Gas-phase, fluidized bed reactor

Description A wide range of polyethylenes is made in a gas-phase, fluidized-bed reactor using proprietary solid and slurry catalysts. The product is in a dry, free-flowing granular form substantially free of fines as it leaves the reactor and is converted to pellet form for sale. Melt index and molecular weight distribution are controlled by... 

Chemically, LLDPE can be described as linear polyethylene copolymers with alpha-olefin comonomers in the ethylene chain. They are produced primarily at low pressures and temperatures by the copolymerization of ethylene with various alpha-olefins such as butene, hexane, octane, etc., in the presence of suitable catalysts. Either gas-phase fluidized-bed reactors or liquid-phase solution-process reactors are used. (In contrast, LDPE is produced at very high pressures and temperatures either in autoclaves or tubular reactors.)... 

The UNIPOL gas phase fluidized bed reactor for the production of polymers was commercialized by Union Carbide (now Dow Chemical) in 1968. This reactor produced high-density polyethylene (HDPE). The UNIPOL process was extended in 1975 to the production of linear low-density polyethylene (LLDPE) and in 1985 to polypropylene production. In the late 1980s, BP Chemicals began licensing its own gas phase Innovene fluidized bed process in competition with Union Carbide. The UNIPOL process currently holds the lion s share of the market with over 120 reaction lines sold or under construction. The reactor design is similar for all types of polymers and is shown conceptually in Fig. 9. A... 

This patent teaches the importance of preparing the same catalysts discussed in patent (VIII) above on silica calcined at a temperature at or above 800°C in order to manufacture ethylene/1-hexene copolymers with a very narrow molecular weight distribution. The catalysts shown in Table 2.6 were evaluated in a continuous gas-phase fluidized-bed reactor with triethylaluminum as cocatalyst to produce LLDPE polyethylene samples with a density of 0.918 g/cc. 

Lo and coworkers reported [46] a silica-supported catalyst that was utilized in a gas-phase fluidized-bed reactor process for the preparation of HOPE and LLDPE using (BuCp)2ZrCl2/MAO and a Davison Grade 955 silica (pore volume of 1.5 cc/g) that was previously calcined at 600 C for 12 hours. A typical catalyst formulation is shown in Table 4.5. 

In a design that most closely resembles the gas-phase fluidized-bed reactor licensed by INEOS as Innovene G and Univation as UNIPOL... 

Figure 5.17 Early gas-phase fluidized-bed reactor design from Phillips Petroleum Co. [32].

The Union Carbide U.S. Patent 4,003,712 on a gas-phase fluidized-bed reactor was issued to Adam R. Miller on January 18, 1977. This patent was a continuation-in-part of two earlier patent applications one filed on August 21, 1967 and another filed on July 29, 1970, both of which were abandoned [36]. This design is shown in Figure 5.21. 

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. 

Recent research development of hydrodynamics and heat and mass transfer in inverse and circulating three-phase fluidized beds for waste water treatment is summarized. The three-phase (gas-liquid-solid) fluidized bed can be utilized for catalytic and photo-catalytic gas-liquid reactions such as chemical, biochemical, biofilm and electrode reactions. For the more effective treatment of wastewater, recently, new processing modes such as the inverse and circulation fluidization have been developed and adopted to circumvent the conventional three-phase fluidized bed reactors [1-6]. 

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. 

The holdups can play an important role in the reactor performance. For example, in a pilot-scale trickle-bed reactor, the liquid holdup can play an important role in changing the nature of the apparent kinetics of the reaction. When homogeneous and catalytic reactions occur simultaneously, the liquid holdup plays an important role in determining the relative rates of homogeneous and catalytic reactions. In a three-phase fluidized-bed reactor, the holdup of the solid phase plays an important role in the reaction rate, particularly when the solid phase is a reactant. The gas holdup, of course, always plays an important role in reactor performance when a gaseous reactant takes part in the reaction. 

Flow maldistribution of the phases can render the evaluation of RTD data very difficult. In some cases, maldistribution may exist in small units but it may not exist in large-scale units (e g., trickle-bed reactors). While in some other cases, such as three-phase fluidized-bed reactors, nonuniform gas distribution in large-scale units may cause undesirable recirculation and dead zones. Uniform gas distribution can usually be achieved in the small-scale fluidized-bed reactor. 

Cocurrent gas-liquid-solid upflow reactor (sometimes referred to as a three-phase fluidized-bed reactor) ... 

The three-phase fluidized-bed reactor with countercurrent mode of operation was used by Pruden and Weber 10 to study the hydrogenation of a-methyl styrene to cumene in the presence of palladium black catalysts. They used low gas velocities so that the gas was dispersed as bubbles in the slurry. They showed that the countercurrent mode of operation was better than the slurry operation (with no liquid flow), due to improved catalyst usage and improved gas holdup characteristics. 

By comparison with a fixed-bed gas-liquid reaction, a three-phase fluidized-bed reactor offers the advantage of very high effective thermal conductivity and, therefore, a more uniform temperature distribution in the reactor. Van Driesen and Stewart139 have demonstrated this for large-scale catalytic desulfurization and hydrocracking of heavy petroleum fractions. 

Three-phase reactors are widely used in hydroprocessing operations and for oxidation reactions. Trickle bed reactors have been widely used for hydrodesulfurization of residue oils, hydrodesulfurization, and hydrocracking of gas oils and in numerous oxidation reactions. Three-phase fluidized bed reactors are also used in coal liquefaction and in Fischer-Tropsch synthesis. It is in these and similar examples that the review presented in this monograph can most pertinently be applied. 

Gas-liquid-solids reactors Stirred slurry reactors, three-phase fluidized bed reactors (bubble column slurry reactors), packed bubble column reactors, trickle bed reactors, loop reactors. 

In order to reduce attrition, recirculation of the particles can be achieved by the gas bubbles (gas lift) instead of by mechanical agitation (bubble column, see Fig. 8.2). External liquid recirculation offers the possibility of improving the gas-liquid mass transfer by special liquid ejector types. In the case of gas and liquid upflow a three-phase fluidized bed reactor is produced. Here particles of a few millimeters are needed to retain them in the reactor and allow separation from the gas/liquid phase. 

Gas-liquid mass transfer in a three-phase fluidized bed reactor. 

GAS-LIQUID MASS TRANSFER IN THREE-PHASE FLUIDIZED BED REACTORS... 

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