Nucleation in continuous flow cooling sonocrystallization for coiled capillary crystallizers

Abstract

Nucleation in continuously operated capillary coiled cooling crystallizers is experimentally investigated under the influence of ultrasound. It was found that there is no sharp boundary but rather a transition zone for nucleation under sonication. For this purpose, a tube with an inner diameter of 1.6 mm and a length of 6 m was winded in a coiled flow inverter (CFI) design and immersed into a cooled ultrasonic bath (37 kHz). The CFI design was chosen for improved radial mixing and narrow residence time distribution, which is also investigated. Amino acid l-alanine dissolved in deionized water is employed in a supersaturation range of 1.10 to 1.46 under quiet and sonicated conditions. Nucleation is non-invasive detected using a flow cell equipped with a microscope and camera. Graphical abstract Since the interest and demand for small-scale, continuous crystallization increases, seed crystals were generated in a coiled tube via sonication and optically investigated and characterized. No distinct threshold for nucleation could be determined in a wide range of supersaturations of l-alanine in water

Continuous Cooling Crystallization in a Coiled Flow Inverter Crystallizer Technology—Design, Characterization, and Hurdles

“…The process pump chosen here is a LabDos ® Easy-Load (HiTec Zang GmbH, Herzogenrath, Germany), which can overcome up to 2 bar pressure drop. Further investigations on the pressure loss in the CFI can be found in References [25,56].…”

“…Processes 2021, 9, 1537 2 of 36 Besides narrowing the MZW [17], ultrasound is used and investigated as a nucleation source to generate seed crystals [23][24][25]. Ultrasound has the benefit that no impurities can enter the system with introduced seed crystals [26,27].…”

“…Sonocrystallization is another current topic for small-scale crystallization. The mechanisms of sonocrystallization in solution, influencing parameters, and overview of continuous sonocrystallization in capillaries are given in References [9,25,65]. The primary influence of ultrasound on crystallization processes and nucleation are reduction of MZW [17] and induction time [66], influence on the polymorphic form [16,[67][68][69], and particle size [9][10][11][12][13][14][15].…”

“…The primary influence of ultrasound on crystallization processes and nucleation are reduction of MZW [17] and induction time [66], influence on the polymorphic form [16,[67][68][69], and particle size [9][10][11][12][13][14][15]. In this work, nucleation was continuously performed in an ultrasonic bath, and nucleation supersaturation thresholds were already investigated in detail for the used model system [25]. The chosen setup is most similar to Eder et al [23]; however, no slug-flow or mixing with the undersaturated solution is applied.…”

“…Two anti-plugging strategies are introduced, and we recount how we tested them. Regarding the analysis, a non-invasive sensor to detect whether suspension or solvent was flowing was implemented, and a lowcost concept with a digital camera on a microscope equipped with a glass measurement-cell enabled non-invasive inline observation of the crystals [25,70]. A feasibility study about the connection of nucleation in an ultrasonic bath connected with a crystal growth part in the CFIC is here described, and hurdles are presented in detail.…”