Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Patent
1996-01-04
1997-04-01
Weber, Thomas R.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
526901, C08F 234
Patent
active
056166621
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of polymerizing and copolymerizing olefins in a fluidized-bed reactor suited for the polymerization of olefins in gas phase and equipped with a stirrer.
Gas-phase polymerization of olefins is conventionally performed in a fluidized bed, where the polymerization reaction occurs in a bed comprised of particles formed during polymerization. The heat generated in the polymerization is removed by circulating through the reactor and the fluidized bed such a circulating gas that, besides the monomers to be polymerized, also frequently contains hydrogen, inert diluents or inert gases. The circulating gas is cooled in a cooler placed outside the reactor and is then passed back to the lower part of the reactor. The circulating gas rising through the reactor keeps the fluidized bed in fluidized state maintaining efficient stirring in the bed. The larger the reactor, the more difficult it is to maintain homogeneous fluidization and stirring throughout the entire volume of the fluidized bed. In fact, it has been found that particularly the lower corners of the reaction space may contain zones with insufficient fluidization and stirring, which causes inferior heat transfer. This in turn may cause local overheating, whereby the temperature of the polymer particles reaches the softening point thus initiating the growth of adhering agglomerated clumps which then stick to the reactor walls. When such agglomerated polymer clumps later detach from the reactor walls, they deteriorate the quality of the product.
Remedy to the above-described problem in fluidized-bed reactors has been attempted through complementing the reactor with different types of mechanical agitator elements which augment the stirring action. A typical agitator device used is an anchor agitator having a pair or greater number of support arms attached to a vertical drive shaft with rite ends of the arms carrying vertically aligned agitator blades at equal distances from the vertical drive shaft. A disadvantage of such a stirrer is that the polymer particles driven by the centrifugal force imposed by the stirrer tend to travel toward the inner walls of the reactor. Consequently, agglomerated polymer clumps begin to adhere to the reactor walls. If the agitator arms are relatively short, a vortex is simultaneously induced in the center zone of the reactor, whereby the circulating gas introduced to the lower part of the reactor preferentially seeks toward this vortex zone. A solution to this problem has been attempted through providing the stirrer with a number of agitator arms of different lengths.
U.S. Pat. No. 4,366,123 discloses an anchor agitator with the main agitator arms designed so long that the agitator blades attached to the ends of the arms move very close to the vertical inner wall of the reactor. Additionally, the agitator has shorter agitator arms carrying agitator blades that move closer to the drive shaft. Furthermore, at least the main agitator blades have a triangular cross section. This design aims at achieving a sufficient stirring efficiency and avoiding vibration effects induced by the rotational movement of the agitator.
U.S. Pat. No. 4,188,132 discloses a spiral stirrer of a fluidized bed reactor comprising the stirrer attached to the end of a drive shaft only at the lower pan of the spiral stirrer which at its lower end over the length of the first turn of the spiral is provided with a separate distributor spiral element. Fresh gas acting as a cooling medium is introduced to the reactor via an inlet channel of the drive shaft. The purpose of said distribution spiral element is to reverse the direction of the gas flow entering the reactor and thus prevent the formation of agglomerated clumps on the bottom of the reactor.
However, the above-cited US patents fail to cure the insufficient fluidization at the reactor comers and inner walls and the adherence of the polymer particles to the inner walls of the reactor, which typically occur in large-diameter reactors. The present invention is especially related
REFERENCES:
patent: 4188132 (1980-02-01), Lenart et al.
patent: 4366123 (1982-12-01), Kato et al.
Andtsjo Henrik
Koskinen Jukka
Sarantila Kari
Takakarhu Jouni
Borealis Polymers OY
Weber Thomas R.
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