METHODS AND SYSTEMS FOR ANALYSING A FLUID MIXTURE

Abstract

A method of obtaining the concentration of one component in a fluid mixture of a plurality of components, comprising leading to one side of a sensing element from a source of said mixture a sample of said mixture, leading to another side of said sensing element from said source another sample of said mixture after having substantially removed said one component therefrom, thereby to cause a monitoring device to which said sensing element is connected to signal the ratio between the concentrations of the one component in the samples at the respective sides of the sensing element.

Claims

1. A method of obtaining the concentration of one component in a fluid mixture of a plurality of components, comprising leading to one side of a sensing element from a source of said mixture a sample of said mixture, leading to another side of said sensing element from said source another sample of said mixture after having substantially removed said one component therefrom, thereby to cause a monitoring device to which said sensing element is connected to signal the ratio between the concentrations of the one component in the samples at the respective sides of the sensing element.

2. A method according to claim 1 and further comprising maintaining no, or at most a substantially negligible, temperature difference between said one side and said other side.

3. A method according to claim 1, wherein said mixture is a product of combustion, said method further comprising arresting, in a flame arresting arrangement, any flame progressing in one or both of said samples towards said sensing element

4. A method according to claim 3, wherein said arresting is performed by thermal conduction at a temperature of no less than 100 C.

5. A method according to claim 4, wherein the temperature at which said arresting is performed is not higher than 150 C.

6. A method according to claim 1 and further comprising filtering solids from one or both of said samples before the sample(s) reach said sensing element.

7. A method according to claim 6, wherein said arresting is performed by alternate flame arrestors of which one is effective whilst another is being cleaned, and wherein also said filtering is performed by said alternate flame arrestors.

8. A method according to claim 1 and further comprising returning the samples after sensing to said source of said gaseous medium.

9. A method according to claim 1, wherein the travel time of said one sample to said one side is substantially equal to the travel time of said other sample to said other side.

10. A method according to claim 9, wherein said travel times are less than the response time of said sensing element.

11. A method according to claim 1, wherein said mixture is a gaseous medium and said one component is water vapour, wherein the removal of the water vapour is performed by passing said other sample through a moisture-removing arrangement.

12. A hod according to claim 11, wherein said other component is oxygen and said sensing element is an oxygen sensing element.

13. A method according to claim 12 and further comprising removing, at said moisture-removing arrangement, substantially all of the moisture in said other sample, but substantially none of any oxidisable species.

14. A method according to claim 11, wherein said moisture-removing arrangement comprises first and second moisture-removing devices of which one is effective whilst another is being re-generated.

15. A method according to claim 14, wherein the switching-over from effectiveness of one of said flame arrestors to cleaning thereof and from one of said moisture-removing devices to regeneration thereof occurs substantially simultaneously.

16. A method according to claim 14, wherein the regeneration of the moisture-removing arrangement is performed by heating the same to evaporate condensed water vapour therein and causing a flow of dry gas to occur therethrough in a direction towards said source.

17. A method according to claim 16, wherein a cooling gas flow is passed through the heated, ineffective, moisture-removing device to cool the same following the re-generation thereof and said flow is passed towards said source through the ineffective flame arrestor to clean the same.

18. A system for use in obtaining the concentration of one component in a fluid mixture of a plurality of components, comprising a forwarding arrangement serving to forward from a source one sample of said mixture, a sensing element arranged to receive the forwarded one sample at one side thereof, as well as to forward from said source another sample of said mixture to another side of said sensing element, a removing arrangement in the path of said other sample towards said sensing element and serving to remove substantially said one component thereof, and a monitoring device connected to said sensing element and arranged to signal the ratio between the concentrations of the one component in the samples at the respective sides of the sensing element.

19. A system according to claim 18, and further comprising a temperature-adjusting device serving to maintain substantially no, or at most a negligible, temperature difference between said one side and said other side.

20. A system according to claim 18, said gaseous medium is a product of combustion and said source is an exhaust channel.

21. A system according to claim 20, and further comprising a flame arresting arrangement arranged in the flow of said samples between said stack and said sensing element.

22. A system according to claim 21, wherein said flame arresting arrangement serves as a filtering arrangement for solids in said one sample and said other sample.

