Mass Transfer Characteristics of Large Oscillating Drops

Abstract

The literature related to mass transfer of solute from large, oscillating drops in liquid-liquid systems has been critically reviewed. A thermostatically-controlled spray column, provided with mirrors, was used to study the hydrodynamics and mass transfer characteristics of single large oscillating drops and drop pairs ascending through water. Up to 25%w/w of acetone was dissolved in toluene, which facilitated investigation of the effect of physical properties upon the frequency and amplitude of drop oscillation and the mass transfer rate. Glass or ptfe nozzles were used to create single, or pairs of, drops (d = 5mm) and their behaviour was investigated, during formation, and during travel. The frequency of droplet oscillation, area change, amplitude and vertical velocities were recorded using either video or high speed cine photography. The range of shapes of large oscillating drops (d => 6mm) and drop pairs were geometrically indefinable and the mode of oscillation was 2, 3 or 4. The frequency of oscillation of drop pairs was best correlated by ω² = 2.8 (We)0.40Q but both the amplitude and frequency of oscillation of droplet pairs were less than for single drops. Published correlations for overall mass transfer coefficients during formation, underestimated the experimental coefficients by 22% for single drops and 55% for drop pairs. A new model formulated for the overall mass transfer coefficient, with the droplet surface area expressed as a function of time of formation, <formula> correlated overall mass transfer coefficients for drop pairs within 5%. Experimental data for single oscillating drops and drop pairs during travel confirmed the limitations of existing models. An enhancement of overall mass transfer coefficient of up to 88% was observed due to collision-rebound effects with drop pairs. A new correlation was developed to account for the effect of drop interactions. <formula> Agreement within 9% was obtained between experimental overall mass transfer coefficients and those predicted from theory where kq was predicted from this correlation and k, from Gamer and Tayeban's correlation for oscillating drops.