Absorption Analyzer

Abstract

The invention relates to analytical chemistry, in particular, to the spectral absorption analysis with a differential method of measuring concentrations of mercury and benzene vapors.

The invention is aimed at creation of an absorption analyzer, which allows to determine the content of mercury and benzene in the carrier gas, with improved analytical performance for benzene.

The aim is achieved with an absorption analyzer, which comprises optically coupled components: a photodetector, an analytical cell, a modulator of radiation polarization and a spectral lamp containing a discharge cavity located between magnet poles and connected with means of electric discharge excitation, buffer gas and mercury placed into the spectral lamp, as well as a gas system connecting a sampling port of the analyzer with an input port of the analytical cell by gas communications, wherein the gas system comprises at least three gas channels connecting the sampling port of the analyzer to the input port of the analytical cell via a gas channels selector, while at least one of these gas channels comprises means for removing the benzene from the gas stream, at least one comprises means for removing mercury from the gas stream, at least one is permeable for mercury, at least one is permeable for benzene and at least one has different permeability rates for mercury and benzene.

3 figures.

Claims

1. An absorption analyzer, which comprises optically coupled components: a photodetector, an analytical cell, a modulator of radiation polarization and a spectral lamp containing a discharge cavity located between magnet poles and connected with means of electric discharge excitation, buffer gas and mercury placed into the spectral lamp, as well as a gas system connecting a sampling port of the analyzer with an input port of the analytical cell by gas communications, wherein the gas system comprises at least three gas channels connecting the sampling port of the analyzer to the input port of the analytical cell via a gas channels selector, while at least one of these gas channels comprises means for removing the benzene from the gas stream, at least one comprises means for removing mercury from the gas stream, at least one is permeable for mercury, at least one is permeable for benzene and at least one has different permeability rates for mercury and benzene.

2. The absorption analyzer of claim 1, wherein the mercury placed in the spectral lamp is enriched with mercury isotope with an even number of neutrons, wherein said isotope constitutes not less than 50% of the total amount of mercury in the spectral lamp.

Description

[0012] The essence of the claimed invention is illustrated by figures:

[0013] FIG. 1. A block diagram of an absorption analyzer.

[0014] FIG. 2. The diagram of spectral location of Zeeman - and .sub.-emission linecomponents .sup.198Hg =254 nm absorption line profile of mercury () and benzene absorption bands .sub.(benzene) (atmospheric pressure).

[0015] FIG. 3. The results of determination of mercury and benzene in the natural gas in one of Russian gas fields by transmitting the natural gas into an analytical cell of the absorption analyzer 1through a filter with activated charcoal, which absorbs the mercury and benzene, 2through a filter that absorbs only mercury, 3directly with no filters.

[0016] Absorption analyzer shown at the block diagram (FIG. 1) consists of the spectral lamp 1, means of electrical discharge excitation 2, the magnet 3, the modulator of radiation polarization consisting of photoelastic modulator 4 with a quartz generator 5 and a polarizer 6, the analytical cell 7, the photodetector 8, and the signal processing unit 9. The gas system of the analyzer comprises the sampling port 10, three gas channels, one of which has the filter 11, which removes mercury and benzene from the gas stream, the other is directly connected to the sampling port 10 and to one of gas ports of the gas channels selector 13, and the third one has the filter 12 which removes mercury from the gas stream, and the gas channels selector 13, which alternately connects the gas channels to the input port of the analytical cell, and the output system 14 connected to the output port of the analytical cell.

[0017] Means of electrical discharge excitation 2 can be made as electrodes installed on the discharge cavity of the spectral lamp and connected to the high frequency excitation generator.

[0018] The analytical cell 7 can be made as a closed volume with the input port and the output port used for feeding and removing the sample gas, and the probing radiation passes through this volume multiple times by means of a system of mirrors. In a particular example, the use of multipath analytical cell with an equivalent length of 960 cm at allows to increase the measurement sensitivity 24 times relative to a length of one path of 40 cm with just a slight decrease of the intensity of the probing radiation.

