Continuous gas-phase polymerization process

Details Continuous gas-phase polymerization process Continuous gas-phase polymerization process

1998-11-30

2001-01-30

Wu, David (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S082000, C526S083000, C526S084000, C526S088000, C526S089000

Reexamination Certificate

active

06180729

ABSTRACT:

The present invention relates to a continuous gas-phase polymerization process for preparing ethylene and propene homopolymers and copolymers, in which ethylene, propene or a mixture of ethylene or propene and C
3
-C
8
-&agr;-monoolefins is polymerized in the polymerization zone of a gas-phase polymerization reactor at from 30 to 125° C. and a pressure of from 1 to 100 bar in the gas phase in a bed of finely divided polymer in the presence of a catalyst, where the reactor gas is circulated to remove the heat of polymerization.
The invention further relates to the use of catalyst poisons as means of preventing deposits in the circulating gas line of gas-phase polymerization plants.
Gas-phase polymerization processes are economical processes for the polymerization of ethylene and propene or for the copolymerization of ethylene or propene with C
3
-C
8
-&agr;-olefins. Such gas-phase polymerization processes can be configured either as gas-phase fluidized-bed processes or as stirred gas-phase processes. Said processes are described, for example, in EP-A-0 475 603, EP-A-0 089 691 and EP-A-0 571 826.
It is characteristic of gas-phase fluidized-bed processes that the bed of polymerizing polymer particles is kept in a fluidized state by introduction of a gas mixture from below. In addition, the heat of polymerization liberated is removed from the reactor by this gas. The reaction gas is cooled in a heat exchanger located outside the reactor and is recirculated to the reactor through a gas distribution plate (circulating gas).
However, the circulating gas carries a certain amount of finely divided polymer from the reactor and into the circulating gas system. These polymer particles contain active catalyst and can thus continue to polymerize in the circulating gas system. If these particles precipitate in the circulating gas system, deposits and accumulations can be formed at these places. On the one hand, these deposits can themselves cause malfunctions (blocking of the cooler, conglutinations at the compressor) and, on the other hand, pieces of these deposits can also become detached again. This is undesirable from two points of view. The detached deposits can quickly block the holes of the gas distribution plate of the reactor and thus require shutdown and costly cleaning. If such pieces of deposit get through the gas distribution plate into the reactor, the product quality is impaired by these particles: specks are formed. Particularly in the case of products for film applications, this can lead to out-of-specification material.
Past attempts to solve this problem were primarily based on reducing the amount of fine polymer dust in the circulating gas. For this reason, many fluidized-bed plants include a cyclone downstream of the reactor outlet to remove this reactive fine dust. However, this cyclone represents a considerable outlay in terms of apparatus and its efficiency in removing very fine dusts, which, however, often contain a very high proportion of catalyst, is often unsatisfactory.
The use of catalyst poisons in gas-phase fluidized-bed polymerizations is already known. Thus, catalyst poisons are used, for example, to stop polymerizations (e.g. in the case of a polymerization reaction which has run out of control, see, for example, EP-A-0 471 497) or for the fine regulation of catalyst activity (see, for example, EP-A-359 444 or EP-A-376 559). However, in continuous processes, volatile compounds, generally gases such as CO
2
, CO or O
2
, are always used because their site of action is believed to be primarily in the reactor and a uniform distribution in the reactor chamber is desirable. These agents are generally unsuitable for targeted inactivation of catalyst-containing fine dusts in the circulating gas line, since they have a considerable influence on the polymerization reaction in the reactor.
It is an object of the present invention to find a way of preventing the formation of polymer deposits in the circulating gas line of gas-phase polymerization reactors without use of complicated apparatus, to increase the running time of such plants and to improve the quality of the polymerization products.
We have found that this object is achieved by a continuous gas-phase polymerization process for preparing ethylene and propene homopolymers and copolymers, in which ethylene, propene or a mixture of ethylene or propene and C
3
-C
8
-&agr;-monoolefins is polymerized in the polymerization zone of a gas-phase polymerization reactor at from 30 to 125° C. and a pressure of from 1 to 100 bar in the gas phase in a bed of finely divided polymer in the presence of a catalyst, where the reactor gas is circulated to remove the heat of polymerization, wherein, to prevent polymer deposits in the circulating gas line, a catalyst poison having a boiling point above the maximum temperature within the circulating gas line is metered into this circulating gas line in at most such an amount that it does not significantly impair the productivity of the catalyst in the reactor.
Furthermore, we have found the use of such catalyst poisons as means of preventing deposits in the circulating gas line of gas-phase polymerization plants.
The process of the present invention can be carried out in a gas-phase fluidized-bed reactor as is described in detail, for example, in EP-A-0 475 603, EP-A-0 089 691 or EP-A-0 571 826, or in a gas-phase stirred reactor. The following description relates predominantly to gas-phase fluidlized-bed processes, but can also be applied analogously to stirred gas-phase polymerization processes. In general, the gas-phase fluidized-bed reactor is a more or less long tube through which circulated reactor gas flows. In general, the circulated reactor gas is fed in at the lower end of the gas-phase fluidized-bed reactor and is taken out again at its upper end. The circulated reactor gas is usually a mixture of ethylene or propene, if desired a molecular weight regulator such as hydrogen and inert gases such as nitrogen and/or saturated hydrocarbons such as ethane, butane or hexane. The reactor gas can further comprise C
3
-C
8
-&agr;-monoolefins such as propene, 1-butene, 1-pentene, 2-methylpentene, 1-hexene, 1-heptene and 1-octene as comonomers. Preference is given to a process in which ethylene is copolymerized with 1-hexene. The flow rate of the reactor gas has to be sufficiently high, on the one hand, to fluidize the bed of mixed, finely divided polymer which is located in the tube and serves as polymerization zone and, on the other hand, to effectively remove the heat of polymerization.
To set constant reaction conditions, the constituents of the reactor gas can be fed into the gas-phase fluidized-bed reactor directly or via the circulated reactor gas. In the process of the present invention, it is advantageous to introduce the catalyst directly into the fluidized bed. It has been found to be particularly advantageous to meter the catalyst a little at a time together with nitrogen or argon directly into the bed of material, using the method described in DE-A-35 44 915.
In order to prevent finely divided polymer being carried from the polymerization zone into the circulating gas system, the gas-phase fluidized-bed reactor used for the process of the present invention has, at its upper end, a widened-diameter calming zone which reduces the velocity of circulating gas. In general, it is advisable to reduce the circulating gas velocity in this calming zone to from one third to one sixth of the circulating gas velocity in the polymerization zone.
After leaving the gas-phase fluidized-bed reactor, the circulated reactor gas is fed to a circulating gas compressor and a circulating gas cooler. These equipment items may be installed in either order. The cooled and compressed circulating gas is then passed via a customary and known gas distribution plate back into the mixed bed of material in the gas-phase fluidized-bed reactor. This results in an extremely homogeneous gas distribution which ensures good mixing of the bed of material.
In the process of the present invention too, the ratio

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