RU2680861C1 - Submersible polarimeter to control aromatic hydrocarbons ratio in light oil products - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to optical instrumentation, and more specifically to optical polarization devices that use the Faraday effect, and can be used in laboratories of petrochemical enterprises, regulatory bodies, universities, research institutes, in technological lines of oil refineries, as well as at oil depots in the centers of motor fuel sales. Claimed submersible polarimeter contains a light source, a first polarizer, a second polarizer in the form of a Wollaston prism, whose transmission planes differ by angle of ±45°. After the second polarizer, a cuvette is installed in the form of a thin-walled pipe with a flange of diamagnetic material. Near the cuvette security windows, holes are made for filling with the test oil. Mirror coating is applied on the second protective window from the side opposite to the light source. Fixed magnet is fixed on the cuvette in the form of cylindrical tube sections with the axial direction of the magnetic field strength. Angle of bifurcation of rays by Wollaston prism 2β satisfies the

condition, where 2β - the angle of bifurcation of rays by a Wollaston prism; A is the width of Wollaston prism; D is the diameter of the light beam; L is the distance from the Wollaston prism to the mirror.

EFFECT: reduced dimensions and simplified design.

1 cl, 3 dwg

Description

The invention relates to optical instrumentation, and more specifically to polarizing devices that measure the angle of rotation of the plane of polarization of light transmitted through a substance with natural or induced optical activity. An immersion polarimeter is proposed to control the proportion of aromatic hydrocarbons in light oil products both during their production and during their sale or storage at oil depots. A polarimeter measures the angle of rotation of the plane of polarization of linearly polarized light transmitted through a substance exposed to a longitudinal magnetic field (Faraday effect) [1].

, на уголA substance placed in a longitudinal magnetic field rotates the plane of polarization of light propagating along the magnetic field vector

at an angle

where: V is the Verdet constant (specific magnetic rotation in the substance [min. / e cm]);

- напряженность магнитного поля в [эрстедах];

- magnetic field strength in [oersteds];

L is the path length of light in matter in [cm];

β is the angle between the direction of the light rays and the direction of the magnetic field vector;

- постоянный конструктивный коэффициент.

- constant design factor.

From equation (1) it can be seen that for a constant coefficient K, the angle of rotation of the plane of polarization of light α is proportional to the Verdet constant of the test substance.

Thus, by the found angle of rotation of the plane of polarization α, one can identify the analyte or find the concentration of the dissolved substance in the solution, if the difference between the Verdet constant of the solvent and the dissolved substance is known.

So, for example, according to previously known values of the Verde constants of heptane (V _hep = 0.0125 min / e cm) and toluene (V _tol = 0.0269 min / e cm), one can find the concentration C% of toluene dissolved in heptane

where V _x is the measured value of the Verdet constant of the mixture.

Given the linear dependence of the angle of rotation of the plane of polarization α and the Verdet constant V (1), we can write:

where: α _x is the angle of rotation of the plane of polarization of the light that has passed the path L in the test solution at a magnetic field Hcosβ;

, L и β.α _hep , α _tol - angles of rotation of the plane of polarization of light of heptane and toluene under the same conditions, that is, under identical

, L and β.

A known method for determining the total content of aromatic hydrocarbons in oil fractions and light petroleum products [2]. According to this method, light oil products are presented in the form of two components: paraffin-naphthenic hydrocarbons (solvent) and aromatic hydrocarbons (dissolved substance).

Aromatic hydrocarbons differ significantly in their physical properties from paraffin-naphthenic constituents of fuels, for example, Cotton-Mouton constant, density, refractive index, dispersion, including the Verdet constant.

Therefore, using the method of group analysis for light petroleum products, formula (3) can be written as:

;where: α _x is the measured angle of rotation of the plane of polarization of the investigated oil at

;

α _mon , α _ar are the average values of the angles of rotation of the plane of polarization of paraffin-naphthenic and, accordingly, aromatic components with the same design coefficient K.

It is known that the measurement of the angle of rotation a of the plane of polarization of linearly polarized light transmitted through a substance located in a longitudinal magnetic field can be performed using simple visual polaroscopes [1]. However, visual polaroscopes are rude instruments, their error in measuring the angle α is in the range from ± 0.05 ° to 0.5 °.

