Why does a liquid segment shift occur in the Archimedes Tube Crystallizer and how does it affect system performance?

The Archimedes Tube Crystallizer (ATC) is a small-scale continuous crystallizer. The ATC features a narrow residence time distribution and flexible residence times, enabling the production of high-quality particles with narrow particle size distributions and tailored particle sizes. The ATC consists of a coiled tube that transports the product in segmented liquid batches via rotation of the apparatus. While operating the ATC, it can be observed that the liquid segments with the suspension shift in position along the apparatus. Thus, the segments occupy different tangential positions within the coils. The hypothesis is that the liquid segment shift is caused by a pressure profile inside the apparatus. Furthermore, it is assumed that the shift in position can help with the suspension of particles, as the direction of the gravitational force on the particle changes. To verify these hypotheses, a mechanistic model of the liquid segment shift was built and used to design an ATC with a predefined pressure profile, showing that the liquid segment shift originates from an internal pressure profile, which depends on the amount of gaseous phase introduced into the apparatus and the coil diameter. The influence of the liquid segment shift on particle suspension was evaluated using CFD based on particle-resolved DNS simulations with two-way coupled particle–fluid interactions, employing a previously validated computational model; these simulations demonstrated improved particle suspension for more strongly shifted liquid segments. The increasingly shifted liquid segments enhance the axial particle distribution from 0.76 to 0.58 (40 rpm), approaching the ideal value of 0.5.