Gas-phase physico-chemical analysis method

Abstract

The present invention relates to a method of physico-chemical analysis of a gas sample comprising a carrier gas in which chlorine gas is diluted, the physico-chemical analysis method comprising: a) a step of converting the chlorine of the gas sample into chlorine dioxide in a conversion vial; b) a step of acquiring an absorption spectrum of the gas sample after carrying out step a) in a measurement cell which is fluidically connected to the conversion vial; and c) a step of mathematically processing the absorption spectrum with a view to deducing the concentration of chlorine dioxide in the gas sample therefrom.

Description

gas phase physicochemical analysis process FIELD OF THE INVENTION

[0001] The invention relates to the field of physicochemical analyses and more particularly to chemical analyses in the gas phase.

[0002] In particular, the present invention relates to a method for the physicochemical analysis of a gaseous sample comprising chlorine with a view to determining the concentration of chlorine in said sample.

[0003] In this respect, the physicochemical analysis method is suitable for measuring reduced dichlorine contents with regard to known state-of-the-art techniques. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0004] The detection and quantification of chlorinated species in gaseous or liquid samples represents an important issue in many industrial and / or domestic fields.

[0005] In this regard, it is possible to implement electrochemical sensors. However, these electrochemical sensors, known to be not very selective, do not allow to distinguish the response of two different chemical species likely to induce a similar effect on the response of the sensor implemented.

[0006] Thus, in order to overcome this selectivity problem, spectroscopy in the ultraviolet range (hereinafter "UV spectroscopy") can be considered. The latter, when it comes to quantifying the chlorine content in a gaseous sample, due to the low absorption level of chlorine in the ultraviolet range, remains of limited sensitivity. In particular, UV spectroscopy generally does not allow the measurement of chlorine contents below 10 ppm in a gaseous sample. The chlorine sensitivity of UV spectroscopy can nevertheless be improved by increasing the optical path associated with the UV spectroscopy device. However, a solution for increasing the optical path without penalizing the compactness of the UV spectroscopy device is to consider the implementation of additional reflection mirrors. However, these mirrors are likely to generate reflection losses and are sensitive to possible dust deposits.These latter aspects are the source of a degradation in the performance of the device considered.

[0007] Thus, an aim of the present invention is to propose a method for the physicochemical analysis of a gaseous sample comprising chlorine, water or hydrogen chloride and having a sensitivity greater than the methods known to those skilled in the art.

[0008] Another aim of the present invention is to propose a physicochemical analysis method which can be implemented by a spectroscopic device which is more compact than the devices known for this type of application.

[0009] Another aim of the present invention is to propose a physicochemical analysis device capable of determining the content of dichlorine, water or hydrogen chloride in a gaseous sample with increased sensitivity compared to known devices of the state of the art. BRIEF DESCRIPTION OF THE INVENTION

[0010] The invention relates to a method for the physicochemical analysis of a gaseous sample which comprises a gaseous main chemical compound, said main chemical compound having an absorption level, called main level, in a predetermined wavelength range, called analysis range, the physicochemical analysis method comprising:

[0011] a) a step of converting the gaseous main chemical compound into a gaseous secondary chemical compound in a conversion ampoule, said secondary chemical compound having an absorption level called secondary level in the analysis range, at least 5 times, more advantageously at least 10 times, or even 20 times higher than the main level in said analysis range;

[0012] b) a step of acquiring an absorption spectrum of the gaseous sample after execution of step a) in a measuring cell fluidically connected to the conversion bulb;

[0013] c) a step of mathematical processing of the absorption spectrum in order to deduce the concentration of secondary chemical compound in the gaseous sample.

[0014] The aims are, at least in part, achieved by a method of physicochemical analysis of a gaseous sample which comprises a gaseous main chemical compound, the main chemical compound comprising one of the chemical compounds chosen from dichlorine, hydrogen chloride, the method of physicochemical analysis comprising:

[0015] a) a step of conversion, advantageously by oxidation, in a conversion ampoule, of the main chemical compound included in the gaseous sample into chlorine dioxide;

[0016] b) a step of acquiring an absorption spectrum of the gaseous sample after execution of step a) in a measuring cell fluidically connected to the conversion bulb;

[0017] (c) a step of mathematical processing of the absorption spectrum in order to deduce the concentration of chlorine dioxide in the gaseous sample.

