Effect of channel size on mass transfer during liquid-liquid plug flow in small scale extractors

In this paper the effect of channel size on the mass transfer characteristics of liquid-liquid plug flow was investigated for capillaries with internal diameter ranging from 0.5 to 2 mm. The extraction of {UO2}2+ ions from nitric acid solutions into TBP/IL mixtures, relevant to spent nuclear fuel reprocessing, was studied for different residence times, dispersed phase fractions, and mixture velocities. It was found that extraction efficiencies increased as the channel size decreased. For a given channel length and for all channel sizes, an increase in mixture velocity decreased the extraction efficiency. The overall mass transfer coefficients (kLα) for all channels varied between 0.049 and 0.29 s−1 and decreased as the channel size increased. The evolution of the kLα along the extraction channel showed a decreasing trend for all the channel sizes. The experimentally obtained mass transfer coefficients were compared with existing models for liquid-liquid and gas-liquid segmented flows from the literature. The results showed good agreement with the empirical correlation proposed for a liquid-liquid system. A finite element model was developed that solved the velocity and concentration fields in the channel for both phases considering a unit cell (one plug and one slug) with periodic boundary conditions at the inlet and the outlet. The model used experimental data for the geometric characteristics of the plug flow and predicted reasonably well the experimentally measured extraction efficiencies (with mean relative error of 11%).