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Table 4: Summary of elementary reactions for ethylene and a-olefins co-polymerization [22]

Table 4 Summary of elementary reactions for ethylene and a-olefins co-polymerization [22]

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Table 1: Polymerization Processes and Reactor Operating Conditions
Table 1: Polymerization Processes and Reactor Operating Conditions
Figure 2: LDPE/EVA process, Basell Polyolefins | (Hydrocarbon Processing, March 2003) Figure 1: LDPE/EVA process, ExxonMobil Chemical Co (Hydrocarbon Processing, March 2003)
Figure 2: LDPE/EVA process, Basell Polyolefins | (Hydrocarbon Processing, March 2003) Figure 1: LDPE/EVA process, ExxonMobil Chemical Co (Hydrocarbon Processing, March 2003)
The subsequent technologies developed focused on achieving lower operating pressure. With the development of catalyst to allow coordination polymerization, slurry and solution phase reactors were invented. These reactors operate at a lower pressure compared to the earlier high-pressure free radical process. The slurry-phase “loop reactor” by Chevron Phillips Chemical Co. operates at 40 atm. Development for low pressure polymerization processes also started the use of comonomers such as 1- butene and 1-hexene to produce linear-low density polyethylene (LLDPE) and high density polyethylene (HDPE). The Chevron Phillips technology is used in eighty-six commercial LLDPE plants and account for 34% of worldwide capacity. Another variant of this technology by Mitsui Chemicals Inc., named CX process is used for production of HDPE and has a worldwide capacity of 4 MMtpy.
The subsequent technologies developed focused on achieving lower operating pressure. With the development of catalyst to allow coordination polymerization, slurry and solution phase reactors were invented. These reactors operate at a lower pressure compared to the earlier high-pressure free radical process. The slurry-phase “loop reactor” by Chevron Phillips Chemical Co. operates at 40 atm. Development for low pressure polymerization processes also started the use of comonomers such as 1- butene and 1-hexene to produce linear-low density polyethylene (LLDPE) and high density polyethylene (HDPE). The Chevron Phillips technology is used in eighty-six commercial LLDPE plants and account for 34% of worldwide capacity. Another variant of this technology by Mitsui Chemicals Inc., named CX process is used for production of HDPE and has a worldwide capacity of 4 MMtpy.
Figure 3: LPE process, Chevron Phillips Chemical Co., LP (Hydrocarbon Processing, March 2003) Figure 4: CX HDPE process, Mitsui Chemical Inc. (Hydrocarbon Processing, March 2003)
Figure 3: LPE process, Chevron Phillips Chemical Co., LP (Hydrocarbon Processing, March 2003) Figure 4: CX HDPE process, Mitsui Chemical Inc. (Hydrocarbon Processing, March 2003)
Figure 5: Fluidized bed reactor (Dye, 1962) Figure 6: Fluidized bed reactor (Goins, 1960) Dye (1962) [4] described one of the earlier reactors as 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 30 atm and 100°C. Goins (1960) [5] carried out ethylene copolymerization in a countercurrent fluidized bed reactor in presence of inert diluent gas. In this process, polymer particles are passed downward in the reactor and monomer mixed with diluent gas is passed counter-currently upward in the reactor and monomer mixed with diluent gas is passed counter currently 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 but were never applied commercially. Nevertheless, they formed the foundation idea of how a fluidized bed reaction process will be and subsequent commercialized processes are very similar to these earlier inventions.
Figure 5: Fluidized bed reactor (Dye, 1962) Figure 6: Fluidized bed reactor (Goins, 1960) Dye (1962) [4] described one of the earlier reactors as 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 30 atm and 100°C. Goins (1960) [5] carried out ethylene copolymerization in a countercurrent fluidized bed reactor in presence of inert diluent gas. In this process, polymer particles are passed downward in the reactor and monomer mixed with diluent gas is passed counter-currently upward in the reactor and monomer mixed with diluent gas is passed counter currently 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 but were never applied commercially. Nevertheless, they formed the foundation idea of how a fluidized bed reaction process will be and subsequent commercialized processes are very similar to these earlier inventions.
Table 2: Fluidized Bed Reactors Processes and Operating Conditions Since the successes of both Union Carbide and BP Chemicals processes, the fluidized bed reactor technology has been applied and improved in other proprietary polymerization processes. Notable examples are combination of slurry loop reactor and fluidized bed reactor in series for production of polyethylene and fluidized beds in series for production of polypropylene. These hybrid systems will be discussed in a later part of this paper.
