GB2156363A - Preparation of olefin polymers - Google Patents

Abstract

In the polymerisation of olefins having at least three carbon atoms, polymer slurry from the reactor is introduced into a devolatilising extruder and compressed so that at least 75% of the liquid medium is separated off and recycled as liquid, thus avoiding the need for compression of the recycle stream. Any remaining liquid is withdrawn in gaseous form.

Description

SPECIFICATION Preparation of olefin polymers This invention is concerned with a process for the preparation of olefin polymers, particularly polypropylene.

The well-known liquid bulk propylene polymerization process is based on the production of a polymer slurry at a temperature of at least 60"C and a pressure of at least 2000 kPa in a liquid medium comprising propylene monomer as the major constitutent. In working- up the slurry withdrawn from the reactor, it is conventional to incorporate a drying step in which unconverted monomer is removed from the polymer by depressurizing which causes the monomer to evaporate. Employing one or more compressors, the evaporated monomer is then recycled to the polymerization reactor.

Since the solids concentration in the polymer slurry is set at a certain maximum level for practical reasons, it follows that a fairly large volume of monomer has to be recycled to the reactor, thus necessitating the use of large, expensive compressors.

The present invention aims at reducing the volume of monomer to be recycled via compressors to the reactor.

Thereto the invention provides a process for the preparation of an olefin polymer which comprises polymerizing an olefin with at least three carbon atoms at a temperature of at least 60 C and a pressure of at least 2000 kPa to form a slurry of polymer in a liquid medium comprising at least 20 % vol. of liquid olefin monomer, withdrawing polymer slurry from the reactor and, without depressurizing, introducing slurry into a devolatilizing extruder in which by compressing the slurry at a temperature below the melting point of the polymer at least 75 % of the liquid medium introduced into the extruder is separated from the polymer, and withdrawn from the extruder through one or more venting openings, this part of the liquid medium being recycled to the polymerization reactor, and in which the remaining part of the liquid medium is separated from the polymer at a temperature above the melting point of the polymer and withdrawn in gaseous form from the extruder through one or more venting openings at a pressure of at least 1000 kPa less then the polymerization pressure, that part of the liquid medium being compressed to a pressure which is at least equal to the polymerization pressure and recycled to the polymerization reactor, Whilst the invention is of particular interest in the production of propylene polymers, both honoand copolymers, it should be understood that other olefin polymers are also applicable, e.g.

those obtained by polymerization of butene-1, butene-2, isobutene and pentene-1. Propylene copolymers comprise those of propylene and higher olefins, e.g. butene-1, as well as ehtylene-propylene copolymers. The latter copolymers can be made in two or more successive reaction stages, e.g. employing a first reactor in which propylene is homopolymerized, and a second reactor in which ethylene and propylene are copolymerized to form a copolymer comprising 40 to 65 % mol of ethylene, balance propylene. In such event, the polymer slurry to be treated in accordance with the present invention is the one withdrawn from the last reactor. To increase the production capacity of the polymerization reactors the polymerization or copolymerization will preferably be effected at a temperature of at least 70 "C and a pressure of at least 2750 kPa.

It is not essential that the liquid medium merely consists of olefin monomer, other hydrocarbon diluents may be employed in addition to the olefin monomer, e.g. propane, butane, isobutane or isopentane; preferably less than 30 % vol of such other hydrocarbon diluent is contained in the liquid medium. However, it is critical that only such additional diluents are employed that will completely evaporate upon depressurizing the molten polymer in the devolatilizing extruder. Thus, in the recycle stream which includes a compressor, gaseous medium must be withdrawn through the low pressure venting opening in this extruder. Preferably this venting opening is operated at atmospheric or subatmospheric pressure.The latter recycle stream should be operated to keep the gas volume to be recycled as low as possible, preferably less than 10 % of all liquid medium entering into the extruder is removed via the relevant low pressure venting opening, even better results are obtained when this percentage is kept below 3.5 %.

In order to reduce the volume of liquid to be separated from the polymer by compressing the polymer slurry in the extruder at a temperature below the melting point of the polymer, it is possible and indeed preferred, to concentrate the slurry before it is passed through the inlet opening of the extruder. This may be effected by employing conventional slurry concentrating equipment, such as a settler, hydrocyclone, centrifuge, or by partial filtering. The separated phase may then comprise either a diluted polymer slurry or a polymer-free liquid phase, both can of course be recycled to the polymerization reactor. Preferred slurry concentration methods are those wherein solids concentration of from 20 to 49 % wt is increased to a solids concentration of from 50 to 75 % wt.It is again essential that any slurry- concentrators are operated at a pressure substantially equal to the pressure prevailing in the polymerization reactor.

