GB1562481A

GB1562481A - Apparatus for analysis of nitrogen oxides

Info

Publication number: GB1562481A
Application number: GB706/78A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-01-09
Filing date: 1978-01-09
Publication date: 1980-03-12
Also published as: IT1105880B; IT7803400A0; FR2414196A1; NL7803398A; ES469719A1; JPS5495295A; BE865134A; DE2813803A1

Description

(54) APPARATUS FOR ANALYSIS OF NITROGEN OXIDES (71) 1, ALBERT EDWARD PROCTOR, a British Subject of "Nerhaven". Taplow Common Road, Burnham, Bucks, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed. to be particularly described in and by the following statement:- The present invention relates to apparatus for use in analysing the concentration of nitrogen oxides in a gas.

Apparatus made in accordance with the invention is especially useful for determining the pollution of air or combustion exhaust.

The presence of nitrogen oxides in the atmosphere in excess of 5 parts per million is nowadays regarded as harmful. In this context there is a need for a simple reliable rugged apparatus which can be used in the vicinity of combustion equipment, for example. as opposed to a laboratory. A general object of this invention is to provide apparatus which will satisfy this need.

In its broadest aspect the invention provides apparatus for use in analysing the concentration of nitrogen oxides in a gas said apparatus including means for converting nitrogen oxides in the gas to be anslysed into nitrogen dioxide and an electrolytic cell adapted to produce an electrical signal proportional to the concentration of the nitrogen dioxide in the converted gas wherein the electrolytic cell is composed of a neutral aqueous electrolyte contained in a sealed chamber, an anode of activated carbon within a perforated jacket or container immersed in said electrolyte, a cathode of platinum supported above the electrolyte for direct contact with the converted gas and means for wetting the cathode with the electrolyte and for establishing electrical contact between the electrolyte and the cathode.

A small battery-powered fan can draw the gas at a controlled flow rate through a sealed chamber containing an oxidizing agent, acting as the conversion means and then through the chamber containing the electrolytic cell. It is useful to provide a simple flow meter to indicate the gas flow rate.

In many applications the gas to be analysed is moist and in these cases it is desirable to initially dry the gas prior to conversion. This can be accomplished by a dehydrating agent in a further sealed chamber. The various chambers can be interconnected by pipes or the like.

The electrolytic or fuel cell may employ a cathode in the form of platinum foil or tape wound in helical fashion around a tubular support. This support may extend into the perforated jacket and may additionally support platinum wire wound around the support inside the jacket to contact with the carbon therein. Platinum leads or wires may then connect the cathode and the anode to terminals on a cap of a housing defining the sealed chamber of the cell and the leads may be conducted through or alongside the tubular support.

Preferably the electrolyte is buffered and is conducted to the cathode by capillary action. Glass fibres, conveniently woven or otherwise meshed, to form a stable structure can serve as the capillary wetting means.

Conveniently the various chambers of the apparatus can be defined by housings made from transparent acrylic resin permitting a visual inspection of their content. The output from the cell may feed an amplifier driving a recording or indicating means. To permit calibration, it is desirable to include in the measuring circuit, means for backingoff the standing e.m.f.. produced by the cell in the presence of pure air.

The invention may be understood more readily and various other features of the invention may become apparent from consideration of the following description.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which is a schematical representation of apparatus made in accordance with the invention.

As shown in the drawing. the apparatus has a main inlet A for receiving the gas to be treated and analyzed and an outlet B for discharging the gas after treatment. The gas may be air. or exhaust gas, containing nitrogen oxides and the apparatus is designed to provide an automatic and continuous measurement of the concentration of these nitrogen oxides. The incoming gas is drawn from the inlet A and through various chambers in the apparatus, described hereinafter. by means of a small blower or fan 4 which is driven by a d.c.

source in the form of a battery (not shown).

The outlet B is here the direct outlet of the fan 4.

