The fluidized bed reactor with a prepolymerization system and its influence on polymer physicochemical characteristics

The fluidized bed reactor with a prepolymerization system and its influence on polymer physicochemical characteristics

F.A.N.Fernandes; L.M.F.Lona

Departamento de Processos Químicos, Faculdade de Engenharia Química, Universidade Estadual de Campinas (UNICAMP), Phone: (+55) (19) 3788-3954, Fax: (+55) (19) 3788-3965, 13083-970, Campinas - SP, Brazil. E-mail: [e-mail], E-mail: [e-mail]

ABSTRACT

This work addresses the influence of a prepolymerization system on the behavior of the fluidized bed reactor used for polyethylene production. Its influence on the polymer's physicochemical characteristics and production was also studied. The results indicate that the use of prepolymerized catalyst particles results in milder temperatures in the fluidized bed reactor, thus avoiding the formation of hot spots, melting of the polymer particle and reactor shutdown. Productivity can be enhanced depending on the operational conditions used in the prepolymerization reactor.

Keywords: gas-phase polymerization, simulation, polyethylene, fluidized bed reactor, effect of prepolymerization.

INTRODUCTION

One of the main problems of gas-phase polymerization in the fluidized bed reactors (FBR) is the overheating of the polymer particle. Particle overheating can lead to extensive particle agglomeration that can clog inlet and outlet orifices, thus forcing the reactor to shutdown. The addition of a prepolymerization system can reduce particle overheating and agglomeration. Prepolymerization is also a way of controlling particle growth and morphology, especially in the early stages of particle development when the concentration gradient inside the particle is very steep. Experimental results show that prepolymerization improves particle morphology and leads to slightly higher production rates (Hutchinson et al., 1992; Chu et al., 2000).

Although several papers have presented FBR models (McAuley et al., 1994; Hatzantonis et al., 2000; Fernandes & Lona, 1999, 2001), little has been done to model the prepolymerization reactor in connection with the FBR. Hence, little information is available in the literature on the entire polymerization reaction system and its capabilities.

Recently, the FBR model has been further developed to take three phases into account: the bubble, the emulsion and the particulate phases (Fernandes & Lona, 2001). This modeling technique offers a more complete understanding of the FBR used in polymerization, allowing study of reactor behavior and prediction of the physicochemical properties of the polymer. Thus, a thorough survey of the potentialities, capabilities and limitations of FBRs can be carried out using new computer-aid techniques prior to lab- and pilot-plant-scale tests.

In this work we have extended the model presented in Fernandes and Lona (2001), including a CSTR as a prepolymerization reactor before the fluidized bed reactor in the gas-phase polymerization of ethylene, to study the influence of prepolymerization on the overall polymerization system. Industrial operational conditions were simulated in order to study the behavior of the system, as well as its influence on productivity and on the physicochemical characteristics of the polymer.

FLUIDIZED BED REACTOR

The fluidized bed reactor has a unique physical design, with gas and polymer particles flowing in opposite directions. The gas is fed in at the base of the reactor and splits into two phases: bubble and emulsion. Catalyst or prepolymer particles are fed in near the top of the reactor, and while the polymerization reaction occurs, the particles grow, increasing in weight and size. Particle segregation occurs in the reactor according to particle weight, so the full-grown polymer particles are removed at the base of the reactor. Nonreacted gases leave the reactor after passing through the disengagement zone.

Prepolymerization is generally carried out in continuous-stirred tank reactors (CSTR) located before the fluidized bed reactor. Catalyst particles are fed into the CSTR along with ethylene and comonomers to yield prepolymer. Afterwards, these prepolymerized catalyst particles are fed into the FBR to complete the ethylene polymerization. A diagram of the industrial fluidized-bed reactor system using a CSTR as the prepolymerization reactor is shown in Figure 1.

This study was carried out using the phenomenological model developed by Fernandes and Lona (2001), modified to include a continuous-stirred tank reactor (CSTR) as a prepolymerization reactor before the main FBR. The FBR model assumes: (a) the fluidized bed comprises bubble, emulsion and particle phases; (b) polymerization reaction occurs only in the particle phase; (c) the emulsion phase is at its minimum fluidizing conditions; (d) gas in excess of that required for minimum fluidization passes through the bed as bubbles; (e) bubbles grow only to a maximum stable size and travel up the reactor in plug-flow regime; (f) the emulsion phase also travels up through the bed in plug-flow regime; (g) polymer particles flow downwards and segregate within the bed according to size and weight; (h) particle diameter is not constant; (i) ethylene, 1-butene, 1-hexene, hydrogen and nitrogen are present as components of the gas phase; (j) copolymerization of the ethylene occurs; (k) the kinetic model takes propagation and chain transfer rates into account; (l) a two-site kinetic model is assumed; (m) radial concentration and temperature gradients within the bed, resistance to heat and material transfer between gas and solids and elutriation are assumed to be negligible. The FBR model and its equations are described in more detail in Fernandes and Lona (2001).

The equations for the CSTR that was included in the polymerization system are shown in Table 1.

The FBR model was modified so that the prepolymer characteristics (polymer moments and mass flow rate) could be used as inputs in the FBR. Prepolymer flow rate replaced catalyst flow rate and the prepolymer moments were used as the initial moments of the polymer in the FBR model.

