GB2237633A

GB2237633A - Determining the concentration of a gaseous component by fluorescence

Info

Publication number: GB2237633A
Application number: GB9021794A
Authority: GB
Inventors: Benoit Pradet; Claude Roche; Bernard Remy; Thierry Berthoud
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1989-10-20
Filing date: 1990-10-08
Publication date: 1991-05-08
Anticipated expiration: 2010-10-08
Also published as: FR2653559A1; GB2237633B; GB9021794D0; JP3009445B2; FR2653559B1; JPH03140843A

Abstract

To determine the concentration of a component in a gaseous mixture flowing within a duct or pipe (10), eg. a toxic component in a gaseous mixture discharged into the atmosphere, a proportion of the mixture is pumped around a sampling circuit (40-48) and back into the duct (10), while a sample is drawn from the sampling circuit through a fluorescence measuring cell (56) by a pump 59, and at intervals the supply from the duct is stopped and replaced by calibrating gas from a device (90). A carrier gas (92) may pass through coils (102) containing crystals of the component to be detected. The temperature of the surrounding cryostat (96) is adjusted to vary the concentration of the component in the calibration mixture. Light from a laser (16) is chopped and directed by fibre optic (66) into the cell (56) and fluorescent light is dissected by fibre optic 78 to a multichannel spectrometer 80 connected to a computer 100. <IMAGE>

Description

APPARATUS FOR THE DETERMINATION BY MOLECULAR FLUORESCENCE OF THE CONCENTRATION OF A COMPONENT TO BE MONITORED IN A FLOWING GASEOUS MIXTURE Description The present invention relates to an apparatus for the determination by molecular fluorescence of the concentration of a component to be monitored in a flowing gaseous mixture. It more particularly applies to the monitoring of the presence of toxic components in a gaseous mixture discharged into the atmosphere.

A gaseous mixture e.g. resulting from the reprocessing of materials is usually discharged into the atmosphere. It is therefore necessary to ensure that there is no toxic gas, even in very low concentrations in the mixture in question. An extremely sensitive concentration measuring apparatus must make it possible to detect any passing beyond a concentration threshold, which is fixed at a very low level for safety reasons.

Fig.l shows a known apparatus for measuring by molecular fluorescence the concentration of a component in a flowing gaseous mixture. The gaseous mixture flows in a main duct 10 in the direction indicated by the arrow. Under the action of a suction pump 12, part of the gaseous mixture is sucked in to a bypass pneumatic circuit having a measuring cell 14. The flow rate and pressure of the gaseous mixture are measured by a flowmeter 22 and a manometer 24 placed at the inlet of the cell 14.

A light beam from an appropriate laser 16 passes completely through the measuring cell 14. Its wavelength is chosen in such a way that in preferred manner it excites the component to be monitored. This light beam is modulated by an optical modulator or chopper 26, which allows a synchronous detection.

The fluorescent light emitted to the outside of the cell 14 through a lateral shield window 18 is filtered by a filter 20 to eliminate the diffused light from the excitation beam.

A photomultiplier 28 supplied by a power supply 30 collects the fluorescent light and supplies on an output an electric signal proportional to the detected light intensity.

A second cell 34 filled with a gaseous mixture of known composition and containing the component to be monitored is placed on the path of the excitation light beam coming from the measuring cell 14. A system 35 is dependent on the manometer 24 and makes it possible to make the pressure within the second cell 34 equal to that in the measuring cell 14.

The excitation beam is absorbed in the second cell 34 and gives rise to a fluorescent light emission on the part of the component identical to the component to be monitored. This fluorescent light emitted outside the cell 34 via a lateral shield window 33 is filtered by a filter 36 eliminating the parasitic light from the diffusion of the excitation beam and then collected by a photomultiplier 38 supplied by the power supply 30. The latter supplies at an output an electric signal proportional to the detected light intensity.

The electric signals from the two photomultipliers 28,38 are supplied on two inputs of processing means 32. The measurement of the fluorescent light intensity of the component having the known concentration makes it possible to deduce the concentration of the component to be monitored on the basis of the light intensity emitted by the latter. The measurement performed on the basis of the second cell 38 constitutes a calibration of the measuring apparatus.

This known apparatus suffers from numerous disadvantages. Its sensitivity does not make it possible to determine concentrations below 1012 molecules/cm3 in an industrial medium, which is not adequate in the case where it is necessary to monitor highly toxic components and which are subject to extremely low discharge standards.

