Polyolefins are polymers produced from olefins such as ethylene and/orpropylene. Although polyolefins can be produced via different productionmethods, the gas-phase polymerization process based on fluidizedbed reactor technology is the most important method for the productionof polyethylene since the 1980’s and also polypropylene is increasinglyproduced via the gas-phase polymerization process. Although fluidizedbed reactors have been employed for several decades in the chemicalindustry, quantitative information on solids motion and macroscopiccirculation patterns is still incomplete.To investigate the macroscopic circulation patterns in a freely bubbling,gas-solid fluidized bed, first the hydrodynamics in two pseudo-2Dcolumns of different width filled with glass beads and Linear Low DensityPolyethylene (LLDPE) particles have been investigated (both exhibitingGeldart B type behavior) experimentally with two optical non-invasivemeasuring techniques. Particle Image Velocimetry (PIV) combined withDigital Image Analysis (DIA) has been developed to determine simultaneouslythe emulsion phase circulation patterns, bubble hold-up, bubblesize and velocity distributions and visual bubble flow rate profiles.The combination of DIA with PIV allows correcting for the influence ofparticle raining through the roof of the bubbles on the time-averagedemulsion phase velocity profiles. The number-averaged emulsion phaseviiSUMMARYcirculation patterns have been measured as a function of fluidization velocity,bed aspect ratio, bed width and bed material. Moreover, with DIAthe average bubble diameter and averaged bubble velocity as a functionof height and fluidization velocity have been determined and found tocorrespond reasonably well with literature correlations. However, thedifference in averaged bubble diameter as a function of the height in thefluidized bed for the two different particle types could not be explainedby the currently available correlations for the bubble diameter. The differencein observed bubble properties is attributed to differences in theparticle collisional properties (coefficients of restitution and the particlefriction coefficient).To verify this hypothesis, the influence of microscopic particle propertieson the hydrodynamics in a bubbling fluidized bed have been investigatedin detail using the Discrete Particle Model (DPM) and theTwo-Fluid Model (TFM). It was concluded that, for the conditions investigated,indeed bubbles are formed due to collisional dissipation ofmechanical energy. Furthermore, the nature (i.e. due to restitutionor friction) of the energy dissipation is important for the shape of thebubbles. In addition it is shown that in a bubbling fluidized bed, theenergy is mainly dissipated by friction between particles and particlesand the wall. The influence of the normal restitution coefficient onthe macroscopic circulation pattern was also investigated with the Two-Fluid Model. The observed influence of the coefficient of restitution inthe normal direction agreed with the influence of the coefficient of restitutionin the normal direction in the DPM. Also the experimental resultsobtained with the PIV combined with DIA measurements for the solidsphase and DIA measurements for the bubble behavior were comparedwith simulations performed with the DPM and the TFM. It was shownthat the trends for the emulsion phase and the bubble phase can bepredicted with the DPM.The solids and bubble behavior in a freely bubbling, three dimensional,gas-solid fluidized bed has been experimentally investigated usingdifferent bed materials, different bed aspect ratios at different superficialgas velocities by performing Positron Emission Particle Trackingviii(PEPT) experiments. The fluidized bed was filled with either glass beadsor with linear low density polyethylene (LLDPE). At lower superficial gasvelocities two distinct vortices appear above each other for both types ofbed material; when the superficial gas velocity is increased, the lowervortex disappears and the top vortex spans the entire length of the bed.Although qualitatively the same phenomena were observed, the timeaveragedsolids phase circulation rate in the fluidized bed filled withLLDPE particles was higher than the time-averaged solids phase velocityin the fluidized bed filled with glass beads. When the bed aspect ratio isincreased from 1 to 1.5, the vortices become elongated without alteringthe solids circulation rate. Differences in the particle-particle collisionalproperties (coefficients of restitution and friction particle coefficients) arebelieved to be the cause of the observed quantitative differences in thebed hydrodynamics via their influence on the bubble properties.Finally, the hydrodynamic behavior of industrial scale bubbling fluidizedbed reactors, a 3D Discrete Bubble Model (DBM) has been used.In the DBM, an Euler-Lagrange model, the bubbles are treated as discreteelements and the bubble trajectories are tracked individually, whilethe emulsion phase is considered as a continuum and is described withthe continuity and Navier-Stokes equations. The main advantage of theDBM is that it fully accounts for the two-way coupling, allowing computationof the prevailing macroscopic circulation patterns in large scalegas-fluidized beds. We have examined the effects of bubble-bubble interactionson the macro-scale velocity profiles using the DBM. It has beenfound that the extent of the macroscopic circulation is significantly increasedby the bubble-bubble interaction forces.