Validation of the model was carried out using data reported in patents (Dechellis & Griffin, 1999; Agapiou et al., 1996). Runs used in the validation are shown in Table 2. Results of the validation simulations are shown in Figure 2.

SYSTEM SIMULATIONS

Simulations were carried out to analyze the influence of prepolymerization. Special attention was given to the mean temperature of the FBR, observing whether or not it would rise above the polymer melting temperature (~ 420 K, temperature at which the polymer melts, clogging the feed and removal points of the reactor, resulting in the shutdown of the reactor). Data for the simulations, obtained from the patent, account for real industrial conditions and are summarized in Tables 3 and 4 (Dechellis & Griffin, 1999). The kinetic model used and the kinetic constants are shown in Table 5.

RESULTS

The results for medium weight average molecular weight polyethylene (Mw ~ 100000 g/mol) indicate that using prepolymerization, the mean temperature in the FBR can be 6 K lower than in the system not using prepolymerization (Figure 2). An increase in prepolymerization time lowers the mean temperature in the FBR, since part of the heat of reaction will be generated in the prepolymerization reactor.

This heat release in the prepolymerization reactor, specially in the early stages of polymer formation is beneficial since better temperature control can be achieved. A controlled temperature in the prepolymerization reactor and a milder temperature in the FBR can be extremely important, since this improves the morphological formation of the particle (the polymer particle develops in a spherical shape) (Hutchinson et al., 1992) and help to prevent hot spots.

Productivity can increase up to 17% when prepolymerization is used, but this rise depends on prepolymerization time and on the monomer concentration in the prepolymerization reactor (Figure 3).

The increase in productivity does not only from the increase in total reactor volume (FBR + CSTR), but also from the increase in catalyst activity (production of polymer per gram of catalyst). The increase in catalyst activity had been previously reported as possible by Hutchinson et al. (1992). This feature can be observed in Figure 3. For monomer concentrations, in the prepolymerization reactor between 50% and 100% of the concentration value used in the FBR, an increase in catalyst activity is observed, and hence, the polymerization system has a higher productivity. However, if the prepolymerization time is long (more than 30 minutes), a major part of the polymerization will be carried out in the prepolymerization reactor, and in this latter case, the total time of reaction increases and productivity decreases (in ton/h). For long prepolymerization periods, the polymer that is transferred to the FBR is already well developed and the FBR is responsible more for chain termination than for propagation of the polymer chain.

Physicochemical characterization of the polymer can change considerably depending on the operational conditions used. In general, the FBR conditions will dictate the polymer characteristics, but for long prepolymerization times and high monomer concentrations in the prepolymerization reactor, the polymer weight average molecular weight can increase 22% (Figure 4). In part this behavior is due to not feeding hydrogen, which acts as a chain transfer agent, into the prepolymerization reactor. Polydispersity has remained constant. This also indicates that the prepolymerization reactor can be used to give special characteristics to the polymer if different conditions are used in the two reactors. For instance, bimodal polymers can be obtained if different concentrations of the chain transfer agent (hydrogen) are used in the reactors.

For low weight average molecular weight polyethylenes (Mw ~ 50000 g/mol), prepolymerization implies slight changes in the behavior of the polymerization system when compared with the production of medium weight average molecular weight polyethylenes.

The FBR can be operated at temperatures 10 K lower than for systems not using prepolymerization (Figure 5). As before, an increase in prepolymerization time reduces the mean temperature in the FBR. Monomer concentration in the prepolymerization reactor does not influence the mean temperature in the FBR.

An increase in polymer productivity is also noticed, but this increase is only obtained for high residence times in the prepolymerization reactor combined with high monomer concentrations. Otherwise productivity tends to decrease (Figure 6).

Again, the physicochemical characteristics of the polymer can considerably change depending on the operational conditions used. For low weight average molecular weight polyethylenes, these changes can be as high as 100% for long prepolymerization times and high monomer concentrations (Figure 7). Polydispersity also increases considerably (Figure 8).

CONCLUSIONS

This study showed the influence of prepolymerization on the behavior of the fluidized bed reactor, related mainly to the temperature, productivity and physicochemical characterization of the polyethylene produced. As was demonstrated, the use of prepolymer particles results in milder temperatures in the FBR. These conditions avoid the formation of hot spots, melting of the polymer and shutdown of the reactor. Productivity and catalyst activity can be enhanced, depending on the operational conditions used in the prepolymerization reactor.

These results confirm industrial observations on the advantages of a prepolymerization system for polymer production, increasing the range of operability of the system without compromising reactor integrity and polymer quality. It also shows that simulations can play an important role in studying process capabilities and limitations, allowing for prediction of the behavior and expectancies of a chemical process in a very short period of time, thus providing a good basis for product development and plant optimization.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the financial support of the Brazilian research funding institution FAPESP - Fundação de Amparo à Pesquisa do Estado de São Paulo.

NOMENCLATURE

Received: July 25, 2001

Accepted: November 8, 2002

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Publication Dates

• Publication in this collection
  25 June 2003
• Date of issue
  June 2003

History

• Received
  25 July 2001
• Accepted
  08 Nov 2002