There are many designs of photoelectric digital polarimeters and saccharimeters designed to measure the angle of rotation of the plane of polarization of light transmitted through an optically active substance, for example, a solution of sucrose, glucose, maltose, fructose and the like. Their error in measuring the angle α is much lower and can reach values from ± 0.005 ° to ± 0.001 °. For example, in the photoelectric (digital) polarimeter for measuring sugar (glucose) in urine [3], a modulation method was used to measure the state of polarization of light passing through a cuvette with an analyzed solution, which allows one to reduce the error in recording the angle of rotation of the plane of polarization to ± 0.002 °.

In terms of the essential features, the closest analogue to the proposed polarimeter is a device for measuring the angle of optical rotation caused by an optically active substance according to US patent No. 5.168.326 [4].

The known device contains a monochromatic light source 1 (Fig. 1), the first simple linear polarizer 2, the transmission plane of which is an angle of ± 45 ° with the transmission planes of the second polarizer installed behind it made in the form of a Wollaston prism 3, installed between the polarizers 2, 3 of the cuvette 4 , with the first 5 and second 6 protective windows. The cuvette 4 is filled with the studied substance 7. In the bifurcated prism 3 light beams there are two collecting lenses 8, 9, in the focal planes of which photodetectors 10 and 11 are installed. Photodetectors 10 and 11 are connected to linear amplifiers 12 and 13, which are connected to the electric unit 14 s indicator 15.

The known device operates as follows. The monochromatic light beam from the source 1 collimates, passes the polarizer 2 and becomes linearly polarized, the polarization azimuth of which is an angle of ± 45 ° with respect to the transmission planes of the rays of the polarizer 3. The linearly polarized light beam after polarizer 2 passes through the cell 4 with the test substance 7 and the Wollaston prism 3. The Wollaston 3 prism divides the light beam into two linearly polarized beams whose polarization planes are mutually perpendicular. Further, the light beams of intensity I ₁ and I ₂ separated by a prism 3 with lenses 8 and 9 are collected on the sensitive elements of the photodetectors 10 and 11.

The electrical signals U ₁ and U _{2 of the} photodetectors 10, 11 are amplified by linear amplifiers 12, 13 and fed to the electronic unit 14.

In the electronic unit 14, the difference between the signals U ₁ -U ₂ , the sum of the signals U ₁ + U ₂ and their ratio are calculated

which is proportional to the measured angle of rotation of the plane of polarization of light α, which occurred as a result of the passage of linearly polarized light through an optically active substance 7.

The measurement result of the angle α is indicated by indicator 15. If the test substance 7 does not have optical activity, then linearly polarized light passes through the polarizer 2 to the substance 7 without changing the azimuth of linear polarization and after the Wollaston prism 3 is divided into two light beams of equal intensity

where I ₀ - the intensity of the light incident on the polarizer 2;

τ is the transmittance of the optical path.

In this case, the potentials U ₁ and U ₂ are equal, the electronic unit 14 gives information about the relation Q = 0, and the zero result of measurements of the angle α is displayed on the digital indicator 15.

If the test substance 7 has optical activity, then at the output of the cuvette 4 the azimuth of linear polarization changes by an angle a and the light intensities I ₁ and I ₂ change according to the Malus law:

In this case, the difference in light intensities will be equal to

and their amount

The potentials U ₁ and U ₂ at the outputs of the amplifiers 12, 13 are proportional to the light intensities I ₁ and I ₂ , therefore, the signal ratio

The electronic unit 14 gives information about the measured angle α, which is indicated on the indicator 15.

The known device according to US patent No. 5.168.326 allows accurate measurements of the angle of rotation of the plane of polarization of light transmitted through a substance with natural optical activity, for example, through a sugar solution. However, this device has a number of significant drawbacks. Firstly, it does not contain signs that make it possible to measure the angle of rotation of the plane of polarization of light induced by a longitudinal magnetic field, cannot measure the Verde constant and, therefore, is not suitable for determining the proportion of aromatic hydrocarbons in light oil products. Secondly, the known device according to US patent No. 5.168.326 provides only a single passage of a linearly polarized light beam through the test substance 7. This means that if the cell 4 with substance 7 is placed in a longitudinal magnetic field, and substance 7 has a natural optical activity , then as a result we get the sum of the effects, that is, the uncontrolled effect of natural optical activity is superimposed on the Faraday effect. This leads to unreliable results of the measured values of the Verde constant and the proportion of aromatic hydrocarbons in light petroleum products. Thirdly, the design of the known device according to US patent No. 5.168.326 does not contain signs that allow it to be immersed in the studied light petroleum products.

