CALCULATION OF THE ADSORPTION COEFFICIENTS OF KRYPTON ON COCONUT ACTIVATED CARBON AT DIFFERENT TEMPERATURES

CALCULATION OF THE ADSORPTION COEFFICIENTS OF KRYPTON ON COCONUT ACTIVATED CARBON AT DIFFERENT TEMPERATURES

Daria Matyukhina

student, Mendeleev University of Chemical Technology,

Russia, Moscow

Veronika Pokalchuk

assistant, 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

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

The presence of sources of radiation risk is a characteristic feature of all facilities involved in activities with radioactive substances. This is due to releases and discharges of radioactivity, which can lead to negative impacts on humans and the environment. Particular attention should be paid to gaseous radionuclides, as the least controlled [1]. Noble radioactive gases (NRGs) are allocated to a separate group. According to the IAEA recommendation [2], the radioactive isotopes of noble gases: argon, krypton and xenon should be considered an important factor of impact on the environment during the operation of nuclear power plants. Isotopes of krypton and xenon are formed during the fission of nuclear fuel. The radioactive isotope Ar-41 is formed when neutrons activate the stable isotope Ar-40, which is contained in the air and some gas streams of NPPs.

Currently, a noble gas delay system (NGDS) is used at NPPs to clean gas blow-offs from NRG, radioactive iodine and aerosols. Its main equipment includes aerosol self-cleaning filters, zeolite filters and adsorber filters. The main unit of this system is an adsorber filter. It is used for highly efficient gas purification from isotopes of radioactive inert gases (xenon, krypton, argon). [3].

The amount of sorbent loaded into the adsorber should be sufficient to retain NRG and decay of all isotopes of argon, xenon, and krypton to an acceptable level of total volumetric activity [4]. The retention time will depend on the nature of the adsorptive, temperature, gas flow rate, and properties of the adsorbent. Replacement of the adsorbent in adsorbers (for example, due to the transition to activated carbon of another grade or another manufacturer) will require the calculation of the adsorption coefficients to ensure the required purification degree of the gas stream from NRG.

The purpose of this study was to obtain data on the adsorption of krypton on the VSK-5 grade activated carbon at various temperatures under static conditions.

Data on gas adsorption on porous materials, as well as measurements of the surface area, the volume of micro- and mesopores of pores, and pore size distribution were obtained on a Quadrasorb SI unit (Fig. 1). VSK-5 activated carbon based on coconut shells produced by “ENPO Neorganika” JSC was chosen as the object of the study. Before the experiment, a sample of activated carbon in cell (2) was cleaned from impurities at the coal degassing station The FloVac Degasser (6) by evacuation at 350°C for 3 hours. After degassing, cell (2) was fixed on one of the analysis stations of the Quadrasorb unit (1) and immersed in a thermostated glass (5) filled with water. The required temperature and coordination of the position of the vessel with the adsorbent were controlled by temperature (3) and immersion (4) sensors. A vacuum pump (7) and a built-in turbomolecular pump created a residual pressure of about 10^-6 mm Hg in the system. Art. Then krypton was supplied from a cylinder (8). The interval of gas pressures was set for the experiment in the software of the installation. Thereafter, the readings of the equilibrium value of the adsorption of krypton at this pressure were taken.

Figure 1. Schematic diagram of a setup for studying adsorption under static conditions. 1 – Quadrasorb unit; 2 – cell with an activated carbon sample; 3 – temperature sensor; 4 – immersion sensor; 5 – thermostatically controlled glass; 6 – The FloVac Degasser degassing station; 7 – vacuum pump; 8 – krypton cylinder

When adsorption equilibrium is reached, the following equation can be used: , where ΔG is the standard change in the Gibbs energy during adsorption, K is the constant of adsorption equilibrium.

The temperature dependence of the adsorption constant can be expressed as follows: .

Obviously, with increasing T, the constant K will decrease.

One of the simplest models for the adsorption of a certain gaseous substance on a homogeneous solid surface is the Langmuir model [6]. It is based on the following assumptions:

· the surface of the adsorbent is a set of energetically homogeneous adsorption centers; the probability of adsorption on each of them is the same;

· the interaction between adjacent adsorbed molecules can be neglected;

· the adsorption process ends with the formation of a layer with a thickness of one molecule on the interface (monomolecular adsorption layer).

Following with this theory, the rate of adsorption from the gas phase V_ad (i.e., the number of molecules adsorbed per unit time) is proportional to the gas pressure and the number of free centers on the surface of the solid. If the total number of centers is A₀, and A centers are occupied during adsorption, then the number of centers that remain free is (A₀–A). That's why: .

The adsorption is dynamically balanced by the desorption process. The desorption rate is proportional to the number of adsorbed molecules: .

In equilibrium, V_ad = V_des, and therefore . Thus, the Langmuir adsorption isotherm equation is actual .

In the range of low gas pressures (p → 0), it reduces to the well-known Henry equation: , where K_H is Henry's constant, which is valid for the initial (linear) section of the adsorption isotherm.

To describe the obtained data set, we used the Henry adsorption model for the range of low concentrations of the adsorptive, as well as the Langmuir adsorption model covering the entire set of experimental points. A search was made for a solution to find the coefficients in the corresponding equations using the least-squares method. Due to the noticeable nonlinearity of the isotherms even in the range of low pressures, the calculated value of the adsorption coefficient according to Henry's model turned out to be significantly dependent on the number of points being processed. Fig. 2a shows a description of the data using the Langmuir equation. It can be seen that all experimental points fit fairly well on the mathematical curves.

Figure 2.

Nevertheless, we plotted the dependences of the obtained constants on the reciprocal temperature to confirm the adequacy of the description of the experimental data by different approaches (Fig. 2b). It can be seen that the points found by Henry's model do not lie on a straight line, and their values are somewhat underestimated. Therefore, in our opinion, the application of the Langmuir adsorption model is more adequate to the obtained results and will allow the most accurate calculation of the radiochromatographic system of the NGDS. The calculation of the adsorber in this system is reduced to determining the volume of activated carbon (V_sorb) sufficient to reduce the concentration of all NRGs due to the time they are retained in the sorbent and not exceeding the control daily emission from the nuclear power plant unit. Based on the data on the normal operation of the system (gas flow rate Q and temperature), the retention time of krypton in coal is calculated using the equation [5]. After that, the activity of radioactive isotopes at the outlet from the adsorber is calculated taking into account the decay constant.

The following conclusions can be drawn based on the results of the study: adsorption isotherms of krypton were obtained for the VSK-5 activated carbon at different temperatures under static conditions; it was found that the Langmuir model is most appropriate for describing the experimental data obtained for the initial section of the isotherm; the calculated values of Henry's constants will make it possible to estimate the minimum required volume of activated carbon in the NGDS. Nevertheless, it should be noted that to clarify the conclusions made, it is necessary to additionally obtain data on the adsorption of argon and xenon on the VSK-5 activated carbon.

References:

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