The innadequate sensitivity leads to long response times. As the detected signal if very weak, the determination of the concentration is obtained by averaging out successive measurements. The number of these measurements increases with the weakness of the signal and when the latter is drowned by noise. During the determination, the concentration of the component can vary significantly, which falsifies the result of the measurement.

To make the calibration effective, the two photomultipliers 28,38 must have similar characteristics, which leads to control complications and calibration difficulties.

This known apparatus does nt make it possible to differentiate between parasitic fluorescent light from other components possibly constituting the gaseous mixture.

The present invention obviates these disadvantages. The concentration measurement and the calibration take place by using the same cell at different times.

The calibration can be carried out at regular intervals but, in addition, as a result of the rapid and easy passage from the "measurement" mode to the "calibration" mode, on a concentration threshold has been exceeded. Therefore the operation of the measuring apparatus is controlled and the reliable measurement optionally allows alarms to be set off.

The sensitivity of the appaaratus according to the invention makes it possible to monitor a component appearing in very low concentrations.

More specifically, the present invention relates to an apparatus for the determination of the concentration of a component to be monitored in a gaseous mixture flowing within a main duct, characterized in that it comprises a pneumatic sampling circuit having a suction pump, said sampling circuit being connected by an inlet and an outlet to the main duct; a pneumatic measuring circuit havinga measuring cell and a suction pump, the measuring cell being suitable for the circulation of the gaseous fixture at a known flow rate and pressure, said measuring circuit being connected on the one hand to the pneumatic sampling circuit and on the other hand to the main duct, in such a way that the gaseous mixture sucked in by the pneumatic sampling circuit is discharged Lnto the main duct; a pneumatic calibrating circuit having a means for supplying a gaseous calibration mixture containing said component in a known and randomly variable concentration, said pneumatic calibration circuit being connected to the pneumatic sampling circuit by a bypass able to isolate the pneumatic sampling circuit from the main duct, the gaseous calibration mixture then flowing in the pneumatic measuring circuit; means for inducing a fluorescence of said component within the measuring cell; means for measuring the light intensity of the fluorescence supplying an electric signal to an output; and means for processing said electric signal, said means being connoted by an input to the output of the intensity measuring means for determining the concentration of the component contained in the mixture flowing within the main duct on the basis of calibration measurzzents performed when the gaseous calibration mixture flows in the pneumatic measuring circuit.

Calibration measurennts are performed on the basis of the fluorescent light emitted by a component identical to that to be monitored1 but contained in a known. concentration in a gaseous calibration mixture.

The calibration measwrements and the determination of the concentration of the component to be monitored are carried out at different times, but using the same measuring system.

The calibration and the measurement are carried out with the same tools, which makes zt possible to confirm the validity of the measurements and improve the accuracy of the determination.

In order to pass fro;n- normal operation to the "calibration" mode, it is merely necessary to interrupt the arrival of the gaseous mixture from the main duct and blow the calibration mixture of known composition into the pneumatic measuring circuit.

According to a variant of the invention, sampling circuit has a pyrolyzer.

If desired, it is thus possible to dissociate the components present in the gaseous mixture and which may contain the component to be monitored. Without the pyrolyzer, the quantity of components to be monitored present in compound form would not be covered by the measurement.

In a preferred manner, the means for supplying the gaseous calibration mixture consists of a cryogenic exchanger.

According to a preferred embodiment, a light trap prevents any return to the interior of the measuring cell on the part of a light beam which has longitudinally traversed the latter.

The invention is described in greater detail hereinafter relative to non-limitative embodiments and with reference to the attached drawings, wherein show: Fig.1 already described, diagrammatically shows a known apparatus for determining the concentration of a component to be monitored in a flowing gaseous mixture.

Fig.2 diagrammatically shows an apparatus according to the invention for determining the concentration of a component to be monitored in a flowing gaseous mixture.

Fig.2 diagrammatically shows an apparatus according to the invention.

The gaseous mixture (diagrammatically represented by an arrow) containing the component to be monitored flows in a main duct 10. A pneumatic sampling circuit is connected in parallel to the duct 10. A suction pump 40 forces part of the gaseous mixture into the sampling circuit at a volume flow rate of 1 l/mm at atmospheric pressure.

A valve 42 positioned at the inlet of the sampling circuit permits the isolation thereof or the regulation of the pressure of the gaseous mixture flowing in said circuit. The latter and the flow rate are controlled with the aid of a manometer 46 and a flow meter 44. The sampling circuit also has an e.g. quartz pyrolyzer 48. If desired, it is possible to dissociate the compounds of the mixture and which may contain the component to be monitored.