[0018] Considering chlorine dioxide is particularly advantageous. Indeed, this gas has a relatively high absorption level compared to chlorine, water or even hydrogen chloride in the 300 nm – 400 nm wavelength range. In particular, in this range, chlorine dioxide absorbs approximately 20 times more than chlorine. Furthermore, chlorine dioxide has a very specific absorption spectrum (in particular, the absorption spectrum of chlorine dioxide includes a plurality of periodically repeated absorption “peaks”) allowing it to be identified with certainty. In other words, chlorine dioxide can be distinguished with certainty from other species likely to be present in the gas sample.

[0019] According to one embodiment, conversion step a) comprises a reaction of the main chemical compound with sodium chlorite and / or sodium sulfate placed in the conversion ampoule.

[0020] According to one embodiment, said analysis method comprises a step a1), interposed between step a) and step b), which comprises the circulation of the gaseous sample from the conversion bulb to the measuring cell.

[0021] According to one embodiment, step b) comprises measuring by means of a spectrograph the absorption spectrum by the gaseous sample of light radiation emitted by a light source.

[0022] According to one embodiment, step a) is preceded by a background measurement step a0) which comprises the measurement of the absorption spectrum of a gaseous sample, called background, formed solely of an optically inert gas.

[0023] Optically inert means a gas that exhibits negligible absorption in the working wavelength range of the spectrograph.

[0024] According to one method of implementation, the diluted chlorine in the gas sample comes from the transformation of diluted chlorinated species in a sample of an aqueous solution.

[0025] According to one embodiment, the chlorinated species include hypochlorous acid and its conjugate base.

[0026] According to one embodiment, the transformation of hypochlorous acid and its conjugate base into dichlorine involves the addition of an acid to the aqueous solution sample.

[0027] According to one embodiment, the absorption spectrum is acquired in a wavelength range between 300 nm and 400 nm.

[0028] According to one mode of implementation, step c) of mathematical processing comprises at least one of the methods chosen from: the calculation of the Fourier transform of the absorption spectrum, a linear regression, a least squares method.

[0029] The invention also relates to a device for physicochemical analysis of a gaseous sample which comprises a gaseous main chemical compound, the main chemical compound comprising one of the chemical compounds chosen from dichlorine, water, hydrogen chloride, the physicochemical analysis device comprising:

[0030] - a conversion bulb which comprises a reagent, called a conversion reagent, configured to convert the main chemical compound of the gaseous sample into chlorine dioxide;

[0031] - spectroscopic means, cooperating with a measuring bulb, and configured to measure an absorption spectrum of the gaseous sample after conversion of chlorine into chlorine dioxide and circulating in said measuring bulb;

[0032] - a processing unit configured to perform a mathematical processing of an absorption spectrum in order to deduce therefrom the concentration of chlorine dioxide in the gaseous sample.

[0033] Other characteristics and advantages of the invention will emerge from the detailed description which follows with reference to the appended figure in which:

[0034] This is a schematic representation of an analysis device capable of implementing the analysis method according to the present invention;

[0035] This is a graphical representation of an absorption spectrum of sodium chlorite between 300 nm and 400 nm, with the vertical axis representing the absorption level (in arbitrary units) and the vertical axis representing the wavelength in nm;

[0036] This is a graphical representation of measurements made over time (horizontal axis) of the chlorine concentration of a gas sample whose carrier gas is nitrogen, and whose nominal chlorine concentration is 17.9 ppm. DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention relates to a method for the physicochemical analysis of a gaseous sample comprising a carrier gas in which a main chemical compound is diluted. In particular, the main chemical compound, according to the terms of the present invention, has an absorption level, called the main level, in a predetermined wavelength range, called the analysis range.

[0038] The physicochemical analysis method according to the present invention comprises a step a) of conversion, in a conversion ampoule, of the main chemical compound of the gaseous sample into a secondary chemical compound in the gaseous state.

[0039] The physicochemical analysis method also comprises the execution of a step b). Step b) comprises in particular the acquisition, in a predetermined wavelength range, called the analysis range, of an absorption spectrum of the gaseous sample after execution of step a) in a measuring cell fluidically connected to the conversion bulb.

