Znd. Eng. C h e m . Res. 1994,33, 449-479
449
REVIEWS Gas Phase Ethylene Polymerization: Production Processes, Polymer Properties, and Reactor Modeling Tuyu Xie, Kim B. McAuley,' James C. C. HSU,and David W. Bacon Department of Chemical Engineering, Queen's University, Kingston, Ontario, Canada K 7 L 3N6
A review of relevant macroscopic and microscopic processes of gas phase ethylene polymerization, both chemical and physical, is given. The commercial technology development of gas-phase ethylene polymerization processes is illustrated through a selective survey of the patent literature. Both advantages and disadvantages of gas phase polymerization processes are addressed, and the challenges of laboratory studies of gas phase polymerization are also outlined. Physicochemical phenomena of ethylene polymerization using heterogeneous catalysts are discussed, including examination of catalyst preparation, polymer morphological development, and elementary chemical reactions. Metallocene-based catalysts and their kinetic performance for olefin polymerizations are also discussed. The current state of the art for reactor modeling of polymerization rate, molecular weight development, reactor dynamics, and resin grade transition strategies is illustrated on the basis of the most recent academic studies. Finally, relationships between resin properties and polymer microstructures as well as characterization methods are described briefly. In particular, temperature-rising elution fractionation technology is emphasized for characterization of ethylene copolymers. The fundamental issues involved in gas phase ethylene polymerization and the& interrelationships are also discussed in some detail. Contents 1. Introduction 2. Gas Phase Polymerization Processes 2.1. Commercial Gas Phase Polymerization Processes 2.2. Experimental Methods 3. Physicochemical Phenomena 3.1. Catalysts for Gas Phase Polymerization 3.2. Polymer Particle Morphology Developments 3.3. Chemical Reactions 4. Reactor Modeling 4.1. Kinetic Modeling 4.2. Dynamic Process Modeling and Control 5. Polymer Properties and Characterization 5.1. Physical and Mechanical Properties 5.2. Polyethylene Characterization 6. Summary 7. References
449 450 450 455 456 456 460 461 463 463 467 469 469 470 474 474
1. Introduction
Polyethylene (PE) is the largest synthetic commodity polymer in terms of annual production and is widely used throughout the world due to its versatile physical and chemical properties. The American Society for Testing and Materials (ASTM) has classified PE into four groups: I (low density) at 0.910-0.925 g/cm3, I1 (also low density) at 0.926-0.940 g/cm3, I11 (high-density copolymers) at 0SSS-5SS5/94/2633-0449$04.50/0
0.941-0.959 g/cm3, and IV (high-density homopolymer) at 0.960 g/cm3and above (Redman,1991). In the literature, however, PE is normally classified as low density (LDPE) at 0.910-0.930 g/cm3 and high density (HDPE) a t 0.9310.970 g/cm3. Low-densitypolyethylene is further classified as low-densitypolyethylene (LDPE) and linear low-density polyethylene (LLDPE) based on polymer chain microstructure and synthesis processes. According to the figures reported in Mod. Plast. (19931, polyethylene production in the United States alone was over 10 million tons in 1992. The annual production of PE in Europe is about 9 million tons (Redman, 1991). The current annual worldwide capacity for PE production is over 30 million tons. Figure 1shows the US. PE production profile over the past-decade. Although the annual rate of increase slowed down slightly at the end of the 1980s, the average annual increase rate is about 8 94 for HDPE and about 5 % for LDPE and LLDPE for the past decade. Consumption of PE is still rising through the 1990s with development of synthesis and processing technology. The main markets and applications of LLDPE and HDPE are summarized in Figure 2 after James (1986) and Foster (1991). Polyethylene is commercially produced exclusively by continuous processes. On the basis of polymerization mechanisms and reactor operating conditions, PE production processes can be classified into at least five process categories as shown in Table 1. Among them, the gas phase polymerization process is the most recently developed and also the most versatile. Since its emergence, this process has been challenging other existing processes for market share, particularly, for production of LLDPE, due to its economic and technological advantages. Many excellent reviews of ethylene polymerization processes have been published (Vandenbergand Perka, 1977;Short, 1981;Choi and Ray, 1985a; Nowlin, 1985; James, 1986; Beach and Kissin, 1986). However, the fundamental issues involved 0 1994 American Chemical Society
450 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 'Ooo
eI-z
zation can be traced back to the 1950s. Dye (1962),whose L patent was filed in 1957, was perhaps the first to adopt a
6000 -
LDPE & LLDPE
o
f HDPE
vj
5
2ooo
y
.""" 80
82
86
84
88 90 TIME, year
92
94
96
Figure 1. U S . polyethylene production profile (data from Mod. Plast. (198C-1993)).