The piping and the various elements of the sampling circuit are made from a material having no chemical affinity with the toxic component to be monitored. Thus, there is no accumulation of said component in the circuit. The piping, as well as the pump 40 and the isolating valve 42 are e.g. made from Teflon. The pyrolyzer 48 is e.g. made from quartz. The flowneter 44 is e.g. made from glass and Teflon.

A pneumatic measuring circuit is connected on the one hand to the pneumatic sampling circuit and on the other to the main duct 10 downstream of the outlet of the sampling circuit. This measuring circuit comprises a flowmeter 50, a valve 52, a manometer 54, a measuring cell 56 and a suction pump 59. In prefered manner the pressure in the cell 56 is below 100 Pa. The pressure must be regulated in such a way that the excitation of the fluorescence takes place under the best possible conditions.

Advantageously, the power of the pump 59 is adequate to obtain a pressure in the cell 5 of approximately 0.1 mbar with a volume flow rate therein of lOOcchmin.

Under the action of the pump 59, part of the gaseous mixture circulating in the sampling circuit is sucked into the measuring circuit.

The flowmeter 50 and the manometer 54 are used for controlling the flow rate and the pressure within the measuring cell 56. The pressure within the cell 56 is regulated with the aid of the valve 52.

The measuring cell 56 has two windows 58,60, e.g. inclined by a Brewster angle with respect to the longitudinal axis of the cell 56, so as to prevent any parasitic reflection into the cell 56 when a light beam passes through the same. The measuring cell 56 is flanked by a lateral shield window for observing the fluorescent light emitted by the component to be monitored in the cell 56.

In the same manner as previously, the materials constituting the measuring circuit have no chemical affinity with the component to be monitored. The flowmeter 50 and the manometer 54 are identical to the flowmeter 44 and the manometer 46 of the sampling circuit.

The valve 52 is e.g. made from Teflon. The measuring cell 56 is e.g. made from Teflon. The windows 58,60 and the shield window 62 are e.g. made from quartz.

A laser 16, which can e.g. be a dye laser, supplies a light beam having a narrow spectral width and which is able to excite in a selective manner the component to be monitored. A chopper 26 modulates the light beam, which permits a synchronous detection of the fluorescent light.

A lens 64 focusses the light beam in to an optical fibre 66 guiding the latter towards the measuring cell 56. Following propagation in the fibre 66, the beam is focussed by a lens 68 within the measuring cell 56, which it penetrates through the window 58 and where it excites the component to be monitored.

There is not a total absorption of the beam. Infact, it is slight in most cases, because the component to be monitored, which is the only one which absorbs the beam, is only present in a very weak concentration. Therefore the light beam traverses the cell and passes out through the second window 60.

A light trap 70 completely absorbs the light beam which has traversed the cell 56, so as to prevent any return of said beam into the cell 56 in such a way as to create parasitic light effects.

Following its excitatton by the light beam, the component to be monitored contained in the gaseous mixture circulating within the cell 56 produces a fluorescent lights whereof part is emitted to the outside via the shield window 62.

A lens 72 positioned in the vicinity of the shield window 62 brings the fluorescent light ht into the form of a parallel beam, which traverses an interferential fiIer 74 making it possible to eliminate most of the light diffused by the excitation light beam.

. lens 76 focusses thyme filtered beam in to an optical fibre, which guides it towards means 80 for measuring the fluorescent light intensity and constituted in preferred manner by a multichannel spectrometer.

The latter has a dispersive element permitting the spreading of the light beam as a function of the wavelength. The spread beam is detected by a photodiode arrays, each of the photodiodes corresponding to a narrow wavelength r nWge.

Therefore the multicmannel spectrometer simultaneously supplies electric signals proportional to the light intensity corresponding to each wavelength range As the fluorescence swavelength of the component to be monitored is known, it is consequently possible to filter (electrically by only taking account of the signal which as of interest) the parasitic light at other wavelengths and in particuler the light residue from the diffusion of the excitation beam and the parasitic fluorescent light emitted by other components present in the gaseous mixture.

The electric signal corresponding to the fluorescence intensity of the component to to be monitored is supplied to an input of the processing means 82 of the micrscomputer type, which determines the concentration on the basis of the measured intensity and calibration curves recorded in a memory.

As will be shown hereinafter, the calibration is carried out via the already discribed measuring circuit. This calibration is carried out on a first occassion during the putting into service of the present apparatus and is repeated every so often. It can also be automatically initiated by the microcomputer 82, when the concentration of the component to be monitored exceeds a predetermined fixed threshold.

In this case and in order to gain time, it is possible to only control one particular operating point without repeating a complete calibration.

