RU2768242C1 - Method of determining gas compressibility factor - Google Patents

Abstract

FIELD: oil, gas and coke-chemical industry.

SUBSTANCE: invention relates to spectroscopy methods and can be used to determine gas compressibility factor. Method of determining gas compressibility factor by Raman scattering spectrometry consists in determining dependence of radiation intensity on gas pressure. Based on the obtained data, a graphical dependence is plotted, and a linear section corresponding to the behaviour of an ideal gas at low pressures of up to 5 MPa is used to determine a coefficient of proportionality relating radiation intensity and the gas density, which is used to determine the compressibility factor dependence on the gas pressure at each experimental point, from which the gas density at a given pressure is determined.

EFFECT: improving measurement accuracy, simplifying the measurement procedure.

1 cl, 6 dwg

Description

The invention relates to spectroscopy methods and can be used to determine the gas compressibility factor.

A known method for determining the coefficient of compressibility of a gas using a gas bypass (Burnett ESJAppl. Mech. Trans. ASME 1936, 58, A146; Hoover AE Determination of Virial Coefficients by the Burnett Method // Journal of Chemical and Engineering Data, Vol. 9, No. 4, 1964, pp. 568-573), selected as an analogue. The objective of the method is to determine the gas compressibility factor Ζ in the equation describing the state of the gas:

where Ρ is the pressure of the gas, V is the volume of the gas, n is the number of moles of the gas, R is the universal gas constant, Τ is the temperature of the gas, and Ζ is the pressure-dependent compressibility of the gas.

The method consists in performing the following actions (Fig. 1):

- temperature control of the experimental setup at a given temperature in thermostat 1;

- pumping out volumes 6a) and b) with a vacuum pump through valve 4;

- filling the volume 6a) with the investigated gas through the open valve 2, fixing the pressure in the container 6 by the pressure sensor 5;

- opening valve 3, bypassing gas into volume 6b), fixing pressure in volumes 6a) and b);

- closing valve 3, pumping out volume 6b) with a vacuum pump through valve 4;

- repeating the gas bypass procedure until the minimum measured pressure is reached.

When gas is bypassed from volume 6a) into a pre-evacuated volume 6b), equation (1) can be written as:

before bypass:

after bypass:

Since the volume V ₂ was initially pumped out, the amount of gas before the bypass and after the bypass did not change, i.e. n ₁ =n ₂ . Dividing equation (3) by equation (2) we get:

where the value (V ₁ +V ₂ )/V ₁ is always constant and is called the hardware constant N.

Thus, after the first bypass, we can write:

after the second:

after the third:

Substituting expression (7) into (6), and the result into (5), we get:

Thus, for the number of bypasses equal to i, we can write:

From expression (9) one can find the dependence of the compressibility factor Z on pressure:

The disadvantage of analog is that to determine the compressibility factor, it is necessary to take into account a large number of measured values: volume (hardware constant), initial pressure, compressibility factor at initial pressure. Each of these quantities contributes to the error in determining the coefficient Z. In addition, when using this method, it is impossible to get rid of the influence of such processes as adsorption and desorption of gas by the walls of the vessel, which leads to a change in the gas concentration in the volume during measurements (i.e., the assumption that the amount of gas before bypass and after bypass is not entirely correct ). The influence of adsorption and desorption also leads to an increase in the error.

Known article "Quantitative analysis of gaseous media by Raman spectroscopy" (Analytics and Control, No. 3-4, 1998 Bazhanov Yu.V. and others), selected as a prototype. In this work, Raman spectroscopy is used to determine the relative concentration of gases in a gas mixture. When a gas mixture is irradiated with laser radiation, additional peaks corresponding to one or another gas are recorded in the spectrum of scattered radiation. The intensity of the recorded peaks is directly proportional to the gas concentration:

where I is the intensity of the registered Raman peak, s is the proportionality factor, n is the concentration of gas molecules in a fixed volume.

The disadvantage of the prototype is that in this method it is required to determine the volume occupied by the gas, and this affects the error and accuracy of volume measurement. Also, the gas concentration is determined not in absolute units (mol / m ³ ), but in relative ones (the mole fraction of gases in the mixture is determined, expressed as a percentage).

The objective of the proposed method is to improve the accuracy of determining the gas compressibility factor Ζ while simplifying the method.

The technical result achieved using the present method is as follows:

- increasing the accuracy of measurements by eliminating the operation of determining the volume occupied by gas;

- simplification of the measurement procedure.

To solve this problem and achieve a technical result, a method is claimed for determining the gas compressibility factor by Raman spectroscopy. The method consists in determining the dependence of the radiation intensity on the gas pressure. According to the data obtained, a graphical dependence is built and, according to the linear section corresponding to the behavior of an ideal gas, a proportionality coefficient is determined that relates the radiation intensity and the gas density. The proportionality factor is used to determine the dependence of the compressibility factor on the gas pressure at each experimental point, which determines the density of the gas at a given pressure.