[0040] Finally, the physicochemical analysis method comprises the execution of a step c) which comprises a mathematical processing of the absorption spectrum in order to deduce the concentration of secondary chemical compound in the gaseous sample. Knowledge of this latter concentration makes it possible to deduce the concentration of the main chemical compound in the gaseous sample.

[0041] According to the terms of the present invention, the main chemical compound has an absorption level, called the main level, in the analysis domain, while the secondary chemical compound has an absorption level, called the secondary level, at least 20 times higher than the main level in said analysis domain.

[0042] This latter aspect makes it possible to improve the sensitivity of dosage and / or analysis of a chemical compound diluted in a carrier gas.

[0043] The remainder of the description is limited to the sole consideration of dichlorine as the main chemical compound. However, this aspect is not such as to limit the scope of the present invention, and those skilled in the art, on the basis of the description which follows, will be able to adapt the concepts to other chemical compounds. In particular, those skilled in the art will be able to consider water or even hydrogen chloride.

[0044] These three compounds (chlorine and hydrogen chloride) react with NaClO2 to form chlorine dioxide.

[0045] Furthermore, and by way of example, the physicochemical analysis method according to the present invention can be implemented by the physicochemical analysis device 10 shown in.

[0046] The physicochemical analysis device 10 notably comprises a conversion bulb 20 in which is placed a reagent, called a conversion reagent, configured to convert the chlorine in the gaseous sample into chlorine dioxide.

[0047] The conversion bulb 20 generally has an elongated shape, and may, for example, be cylindrical. The conversion bulb 20 comprises at one of its ends a first inlet 20a and at the other of its ends a first outlet 20b.

[0048] The conversion bulb 20 comprises in its volume a reagent, called a conversion reagent, configured to convert, by chemical means, the chlorine likely to circulate in gaseous form in said bulb into chlorine dioxide.

[0049] The conversion reagent may include sodium chlorite (NaClO2), for example in powder form or in crystal form. Also, to prevent the entrainment of sodium chlorite by the carrier gas during its flow, it may be considered to use membranes, for example PTFE membranes.

[0050] The physicochemical analysis device 10 also comprises spectroscopic means configured to measure an absorption spectrum of the gaseous sample after conversion of the chlorine into chlorine dioxide.

[0051] More particularly, the spectroscopic means cooperate with a measuring bulb 30 for the purpose of measuring the absorption spectrum when the gaseous sample is in said measuring bulb.

[0052] More particularly, the measuring bulb 30 of generally elongated shape comprises two ends 31a and 31b (opposite one another) each provided with a window 32a and 32b transparent to the wavelengths included in the analysis domain. These two windows 32a and 32b are parallel to each other.

[0053] The conversion bulb may comprise a main body, tubular in shape and hermetically sealed at its ends.

[0054] The main body can advantageously be transparent and comprise, for example, PMMA or glass.

[0055] The spectroscopic means comprise a spectrograph 61 and a light source 60 configured to emit light radiation covering the analysis domain.

[0056] In particular, the light source 60 is arranged opposite the window 32a in order to allow illumination of a gas sample likely to be found in the measuring bulb 30.

[0057] The spectrograph 61 is, for its part, arranged opposite the window 32b and is arranged to measure the absorption of light radiation by the gaseous sample in the analysis domain. It is understood, without it being necessary to specify it, that a spectrograph measures an absorption spectrum as a function of the wavelength. The spectrograph 61 is advantageously connected to a computer 62 configured to process and / or analyze the spectra collected by said spectrograph 61.

[0058] By "computer" is meant an electronic device configured to perform automatic processing of a spectrum collected by the spectrograph. The computer 62 may comprise a memory space, a processor, a motherboard, a display means, a control interface.

[0059] Furthermore, the measuring bulb 30 is fluidically connected to the conversion bulb 20. In particular, a fluid connection means connects the first outlet 20b to an inlet, called the second inlet 30a, arranged near the end 31a of the measuring bulb 30. The fluid connection means may in particular comprise a fluid connection tube, called the first tube 40a. The fluid connection means may also be provided with a three-way valve (or solenoid valve) 50. Thus, the fluid connection means may comprise a tube 40b connecting the first outlet 20b to the three-way valve 50, the tube 40a connecting the three-way valve 50 to the second inlet 30a, and finally a tube 40c connecting a carrier gas source 51 to the three-way valve 50.