Table 2: Fluidized Bed Reactors Processes and Operating Conditions Since the successes of both Union Carbide and BP Chemicals processes, the fluidized bed reactor technology has been applied and improved in other proprietary polymerization processes. Notable examples are combination of slurry loop reactor and fluidized bed reactor in series for production of polyethylene and fluidized beds in series for production of polypropylene. These hybrid systems will be discussed in a later part of this paper.
Figure 8 BP Chemicals fluidized bed reactor process (Dormenval et al, 1975; Havas and Magin, 1976; Havas and Lalanne-Magne, 1992) Figure 7: Union Carbide fluidized bed reactor (Miller 1977; Levine and Karol, 1977; Karol and Wu, 1978; Wagner et al., 1981; Jorgensen et al., 1982 ) latter. The original Naphtachimie design was very similar to the Union Carbide design with exception of a pre-polymerization step. Monomers, catalysts and hydrogen were first activated in an n-heptane phase using a mechanically stirred vessel. The catalytically active solids or “pre-polymers” were decanted and subsequently fed to the fluidized bed reactor instead of solid catalyst as in the case of UNIPOL process. This major distinction was incorporated in the latter BP designs until the development of metallocene catalyst.
Figure 8 BP Chemicals fluidized bed reactor process (Dormenval et al, 1975; Havas and Magin, 1976; Havas and Lalanne-Magne, 1992) Figure 7: Union Carbide fluidized bed reactor (Miller 1977; Levine and Karol, 1977; Karol and Wu, 1978; Wagner et al., 1981; Jorgensen et al., 1982 ) latter. The original Naphtachimie design was very similar to the Union Carbide design with exception of a pre-polymerization step. Monomers, catalysts and hydrogen were first activated in an n-heptane phase using a mechanically stirred vessel. The catalytically active solids or “pre-polymers” were decanted and subsequently fed to the fluidized bed reactor instead of solid catalyst as in the case of UNIPOL process. This major distinction was incorporated in the latter BP designs until the development of metallocene catalyst.
Figure 9: Industrial scale UNIPOL reactor (Univation Technologies, 2004) Fouling of the cooler (polymer fines buildup in the exchanger tubes) often becomes the criteria of 2 shutdown; hence the reconfiguration allows longer service time between cooler cleaning. Putting the -ooler downstream of the compressor also allows the comonomers (more notably 1-hexene) to condense partially in the cooler. This condensation phenomenon removes a lot of heat from the system and enables uigher production rate. The cyclone shown in both the simplified flow diagrams has the function ot removing polymer fines and reduces the fouling of the cooler or compressor blades. This equipment 10wever is optional and is omitted in some newer plants built in the last decade. The following oaragraphs are detailed description of the earlier configurations by Union Carbide and BP Chemicals.
Figure 9: Industrial scale UNIPOL reactor (Univation Technologies, 2004) Fouling of the cooler (polymer fines buildup in the exchanger tubes) often becomes the criteria of 2 shutdown; hence the reconfiguration allows longer service time between cooler cleaning. Putting the -ooler downstream of the compressor also allows the comonomers (more notably 1-hexene) to condense partially in the cooler. This condensation phenomenon removes a lot of heat from the system and enables uigher production rate. The cyclone shown in both the simplified flow diagrams has the function ot removing polymer fines and reduces the fouling of the cooler or compressor blades. This equipment 10wever is optional and is omitted in some newer plants built in the last decade. The following oaragraphs are detailed description of the earlier configurations by Union Carbide and BP Chemicals.
Figure 10: Ethylene polymerization using Ziegler-Natta catalyst Ziegler-Natta catalysts have many variations but are generally TiCl, supported on MgCl, combined with a variety of electron donors and cocatalysts. Ziegler-Natta catalysts account for most of the polyethylene produced industrially. In operations, a trialkyl aluminum co-catalyst (typically triethyl aluminum) is added separately to the reactor feed. Figure 10 gives an illustration of how polyethylene is made using Ziegler-Natta catalysts in a fluidized bed reactor.
Figure 10: Ethylene polymerization using Ziegler-Natta catalyst Ziegler-Natta catalysts have many variations but are generally TiCl, supported on MgCl, combined with a variety of electron donors and cocatalysts. Ziegler-Natta catalysts account for most of the polyethylene produced industrially. In operations, a trialkyl aluminum co-catalyst (typically triethyl aluminum) is added separately to the reactor feed. Figure 10 gives an illustration of how polyethylene is made using Ziegler-Natta catalysts in a fluidized bed reactor.
