(52d) Experimental and Computational Investigation of Slow and Dense Granular Flow in Moving/Pebble Bed Type Reactors

Moving bed reactors are used in chemical and  petrochemical  industries in situations where there is a need to replace deactivated catalysts with new or regenerated catalysts. They are also under consideration as a 4th generation nuclear reactor known as Pebble Bed Reactor (PBR). The granular flow in moving/pebble bed reactors is of slow and dense type. The basic physics governing such flows is not yet fully understood and relies on experimental and computational investigations to extract useful information. It is important to study the movement of solids in moving/pebble beds, which can provide useful information about Lagrangian trajectories, residence time distributions, velocity fields, and the presence and extent of dead zones, if any. Conventional optics based velocimetry techniques are of limited use for investigation of granular flow in these reactors; as these systems are dense and opaque.  Radio-isotopes based non-invasive techniques such as radioactive particle tracking (RPT) are suitable for such applications and can provide detailed information about solids flow fields, overall and local residence time distribution, stagnant zones and their sizes, and many other parameters. Discrete element method (DEM) based simulations can provide complete information in 3-D about granular flow in these reactors. DEM simulations rely on calculation of contact forces which are evaluated using phenomenological models. There is a lack of contact force models developed from the first principles and this demands assessment of contact force models with experimental bench-mark data. The design and development of a continuous pebble re-circulation experimental set-up, which simulates the flow of solids in a moving/pebble bed test reactor that measures one foot in diameter and one foot in height, is carried out as a part of this work. Advanced radioisotopes based flow visualization techniques such as RPT are implemented around this continuous pebble recirculation experimental set-up mimicking cold flow operation of moving/pebble bed reactors. RPT experiments provided crucial information about solids movement in terms of Lagrangian trajectories, velocities, residence times etc. EDEM™ (Experts of Discrete Element Method) is a commercial DEM based software from DEM Solutions Ltd., UK and is used to simulate a slow and dense granular flow. Hertz-Mindlin contact model (with no-slip) is used in these DEM based simulations. Obtained RPT results served as a benchmark data for the assessment of contact force model used in these DEM based simulations. EDEM^TM based simulation results are in fair agreement with RPT experiments results.