RU2544264C1 - Method of gas analysis of natural gas - Google Patents

Abstract

FIELD: instrument engineering.

SUBSTANCE: invention is designed for qualitative and quantitative analysis of natural gas (NG). The method comprises irradiating the gas linearly with polarized monochromatic laser radiation and the simultaneous registration of m spectra of spontaneous Raman scattering (SRS) of standard gas components included in the NG. For them the integrated intensity of the irradiating laser radiation I_i, i=1…m is additionally recorded, and the values of relative concentrations of the components of the analysed NG from its SRS spectrum are determined by the formula which comprises the contributions of SRS spectra of the reference gas components in the registered SRS spectrum of NG, calculated by the least square method. At that from the spectrum registered by the multichannel photodetector, the ranges of 300-2500 cm^-1 and 3400-3750 cm^-1 are used.

EFFECT: simplified determining and increase in accuracy.

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Description

The invention relates to the field of analytical instrumentation and is intended for qualitative and quantitative analysis of natural gas (GHG).

The chemical composition of GHGs varies significantly depending on its field. Along with this, its calorific value, and hence its value, changes. For this reason, the determination of the composition of GHGs with a high degree of accuracy is a very urgent task for enterprises engaged in its extraction and transportation.

At the same time, quantitative analysis of GHGs is one of the most difficult tasks of gas analysis due to the large number of components included in its composition, as well as a significant dispersion of their concentrations.

The most common method for determining the chemical composition of GHGs today is chromatographic analysis [Buzanovsky V.A., Hovsepyan A.M. Information-measuring systems of the composition and properties of natural gas // Territory Neftegaz, 2007, No. 8, p. 36-43]. The main disadvantages of this method are the need to have a carrier gas (for example, He or Ar) for gas chromatographic separation, a long analysis time, as well as the degradation of the characteristics of the main components (detectors, columns) over time and the need for periodic calibration of the instrument .

In addition, a known method based on laser absorption spectroscopy [RU 2441219, 01/27/2012]. This method is free from the disadvantages of chromatographic analysis, but it has a number of its own. First of all, they include the need for preliminary information on the composition of the analyzed gas, as well as the need to have several lasers operating in different wavelength ranges, which will ultimately lead to a significant increase in the cost of the gas analyzer. In addition, in this way it is impossible to determine the concentration of homonuclear molecules that make up the GHG (for example, N ₂ , H ₂ , etc.), the determination of the content of which is fundamentally important.

, где j - номер спектрального компонента, k - номер пиксела, $$a_k^j$$

- вклад j-го компонента в интенсивность, регистрируемую k-м пикселом, d_j - коэффициент, сочетающий в себе сечение рассеяния j-го компонента σ_j и аппаратную функцию пропускания оптических элементов, n - абсолютная концентрация молекул того сорта, частоте колебаний которого соответствует данная спектральная компонента, i_k - интенсивность, зарегистрированная k-м пикселом, $$I_k^{(f)}$$

- величина фона, J - интенсивность возбуждающего излучения. Данная система избыточна, поскольку имеет число уравнений, равное общему числу пикселов, и число неизвестных, равное полному числу компонент природного газа N. Поэтому из нее выделяют подсистему с N уравнениями, каждое из которых соответствует пикселу, регистрирующему максимум одной из спектральных линий. Интенсивность возбуждающего излучения исключается путем перехода к относительным концентрациям и нормировке их суммы на 100%.The closest to the principle of action is the analysis method based on the use of spectroscopy of spontaneous Raman scattering (SCR) [Bazhanov Yu.V. et al. Quantitative analysis of gaseous media by Raman spectroscopy // Analytics and Control, 1998, No. 3-4, p.65-74]. Its main advantage is the absence of consumables, as well as the control of all molecular components of natural gas using a single laser with a fixed wavelength. The essence of this method is to irradiate the analyzed GHG with linearly polarized monochromatic radiation and simultaneously record its Raman spectrum in the range of 0-4200 cm ^-1 , where all the bands of the molecules fall. Further, the process is reduced to the following. The system of equations is compiled

