Ferrofluidic countercurrent slug flow liquid-liquid extraction in millichannels

The core strength of liquid-liquid extraction (LLX) lies in multistage countercurrent operation, where even low selectivity can yield high purity with enough stages. Millichannel extractions are interesting for small scale laboratory LLX process development but at milliscale countercurrent flow is challenging due to the dominance of viscous and interfacial forces, channel blockage and phase separation difficulties. This study investigated the use of ferrofluids and magnetic forces to achieve countercurrent slug flow extraction in a millichannel extractor. Using an aqueous ferrofluid (AqFF) and n-heptane as a model system, the feasibility conditions for countercurrent flow at the millimeter channel scale and LLX performance were investigated. The effect of volumetric flow rate ratios (1:1 and 4:1), organic phase flow rates (0.20 and 1.00 mL/min), and slug flow velocities (0.44, 0.78, and 1.12 cm/s) on the operational stability was studied in 2.5mm diameter channels. The extraction behavior was investigated by extracting guaiacol from the AqFF to n-heptane. The number of theoretical stages (NTS) and the length equivalent to a theoretical stage (LETS) were determined to quantify performance. The highest NTS obtained was 2.12 ± 0.19, corresponding with a LETS of 11.76 ± 1.07 cm at a 1:1 volumetric flow rate ratio, 0.20 mL/min n-heptane flow, and 0.78 cm/s slug velocity. This work demonstrates that a magnetic driving force can enable stable slug flow countercurrent multistage LLX at millimeter scales, opening up new opportunities for LLX investigations.