23. A system according to claim 22, wherein said flame arresting arrangement comprises a thermally conductive base, a sintered particles arrangement thermally conductively connected with said base and providing sufficient heat transfer area for quenching of a flame, a heater thermally connected with said base and said particles, a temperature sensor thermally connected with said base and said particles, and a control device which is connected to said temperature sensor and which serves in use to prevent the temperature of said base and said particles from falling below a lower threshold and from rising above a higher threshold.

24. A system according to claim 23, wherein said heater has a heating capability of said flame arresting arrangement no higher than said higher threshold.

25. A system according to claim 24, wherein said higher threshold is 150 C.

26. A system according to 21, wherein said flame arresting arrangement comprises first and second flame arrestors, said system serving to switch the first arrestor to a cleaning condition and the second arrestor to an effective condition, and vice-versa, alternatingly.

27. A system according to claim 18 and further comprising return ducting extending downstream from said sensing element towards said source for leading said samples to said source.

28. A system according to claim 27, wherein said return ducting debouches in said exhaust channel at a location such that the gaseous material exiting therefrom substantially avoids the mixture being sampled therefrom.

29. A system according to claim 18, wherein said sensing element comprises a sensing plate and first and second chambers at respective opposite sides of said plate for having said one sample and said other sample respectively conducted therethrough, said monitoring device serving to detect an electromotive force across said sides.

30. A system according to claim 18, wherein said mixture is a gaseous medium and said one component is water vapour.

31. A system according to claim 30, wherein said sensing element is an oxygen sensing element.

32. A system according to claim 30, wherein said removing arrangement comprises a molecular sieve arrangement of a sieving size to obstruct the passage of water vapour molecules therethrough but to allow the passage therethrough of smaller molecules.

33. A system according to claim 32, wherein said removing arrangement comprises first and second removers, said system serving to switch the first remover to a regeneration condition and the second remover to an effective condition, and vice-versa, alternatingly.

34. A system according to claim 33, and further comprising a heating arrangement serving to heat the relevant ineffective, remover and supply ducting for supplying dry gas to that remover, to re-generate the same.

35. A system according to claim 34, wherein said supply ducting serves to supply cooling air to said relevant, ineffective remover following regeneration thereof and to supply said cooling air to the relevant, ineffective flame arrestor to clean the same.

36. A flame arresting arrangement comprising a thermally conductive base, sintered particles thermally conductively connected with said base and providing sufficient heat transfer area for quenching of a flame, a heater thermally connected with said base and said particles, a temperature sensor thermally connected with said base and said particles, and a control device which is connected to said temperature sensor and which serves in use to prevent the temperature of said base and said particles from falling below a lower threshold and from rising above a higher threshold.

37. A flame arresting arrangement according to claim 36, wherein said lower threshold is 100 C.

38. A flame arresting arrangement according to claim 36, wherein said heater has a heating capability of said flame arresting arrangement no higher than said higher threshold.

39. A flame arresting arrangement according to claim 38, wherein said higher threshold is 150 C.

40. A flame arresting arrangement according to claim 36, and comprising first and second flame arrestors switchable between a cleaning condition for the first arrestor and an effective condition for the second arrestor, on the one hand, and vice-versa, on the other hand, alternatingly.

41. An electrochemical oxygen sensor comprising a sensing plate and first and second chambers at respective opposite sides of said plate for having conducted therethrough respective differing gaseous samples containing respective differing concentrations of oxygen gas, and a device for detecting an electromotive force across said sides, said sensing plate and said chambers being arranged substantially symmetrically with substantial sources and sinks for heat transfer.

42. A sensor according to claim 41 and in the form of a zirconia oxygen-sensing element.

Description

[0116] In order that the invention may be clearly understood and readily carried into effect, reference will be made, by way of example, to the accompanying drawings, in which:

[0117] FIG. 1 shows diagrammatically, an example of a known system for sensing of the water vapour content of exhaust gas from an oven, kiln or drier;

[0118] FIG. 2A shows diagrammatically a first version of a known, improved system;

[0119] FIG. 2B shows diagrammatically a second version of the known, improved system;

[0120] FIG. 3 shows diagrammatically a known zirconia oxygen sensor;

[0121] FIG. 4 shows an example of the present invention and constituting a further improvement over the known versions shown in FIGS. 2A and 2B;

[0122] FIGS. 5A and 5B show diagrammatically axial sections taken orthogonally relative to each other through a zirconia oxygen sensor constituting an inventive improvement upon the known sensor of FIG. 3;

[0123] FIG. 6 shows diagrammatically and in greater detail an example according to the present invention and constituting a development of the inventive example shown in FIG. 4; and

[0124] FIG. 7 shows diagrammatically a principle of the present invention.

