QUANTIFICATION OF CHANNELING IN POLYETHYLENE RESIN FLUID BEDS USING X-RAY COMPUTER-ASSISTED TOMOGRAPHY (CAT)

The determination of fluid distributions in fixed and fluidized beds w as given a new dimension with the implementation of tomographic imagin g techniques. Of particular interest is the time-averaged macroscopic determination of solids and gas in gas-phase polymerization reactors. The voidage distribution coupled with appropriate kinetic models can p rovide an estimate of deviation from expected fluid behaviour, which i s due to the presence of hot spots or gas by-passing. In this study, t he fluidization characteristics of commercially available LLDPE and HD PE resins were investigated using X-ray CAT scanning. The experiments were run in a small column with a diameter of 10 cm (D) and variable b ed heights at L/D ratios of one, two and three. The fluidization veloc ities were varied between one and three times the minimum fluidization velocity for each sample. All experiments were performed at ambient c onditions. The X-ray CAT scanner images describe the solid and gas dis tributions at a resolution of 400 mu m by 400 mu m in cross-section an d 3 mm in thickness. Hundreds of images were collected and analysed. I t is very important to note that in most cases the slice-average voida ge is constant through the column length. However, there is considerab le radial variability within each slice. This variability is measured as a function of position, and in fixed positions as a function of tim e. In this paper, we try to quantify these observations through compar isons of the voidage distribution of a fluid bed to the expected corre sponding distribution of a uniform bed. Deviations from the expected m ean are classified in distinct categories. The areas of consistently h igh gas concentration are identified as areas of gas channelling. Havi ng identified these areas, we proceeded with the determination of the formation and propagation of gas channels in a fluid bed both in the s patial and temporal domains. We found that the simple fluid-bed system s we used in the laboratory exhibited a complicated gas channelling pi cture at relatively low L/D and relatively low fluidization numbers. C hannels can appear in the bed and can have a variety of characteristic s and relative positions in the bed while the operating conditions var y only slightly. The implication of such channels in the operation of gas-phase polymerization reactors is also presented. (C) 1997 Elsevier Science Ltd. All rights reserved.