Continuous Sonocrystallization of Acetylsalicylic Acid (ASA): Control of Crystal Size

A continuous tubular crystallizer system with an inner diameter of 2.0 mm and an overall length of 27 m was used to generate acetylsalicylic acid seeds in situ from ethanolic solution via cooling and ultrasound irradiation and to grow the crystals in the tubing with a controlled temperature trajectory. In order to minimize the residence time distribution, air bubbles were introduced into the system to generate a segmented gas-slurry flow. The narrow residence time distribution and the tight temperature control in the small tubing due to the large surface to volume ratio resulted in relatively narrow crystal size distributions of the product. Generally, all experiments clearly demonstrated significant crystal growth for the product crystals in comparison to the seeds and yielded product masses on the g/min scale. Furthermore, it was demonstrated that the size of the product can be easily controlled via fines removal by dissolution due to rapid heating and varying the mass of seeds per mL of solution.