Liquid slurry polymerization with catalyst

Liquid Slurry Polymerization with Heterogenized Cp Ti(OCH3)3 Catalyst... 

The earliest commercial methods used slurry polymerizations with liquid hydrocarbon diluents, like hexane or heptane. These diluents carried the propylene and the catalyst. Small amounts of hydrogen were fed into the reaction mixtures to control molecular weights. The catalyst system consisted of a deep purple or violet-colored TiCls reacted with diethyl aluminum chloride. The TiCb was often prepared by reduction of TiCU with an aluminum powder. These reactions were carried out in stirred autoclaves at temperatures below 90 °C and at pressures sufficient to maintain a liquid phase. The concentration of propylene in the reaction mixtures ranged between 10-20%. The products formed in discrete particles and were removed at 20-40% concentrations of solids. Unreacted monomer was withdrawn from the product mixtures and reused. The catalysts were deactivated and dissolved out of the products with alcohol containing some HCl, or removed by steam extraction. This was followed by extraction of the amorphous fractions with hot liquid hydrocarbons. 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

Dicyclopentadiene is also polymerized with tungsten-based catalysts. Because the polymerization reaction produces heavily cross-linked resins, the polymers are manufactured in a reaction injection molding (RIM) process, in which all catalyst components and resin modifiers are slurried in two batches of the monomer, The first batch contains the catalyst (a mixture of WClg and WOCl4), additives, and fillers the second batch contains the co-catalyst (a combination of an alkylaluminum compound and a Lewis base such as ether), antioxidants, and elastomeric fillers. Mixing two liquids in a mold results 111 a rapid polymerization reaction. 

The similar, older slurry process uses a less active catalyst. The monomer is dissolved in isooctane, the titanium catalyst and aluminium cocatalyst are added and this mixture is fed to the reactor which is maintained at 70°C. The inorganic corrosive (Cl) residues are removed in a washing step with alcohols. The atactic material is removed by extraction. A third process employs propene as the liquid in combination with a high activity catalyst. The Himont Spheripol process, which uses spherical catalyst particles, gives spherical polymer beads of millimetre size that need no extrusion for certain purposes. A more recent development is the gas-phase polymerization using an agitated bed. All processes are continuous processes, where the product is continuously removed from the reactor. Over the years we have seen a reduction of the number of process steps. The process costs are very low nowadays, propene feed costs amounting to more than 60% of the total cost. 

Precipitation polymerization, also known as slurry polymerization, involves solution systems in which the monomer is soluble but the polymer is not. It is probably the most important process for the coordination polymerization of olefins. The process involves, essentially, a catalyst preparation step and polymerization at pressures usually less than 50 atm and low temperatures (less than 100°C). The resultant polymer, which is precipitated as fine floes, forms a slurry consisting of about 20% polymer strspended in the liquid hydrocarbon employed as solvent. The polymer is recovered by stripping off the solvent, washing off the catalyst, and if necessary, extracting any undesirable polymer components. Finally, the polymer is compounded with additives and stabilizers and then granulated. 

Although the Phillips and Standard Oil processes can be used to prepare polypropylene, the polymer yields tend to be low and it appears that these processes have not been used for commercial production of polypropylene. Until about 1980, polypropylene has been produced commercially only by the use of Ziegler-Natta catalysts. Commonly a slurry process is used and is carried out in much the same manner as described previously for the preparation of polyethylene (see section 2.3.2(b)). In the case of polypropylene, some atactic polymer is formed besides the required isotactic polymer but much of this atactic material is soluble in the diluent (commonly heptane) so that the product isolated is largely isotactic polymer. Recently, there has been a marked shift towards processes involving gas phase polymerization and liquid phase polymerization. Few details of these newer processes have been published. Gas phase processes resemble those described previously for the preparation of polyethylene (see section (2.3.2(b)) and swing plants are now feasible. In liquid phase processes polymerization is conducted in liquid propylene, typically at 2 MPa (20 atmospheres) and 55°C. Concurrently with these developments, new catalyst systems have been introduced. These materials have very high activity and the reduced levels that are required make it unnecessary to remove catalyst from the final polymer. Also, the new catalyst systems lead to polypropylene with higher proportions of isotactic polymer and removal of atactic polymer is not necessary. 