The inlet A is connected through a valve 5. which can be selectively opened or closed to a branched conduit or pipe 6 which has a main limb which leads into a first chamber 31 defined by a housing 1. A second conduit or pipe 7 leads from the first chamber 31 to a second chamber 32 defined by a housing 2.

A third conduit or pipe 8 leads from the chamber 32 to a third chamber 33 defined by a housing 3. The pipe 8 is provided with a flow meter 11 adapted to directly indicate the flow rate of the gas passing through the apparatus. A fourth conduit or pipe 9 leads from the chamber 33 to the inlet of the fan 4. All the conduits or pipes are preferably made from synthetic plastics.

The housings 31, 33, 33 are conveniently provided with main walls of cylindrical shape. Each housing 31, 32, 33 also has a bottom wall la, 2a, 3a, respectively. The bottom wall 1 a, 2a, 3a may be formed integrally with or fixed to the main wall or else detachably affixed thereto. Each housing 31. 33, 33 also has a top wall or cover lb. 2b. 3b respectively which is detachably mounted to the main wall. The covers lb, 2b, 3b are hermetically sealed to the main wall of the respective housing 31, 32, 33 with the aid of seals in the form of Orings l c. 2c, 3c. respectively to seal the chambers 1, 2, 3 from the surroundings.The housings 31, 32, 33 defining the chambers 1, 2, 3 and composed of the main walls, the bottom walls la, 2a, 3a, and the covers 1b, 2b, 3b are made from a material, preferably a synthetic material, capable of resisting shock and chemical action. Suitable synthetic materials are various cross-linked polyesters and acrylic resins. A transparent acrylic resin is especially suitable since it additionally permits visual inspection of the contents of the housing.

A further inlet C is connected through a valve 27, which can be selectively opened or closed. to the conduit 6. With the valve 5 closed and the valve 27 opened an inert gas can be passed through the apparatus as a flushing agent. During normal operation when the gas fed to the inlet A is analysed, the valve 5 is opened and the valve 27 is closed.

The first chamber 31 is a dehydration chamber wherein moisture is initially removed from the gas to be analysed.

The first chamber 31 contains a suitable dehydrating agent or substance 39, such as calcium chloride, although a mass of small glass spheres or beads is also effective in removing moisture from the gas by condensation. It is possible to provide a suitable cooling means for cooling the chamber 31 therebv to enhance the dehydration process. The liquid formed in the chamber 31 collects on the bottom wall la of the housing 31. A drain tube with a valve 10, which can be selectively opened or closed. enables the liquid to be removed from the chamber 31 from time to time.

The second chamber 32 is an oxidation chamber wherein the nitrogen oxides in the dried gas taken from the chamber 31 are converted into nitrogen dioxide (NO2). The second chamber 32 contains material 40 for effecting the conversion of the nitrogen oxides. The material 40 may comprise one or more oxidizing agents such as CrO3, sodium dichromate and potassium permanganate. Conveniently, the oxidizing agent is supported on an inert carrier. such as glass fibres or pumice, although the oxidizing agent may be in the form of fine solid particles.

The third chamber 33 contains a fuel cell or an electrolytic measuring cell designed to react galvanically to the nitrogen dioxide to thereby produce an e.m.f., directly proportional to the NO2 contained in the gas taken from the chamber 32. As shown. the chamber 33 contains a central tubular support 12 fixed at its lower end to a jacket or cup 13 surrounding the lower region of the support 12. The wall of the cup 13 is perforated with a large number of openings 14. The cup 13 and the support 12 are carried by a plug 15 resting on the lower wall 3a of the housing 3. The support 12 extends through the cover 3b of the housing 3 and the interior of the support 12 is closed off with the aid of a cap 16. The support 12 is sealed with respect to the cover 3b and conveniently these parts can be secured together. It is possible to provide a screwthreading or a friction fit between the support 12 and the cover 3b while maintaining the requisite sealing of the chamber 33 from its surroundings. It is preferable to make the support 12, the cup 13 and the cap 16 from the same material as the housing 3. e.g., acrylic resin.

