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Effect of condensable materials during the gas phase polymerization of ethylene on supported catalysts

Abstract : Fluidized bed reactors (FBR) are the only commercially viable technology for the production of polyethylene in the gas phase since the polymerization is highly exothermic and the FBR is the only type of gas phase reactor that offers adequate possibilities of heat transfer. The highly exothermic nature of this polymerization effectively poses many problems for gas phase operation and can limit the production of a certain process. However, in recent years the fluidized bed processes have been improved with new technologies. In particular, the addition of inert (usually liquefied) hydrocarbons allows one to increase the amount of heat removed from the reactor. These compounds increase the heat capacity of the gas phase and, if injected in liquid form, also evaporate and thus absorb even more heat from the reaction medium efficiently. This is known as a condensed mode operation. In it, one uses compounds that can be liquefied in the recycle condenser, and which are called Induced Condensing Agents (ICA). The use of ICA is extremely important from an industrial point of view. The injection of ICA can have many different physical effects at the level of the growing polymer particles. For instance, adding these compounds can cause changes in solubility and other physical properties, which can facilitate the transport of ethylene and hydrogen to the active sites of the catalysts. It is thus very important that the physical phenomena related to the sorption equilibrium of the monomer(s) and other species from the gas phase to the polymer phase, and their diffusion on the polymer matrix at the active sites should be accounted for. In addition to having an effect on the kinetics, these phenomena can also impact the structure of the polymer molecules and consequently qualify the characteristics of the polymer. Identifying the behavior of these phenomena under process conditions and control variables of the hydrogen/ethylene ratio and the comonomer/ethylene ratio with ICA are central objectives of this study. A series of ethylene homo- and co-polymerizations in the gas phase were carried out using a commercial Ziegler-Natta catalyst in the presence of ICA (propane, n-pentane, and n-hexane). We investigated the effect of temperatures, the partial pressure of ICA, hydrogen, and comonomers on the behavior of the polymerization. It was found that adding ICA significantly increased the reaction rate and average molecular weights at a given temperature. It was also unexpectedly observed that increasing the reactor temperature in the presence of an ICA actually led to a decrease in the overall reaction rate. These results were attributed to the socalled cosolubility effect. In reactions in the presence of different hydrogen concentrations, for an ICA/C2 ratio much larger than the H2/C2 ratio, the effect of ICA on ethylene solubility can counteract the decrease in average molecular weight caused by the presence of hydrogen. The impact of ICA on the rates of copolymerization reactions is more pronounced in the initial stages, losing strength due to the effect of the comonomer. Finally, an evaluation of the kinetics of crystallization under isothermal conditions for mixtures of different ICA:HDPE concentrations showed that the crystallization time is significantly higher for systems rich in ICA than for dry polymer
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Fabiana Nascimento de Andrade. Effect of condensable materials during the gas phase polymerization of ethylene on supported catalysts. Chemical and Process Engineering. Université de Lyon, 2019. English. ⟨NNT : 2019LYSE1016⟩. ⟨tel-02091507⟩

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