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Kinetics, Catalysis, and Reaction Engineering
Modeling Condensed Mode Operation for Ethylene Polymerization: Part III. Mass and Heat Transfer Arash Alizadeh, Farhad Sharif, Morteza Ebrahimi, and Timothy F.L. McKenna Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00330 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018
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Manuscript Type: Article
Modeling Condensed Mode Operation for Ethylene Polymerization: Part III. Mass and Heat Transfer Arash Alizadeh a,b,c, Farhad Sharif a, Morteza Ebrahimi a, Timothy F.L. McKenna b,*
a
Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, PO Box 15875-4413, 424 Hafez Ave, Tehran, Iran b
C2P2 – LCPP Group, UMR5265 CNRS, ESCPE Lyon, Université de Lyon, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne, France
c
Dutch Polymer Institute DPI, PO Box 902, 5600 AX, Eindhoven, Netherlands
*
Author to whom correspondence should be addressed:
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Abstract The polymer flow model is employed to predict the transient behavior of the concentration and temperature profiles within the growing particles during gas-phase ethylene polymerization in the absence and presence of n-hexane as the induced condensing agent. It is demonstrated that by accurate estimation of the model parameters, one can precisely describe the impact of gas phase composition on the reaction rate. The simulation results show that the experimentally observed change in the polymerization rate under the various operating conditions is the direct result of alteration in the average concentration of ethylene in the particle; the average ethylene concentration not only depends on its equilibrium concentration but is also controlled by the significance of the mass transfer resistance through the growing particles. This is while the effect of partial pressure of n-hexane and ethylene on the thermal behavior of the particle is found to be insignificant for the studied experiments.
Figure for Abstract:
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1. Introduction Polyethylene (PE), with a projected production capacity of more than 100 million tons in 2020, will continue to be the most widely used plastic resin in the world for some time to come, and low-pressure catalytic processes are expected to account for more than three-quarters of this production.1 Today, the heterogeneously-catalyzed gas-phase polymerization of ethylene in fluidized bed reactors (FBRs) is the most commonly used technology for the production of PE. One of the principle reasons for this wide spread use of gas phase processes is the wide range of products that it can be used to manufacture. FBRs are the only reactors used to manufacture PE in gas phase at the commercial scale (with the very recent exception of the Hyperzone recirculating reactor) since they provide much higher gas-particle relative velocities than stirred beds, and therefore acceptable rates of heat removal.2 Nevertheless, the highly exothermic nature of ethylene polymerization and the poor heat transfer characteristics of the gas phase (in comparison with the slurry phase) can pose significant problems for heat removal, and thus are the main factors limiting production rates in PE processes. Due to ever-increasing demand for PE, it has become necessary to improve the heat removal efficiency of FBRs, and for reasons discussed elsewhere the most appropriate way to do this is to use so-called condensed mode cooling.2 In this method, induced condensing agents (ICAs), usually alkanes, are added to the reactor recycle stream. Along with any comonomers already present in the process gas, these ICAs are partially liquefied in an external heat exchanger, and then injected into the reactor in the form of small droplets along with the gaseous components.3-5 The evaporation of the liquid droplets inside the reactor absorbs additional polymerization heat and thus results in the enhanced production capacity of the unit. It is also common practice to run a fluidized bed reactor under conditions where chemically inert alkanes are added, but not liquefied in order to increase the heat capacity of the gas stream, thereby allowing more heat to be removed.6 It is also widely accepted that the evaporation of liquids injected into the reactor is relatively rapid, and occurs in the first meter or so of the bed above the distributor plate.7 In other words, even if an alkane is liquefied before being fed into the reactor, it will be present as a vapor in most of the bed. For this reason, we will focus exclusively on the role of vaporized ICA in the remainder of this paper. Recall that an ICA does not directly influence the chemical nature of the active sites, but as recent papers from our group have shown, they can have a major impact on the rate of polymerisation and polymer properties.8-11 Therefore, in the present series of the modeling studies, it is attempted to provide a phenomenological description for the observed impact of ICAs. In this respect, the current paper represents an extension of the interpretative scope of the first part.12 3 ACS Paragon Plus Environment
