Improved single phase modeling of propylene polymerization in a fluidized bed reactor

Abstract

An improved model for the production of polypropylene in a gas phase fluidized bed reactor was developed. Comparative simulation studies were carried out using the well-mixed, constant bubble size and the improved models. The improved model showed different prediction characteristics of polymer production rate as well as heat and mass transfer behavior as compared to other published models. All the three models showed similar dynamic behavior at the startup conditions but the improved model predicted a narrower safe operation window. Furthermore, the safe ranges of variation of the main operating parameters such as catalyst feed rate and superficial gas velocity calculated by the improved and well mixed models are wider than that obtained by the constant bubble size model. The improved model predicts the monomer conversion per pass through the bed which varies from 0.28 to 5.57% within the practical ranges of superficial gas velocity and catalyst feed rate.

Introduction

The various advantages of fluidized bed reactors (FBR) such as their ability to carry out a variety of multiphase chemical reactions, good mixing of particles, high rate of mass and heat transfer and their ability to operate in continuous state have made it one of the most widely used reactors for polyolefin production. Consequently, considerable attention has been paid to model propylene polymerization in fluidized bed reactors. Fig. 1 illustrates the schematic of an industrial gas-phase fluidized bed polypropylene reactor. As shown in this figure, small particles of Ziegler–Natta catalyst and triethyl aluminum co-catalyst are charged continuously to the reactor and react with the reactants to produce a broad distribution of polymer particles. The catalyst particles are porous and are composed of small sub-fragments containing titanium active metal. As the monomer diffuses through the porous catalyst, it polymerizes by reaction on the active sites of the catalyst surface. The catalyst fragments become dispersed during the polymerization and the particles grow into the final polymer product (Zacca, Debling, & Ray, 1996). The feed gas, which consists of propylene, hydrogen and nitrogen, provides the fluidization through the distributor, acts as the heat transfer medium and supplies the reactants for the growing polymer particles. The fluidized particles disengage from the unreacted gases in the disengaging zone. The solid-free gas is combined with fresh feed after heat removal and recycled back to the gas distributor. The polypropylene product is continuously withdrawn from near the base of the reactor and above the gas distributor. The unreacted gas is recovered from the product which proceeds to the finishing section of the plant.

In heterogeneous systems, polymerization occurs in the presence of different phases with inter-phase mass and heat transfer and chemical reaction. Phenomena such as complex flow of gas and solids, kinetics of heterogeneous polymerization and various heat and mass transfer mechanisms must be incorporated in a realistic modeling approach. Several different methods for describing the hydrodynamics of the fluidized bed polyolefin reactor have been proposed in the literature. McAuley, Talbot, and Harris (1994) and Xie, McAuley, Hsu, and Bacon (1994)considered the fluidized bed polyolefin reactor as a well mixed reactor. They compared the simple two-phase and the well mixed models at steady state conditions and showed that the well mixed model does not exhibit significant error in the prediction of the temperature and monomer concentration in the reactor as compared with the simple two-phase model at steady state conditions. Choi and Ray (1985) presented a simple two-phase model in which the reactor consists of emulsion and bubble phases. They assumed that the polymerization reaction occurs only in the emulsion since the bubbles are solid-free. Fernandes and Lona (2001) proposed a heterogeneous three-phase model that considers bubble, emulsion and particulate phases with plug flow behavior. Hatzantonis, Yiannoulakis, Yiagopoulos, and Kiparissides (2000) considered the reactor being comprised of perfectly mixed emulsion phase and a bubble phase divided into several solid-free well-mixed compartments in series. Alizadeh, Mostoufi, Pourmahdian, and Sotudeh-Gharebagh (2004) proposed a tanks-in-series model to represent the hydrodynamics of the reactor. Harshe, Utikar, and Ranade (2004) developed a comprehensive mathematical model based on the mixing cell framework to simulate transient behavior in the fluidized bed polypropylene reactors. This dynamic model was coupled with a steady state population balance equation. Rigorous multi-monomer, multisite polymerization kinetics was incorporated in the model. Ibrehem, Hussain, and Ghasem (2008) proposed a fluidized bed comprising of bubble, cloud, emulsion and solid phases and considered the polymerization reactions occurring in the emulsion and solid phases. Their model also accounts for the effect of catalyst particles type and porosity on the rate of reaction.

In the present study, these previous works on the modeling of gas-phase olefin polymerization fluidized-bed reactors were extended to account for the dynamic behavior of propylene polymerization in fluidized bed reactors. To describe the homopolymerization of propylene over a heterogeneous Ziegler–Natta catalyst, a two-site homopolymerization kinetic scheme was employed. Extensive simulations were carried out to determine the influence of key process parameters (such as superficial gas velocity, catalyst feed rate, feed composition and feed temperature) on the dynamic response of the reactor. In order to obtain a better understanding of the reactor performance, a comparative study between the well-mixed, constant bubble size and improved model was carried out.