RU2244291C2 - Two-component optical gas analyzer - Google Patents

Abstract

FIELD: measuring engineering.

SUBSTANCE: device has a temperature controlled source, quartz plate, mechanism for orientation of the plate, meter of radiance intensity , and control block.

EFFECT: enhanced accuracy.

3 cl, 3 dwg

Description

The invention relates to the field of analytical instrumentation and can be used to control the composition of gaseous media that absorb ultraviolet and visible spectral radiation.

Measurement of the concentration of molecular components of the gas mixture is an urgent task in the field of control of technological processes in production and power engineering and is important for environmental control of emissions of polluting gases into the atmosphere. To solve this problem, various physicochemical methods of gas analysis are used, among which optical spectroscopic methods are among the most accurate and reliable. However, the currently known technical solutions in the field of creating optical gas analyzers do not always satisfy consumers' requirements for such characteristics as sensitivity, reliability and long-term stability of metrological characteristics.

A known optical gas analyzer containing a broadband radiation source, a radiation beam forming unit, a cuvette, a thermostatically controlled interference filter, equipped with a mechanism for changing its spatial orientation with the possibility of fixing two orientations, a radiation intensity meter and a processing and control unit [1]. In this case, the radiation source, a light filter and a radiation intensity meter are selected operating in the ultraviolet or visible spectral ranges. The main disadvantages of this gas analyzer are, firstly, the ability to measure only one component of the gas mixture, and its measurement is possible only if its absorption band does not overlap with the absorption bands of other components of the gas mixture, and secondly, there is an error in the concentration measurement due to temperature uncontrolled gas mixture and, thirdly, an increase in the measurement error of concentration over time, due to uncontrolled temporary drift of the interference parameters optical filter and radiation intensity meter.

The closest to the invention in technical essence is an optical gas analyzer, which contains a broadband radiation source, a radiation beam forming unit, a thermostated cuvette, a thermostatic interference filter, equipped with a mechanism for changing its spatial orientation with the possibility of fixing three orientations, a radiation intensity meter and a processing and control unit [ 2]. In this case, the radiation source, a light filter and a radiation intensity meter are selected operating in the ultraviolet or visible spectral ranges.

The main disadvantages of this gas analyzer are the increase in the error in measuring the concentrations of the components of the gas mixture, which is due, firstly, to the uncontrolled temporal drift of the passband position and transmittance of the interference filter and, secondly, to changes in the sensitivity of the radiation intensity meter from fluctuations in external temperature.

Thus, both the analog and the prototype have a significant error in measuring the concentrations of the controlled components of the gas mixture, which limits the possibility of their use for long-term continuous control of multicomponent gas mixtures in technological processes.

The invention is directed to the creation of a measuring device with high sensitivity, providing the ability to measure two components of the gas mixture and having a low error during their long-term continuous measurements.

In accordance with the task, the claimed device comprises a broadband radiation source sequentially located on the optical axis, a radiation beam forming unit, a thermostated cuvette, a radiation intensity meter and a processing and control unit connected to it.

The device differs from the prototype in that a quartz prism monochromator is inserted between the cuvette and the radiation intensity meter, in front of the output slit of which there is a quartz plate coupled to a mechanism for changing the spatial orientation of the plate, which is connected to the control output of the processing and control unit. In this case, the quartz prism of the monochromator is removed from the position of the minimum deviation by 20 ± 5 ° towards large angles of incidence of the radiation beam on the first face of the prism, and the mechanism for changing the spatial orientation of the quartz plate is made with the possibility of fixing three predetermined plate orientations. In addition to this, the radiation intensity meter is enclosed in a temperature control unit.

The introduction of a prism monochromator into the gas analyzer instead of an interference filter allows avoiding an increase in the error in measuring the concentrations of the components of the gas mixture due to uncontrolled temporal drift of the passband position and transmittance of the interference filter, since the prism, as a dispersing element of the prism monochromator, does not have these disadvantages.

Removing the monochromator prism from the position of the deviation minimum towards large angles of incidence of the radiation beam on the first face of the prism improves the resolution of the monochromator, since the spectral lines (image of the entrance slit) become narrower with almost the same dispersion. The choice of the angle of 20 ± 5 °, at which the prism was removed from the position of the minimum deviation, is due to the fact that at these angles the resolution of the monochromator increases by 2-2.5 times, and the increase in light loss due to reflection from the first face of the prism does not exceed 30% [3 ].

