CALCULATION OF HENRY'S CONSTANTS FROM THE OUTPUT CURVES OF NOBLE GASES DYNAMIC ADSORPTION ON THE ACTIVATED CHARCOAL VSK-5

CALCULATION OF HENRY'S CONSTANTS FROM THE OUTPUT CURVES OF NOBLE GASES DYNAMIC ADSORPTION ON THE ACTIVATED CHARCOAL VSK-5

Sofia Neberekutina

student, Mendeleev University of Chemical Technology,

Russia, Moscow

Alexey Merkushkin

candidate of chemical sciences, associate Professor, Mendeleev University of Chemical Technology,

Russia, Moscow

Alexander Obruchikov

candidate of technical sciences, associate Professor, Mendeleev University of Chemical Technology,

Russia, Moscow

Radioactive noble gases (RNG) form 55% to 90% of the total activity of nuclear power plant emissions [1]. Most of them have a half-life of less than a few days, which can significantly reduce the activity of gaseous emissions, producing a delay of gases in charcoal adsorbers before being released. The exception is Kr-85 with a half-life of 10.77 years, which can accumulate in the atmosphere. Radioactive noble gases make the greatest contribution to the effective dose formation of the critical group of the population during the operation of the VVER and RBMK types nuclear reactors [2].

Thus, the study of the properties and methods of capturing RNG is a serious technological problem to optimize the radiological protection of the population and the environment.

The purpose of this work is to determine the coefficients of adsorption of argon, krypton and xenon on activated charcoal VSK-5 under dynamic conditions.

A facility was created to study the noble gases adsorption under dynamic conditions, which a schematic diagram is shown in Fig. 1. The main units of the facility are a thermostatically controlled column with a sorbent (1), as well as an OmniStar Pfeiffer mass spectrometric gas analyzer (4).

Figure 1. Schematic diagram of the facility for studying the noble gases adsorption under dynamic conditions: 1 – adsorption column; 2 – thermostat; 3 – compressor; 4 – gas analyzer; 5 – personal computer; 6 – column with silica gel; 7 – column with charcoal; 8 – stand with regulators of carrier gas flow; 9 – input of a mixture of inert gases

The output curves of dynamic adsorption were obtained as time dependences of the relative concentration of the noble gas in the frontal mode.

The data on the adsorption coefficients were obtained by pulsing a mixture of inert gases into the column (displacement mode). In this case, it becomes possible to obtain the output curves of several components of the mixture simultaneously (Fig. 2). With a pulsed supply of an inert gas, the definition of the Henry constant (K_H) is reduced to solving the Shilov equation [3], from which it follows that the Henry constant is directly proportional to the retention time of the adsorptive and is related to it by relationship (1), which follows from the theory of equilibrium chromatography.

where t – is the corrected release time of the adsorptive, taking into account the correction for the carrier gas velocity through the free volume and representing the difference between the release time (peak maximum) of the adsorptive and the time of helium release.

During the tests, the values of the exit time of each gas in the chromatographic column were obtained (Fig. 2). Helium, as an insorbable gas, left the column along with the air flow and was a component of the mixture, which was used to determine the retention time of the remaining noble gases in the sorbent layer.

Figure 2. Output curves of noble gases obtained with dynamic adsorption on VSK-5 at 20°С

After processing the experimental data, the following values of Henry's constants (Table 1) were obtained. It can be seen that the values of the constants a increase in the Ar-Kr-Xe series, which is explained by an increase in the atomic radius of a chemical element. It is also obvious that with an increase in temperature, the sorption capacity decreases for all noble gases. Based on these data, the dependence of the logarithm of the constants values on the reciprocal temperature was obtained, which is in good agreement with the theoretical concepts of the independence of the change in the enthalpy and entropy of adsorption on temperature. In other words, the linearity of these dependences makes it possible to conclude about the adequacy of the chosen method for determining the adsorption coefficients.

The difference in the values of Henry's constants indicates a different degree of sorption on activated charcoal, that is, the possibility of separating gas components.

Table 1.

Calculated values of Henry's constants in the temperature range 15-60 °С

Figure 3. Dependence of the logarithm of Henry's constant on the reciprocal temperature

A quantitative characteristic for this is the selectivity coefficient: the ratio of the distribution coefficients or Henry's constants of two substances, the values of which are presented in Table 2.

Table 2.

Values of selectivity coefficients at different temperatures

It can be seen that with decreasing temperature, the values of the selectivity coefficients increase. Obviously, their highest values refer to the Xe/Ar pair. The data obtained will make it possible to further develop an approach to the separation of target components from the radioactive gas mixture of the NPP noble gas delay system. In other words, it will be possible to select the necessary conditions for the isolation and storage of the long-lived isotope Kr-85, as well as to retain stable isotopes of xenon, a rather expensive gas used in medicine and industry.

The work was supported by Mendeleev University of Chemical Technology of Russia. Project Number 2020-008.

References:

1. Pyshkina M.D. The determination of main dose-forming nuclides in NPP PWR and VVER releases // Biosfernaya sovmestimost': chelovek, region, tekhnologii. 2017. T. 18, No 2. P. 98-107.
2. INPRO Methodology for Sustainability Assessment of Nuclear Energy Systems: Environmental Impact, IAEA, Vienna, IAEA Nuclear Energy Series № NG-T-3.15. 2016. P. 40-44.
3. Matveikin V.G., Pogonin V.A., Putin S.B., Skvortsov S.A. Matematicheskoe modelirovanie i upravlenie protsessom korotkotsiklovoi beznagrevnoi adsorbtsii. Moskva: Mashinostroenie-1, 2007. 140 s.