I
0.97 0.96 -
- 0.95 rn
0.94
-
v)
z W
0 0.93 -
SHEET
0.92 -
0.91 0.90 1
0.01
1.oo 10.0 MELT INDEX, 9/10 min
0.10
100
I
Figure 2. Markets and applications for low-pressure polyethylene.
in gas phase polymerization processes have not been comprehensively discussed. With development of commercial gas phase polymerization processes, the importance of more complete understanding of gas phase polymerization processes has been recognized by the academic community. Significant experimental and reactor modeling work has been carried out in recent years. In the present review, the authors focus on the main issues involved in gas phase ethylene polymerization processes, including commercial production technology development, physicochemical phenomena, kinetictdynamic reactor modeling and control; polymer properties and characterization, as well as their interrelationships. 2. Gas Phase Polymerization Processes
The distinguishing characteristic of gas phase polymerization is that the system does not involve any liquid phase in the polymerization zone. Polymerization does occur at the interface between the solid catalyst and the polymer matrix, which is swollen with monomers during polymerization. The gas phase plays a role in the supply of monomers, mixing of polymer particles, and removal of reaction heat. Hence, gas phase polymerization is also called dry polymerization in some patents (Dormenval et al., 1975; Havas and Mangin, 1976). In this section, commercial gas phase polymerization processes and experimental studies of gas phase polymerization are discussed. 2.1. Commercial Gas Phase Polymerization Processes. The invention of gas phase ethylene polymeri-
fluidized bed reactor for gas phase olefin polymerization. The original reactor consisted of three concentric superimposed vertical sections. Polymer particles were discharged through an extruder, which was connected to the bottom section of the reactor. The reactor was operated at a pressure of 30 atm and a temperature of 100 "C for ethylene polymerization. Goins (1960)carried out ethylene copolymerization in a countercurrent fluidized bed reactor in the presence of inert diluent gas. In this process, polymer particles are passed downward in the reactor and monomer mixed with diluent gas is passed countercurrently upward in the reactor and monomer mixed with diluent gas is passed countercurrently upward through a series of vertical fluidized bed reaction zones. The reaction zones can be controlled independently by taking off-gas from the last reaction zone, cooling it, and recycling portions of such off-gas to each of the reaction zones. Both patents (Dye, 1962;Goins, 1960)were assigned to Phillips Petroleum Company. Schmid et al. (1967) carried out ethylene polymerization in a stirred fluidized bed reactor. In this configuration, polymer particles are moved in the direction of monomer flow by stirring, and reaction heat is removed by cooling the walls of the reactor, by the gas stream, and by introduction of liquified monomers. The patent of Schmid et al. was assigned to BASF. The benefits of gas phase polymerization were recognized by these pioneer inventors. Although these inventions have not been directly applied in commercial gas phase polymerization processes, the fundamental ideas demonstrated by these inventors provided the foundation for the later commercial gas phase process development. The first commercial gas phase polymerization plant using a fluidized bed reactor was constructed by Union Carbide in 1968 at Seadrift, TX (Rasmussen, 1972; Batleman, 1975; Burdett, 1988). This process was developed initially for HDPE production. The success of this novel technology led to the extension of the process to LLDPE, which was produced initially on a commercial reactor in 1975 (Davis, 1978; Burdett, 1988). The Union Carbide gas phase process, commonly called UNIPOL, has been licensed worldwide with more than 25 licenses operating in 14 different countries (Burdett, 1988). Production of LLDPE using gas phase processes is more difficult than production of HDPE because the difference between the melting point and polymerizationtemperature is much narrower for LLDPE. The catalyst types and equipment design developed for HDPE cannot be used to produce LLDPE because of the potential for agglomeration of polymer particles. Hence, significant engineering and chemistry research has been required to assure the success of gas phase LLDPE production. According to Karol (19831,the keys to the success of the UNIPOL technology for LLDPE production are the proprietary catalysts that operate at low pressure and low temperature and which are suitable for use in a gas phase fluidized bed reactor. Union Carbide Corporation received the 1979Kirkpatrick Chemical Engineering Achievement Award in recognition of the innovation of the UNIPOL process (Chem. Eng., 1979). The UNIPOL process has been described in several US. Patents (Miller, 1977; Levine and Karol, 1977; Karol and Wu, 1978; Wagner et al., 1981; Jorgensen et al., 1982). A simplified flow diagram is shown in Figure 3. The fluidized bed reactor consists of a reaction zone and a disengagement zone. The reaction zone has a height to diameter ratio of about 6-7.5. The disengagement zone has a diameter to
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 451 Table 1. Polymerization Processes and Reactor Operating Conditions
reactor type
conventional high-press. process tubular or autoclave
high-press. bulk process autoclave
solution Polymn CSTR
reactor press., atm temp, "C polymn mech loci of polymn density, g/cm3 melt index, g/10 min reference
1200-3000 130-350 free radical monomer phase 0.910-0.930 0.10-100 Doak (1986)
600-800 200-300 coordination monomer phase 0.910-0.955 0.80-100 Grunig and Luft (1986); Villermaux et al. (1989)
100 140-200 coordination solvent 0.910.970 0.50-105 James (1986)
GAS RECYCLE
1
CATALYST FEEDER
I
G
-
GAS FEED 7
EXCHANGER
Figure 3. Union Carbide gas phase ethylene polymerization process-UNIPOL (Miller, 1977;Levine and Karol, 1977;Karol and Wu, 1978; Wagner et al., 1981; Jorgensen et al., 1982).
