Ultrasound assisted continuous processing in microreactors with focus on crystallization and chemical synthesis: A critical review

Traditionally, chemical industries including pharmaceuticals and food industries rely on batch or semi-continuous operations for the production of variety of chemicals. However, continuous processes are attractive due to smaller reactor volumes, lower operating and labor costs, and better product reproducibility. Recently there has been an increasing trend towards developing novel reactor designs and processes based on continuous flow. Reactors with micro and milli structured elements are also increasingly applied for continuous processing due to their advantages like better heat and mass transfer, large specific surface area, safety and sound scale-up approaches. But these reactors have limitations in handling solids, especially when solid particles (catalysts, solid reactants, products or by-products) are involved in a chemical reaction or when microreactors are applied for intensification of crystallization. One of the proposed solutions is based on the use of power ultrasound as the size of microreactors and size of bubbles generated by ultrasound during the process acoustic cavitation fall in the same range. The present review focuses on the state-of-the-art ultrasonic flow processes and their combination with microreactors aplied for crystallization and chemical synthesis. Ultrasound applications to different micro or milli-fluidic continuous devices, such as slug-flow crystallizers, plug flow, and channels have been discussed. Guidelines on the application of ultrasound in the fields of crystallization and chemical synthesis have been provided in terms of best-operating conditions as well as discussion on the reactor designs. It appears that ultrasound-assisted continuous processes or their combination with microreactors can be an effective way forward giving significant intensification benefits. [Display omitted] •Using ultrasound to mitigate the drawbacks of micro-reactors and obtain synergism.•Intensified processing demonstrated for coupling ultrasound with micro-reactors.•Guidelines for the design and operating conditions provided.•Continuous operation is key to commercial applications.