The continuous extraction of rhodamine 6G into water from solution in pentanone was achieved in both milli- and microfluidic devices. Continuous extraction is possible through cocurrent laminar flow of the organic and aqueous streams stabilized by a hydrogel defining one wall of the channel. Water flow rates of 10 ml/h to 40 ml/h were used with a pentanone flow rate of 40 ml/h. The mass transfer coefficient for this system was found to be (1.7 ± 0.8) × 10-3 cm/s. Milli-fluidic LLE with cocurrent laminar flow of immiscible liquids provides a suitable geometry to accurately determine mass transfer coefficients due to the well defined interface between the phases.
After accurate determination of the mass transfer coefficient for the system was achieved, droplet-based extraction was performed and a milli-fluidic method capable of continuous passive separation of the emulsions with an efficiency of ~90 % was utilized to recover the aqueous phase. The overall scheme is an efficient method for liquid liquid extraction. To achieve continuous passive separation, a device with opposing channel walls of disparate hydrophobicity is used to stabilize co-laminar flow of the solvent and water. The disparity in hydrophobicity of the channel walls is accomplished by defining one length of the channel with a hydrogel, in this case polyethylene glycol. Emulsion separation is facilitated by introducing the emulsion at the water/hydrogel interface. Advantages of performing separations at the millli-fluidic scale are presented. The scale up of this continuous, passive emulsion separation method to several mL/min was achieved.
To demonstrate the utility and robustness of the emulsion separation scheme, experiments used high oleic sunflower oil or mineral oil as the continuous phase and aqueous solutions of methylene blue, crystal violet, or dextran with methylene blue as the dispersed phase. The water-in-oil emulsion was created within a T-junction device and the emulsion was transported to a milli-fluidic device where passive emulsion separation was achieved by the co-current laminar flow of oil and water. Co-current laminar flow was stabilized by defining one channel length with a hydrogel layer. Introduction of the emulsion at the hydrogel–water interface induced emulsion break up. The maximum emulsion separation throughput was defined by a decrease in emulsion separation efficiency to below 80%, a loss of stable parallel flow, or the creation of an emulsion. A maximum emulsion separation throughput of 1.4 metric tons per year was achieved.