MONITORING THE FLUIDIZATION CHARACTERISTICS OF POLYOLEFIN RESINS USING X-RAY COMPUTER ASSISTED TOMOGRAPHY SCANNING

摘要

Flow visualization of chemical reactor phenomena was given a new direction with the implementation of process tomography techniques These techniques can provide valuable information on flow phenomena in heterogeneous reactors. In particular. visualization and understanding of flow phenomena in gas phase polymerization reactors can aid us in achieving a better temperature distribution in the reactor and higher quality of products. In this study, the fluidization characteristics of a series of commercially available LLDPE, HDPE and IPP resins were investigated. The experiments were run in a small column of IO cm in diameter (D) and variable bed heights (L), at L/D ratios of one. two and three. The fluidization velocities were varied between one and three times the minimum fluidization velocity of each sample. The particle size distributions varied from very narrow mesh sizes to full panicle size distributions as they came out of commercial gas phase polymerization reactors. The resins also varied in sphericity. In several samples, there was some rubber present in the panicles resulting in a ''stickiness'' characteristic of the resin. All experiments were performed at ambient conditions and inside the x-ray Computer Assisted Tomography (CAT) scanner facility. The CAT scanner images provided areal gas voidage distributions as a function of position and operating conditions at a resolution of 400 mu m by 400 mu m in cross section and 3 mm in thickness. A large number of images were collected and analyzed. Experimental results clearly established a set of fluidization experiments that gave relatively uniform voidage distributions, whereas other experiments had extremely non-uniform voidage distributions. Segregation and channeling were also observed under various operating conditions. It was found that certain regions of the bed were characterized by high voidage while others by very low voidage. These regions varied with experiments using the same resin but different velocities and L/Ds. The spatial variability of the voidage was also described in terms of statistical moments, frequency distributions and variograms. The voidage distributions were found to vary from wide unimodal to tri-modal of the same range for experimental conditions that, at first sight, did nor appear to be significantly different. This paper describer the various categories of voidage distributions observed and provides some first correlations between voidage and operating parameters. A discussion of the implications of such phenomena to the performance of a gas phase polymerization reactor is also presented. [References: 8]