[0060] The measuring bulb 30 also comprises an outlet, called the second outlet 30b, arranged near the end 31b. This second outlet 30b can in particular be connected to a pump 52 by means of a fluid connection tube, called the second tube 41. This latter arrangement can in particular be implemented for the purposes of purging and / or transferring the gaseous sample from the measuring bulb.

[0061] The remainder of the statement is devoted to the description of the physicochemical analysis process of a gaseous sample which includes gaseous dichlorine (main chemical compound).

[0062] The physicochemical analysis method according to the present invention comprises a step a) of converting the chlorine in the gas sample into chlorine dioxide in the conversion ampoule. In this regard, the gas sample can be introduced, for example by means of a pump, into the conversion ampoule via the first inlet 20a.

[0063] It is understood that since the process is implemented for dosing purposes, the volume injected is known, as are the pressure and flow rate of the fluid.

[0064] When performing step a), the three-way valve 50 may be in a position closing the first outlet 20b.

[0065] This 50 three-way valve allows for both continuous and sequenced operation.

[0066] In this step a), the diluted chlorine in the gaseous sample is converted into chlorine dioxide using the conversion reagent present in the conversion funnel.

[0067] According to an advantageous embodiment, the conversion reagent comprises sodium chlorite, in the form of powder or single crystals.

[0068] The reaction between chlorine and sodium chlorite is written as follows:

[0069] Cl2+ 2 NaClO22 ClO2+ 2 NaCl

[0070] The physicochemical analysis method also comprises a step b) of acquiring an absorption spectrum of the gaseous sample after execution of step a) in the measuring cell 30.

[0071] In this regard, the gas sample is transferred (step a1) from the conversion bulb 20 to the measuring cell 30 via the fluid connection means. In particular, this transfer may involve positioning the three-way valve 50 so as to allow the circulation of the gas sample from the first outlet 20b to the second inlet 30a.

[0072] The acquisition of the absorption spectrum comprises illumination, by the light source 60, of the gaseous sample present in the measuring cell 30 and collection of the light signal, after absorption by the gaseous sample, by the spectrograph 61.

[0073] Since the secondary chemical compound includes chlorine dioxide, the absorption spectrum is advantageously collected between 300 nm and 400 nm.

[0074] In this wavelength range, chlorine dioxide exhibits a specific signature (as illustrated in). In particular, the absorption spectrum of chlorine dioxide includes a plurality of periodically repeated absorption "peaks" that allow it to be identified with certainty. In other words, chlorine dioxide can be distinguished with certainty from other species that may be present in the gaseous sample.

[0075] The invention also comprises a step c) of mathematical processing of the absorption spectrum in order to deduce therefrom the concentration of chlorine dioxide in the gaseous sample.

[0076] Knowing the chlorine dioxide concentration in the gas sample provides access to the initial chlorine concentration in the gas sample.

[0077] Step c) of mathematical processing may comprise a calculation by Fourier transform of the absorption spectrum. In particular, the Fourier transform of the spectrum is a bell-shaped curve whose area is representative of the concentration of chlorine dioxide present in the gaseous sample. Those skilled in the art will recognize that calibration curves and / or charts may be used for the interpretation of the measurements carried out.

[0078] Alternatively, the mathematical processing step may include: linear regression, least squares method.

[0079] Advantageously, the mathematical processing step c) can be carried out by means of the calculator 62.

[0080] Thus, the present invention proposes to determine the concentration of chlorine diluted in a carrier gas by converting it into chlorine dioxide. The latter has, between 300 nm and 400 nm, an absorption level 20 times higher than that of chlorine, and thus makes it possible to measure chlorine concentrations in a sample below the thresholds usually accepted when implementing analysis techniques known to those skilled in the art.