A thin platinum wire 19 is wound in a helical fashion around the lower region of the support 12 within the cup 13. The space between the cup 13 and the support 12 contains activated carbon in the form of activated graphite granules 42 contacting the wire 19. The wire 19 extends through a boring in the wall of the support 12 at the lower region of the latter and passes up through the support 12 to extend through the cap 16 and connect with a terminal 17.

The terminal 17 can extend through the cap 16 to contact the wire 19 if preferred. In the illustrated case however the wire 19 is sealed to the cap 16 where it passes through the latter and preferably the wire 19 is also sealed to the support 12 where it passes .therethrough to commence the helical cohfiguration.

A thin platinum foil or tape 20 is also wound in a helical fashion around an intermediate region of the support 12 above the cup 13. Similarly to the wire 19 the tape 20 extends through the wall of the support 12 and passes up through the support to connect with a second terminal 18. As with the wire 19, the terminal 18 can extend through the cap 16, but, as illustrated, the tape 20 is sealed to the cap 16 or to the cap 16 and the support 12. These measures ensure that the sealing between the chamber 33 and its surroundings is maintained.

A neutral buffered aqueous electrolyte 21 is contained in the space between the cup 13 and the lower part of the chamber 33. A layer of glass fibres 22 conveniently woven together is formed into a sheath extending around the tape 20 on the support 12 and widening out at the bottom to directly contact the electrolyte 21. The glass fibres 22 tend to lift electrolyte by capillary action.

Hence, the tape 20 is wetted and maintained in electrical contact with the electrolyte 21 to form the cathode of the measuring cell.

The terminal 18 is thus the cathodic terminal. The electrolyte 21 is also in electrical contact with the graphite 42 via the openings 14 in the wall of the cup 13.

The graphite 42 forms the anode of the measuring cell which is connected via the wire 19 to the terminal 17 forming the anodic terminal.

The electrolyte 21 may consist of a solution containing 3.0 mols of KC1, 0.1 mols of K2HPO4 and 0.1 mols of KH2PO4 per litre.

The gas containing NO2 enters the open region of chamber 33 via the conduit 8 and is led to the moistened platinum cathode.

The following galvanic reaction then takes place: NO2+H20+2e=NO+20H'.

The liberated hydroxyl radicals pass through the electrolyte of the measuring cell and are collected at the anode, thus: C+20'H'=CO+H20+2e.

An e.m.f. proportional to the quantity or concentration of NO2 is thus generated across the terminals 17, 18. This e.m.f. can be measured and/or displayed by any suitable arrangement.

The drawing depicts one arrangement where the terminals 17. 18 are connected to a series measuring circuit composed of a potentiometer 49, a compensating bridge 26 with a d.c. battery 30 and a micro-ammeter 25. An amplifier 23 driven by a voltage developed across the potentiometer feeds a display instrument 24 which conveniently presents a digital display directly indicative of the prevailing quantity of NO2. It is also possible to have a monitoring or recording instrument which produces a curve on recording paper for example, showing the NO2 content over a period of time.

The bridge 26 enables the measuring circuit to be reliably calibrated and set to back-off the standing e.m.f. created by the cell. This calibration is best carried out when pure air, i.e., air free of nitrogen oxides, is passed through the apparatus. In this way the recording instrument can be set to record a zero NO2 content prior to actual operation. It is possible to include one or more temperature-sensitive components in the bridge 26 to automatically compensate for temperature change.

The entire apparatus as described can be mounted in a conveniently-portable protective casing and is independent of mains supply. This enables a user to make measurements of nitrogen oxides in combustion products, for example, in situ.

In this way safe reliable monitoring of nitrogen oxides can be carried out. In certain cases adjustment of a combustion process can be effected to achieve maximum thermal efficiency and the apparatus as described can be useful in enabling a suitable compromise between maximum efficiency and the lowest concentration of nitrogen oxides.