where j is the number of the spectral component, k is the number of the pixel, $$a_k^j$$

is the contribution of the jth component to the intensity recorded by the kth pixel, d _j is the coefficient combining the scattering cross section of the jth component σ _j and the hardware transmission function of the optical elements, n is the absolute concentration of molecules of the sort whose vibration frequency corresponds to given spectral component, i _k is the intensity recorded by the kth pixel, $$I_k^{(f)}$$

is the background value, J is the intensity of the exciting radiation. This system is redundant because it has the number of equations equal to the total number of pixels and the number of unknowns equal to the total number of components of natural gas N. Therefore, a subsystem with N equations is allocated from it, each of which corresponds to a pixel recording the maximum of one of the spectral lines. The intensity of the exciting radiation is eliminated by switching to relative concentrations and normalizing their sum by 100%.

The main disadvantage of this approach is the need for knowledge of the scattering cross sections σ _{j of the} components at the selected pixels with very high accuracy, which is a very non-trivial task. In addition, due to the small number of equations, in this method it is not possible to correctly take into account random fluctuations of light signals (for example, shot noise), as a result of which the analysis accuracy is low.

The problem to which the invention is directed is to create a method for gas analysis of natural gas, based on Raman spectroscopy, which is stable to light fluctuations of signals and does not require information on the Raman scattering cross sections of its components. EFFECT: simplified procedure and increased accuracy of gas analysis of natural gas.

, где a _i - вклады спектров СКР эталонных газовых компонентов $$J_i^{pix}$$

в зарегистрированный спектр СКР ПГ J^pix, вычисленные с помощью метода наименьших квадратов из системы уравнений $$J^{pix}={\displaystyle \sum _{i=1}^m{a}_i{J}_i^{pix}}$$

(pix соответствует номерам элементов используемого многоканального фотоприемника, обеспечивающих регистрацию спектра в диапазонах 300-2500 см^-1 и 3400-3750 см^-1), N_i - величина абсолютной концентрации молекул сорта i в его эталонном спектре, определяемая из соотношения $$N_i=\frac{P_i}{k{T}_i{Z}_i(P_i,T_i)}$$

, где k - коэффициент Больцмана, P_i, T_i - соответственно давление и температура эталонного газа i в кювете при регистрации его спектра СКР, Z_i (P_i, T_i) - коэффициент сжимаемости газа i при давлении P_i и температуре T_i.The specified result is achieved by the fact that, as in the prototype, the natural gas is irradiated with linearly polarized monochromatic radiation and its Raman spectrum is simultaneously recorded in the range of 0-4200 cm ^-1 . But, unlike the prototype, before registering the Raman spectra of the analyzed GHG samples, m Raman spectra of the standard gas components that make up the GHG are recorded once, and the integral intensity of the irradiating laser radiation I _i , i = 1..m is additionally recorded for them, and the values the relative concentrations of the components of the analyzed GHG from its spectrum of SCR are determined by the formula $$b_{\mathrm{one}}=\frac{N_i{I}_i{a}_i}{{\displaystyle \sum _{i=\mathrm{one}}^m{{\displaystyle N}}_i{{\displaystyle I}}_i{{\displaystyle a}}_i}}\cdot \mathrm{one\; hundred}\%$$

, where a _i are the contributions of the Raman spectra of the reference gas components $$J_i^{pix}$$

into the recorded spectrum of the Raman spectrum of the GHG J ^pix calculated using the least squares method from the system of equations $$J^{pix}={\displaystyle \sum _{i=\mathrm{one}}^m{a}_i{J}_i^{pix}}$$

(pix corresponds to the numbers of the elements of the multichannel photodetector used, which provide registration of the spectrum in the ranges of 300-2500 cm ^-1 and 3400-3750 cm ^-1 ), N _i is the absolute concentration of molecules of variety i in its reference spectrum, determined from the ratio $$N_i=\frac{P_i}{k{T}_i{Z}_i(P_i,T_i)}$$

where k is the Boltzmann coefficient, P _i , T _i are the pressure and temperature of the reference gas i in the cell, respectively, when registering its Raman spectrum, Z _i (P _i , T _i ) is the compressibility coefficient of gas i at pressure P _i and temperature T _i .