A new device is proposed, free of the above disadvantages.

The immersion polarimeter contains a source of a monochromatic, collimated beam, light, and a first linear polarizer, a second polarizer made in the form of a Wollaston prism, installed along the rays of light. The transmission plane of the first polarizer is an angle of ± 45 ° with the transmission planes of the second polarizer. A cuvette with first and second protective windows, which is filled with the test substance, is installed between the polarizers. After the Wollaston prism, two collecting lenses are installed in separated beams of light, in the focal planes of which two photodetectors are installed. Photodetectors are connected to two linear amplifiers that are connected to an electronic unit with an indicator. The proposed polarimeter has the following distinctive features. The cuvette is made in the form of a thin-walled pipe with a flange made of diamagnetic and oil-neutral material. Rectangular openings are made directly near the protective windows in the cell tube. The second protective window has a mirror coating on the side opposite from the light source. A permanent magnet is fixed on the cuvette in the form of a segment of a cylindrical pipe made of a material with high coercive force, and the direction of the magnetic field is axial, that is, along the axis of the cylindrical pipe. The outer surfaces of the permanent magnet are protected by diamagnetic material.

The angle of beam dilution by the prism of Wollaston 2β satisfies the condition

where: 2β is the angle of the bifurcation of rays by the Wollaston prism;

A is the width of the Wollaston prism;

D is the diameter of the light beam;

L is the distance from the Wollaston prism to the mirror.

In FIG. 1 shows a structural diagram of a known device according to US patent No. 5.168.326.

In FIG. 2 shows a block diagram of the proposed immersion-type polarimeter to control the proportion of aromatic hydrocarbons in light petroleum products.

In FIG. 3 shows the design of the proposed polarimeter assembly.

The proposed submersible polarimeter for controlling the proportion of aromatic hydrocarbons contains a source of a monochromatic, collimated light beam 1 (Fig. 2), for example, in the form of a laser module and installed along the rays of the first simple linear polarizer 2, the second linear polarizer 3, made in the form of a Wollaston prism. The transmission plane of the first polarizer is an angle of ± 45 ° with the polarization planes of the rays emerging from the second polarizer 3. After the polarizer 3, a cuvette 4 is installed, which is made in the form of a thin-walled pipe with a flange made of diamagnetic and oil-neutral material. The cuvette 4 contains the first 5 and second 6 protective windows made of optical glass and is filled with the test substance 7, for example, motor fuel. On the second protective window 6, a mirror coating is applied on the side opposite from the light source. In both light beams separated by a Wollaston prism 3, light-collecting lenses 8 and 9 are installed, in the focal planes of which the sensitive layers of photodetectors 10, 11 are fixed. Photodetectors 10 and 11 are connected to linear amplifiers 12, 13, respectively, which, in turn, are connected to an electronic a block 14 containing an indicator 15. In the tube of the cell 4 immediately after the protective window 5 and in front of the protective window 6, rectangular holes 16, 17 are made. (Fig. 3) A permanent magnet is fixed to the tube of the cell 4 between holes 16 and 17 in the form of a set of cutting cylindrical pipes 18 from a material with a large coercive force, for example, Nd ₂ Fe ₁₄ B and with the axial direction of the magnetic field strength. The outer surfaces of the permanent magnet 18 are protected by a diamagnetic material 19, the ends of which are sealed with a compound 20.

Structurally, the proposed submersible polarimeter to control the proportion of aromatic hydrocarbons in light petroleum products consists of four main parts (Fig. 2, Fig. 3):

- the immersion part of the polarimeter, which contains the tube of the cell 4 with the protective windows 5, 6 fixed to it, a permanent magnet 18 and a Wollaston prism 3;

- a reservoir 21 with a test substance 7;

- the outer part, which contains a housing 22, in which a light source 1, a polarizer 2, lenses 8, 9, photodetectors 10, 11 are fixed;

- the electronic part in which the amplifiers 12, 13, the electronic unit 14 and the indicator of the measurement results 15 are fixed.

