(517d) Effects of Different Additives on Fluid Dynamics and Mass Transfer in Single Drop Extraction

Extraction processes in liquid/liquid systems rely strongly on the characteristics of the interface and the occurring transport phenomena. Additives, such as surface-active substances or nanoparticles, which are added intentionally or occur as impurities, may influence fluid dynamics and mass transfer substantially. Residence times and hold-up in liquid/liquid extractors may increase while the additives impose an additional resistance against mass transfer or promote the same by inducing interfacial effects like Marangoni convection. Since the interfacial phenomena can have contrary effects on the mass transfer in multiphase systems, the prediction of their occurrence and process variables linked to them based on theoretical or empirical basis is a challenging task.

The present work focuses on the characterization of fluid dynamics and mass transfer in the presence of different surface-active substances. Due to the interaction between the transport processes, fluid dynamic measurements of single droplets can act as an indicator for the prediction of interfacial phenomena and mass transfer rates. Measurements of single drops are done in a drop rising cell of one meter length. Droplet paths and velocities were detected optically with a high-speed camera. Mass transfer was determined by collecting drops at different heights and analyzing the concentration of the transfer component.

Acetic acid was used as a transfer component and fluid dynamics and mass transfer from the disperse 1-octanol phase to the continuous water phase were measured for different drop sizes and initial concentrations of the transfer component. As additives, the surfactant Butyldiglycol (C4E2) and the partly hydrophobic silica nanoparticles HDK H20 were chosen. Drop rising velocity, droplet path and concentration for different contact times were measured and compared between the ternary reference system and the two systems with additives.

At high concentrations of acetic acid the pure system showed lateral displacement resulting in a break out from linear droplet paths, which could indicate Marangoni effects. For low concentration no Marangoni effect occurred. Surfactant and nanoparticle systems were, therefore, compared for low concentrations of the transfer component.

Upon addition of C4E2 without transfer component the rising paths of the drops likewise indicated Marangoni convection. Stunningly, the dependencies of concentration and drop size were reversed compared to the standard reference system toluene/water/acetone known for Marangoni effects. With acetic acid as transfer component the behavior became even more chaotic, which indicates a strong influence on the mass transfer.

When adding nanoparticles, no influence on the fluid dynamics is observed in the nanoparticle concentration range investigated. Although the fluid dynamics stayed the same a decrease of mass transfer was measured with increasing nanoparticle concentration.