Polyolefins are polymers produced from olefins such as ethylene and/or propylene. Although polyolefins can be produced via different production methods, the gas-phase polymerization process based on fluidized bed reactor technology is the most important method for the production of polyethylene since the 1980’s and also polypropylene is increasingly produced via the gas-phase polymerization process. Although fluidized bed reactors have been employed for several decades in the chemical industry, quantitative information on solids motion and macroscopic circulation patterns is still incomplete. To investigate the macroscopic circulation patterns in a freely bubbling, gas-solid fluidized bed, first the hydrodynamics in two pseudo-2D columns of different width filled with glass beads and Linear Low Density Polyethylene (LLDPE) particles have been investigated (both exhibiting Geldart B type behavior) experimentally with two optical non-invasive measuring techniques. Particle Image Velocimetry (PIV) combined with Digital Image Analysis (DIA) has been developed to determine simultaneously the emulsion phase circulation patterns, bubble hold-up, bubble size and velocity distributions and visual bubble flow rate profiles. The combination of DIA with PIV allows correcting for the influence of particle raining through the roof of the bubbles on the time-averaged emulsion phase velocity profiles. The number-averaged emulsion phase circulation patterns have been measured as a function of fluidization velocity, bed aspect ratio, bed width and bed material. Moreover, with DIA the average bubble diameter and averaged bubble velocity as a function of height and fluidization velocity have been determined and found to correspond reasonably well with literature correlations. However, the difference in averaged bubble diameter as a function of the height in the fluidized bed for the two different particle types could not be explained by the currently available correlations for the bubble diameter. The difference in observed bubble properties is attributed to differences in the particle collisional properties (coefficients of restitution and the particle friction coefficient). To verify this hypothesis, the influence of microscopic particle properties on the hydrodynamics in a bubbling fluidized bed have be investigated in detail using the Discrete Particle Model (DPM) and the Two-Fluid Model (TFM). It was concluded that, for the conditions investigated, indeed bubbles are formed due to collisional dissipation. Furthermore, the nature (i.e. due to restitution or friction) of the energy dissipation is important for the shape of the bubbles. In addition it is shown that in a bubbling fluidized bed, the energy is mainly dissipated by friction between particles and particles and the wall. The influence of the normal restitution coefficient on the macroscopic circulation pattern was investigated with the Two-Fluid Model. The observed influence of the coefficient of restitution in the normal direction agreed with the influence of the coefficient of restitution in the normal direction in the DPM. Also the experimental results obtained with the PIV combined with DIA measurements for the solids phase and DIA measurements for the bubble behavior were compared with simulations performed with the DPM and the TFM. It was shown that the trends for the emulsion phase and the bubble phase can be predicted with the DPM. The solids and bubble behavior in a freely bubbling, three dimensional, gas-solid fluidized bed has been experimentally investigated using different bed materials, different bed aspect ratios at different superficial gas velocities by performing Positron Emission Particle Tracking (PEPT) experiments. The fluidized bed was filled with either glass beads or with linear low density polyethylene (LLDPE). At lower superficial gas velocities two distinct vortices appear above each other for both types of bed material; when the superficial gas velocity is increased, the lower vortex disappears and the top vortex spans the entire length of the bed. Although qualitatively the same phenomena were observed, the timeaveraged solids phase circulation rate in the fluidized bed filled with LLDPE particles was higher than the time-averaged solids phase velocity in the fluidized bed filled with glass beads. When the bed aspect ratio is increased from 1 to 1.5, the vortices become elongated without altering the solids circulation rate. Differences in the particle-particle collisional properties (coefficients of restitution and friction particle coefficients) are believed to be the cause of the observed quantitative differences in the bed hydrodynamics via their influence on the bubble properties. Finally, the hydrodynamic behavior of industrial scale bubbling fluidized bed reactors, a 3D Discrete Bubble Model (DBM) has been used. In the DBM, an Euler-Lagrange model, the bubbles are treated as discrete elements and the bubble trajectories are tracked individually, while the emulsion phase is considered as a continuum and is described with the continuity and Navier-Stokes equations. The main advantage of the DBM is that it fully accounts for the two-way coupling, allowing computation of the prevailing macroscopic circulation patterns in large scale gas-fluidized beds. We have examined the effects of bubble-bubble interactions on the macro-scale velocity profiles using the DBM. It has been found that the extent of the macroscopic circulation is significantly increased by the bubble-bubble interaction forces.
|Qualification||Doctor of Philosophy|
|Award date||20 Dec 2010|
|Place of Publication||Eindhoven|
|Publication status||Published - 2010|