNO3 to give 50 ml) 4 ml Thorium nitrate solution (as a 1.69 g/ml density solution in water) 2 ml C) Formation of the detector element pellet A suitable platinum wire coiled into helical form and cleaned by a simple heat treatment at 5000--6000C in a mixture of air and ligroin vapour was pelletised in the following manner.

The coil was first dipped into the carrier/catalyst slurry (see (B) above), then removed. It was then piaced in an atmosphere of a mixture of ligroin vapour and air, a current was passed through the coil sufficient to raise its temperature to about 5000--6000C, and the coil was maintained at that temperature for a few seconds (sufficient to allow the catalyst to decompose), whereupon it was allowed to cool to room temperature. It then bore a coating of the slurry solids in the form of a porous bound mass.

This was repeated five times (making 6 in all).

The thus-coated coil was then dipped into the carrier solution (see (A) above), removed, and heated to about 5000--6000C in a ligroin vapour/air mixture (again, by using the coil as its own heating element), left for a few seconds, and then allowed to cool. This was repeated four times (a total of 5 in all) resulting in an outer coating of "fused" but porous silica.

Example 2: Preparation of an inventive detector element pellet A second pellet was made in a manner very similar to that used in Example 1.

A) Preparation of the carrier slurry A first slurry of colloidal silica was made by mixing the following ingredients.

Colloidal silica 3.9 g Thorium nitrate (as a saturated 50:50 methanol/acetone solution) 2 ml Aluminium nitrate (as a 1.37 g/ml density solution in water) 1 ml Acetone 5 ml Water 20 ml B) Preparation of the carrier/catalyst slurry To 5 ml of the carrier slurry of (A) above there were added: Ammonium chloropalladite solution (12.8 g Ammonium chloropalladite dissolved in N/3 HNO3 to give 50 ml) 4 ml Thorium nitrate solution (a 1.69 g/ml density solution in water) 2 ml C) Preparation of the outer coating carrier slurry To 10 ml of the carrier slurry of (A) above there were added: 50:50 Acetone/ethanol 2.5 ml D) Formation of the detector element pellet (i) Fiigh power pellet A high power detector pellet was prepared in much the same way as in Example 1.

The 0.002 in wire coil was heated to 5000-- 6000 C, fired in air/ligroin vapour mixture, cooled, and dipped into the carrier/catalyst mixture. It was then reheated to 5000--6000C, fired again in ligroin, and re-dipped in the carrier catalyst mixture. The procedure was repeated until the coil had been dipped a total of six times. The carrier/catalyst-coated coil was then washed in acetone, heated (again to 5000--6000C) and cooled.

The thus-coated coil was then dipped in the outer coating slurry, heated to about 9000C and allowed to cool. This procedure was repeated until the coated coil had been dipped (in the outer coating slurry) a total of five times.

The coil was then heated to 9000C, cooled, washed in acetone, and reheated to about 7000C.

It was then ready for use.

(ii) Low power pellet In much the same way (as for the high power pellet), another pellet was made using a 0.001 in wire coil. It was given a total of 8 carrier/catalyst dippings and a total of 5 outer coating dippings.

The Tests A) The apparatus The inventive detector element pellets made according to the above-described procedures were each subjected to an accelerated poison test (described hereinafter), being compared with a Prior Art pellet of the type consisting of a similar helical platinum wire pelletised in alumina and having an outer layer of palladium mixed with thorium oxide (such pellets are commercially available from English Electric Valve Co. Limited as the VQ3-high power and VQ2-low power).

In each case the element was used in a bridge together with an 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 to zero by the adjustment of the slider on a potentiometer (44). Terminals (as 45) allow 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 on or in the outer porous carrier layer 12, whilst the smailer combustible gas molecules tend to diffuse through to the inner catalyst layer 11 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 detector elements was first tested with an atmosphere of 1 vol.% methane in air at room temperature (about 200C) 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). At intervals the detector's response to a 1 vol.% in air methane atmosphere (without HMDS) was tested (using the circuit of Figure 4) and converted into a percentage sensitivity figure, indicating the degree of poisoning, and plotted to give the graphs of Figures 5A and B. 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.

The described Test was effected on three low power pellet devices standard VQ2 device, an inventive pellet device made in accordance with the procedure of Example 1 (and thus having a carrier of a silica/zeolite mixture) and an inventive pellet device made in accordance with the procedure of Example 2 (and thus having a carrier of silica only). It was also effected in three high power devices standard VO3 device, and two inventive pellets (one according to Example 1, the other according to Example 2).

C) The results The results of the Tests are shown graphically in Figures 5A and B. It will immediately be apparent that the detector elements of the invention performed very well, losing relatively little of their original sensitivity over the duration of the Tests. In the low power Tests the Prior Art VQ2 device lost as much as 80% in 45 minutes while the two inventive devices lost no more than 10% in 100 minutes, and in the high power Tests the Prior Art VQ3 device lost 50% in 40 minutes while the two inventive devices lost no more than 45% in 10 hours. In a real life situation this could mean that the VQ2 or 3 would need replacing every day, while the inventive detector would possibly last for several months.

Claims (9)

Claims

1. A combustible-gas detector element comprising a heatable wire filament embedded in a pellet formed overall of an oxidation catalyst and a porous non-catalytic inert carrier therefor, wherein the carrier, at least in part, is-or is formed from-colloidal silica.

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

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

4. A detector element as claimed in any of the preceding claims, wherein the colloidal silica has an average single particle size of around 0.04 micrometre.

5. A combustible-gas etector element as claimed in any of the preceding claims and substantially as described hereinbefore.

6. A process for the preparation of a combustible-gas detector element as claimed in any of the preceding claims, which process involves dipping the filament into a slurry of carrier and catalyst followed by a curing heat treatment, this being repeated as many times as is appropriate, the finished catalytically-active core then being given an outer coating of carrier only by similar repeated dipping (and curing) in a carrier-only slurry.

7. A process as claimed in calim 6 and substantially as described hereinbefore.

8. A detector element as claimed in any of Claims 1 to 5, whenever made by a process as claimed in either of Claims 6 and 7.

9. Apparatus for the detecting of combustible gases, which apparatus includes a detector element pellet as claimed in any of Claims 1 to 5 and 8.