[0081] This is a graphical representation of measurements over time (horizontal axis) of the chlorine concentration of a gas sample whose carrier gas is nitrogen, and whose nominal chlorine concentration is 17.9 ppm. The measurements obtained over time are close to the nominal concentration and have a standard deviation of 7.39x10 -2This standard deviation, relatively low compared to the nominal concentration, is indicative of an increased sensitivity of the physicochemical analysis method according to the present invention compared to the methods known in the state of the art.

[0082] Furthermore, the periodic absorption spectrum shape gives the process improved selectivity.

[0083] The physicochemical analysis method may comprise a step a0), preceding step a), of background measurement which comprises the measurement of the absorption spectrum of a gaseous sample, called background, formed solely of the carrier gas. This step a0) comprises in particular the acquisition of an absorption spectrum of optically inert gas present in the measuring cell according to terms identical to step b).

[0084] The optically inert carrier gas may in particular have been taken from the gas source 51.

[0085] The invention can also be implemented to determine the concentration of free chlorine and total chlorine in aqueous solution. In particular, the diluted chlorine in the gaseous sample can come from the transformation of diluted chlorinated species in a sample of an aqueous solution.

[0086] More particularly, the chlorinated species may include hypochlorous acid and its conjugate base. In this regard, the transformation of hypochlorous acid and its conjugate base into chlorine may include the addition of an acid, for example hydrochloric acid, to the aqueous solution sample.

[0087] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Method for physicochemical analysis of a gaseous sample which comprises a gaseous main chemical compound, the main chemical compound comprising one of the chemical compounds chosen from dichlorine, hydrogen chloride, the physicochemical analysis method comprising:a) a step of conversion, advantageously by oxidation, in a conversion bulb, of the main chemical compound included in the gaseous sample into chlorine dioxide;b) a step of acquisition of an absorption spectrum of the gaseous sample after execution of step a) in a measuring cell fluidically connected to the conversion bulb;c) a step of mathematical processing of the absorption spectrum in order to deduce therefrom the concentration of chlorine dioxide in the gaseous sample. Physicochemical analysis method according to claim 1, wherein the conversion step a) comprises a reaction of the main chemical compound with sodium chlorite and / or sodium sulfate placed in the conversion ampoule. Physicochemical analysis method according to claim 1 or 2, wherein said analysis method comprises a step a1), interposed between step a) and step b), which comprises the circulation of the gaseous sample from the conversion bulb to the measuring cell. Physicochemical analysis method according to one of claims 1 to 3, in which step b) comprises measuring by means of a spectrograph the absorption spectrum by the gaseous sample of light radiation emitted by a light source. Physicochemical analysis method according to one of claims 1 to 4, in which step a) is preceded by a background measurement step a0) which comprises the measurement of the absorption spectrum of a gaseous sample, called background, formed solely of an optically inert gas. Physicochemical analysis method according to one of claims 1 to 5, in which the main chemical compound comprises dichlorine originating from the transformation of chlorinated species diluted in a sample of an aqueous solution. A physicochemical analysis method according to claim 6, wherein the chlorinated species comprise hypochlorous acid and its conjugate base. A physicochemical analysis method according to claim 7, wherein the transformation of hypochlorous acid and its conjugate base into dichlorine comprises the addition of an acid into the aqueous solution sample. Physicochemical analysis method according to one of claims 1 to 8, in which the absorption spectrum is acquired in a wavelength range between 300 nm and 400 nm. Physicochemical analysis method according to one of claims 1 to 9, in which step c) of mathematical processing comprises at least one of the methods chosen from: the calculation of the Fourier transform of the absorption spectrum, a linear regression, a least squares method. Device for physicochemical analysis of a gaseous sample which comprises a gaseous main chemical compound, the main chemical compound comprising one of the chemical compounds chosen from chlorine, water, hydrogen chloride, the physicochemical analysis device comprising:- a conversion bulb which comprises a reagent, called conversion, configured to convert the main chemical compound of the gaseous sample into chlorine dioxide;- spectroscopic means, cooperating with a measuring bulb, and configured to measure an absorption spectrum of the gaseous sample after conversion of the main chemical compound into chlorine dioxide and circulating in said measuring bulb;- a processing unit configured to carry out a mathematical processing of an absorption spectrum in order to deduce therefrom the concentration of chlorine dioxide in the gaseous sample.

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