Gas distribution and particle mobility in a model fluid-bed polymer recycling reactor

The effect on fluidisation quality of a number of sieve-tray and tuyere distributor designs has been investigated in a cold scale model of a proposed pyrolytic fluid-bed polymer recycling reactor. Fluidisation quality in a scale model of the proposed reactor design was assessed visually, by comparing bed pressure drop to solids weight per unit area, by measuring activity of individual nozzles and by the use of probes measuring film heat transfer coefficients at a number of locations across the bed. It was found that all the distributors gave good-quality fluidisation at fluidising velocities greater than three times the minimum-fluidising value, except in a dead zone near the distributor. This appears to support the standard models of fluid-bed distribution, however a more surprising observation was that uniform activity of all orifices did not necessarily mean that fluidisation was uniform elsewhere in the bed. These dead areas could trap incompletely reacted polymer particles and hence favour the accumulation of carbonised plastic agglomerates in the full-scale reactor bed. fect of the configuration of internals on the dispersion coefficient of polymer particles was also appraised, by injecting a representative polymer sample, defluidising the bed, dissecting it and recording the position of sample particles. Experiments using Positron Emission Particle Tracking (PEPT) were conducted, giving insight into circulation patterns of the particles in the bed. Both dispersion experiments and PEPT demonstrated the presence of recirculation vortices in the free space between successive stacks of internals and extending into the stacks. These results suggest injecting the polymer at several locations, in order to make use of these vortices for dispersion without overloading them during the initial melting stage.