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Seed loading effects on the mean crystal size of acetylsalicylic acid in a continuous‐flow crystallization device

This study investigates the effects of seed loading on the mean crystal size of the model substance, acetylsalicylic acid, crystallized from ethanol in a continuously seeded tubular crystallizer. A hot, highly concentrated ethanolic acetylsalicylic acid solution was mixed with an acetylsalicylic acid‐ethanol seed suspension. Subsequent cooling of the slurry in the tubing promoted supersaturation and hence crystal growth. The tubular shape of the 15 m‐long crystallizer with an inner diameter of 2 mm enabled narrow residence time distributions of the crystals in the pipe and excellent temperature control in the radial direction and along the tubing. Crystals entering the crystallizer had both identical growth conditions in each section and about the same time for crystal growth. Narrow crystal size distributions were achieved with decreasing differences in the volume‐mean‐diameter sizes of the seed and product crystals as seed loadings increased. Decreasing the seed size had a similar effect as increasing the seed loading, since in that case the same amount of seed mass resulted in more individual seed particles. Altering the arrangement of the coiled crystallizer with respect to spatial directions (horizontal, vertical) did not lead to a significantly different outcome. All experiments produced considerably larger product crystals in comparison to the seeds despite relatively short crystallization times of less than 3 min. Moreover, product mass gains of a few hundred percent at a g/min‐scale were achieved. Similarities in product crystal samples taken at different times at the outlet of the crystallizer showed that steady‐state conditions were rapidly reached in the continuous flow crystallization device. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)