Gas-Phase Fluidized-Bed Polymerization

The activated catalyst precursor was evaluated in a continuous, gas-phase fluid-bed reactor to produce a linear low-density ethylene/1-butene copolymer (LLDPE) with Melt Index (1215 ) of 2.9 and a density of 0.921 g/cc, utilizing a 1 -butene/ethylene and hydrogen/ethylene gas-phase molar ratio of 0.375 and 0.266, respectively, and a production rate (space time yield) of 5.3 lbs. PE/hr/ft of reactor volume. The granular polyethylene had a residual titanium level of 3-5 ppm and high bulk density of 26.2 Ibs./ft when the catalyst precursor was preactivated with Tri(n-hexyl) aluminiun at an Al/Ti molar ratio of 6.6. 

A summary of the activity of the Mg/Ti-based catalysts identified in the four patents summarized above is shown in Table 2.3. 

The activity of these second generation Ziegler catalysts is 10-100 times more active on a titanium basis than the first-generation Ziegler catalysts, which created significant growth opportxmities in the polyethylene industry. 

Patent Example (year issued) Titanium-based Activity KgPE/gTi/hr Company/Location 

3 Impact of High-Activity Mg/Ti Ziegler Catalysts on the Polyethylene Industry 

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The technologies suitable for LLDPE manufacture include gas-phase fluidized-bed polymerization, polymerization in solution, polymerization in a polymer melt under high ethylene pressure, and sluny polymerization. Most catalysts are fine-tuned for each particular process. 

In 1977 Phillips Petroleum Co. developed blends of Z-N-LLDPE using C2+4 and C2+6 copolymers which were disclosed as suitable for pipe extrusion (Larsen 1982). The same year, Mobil Oil announced reactor blends of PE (copolymer of C2+4 and/or C2+6) for production of blown films that exhibited improved MD/TD tear balance. The latter materials were produced in a multistage, gas-phase, fluidized bed polymerization process with in situ blending. The resulted bimodal MWD resins had 0.35 to about 0.75 wt% of a higher-MW component (Ah et al. 1994). 

HOPE with butene and/or hexene, were produced in a multistage, gas-phase, fluidized bed polymerization, where blending occinred in situ. The resulted bimodal molecular weight distribution resin had 35-75 wt% of the higher-molecular-weight component. The blown films had Improved MD/TD tear balance... 

Hendrickson G. Electrostatics and gas phase fluidized bed polymerization reactor wall sheeting. Chemical Engineering Science 2006 61 1041-1064. 

UNIPOL [Union Carbide Polymerization] A process for polymerizing ethylene to polyethylene, and propylene to polypropylene. It is a low-pressure, gas-phase, fluidized-bed process, in contrast to the Ziegler-Natta process, which is conducted in the liquid phase. The catalyst powder is continuously added to the bed and the granular product is continuously withdrawn. A co-monomer such as 1-butene is normally used. The polyethylene process was developed by F. J. Karol and his colleagues at Union Carbide Corporation the polypropylene process was developed jointly with the Shell Chemical Company. The development of the ethylene process started in the mid 1960s, the propylene process was first commercialized in 1983. It is currently used under license by 75 producers in 26 countries, in a total of 96 reactors with a combined capacity of over 12 million tonnes/y. It is now available through Univation, the joint licensing subsidiary of Union Carbide and Exxon Chemical. A supported metallocene catalyst is used today. 

A solution polymerization process was especially useful to commercially produce LLDPE products for the market, while the Phillips loop reactor was somewhat limited in producing LLDPE with relatively lower levels of comonomer in order to avoid process problems, as the solubility of the LLDPE increased in the slurry solvent. The LLDPE became commercially more important in 1977 when Union Carbide developed a high-activity, silica-supported Ziegler catalyst that was utilized to manufacture LLDPE in a gas-phase, fluidized-bed process. 

The first stage of the Spheripol process consists of polymerization in liquid propylene. Usually, two loops are used in series to narrow the residence-time distribution of the catalyst particles. For the ethylene-propylene copolymer (EPR) stage, the Spheripol process (Fig. 2.33) utilizes a gas phase fluidized bed reactor (FBR). The liquid propylene/ polymer suspension from the first reactor is flashed to gas/solid conditions prior to entering the second stage. The second stage operates at pressures of 15-35 atm, which is often close to the dew point of the gas. Elevated temperatures of approximately 80°C are used to provide a reasonable amount of copolymer contents in the final product. 