Figure 11: Inorganic oxide supported bis(cyclopentadienyl) chromium [II] (Karol et al., 1978) Figure 12: A metallocene structure (Winter et al., 2002) (21). Phillips catalysts are mostly composed of chromium oxide supported on silica or silica/alumina. It is responsible for a considerable fraction of the HDPE produced. In operations, Phillips catalysts do not require separate addition of co-catalyst. It is however more exothermic and heat removal needs to be efficient.
Figure 11: Inorganic oxide supported bis(cyclopentadienyl) chromium [II] (Karol et al., 1978) Figure 12: A metallocene structure (Winter et al., 2002) (21). Phillips catalysts are mostly composed of chromium oxide supported on silica or silica/alumina. It is responsible for a considerable fraction of the HDPE produced. In operations, Phillips catalysts do not require separate addition of co-catalyst. It is however more exothermic and heat removal needs to be efficient.
Table 3: Relationship between molecular structure and properties of polyethylene [15]
Table 3: Relationship between molecular structure and properties of polyethylene [15]
Figure 14: Relationship between polymer properties and resin molecular weight. Figure 13: Relationship between polymer properties and resin density
Figure 14: Relationship between polymer properties and resin molecular weight. Figure 13: Relationship between polymer properties and resin density
3.2. Mathematical Model
3.2. Mathematical Model
Figure 16: Micrograph of of the surface ofa prepolymer p: particle. The particle is clearly an assembly of smaller, spherical structures as described by the multigrain model (McKenna and Soares, 2001). Figure 15: Schematic showing diffusion phenomenon in a multigrain model (McKenna and Soares, 2001) polymer chains push the previously formed polymer layer, thus increasing the radius of the microparticles and consequently the size of the macroparticles. Electron microscopy studies confirm the formation of primary and secondary structures in polymerizations with Ziegler—Natta catalysts. For the multigrain model, the radial profile of monomer concentration in the secondary particle (macroparticle) is described by the well-known diffusion-reaction equations in spherical co-ordinates:
Figure 16: Micrograph of of the surface ofa prepolymer p: particle. The particle is clearly an assembly of smaller, spherical structures as described by the multigrain model (McKenna and Soares, 2001). Figure 15: Schematic showing diffusion phenomenon in a multigrain model (McKenna and Soares, 2001) polymer chains push the previously formed polymer layer, thus increasing the radius of the microparticles and consequently the size of the macroparticles. Electron microscopy studies confirm the formation of primary and secondary structures in polymerizations with Ziegler—Natta catalysts. For the multigrain model, the radial profile of monomer concentration in the secondary particle (macroparticle) is described by the well-known diffusion-reaction equations in spherical co-ordinates:
where T,o is the initial temperature in the microparticle. Equations for intraparticle monomer concentration and temperature radial profiles for the polymer flow model are similar to the ones for the macroparticles of the multigrain model. It is either assumed that the mass transfer resistance at the level of the microparticles is negligible or that it can be accounted for indirectly in the effective diffusivity Der, so that one can use Eq. (3) with a monomer concentration in equilibrium with the concentration of the macroparticle to calculate the radial variation of the polymerization rate. The multigrain model is certainly the most studied expansion model for polymerization of olefins with supported Ziegler—Natta catalysts. It has been used extensively to predict heat and mass transfer resistances for homo- and copolymerization of ethylene, propylene, and higher o-olefins, in slurry and gas-phase reactors. Similarly, for the microparticles, the temperature profile is given by
where T,o is the initial temperature in the microparticle. Equations for intraparticle monomer concentration and temperature radial profiles for the polymer flow model are similar to the ones for the macroparticles of the multigrain model. It is either assumed that the mass transfer resistance at the level of the microparticles is negligible or that it can be accounted for indirectly in the effective diffusivity Der, so that one can use Eq. (3) with a monomer concentration in equilibrium with the concentration of the macroparticle to calculate the radial variation of the polymerization rate. The multigrain model is certainly the most studied expansion model for polymerization of olefins with supported Ziegler—Natta catalysts. It has been used extensively to predict heat and mass transfer resistances for homo- and copolymerization of ethylene, propylene, and higher o-olefins, in slurry and gas-phase reactors. Similarly, for the microparticles, the temperature profile is given by
Figure 17: Grade changeover scheme for ethylene polymerization One of the greater attractions of the fluidized bed reactor technology is the ability to produce polymers with a wide range of density and melt flow index combination. This attraction is severely undermined by the production loss involved in transition between resin grades. It will be ideal to switch between grades “seamlessly” without any off-specification product while maintaining the production rate. In practice, grade specifications rarely overlap, hence generating off-specification products during transition. The production rate is also sacrificed to minimize these off-specification products, which can only be sold for a small fraction of the prime grade resins. Grade transitions pose quality and capacity utilization challenges.
