[CCoE Notice] ChBE Seminar: "Olefin Polymerisation Reactors: Heat Transfer, Lab Tools and Condensed Mode Operation"

[Grayson, Audrey A]

ChBE Dept. Seminar
10:30am-11:30am, Friday, November 22, 2013
Rm W122
"Olefin Polymerisation Reactors: Heat Transfer, Lab Tools and Condensed Mode Operation"
Timothy McKenna
Universite de Lyon


ABSTRACT: In the case of a fluidized bed reactor for the production of polyolefins, the injection of gas will fluidize the particles in the bed, and will cause a complex flow field to develop, leading to the mixing (and perhaps segregation) of the powder.  Particles can therefore be exposed to different velocity fields, and thus heat transfer conditions.  The heat transfer conditions will have an impact on the particle temperature and, therefore, on intraparticle rate of polymerisation and on the polymer molecular weight.  Depending on temperature, molecular weight, and *-olefin content, the polymer may soften if overheated.  If the mixing conditions in the particle cloud favour particle-particle contact, then the softened particles can eventually agglomerate, leading to lump or chunk formation.  If this becomes serious, then bed operation can become compromised and one finds a single particle with a volume of several cubic metres!



For a number of reasons that will be discussed in the seminar, one of the major stumbling blocks in developing an accurate description of the particle, and the impact of the reactor environment on the particle is a lack of appropriate experimental tools.  To overcome (at least in part) this challenge, our group has adapted a specially conceived packed bed stopped flow minireactor (2.5mL) suitable for short (down to 0.1s) pulsed gas phase polymerisations, and another adapted for solution or slurry polymerisations. In the gas phase reactions we have studied the influence of temperature, gas solid relative velocities, olefin concentration and bed particle properties on the nascent polymer properties and on the short time activity profile of different metallocenes supported on silica particles.



In a related vein, the rate of polymer production is limited by the rate at which the heat of the polymerisation can be removed from the reactor. In the condensed mode operation, an inert condensing agent (ICA) like pentane or hexane is added to the feed stream, and provides the possibility of partial condensation of reactor feed in an external heat exchanger. By vaporisation of the liquefied portion of feed stream in the reactor, a higher amount of heat can be removed from the reaction environment.  We used a lab-scale gas phase reactor, the effect of presence of ICA (hexane in the current study) in the gas phase composition during ethylene polymerisation on heterogeneous catalyst is investigated. The catalyst used is conventional Ziegler-Natta supported on MgCl2. The presence of ICA in the gas phase composition is believed to increase the diffusivity of ethylene through the polymer phase.  The raised ethylene diffusivity will induce a lower level of reactant concentration gradient through the growing polymer particle. The enhanced ethylene solubility and diffusivity in the polymer phase in the presence of ICA results in higher rate of polymerisation thanks to higher availability of monomer at the active sites.

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