Experimental investigation of the solid phase residence time distribution in screw extractors

A horizontal screw extractor was designed for the continuous counter-current extraction of artemisinin from dried and ground Artemisia (A.) annua L. leaves. Artemisinin is a drug utilised in the treatment of malaria, a life-threatening disease that causes millions of cases and hundreds of thousands of deaths worldwide. This study examines the impact of the screw angle on the residence time distribution of the solid phase, represented by A. annua L. leaves, during the continuous counter-current process within this screw extractor using the tracer method and theoretical model. For this purpose, the screw angle of an original screw is modified and reduced by 10 % and 20 %, leading to two modified screws. The results of the experimental study demonstrate a nearly constant standard deviation of the normalised residence time distribution function, while the mean residence times increase significantly with lower screw angle. This results in Bodenstein numbers greater than 100, approaching a hypothetical plug-flow behaviour with low back-mixing effects and thus enhances an efficient transport of bulk material through the extractor increasing extraction efficiency.