height ratio of about 1-2. To maintain a viable fluidized bed, superficial flow through the bed is about 2-6 times the minimum flow required for fluidization. It is essential that the bed always contain polymer particles to prevent the formation of localized "hot spots" and to entrap and distribute the powdery catalyst. On startup, the reaction zone is usually charged with a base of polymer particles before gas flow is initiated. The catalyst is stored in a catalyst feeder under a nitrogen blanket. Catalyst is injected into the bed at a rate equal to its consumption rate at 114 to 314 of the height of the bed. Catalyst concentration in the fluidized bed is essentially equal to the catalyst concentration in the product, namely on the order of about 0.005-0.50% of bed volume. Fluidization is achieved by a high rate of gas recycle to and through the bed, typically on the order of about 50 times the rate of feed of make-up gas. The pressure drop through the bed is typically on the order of 1 psig. Make-up gas is fed to the bed at a rate equal to the rate at which polymer particles are withdrawn. A gas analyzer, position above the bed as shown in Figure 3, determines the composition of the gas being recycled, and the make-up gas composition is adjusted accordingly to maintain an essentially steady-state gaseous composition within the reaction zone. The gas which has not been consumed in the bed passes through the enlarged disengagement zone where entrained particles drop back into the bed. Particle entrainment is further reduced by a cyclone and a filter to avoid deposition of polymer on heat-transfer surfaces and compressor blades. Polymerization heat is removed by a heat exchanger before the recycle gas is compressed and returned to the reactor. The fluidized bed can maintain itself at
-
slurry polymn loop or CSTR 30-35 85-110 coordination solid 0.930.970 loo0
PARTICLE SIZE, micron
Figure 12. Particle size replication of ethylene polymerizationwith silica-supportedZiegler-Nattacatalyst (Munoz-Escalonaet al., 1988).
scanning electron microscopy and found that the catalyst consists of small particles with a rough surface. However, the internal structure of the catalyst was not shown. If the catalyst is impregnated on silica support, the particle size is in the range of 30-250 pm (Bailly and Speakman, 1986; Munoz-Escalona, 1988). The average polymer particle size is about 20 times larger in diameter than the catalyst particle size (Galli et al., 1981, 1984; Karol et al., 1987). Kakugo et al. (l988,1989a,b) recently studied internal morphologydevelopment of polypropylene particles using transmission electron microscopy. Their results indicate that each polymer primary particle has a nucleus 50-170 A in diameter, which is believed to be the catalyst primary particle. It is interesting to notice that the catalyst primary particle size identified in the polymer particle is in excellent agreement with catalyst crystal particle size estimated by Chien (19871, as shown in Table 8. This suggests that the minicrystal size of a catalyst achieved by ball-milling cannot be reduced further during polymerization, or that the Ti catalyst active center is fixed on the surface of this mini MgClz crystal and not inside the minicrystal. The diameter of a polymer primary particle is about 0.2 pm. The fine polymer globules, about 1 pm in diameter, are formed by fusion of several polymer primary particles. The gross polymer particles are composed of these primary particle aggregates. Weist et al. (1989) found that the pore distribution of silica supported catalyst does not change at the initial stage of polymerization. However, the pore size of a catalyst increases with an increase in polymer yield, indicating fragmentation during polymerization. The SEM photographs of polymer particles show that the catalyst particles are completely encapsulated by polymer in the early stage (
