Suspension polymerization for preparing polyolefins

Details Suspension polymerization for preparing polyolefins Suspension polymerization for preparing polyolefins

1998-09-18

2001-03-20

Teskin, Fred (Department: 1713)

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

Synthetic resins

Removing and recycling removed material from an ongoing...

C526S065000, C526S067000, C526S088000, C526S348600

Reexamination Certificate

active

06204345

ABSTRACT:

The invention relates to an olefin polymerization process for preparing ethylene copolymers by polymerizing ethylene with, as comonomer, up to 10% by weight, based on the total amount of the monomers, of a 1-olefin of the formula R—CH═CH
2
, where R is a straight-chain or branched alkyl radical having from 1 to 4 carbon atoms, in a polymerization reactor and in suspension with a dispersion medium at a temperature of from 20 to 100° C. and a pressure of from 2 to 20 bar, in the presence of a catalyst, followed by separation of the dispersion medium from the polymer, and with reduced production of wax, and in which there is selective precipitation of wax onto the polymer, giving improved yield.
Olefin polymerization is a structure-building reaction which—starting with the monomer via oligomeric compounds—finally forms the high-molecular-weight polyolefins. Because of its nature, the reaction always gives a distribution of molecular weights including, even if in small amounts, low-molecular-weight constituents, i.e. oligomeric compounds with chain lengths of up to 60 carbon atoms.
It is known that these “waxy constituents” remain on the grains of the polymeric product in polymerization processes carried out in the gas phase. However, in polymerization processes conducted, in contrast, in a dispersion medium, the low-molecular-weight polymeric compounds become dissolved to some extent in the dispersion medium.
Suspension polymerization processes are operated with a “high-boiler” as dispersion medium. Commonly used dispersion media are petroleum cuts with a boiling point of above 60° C. (at 1 bar), such as hexane, isooctane, nonane, etc. In the suspension polymerization process, the polymer is mechanically removed from the dispersion medium after the polymerization reaction. Some portion of,. or all of, the dispersion medium used is reintroduced into the polymerization reactors. The portion of the dispersion medium which, if desired, is not reintroduced into the reactors is generally regenerated by distillation, during which the dissolved waxy constituents appear as a paste-like to solid residue which has to be disposed of.
However, there are also versions of the process in which, during the subsequent course of the process, the wax removed by distilling the dispersion medium is, for example, sprayed onto the product and utilized together therewith. However, in particular for high-quality products, this measure has an adverse effect on processing and quality.
A further difficulty is that in the case of polymerizations which are carried out continuously certain polyolefin product types can be prepared only using two reactors connected in series, in which different reaction conditions are established. The reaction rich in comonomer is generally carried out in the second reactor. The manner of interconnection of the apparatus and the requirements of the mixing specification make it essential to discharge a high proportion of dispersion medium out of the polymerization process and to regenerate the same by distillation. One of the results of this is that a considerable amount of wax is produced, and this can be up to 3% by weight of the starting materials.
The wax which is discharged comprises, inter alia, polymer molecules with chain lengths above C-24, preferably above C-30 and in particular above C-40, which can be processed with the high-molecular-weight polymer in downstream application sectors, without sacrificing quality. However, those oligomeric compounds with a chain length of up to C-24 have proven disadvantageous for the processing stage.
The object of the invention is to find a process for removing these oligomers up to a chain length of 24 carbon atoms.
The invention relates to a process of the type mentioned at the outset which comprises, after carrying out the polymerization and before removing the dispersion medium from the polymer, partly evaporating the suspension stream, with the result that some of the wax formed in the polymerization is adsorbed onto the polymer, and feeding the resultant vapor stream directly into the reactors after condensation or back into the process after distillative removal of the comonomer.
Surprisingly, it has been found that in the novel process the fractions of wax produced which, although dissolved by the dispersion medium, do not bring about a reduction in quality, are adsorbed onto the polymer, thus increasing the yield of the process and reducing the need to dispose of wax.
The adsorbed wax fractions are generally composed of oligomers above C-24, but preferably of oligomers above C-30 and in particular above C40.
During the partial evaporation, from ⅕ to ½ of the dispersion medium, preferably from ¼ to ⅓ of the dispersion medium used in the process, is generally drawn off in the vapor stream as vapor.
The evaporation preferably takes place at reduced pressure, and the pressure during the evaporation is from 0.1 to 0.8 bar, in particular from 0.3 to 0.5 bar.
The novel polymerization uses, as comonomer, a proportion of up to 10% by weight, based on the total amount of the monomers, of a 1-olefin of the formula R—CH═CH
2
, where R is a straight-chain or branched alkyl radical having from 1 to 4 carbon atoms. Comonomers of preferred suitability are butene, pentene, hexene and 4-methyl-1-pentene.
In order to avoid impairing the properties of the end product, care must be taken at the start of the polymerization, i.e. in the first reactor, that a possible comonomer concentration which may in some circumstances find its way into the reactor via regenerated dispersion medium does not exceed 0.04% by weight at a pressure of 8 bar and a temperature of 80° C. The comonomer concentration is generally from 0 to 0.04% by weight, preferably from 0 to 0.03% by weight.
The novel process contributes to increased cost-effectiveness of the polymerization process for polyolefins and to reduction of environmental pollution via lower amounts of wax for disposal. Since disposal also gives rise to considerable costs, cost-effectiveness is further increased by producing lower amounts of wax.
The dispersion media used in the novel process are cuts from a fraction of saturated hydrocarbons with a boiling point (at 1 bar) of above 60° C. Examples of these are hexane, isooctane, nonane and others.
The novel process is described below in greater detail using FIGS.
1
and
2
.


REFERENCES:
patent: 3126365 (1964-03-01), Hooker
patent: 3502633 (1970-03-01), Schwaar et al.
patent: 561782 (1963-04-01), None
patent: 570514 (1959-02-01), None
patent: 653951 (1962-12-01), None
patent: 1420678 (1968-11-01), None
patent: 1520556 (1969-04-01), None
Database WPI, Section Ch, Week 9731, Derwent Publications Ltd., London, GB; Class A17, AN 97-337077, XP 002089173.
Chem. Zentralbl. 1964/Heft—25/26.
Chem. Zentralbl. 1965/Heft—5.
Chem. Zentralbl. 1964/Heft—47.

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