WO1990004268A1

WO1990004268A1 - Apparatus to provide oxygen-free gas

Info

Publication number: WO1990004268A1
Application number: PCT/GB1989/001203
Authority: WO
Inventors: Peter Simmonds
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1988-10-13
Filing date: 1989-10-12
Publication date: 1990-04-19
Anticipated expiration: 1991-04-13
Also published as: GB8823986D0

Abstract

A nitrogen gas source, suitable as a field source, comprises a metal air battery having an inlet and outlet to allow a flow of air to pass over its porous cathode. Operation of the battery causes the cathode to remove the oxygen, leaving an oxygen-free gas supply. The output gas may then pass through a chemical filter containing charcoal or molecular sieves to further purify the nitrogen gas. The source is particularly suited for use with an electron capture detector for monitoring tracer substances in air when the cathode is coated with a small amount of stationary phase material of the type used in gas chromatography to minimise absorption of the tracers. The battery is constructed from two identical air cathodes (1) and a single sheet of super purity (99.99 %) aluminium (2) which acts as the positive electrode. Each air cathode (1) is a conducting nickel screen which has a layer of active carbon (3) coated on one side which is exposed to electrolyte. The other side of the cathode is coated with a layer (4) of water-impermeable, gas-porous plastics material and is exposed to a flow of air during operation of the battery. Each air cathode (1) is attached to a plastics sheet (5) using a double-sided adhesive tape (6) which has previously been cut into a grid pattern so as to define an air channel (7). Holes (11) are drilled in each external face of the plastics sheets, interconnecting with the air channels on both sides of the cathode, and fitted with gas inlet (12) and outlet (13) pipes. Air is introduced by means of a small pump.

Description

Apparatus to Provide Oxyσen-Free Gas

Ihe invention relates to the removal of oxygen from amixture of gases and in particular to the removal of oxygen from ambient air to provide a supply of oxygen-free gas.

T-heselective removal or separationof oxygenfromambient air isofpractical interest for many applications and several large scale processes exist commercially. However, in a number of field-portable analytical instruments it isoftendesirabletoprovidea supply ofpurenitrogengasona relatively small scale for proper operation of the instrument. This requirement is normally met by using high pressure gas cylinders and appropriate gas regulators. For continuous analysers which incorporate an electron capture detector in their resign, complete removal of oxygen from the incoming ai^'r stream is mandatory since oxygen itself is an electron absorber. Such an apparatus which selectively removes oxygen from a continuous flow of ambient air by catalytic reaction with hydrogen to form waterhas been described by Simmonds et al in J. of Chrcmatog.126 (1976) 3-9, and in US Patent No 4,304,752. However, thepenalty of such arrangements is that a cylinder of hydrogen gas is still required.

The object of the present invention is to provide a small portable device which can generate nitrogen directly from ambient air without the need for high pressure cylinders.

T-he invention provides apparatus toprovideanoutput flowof oxygen-freegas comprising: a) A metal air battery having a porous air cathode; b) Inputmeansto introduceaflowofair overthecathodeof thebattery; and c) Output means to collect the nitrogen gas after the removal of oxygen by the cathode of the battery.

Advantageously the output gas ispassedthrough a chemical filter containing charcoal or molecular sieves to further purify the nitrogen gas.

Where it is required tomonitor the presence of tracer substances in ambient air directly with an electron-capture detector it is desirable to coat the cathode with a small amount of stationary phase material of the type used in gaschromatography tominimiseabsorptionof the tracers. ^"When thetracer is ahalocarbon the cathodemaybecoatedwithapolyglycol stationaryphase such as "Carbowax" (Registered Trade Mark) . Mien used as a tracer monitor the nitrogen cannot be purified by charcoal or molecular sieve filters since these would remove the tracers.

The invention will now be described by way of exaπple only with reference to the accompanying drawings of which:

Figure 1 is an exploded perspectiveview of apparatus for producing nitrogen gas; and

Figure2 is a schematicdiagramshowing theapparatusofFigure1 incorporated in an equipment for detecting tracers in the atmosphere.

1-Ietal air batterieswhich employ specialisedporous cathodes are designed to use oxygen of the air as the reactive materiel. Several types of metal air batteries are available commercieύlly, including those which use either aluminium or zinc as thepositive electrode. The electrochemical reactions occuring in the aluminium air battery are described by the following equation:

__L + 302 +6H20 =4A1 (OH)3

The inventor has realised that this battery, when suitably designed, can be used as a continuous flow source of nitrogen for detector systems.