The flanges of the cuvette 4 and the housing 22 are larger in diameter than the diameter of the neck of the reservoir 21, which in laboratory conditions can be played by an ordinary glass (Fig. 3) and in production lines, a fragment of the main pipeline through which the light oil product moves.

The proposed submersible polarimeter to control the proportion of aromatic hydrocarbons in light petroleum products works as follows.

A monochromatic, collimated and partially polarized light beam from the laser module 1 (Fig. 2) passes through the polarizer 2, becomes linearly polarized with a polarization azimuth of + 45 ° (or -45 °) with respect to the transmission planes of the Wollaston prism 3, is divided by two identical in intensity of the light beam with mutually orthogonal polarization azimuths, which pass the cell 4 with the test substance 7, are reflected from the mirror 6, pass the test substance 7 again, polarizer 2, are collected by lenses 8, 9 and perceived photodetectors 10 and 11.

Under the influence of the longitudinal (axial) magnetic field of the permanent magnet 18 in the test substance 7, the effect of rotation of the plane of polarization of linearly polarized light in each beam by an angle:

- вектор напряженности продольного магнитного поля;Where:

- vector of longitudinal magnetic field strength;

V is the Verdet constant of the test substance 7;

2L - the path traveled by light in the test substance in the cell 4 long L.

и

после перемножения матриц Мюллера, характеризующих воздействие каждого элемента оптики на поляризованный свет согласно уравнению:The dependence of the light intensity I ₁ and I ₂ , perceived by the photodetectors 10, 11, on the angle of rotation of the plane of polarization and the investigated substance 7 can be found by the first parameters of the Stokes vectors

and

after multiplying the Muller matrices characterizing the effect of each element of optics on polarized light according to the equation:

- вектор Стокса, характеризующий свет от источника 1;Where:

is the Stokes vector characterizing the light from source 1;

Partially polarized light from source 1 can be represented by the Stokes vector

where: I ₀ - light intensity studied by source 1;

p is the degree of polarization of light from source 1;

γ is the azimuth of predominant partial polarization.

The azimuth γ of the predominant polarization of light I ₀ from the source, made in the form of a laser module, can vary with changes in temperature and supply current. By passing such light through a linear polarizer 2, the state of polarization of the light incident on the prism of Wollaston 3 is stabilized.

на вектор

получаем новый вектор'' Hack multiplying the matrix

on vector

we get a new vector

which is characterized by a stable azimuth of linear polarization of 45 ° and does not depend on the change in the azimuth of the preferred polarization γ and the degree of polarization p of the light source 1, since γ and p affect only the magnitude of the light intensity, and not the polarization state of the light incident on the Wollaston prism 3.

After the Wollaston 3 prism, two light beams separated by an angle of 2β come out, which can be represented by vectors

that is, one light beam is linearly polarized in the horizontal plane (plus sign), and the other in the vertical plane (minus sign).

Further, after twofold passage of linearly polarized light beams with mutually orthogonal planes of polarization through the test substance 7, we obtain light beams that are represented by Stokes vectors

and after passing through the polarizer 2 Stokes vectors

The photodetectors 10, 11 perceive the intensity of the light beams I ₁ and I ₂ , which are characterized by the first parameters of the Stokes vector, that is:

Amplifiers 12, 13 operate in a linear mode, therefore, at their outputs, signals in the form of signal potentials U ₁ and U _{2 are} proportional to the light intensities I ₁ and I ₂ .

In the electronic unit 14 are calculated: the ratio of the difference of the signals U ₁ and U ₂ to the sum of these signals

desired angle of rotation of the plane of polarization

as well as the percentage of aromatic hydrocarbons in light petroleum products

магнита 18 и длине пути 2L, пройденном светом в кювете 4 длиной L;where: α _x is the measured angle of rotation of the plane of polarization of light by the investigated substance 7 at field strength

magnet 18 and a path length of 2L, passed by light in a cuvette 4 of length L;

и 2L;α _ar , α _PN - a priori known average values of the angles of rotation of the plane of polarization of light of aromatic and paraffin-naphthenic components, measured at the same values

and 2L;

K _T - calibration factor, depending on the type of oil product.

