2D Path of a Radioactive Tracer within a Lamellar Settler: Experiments and Modeling

Determining and controlling the trajectory of solid flocs in a settler are of paramount importance to ensure the desired solid-liquid separation. This work evinces the feasibility of determining the motion of solid particles in a lamellar settler by using radioactive particle tracking and extracting relevant dynamic information from an approximate two-dimensional (2D) reconstruction. The experimental procedure involves simultaneously scanning different regions of the settler with scintillation detectors located at a given distance from the examined vessel while a radioactive tracer particle is freely moving inside. The complete reconstruction considers the system geometry, the attenuation of the medium (water), the activity of the radioactive tracer, and the efficiency of the detectors, determined by calibration. The approximate reconstruction considers that the detectors located closer to the tracer are the ones that record the largest number of counts. Then, searching for the detector with the highest number of counts and considering the ratio of the intensity measured by the closest ones, an approximate tracer axial bidimensional location is estimated. Compared to the results given by the radioactive particle tracking (RPT) technique, although the information extracted is more limited, the approximate experimental procedure has the advantage of not requiring a calibration stage, which could be complex for systems with varying solid holdup. Analysis of the particles' trajectories obtained with this methodology can be quite useful to assess a proper design of the settler. For instance, the upper limit of the working flow rate and the mean residence time of the flocs before settling can be determined. The particle motion is simulated with a volume-of-fluid (VOF) method implemented in Gerris open-access software, and compared to the experimental results.