Abstract
The instantaneous concentration of a solute in single toluene droplets in water phase was measured contactless as function of the exchange time during non-stationary mass exchange by means of a modified liquid scintillation technique using two photomultipliers in coincidence, a three channel spectrometer and multichannel analyser. The interchange of fluid mechanics and mass exchange from the toluene droplet into the water phase was measured with acetic acid and dioxane as solutes under the following conditions: 1. 1. The droplet is held on a stainless steel capillary in stagnant water (Reynolds number, Re ≅ 0). 2. 2. The droplet is held on a stainless steel capillary in water flow (32 < Re < 215). 3. 3. The droplet is formed on a stainless steel capillary in water flow and then removed from the capillary. The mass exchange is investigated on the freely suspended droplet (64 < Re < 215). 4. 4. The droplet is formed on a stainless steel capillary in stagnant water and then released from the capillary and the mass exchange is investigated on the freely suspended droplet (64 < Re < 215) 5. 5. The droplet is formed on a stainless steel capillary in stagnant water solution, which is in distribution equilibrium with the toluene with regard to the solute. The droplet is then released and the mass exchange is investigated on the freely suspended droplet (64 < Re < 215). In all of these cases the exchange process was not considered during the droplet formation, but only after the final droplet volume was attained. Therefore the droplet radius was constant during the exchange process. The concentration-exchange time and/or dimensionless concentration—Fourier-number—functions as well as the integral and/or instantaneous Sherwood number—Fourier number—functions were calculated by stationary and non-stationary models and compared with the measurements. The precise and instantaneous measurements of the solute concentration made it possible to recognize subtle differences of mass exchange mechanisms. For the first time the existence of the stepwise change of the instantaneous Sherwood number with increasing exchange time could experimentally be proved.
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