BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a process for supply of a polymerization catalyst component to a polymerization reactor, and, in particular, in which a catalyst component is mixed with a diluent stream and passed to a polymerization reactor.

2. Description of the Prior Art

The catalytic polymerization of olefin monomers to produce polymers is well known and a number of processes are operated industrially, including in gas, solution and slurry phases. A catalyst or catalyst system typically comprises several catalyst components such as a transition metal containing catalyst (often referred to simply as a “procatalyst”), a metal alkyl cocatalyst or modifiers. In continuous commercial processes all catalyst components and monomer(s) are provided to a reactor.

Because of the relatively high productivity of modern polymerization catalysts it is not necessary, nor economic, to try to recover the catalyst components from the product, and so a continuous process must be supplied to replace the catalyst components withdrawn with the product, either continuously or discontinuously, with fresh catalyst components. Another result of the relatively high productivity of modern polymerization catalysts is that relatively small amounts of catalyst components need to be supplied.

Depending on the process the catalyst may be provided in supported or unsupported forms, and may be injected directly or mixed with other components of the reaction. In many instances it is advantageous to mix catalyst with a diluent liquid to form a slurry which is then passed to the reactor, not least because it is generally easier to control the addition of catalyst to the reactor by metering of a dilute catalyst slurry using a pump.

EP 1660231 relates to a process for preparing and supplying a catalyst slurry to a polymerization reactor in which polyethylene is prepared. The catalyst is initially in the form of a “concentrated” slurry and is diluted in an agitated mixing vessel to form a diluted catalyst slurry. This diluted slurry is then pumped to a reactor using a membrane pump.

The volume of the mixing vessel in EP 1660231 is relatively large, being sufficient to prepare a large batch of diluted catalyst slurry, including sufficient volume to fill a day tank whilst a new batch is prepared.

Whilst the system of EP 1660231 enables large batches of catalyst to be prepared and passed to a reactor it is necessary to carefully manage its inventory, both to ensure that a new batch of diluted catalyst is prepared before the old batch is all used, and also to ensure that not too much diluted catalyst is prepared prior to a change in catalyst, which might result in unused catalyst having to be dumped to prepare the batch of different catalyst. The process in EP 1660231 is also equipment “heavy”, requiring a number of agitated vessels and interconnecting pipes, as well as both a pump for transferring the dilute slurry from the mixing vessel to the reactor and additional pumps or metering valves for transferring the concentrated slurry to the mixing vessel in the first place.

SUMMARY OF THE INVENTION

The present invention provides an improved process for preparing a diluted slurry containing a catalyst component which can be performed continuously in a relatively small chamber and which uses the flow of the diluent stream to provide adequate mixing as well as the transfer of the mixed stream to the downstream reactor.Thus, in a first aspect, the present invention provides a process for supply of a polymerization catalyst component to a polymerization reactor which comprises:a. providing a first stream comprising the catalyst component in a first line, which first line is connected to and downstream of a pump outlet or of a flow control valve,b. providing a diluent stream in a second line,c. contacting the first stream and the diluent stream to form a mixed stream and passing the mixed stream to a polymerization reactor,
characterised in that the mixing of the first stream and the diluent stream takes place by providing the first stream from the first line and the diluent stream from the second line separately to a mixing chamber which has an enlarged cross-section compared to the first and second lines, and further characterised in that at least one of the following applies:i) the volume of the mixing chamber is such that the residence time, based on the total volumetric flow rate of the mixed stream, is less than 5 seconds,ii) the volume of the mixing chamber is less than 150 cm3,iii) the mixing chamber is not mechanically agitated,iv) the mixing chamber has a cover which can be removed to allow cleaning of the mixing chamber in situ.