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In the comprehensive experimental study by the authors,9 it was observed that the instantaneous rate of ethylene polymerization is promoted in the presence of various commonly used ICA compounds. This observation was attributed to the well-known cosolubility effect. The thrust of our first modeling paper,12 was therefore to quantify the equilibrium concentration of ethylene in the polymer phase in the absence and presence of n-hexane as the ICA. The Sanchez-Lacombe (SL) and PC-SAFT EoS were used in this previous paper to model the equilibrium solubility data of binary systems of ethylene-PE and n-hexanePE, and the ternary system of ethylene-n-hexane-PE obtained using the pressure-decay technique of Yao et al.13,14 The thermodynamic simulations demonstrated that the equilibrium concentration of ethylene in the amorphous phase of PE rises with the partial pressure of n-hexane (i.e., the cosolubility effect). It nevertheless became clear that the cosolubility phenomenon cannot be only explanation for the impact of ICA on the polymerization rate, particularly at the beginning of the polymerisation when the enhancement effect is much greater than predicted based on changes in solubility. It was also noticed that the magnitude of the rate enhancement eventually diminishes until it reaches a value very close to that predicted by either PC-SAFT or SL EoS. The deviation of experimental data from the equilibrium prediction suggests that the effects of mass and heat transfer resistances are likely important and must also be taken into account; n-hexane improves the heat capacity of gas phase and can substantially increase the ethylene diffusivity through the polymer phase due to its plasticization effect. This, in turn, indicates that to thoroughly analyze the influence of gas phase composition on the polymerization rate, a single particle modeling approach needs to be adapted as the framework to integrate the effects of thermodynamics and transport phenomena. A variety of single particle models (SPMs) with different degrees of complexity can be found in the open literature. It would be beyond the scope of this study to provide a full overview of the SPMs. However, the interested reader is directed to the recent review paper by the authors that thoroughly covers this topic.15 Among the various SPMs, the polymer flow model (PFM) and multi-grain model (MGM) can be considered as the most widely adapted and used models in this field during the last three decades.16 The PFM offers the advantage of a relatively simpler mathematical formulation than the MGM without any great reduction in applicability, and this substantially facilitates the numerical solution of the resulting PDEs. Given the explanation above, the PFM approach originally developed by the group of Kiparissides17 will be adapted in the present study. To the best of our knowledge this is the first time that an SPM is being utilized as the platform to investigate the impact of gas phase composition (primarily being composed of
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ethylene and n-hexane) on the quality of mass and heat transfer in the growing particles and ultimately polymerization rate. The article is organized as follows. First, the experiments from an already published work,9 designed to systematically investigate the effect of n-hexane, as one of the most commonly used ICAs, on the polymerization rate will be presented. In the following section, a process model based on the SPM is developed to describe the transient behavior of the concentration and temperature profiles within the growing particles during the course of reaction. As the accurate estimation of the ethylene diffusivity in the particle plays a critical role in the performance of the developed model, a phenomenological framework is presented in the succeeding section to calculate the diffusivity of penetrants in the porous PE particles. At the next step, all of the parameters of process model are determined under the various operating conditions studied in this paper. Subsequently, the developed model is applied to predict and describe the effect of process condition on the polymerization rate. Finally, by implementing the process model, a phenomenological explanation is provided for the experimentally observed trends.
2. Experimental Section
2.1. Polymerization Experiments The experimental data used to validate the novel modelling approach presented in the current study have been previously published in a paper from our group.9 The list of experiments considered in this work and the gas phase composition of each polymerization reaction is summarized in Table 1. The run number assigned to each polymerization reaction under the specified operating condition in Table 1 will be used consistently to refer to that reaction condition throughout the current paper. Polymerizations were run at 7 and 12 bars using a commercial Ziegler-Natta catalyst, with a gas phase temperature of 80°C (± 1°C). The interested reader if referred to reference 9 for more details on the experimental procedure, reactor setup and monitoring. Table 1. The summary of the polymerization reactions analyzed in the current study with the partial pressure of the components (in bar) present in the gas phase composition at each reaction condition. All runs contain