One version of an aluminium air battery designed ^■to efficiently and selectively remove oxygen froma flowingair stream, therebyprovidingaflow of reasonably pure nitrogen, is shown in Figure 1. The battery is constructed from two identical air cathodes (1) and a single sheet of sυj.^*er purity (99.99%) aluminium (2) whichactsasthepositiveelectrode (materials obtained commercially fro Alupower Inc, Bernardsville, NJ, USA). Eachair cathode (1) is a conducting nickel screen which has a layer of active carbon (3) coated on one sidewhich isexposed toelectrolyte. Theother side of the cathode is coated with a layer (4) of water-impermeable Teflon (Registered Trade Mark) and is exposed to a flow of air during operation of the battery. Thecathodes arebydroi-hobic sothat electrolytedoesnotpenetrate. Theair cathodes (1) are chemically treated with a solution of "Carbowax" (0.5% in ethanol) to reduce adsorption before assembly. Each air cathode (1) is attached to a Perspex (RegisteredTrade Mark) sheet (5) using a double-sided adhesive tape (6) which has previously been cut into a grid pattern so as to define an air channel (7). This air channel (7) is typically 0.5-1.0 cmwide and with a depth of 0.6 mm determined by the thickness of the adhesive tape. The two air cathodes (1) arebondedwiththedouble sidedtape (6) toa central U- shaped section of perspex (8), which has a small hole (9) drilled through one side. This hole is aligned with similar size holes (10) drilled in each of the air cathode/perspex sheets (5) so as to interconnect the two air channels on each side of the battery. Further holes (11) are drilled in each external face of theperspex sheets and fittedwith gas inlet (12) and outlet (13) pipes. Air introduced by means of a small pump (not shown) therefore flows first through one side of thebattery passing over thesurfaceofoneof the air cathodes (1) exposed by the channel (7) cut in the adhesive tape, and then through the similar channel (7) defined by the second air cathode (1). The total internal volume of the combined air channel (7) is less than 5ml so as to maintain a reasonably short time constant for the range of flow rates used. The active carbon sides (3) of the air cathode form two sides of an inner chamber defined by the ϋ-shaped perspex member (8) which contains the aluminium anode (2) and is filled with an electrolyte solution of 15% sodium chloride (typically) . Appropriate electrical connections are made to each of the two cathodes (1) using stainless steel screws (14) as well as a third screw (15) which passes through the ϋ-shaped perspex member (8) into the central aluminium anode (2) . The entire battery is provided with a perspex lid (16) which is drilled with four large holes (18) and sealed with double sided adhesive tape to the main battery. The inside of theperspex lid (16) is fitted with aporous Teflon sheet (17) toprevent any loss of electrolyte. The porous Teflon is carefully selected to allow emission of any gases, such as hydrogen, which may be formed as by-products during the electrolytic process.

The entire battery thus described is portable and also extremely simple and flexible, since the dimensions of the cathodes, size of the air channels, and overall sizeof thebatterycanall beeasilychanged toacconmodatedifferent required flow rates. In the preferredmode of operation the two cathodes do not remove oxygen at equal rates since the current densities through each cathode are adjusted by the selection of appropriate load resistors. Thus themajority of the oxygen in the flow of airwhichpasses through thebattery is removed by the first cathode, while the second cathode acts as a final scavenger of any residual oxygen. However, it is important that the load resistors are chosen so that the current density maintained in the first cathode is not excessive for the flow rate used. It may be appropriate to periodically reverse the gas flow throughthecathodes soas toexposethetwo cathodes to the same overall current density and usage. The air cathodes appear tohave a lifetime of many months provided they are not operatedwith excessive current densities. _he aluminium anode is consumed during electrolysis and the 6 mm thick plates used in the above design contain approximately 100 grans of aluminium which will consume a theoretical electrochemical equivalent of about 90 litres of oxygen. In practice, continuous operation of thebattery islimitedbytheproduction ofaluminium hydroxide (see equation above) which accumulates in the electrolyte as a white flocculant precipitate. After about twelve hours of continuous operation a sufficiently dense precipitate of the aluminium hydroxide has accumulated, as a sludge at the bottom of the battery, to reduce the efficiency of the air cathodes. This problem can be easily solved by temporarily interrupting operation of thebattery and simplywashing out the old electrolyte and precipitate with distilled water. The battery can then be rechargedwith fresh electrolyte and operation resumedby reconnecticn of the load resistors. Where necessary two batteries could be provided in series so that one can be switched into use as the other is removed for cleaning and rechargingwith fresh electrolyte. Alongeroperating timemay be achieved for a single battery by using a large reservoir of fresh electrolyte which can be recirculated within the battery itself. Alternatively, adesign of acrystalizer/separator hasbeen described inthe prior art for the continuous removal of the aluminiumhydroxide precipitate formed in this type ofbattery (GMScamens et al, 21st International Society Energy Conversion Conference, San Diego, California, 1986).

