The influence of surface tension on F2-Bubble Morphology at detachment from carbon electrodes in KF-2HF molten fluoride salt

Abstract

Molecular F2 and H2-bubbles were generated electrochemically on amorphous carbon electrodes in KF-2HF molten salt to study the effect of surface tension (ϒ) on the profile of a bubble. F2 is used for the purification and enrichment of fuel in the nuclear industry. Its generation is energy-intensive because of electrical resistances caused by the adhesion of F2-bubbles on the electrode surface. The dimensions and contact angles of generated F2-bubbles were measured and a relationship was developed to study how the profile changes with wetting conditions. F2 was also generated on a large (~4.0 cm2) carbon electrode to observe how the process of bubble formation changes when the size of bubbles at detachment are not restricted by the size of the electrode. The results from these studies revealed inconsistencies with the mixed-phase fluidized layer' model, which could not explain a spherical-to-lenticular transition in bubble shape that was observed. The fluid dynamics of rising bubbles in molten KF-2HF were characterized using the rise velocities (Vt) and aspect ratios (Eb) of F2 and H2-bubbles for a range of bubble sizes. Correlations for Vt, Eb, and drag coefficients (Cd) were evaluated for their accuracy in predicting bubble behavior in this molten salt. A range of -values were obtained by fitting the correlations to the F2 and H2-data sets. A numerical model based on a force balance calculation was developed to estimate the interfacial tension of spherical and lenticular F2-bubbles for different stages of electrode passivation. The calculated ϒGL-value for KF-2HF (0.11 N/m) fit into the trends for surface tension of simple 1:1 and 1:2 molten halide salts. In addition, the GL value was within the range obtained from the correlation fitting. This body of work is the first: 1) study of the effect of wetting on the shape and detachment of lenticular bubbles in a molten salt; 2) comprehensive study of bubble morphology and dynamics in a molten salt; and, 3) estimate of the surface tension of KF-2HF.