Quantification of Channeling in Polyethylene Resin Fluid Beds Using X-Ray Computer Assisted Tomography (CAT)

ABSTRACT

The determination of fluid distributions in fixed and fluidized beds was given a new dimension with the implementation of tomographic imaging 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 provide an estimate of deviation from expected fluid behaviour, which is due to the presence of hot spots or gas by-passing. In this study, the fluidization characteristics of commercially available LLDPE and HDPE 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 bed heights at L/D ratios of one, two and three. The fluidization velocities were varied between one and three times the minimum fluidization velocity for each sample. All experiments were performed at ambient conditions. The X-ray CAT scanner images describe the solid and gas distributions at a resolution of 400 μm by 400 μm in cross-section and 3 mm in thickness. Hundreds of images were collected and analysed. It is very important to note that in most cases the slice-average voidage is constant through the column length. However, there is considerable radial variability within each slice. This variability is measured as a function of position, and in fixed positions as a function of time. In this paper, we try to quantify these observations through comparisons of the voidage distribution of a fluid bed to the expected corresponding distribution of a uniform bed. Deviations from the expected mean are classified in distinct categories. The areas of consistently high gas concentration are identified as areas of gas channelling. Having identified these areas, we proceeded with the determination of the formation and propagation of gas channels in a fluid bed both in the spatial and temporal domains. We found that the simple fluid-bed systems we used in the laboratory exhibited a complicated gas channelling picture at relatively low L/D and relatively low fluidization numbers. Channels can appear in the bed and can have a variety of characteristics and relative positions in the bed while the operating conditions vary only slightly. The implication of such channels in the operation of gas-phase polymerization reactors is also presented.

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