Towards resource-efficient washing within the continuous vacuum screw filter

Continuous manufacturing offers substantial potential for process intensification in pharmaceutical production, including improved product quality, reduced waste, and enhanced flexibility. While continuous crystallization technologies are well established, continuous particle isolation remains a bottleneck for realizing end-to-end continuous crystal process chains. To address this gap, the modular continuous vacuum screw filter was developed integrating continuous filtration, washing, and drying. In this study, the two-stage washing process within this apparatus was systematically optimized for particles in the size range of 355–560 μm with the objective of minimizing washing liquid consumption while fulfilling the requirements for continuous particle isolation, including high purification performance to residual impurity loading of less than 5 ppm, residual moisture below 1%, and maintenance of the particle size distribution. Based on wash curve analysis, various washing liquid dosing units were evaluated with respect to washing efficiency. The results demonstrate that capillary-based dosing units provide significantly higher washing efficiency than nozzle-based dosing units due to gentler application of the washing liquid to the filter cake surface. Using a capillary with circular outlet, the washing liquid consumption in each washing step was reduced by 80%, corresponding to a wash ratio of 4, while achieving the product quality specifications. Overall, optimization of the two-stage washing process enabled a substantial reduction in washing liquid consumption, improved product quality in terms of product purity and residual moisture, as well as elimination of an additional drying step, highlighting the ecological and economic potential of the continuous vacuum screw filter for continuous particle isolation.