Analysis of Crystal Size Dispersion Effects in a Continuous Coiled Tubular Crystallizer: Experiments and Modeling

Abstract

Continuous processing gains importance in the fine chemical and pharmaceutical industries where crystallization is an important downstream operation. Seeded cooling crystallization of the L-alanine/water system was investigated under similar conditions, i.e., temperature interval, cooling rate, and seed material, both in a stirred batch vessel and in a continuous plug flow crystallizer in the coiled flow inverter (CFI) design with horizontal helical tube coils (ID = 4 mm) and frequent 90°bends of the coils. Short-cut calculations based on characteristic time scales and the Damkoḧler number allow for comparing the batch and continuous crystallization processes. The experimental results reveal crystal growth and growth rate dispersion to be dominating on the product crystal size distribution (CSD). However, at low flow rates of approximately 31 g min −1 , a moving sediment flow of the slurry was present in the CFI crystallizer, resulting in further size dispersion effects. Elevated flow rates of approximately 40 g min −1 resulted in a more homogeneous suspension flow and a product CSD comparable to batch quality. Simulation studies based on a population balance equation model strengthen the hypothesis of the solid phase residence time distribution (RTD S ) to be more spread in the moving sediment flow regime, leading to a wider product CSD.

Continuous Downstream Processing of Amino Acids in a Modular Miniplant

“…These measures cannot be estimated directly from process data. More sophisticated tools, i.e., population balance equation modeling , , , are required to perform this task.…”

“…). This indicates a nonhomogeneous radial distribution of the crystals in the crystallizer, e.g., a moving sediment flow regime, as found in previous laboratory‐scale studies (see Sect. ).…”

“…Due to coiling of the process tube, Dean vortices are superimposed on the parabolic laminar flow profile, which aids radial mixing, heat transfer , and particle fluidization , with a narrow residence time distribution (RTD) close to that of ideal plug flow , . The unseeded and seeded cooling crystallization of L ‐alanine from aqueous solution was investigated. Although primary nucleation in unseeded operation was unstable and led to rapid precipitation at restrictions and clogging of the device, continuous seeding stabilized the process.…”

“…Although primary nucleation in unseeded operation was unstable and led to rapid precipitation at restrictions and clogging of the device, continuous seeding stabilized the process. Furthermore, a moving sediment flow regime at lower flow velocities and a homogeneous flow regime at elevated flow velocities were observed . In the homogeneous flow regime, a narrow RTD of the solid phase can be achieved, which leads to a product crystal size distribution (CSD) comparable to that of a comparable batch product .…”

Equipment and Separation Units for Flow Chemistry Applications and Process Development

“…The temperature of the process medium is recorded via non‐invasive K‐type thermocouples. The flow rate at which a homogeneous suspension flow could be ensured is about 40 g min −1 (2.4 kg h −1 ) for the model system L ‐alanine/water and crystals with a size of around 100 µm (1 × 10 −6 m) for this apparatus , , .…”