Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-07-31
2003-05-13
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S083000, C526S084000, C526S226000, C526S348000, C526S352000, C422S131000
Reexamination Certificate
active
06562915
ABSTRACT:
The present invention relates to a process for the polymerization of ethylenically unsaturated monomers with introduction of inhibitors and to the use of the inhibitors.
Polymerization processes in which ethylene polymers are produced by homopolymerization of ethylene or copolymerization of ethylene and compounds which are copolymerizable with ethylene at from 150 to 350° C. and pressures of from 1500 to 3500 bar in continuously operated polymerization reactors in the presence of polymerization initiators which decompose to form free radicals are known. A description of this process and a corresponding process flow diagram may be found in Ullmanns Encyclopädie der technischen Chemie, Verlag Chemie GmbH, Weinheim, Volume 19 (1980), pages 169-178.
In this process, fresh monomer gas is usually compressed in a 5- or 6-stage precompressor from about atmospheric pressure to about 200-300 bar, and the compressed gas is cooled in an intermediate cooler after each stage to remove the heat of compression. In a postcompressor, the gas is compressed to a final pressure of from about 1500 to 3500 bar. In the reactor, namely a tube reactor or autoclave, the polymerization is started by means of initiators. In the tube reactor process, oxygen is frequently used as initiator. Use is also made of initiators which decompose to form free radicals, e.g. peroxides, and oxygen can be combined with peroxides. The monomer gas in which the initiator is already present is heated in a preheater at the reactor inlet to the temperature necessary for commencement of polymerization, for example from 170 to 190° C. In the autoclave reactor, the polymerization is started by introducing initiator solution into the preheated reactor. The monomer conversion in the reactor is usually not more than 20%. After leaving the reactor, the reaction mixture is depressurized by means of a pressure maintenance valve to 200-300 bar into a high-pressure product separator and from there decompressed by means of a product valve to from 1 to 3 bar into a low-pressure product separator. Part of the unreacted monomer gas is separated off by the high-pressure product separator and is returned to the suction side of the postcompressor. The proportion of monomer still present in the polymer is largely removed by outgassing in the low-pressure product separator. The monomer gas substreams which are contaminated by byproducts of the polymerization are recirculated in the high-pressure and low-pressure circuits via coolers and separators, with wax-like and liquid substances being separated off and the recirculated monomer gas at the same time being cooled to a temperature suitable for recompression.
Premature polymerization frequently occurs in the precompressors and compressors in the compression of the ethylenically unsaturated monomer, even prior to initiation of the polymerization, and this leads to deposit formation and makes it necessary for the compressors to be cleaned frequently at short intervals. Owing to the deposit formation and the associated temperature increase in the region of the compressors, the performance of the compressors drops. However, very constant operating conditions are necessary for constantly high product quality. In the region of the high-pressure product separator, the formation of free radicals leads to grafting onto the polymers formed as a result of undesirable chain branching and crosslinking reactions and thus to a reduction in the product quality.
To avoid undesirable polymerization reactions, inhibitors (stabilizers) can be added to the monomer gas mixture. DE-A 196 22 441 describes a process for compressing ethylenically unsaturated monomers to a pressure of from 200 to 5000 bar in the absence of a polymerization initiator, in which compression is carried out in the presence of a sterically hindered amine derivative, for example derivatives of tetramethylpiperidine oxide, as inhibitor. The inhibitors are introduced as solutions, for example in isododecane. The inhibitor is, for example, introduced downstream of the precompressor at 220 bar.
It is an object of the present invention to provide a gaseous inhibitor which can be metered in simply.
We have found that this object is achieved by a process for the polymerization or copolymerization of ethylenically unsaturated monomers in the presence of polymerization initiators which decompose to form free radicals in a continuously operated polymerization apparatus comprising a fresh gas feed line (a), a precompressor (b), a postcompressor (c), a reactor (e), a pressure maintenance valve (f), a high-pressure circuit (g) having a high-pressure product separator (i) and a high-pressure circulation gas return line (l), a product valve (m) and a low-pressure circuit (n) having a low-pressure product separator (o) and a low-pressure circulation gas return line (q), wherein nitrogen monoxide or oxygen is introduced as inhibitor into the high-pressure circuit (g), into the low-pressure circuit (n) and/or the precompressor (b).
BRIEF DESCRIPTION OF THE DRAWINGS
A simplified flow diagram of the polymerization apparatus which is preferably used is shown in FIG.
1
. This corresponds, except for the introduction of inhibitor, to the prior art described at the outset. The figure also shows possible points, designated by (r), for introduction of the inhibitor.
The reactor (e) can be any customary high-pressure polymerization reactor, for example a tube reactor or an autoclave. Preference is given to using a tube reactor. All possible types of tubular polymerization reactors can be employed. The reactor (e) can, for example, be a single-zone reactor in which all the monomer gas including initiators, molar mass regulators and, if desired, comonomers is introduced at the reactor inlet. The tube reactor is preferably a multizone reactor with further introduction of cold gas and initiator. The polymerization apparatus can comprise further equipment items. For example, it may have an aftercooler (h) between the pressure maintenance valve (f) and the high-pressure product separator (i). If the reactor (e) is configured as a tube reactor, then the apparatus has a preheater (d) in front of the reactor inlet. If the tube reactor is a multizone reactor, further points for introduction of monomer gas and/or for introduction of further initiators are provided on the reactor. Further customary equipment items may be present. Thus, the high-pressure circuit can include a high-pressure separator (k) and the low-pressure circuit can contain a low-pressure separator (p) for separating off wax-like or liquid byproducts from the circulating gases. The compressors can have intermediate pressure bottles and intermediate coolers between the individual compressor stages. Of course, more than one of each of the equipment items may be present.
The inhibitor can be introduced at any point into the high-pressure circuit (g), into the low-pressure circuit (n) and/or the precompressor (b). Introduction into the high-pressure circuit, for example into the aftercooler, requires the application of high pressure, for example from 200 to 300 bar, and can be carried out using a compressor. Owing to the associated high equipment cost, introduction into the high-pressure circuit is less preferred.
In a preferred embodiment of the process of the present invention, the inhibitor is introduced into the low-pressure circuit. Here, it is introduced at a low pressure of preferably from 1 to 3 bar, particularly preferably about 1.5 bar. It can be introduced at any point on the low-pressure circuit, for example in the region of the low-pressure circulation gas return line. The inhibitor can also be introduced with the fresh gas. The simple equipment required for introduction is advantageous. Owing to the low pressure, the inhibitor can, for example, be introduced directly from a gas bottle via a reduction valve.
In a further preferred embodiment of the process of the present invention, the inhibitor is introduced into the precompressor. This can be carried out at any stage of the precompressor, but preferabl
Deckers Andreas
Evertz Kaspar
Littmann Dieter
Mähling Frank-Olaf
Sutoris Heinz Friedrich
Basell Polyolefine GmbH
Cheung William
Keil & Weinkauf
Wu David W.
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