Further purification of the product nitrogen gas to remove trace impurities can be achieved by passing the gas through a chemical filter containing charcoal or molecular sieves and will be obvious to those familiar with the art. For flow rates in the region of 50 ml/min, oxygen can be continuously removed in a single stepwith a residual concentrationbelow5 ppm andwith a cathode current density ofbetween15-30 ma cm2. The amount of oxygenwhich can be removed is directly related to the available cathode surface area, and for the flow rates described here a surface area of about150 cm2 is required. Where it isdesirable tomonitor thepresenceof tracer substancesdirectly in a flowing stream of ambient air with an electron capture detector then, in addition to continuously removing oxygen, a second problemarises due to the possible adsorption of thetracersubstanceswithin theair cathode. This is because most air cathodes are constructed with active high surface area carbon powders as one of their principal components. Thus in order to produce a practical device, the air cathode must be chemically modified to prevent adsorption of the tracer substances. In this invention coating the cathode with a small amount of stationary phase of the type used in gas chromatography has proven to be the roost effective in reducing adsorption. It is equally important that this chemical treatment does not adversely affect the efficient removal of oxygen by the cathode. The tracers most commonly used with field-portable electron capture based instruments are halocarbons and more specifically perfluorinateo compounds such as sulphur hexafluoride, and perflυoromethylcylohexane. Adsorption of this class of substancecanbe essentially elindnatedbytreatment oftheaircathodewitha "Carbowax" stationary phase, without any impairment of cathode efficiency. When the air battery is used in the above configuration as a continuous analyser the product nitrogen gas cannot be further purified by using charcoal or molecular sieve filters, since thesewould remove the tracers of interest.

The invention described above is an effective source of nitrogen gas derived from the ambient air and suitable for some portable instruments. For continuous analysers designed to detect the presence of perfluorinated tracersintheatmosphere, thealuminiumairbatterymustbeconfiguredintoa flow arrangementwhich candeliver cleandry nitrogen containing the tracers ona real timebasistoanelectroncapturedetector (ECD). Oneembodimentof such an arrangement, which is suitable as a field-portable battery operated instrument, is illustrated in Figure 2.

Theair tobemonitored for tracers is drawn through asuitablenozzle (20) by a small pump (21) and is divided into two streams at a T-junction (22). One stream flows through the aluminium air battery (23), where oxygen is selectively removed from the gas and then passes through a permeation dryer _Λ_ ,_{1. m^0} 90/04268

(24) constructed fromwater-permeabletubingmade fromaperfluorosulphonic acid polymer such as Kafion (Registered Trade Mark) which is externally purged with a counter-current flowof dry air. This dryer (24) removes any water vapour from the gas stream and typically delivers gas dried to a dew point ofless than-30 degreesC. Thedriedgas stream, whichisessentially now only nitrogen, next passes into a small heated catalytic reactor (25) containingpalladisedasbestosorapalladisedmolecularsieve. Thisserves toremoveanyunwantedminorelectronabsorbingimpurities, eithernaturally presentintheatmosphere, orreleasedfromplasticsandchemicalsusedinthe construction of various components. The catalytic reactor (25) does not reactwithorremoveanyoftheperfluorinatedcompoundsmostcoirmonlyusedas tracers. Thedryandnowpurifiednitrogengasnextpasses intoanelectron capture detector (26) where the presence of any tracer compounds is continuously recorded. The second air stream which leaves the T-juncticn (22) is first regulatedby a flowcontroller (27). Itnextpassesthrougha drying tube (28) containing amolecular sieve and then flows through a gas channel (2S) surrounding the Mafion permeation dryer (24). With this arrangement it isnotnecessarytouseaseparatesupplyofgasasapurgegas for theEafion dryer, since apartoftheincomingairstream, firstdriedby passage through a molecular sieve, is used for this purpose. The only essential requirement for obtaining a dry flow of nitrogen into the BCD is thattheexternalflowofpurgegasisatleast1.5timestheinternalnitrogen flow. There is a further benefit that arises fromthe use of the aluminium battery and which contributes significantly to the portability of the continuous trace analyser depicted schematically in Figure 2. The total currentdrainofthetwoloadresistorsisabout2.6amperesforinputairflow rates of 50 cc/ in, and this can be usedvery efficiently as apower source (30) for heating of thecatalytic reactor, whichwould otherwise require an additional and independent source of a battery power.

