Counter-Current Extraction in Microchannel Flow: Current Status and Perspectives

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.