Effect of temperature on the efficiency of the electrolyzer

Аuthors

Kropochev E. V.^1*, Brendakov V. N.²

1. Tomsk State University, 36 Lenin Ave., Tomsk, Tomsk region, 634050, Russia
2. 2Seversky Technological Institute - branch of National Research Nuclear University “MEPhI”, Seversk, Russian Federation

*e-mail: messive@yandex.ru

Abstract

Fluorine gas is widely used in many sectors of the national economy, in chemical production, agriculture, space industry, nuclear power engineering. Obtaining pure fluorine is a labor-intensive and highly expensive activity. One of the promising areas is electrolysis. Medium-temperature electrolysis of potassium dihydrofluoride melt is characterized by a minimum content of impurity HF in the anode gas, lower corrosive action of the melt. Industrial production of fluorine is a complex electro-hydro-thermal-chemical reaction in which many different factors interact, which complicates the implementation of large-scale experimental studies. The issue of improving the technology and equipment for fluorine production makes it relevant to conduct numerical studies and mathematical modeling of the technology for obtaining fluorine in medium-temperature electrolyzers. This contributes to increasing the competitiveness of production and meeting the growing demand for fluorine. The paper considers a mathematical model of the process of electrolytic production of fluorine, consisting of a description of three interrelated processes. The hydrodynamics of potassium trifluoride melt is presented by the Navier–Stokes equation system written for a mixture of different-phase components. An algebraic slip model is used to close the system. The thermodynamic situation inside the electrolysis cell is estimated based on the energy transfer equation for a phase mixture. The electrochemical kinetics of the created mathematical model in the considered area is based on Faraday's laws for electrolysis. The Butler-Folmer equation is used to calculate the current density at the boundary with the electrodes. A balance equation is used to estimate the concentrations of the oxidizer and reducer in the working area. The created mathematical model is implemented numerically in the OpenFOAM software environment. This computational toolkit was chosen as a working tool because it is an open-source system with flexible customization options that allow creating individual and potentially accurate solutions for specific applications. When performing test calculations using the created model, variable parameters were selected that most significantly affect the electrolysis process. This is the temperature of the melt inside the electrolysis cell T = 85÷105 °С, the mass fraction of hydrogen fluoride in the melt CHF = 38÷41 wt. % and the current density on the anode ia = 1÷2 kA/m2. The adequacy of the constructed model was verified by comparing the number of electrochemical equivalent of fluorine calculated on the basis of the electrolysis process implemented according to the model with the known theoretical value. Good agreement, more than 95 %, indicates the operability of the created model. Using the mathematical model of electrolysis, calculations were performed to assess the effect of temperature on the efficiency of the modeled process. The amount of fluorine formed in the electrolyzer, recalculated per 1 kW h of consumed electricity, was taken as the efficiency of the process of obtaining electrolysis fluorine. The calculations showed a significant effect of temperature on the efficiency of the electrolysis process.

Keywords:

mathematical modeling, medium-temperature electrolyzer, three-component electrolysis model, elec-trolyzer performance for fluorine, efficiency of the technological process

References

1. Galkin NP, Krutikov AB. Fluorine technology. Moscow: Atomizdat; 1968. 188 p. (In Russ.).
2. Belyaev VM. Study of the process of obtaining fluorine by electrolysis of the KF-HF system in electrolyzers with forced circulation of the electrolyte. PhD. thesis. Tomsk: TPU; 1974. 190 p. (In Russ.).
3. Liventsov SN. Development of a mathematical model of the technological process of electrolytic production of fluorine in STE-20 devices. Izvestiya Tomskogo politekh-nicheskogo universiteta. 2002;305(3):408–415. (In Russ.)
4. Liventsova NV. Optimization of the technological pro-cess of obtaining fluorine. Izvestiya Tomskogo politekhniche-skogo universiteta. 2007;311(3):45–48. (In Russ.).
5. Zusailov YuN. Quality control of products in the produc-tion of carbon anodes, fluorine and uranium hexafluo-ride. Angarsk: Angarsk State Technical University; 2017. 267 p. (In Russ.).
6. Loitsyanskii LG. Mechanics of liquids and gases. textbook for universities. (eds.). Moscow: Nauka; 1987. 840 p. (In Russ.).
7. Manninen M., Taivassalo V., Kallio S. On the mixture model for multiphase flow. Technical Research Centre of Finland: VTT Publications; 288. 1996. 67 p.
8. Molchanov AM. Thermophysics and dynamics of liquids and gases. Special chapters. Moscow: MAI; 2019. 152 p. (In Russ.).
9. Lukomsky YuYa, Hamburg YuD. Physicochemical foun-dations of electrochemistr. Dolgoprudny: Intellect; 2008. 424 p. (In Russ.).
10. Newman JS, Thomas-Alyea KE. Electrochemical systems. Hoboken, New Jersey, USA: John Wiley & Sons, Inc. 2004. 647 p.
11. Sinkov KF. Development of hydrodynamic models of multiphase flows in pipelines: PhD. thesis. MIPT. Mos-cow; 2016. 130 p. (In Russ.).
12. Johnson A. Thermodynamic aspects in the course of in-organic chemistry. Moscow: Mir; 1985. 134 p.
13. Greenshields C. OpenFOAM v7 User Guide. 2019.
14. Belyanin AV, Nagaytseva OV, Liventsova NV et al. De-velopment of a model of the thermal process of an elec-trolyzer for the simulator of the operator of an automated process control system for fluorine production. Izvestiya Tomskogo politekhnicheskogo universiteta. 2009;315(4): 38–42. (In Russ.).
15. Petriy OA. Electrochemical equivalent. Brief Chemical Encyclopedia. Vol. 5. Moscow: Sov. Enzik. 979 p. (In Russ.).