Particle Lagrangian CFD Simulation and Experimental Characterization of the Rounding of Polymer Particles in a Downer Reactor with Direct Heating

Abstract

Polypropylene (PP) powders are rounded at different conditions in a downer reactor with direct heating. The particles are fed through a single central tube, while the preheated sheath gas is fed coaxially surrounding the central aerosol jet. The influence of the process parameters on the quality of the powder product in terms of particle shape and size is analyzed by correlating the experimental results with the flow pattern, residence time distribution of the particles and temperature distribution predicted by computational fluid dynamics (CFD) simulations. An Eulerian–Lagrangian numerical approach is used to capture the effect of the particle size distribution on the particle dynamics and the degree of rounding. The simulation results reveal that inlet effects lead to inhomogeneous particle radial distributions along the total length of the downer. The configuration of particle/gas injection also leads to fast dispersion of the particles in direction of the wall and to particle segregation by size. Broad particle residence time distributions are obtained due to broad particle size distribution of the powders and the particles dispersion towards the wall. Lower mass flow ratios of aerosol to sheath gas are useful to reduce the particle dispersion and produce more homogenous residence time distributions. The particles’ residence time at temperatures above the polymer’s melting onset is determined from the simulations. This time accounts for the effective treatment (rounding) time of the particles. Clear correlations are observed between the numerically determined effective rounding time distributions and the progress of shape modification on the particles determined experimentally.

Particle Residence Time Distribution in a Concurrent Multiphase Flow Reactor: Experiments and Euler-Lagrange Simulations

“…However, the short contact time and the residence time distribution of the involved phases, especially the solid phase, are rarely assessed. Gomez et al [28] investigated the particle residence time distribution in a directly heated downer reactor under different operating conditions, using the Euler-Lagrange approach (CFD-DEM). The mean particle size used in their work was d 32 = 87.8 µm.…”

“…So far, a combined study comprising experimental and numerical investigations of the particle residence time distribution in a downer reactor has not been reported in the literature. Though Brust et al [34] and Gomez et al [28] already accomplished measurements of the residence time, both authors focused on the measurement of the sheath gas behavior. Thus, no investigation of the solid phase has been achieved.…”