A continuously operated helically coiled flow tube (HCT) crystallizer is investigated for crystal growth. Inline video-imaging is used for crystal shape analysis and residence time estimation of potash alum. The main finding is that there is a size-dependent particle residence time. Large particles move faster through the HCT than small particles. Consequently, small crystals have more time to grow in the HCT. Physical reasons for this behavior are proposed, for example small-scale flow characteristics. In a direct numerical simulation of the instationary Navier-Stokes equations, velocity fluctuations and a secondary flow are identified. The presented flow field may have a different impact on the particles and cause the size-dependent particle residence time. A particle size dependent residence time may potentially narrow the crystal size and shape distribution in such a process, frequently a desired feature in solids' production.

In order to fully characterize crystal aggregates, the orientation of primary particles has to be analyzed. A procedure for extracting this information from three-dimensional microcomputed tomography (µCT) images was recently published by our group. We here extend this method for asymmetrical crystals and apply it for studying the disorientation angle distribution of four potash alum crystal samples that were obtained under various experimental conditions. The results show that for all considered supersaturation profiles, primary particle pairs tend to have the same orientation significantly more often than in theoretical considerations, in which the orientations of primary particles are assumed to be distributed randomly.

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A helically coiled flow tube crystallizer was investigated as a novel device for the shape-selective generation of crystals with potassium alum as the model solid. The shape evolution of the crystal population was analyzed for growth-dominated seeded cooling crystallization. The macro-mixing behavior was characterized by residence time distribution measurements for the fluid phase and the crystals. At the crystallizer outlet, a flow-through microscope was used to analyze the crystal size and shape distribution, where the 3D shape of individual crystals was estimated from 2D projections. The experiments showed a separation of the particle population according to the crystal size. This process allows the controlled shaping of crystals under continuous operating conditions.

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