, соответственно, решив которую, можно найти вклады спектров СКР отдельных компонентов a _i в итоговый спектр СКР и, следовательно, концентрации всех молекулярных составляющих анализируемой газовой среды. Очевидно, что данная система является переопределенной и ее решение целесообразно проводить методом наименьших квадратов, в результате чего помимо значений a _i возможно получить и случайные погрешности для данных величин. Однако для решения задачи определения компонентного состава ПГ данным способом необходимо один раз заранее зарегистрировать спектры СКР эталонных компонентов, на которые будет раскладываться полученный спектр СКР ПГ. Стоит отметить, что согласно [ГОСТ 31371.7-2008] основными составляющими ПГ, требующими контроля, являются: метан, этан, пропан, н-бутан, изо-бутан, н-пентан, изо-пентан, неопентан, гексан, азот, кислород, углекислый газ, пары воды. Таким образом оценки содержания данных компонентов достаточно для определения требуемых характеристик ПГ (теплота сгорания, плотность и т.п.).The proposed method is based on the fact that the recorded intensity of the SCR signal from component i linearly depends on the concentration of molecules of this type in the analyzed volume and the intensity of the exciting (irradiating) laser radiation. At the same time, SCR signals in multicomponent gas media, overlapping each other, have the property of additivity. In other words, the total recorded Raman spectrum is the sum of the Raman spectra of the individual components. Those. for the case of multi-channel recording, the system of equations $$J^{pix}={\displaystyle \sum _{i=\mathrm{one}}^m{a}_i{J}_i^{pix}}$$

, respectively, having solved which, one can find the contributions of the Raman spectra of the individual components a _i to the final Raman spectrum and, therefore, the concentration of all molecular components of the analyzed gas medium. Obviously, this system is overdetermined and it is advisable to carry out its solution using the least squares method, as a result of which, in addition to the values of a _i, it is possible to obtain random errors for these quantities. However, in order to solve the problem of determining the component composition of GHGs using this method, it is necessary to register the Raman spectra of the reference components once into which the resulting spectrum of GHGs of GHGs will be decomposed. It is worth noting that according to [GOST 31371.7-2008] the main GHG components that require control are: methane, ethane, propane, n-butane, iso-butane, n-pentane, iso-pentane, neopentane, hexane, nitrogen, oxygen, carbon dioxide, water vapor. Thus, an assessment of the content of these components is sufficient to determine the required GHG characteristics (calorific value, density, etc.).

, где k - коэффициент Больцмана, P_i, T_i - давление и температура эталонного газа в кювете при регистрации его спектра СКР, Z_i (P_i, Ti) - коэффициент его сжимаемости при давлении P_i и температуре T_i. Стоит отметить, что Z_i (P_i, T_i) является табличной величиной и может быть взята из [ГОСТ 31369-2008. Газ природный. Вычисление теплоты сгорания, плотности, относительной плотности и числа Воббе на основе компонентного состава].It is worth noting that absolutely all lasers used for excitation in SCR gases have a power instability over time, which, as a rule, varies between 1-15%. In this regard, it is advisable to control the radiation power when recording the Raman spectra of the reference components. In addition, for the implementation of high-precision measurements, it is necessary to take into account the degree of non-ideal gas. For this reason, the absolute concentration of molecules in the reference gas i should be determined by the formula $$N_i=\frac{P_i}{k{T}_i{Z}_i(P_i,T_i)}$$

Where k - Boltzmann coefficient, P _i, T _i - pressure and temperature of the master gas in the cell when registering its Raman _{spectrum, Z i (P i, Ti} ) - compressibility factor at pressure P _i and the temperature T _i. It should be noted that Z _i (P _i , T _i ) is a tabular value and can be taken from [GOST 31369-2008. Natural gas. Calculation of the calorific value, density, relative density and Wobbe number based on the component composition].