[0125] Referring to FIG. 4, which shows the present invention schematically, the following reference numerals indicate the following items: [0126] 14 stack or duct; [0127] 28 gases to be measured; [0128] 30 WET sample stream; [0129] 40 DRY sample stream; [0130] 36 water removal; [0131] 38 water disposal; [0132] 48 zirconia sensor outer casing; [0133] 50 temperature sensor; [0134] 44 heater; [0135] 42 ZrO.sub.2/Yt.sub.2O.sub.3 ceramic rectangular plate with platinum catalytic coating on inside and outside.

[0136] The moisture vapour content of the sample gases from this sensor 50 is then:

[00009]$\begin{array}{cc}{v}_{H.Math..Math.2.Math.O}=1-\exp \left[\frac{\mathrm{zFE}}{{\mathrm{RT}}_s}\right]& \left(\mathrm{Equation}.Math..Math.9\right)\end{array}$

[0137] This is a very simple relationship between sensor EMF and the sample water vapour content, particularly when compared to Equation (8) which applies to known sensor configurations.

[0138] FIGS. 5A and 5B show the zirconia oxygen sensor of FIG. 4 in more detail, with the following reference numbers indicating the following items: [0139] 30 WET sample inlet; [0140] 40 DRY sample inlet; [0141] 52 sample gaseous returned to process; [0142] 50 temperature sensor; [0143] 42 ZrO.sub.2/Yt.sub.2O.sub.3 ceramic plate with platinum catalytic coating on inside and outside; [0144] 54 dividing plate to create separate DRY and WET measurement chambers, and to support the items 42, 44 and 50; [0145] 44 heater element.

[0146] The operating temperature of each face of the plate 5 will depend on a heat balance between energy supplied to that face [by (i) the heater 44 and (ii) the incoming sample 52] and energy lost from that face to the surroundings.

[0147] Therefore, in this embodiment: [0148] The temperature of the incoming WET and DRY sample gas streams 30 and 40 is arranged to be substantially the same. Differences in flow rate of the WET and DRY samples (due to water removal from the DRY sample) can be shown to have a negligible effect on the heat balance. [0149] The location of the heater element 44 is symmetrical with respect to the two sensing faces of the plate 42, and the whole sensor assembly has a plane of symmetry through the centre of the ceramic plate 42 and the heater 44. This ensures the same radiation convection and conduction heat transfer to each face of the plate 42. In particular, as the emissivities of the various surfaces within the sensor change due to ageing (e.g. tarnishing of the inside of the sensor casing 48) the changes to net radiation heat transfer received by the two faces of the plate 42 will be the same and so will not create a temperature difference between the two faces. [0150] The sensor assembly is insulated to avoid differences in heat loss to the surroundings from the WET and DRY sides of the outer sensor casing 48.

[0151] In order to make the sensor suitable for the widest possible range of industrial application, the following issues are addressed: [0152] All sample gas connections to the process to include a flame arrestor; [0153] All sample gas entering the sensor to be filtered. The filters should be self-cleaning, to permit continuous operation of the sensor system for at least a year without operator intervention; [0154] All sample gas species to be returned to the process (in case they are toxic or flammable); [0155] The gas drying system (e.g. a bed of molecular sieve) to be self-regenerating, to permit continuous operation of the sensor system for at least a year without operator intervention; [0156] The travel time for the sample gas from the process to both sides of the sensor to be the same.

[0157] The sensor configuration shown in FIG. 6 addresses these requirements. The following reference numerals therein indicate the following items: [0158] 14 stack or duct; [0159] 28 process gases to be sampled; [0160] 30 supply lines for WET sample; [0161] 40 supply lines for DRY sample; [0162] 52 sample return line; [0163] 56 thermally conductive block, losing heat to ambient air; [0164] 58 temperature sensor, used in conjunction with heater element 60 to control block 56 at 120 C.; [0165] 60 heater element; [0166] 62 flame arrestor/filter element; [0167] 64 heater element; [0168] 36 drierbed of molecular sieve (e.g. type 3A) to absorb water selectively; [0169] 66 thermal insulation; [0170] 68 3-way solenoid valve; [0171] 701 instrument air supply (e.g. 7 barg); [0172] 72 flow restrictor to provide suitable flow for purging driers 36 during regeneration; [0173] 50 temperature sensor; [0174] 42 ZrO.sub.2/Yt.sub.2O.sub.3 ceramic plate with catalytic porous platinum coating on both faces; [0175] 44 heater; [0176] 48 outer housing of zirconia sensor; [0177] 76 thermal insulation; [0178] 78 positive displacement sampling pumps (piezoelectric or similar).

