Study of Sorption, Heat and Mass Transfer During Condensed Mode Operation of Gas Phase Ethylene Polymerization on Supported Catalyst

Abstract

In the current thesis study it is intended to investigate the potential effect of the inert condensing agent (ICA) of n-hexane used in condensed mode operation on the solubility of ethylene in produced polyethylene (PE) and consequently the quality and rate of gas phase ethylene polymerization on supported catalyst under reactive conditions. This is the first time for such a study. Performing the set of designed polymerization reaction experiments using a lab-scale stirred-bed gas phase reactor, it is observed that the instantaneous rate of ethylene polymerization increases in the presence of n-hexane, thus supporting the initial speculation of the effect of n-hexane on the enhancement of the ethylene solubility in polymer known as “cosolubility” phenomenon. In order to have a better picture and understanding, the averaged instantaneous rate of polymerization in presence of n-hexane is normalized with the one without any n-hexane. Consequently, this helps to see that while the effect of n-hexane increases proportionally to its partial pressure in the gas phase composition, this effect is more pronounced at the initial steps during the course of polymerization. In the current thesis study for the first time, the Sanchez-Lacombe EOS as one of the most widely applied thermodynamic models in polymer industry is adapted and developed in order to study not only the solubility but also concentration of ethylene in polyethylene in the absence and presence of an inert condensing agent in order to quantify the speculated cosorption phenomenon under the reactive polymerization condition. By incorporating this thermodynamic model to describe the solubility of ethylene in polymer into a single particle model like Polymer Flow Model (PFM) to estimate the concentration and temperature gradient through a growing polymer particle, it is ultimately attempted to predict the effect of change in the process operating condition by addition of n-hexane as the ICA to the gas phase composition. Finally in the current thesis study, it is demonstrated how the thermal effect associated with the heat of sorption of ICAs can have a positive effect in terms of avoiding particle over-heating under certain circumstances like its temporary exposition to the defluidized regions inside a fluidized bed reactor (FBR) as a possible undesirable operating condition for this type of reactor set-ups.