WHAT I CLAIM IS: 1. Apparatus for use in analysing the concentration of nitrogen oxides in a gas; said apparatus including means for converting nitrogen oxides in the gas to be analysed into nitrogen dioxide and an electrolytic cell adapted to produce an electrical signal proportional to the concentration of the nitrogen dioxide in the converted gas wherein the electrolytic cell is composed of a neutral aqueous electrolyte contained in a sealed chamber, an anode of activated carbon within a perforated jacket or container immersed in said electrolyte, a cathode of platinum supported above the electrolyte for direct contact with the converted gas and means for wetting the cathode with the electrolyte and for

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

boring in the wall of the support 12 at the lower region of the latter and passes up through the support 12 to extend through the cap 16 and connect with a terminal 17.

The terminal 17 can extend through the cap 16 to contact the wire 19 if preferred. In the illustrated case however the wire 19 is sealed to the cap 16 where it passes through the latter and preferably the wire 19 is also sealed to the support 12 where it passes .therethrough to commence the helical cohfiguration.

A thin platinum foil or tape 20 is also wound in a helical fashion around an intermediate region of the support 12 above the cup 13. Similarly to the wire 19 the tape 20 extends through the wall of the support

12 and passes up through the support to connect with a second terminal 18. As with the wire 19, the terminal 18 can extend through the cap 16, but, as illustrated, the tape 20 is sealed to the cap 16 or to the cap

16 and the support 12. These measures ensure that the sealing between the chamber 33 and its surroundings is maintained.

A neutral buffered aqueous electrolyte 21 is contained in the space between the cup 13 and the lower part of the chamber 33. A layer of glass fibres 22 conveniently woven together is formed into a sheath extending around the tape 20 on the support 12 and widening out at the bottom to directly contact the electrolyte 21. The glass fibres 22 tend to lift electrolyte by capillary action.

Hence, the tape 20 is wetted and maintained in electrical contact with the electrolyte 21 to form the cathode of the measuring cell.

The terminal 18 is thus the cathodic terminal. The electrolyte 21 is also in electrical contact with the graphite 42 via the openings 14 in the wall of the cup 13.

The graphite 42 forms the anode of the measuring cell which is connected via the wire 19 to the terminal 17 forming the anodic terminal.

The electrolyte 21 may consist of a solution containing 3.0 mols of KC1, 0.1 mols of K2HPO4 and 0.1 mols of KH2PO4 per litre.

The gas containing NO2 enters the open region of chamber 33 via the conduit 8 and is led to the moistened platinum cathode.

The following galvanic reaction then takes place: NO2+H20+2e=NO+20H'.

The liberated hydroxyl radicals pass through the electrolyte of the measuring cell and are collected at the anode, thus: C+20'H'=CO+H20+2e.

An e.m.f. proportional to the quantity or concentration of NO2 is thus generated across the terminals 17, 18. This e.m.f. can be measured and/or displayed by any suitable arrangement.

The drawing depicts one arrangement where the terminals 17. 18 are connected to a series measuring circuit composed of a potentiometer 49, a compensating bridge 26 with a d.c. battery 30 and a micro-ammeter 25. An amplifier 23 driven by a voltage developed across the potentiometer feeds a display instrument 24 which conveniently presents a digital display directly indicative of the prevailing quantity of NO2. It is also possible to have a monitoring or recording instrument which produces a curve on recording paper for example, showing the NO2 content over a period of time.

The bridge 26 enables the measuring circuit to be reliably calibrated and set to back-off the standing e.m.f. created by the cell. This calibration is best carried out when pure air, i.e., air free of nitrogen oxides, is passed through the apparatus. In this way the recording instrument can be set to record a zero NO2 content prior to actual operation. It is possible to include one or more temperature-sensitive components in the bridge 26 to automatically compensate for temperature change.

The entire apparatus as described can be mounted in a conveniently-portable protective casing and is independent of mains supply. This enables a user to make measurements of nitrogen oxides in combustion products, for example, in situ.