The introduction of a quartz plate equipped with a mechanism for changing its spatial orientation into a monochromator in front of its output slit allows you to scan the spectrum in the plane of the output slit without rotating the prism. A quartz plate with a mechanism for changing its spatial orientation is an image shifting device and allows high-precision scanning of the spectrum in a small wavelength range. The wavelength range is determined by the thickness of the quartz plate and the angle of its rotation about an axis parallel to the exit slit. Since the gas analyzer does not need to scan the spectrum over a large portion of wavelengths, the proposed spectrum scanning device is simpler than the standard prism rotation mechanism and allows the spectrum to be scanned with higher accuracy.

The ability to fix three predetermined plate orientations using a mechanism for changing the spatial orientation of a quartz plate allows separate determination of the concentrations of two components of the gas mixture. The connection of the mechanism for changing the spatial orientation of the plate to the control output of the processing and control unit allows you to fix the orientation of the plate specified by the algorithm of the control program.

Thermostating of the radiation intensity meter allows avoiding additional measurement errors associated with a change in the sensitivity of the radiation intensity meter from external temperature fluctuations.

Figure 1 presents a block diagram of a gas analyzer. Figure 2 shows the absorption spectra of two gas components (nitric oxide NO and sulfur dioxide SO ₂ ) and preferred, from the point of view of obtaining maximum measurement accuracy, portions of the transmission spectrum of the monochromator specified by the fixed positions of the quartz plate. Figure 3 shows the principle of scanning the spectrum by rotating the quartz plate.

The gas analyzer contains a broadband radiation source 1, a radiation beam forming unit 2, a thermostated cuvette 3, a quartz prism monochromator 4, a quartz plate 5 and a radiation intensity meter 6. Quartz plate 5 is coupled with a mechanism for changing its spatial orientation 7. The output of the radiation intensity meter 6 is connected to the input of the processing and control unit 8. The mechanism for changing the spatial orientation of the plate 7 is connected to the control output of the processing and control unit 8.

In the implemented device for determining the concentrations of nitric oxide (NO) and sulfur dioxide (SO ₂ ) in the flue gases of fuel-burning plants, an LD2 (D) discharge lamp was used as a broadband radiation source 1. The radiation beam forming unit 2 is a spherical concave mirror focusing the radiation of the lamp onto the entrance slit of the monochromator 4. A temperature-controlled cell 3 with a length of 40 cm and a diameter of 3 cm is placed between the radiation beam forming unit 2 and the entrance slit of the monochromator 4. The input and output optical windows of the cell are made from optical quartz, transparent in the ultraviolet region of the spectrum. The temperature regulator of the cell 3 is set to a temperature of 40 ° C. Monochromator 4 consists of an entrance slit 2 mm wide, a collimator mirror lens, a quartz prism with a refracting angle of 60 °, a chamber mirror lens and an output slit 1 mm wide. A quartz plate 5 1 cm thick is placed between the chamber specular lens and the output slit of the monochromator. An F-29 photocell was used as a radiation intensity meter 6.

The mechanism for changing the spatial orientation of the quartz plate 7 consists of a DSHI-200 stepper motor with a control unit and optocouplers, the emitter and receiver of which are spatially separated by a positioning mask filter (an opaque disk with a narrow radial slot), mounted, like a quartz plate, on the motor shaft.

The structural elements of the processing and control unit 8 are based on commercially available microchips and other electrical radio elements: operational amplifiers K544UD1A and K140UD1208, voltage-frequency converter K1108PP1, temperature sensor TMP01, pressure sensor MPX4115, microcontroller ATmega-103, etc.