Although the metal-air battery has been described as having an aluminium anode other suitable electro-positivemetals canbeusedsuch asmagnesium, lithium or zinc. In addition the preferred electrolyte, sodium, cliloride, could be replaced by potassium hydroxide which would allow use of higher current densitiesalthoughtreatment oftheanodewouldprobablyberequired to inhibit too rapid deterioration. If the use of tracers different from thosedescribediscontemplatedthenitmaybenecessarytotreatthecathodes with different stationary phase materials as would be apparent to those skilled in the art.

The perspex battery container may alternativelybeproduced as amoulding so as to simplify manufacture and reduce cost.

Claims

1. An apparatus to provide an output flow of oxygen-free gas comprising: a) a metal air battery having a porous air cathode; b) inputmeanstointroduce aflowofairoverthecathodeof thebattery; and c) output means to collect the nitrogen gas after the removal of oxygen by the cathode of the battery.

2. An apparatus as claimed in claim 1 wherein the output gas is passed through a chemical filter containing charcoal or molecular sieves to purify the nitrogen gas.

3. An apparatus as claimed in claim1 for use inmonitoring the presence of tracer substances with an electron-capture detector wherein the cathode is coated with a small amount of stationary phase material to minimise absorption of the tracers.

4. An apparatus as claimed in any onepreceding claimwherein the nitrogen gas is passed through a permeation drier to remove water vapour.

5. Anapparatus as claimed inany onepreceding claimwhereinthebattery is forroed with a single sheet anode with an air cathodedisposed on each side of the anode and an air passage provided to circulate air over one cathode and then over the second cathode. . Anapparatus asclaimedinclaim5whereinloadresistors areconnectedto the two cathodes such that most oxygen is removed by the first cathode.

7. An apparatus as claimed in claim 5 or 6 wherein the two cathodes are attached to respective sides of the anode bymeans of double-sided adhesive tape formed with a grid pattern so as to define serpentine air passages between the anode and cathodes.

8. An apparatus as claimed in any one preceding claim wherein the batters- casing is formed from a plastics moulding.

9. An apparatus as claimed in claim8 whereinthebattery casing isprovided with holes for providing respectively an inlet of ambient air to a first air passage between the anode and a first cathode of the battery, a connection fromthe first airpassage to the second airpassagebetweentheanodeandthe second cathode of the battery, and an outlet for nitrogen.

10. An apparatus as claimed in claim 8 or 9 wherein the battery casing is providedwithmeans to allow escape of gasesproduced during operationof the battery.

11. Detection equipment including apparatus as claimed inany onepreceding claim wherein there is included: an electron capture detector; and means connecting the output flow of oxygen-free gas to the detector.

12. Detection equipnent as claimed in claim11 wherein there is provided a means for drying the gas prior to connection to the detector.

13. Detection equipment as claimed in claim 12 wherein the drier is a permeation drier comprising water-permeable tubing made from a perfluorosulphonic acid polymer.

14. Detection equipment as claimed in claim13 includingmeans toprovide a counter-current flow of dry air to purge the permeation drier.

15. Detection equipment as claimed in claim14 wherein the counter-current flow of air is purified by passage through a catalyst of palladium or platinum.

16. Detection equipment as claimed in claim 15 wherein there is provided means toheat the catalyst withpower derived fromoperation ofthemetal-air battery.