The invention is of particular importance when the polymerization is effected with advanced high activity/high selectivity MgCl2-supported coordination catalysts, e.g. of the type disclosed in British Patent Specification 1,559,194. In that event the polymer slurry is directly passed from the polymerization reactor to the extruder or to intermediate slurry concentrators, hence, excluding the use of any and all equipment for catalyst deactivation and washing and equipment for polymer extraction to remove atactics.

In the extruder the polymer slurry is compressed, basically by volume reduction. This can be effected when employing a single or twin screw with a gradually decreasing pitch or with a gradually increasing screw core diameter. As a result of the compressing action liquid is squeezed out of the polymer thus yielding a nearly dry mass of compacted polymer particles. Melting of the polymer should be avoided at this stage, this can be achieved by operating external cooling means, whenever required. It is however not critical that the liquid medium separated from the polymer at this stage be vented from the extruder in the form of a liquid phase. Partial evaportion is allowed and when employing mixed liquid media, such as a mixture of propylene and propane, one or both of these constituents may preponderantly be present in vaporized form.

Since compressing the slurry will normally result in an increased pressure it will be clear that the pressure in the venting opening or openings in this stage of the extruder is higher than the pressure previaling in the polymerization reactor. Therefore no compression of this recycle stream is required for conveying recycled medium to the reactor.

It is thus seen that the only compression needed in the recycle streams comprising recovered monomer is the one in the gaseous streams withdrawn from the extruder at the stage when molten polymer is depressurized. Since this stream comprises less than 10% of the monomer that has been introduced into the extruder it follows that with this invention an important reduction in compression capacity needed for monomer recycle has been achived.

Example Propylene was continuously polymerized at a temperature of 75 "C and a pressure of 3020 kPa with the aid of a high activity/high selectivity MgCI2-supported coordination catalyst, in a stirred reactor to form a homopolymer slurry (40 %wt solids) in liquid medium comprising 90 % vol of propylene and 10 % vol of propane. The slurry directly pumped, from the reactor at a rate of 28125 kg/hr into a solidslliquids settler which was assembled to the feed hopper of a single-screw devolatilizing extruder. A diluted polymer slurry (24.6 %wt solids) was withdrawn from the settler at a rate of 15194 kg/hr and recycled to the reactor. A concentrated slurry (58 %wt solids) was passed through the hopper into the extruder. The entire system of piping, settler and hopper was kept at a pressure of 3020 kPa.

The extruder was provided with a first, unheated, compressing section in which the pitch of the screw was gradually reduced to provide for a reducing volume. Thus, liquid was squeezed out from the slurry to yield an almost dry compacted polymer powder. The liquid was withdrawn through two narrow venting openings, at a rate of 5306 kg/hr and a pressure above 3025 kPa. This liquid was recycled to the polymerization reactor. In a subsequent section of the extruder the polymer was melted, compressed, depressurized to atmospheric pressure, compressed again and extruded through a number of orifices provided with a facecutter to produce polymer granulate at a rate of 7504 kg/hr. From the atmospheric zone a gaseous stream of propane and propene was withdrawn through a single venting opening at a rate of 121 kg/hr. With the aid of a small compressor this stream was recycled to the reactor.

Claims (5)

1. A process for the preparation of an olefin polymer which comprises polymerizing an olefin with at least three carbon atoms at a temperature of at least 60 "C and a pressure of at least 2000 kPa to form a slurry of polymer in a liquid medium comprising at least 20 % vol of liquid olefin monomer, withdrawing polymer slurry from the reactor and, without depressurizing, introducing slurry into a devolatilizing extruder in which by compressing the slurry at a temperature below the melting point of the polymer at least 75 % of the liquid medium introduced into the extruder is separated from the polymer, and withdrawn from the extruder through one or more venting openings, this part of the liquid medium being recycled to the polymerization reactor, and in which the remaining part of the liquid medium is separated from the polymer at a temperature above the melting point of the polymer and withdrawn in gaseous form from the extruder through one or more venting openings at a pressure of at least 1000 kPa less than the polymerization pressure, that part of the liquid medium being compressed to a pressure which at least equal to the polymerization pressure and recycled to the polymerization reactor.

2. A process as claimed in claim 1, in which the liquid medium withdrawn from the extruder in gaseous form represents less than 10 % of the liquid medium introduced into the extruder.

3. A process as claimed in claim 1 or 2, in which, before being introduced into the extruder, the slurry is concentrated from a solids concentration of from 20 to 49 %wt a solids concentration of from 50 to 75 %wt, and liquid medium removed from the slurry is recycled to the polymerization reactor.

4. A process as claimed in claims 1-3, in which the medium in gaseous form is withdrawn from the extruder at atmospheric or subatmospheric pressure.

5. A process as claimed in claim 1 and substantially as hereinbefore described with particular reference to the Example.