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

Flow visualization of chemical reactor phenomena was given a new direc tion with the implementation of process tomography techniques These te chniques can provide valuable information on flow phenomena in heterog eneous reactors. In particular. visualization and understanding of flo w phenomena in gas phase polymerization reactors can aid us in achievi ng a better temperature distribution in the reactor and higher quality of products. In this study, the fluidization characteristics of a ser ies of commercially available LLDPE, HDPE and IPP resins were investig ated. 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 t he minimum fluidization velocity of each sample. The particle size dis tributions varied from very narrow mesh sizes to full panicle size dis tributions as they came out of commercial gas phase polymerization rea ctors. 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 ambien t conditions and inside the x-ray Computer Assisted Tomography (CAT) s canner facility. The CAT scanner images provided areal gas voidage dis tributions as a function of position and operating conditions at a res olution 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 r esults clearly established a set of fluidization experiments that gave relatively uniform voidage distributions, whereas other experiments h ad extremely non-uniform voidage distributions. Segregation and channe ling were also observed under various operating conditions. It was fou nd that certain regions of the bed were characterized by high voidage while others by very low voidage. These regions varied with experiment s 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 distribut ions were found to vary from wide unimodal to tri-modal of the same ra nge for experimental conditions that, at first sight, did nor appear t o be significantly different. This paper describer the various categor ies of voidage distributions observed and provides some first correlat ions between voidage and operating parameters. A discussion of the imp lications of such phenomena to the performance of a gas phase polymeri zation reactor is also presented.