The absorption spectra of nitric oxide (NO) and sulfur dioxide (SO ₂ ) are located in the near ultraviolet region of the spectrum (FIG. 2), and the spectrum of nitric oxide (20) is a set of separate well-resolved electron-vibrational bands, one of which is shown in FIG. .2, and the spectrum of sulfur dioxide (21) has the form of a single band with a weakly resolved vibrational structure [4]. In this case, the absorption bands of nitric oxide are completely overlapped by the strip of sulfur dioxide. As the analytical region of the spectrum in the device proposed in this application, the spectral range of 225-230 nm was used. The choice of this part of the spectrum is due to the fact that the absorption cross sections of nitric oxide and sulfur dioxide in it have comparable values (~ 10 ^-18 cm ² ), which makes it possible to measure both components of the gas mixture with approximately the same accuracy. When measuring nitric oxide and sulfur dioxide, the proposed device used three fixed positions of a quartz plate corresponding to parts of the transmission spectrum of a monochromator with centers of 230 nm, 227 nm and 224 nm (22, 23 and 24, respectively). Scanning the spectrum by rotating the quartz plate is illustrated in FIG. 3.

The gas analyzer operating mode is determined by the program located in the memory of the microcontroller of the processing and control unit (8). The gas analyzer operates as follows. When it is turned on, the source (1) begins to emit optical radiation, which is generated by the block (2) into a beam passing through the thermostated cell (3) and focuses on the entrance slit of the monochromator (4). The radiation passing through the entrance slit of the monochromator enters the collimator specular lens, which forms a beam close to parallel and directs it to the front face of the prism, which is extracted from the position of the minimum deflection by 20 ± 5 ° towards large incidence angles. After passing through a prism, the radiation decomposed into the spectrum enters a chamber mirror lens, which directs it to a quartz plate (5) and then focuses in the plane of the exit slit of the monochromator. The radiation in the spectral region identified by the output slit of the monochromator is recorded by a radiation intensity meter (6). The measured signal enters the processing and control unit (8).

The gas analyzer operates in a continuous mode, during which a cyclic control program algorithm is worked out. At the beginning of the cycle, the processing and control unit (8) gives a command to the mechanism for changing the spatial orientation of the quartz plate (7) to set it to the position corresponding to passing through the output slit of the monochromator (4) a portion of the spectrum centered at 230 nm (22 in FIG. 2). After that, the radiation intensity meter (6) records the signal (I ₁ ), the value of which is recorded in the memory of the microcontroller of the processing and control unit (8). Then, the processing and control unit (8) gives the command to install the quartz plate (5) in the position corresponding to the transmission of the spectral section centered at 227 nm (23 in FIG. 2). The radiation intensity meter (6) registers the signal (I ₂ ), the value of which is recorded in the memory of the microcontroller of the processing and control unit (8). Next, the processing and control unit (8) gives the command to install the quartz plate (5) in the position corresponding to the transmission of the spectrum with the center 224 nm (24 in figure 2). The radiation intensity meter (6) registers the signal (I ₃ ), the value of which is recorded in the memory of the microcontroller of the processing and control unit (8). Based on the obtained values of I ₁ , I ₂ and I _3, the processing and control unit (8) calculates the concentration of nitrogen oxide molecules N _NO and sulfur dioxide N _SO2 in the gas mixture by solving the system of equations

I ₂ / I ₁ = a ₁ + a ₂ N _NO + a ₃ N _SO2 + a ₄ N $\genfrac{}{}{0ex}{}{\text{2}}{\text{NO}}$ + a ₅ N $\genfrac{}{}{0ex}{}{\text{2}}{\text{SO2}}$ + a ₆ N _NO N _SO2

I ₃ / I ₁ = b ₁ + b ₂ N _NO + b ₃ N _SO2 + b ₄ N $\genfrac{}{}{0ex}{}{\text{2}}{\text{NO}}$ + b ₅ N $\genfrac{}{}{0ex}{}{\text{2}}{\text{SO2}}$ + b ₆ N _NO N _SO2 ,

where a _i and b _i are constant coefficients, the numerical values of which are determined during the calibration of the gas analyzer. Calibration of the gas analyzer consists in measuring the values of I ₁ , I ₂ and I ₃ in reference gas environments with known concentrations of molecules of nitric oxide N _NO and sulfur dioxide N _SO2 and the subsequent determination of the coefficients a _i and b _i by the least squares method.

