Process for the reduction of reactor fouling

Details Process for the reduction of reactor fouling Process for the reduction of reactor fouling

1999-09-08

2001-07-31

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...

C526S194000, C526S318600, C526S329000

Reexamination Certificate

active

06268442

ABSTRACT:

TECHNICAL FIELD
The present invention concerns anti-fouling, and more particularly a process for the reduction of reactor fouling at radical initiated high pressure polymerisation of ethylene (co)polymers.
TECHNICAL BACKGROUND
Polymerisation of ethylene homopolymers and copolymers (referred to below as ethylene (copolymers) by radical initiated high pressure polymerisation is well-known in the art. Generally, the polymerisation of the monomer(s) is carried out at a temperature of about 100-300° C. and at a pressure of about 100-300 MPa in the presence of a radical initiator in a polymerisation reactor. Usually the polymerisation is carried out continuously, preferably in a tubular reactor or a stirred tank reactor.
During polymerisation fouling of the reactor may occur, especially when copolymerising ethylene with polar comonomers. The fouling manifests itself as unstable and inhomogeneous production, with formation of gels and build up of polymer deposits on the inner surfaces of the reactor. The gels and polymer deposits, when released from the reactor surfaces will contaminate the final polymer and compromise its quality. Also, the unstable production due to fouling makes it difficult to produce a polymer with a consistent and reproducible quality. Although reactor fouling may occur when producing ethylene homopolymers as well as copolymers, as mentioned above, it is particularly pronounced in connection with polymerisation of copolymers of ethylene and polar comonomers which polymerise more easily than ethylene. By way of example such polar comonomers comprise &agr;,&bgr;-unsaturated carboxylic acids having 3-8 carbon atoms, anhydrides thereof, or esters thereof with aliphatic alcohols having 1-8 carbon atoms, such as (meth)acrylic acid, methyl(meth)acrylate, ethyl(meth)acrylate, and butyl(meth)acrylate. The expression “(meth)acrylic acid” includes both acrylic acid and methacrylic acid. Similarly, “alkyl(meth)acrylate” includes alkyl acrylates as well as alkyl methacrylates. Other polar comonomers such as vinylesters of saturated carboxylic acids having 1-4 carbon atoms and in particular vinyl acetate are less prone to cause problems with reactor fouling. This is probably due to their lower reactivity which usually leaves some of the comonomer in the polymerisation mixture as a solvent. However, when making high molecular weight copolymers reactor fouling is encountered also with this type of comonomer. Examples of polar comonomers that normally do not cause fouling problems are unsaturated silane compounds, such as vinyl trimethoxy silane, vinyl triethoxysilane, gamma-(meth)acryloxypropyltrimethoxysilane and gamma-(meth)acryloxypropyltriethoxysilane.
The above-mentioned reactor fouling problem is more pronounced the higher the amount of polar comonomer is. Generally, fouling starts to be a problem already at a comonomer content of about 15% by weight, and at a comonomer content of about 20 to 25% by weight it is difficult to carry out polymerisation and recover the polymer due to heavy fouling of the reactor and contamination of the polymer. Thus, when polymerising ethylene and ethyl acrylate problems with fouling and unstable production start at about 15-16% by weight of ethyl acrylate and get increasingly more serious up to about 25% by weight of ethyl acrylate when fouling makes production almost impossible. Similarly, it is not possible to copolymerise ethylene and methyl acrylate at higher methyl acrylate contents than about 20% by weight.
It has been suggested to solve the problem of reactor fouling in different ways, e.g. by periodic reactor scraping and addition of additives such as adhesion inhibitors. According to EP-A-0,460,936 it is e.g. proposed to solve the problem by introducing into the copolymerisation reactor a solvent consisting essentially of methanol in an amount of 2-25% by weight of the total material flowing through the reactor. A disadvantage with the addition of methanol is that it acts as a chain transfer agent and reduces the molecular weight of the copolymer.
Although the solutions proposed by the prior art may alleviate the fouling problem to some degree, so far no truly effective solution has been disclosed. Because of the seriousness of the problem, an effective solution to the reactor fouling problem would be an important advantage both from a technical and economical point of view.
SUMMARY OF THE INVENTION
It is an object of the invention to overcome the disadvantages of the prior art and alleviate the above problem of reactor fouling at radical initiated high pressure (co)polymerisation of ethylene.
The object of the invention is achieved by carrying out the polymerisation in the presence of an adhesion reducing silicon containing compound.
The present invention thus provides a process for the reduction of reactor fouling at radical initiated high pressure polymerisation of ethylene (co)polymers, characterised in that an adhesion reducing silicon containing compound is added to the polymerisation reactor.
Further characteristics and advantages of the invention appear from the appended claims and the following description.
DETAILED DESCRIPTION OF THE INVENTION
Addition of the inventive silicon containing compounds eliminates reactor fouling already at very small amounts of addition to the polymerisation reactor. Generally, it is preferred to add the silicon containing compound in an amount of about 0.001-3% by weight, more preferably about 0.005-2% by weight, and still more preferably about 0.01-1% by weight, such as about 0.1-1% by weight, based on the weight of the polymer produced. The adhesion reducing silicon containing compound may be added in any suitable way to the reactor, e.g. continuously or batchwise; separately or together with the other polymerisation components (e.g. dissolved in a monomer); etc. Preferably the silicon containing compound is added continuously during the polymerisation together with one or more of the monomers to be polymerised. The point of addition of the silicon containing compound is preferably upstream (i.e. on the suction side) of the compressor feeding monomer(s) to the polymerisation reactor.
The addition of the silicon containing compound according to the invention makes possible stable polymerisation without reactor fouling. Particularly, the invention makes it possible to achieve stable polymerisation without reactor fouling when polymerising copolymers of ethylene and certain polar comonomers with higher contents of comonomer than has hitherto been possible. Thus, it is possible according to the invention to produce ethylene-methyl acrylate copolymers (EMA) with 30% by weight of methyl acrylate without any significant fouling. Further, an increased conversion rate, i.e. the percentage of the monomer(s) fed to the reactor that are polymerised, may be achieved according to the invention. As seen from Example 2 the conversion rate of about 30% without silicon compound present could be increased by about 1-2 percent units which is of great economical importance. As a further advantage of the present invention polymers having a higher molecular weight may be produced. Thus, according to the invention high molecular weight ethylene-vinyl acetate copolymers (EVA) having a melt flow rate (MFR) below about 0.3 may be produced in a stable fashion, while polymerisation of such high molecular weight EVA:s normally is so unstable due to reactor fouling as to be all but impossible.
Although the present invention is of particular advantage for the production of ethylene copolymers, it is also very favourable for the production of ethylene homopolymers where an improved and more uniform product quality may be obtained.
As mentioned above, the invention resides in adding an adhesion reducing silicon containing compound to the polymerisation reactor. By the expression “adhesion reducing silicon containing compound” used herein is meant a silicon containing compound that reduces the adhesion between the inner metal surface of the reactor and the polymer produced in the reactor during polymerisation.
In order to

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