An increase in measurement accuracy is achieved due to the fact that in order to determine the compressibility factor, it is not necessary to determine the volume occupied by the gas. Accordingly, the volume measurement error does not affect the final result. In addition, since the gas density is determined directly from the intensity of the peak in the Raman spectrum, the measurement result is not affected by such factors as gas adsorption and desorption by the walls of the vessel, which also leads to an increase in the measurement accuracy.

The presence of only one volume with gas, as well as the fact that when implementing this method there is only one operation - increasing the gas pressure in the tank, leads to a significant simplification of the measurement procedure.

In FIG. 1 shows a diagram of the installation for determining the compressibility factor by the gas bypass method.

In FIG. 2 is a diagram of a setup for determining the compressibility factor of a gas using Raman spectroscopy.

In FIG. Figure 3 shows the dependence of the intensity of the peak Q ₁ (1) in the hydrogen spectrum on pressure.

In FIG. 4 shows the initial linear section of the dependence, which is used to determine the proportionality coefficient.

In FIG. 5 shows the linear dependence of the gas density on the intensity of the peak in the spectrum,

In FIG. 6 shows the dependence of the hydrogen compressibility factor on pressure, determined by (14)

In FIG. 1 and 2, the following designations are accepted: 1 - thermostat, 2-4 - manual valve, 5 - pressure sensor, 6 (a, b) - containers with known volumes (Fig. 1), 6 - container (Fig. 2), 7 - optical probe, 8 - optical fiber for radiation transmission.

The inventive method (Fig. 2) includes the following operations:

- temperature control of container 6 at a given temperature in thermostat 1 and its pumping out through valve 3;

- filling the container 6 with the test gas through the valve 2, closing the valve 4;

- registration of the spectrum of Raman scattering of light at a given pressure with the help of an optical probe 7 and optical fibers 8 installed in the vessel;

- opening the valve 4, increasing the pressure in the tank 6;

- re-registration of the spectrum at a fixed pressure until the maximum possible pressure is reached.

At gas pressures up to 5 MPa, the compressibility coefficient is ≈1, so the state of the gas can be described by the equation of state for an ideal gas:

where Ρ is the gas pressure, ρ is the gas density (ρ=n/V), R is the Boltzmann constant, Τ is the gas temperature.

Based on (12), one can calculate the gas density ρ for each experimental point obtained at low pressures (up to 5 MPa). Taking into account (11), one can determine the proportionality coefficient that relates the gas density and the peak intensity in the Raman spectrum. This calibration makes it possible to determine directly the gas density (mol/m ³ ) from the intensity of the Raman peak. After that, equation (1) can be rewritten as:

where P is the gas pressure; k is the proportionality coefficient relating the gas density and the intensity of the peak in the Raman spectrum, I is the intensity of the recorded peak in the Raman spectrum, R is the universal gas constant, Τ is the gas temperature, Ζ is the pressure-dependent gas compressibility factor. Thus, the coefficient Ζ is defined as:

The author has developed and experimentally tested a method for determining the gas compressibility factor by Raman spectroscopy. The following equipment was used during testing:

- a heat and cold chamber with an accuracy of maintaining the set temperature of ± 0.1 ° С;

- two pressure sensors, one with a measurement range of 0-10 MPa (relative error 0.025%), the other with a measurement range of 0-400 MPa (relative error 0.5%);

- solid-state laser with a wavelength of 532 nm;

- monochromator-spectrograph with a cooled matrix;

- optical probe, which allows to study gases at pressures up to 400 MPa.

Hydrogen (purity 99.99%) was chosen as the object of research.

The dependence of the intensity of the peak Q ₁ (1) in the hydrogen spectrum on pressure is shown in Fig. 3 (at each pressure value, 5 Raman spectra were recorded). The initial linear section of this dependence, which is used to determine the proportionality coefficient relating the gas density and the peak intensity in the Raman spectrum, is shown in Fig. 4.

The linear dependence of the gas density on the intensity of the peak in the spectrum, the slope of which is determined by the coefficient of proportionality (determined from the graph in Fig. 4), is shown in Fig. five.

The dependence of the hydrogen compressibility factor on pressure, determined by (14), is shown in Fig. 6. The relative error in determining the compressibility coefficient does not exceed 2%.

Claims (1)

A method for determining the gas compressibility coefficient by Raman spectrometry, which consists in determining the dependence of the radiation intensity on the gas pressure, according to the data obtained, a graphical dependence is plotted and, according to the linear section corresponding to the behavior of an ideal gas at low pressures up to 5 MPa, a proportionality coefficient is determined that relates the radiation intensity and the density of the gas, which is used to determine the dependence of the compressibility factor on the gas pressure at each experimental point, which determines the density of the gas at a given measurement pressure.

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