[0019] The magnet is made of a material with high magnetic remanence magnetization in the form of two discs parted by a separator. The discharge cavity of the spectral lamp 1 is placed in the gap between the discs. Discs are magnetized so that the gap side of the one is the south pole, and the gap side of the otherthe north pole. For coupling radiation out of the spectral lamp 1 one of the discs has a hole that allows to extract the radiation along the lines of magnetic force towards the optical axis.

[0020] The spectral lamp 1 design and methods of its connection to the exciter generator are discussed in detail in (4).

[0021] The signal-processing unit 9 contains amplifiers and detectors that filter signals at the modulation frequency and at direct current. After analog-to-digital conversion these signals come to the microprocessor for further signal processing, formation of an analytical signal and display of the measured mercury concentration in the analytical cell.

[0022] If the input pressure of the sample gas at the sampling port 10 is equal to the exit pressure at the output system 14, for example, is equal to atmospheric pressure, then the sampling port 10 includes dust filter and communications to transport the sample gas, and the output system 14 includes a gas pump, for example, diaphragm pump, that provides intake of the sample gas from the point of interest and its pumping through analytical cell and output communications. If the input pressure of the sample gas at the sampling port 10 exceeds the exit pressure at the output system 14, the sampling port 10 comprises a reducer which reduces the sample gas pressure down to an acceptable level and provides the desired flow rate through the analytical cell 7. The output system 14 contains only communication for the disposal of sample gas after it has passed through the analytical cell.

[0023] The filter 11, which removes mercury and benzene from a stream of the sample gas is formed as a volume with input and output ports, filled with activated charcoal in granular form.

[0024] The filter 12, which removes mercury from the sample gas stream is formed as a volume having input and output ports, which has the fabric impregnated I+KI placed inside. The efficiency of such a filter is presented in Table 2, which shows the dependence of permeability rates for benzene and mercury versus time.

TABLE-US-00002 TABLE 2 Exposuretime, hours 1 2 4 8 12 24 48 BenzenePermeability 102 2 100 1 99 1 98 1 98 1 101 1 97 1 Rate, % MercuryPermeability 0.04 0.02 0.5 0.3 0.7 0.2 0.8 0.2 0.8 0.2 0.4 0.2 0.9 0.2 Rate, %

[0025] From the results given in Table 2, it follows that the mercury absorption filter efficiency is not less than 99%, the loss of benzene transmitted through the filter does not exceed 3%. Experience with this type of filters has shown that they are able to operate for a long time (at least a year).

[0026] Gas channels selector 13 can be made as a single four-way valve, which has three input ports connected to respective gas channels, and an output port connected to the input port of an analytical cell. The selector can be also made in the form of combinations of two and three-way valves.

[0027] Consider the analyzer operation with the spectral lamp with the isotope .sup.204Hg, the spectral position of the resonance emission line of which does not coincide with the spectral position of the maximum absorption line profile of mercury and a local maximum of the absorption band of benzene. To determine the concentration of mercury and benzene three measurements are made. The first one is made with the sample gas passing through a channel with the filter 11 removing mercury and benzene. The resulting level is taken as a value of zero concentration of mercury and benzene. The second measurement is carried out with the sample gas passing through a channel with the filter 12 removing only mercury. The procedure of measurement is as follows. In a magnetic field of the magnet 3 the emission resonance line of mercury =254 nm is split into unshifted -component and two shifted -components (FIG. 2). In the observation of the radiation spectral lamp 1 along lines of the magnetic field the components .sub.+ and .sub. are observed with circular polarization clockwise and counterclockwise, respectively. The magnitude of magnetic field is selected in such a way that .sub.30 component is shifted into the area of the maximum of the resonance absorption line of mercury and thus this component performs a role of the analytical line. Other -component of the mercury emission line is at the slope of the mercury absorption line profile where the absorption cross section is less than its maximal value. The second component plays a role of the reference line. To separate the intensities of .sub.+- and .sub.components the photoelastic modulator 4 and the linear polarizer 6 are used. In the absence of mercury atoms in the analytical cell 7 the intensity of .sub.+ and .sub.-components are almost equal. In the presence of absorbing atoms the intensity of .sub.+component decreases as its spectral position gets into the absorption maximum, and the intensity of -component remains almost the same since it is in a region where the absorption cross section is less than the maximal value. As a result, there is a frequency modulation signal S.sub., related to concentration of the atoms in the analytical cell. To ensure the selectivity the signal S.sub.0 is used as the normalization signal, which is proportional to the direct current of the photodetector 8. The signals S.sub. and S.sub.0 are separated in the signal processing unit 9, and the signal S=S.sub./S.sub.0 is calculated. The result of the second measurement S.sub.2 is associated with the signal S by the following relation [5]:

