Gas phase polymerization process

Details Gas phase polymerization process Gas phase polymerization process

2000-11-16

2002-03-19

Teskin, Fred (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S082000, C526S901000

Reexamination Certificate

active

06359084

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a process for the gas phase polymerisation of alpha-olefins in a fluidised bed reactor and in particular to a process for the introduction of a deactivating agent into the reactor whilst maintaining a substantially constant polymerisation rate.
It is known to polymerise one or more alpha-olefins, such as ethylene or propylene, in the gas-phase in a fluidised bed reactor, in the presence of a catalyst based on a transition metal belonging to the groups IV, V or VI of the Periodic Table of the Elements; in particular in the presence of a catalyst of the Ziegler-Natta type, chrominum oxide type or a metallocene catalyst. Catalyst particles, together with growing and formed polymer particles are kept in a fluidised and/or agitated state in a gaseous reaction mixture containing the alpha-olefin or alpha-olefins, which are introduced continuously into the reactor. The catalyst is introduced continuously or intermittently into the reactor while the polymer constituting the fluidised bed is withdrawn from the, reactor, also continuously or intermittently. The heat of the polymerisation reaction is essentially removed by the gaseous reaction mixture, which may be passed through a heat transfer means before being recycled into the reactor.
It is also known that in a gas-phase fluidised bed polymerisation reaction system, small variations in reaction conditions can have an adverse effect on the polymerisation reaction. For example, poor or loss of fluidising gas flow can cause catalyst particles, formed polymer particles and growing polymer particles to be insufficiently cooled by the gaseous reaction mixture passing through the reactor and this can give rise to the appearance of lacalise zones of catalyst particles and/or polymer particles e.g hot spots having increased temperature compared to the average temperature of the fluidised bed. These zones of increased temperature are likely to occur in the vicinity of, especially at or close to the reactor walls. Where a polymerisation reactor is equipped with a fluidisation grid the zones will typically occur in the region 0.25D to 0.75D above the grid (where D is the diameter of the reactor). Such zones are usually detected by the use of thermocouples or temperature indicators either attached to the reactor wall or inserted a small distance into the reactor itself.
The appearance of such zones of increased temperature can lead to the melting of the polymer particles resulting in the formation of agglomerates of molten polymer. If the reaction conditions are corrected sufficiently early, for example by lowering the polymerisation temperature or pressure, or by reducing the rate at which catalyst is supplied to the reactor in order to restrict such adverse effects, the amount and size of the agglomerates formed can be reduced to a certain extent. During this period, however, it will not be possible to avoid a drop in the polymer production and a probable deterioration in the quality of the polymer manufactured.
If the formation of agglomerates becomes severe there is a danger of creating irreversible process problems which will require the reactor to be shut down. One way in which the polymerisation process can be terminated quickly to avoid this is to introduce a deactivating agent whose purpose is to kill or terminate the fluidised bed polymerisation.
European Patent EP-B-0 471 479 discloses a process for completely stopping a gas-phase alpha-olefin polymerisation reaction carried out with the aid of a chromium oxide-based catalyst by introducing a deactivating agent such as oxygen, ammonia or carbon monoxide into the polymerisation reactor
U.S. Pat. No. 4,306,044 discloses a system for introducing carbon dioxide into a gas-phase olefin polymerisation process to at least reduce the rate of the polymerisation reaction.
SUMMARY OF THE INVENTION
A process for the gas-phase polymerisation of an alpha-olefin has now been found which makes it possible to overcome, or at least mitigate, the above-mentioned disadvantages. In particular, the process makes it possible to treat, e.g deactivate, zones of increased temperature compared to the average temperature of the bed thereby controlling the formation of agglomerates. Moreover, the process allows the zones to be treated without killing or deactivating the entire fluidisation bed. Furthermore, the process according to the present invention obviates the need to stop production or shut down the reactor. Furthermore, the process of the present invention uses less deactivating agent than is theoretically required to slow down or stop an alpha-olefin polymerisation reaction.
The present invention therefore relates to a process for the gas-phase polymerisation of one or more alpha-olefins conducted in the presence of a catalyst in a reactor having a fluidised bed in which process, one or more zones having a temperature greater than the average bed temperature are formed at or in close proximity to the reactor wall, characterised in that said zones are deactivated by the introduction into the reactor of an effective quantity of a deactivating agent.
DETAILED DESCRIPTION OF THE INVENTION
The deactivating agent may be selected from a wide variety of products which are capable of reducing the polymerisation rate of an alpha-olefin in the presence of a catalyst based on a transition metal e.g a catalyst of the Ziegler-Natta type or a metallocene catalyst, or a chromium oxide type catalyst. The deactivating agent can be selected especially from polymerisation inhibitors or from poisons known for this type of reaction. Deactivating agents which can be selected in particular are carbon monoxide, oxygen and water, especially carbon monoxide.
The deactivating agent may be employed alone or, preferably, diluted in an inert gas such as nitrogen. When carbon monoxide is employed as the deactivating agent, it may be employed in the form of a gaseous mixture with an inert gas such as nitrogen. A mixture of two or more deactivating agents may be employed.
The deactivating agent is introduced into the polymerisation reactor in a quantity which is effective to deactivate zones of increased temperature at or in close proximity to the reactor walls such as hot spots and/or fouling without substantially affecting the polymerisation rate. For this purpose the deactivating agent is preferably introduced into the polymerisation reactor in an amount such that the weight ratio of the deactivating agent to the catalyst is in the range 0.0002-0.0011:1, preferably 0.0004-0.0010:1, especially 0.0007-0.0009:1. For example it has been found to be possible to deactivate hot spots and fouling without reducing the polymerisation reaction by introducing into the polymerisation reactor 0.0004-0.0011 g of carbon monoxide per gram of catalyst. The use of a quantity of deactivating agent which is too large would have the effect of stopping the polymerisation reaction. The minimum quantity of deactivating agent necessary for stopping a polymerisation reaction can be obtained by previous experimentation performed in a gas phase reactor working with known quantities of catalyst and of deactivating agent.
The deactivating agent is introduced into the polymerisation reactor over a relatively short period of time typically less than 5 minutes. The period of introduction of the deactivating agent is advantageously as short as possible and is preferably shorter than one minute and more preferably shorter than 30 seconds. The deactivating agent may be introduced intermittently throughout the polymerisation reaction i.e it may be introduced as and when required to deactivate any increased temperature zones which form throughout the polymerisation reaction. Furthermore, the feeding of catalyst and/or olefin into the polymerisation reactor need not be discontinued.
The deactivating agent may be introduced directly into the polymerisation reactor, preferably into a zone of the reactor where the deactivating agent is dispersed rapidly, for example underneath a fluidisation grid. Advantageously it

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