To obtain reliable measurement results of the proportion of aromatic hydrocarbons in light oil products, the proposed polarimeter is calibrated according to known standards. The standards can be used as chemically pure n-heptane - representative paraffinic hydrocarbon (Verde constant V _hep = 0.0125 m / e cm), reagent grade toluene _(tol constant Verde V = 0.0269 m / e cm) - a typical representative aromatic hydrocarbons and toluene 50% solution in n-heptane (Verde constant V _rm = 0.0197 m / e cm).

The essence of calibration is that after assembling the polarimeter in his cuvette 4, i.e. chemically pure n-heptane and toluene are sequentially poured into a glass 21 (Fig. 2). In this case, each time the values of the measured angles α _tol , α _hep as constant values are entered into the processor of the electronic unit 14. In order to verify the correctness of entering information on α _tol and α _hep in cell 4 (in glass 21), 50% toluene solution is poured in n-heptane. In this case, the readings of indicator 15 should be

Subsequently, during operation of the polarimeter, the values of α _tol and α _hep are retained.

Calibration coefficient K, is established experimentally by reference samples of a specific group of transparent petroleum products. Therefore, before starting to measure the proportion of aromatic hydrocarbons, the operator selects the operating mode “PETROL”, “KEROSIN”, “DIESEL”, etc. using the menu

The main advantages of the proposed device are the following:

Firstly, the proposed polarimeter is submersible, that is, its cuvette 4 (Fig. 2, Fig. 3) is in the studied light oil product 7, has openings 16, 17 for the circulation of the product. This achieves a certain universality of its use. For example, a polarimeter can be used in laboratories when an ordinary glass is used as reservoir 21, in main pipelines, when continuous monitoring of the fuel compounding process is required, or in reservoirs, for example, in vehicle tanks, tankers, transported motor fuel, in oil tank cars , etc.

Secondly, the small size and simplicity of the design of the proposed polarimeter make it possible to use it as a sensor that controls the proportion of aromatic hydrocarbons in transparent petroleum products.

Thirdly, the use of a permanent magnet in the form of a set of cylindrical pipes 18 made of a material with high coercive force allows the immersion part of the small volume polarimeter to be displaced, which leads to the displacement of a small volume of the studied oil product 7 from the tank (glass) 21.

Fourth, the presence of a small-sized powerful permanent magnet 18 allows you to reduce power consumption to a record level (less than 5 watts), and use tiny batteries or batteries as a power source.

The proposed polarimeter for controlling the proportion of aromatic hydrocarbons in light oil products can be widely used in laboratories of oil refineries, oil depots of controlling organizations, universities and research institutes.

INFORMATION SOURCES

1. Volkova EA Polarization measurements. Publishing House of Standards, M. 1974, p. 57.

2. RF patent No. 2163717.

3. RF patent No. 2029258.

4. US Patent No. 5.168.326.

Claims (6)

An immersion polarimeter to control the proportion of aromatic hydrocarbons in light oil products, containing a monochromatic, collimated light source and installed on the fly the first simple linear polarizer, the second polarizer in the form of a Wollaston prism, the transmission planes of which differ by an angle of ± 45 °, installed after the second polarizer cuvette with first and second protective glass windows and filled with the test substance, two light-collecting lenses, two photodetectors, two linear signal amplifiers in photodetectors, an electronic unit and an indicator, characterized in that the cuvette is made in the form of a thin-walled pipe with a flange made of diamagnetic and oil-neutral material, rectangular holes are made directly at the protective windows in the cuvette, on the second protective window, the side opposite to the light source is applied mirror coating, a permanent magnet is fixed on the cuvette in the form of a set of pieces of cylindrical pipes made of a material with a large coercive force and with an axial direction of tension magnetically field, the outer surfaces of the permanent magnet are protected by diamagnetic material, and the angle 2β of the bifurcation of rays by the Wollaston prism satisfies the condition

where: 2β is the angle of the bifurcation of rays by the Wollaston prism; A is the width of the Wollaston prism; D is the diameter of the light beam; L is the distance from the Wollaston prism to the mirror.

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