[00001]$\begin{array}{cc}{S}_2=-\frac{b}{2}.Math.\ln \left(\left(b-S\right)/\left(b+S\right)\right)& \left(1\right)\end{array}$

[0028] where b is a normalization constant that depends on the parameters of the analyzer. Then, the concentration of mercury atoms in the sample gas C.sub.Hg is associated with the received signal S.sub.2 by a simple equation:

C.sub.Hga.sub.HgS.sub.2(2)

[0029] where a.sub.Hg=1/Q.sub.HgL is a calibration coefficient determined during the calibration of the analyzer for mercury, Q.sub.Hg is the differential absorption cross section of .sub. and .sub.+-components by mercury atoms, L is the length of the analytical cell.

[0030] The third measurement is made while the sample gas passing through a channel with no filter installed. During the processing using the above-stated algorithm,an analytical signal S.sub.3 is obtained which is the sum of the analytical signal S.sub.Hg produced by mercury and S.sub.benzene produced by benzene:

S.sub.3=S.sub.Hg+S.sub.benzene(3)

[0031] Because the analytical signal produced by mercury is obtained in the second measurement, the concentration of benzene C.sub.benzene is defined as follows:

C.sub.benzene=a.sub.benzene(S.sub.3S.sub.2)(4),

[0032] where a.sub.benzene=1/Q.sub.benzeneL is a calibration coefficient determined during the calibration of the analyzer for benzene, Q.sub.benzene is a differential absorption cross section of .sub.+- and .sub.-component by benzene molecules, L is the length of the analytical cell.

[0033] For the analyzer with a spectral lamp enriched by mercury isotope .sup.204Hg, the following analytical characteristics were obtained:

[0034] the detection limit (criterion 3 noise for a blank signal): [0035] for benzene 1 mg/m3 at 1 second signal averaging and 0. 2 mg/m3 at 30 seconds averaging, which is below the Occupational Exposure Limit of benzene concentration in the air of industrial zone (3. 2 mg/m3) [0036] for mercury 2 ng/m3 at 1 second signal averaging and 0. 6 ng/m3 at 30 seconds averaging, which is below its background level in the ambient air (1-2 ng/m3). [0037] The dynamic range of measured concentrations was about 10.sup.4.

[0038] The operation of the absorption analyzer was demonstrated by determination of mercury and benzene in one of the gas fields in Russia. Gas was taken to a separator, in which various impurities were removed from natural gas, and was fed to the analyzer. The measurement results are shown in FIG. 3. It is seen from the data provided, the concentration of benzene obtained using the developed analyzer amounted to 80010 mg/m3, which is in good agreement with the data obtained by gas chromatography for the natural gas at the point of sampling (800 mg/m3).

[0039] Thus, the present invention allows to create an absorption analyzer, which can be used to determine mercury below its background level in the air and benzene below its Occupational Exposure Limit; to reduce the detection limit of benzene by using absorption spectroscopy with the direct Zeeman effect and multipath cell; to reduce the detection limit due to enriching mercury in the spectral lamp with mercury isotope with an even number of neutrons.

REFERENCES

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