[0179] Important features of this embodiment are: [0180] The functionality of the flame arrestor and filter arrangement 62 is combined. Thermally conductive elements 62 (e.g. porous plugs made from small sintered metal spheres, as used for pneumatic air vent silencers) through which the sample gas must pass, provide sufficient heat transfer area for quenching of a flame. These elements are housed in a thermally conductive block 56 that loses heat to its ambient surroundings. The block 56 is controlled at a temperature in the range 100-150 C. using a heater element 60, which ensures that no water vapour condenses before it reaches the zirconia sensor and that the maximum allowable temperature of 150 C. for operation of the flame arresters 62 is not exceeded. The heater element 60 is sized so that under no circumstances can it generate sufficient power to raise the block above 150 C., and hence the system is fail-safe. [0181] Regeneration of the alternating molecular sieve bed driers 36 is combined with cleaning of the filter/flame arrestors 62. A valve system is used to switch automatically between two filter/drier units 62, 64 at regular intervals. The filter/drier unit 62, 64 that is being regenerated is heated to a suitable temperature (typically 175-260 C.) using a heater 36 whilst being purged with a low flow rate of dry air, typically from an instrument air supply 70, which is restricted by an orifice 72. At the end of the regeneration period a much higher pressure of dry air is used (by bypassing the instrument air around the orifice 72) for a relatively short period of time (perhaps 5 seconds) to remove any particles of process material that have accumulated on or inside the filter/flame arrestor 62, and to cool down the drier 36. [0182] The travel times of the WET and DRY samples from the stack 14 to the zirconia sensor 42 should be similar (to within perhaps a second), even though some or perhaps most of the initial WET sample gas volumetric flow rate is removed in the drier 36. This is required to ensure that the gas streams entering each side of the sensor 42 are synchronized, and therefore do not give an erroneous transient response to a step change in stack gas composition. For example, if the gas being sampled is 90% (vol) water vapour, the volumetric flow of the wet sample being drawn from the stack must be approximately ten times that of the DRY sample to ensure that the travel time between the stack and the zirconia sensor are the same. This can be achieved, for example, by: [0183] (i) Using two identical positive displacement (i.e. constant volume) sampling pumps 78, positioned as shown in FIG. 6. This ensures that the travel time between the drier 36 and the zirconia sensor 42 for the DRY sample is the same as that for the WET sample from a point adjacent to the drier to the zirconia sensor 42. [0184] (ii) Arranging for the travel times between the stack 14 and the drier 36 to be as short as possible (of the order of a second). This can be achieved by appropriate sizing of the pumps 78 and the diameters of the sampling tubes (30,40). Any residual lack of synchronicity in analysis of the two samples can be smoothed using suitable time-averaging software algorithms (typically over a period of 2-5 seconds). [0185] The ends of the various sample gas pipes 30, 40 and 52 are arranged in the stack 14 so that sample gas or instrument air passing through one or more of them back into the stack 14 does not cause an unrepresentative sample of the exhaust gas 28 to be drawn into the sensor through the pipes 30 and 40.

[0186] In industrial applications where there are negligible quantities of oxidisable species, or where a less accurate measurement of water vapour content will suffice for process control purposes, the system shown in FIG. 6 may be used with a conventional condenser in place of the molecular sieve driers 36 to remove water from the DRY gas sample. It would be necessary, however, to measure the condenser temperature.

[0187] If, in addition, there is no need for filtration of the incoming sample gases, then the use of a condensing system in place of molecular sieve driers 36 will eliminate the requirement for instrument air and associated solenoid valves 68.

[0188] Finally, if all of the above conditions exist, and in addition the sample gas can never be a combustible mixture, then the flame arrestor/filter assembly can be omitted.