In order to fully exploit this catalyst system a unique commercial polymerization process was developed, Himont s Spheripol process is, in practice, a two-stage hybrid process consisting of both liquid and gas phases. Homopolymerization takes place in the liquid slurry phase, and after removal of unreacted monomer and solvent, the solid, porous particles pass into the gas phase part of the process in which the introduction of other monomers allows copolymerization to take place within the solid, spherical particle. This growing polymer particle has become a "reactor granule" and represents a revolution in the development of Ziegler-Natta catalysis. Since polymerization can take place within a solid polymer shell, the mechanical containment aspects of the polymerization process became secondary. Bulk, gas phase, and slurry processes are all equally adaptable for use with this catalyst system and are chosen based on economics, mechanical reliability and reaction control criteria in order to maximize reactivity and productivity. It removes almost all of the previous process constraints on Ziegler-Natta catalysis, allowing reactor-made resins to be produced with... 

Slurry-phase polymerization involves a solid from the beginning of the polymerization process. An important example is the production of high-density polyethylene (HOPE) using immobilized, solid Ziegler-Natta catalysts in a solvent (typically liquid alkanes). Slurry phase polymerization is carried out in stirred tank reactors or loop reactors where the three-phase system is pumped with high flow velodties to reduce the probability of reactor fouling (for details see Section 6.20). 

The polymerization of olefins with coordination catalysts is performed in a large variety of polymerization processes and reactor configurations that can be classified broadly into solution, gas-phase, or slurry processes. In solution processes, both the catalyst and the polymer are soluble in the reaction medium. These processes are used to produce most of the commercial EPDM rubbers and some polyethylene resins. Solution processes are performed in autoclave, tubular, and loop reactors. In slurry and gas-phase processes, the polymer is formed around heterogeneous catalyst particles in the way described by the multigrain model. Slurry processes can be subdivided into slurry-diluent and slurry-bulk. In slurry-diluent processes, an inert diluent is used to suspend the polymer particles while gaseous (ethylene and propylene) and liquid (higher a-olefins) monomers are fed into the reactor. On the other hand, only liquid monomer is used in the slurry-bulk pro-... 

Slurry-diluent processes use an inert diluent to suspend the polymer particles. Although the diluent does not directly affect the polymerization, it has been shown that different diluents might change catalyst behavior, probably due to electronic interaction with the active sites. Gaseous monomers and hydrogen are continuously bubbled through the diluent. Liquid a-olefin comonomers, diluent, catalysts, and cocatalyst are continuously fed into the reactor. Alternatively, liquefied propylene can be fed into the reactor (slurry-bulk process). Except from this difference, all other conditions are similar to the slurry-diluent process. 

The preliminary results on the polymerization of ethylene have clearly shown that the concentrations of ethylene in the stilvent (Csx ), anpartial pressure or concentration of ethylene above the solvent Csv)t zhould be used when activities are compared for different solvents or with activities determined in a gaa-phase reactor. It has also been shown that high-activity catalysts of the type used in this study when used in conjunction with TEAL deactivate much more rapidly in gas-phase reactors than in slurry reactors. The high-activity catalysts did not deactivate when IPRAL was used as a co-catalyst, but the normalized activities in the gas-phase reactor were lower than those in the liquid-phase reactor. The shapes of the activity-time profiles were also different, t.e. the activation rates were not the same for the gas and slurry systems. 

In the sluny process/ a solution of about 2-6 wt% of the monomers in solution in a hydrocarbon such as isobutane, n-pentane or n-hexane is slurred with the catalyst, and the polymerization proceeds in a loop reactor achieving conversions 97%. Typical co-monomers with ethylene can be butene-1, hexene-1, 4-methylpentene-1, and octene-1. The catalyst is suspended in the solution, which is then pumped as turbulent slurry through relatively narrow tubes. The polymer is essentially insoluble in the liquid diluent and forms a sluny containing up to 30 wt% of small polymer particles, which are removed continuously. 

---