Explosive boiling-up of a calcium hydroxide solution in water during pulsed electrolysis

Аuthors

Vinogradov V. E.

Institute of Thermal Physics of the Ural Branch of the Russian Academy of Sciences , Amundsena St., 107a Ekaterinburg, 620016, Russia

e-mail: vinve@mail.ru

Abstract

Pulse electrolysis is considered by one of the methods of producing oxygen and hydrogen. With high current densities of short current pulses it is possible to realize significant supersaturation of the electrolyte on the electrode surface with gaseous electrolysis products, followed by the explosive boiling of a liquid. Supersaturations comparable to those predicted by homogeneous nucleation theory have been obtained for potassium hydroxide (KOH) solution and solutions of the salts Na2SO4, NaCl, and KI. These studies were performed at atmospheric pressure and primarily at room temperature. In this paper, the influence of temperature and pressure on the explosive boiling of a calcium hydroxide (Ca(OH)2) solution in water during pulsed electrolysis has been investigated. A fluoroplastic cell of volume 15 cm3 containing a 0,1 % calcium hydroxide solution in water was placed in a pressure chamber thermostatted to within 1 °С. The pressure in the chamber varied from 0,1 to 2,0 MPa. A platinum wire with a diameter of 30 μm and a length of 1 cm was used as the anode. A stainless steel plate with a thickness of 1 mm and a width of 6 mm served as the cathode. A rectangular pulse generator with an amplitude of up to 600 volts and a duration of up to 100 μs allowed an electrolysis current density of up to 107 A/m2 to be generated at the anode. Current oscillograms were recorded during the electrolysis process. Electrolyte boiling-up at the cathode was accompanied by a sharp drop in the electrolysis current. The nucleation rate observed in the experiments was calculated based on an analysis of the oscillograms in the region of the sharp current drop. It was found to be ~1020 m–3 s–1. The oxygen concentration at the anode during electrolysis was calculated from the current oscillograms. The temperature dependence of the oxygen concentration at the anode at the moment of electrolyte boiling-up was obtained in the temperature range from 25 to 100 °С at constant pressure. The results obtained have been analyzed within the framework of homogeneous nucleation theory. For this purpose, oxygen concentrations in water causing the solution boiling-up with the nucleation rate observed in experiments have been calculated using formulas of homogeneous nucleation theory. The results are consistent within the experimental error and the data on oxygen diffusion in water and the Henry constant used in the calculations. It has been concluded that electrolyte boiling-up occurs predominantly at fluctuation boi-ling sites. The dependence of the oxygen concentration at the anode at the moment of explosive boiling of the solution on pressure has been obtained in the pressure range from 0,4 to (308)2,0 MPa at a constant temperature (~87 °C). The results obtained are compared with calculations based on formulas of homogeneous nucleation theory. A significant discrepancy between the experimental and the calculated results is observed. While the calculations based on formulas of homogeneous nucleation theory yields a weak dependence of concentration on pressure, the experiment, on the contrary, shows a significant increase in concentration with pressure. The reasons for this discrepancy may be both a possible unaccounted error or a lack of data on the pressure dependence of the diffusion coefficient and the Henry's constant. In any case, the results on the pressure dependence of explosive boiling of the electrolyte during pulsed electrolysis require further experimental verification.

Keywords:

pulse electrolysis, explosive boiling-up, extreme superheating, nucleation, calcium hydroxide

References

1. Naohiro Shimizu, Souzaburo Hotta, Takayuki Sekiya et al. A novel method of hydrogen generation by water electrolysis using an ultra-short-pulse power supply. Journal of Applied Electrochemistry. 2006;36:419.

2. Schastlivtsev AI, Dunikov DO, Borzenko VI et al. Hydrogen-oxygen units for power engineering. High Temperature. 2020;58(5):733–743. DOI: 10.1134/S0018151 X20050077

3. Vinogradov VE, Pavlov PA. Phase transition under fast electrolysis conditions. High Temperature. 2024;62(2): 193–198.

4. Svetovoy VB, Sanders RGP, Lammerink TSJ et al. Combustion of hydrogen-oxygen mixture in electroche-mically generated nanobubbles. Phys. Rev. E. 2011;84. DOI: 10.1103/PhysRevE.84.035302

5. Svetovoy VB, Sanders RGP, Elwenspoek MC. Transient nanobubbles in short-time electrolysis. Journal of Physics: Condensed Matter. 2013;25. DOI: 10.1088/0953-8984/ 25/18/184002

6. Sukhotin A.M. Handbook of Electrochemistry. Leningrad: Chemistry; 1981. 488 p.

7. Mitrofanov SM, Pavlov PA. Geometrical characteristics of non-stationary boiling crisis. High Temperature. 2006;44(5):720–728. DOI: https://doi.org/10.1007/s107 40-006-0087-y

8. Pavlov PA. Dynamics of boiling of highly superheated liquids. Sverdlovsk: Ufa Scientific Center of the USSR Academy of Sciences; 1988. 244 p.

9. Reed R, Prausnitz J, Sherwood T. Properties of gases and liquids: A reference manual: Chemistry; 1982. 592 p.

10. Carslaw G, Eger D. Thermal conductivity of solids. Moscow: Nauka; 1964. 487 p.

11. Skripov VP. Metastable liquid. Moscow: Nauka; 1972. 312 p.

12. Verhallen PTHM, Oomen LJP, Elsen AJJMvd et al. The diffusion coefficients of helium, hydrogen, oxygen, and nitrogen in water determined from the permeability of a stagnant liquid layer in the quasi-steady state. Chemical Engineering Science. 1984;39(11):1535–1541.

13. Namiot AYu. Solubility of gases in water: A reference manual. Moscow: Nedra; 1991. 167 p.