Abstract:The objective of microfluidic droplet flow is the control and manipulation of fluids on the sub-mm scale. This article describes the microfluidic droplet flow in a liquid-liquid contacting unit utilizing microchannels and millichannels and their counter-current arrangement. A contacting module consists of a droplet generator, a channel to enable an intensified mass transfer between two fluids and a liquid-liquid separation device. Experiments were performed with deionized water as a continuous phase and various organic solvents as a dispersed phase. The organic droplets are generated by direct coflowing injection through a needle located in the center of the microchannel. Droplet generation and flow patterns were optically investigated in straight and winding channels. The adjacent separation unit continuously splits the organic, dispersed phase. The combined effects of gravity, wetting characteristics of the wall material and capillary forces nearly completely separate the two fluids over a wide range of flow rates. Based on the mixer-settler system, a microfluidic extraction unit is proposed that enables a multistage counter-current arrangement.
Liquid-liquid extraction is one of the most important unit operations with a broad field of applications. During the past few years, research activities have been increasing in the area of microextraction due to the evident advantages of microchannel equipment. While there is a sweeping number of publications on the topic of the procedure of microextraction using cocurrent flow, there are still some difficulties in accomplishing multistage processes as the countercurrent extraction, such as mixer-settler arrangements. This is due to the fact that it is difficult to achieve a continuous stable phase separation with high throughput. Additionally, it is also challenging to balance the pressure loss with micropumps after every stage. Both of these processes are essential for the countercurrent extraction and, therefore, at the current state of affairs, they pose a bottleneck. This field of research bears a high development potential in order to improve these processes using microchannel equipment and to realize a multistage countercurrent extraction with high effectiveness. In this paper, different phase separation devices and their particular separation principles are presented whereas the focus lies on the continuous separation. Additionally, some experimental as well as theoretical concepts for the conduct of a multistage countercurrent extraction are outlined.
Enantioselective liquid-liquid extraction (ELLE) is a powerful alternative to common technologies for separation of racemic mixtures. The first application of ELLE for aromatic acids in liquid-liquid extraction columns is described. The ELLE is investigated experimentally and theoretically for phenylsuccinic acid (PSA) as a representative for aromatic acids. A racemic mixture of (R/S)-PSA is separated with hydroxypropyl-b-cyclodextrin as selector molecule. The ELLE obtained the highest operative selectivity (a op 5 1.8-2) for low pH-values and temperatures. Because of the low operative selectivity, a countercurrent process is necessary to separate both enantiomers completely. The countercurrent process is investigated in process intensified extraction columns (Ø in 5 15 mm) with a high number of equilibrium stages. The experiments demonstrate a good symmetric separation with an enantiomeric excess of 60% and yields of 80% for both enantiomers. Finally, the back extraction is investigated to recycle the selector molecule and increase the efficiency.
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