(601i) CFD Simulation of Solid-Liquid Separation in Lab-Scale Centrifuge and Comparison to Experimental Data

In the context of our fight against man-made climate change, the use of nuclear energy is a first-choice solution. The sustainability of this solution partially lies on the reprocessing of nuclear spent fuel which limits both the need to mine new rare fissile materials and the volume of wastes. In France, the reprocessing is based on the PUREX process which itself requires the use of many and varied devices among which the centrifuge. The work reported here focuses on the centrifuge whose function is to stop the particles that were not dissolved by the nitric dissolution step. Above a certain size those particles are not desirable in the liquid-liquid extraction devices responsible for recovering the noble materials. The high levels of radioactivity involved usually prohibit the basic measurements required for a better understanding and optimization of the devices.

In this context, the laboratory of chemical engineering and instrumentation of CEA Marcoule undertook a numerical simulation of centrifuges supported by experimental studies. Data acquisitions were performed on a CEPA LS laboratory scale centrifuge produced by the CEPA Company. Experimental studies gave access to the efficiency of the centrifuge under operating conditions ranging from operating conditions of interest to so-called degraded conditions. Three types of powder were used for this study so as to simulate the insoluble real particles and to provide a variation of sizes and density. Two feed geometries were tested to evaluate the benefits of tangential feed compared to axial feed.

The numerical model used for the simulations was implemented in OpenFoam. It handles three-phase-flows involving gas, water and particles. It is based on the combination of a Volume Of Fluid[3] (VOF) solver with a lagrangian particle tracking solver using the Multi-PhaseParticle-In-Cell[1] (MPPIC) method to account for massive particle-particle collisions as the filtration cake grows at the bowl’s wall. The bowl rotation was modelled by the Multi-Reference-Frames (MRF) method.

The numerical model has been validated under operating conditions of interest. Comparison of simulation results with the experimental data showed a good match in the estimation of the device’s efficiency for two powders over three, for diameters greater than a certain threshold, at a specific centrifugal accelerations interval and two feed rates. Outside of these conditions, the numerically estimated efficiency differs from the experimental one though still of interest. For he last powder, the model failed matching the escaping experimental mass while giving good trends regarding the cutoff diameter highlighting some limitations when the density difference between the liquid-phase and the simulant is too small.

1 Andrews and O'Rourke, International Journal of Multiphase Flow 22, 379 (1996)

3 Hirt and Nichols, Journal of Computational Physics. 39, 201 (1981)