DK147423B - PROCEDURE FOR SUSPENSION POLYMERIZATION OF VINYL CHLORIDE - Google Patents

Description

in 147423

The invention relates to a process for suspending polymerization of vinyl chloride either alone or in admixture with up to 20% by weight of copolymerizable monomers, during which a flushing of the reactor ceiling with water is carried out.

It is well known that one of the major problems in suspension polymerization of vinyl chloride, either alone or in mixtures with other monomers, is due to the coatings of polymers which are deposited on the walls of the reactor and which, if not removed, contaminate the product from the following polymerizations. In any case, the removal, either manually or mechanically, of high pressure water requires opening the reactor, wasting one or more hours.

Recently, a number of patents have been issued claiming to have solved the problem of coatings on the reactor walls in contact with the liquid phase.

Generally speaking, special preparations are used which prevent the deposition and which make it necessary to purify after just approx. 10 polymerizations.

However, the problem of the coatings on the parts of the reactor in contact with the gas phase is still not solved with these preparations and the tendency to utilize the reactor to the extreme (high degree of filling, vigorous stirring, high distillation rate and the residual monomer and other), has exacerbated the problem because a stronger foam and syringe is formed, thereby depositing polymer in the ceiling of the reactor. After a few polymerizations, the nozzles begin to become clogged with the risk of large chunks of polymer being torn off and preventing the discharge, which adversely affects the density of the bottom valves. In this way, the filling and emptying operations are affected and the safety of the reactor during the polymerization becomes problematic.

All in all, the coatings in the ceiling of the reactor make the possible use of an anti-soiling preparation ir 2 147423 relevant for the purpose: to open the reactor as rarely as possible.

German Patent Publication No. 2,239,942 discloses a method for polymerizing vinyl chloride, during which, during the polymerization, a flushing of the reactor ceiling with water is carried out to counteract deposition of coatings. This flushing is carried out using relatively low pressures, 2 namely 2-3 kg / cm, and throughout the polymerization period, 10 even when the water supply is pulsed. Such rinsing with water, carried out throughout the reaction time, is only capable of permanently preventing the deposition of the polymer by batches with low filling coefficient, slow stirring and the like, i.e. in polymerization processes having little industrial interest.

Moreover, the nozzles used in the German publication publication have the disadvantage that they are highly prone to dirt and clogging.

It has now been found that the disadvantages mentioned can be avoided by the process of the present invention, so that it is not necessary to open the reactor with each polymerization to examine the ceiling and possibly to remove the coatings. The process according to the invention is characterized in that the rinsing 25 is started with water when the conversion rate has reached 15-25% and that this rinsing is continued using water with a pressure of at least 15 kg / cm and in an amount of at least 1000 liters of water per at least 40%, after which, when the polymerization is complete, a wash with water under a pressure of at least 100 kg / cm is carried out. The latter washing is carried out e.g. when the batch is depleted or in any case when the autoclave is closed.

When said rinsing is carried out using a flow rate of at least 1000 liters per hour, only light coatings are obtained.

3 167423 which can be easily removed by the final washing at high pressure, without any difficulty in emptying or the density of the bottom valve.

If rinsing is only started when the conversion rate is above 25%, it does not prevent the deposition of large lump coatings, and premature operations offer no advantage as the reduced free volume available in the usual industrial polymerizations. does not allow the desired flow rate of water to be maintained.

The washing with high pressure water is absolutely necessary to prevent the small residual coating from becoming thicker when the following batches are applied.

The following example and Comparative Example 15 serve to further describe the necessity for the simultaneous application of the two mentioned measures.

EXAMPLE 3 2o A 25m autoclave equipped with a stirrer and one

The Pfandler anti-swelling device filled up to 90% of its volume with equal volumes of water and an 85/15 mixture of vinyl chloride and vinyl acetate. After rinsing the ceiling with water under a pressure of 25 kg / cm, when the conversion rate was from 20% to 40%, 3 kg of crusts which were uniformly distributed over the entire surface were removed.

Then, washing with water under a pressure 2 of 300 kg / cm was used, using adjustable nozzles.

The autoclave air was completely clean and there were no problems with the emptying or the tightness of the bottom valve.

Comparative Example In the same autoclave as in the previous example, the filling rate and the ratio of water to vinyl chloride were unchanged as in the embodiment.

After flushing the ceiling with water under a pressure of 30 kg / cm 2, when the conversion rate was from 40-65%, 10 kg of crust was collected in the form of coarse lumps.

Washing, during the following polymerizations, with water under a pressure of 350 kg / cm, significantly reduced the soiling, but significantly prolonged the emptying times, requiring manual operation to remove the polymer clogging the bottom valve.

The above-described method can be used without limitation on autoclaves of any size in the industry and for all formulations known in the art, including those used in patented anti-fouling methods.

The polymerizations to which the present process can be used are polymerizations of vinyl chloride either alone or in mixtures with other monomers containing 20% or less comonomer. The comonomers are all such as are known to those skilled in the art that can be copolymerized with vinyl chloride.

For example, they may be vinylidene chloride, vinyl acetate, vinyl butyrate, methyl acrylates and metacrylates, butyl acrylates and metacrylates, isooctyl acrylates and metacrylates, diethyl and dipropyl maleate and fumarates, styrene, ethylene, propylene, butylene, vinylethyl, vinyl ethyl, diallyl maleate, acrylonitrile, methacrylonitrile and others.

In practice, when applying what is described above, it is necessary to keep in mind a few things which aim to utilize the rinse and wash water to the utmost without interfering unduly with the proper conduct of the reaction.

Eg. For example, fixed installations projecting into the interior of the autoclave will be preferred locations where polymer coatings can be built up. This significantly reduces the effect of the rinse and wash.

Moving installations of the kind depicted in the drawing should therefore be preferred, characterized by water (below or low).