Static reduction

Details Static reduction Static reduction

2002-11-01

2003-10-07

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S070000, C526S901000, C526S348000, C526S170000, C526S131000, C526S141000

Reexamination Certificate

active

06630548

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a polymerization process for olefins. In particular, the invention relates to a process that reduces reactor static.
BACKGROUND OF THE INVENTION
Reactor fouling is a known problem in the gas-phase polymerization of olefins. The fouling is due to static charge, which builds and causes solids to adhere to surfaces. The buildup of solids on the surfaces of the polymerization reactor decreases productivity and can decrease the quality of the product. The problem is sometimes manifested as sheeting. By “sheeting” is meant the adherence of fused catalyst and resin particles to the walls of the reactor. The sheets will eventually dislodge from the wall and can result in reactor plugging. Reactor buildup can be extensive enough to require shutting down the equipment for cleaning.
Several solutions to this problem have been developed, but they all have drawbacks. One partial solution is to polish the surface of the reactor in order to avoid polymer adhesion. However, this kind of polishing is expensive and the effect is not durable.
Another solution is to add antistatic or antifouling agents into the polymerization medium in order to reduce polymer buildup on the surface of the reactor. However these can be catalyst poisons or can affect polymer properties since they remain in the polymer.
In PCT application WO 93/23436, European Pat. No. 0307074, and PCT application WO 97/49771 reactor coatings are disclosed. The application of coatings adds to the cost and requires reactor down time while the coatings are applied. Coatings are not completely durable and the coating material can contaminate the product polyolefin.
U.S. Pat. No. 6,300,429 uses a falling film of water around the periphery of a fluid-bed olefin polymerization reactor to cool the wall and reduce the temperature in the fluid bed near the inside surface of the wall. Cooling has the effect of reducing static in the reactor, which in turn ameliorates a sheeting problem and can enhance production by facilitating control of the relation of the reactor temperature and the dew point of recycled gas. The process may be used concurrently with a gas cooling and condensing recycle system wherein at least some of the condensed recycle gas is injected in the vicinity of the internal wall surface. There is some added cost and complexity with this process and the results are not completely satisfactory.
U.S. Pat. No. 5,277,245 discloses a method for enhancing heat transfer in a bed of powder comprising cohesive Geldart type C particles, such as phosphors or ultrafine ceramic powders, being fluidized. The fluidizing gas is selected so as to comprise a sufficient amount of helium, hydrogen, or mixtures thereof for obtaining a thermal conductivity of the fluidizing gas of at least four times that of nitrogen for enhancing heat transfer between the wall and the bed. There is no mention of any reaction or polymerization processes. Nor is there any indication of reduced static or of reduced wall buildup. The focus seems to be merely the ability to fluidize these difficult-to-fluidize powders.
U.S. Pat. No. 6,212,794 also talks about the difficulty to fluidize certain powders such as Geldart group C powders and discloses the use of low molecular weight gases such as hydrogen, helium, deuterium and tritium. Again there is not any indication of reduced static or of reduced wall buildup. The focus seems to be merely the ability to fluidize these powders.
U.S. Pat. No. 4,003,712 discloses a fluidized bed reactor where the catalyst is stored in a reservoir under a nitrogen blanket and injected at a point about ¼ to ¾ of the height of the bed and above the distribution plate. All or part of the make up feed stream is used to carry the catalyst into the bed.
U.S. Pat. No. 5,240,683 and references cited therein disclose various devices for feeding a catalyst powder into a fluidized bed reactor. In order to give an improved feed, previous discontinuous particle delivery systems are made more continuous by using an intermediate chamber in the catalyst delivery system. While gases are used to convey the solid particles, there is no indication of any criticality to the choice of gas. The examples in the patent use a mixture of ethylene, 1-butene, hydrogen and nitrogen.
U.S. Pat. No. 6,111,034 and references cited therein teach the correlation between static buildup and reactor fouling or sheeting. They control static buildup by addition of water to dissipate the charge. While this is effective at reducing the static, water can have a deleterious effect on catalyst activity and in some situations can completely deactivate the catalyst.
Despite the importance of olefin polymerizations and the considerable work that has been done to find methods to reduce static and consequent buildup on reactor walls, there are still drawbacks to current methods. Even an incremental improvement can have a large economic effect since it decreases reactor downtime and can improve polyolefin quality.
SUMMARY OF THE INVENTION
This invention is a gas-phase process for polymerizing an olefin. Static is reduced by feeding the catalyst into the reactor in a stream of gas that comprises at least 75 volume % of a noble gas.
DETAILED DESCRIPTION OF THE INVENTION
The invention is an olefin polymerization process. Suitable olefins are C
2
-C
20
&agr;-olefins, such as ethylene, propylene, 1-butene, 1-hexene, 1-octene and mixtures thereof. Preferred olefins are ethylene, propylene and mixtures thereof with a-olefins such as 1-butene, 1-hexene and 1-octene.
The polymerizations are done in the gas phase. Preferably, they are done in continuous reactors. There are many suitable reactor designs for the polymerization of olefins in the gas phase. The gas-phase, fluidized-bed reactors for which this invention is most useful are those such as described in U.S. Pat. Nos. 4,003,712, 4,482,687, 4,803,251, 4,855,370, 5,428,118 and 5,733,510, the teachings of Which are incorporated herein by reference. These and other preferred reactors are characterized by a straight cylindrical section topped by a bulbous expanded section and are widely used for polymerizing olefins. They are large reactors having relatively thick steel walls, i.e. about 2-10 cm thick. These reactors are useful for the polymerization of olefins in the gas phase. A catalyst is also introduced, and as polymerization proceeds, small particles of polymer product are formed, which are suspended in the gas as a fluid bed.
Usually, the fluidized bed is supported by upwardly flowing gases. The gases comprise monomers and usually one or more inert gases such as nitrogen, ethane, propane, butane, isobutane and hexane. The gases may also contain effective amounts of components that may affect the reaction such as moderators, chain terminators, and static-control agents. Those components include hydrogen, water vapor, carbon monoxide, carboxyl-containing compounds such as ketones, aldehydes and carboxylic acids, and hydroxyl-containing compounds such as alcohols and glycols including methanol, ethanol, isopropanol, propanol and ethylene glycol. In the present invention, the gas components preferably also include noble gas present in the recycle gas from use in the catalyst feed section.
The superficial velocity of the upwardly flowing gas is preferably sufficient to support and fluidize the bed. The minimum velocity for fluidization will be dependent upon the density and size of the polymer particles and the density and viscosity of the fluidizing gas. Especially since the fluidized bed will contain a range of particle sizes, the superficial velocity should not be so high as to result in undue removal of polymer particles from the bed. Preferably, the velocity of the fluidizing gas during operation at or near the maximum design height of the bed is such that the top surface of the bed is sufficiently turbulent that particles are thrust into the expanded section. These particles include those that are typically not entrained by the fluidizing gas. These particles thrust into the ex

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