Figure 17: Grade changeover scheme for ethylene polymerization One of the greater attractions of the fluidized bed reactor technology is the ability to produce polymers with a wide range of density and melt flow index combination. This attraction is severely undermined by the production loss involved in transition between resin grades. It will be ideal to switch between grades “seamlessly” without any off-specification product while maintaining the production rate. In practice, grade specifications rarely overlap, hence generating off-specification products during transition. The production rate is also sacrificed to minimize these off-specification products, which can only be sold for a small fraction of the prime grade resins. Grade transitions pose quality and capacity utilization challenges.
Figure 18: Ethylene purification train To ensure the ethylene feed is free of the poisons, the supply has to be purified using a series of adsorption beds. A typical purification train will have a palladium bed to remove acetylene, a copper oxide bed to remove carbon monoxide, a copper bed to remove oxygen and molecular sieves to remove moisture. For the same reason, comonomers also require a purification step. Comonomers are usually degassed using a stripper column and dried using molecular sieve. The purification train poses a capital investment cost and operating costs that is not incurred in other processes.
Figure 18: Ethylene purification train To ensure the ethylene feed is free of the poisons, the supply has to be purified using a series of adsorption beds. A typical purification train will have a palladium bed to remove acetylene, a copper oxide bed to remove carbon monoxide, a copper bed to remove oxygen and molecular sieves to remove moisture. For the same reason, comonomers also require a purification step. Comonomers are usually degassed using a stripper column and dried using molecular sieve. The purification train poses a capital investment cost and operating costs that is not incurred in other processes.
Figure 19: Spherilene gas-phase technology (Hydrocarbon Processing, 2003) Product density is controlled from less than 0.9 g/ml (VLDPE) to more than 0.96 g/ml (HDPE). MI capability ranges from 0.01 to > 100 g/ 10 min. Because of the two fluidized bed reactors setup, this technology is able to produce bimodal grades (MI, density) and specialty polymers based on new molecular models. For example, terpolymer grades which have better properties compared to conventional 1-butene LLDPE/VLDPE and hexene “quattropolymer” to replace hexene based LLDPE. LDPE replacement grades for clarity and shrink film applications have also been developed. There are 8 plants using this technology and a combined capacity of 1.8 MMtpy.
Figure 19: Spherilene gas-phase technology (Hydrocarbon Processing, 2003) Product density is controlled from less than 0.9 g/ml (VLDPE) to more than 0.96 g/ml (HDPE). MI capability ranges from 0.01 to > 100 g/ 10 min. Because of the two fluidized bed reactors setup, this technology is able to produce bimodal grades (MI, density) and specialty polymers based on new molecular models. For example, terpolymer grades which have better properties compared to conventional 1-butene LLDPE/VLDPE and hexene “quattropolymer” to replace hexene based LLDPE. LDPE replacement grades for clarity and shrink film applications have also been developed. There are 8 plants using this technology and a combined capacity of 1.8 MMtpy.
Figure 20: Dow Chemical Co. UNIPOL PP technology (Hydrocarbon Processing, 2003) The UNIPOL PP process was developed by Dow Chemical to produce homopolymers, random copolymer and impact copolymer polypropylene. A wide range of polypropylene is made in a gas phase fluidized bed reactor using proprietary catalysts. Melt flow index, isotactic level and molecular weight distribution are controlled by utilizing the proper catalyst, adjusting operating conditions and adding molecular weight control agent. Random copolymers are produced by adding ethylene or 1-butene. Ethylene addition to a second reactor in series is used to produce the rubber phase of impact copolymers.
Figure 20: Dow Chemical Co. UNIPOL PP technology (Hydrocarbon Processing, 2003) The UNIPOL PP process was developed by Dow Chemical to produce homopolymers, random copolymer and impact copolymer polypropylene. A wide range of polypropylene is made in a gas phase fluidized bed reactor using proprietary catalysts. Melt flow index, isotactic level and molecular weight distribution are controlled by utilizing the proper catalyst, adjusting operating conditions and adding molecular weight control agent. Random copolymers are produced by adding ethylene or 1-butene. Ethylene addition to a second reactor in series is used to produce the rubber phase of impact copolymers.
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