In this way safe reliable monitoring of nitrogen oxides can be carried out. In certain cases adjustment of a combustion process can be effected to achieve maximum thermal efficiency and the apparatus as described can be useful in enabling a suitable compromise between maximum efficiency and the lowest concentration of nitrogen oxides.

WHAT I CLAIM IS: 1. Apparatus for use in analysing the concentration of nitrogen oxides in a gas; said apparatus including means for converting nitrogen oxides in the gas to be analysed into nitrogen dioxide and an electrolytic cell adapted to produce an electrical signal proportional to the concentration of the nitrogen dioxide in the converted gas wherein the electrolytic cell is composed of a neutral aqueous electrolyte contained in a sealed chamber, an anode of activated carbon within a perforated jacket or container immersed in said electrolyte, a cathode of platinum supported above the electrolyte for direct contact with the converted gas and means for wetting the cathode with the electrolyte and for

establishing electrical contact between the electrolyte and the cathode.
2. Apparatus according to claim 1 and further comprising means for causing the gas to flow in succession through the converting means and the electrolytic cell.
3. Apparatus according to claim 2, wherein the means for causing the gas to flow is in the form of a battery-powered fan.
4. Apparatus according to claim 1, 2 or 3, wherein the cathode is in the form of platinum foil or tape wound in helical fashion around a tubular support.
5. Apparatus according to claim 4, wherein the support extends into or through the perforated jacket and platinum wire is wound in helical fashion around the support inside the jacket to establish connection with the anodic carbon.
6. Apparatus according to any one of claims 1 to 5, wherein the electrolyte is a buffered solution.
7. Apparatus according to any one of claims 1 to 6, wherein the means for wetting the cathode comprises material which inherently causes the electrolyte to rise by capillary action to wet the cathode.
8. Apparatus according to claim 7, wherein said wetting-means material is in the form of glass fibres.
9. Apparatus according to claim 8, wherein the glass fibres are woven into a stable structure.
10. Apparatus according to claim 9, when appended to claim 4, wherein the woven structure takes the form of a sheath partly surrounding the tubular support.
11. Apparatus according to claim 10, wherein the sheath is tapered to widen out towards the electrolyte.
12. Apparatus according to any one of claims 1 to 11, wherein a cylindrical housing with a cap defines the sealed chamber and a pair of terminals are provided on the cap and are connected with the anode and cathode respectively with platinum wire.
13. Apparatus according to claim 12, wherein pipes extend into the sealed chamber to provide ingress and egress for the converted gas.
14. Apparatus according to claim 12 wherein the cylindrical casing and the electrolytic cell are substantially as depicted in the accompanying drawing.
15. Apparatus according to any one of claims I to 14, wherein the converting means comprises an oxidizing agent contained in another sealed chamber provided with an ingress and egress for the gas which is converted in said other chamber.
16. Apparatus according to any one of claims 1 to 15. and further comprising means for drying the gas prior to conversion.
17. Apparatus according to claim 16, wherein the drying means comprises a dehydrating agent contained in a further sealed chamber provided with selectivelyoperable drainage means.
18. Apparatus according to any one of claims 1 to 17, wherein one or more of the sealed chambers is defined by housing which is made from a synthetic material.
19. Apparatus according to claim 18, wherein at least the first-mentioned chamber is defined by a housing made from a transparent acrylic resin.
20. Apparatus according to any one of claims 1 to 19, and further comprising a flow meter for indicating the flow rate of the gas.
21. Apparatus according to any one of claims 1 to 20, and further comprising means for indicating or recording the electrical signal produced by the electrolytic cell.
22. Apparatus according to claim 21, wherein the indicating or recording means is included in a measuring circuit which is provided with calibrating means for backing-off the e.m.f. produced by the cell when pure air is supplied to the cell.
23. Apparatus according to claim 22, wherein the measuring circuit is substantially as depicted in the accompanying drawing.
24. Apparatus for use in analysing the concentration of nitrogen oxides in a gas substantially as described and as illustrated in, the accompanying drawing.