When polymer powders are conveyed, static electricity may be formed. This is generally undesirable, and it for example may have harmful effects on the flow properties of the powder. Problems occur especially when polymers are conveyed pneumatically.

WO 96/18662 describes a multistage process for preparing polyethylene. Ethylene is first polymerized in liquid phase in loop reactors and then in a gas phase reactor. After the loop reactors gases and liquids are removed from the reaction mixture in a flash stage and only the solid polymer powder is conveyed into the gas phase reactor.

Static electricity is a known problem also in fluidized bed reactors. This causes for example sheeting during the polymerization of alpha-olefins. US 4 803 251 describes a method for reducing such sheeting by adding a charge generating chemical directly into the reactor. The chemical is an alcohol containing up to 7 carbon atoms, oxygen or nitric oxide, if negative static charge is indicated, or a ketone containing up to 7 carbon atoms, if positive static charge is indicated.

General description of the invention Now a method according to claim 1 has been invented. Some preferable embodiments of the invention are defined in the other claims.

According to the invention, an antistatic chemical is added into the flow of the polymer powder. Depending on the level of the charge, different chemicals may be added. If the charge is positive, a negative charge generating chemical is added, and if the charge is negative, a positive charge genetrating chemical is added. The antistatic chemical especially improves the flow properties of the powder.

The invention can be used for example in multistage polymerizing processes, when polymer powder has to be conveyed from one process stage to another.

Brief description of the drawings The enclosed drawings form a part of the written description.

Figure 1 is a schematic representation of a part of a multistage polymerization process.

Figure 2 is a schematic representation of the feeding system of polymer powder into the gas phase reactor of the process.

Detailed description of the invention The invention relates to processes in which polymer powder is conveyed especially pneumatically through a pipeline system, typically from one vessel to another. The first vessel may be for example a separator, and the second vesssel a reactor. The pipeline system comprises a conduit and possibly also additional vessels for storing and feeding the polymer powder. A pneumatic system is provided with suitable means for feeding carrier gas into into the pipeline. The carrier gas is preferably inert. The carrier gas may be separated from the powder before the second vessel. In this case the powder is fed into the second vessel by separate means, such as by gravity flow or by suitable conveying means. The carrier gas is usually recycled.

The polymer may be especially polyalkylene, such as polyethylene or polypropylene.

Antistatic chemical is fed into the pipeline system in order to eliminate the formation of static electricity during the conveyance. Preferably, the charge of the powder is measured in the second vessel or in the pipeline equipment and the feed of the chemical is controlled based on the measurement. Preferably, the chemical is water, or ketone, if the charge is positive, and an alcohol, oxygen or nitric oxide, if the charge is negative. The alcohol contains preferably 1 to 10 carbon atoms. The chemical is preferably fed into the pipeline equipment in the first possible connection.

The amount of the antistatic chemical may be, for example, 0.1-100, preferably 1- 10, and most preferably 2-5, moles per ton of the powder.

According to the multistage polymerization process of Figure 1, reaction mixture 1 from a first polymerization stage is fed into a flash separator 2. The mixture contains polymer and gases in a liquid medium comprising low boiling hydrocarbons. In the flash separator the hydrocarbons are evaporated and the polymer is recovered as a solid powder. The evaporated hydrocarbons are returned into the firststage. The polymer powder obtained from the separator also contains the solid catalyst from the first polymerization stage.

The first stage process is preferably such as described in WO 96/18662 and comprises two successive loop reactors for carrying out a liquid polymerization.

From the separator 2, the polymer powder is pneumatically conveyed via a conduit 3 into a first feed tank 4. The nitrogen used to convey the polymer powder is dilivered by a compressor 5. In the first feed tank nitrogen is separated from the powder and returned to the compressor. The powder from the first feed tank is led by gravity flow into a second feed tank 6, from which it is fed pneumatically into a gas phase reactor 7.

The pneumatic conveyance from second feed tank 6 to reactor 7 can be carried out by any convinient means. One method to do is presented in Figure 2. Initially, valves 13,14 and 15 are closed. The valves isolate a section 16 from the rest of the piping. When valve 13 is opened, powder falls into section 16. Then valve 13 is closed. Next, valve 14 is opened and section 16 is pressurised with nitrogen to a pressure which is higher than that in the reactor. When the pressure has been stabilised, valve 14 is closed. Now valve 15 is opened, and the high pressure in section 16 causes its contents to be discharged via the conduit into the reactor.

The lower part of reactor 7 contains a bed of polymer particles. The bed is kept in a fluidized state by a gas flow moving upwards in the reactor. The unreacted gas is collected from the top of the reactor and pressurised using a compressor. The pressurised gas is then recycled into the bottom part of the reactor thus establishing the upwardly moving gas flow. Monomer and other possible components such as comonomers and hydrogen can be fed into the lower part of the reactor. The product is removed from the reactor into a product tank 8.

The pipeline system is earth connecte 9 in order to reduce static electricity. The formation of the static electricity depends on the nature of the polymer, reaction conditions, conveying conditions, catalyst, and on the materials of the conveying

system. In most cases the charge is positive, but especially certain impurities in the polymerization gases cause negative charges.

Reactor 7 is provided with one or several charge indicators 10 in order to measure the possible static charge of the polymer powder. Depending on the measured charge, antistatic chemical is added by dosing equipment 11 into the nitrogen flow after the compressor 4. The amount of the chemical is controlled through a line 12 in accordance with the level of the measured charge.

The antistatic chemical is water, if the measured charge is positive, and propanol, if the charge is negative.

When the described equipment was used for preparing bimodal polyethylene without adding any antistatic chemical, disturbances due to static electricity occurred especially in the flow between the feed tanks 5 and 6. As a consequence, the polymer feed into gas phase reactor 7 oscillated and occasionally completely stopped. Since the polymer also contains the active catalyst, the result of the disturbances was that the amount of catalyst in the fluidised bed started to oscillate.

This in turn led to that the reaction in the gas phase reactor became unstable, which led to lower production rates and variations in the quality of the product. A lot of manual operation was needed to reestablish a stable polymer flow into the reactor.

These problems could be avoided, when the described system for adding the antistatic chemical was introduced. Static electricity was thus eliminated also in the reactor, and the stability of the polymerization was accordingly increased. The typical capacity was such that the production rate in the first stage was 6 tons per hour. The nitrogen feed was 1.2 tons per hour. The dose of the antistatic chemical needed to eliminate the static electricity was on an average 20 moles per hour (about 3.3 mol per ton of the powder).