In turn, to increase the accuracy of measurements of a _i , the system of equations to be solved should include only those equations for which the spectral range is 300–2500 cm ^–1 and 3400–3750 cm ^–1 . This circumstance is caused by the fact that the predominant component in natural gas is methane, the content of which is 80-95%. In turn, all the hydrocarbons in the Raman spectrum have intense bands in the region of 2500-3400 cm ^-1 . However, despite this use of this spectral region, the calculation errors increase due to the separation of weak bands of heavy hydrocarbons against the background of very intense methane bands in this region. The remaining parts of the spectrum (250-2500 cm ^-1 and 3400-3750 cm ^-1 ) are quite suitable for analysis, since there are bands of all the main components of natural gas, including water vapor (3652 cm ^-1 ), and the intensity of the ν ₂ band (1534 cm ^-1 ) of the methane molecule does not explicitly prevail over the intensities of the bands of other natural gas molecules.

Figure 1 shows a diagram of a device for implementing the proposed method (1 - laser, 2 - beam splitter, 3 - photodetector, 4 - lens, 5 - cuvette for gas inlet, 6 - pressure gauge, 7 - temperature meter, 8 - laser radiation trap , 9 - a lens for collecting scattered light, 10 - a light filter, 11 - a spectral device, 12 - an electronic control unit). Figure 2 shows the spectrum of SCR GHG in the region of 700-1900 cm ^-1 , as well as the contributions of its individual components to the total spectrum of SCR.

, где k - коэффициент Больцмана, Z_i (P_i, T_i) - коэффициент сжимаемости газа i при давлении P_i и температуре T_i, вычисляется N_i - величина абсолютной концентрации молекул сорта i в его эталонном спектре. Данная процедура поочередно осуществляется для всех компонентов природного газа.The method is as follows. Prior to the analysis of GHG samples, one-time recording of the Raman spectra of individual components of natural gas is carried out i. For this purpose, linearly polarized exciting radiation from the laser 1 is incident on the beam splitter plate 2, which directs part of the radiation to the photodetector 3, which determines the integral radiation power I _i during the time of recording one spectrum. In turn, the main part of the laser radiation is focused by the lens 4 to the center of the cell 5, filled with the reference gas component i. The pressure P _i and the temperature T _{i of the} gas in the cuvette are controlled by a pressure gauge 6 and a temperature meter 7, respectively. The laser radiation transmitted through the cell is absorbed by trap 8, and the scattered radiation from the center of the cell at an angle of 90 degrees to the exciting radiation is collected by the lens 9 and directed through the filter 10, which attenuates the light at the laser radiation frequency, to the input of the spectral device 11, which simultaneously records the Raman spectrum in range 0-4200 cm ^-1. Then, the recorded Raman spectrum of the reference gas component, together with data on its pressure and temperature during registration, as well as data on the corresponding integrated power of the exciting radiation, is sent to the memory of the electronic control unit and according to the ratio $$N_i=\frac{P_i}{k{T}_i{Z}_i(P_i,T_i)}$$

where k is the Boltzmann coefficient, Z _i (P _i , T _i ) is the compressibility coefficient of gas i at pressure P _i and temperature T _i , N _i is calculated - the absolute concentration of molecules of variety i in its reference spectrum. This procedure is alternately carried out for all natural gas components.