The solution of the given system of equations is carried out by the microcontroller of the processing and control unit (8) and is iterated in which the values of N ⁰ NO and N are used as the “zero” approximation $\genfrac{}{}{0ex}{}{\text{0}}{\text{SO2}}$ being a solution to a simplified system of linear equations

I ₂ / I ₁ = a ₁ + a ₂ N $\genfrac{}{}{0ex}{}{\text{0}}{\text{NO}}$ + a ₃ N $\genfrac{}{}{0ex}{}{\text{0}}{\text{SO2}}$

I ₃ / I ₁ = b ₁ + b ₂ N $\genfrac{}{}{0ex}{}{\text{0}}{\text{NO}}$ + b ₃ N $\genfrac{}{}{0ex}{}{\text{0}}{\text{SO2}}$ .

This iterative algorithm provides an unambiguous calculation of the concentrations of molecules of nitric oxide N _NO and sulfur dioxide N _SO2 in gas mixtures with an accuracy of 1 mg / m ³ after two iterations. The obtained values of the concentrations of molecules of nitric oxide N _NO and sulfur dioxide N _{SO2 are} recalculated for standard conditions (temperature 20 ° С and pressure 101.3 kPa) and then displayed on a digital display of the processing and control unit (8) and analog current outputs (0-5 mA) gas analyzer. This completes the cycle of the control program algorithm. The entire cycle takes about 10 seconds. The cyclic algorithm of the control program is repeated until the supply voltage is applied to the gas analyzer.

The above expressions for the ratios of the measured signals I ₂ / I ₁ and I ₃ / I ₁ were obtained in the standard way under the assumption that the Bouguer law is valid for a small (less than 0.2) absorption coefficient of optical radiation. In these expressions, the spectral distribution of the radiation intensity of the source (1), the spectral distribution of the sensitivity of the receiver (6), the spectral distribution of the absorption cross sections of nitric oxide (20 in FIG. 2) and sulfur dioxide (21 in FIG. 2), and the values of the spectral regions (22) are taken into account , 23 and 24 in figure 2).

The model of the proposed gas analyzer and the DOG-3 gas analyzer (prototype) were tested in laboratory conditions, during which the long-term stability of their readings was checked using a reference gas mixture containing 511 mg / m ³ nitric oxide and 486 mg / m ³ sulfur dioxide (the rest nitrogen) under standard conditions (temperature 20 ° С and pressure 101.3 kPa). The test results are recorded in the test report. Tests have shown that over the period of continuous operation of gas analyzers (27 days), the increase in the concentration measurement error was:

- NO - 2.5 mg / m ³ (0.5%) for the proposed layout and 10 mg / m ³ (1.7%) for “DOG-3” (prototype);

- SO ₂ - 3.5 mg / m ³ (0.7%) for the proposed layout and 25 mg / m ³ (4.9%) for “DOG-3” (prototype).

Thus, the gas analyzer can automatically, continuously and in real time measure the concentrations of two components of the gas mixture, while the gas analyzer has improved long-term stability compared to the prototype due to thermal stabilization of the radiation intensity meter and replacement of the interference filter with a quartz monochromator with an original high-precision spectrum scanning device .

Bibliography

1. RF patent No. 2029288, 02.20.1995. Bull. No. 5.

2. Certificate for a utility model of the Russian Federation No. 19169, 06/21/1999.

3. Lebedeva VV The technique of optical spectroscopy. M .: Publishing house of Moscow State University, 1986. С.201.

4. Okabe X. Photochemistry of small molecules. M .: Mir, 1981. 500 s.

Claims (4)

1. A two-component optical gas analyzer containing a broadband radiation source sequentially located on the optical axis, a radiation beam forming unit, a thermostated cuvette, a radiation intensity meter and a processing and control unit connected to it, characterized in that a quartz crystal is inserted between the cuvette and the radiation intensity meter a prism monochromator, in front of the exit slit of which a quartz plate is placed, coupled with a mechanism for changing the spatial orientation plate, which is connected to the control output of the processing and control unit. 2. The device according to claim 1, characterized in that the quartz prism of the monochromator is removed from the position of the minimum deviation of 20 ± 5 ° towards large angles of incidence of the radiation beam on the first face of the prism. 3. The device according to claim 1, characterized in that in it the mechanism for changing the spatial orientation of the quartz plate is made with the possibility of fixing three predetermined orientations of the plate. 4. The device according to claim 1, or 2, or 3, characterized in that in it the radiation intensity meter is thermostatically controlled.

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