UNIPOL [UNIon Carbide POLymerization] A process for polymerizing ethylene to polyethylene and propylene to polypropylene. It is a low-pressure, gas-phase, fluidized-bed process, in contrast to the Ziegler-Natta process, which is conducted in the liquid phase. The catalyst powder is continuously added to the bed, and the granular product is continuously withdrawn. A comonomer, such as 1-butene, is normally used. 

Vinyl chloride is polymerized in bulk, suspension, emulsion, or in the gas phase. The bulk polymerization is a precipitation polymerization. To prevent excessive heat buildup because of the heat of polymerization, it is carried out in two stages. In the gaseous phase polymerization, prepolymerized PVC is loaded with vinyl chloride below the saturated vapour pressure and then further polymerized continuously in a fluidized bed or cascade process. Emulsion polymerizates always contains foreign material, and so, are only used as pastes. 

Union Carbide, now part of Dow Chemical, was the first company to commercialize the technology for polyolefin production using fluidized-bed gas-phase reactors. Since polymerization occurs in the gas phase, separation of the unreacted monomer from the polymer product is achieved simply by flashing off the monomer. Any low molecular weight polymer formed remains in the polymer particles and no further separation is necessary. The process only requires a fluidized-bed gas-phase reactor, a product discharge system to get... 

Polymerization in the Gas Phase. Many polymerization catalysts can be adapted for use in the gas phase. A gas-phase reactor contains a bed of small PE particles that is agitated either by a mechanical stirrer or by employing the fluidized-bed technique. These processes are economical because they do not requite solvent tecitculation streams. 

As an example of the chemical signihcance of the process technology, the products of die Fischer-Tropsch synthesis, in which a signihcant amount of gas phase polymerization occurs vary markedly from hxed bed operation to the fluidized bed. The hxed bed product contains a higher proportion of straight chain hydrocarbons, and the huidized bed produces a larger proportion of branched chain compounds. 

Gaseous monomers can polymerize in the gas phase in the presence of a fluidized catalyst bed. As polymer forms, hot gas forces the newly made material out of the reactor to a collector. Figure 2.15 shows a simplified schematic diagram of a generic polymerization reactor. 

Metallocenes are homogeneous catalysts that are often soluble in organic solvents. Therefore, polymerization can occur via a solution process with a non-polar diluent dissolving the propylene gas, the catalyst, and the co-catalyst system. They can also be adsorbed onto an inert substrate which acts as part of the fluidized bed for gas phase polymerization processes. 

An advantage of this approach to model large-scale fluidized bed reactors is that the behavior of bubbles in fluidized beds can be readily incorporated in the force balance of the bubbles. In this respect, one can think of the rise velocity, and the tendency of rising bubbles to be drawn towards the center of the bed, from the mutual interaction of bubbles and from wall effects (Kobayashi et al., 2000). In Fig. 34, two preliminary calculations are shown for an industrial-scale gas-phase polymerization reactor, using the discrete bubble model. The geometry of the fluidized bed was 1.0 x 3.0 x 1.0 m (w x h x d). The emulsion phase has a density of 400kg/m3, and the apparent viscosity was set to 1.0 Pa s. The density of the bubble phase was 25 g/m3. The bubbles were injected via 49 nozzles positioned equally distributed in a square in the middle of the column. 

For the discrete bubble model described in Section V.C, future work will be focused on implementation of closure equations in the force balance, like empirical relations for bubble-rise velocities and the interaction between bubbles. Clearly, a more refined model for the bubble-bubble interaction, including coalescence and breakup, is required along with a more realistic description of the rheology of fluidized suspensions. Finally, the adapted model should be augmented with a thermal energy balance, and associated closures for the thermophysical properties, to study heat transport in large-scale fluidized beds, such as FCC-regenerators and PE and PP gas-phase polymerization reactors. 

Spheripol A process for making polypropylene and propylene co-polymers. Homopolymerization is conducted in the liquid phase in a loop tubular reactor co-polymerization is conducted in the gas phase in a fluidized-bed reactor. The catalyst is treated with a special silane. The product is in the form of beads of up to 5 mm in diameter. Developed by Montecatini, Italy, and first licensed by Himont, United States, and Mitsui Petrochemical Industries, Japan. In 1989, 29 licenses had been granted worldwide. Now offered for license by Montell, a joint venture between Montedison and Shell. See also Addipol. 

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