, где a _i - вклады спектров СКР эталонных газовых компонентов $$J_i^{pix}$$

в зарегистрированный спектр СКР ПГ J^pix, вычисленные с помощью метода наименьших квадратов из системы уравнений $$J^{pix}={\displaystyle \sum _{i=1}^m{a}_i{J}_i^{pix}}$$

, где pix соответствует номерам элементов используемого многоканального фотоприемника, обеспечивающих регистрацию спектра в диапазонах 300-2500 см^-1 и 3400-3750 см^-1.After that, the analyzed GHG is introduced into the cuvette, its Raman spectrum is recorded in a similar manner, except that its pressure, temperature and power of the exciting radiation are not controlled. In the electronic control unit, the values of the relative concentrations of the components of the analyzed GHG from its Raman spectrum are determined by the formula $$b_{\mathrm{one}}=\frac{N_i{I}_i{a}_i}{{\displaystyle \sum _{i=\mathrm{one}}^m{{\displaystyle N}}_i{{\displaystyle I}}_i{{\displaystyle a}}_i}}\cdot \mathrm{one\; hundred}\%$$

, where a _i are the contributions of the Raman spectra of the reference gas components $$J_i^{pix}$$

into the recorded spectrum of the Raman spectrum of the GHG J ^pix calculated using the least squares method from the system of equations $$J^{pix}={\displaystyle \sum _{i=\mathrm{one}}^m{a}_i{J}_i^{pix}}$$

, where pix corresponds to the element numbers of the multichannel photodetector used, which provide registration of the spectrum in the ranges of 300–2500 cm ^–1 and 3400–3750 cm ^–1 .

Due to the large number of equations used, the proposed method is resistant to light fluctuations of the signals, as well as to the appearance of new components in the GHG. It is also worth noting that this method, unlike the prototype, does not require taking into account the background level, since the background in the recorded spectrum of the GHG Raman spectra, as well as the Raman signals, will be fully compensated by contributions from other components. In addition, when using the proposed method, there is no need to determine the design parameters of the gas analyzer (angle of scattered radiation collection, spectral transmittance). However, it is only necessary that the spectra of the reference gases and the spectrum of the gas mixture be recorded with the same setting of the used Raman gas analyzer.

Thus, the application of the proposed method of gas analysis of natural gas greatly simplifies the procedure for its analysis, contributes to its automation, and also provides high measurement accuracy.

Claims (1)

The process gas analysis natural gas (NG) comprising its irradiation with linearly polarized monochromatic laser radiation and its simultaneous registration of the spontaneous Raman scattering (TFR) multichannel photodetector in the range 0-4200 cm ^-1, characterized in that to register the spontaneous Raman spectra of analyzed samples is recorded GHG m spontaneous Raman spectra of the reference gas components included in the PG, and for them to additionally record integral intensity of the irradiating laser beam i _{i, i} = 1 ... m, and were ins relative concentrations of analyte GHG components of its Raman spectrum are determined by the formula $$b_{\mathrm{one}}=\frac{}{}\frac{N_i{I}_i{a}_i}{{\displaystyle \sum _{i=\mathrm{one}}^m{{\displaystyle N}}_i{{\displaystyle I}}_i{{\displaystyle a}}_i}}\cdot \mathrm{one\; hundred}\%$$

, where a _i are the contributions of the Raman spectra of the reference gas components $$J_i^{pix}$$

into the recorded spectrum of the Raman spectrum of the GHG J ^pix calculated using the least squares method from the system of equations $$J^{pix}={\displaystyle \sum _{i=\mathrm{one}}^m{a}_i{J}_i^{pix}}$$

(pix corresponds to the numbers of the elements of the multichannel photodetector used, which provide registration of the spectrum in the ranges of 300-2500 cm ^-1 and 3400-3750 cm ^-1 ), N _i is the absolute concentration of molecules of variety i in its reference spectrum, determined from the ratio $$N_i=\frac{P_i}{k{T}_i{Z}_i(P_i,T_i)}$$

where k is the Boltzmann coefficient, P _i , T _i are the pressure and temperature of the reference gas i in the cell, respectively, when registering its Raman spectrum, Z _i (P _i , T _i ) is the compressibility coefficient of gas i at pressure P _i and temperature T _i .

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