Thus, the reliability of the measurement is controlled virtually instantaneously because, as will be seen, the passage from the "measurement" mode to the "calibration" mode is is simple and fast.

The calibration consists of measuring fluorescent light intensities emitted by a component identical to that to be monitored and contained in a gaseous calibration mixture having a known concentration, the actual pressure being determined. The various measurements are carried out by varying these different parameters and calibration curves are recorded in a microcomputer memory. The intermediate values between the two curves are extrapolated.

In order to carry out calibration measurements, the sampling circuit is isolated from the main duct 10 and connected to a pneumatic calibration circuit by a bypass. The latter consists of three valves 42,86,88, e.g.

of Teflon and which are manually or automatically controlled by the microcomputer 82 to which they are connected. The isolating valve 42 makes it possible to isolate the intake of the sampling circuit from the main duct 10.

The pneumatic calibration circuit has a means for supplying a gaseous calibration mixture containing said component in known concentration and which can be varied at random. In the embodiment shown in Fig.2, said means comprises a cryogenic exchanger 90.

When the valve 42 isolates the sampling circuit from the main duct, the valve 86 is open and releases a gaseous mixture, e.g. chosen from air, argon or nitrogen contained under pressure in a reservoir 92. The releaseJ pas circulates in a duct 94 immersed in a cryogenic fluid 96 contaimel in a hermetically sealed container. Within the latter, the tenperatue of the cryogenic fluid 96 is determined by that of a thern@stor 92 immersed in the fluid 96 and connected to a power supple controlled by the microcomputer 82. The temperature of the fluid 96 @@@ range between -100 and +10 C.

Within the cont@@er, the duct 94 has a twisted portion containing crystals 102 of aa component identical to that to be monitored. The vapour tension of the crystals is known, so that it is possible to vary at rando@ and in given manner the concentration of the gassous component wIti the gaseous mixture flowing within the duct 94 simply by varying the @emperature of the cryogenic fluid 96.

The duct 94 is conmected to the sampling circuit and when the valve 42 is closed and the valve 86 and 88 are open, a calibration gaseous mixture of knave = imposition and concentration flows in the sampling circuit and in the measuring circuit.

Thus, calibration curves are obtained at different concentrations with the aid of the same measuring apparatus as used for determining the concentration.. This obviates any setting difficulties and inaccuracies.

The return to the "measurement" mode takes place by closing the valve 86,88 and opening the valve 42.

The apparatus arcording to the invention has a high sensitivity and makes it possible to determine extremely low concentrations. The measurement with is automatically verified on exceeding a threshold, is reliable aoni makes it possible to trigger alarm devices. A pyrolyzer also makes it possible to determine the concentration of the component to be monitored and which is noramally contained in a compound said measurement mot normally being carried out.

It is obvious that the invention is not limited to the exemplified embodiment described and covers all variants thereof.

Claims

1. Apparatus for the determination of the concentration of a component to be monitored in a gaseous mixture flowing within a main duct (10), characterized in that it comprises a pneumatic sampling circuit having a suction pump (40), said sampling circuit being connected by an inlet and an outlet to the main duct (10); a pneumatic measuring circuit having a measuring cell (56) and a suction pump (59), the measuring cell (56) being suitable for the circulation of the gaseous mixture at a known flow rate and pressure, said measuring circuit being connected on the one hand to the pneumatic sampling circuit and on the other hand to the main duct (10), in such a way that the gaseous mixture sucked in by the pneumatic sampling circuit is discharged into the main duct (10); a pneumatic calibrating circuit having a means (90) for supplying a gaseous calibration mixture containing said component in a known and randomly variable concentration, said pneumatic calibration circuit being connected to the pneumatic sampline circuit by a bypass able to isolate the pneumatic sampling circuit from the main duct, the gaseous calibration mixture then flowing in the pneumatic measuring circuit; means (80) for inducing a fluorescence of said component within the measuring cell; means for measuring the light intensity of the fluorescence supplying an electric signal to an output; and means (82) for processing said electric signal, said means (82) being connected by an input to the output of the intensity measuring means (80) for determining the concentration of the component contained in the mixture flowing within the main duct on the basis of calibration measurements performed when the gaseous calibration mixture flows in the pneumatic measuring circuit.

2. Apparatus according to claim 1, characterized in that the pneumatic sampling circuit incorporates a pyrolyzer (48).

3. Apparatus according to claim 1, characterized in that the means (90) for supplying the gaseous calibration mixture comprises a cryogenic exchanger.

4. Apparatus according to claim 1, characterized in that a light trap (70) prevents any return into the interior of the measuring cell of a light beam which has already traversed the latter longitudinally.