IE44309B1 - Polymerization - Google Patents

Abstract

In the process, the vault of the reactor is kept, during the reaction, under a continuous flow of water at a pressure of >/= 15 kp/cm<2> and, after the reaction has taken place, the residual encrustations on the reactor vault are removed mechanically with high-pressure water at >/= 100 kp/cm<2>. As means for carrying out the process, a device comprising a hollow shaft (1) having nozzles (3) and having axial and rotary movement is used. The process is applied in polymerisations of vinyl chloride by itself or as a mixture with other monomers containing 20 % by weight or less of comonomer.

Description

This invention relates to a process for polymerizing at least one monomer in a reaction vessel in which a gas phase exists in an upper region of the vessel. More particularly, the invehtion relates to the removal of deposits from the vzall (s) in the upper region of the vessel. The polymerization can be, but need not be, that of vinyl chloride in suspension, either alone or in admixture with at least one other monomer.

It is known that one of the major problems in the polymerization in suspension of vinyl chloride, either alone or in admixture with at least one other monomer, is caused by the deposits or incrustations of polymer which are deposited on the reactor walls and which, if they are not removed, pollute the product of subsequent batches. The removal, either manual or with the aid of water under high pressures, has required the reactor to be opened and for times of one hour or more to be wasted.

Recently, there have been disclosed various solutions to the problem of the incrustations on those regions of the wall of the reactor which are in contact with the liquid phase. Generally speaking, . specific formulations are involved, which prevent the deposition, and require cleaning operations only after some ten batches.

The tendency to exploit reactors to the utmost (e.g. by high filling coefficients, vigorous stirring, and high distillation speeds of the residual monomer) has aggravated the phenomenon of the incrustations on those parts of the reactor which are in contact with £4309 the gaseous phase, by encouraging a more intense formation of foams and splashes which deposit the polymer on the reactor ceiling. After a few batches, the nozzles used for introducing jets of water begin to become clogged, with the danger that large lumps of polymer are detached and hinder the discharge of the reactor contents and compromise the tightness of the bottom valves; thus the correct performance of the loading and unloading operations is impaired and the safety of the reactor during polymerization becomes questionable.

According to the present invention, there is provided a process for polymerizing at least one monomer in a reaction vessel in which a gas phase exists in the upper region of the vessel, in which process during at least part of the reaction, the internal wall defining the upper region of the vessel is continuously washed with water under a relatively low pressure, the washing with water under low pressure commencing before 25% conversion of the monomer(s) has occurred; and, when the reaction is complete, subjecting the internal wall defining the upper region of the reactor to the action of water under a relatively high pressure, in order to remove any residual deposits from 2o that region.

The monomer(s) being polymerized may comprise vinyl chloride.

When operating in accordance with the present invention, it is not necessary to open, with each batch, the reactor for a visual inspection of its ceiling and for possibly removing the incrustations. Preferably, 4309 during at least part of the reaction, a continuous flushing of the reactor ceiling is carried out with water under pressure of at least 15 kilograms/square centimetre and preferably the reaction is followed by washing (for example during the batch discharge and in any case when the autoclave is sealed) with water under a pressure of at least 100 kilograms/square centimetre.

More particularly, the washing, if started when the conversion is equal to or greater than 15% but less than 25% and particularly with a rate of flow of at least 1,000 litres per hour, and continued until the conversion has attained at least 40%, permits only light incrustations to form, v/hich can be stripped with water under a high pressure without difficulty, both as regards the unloading and the tightness of the bottom valve.

Cleaning operations which are started when the conversion is greater than 25% do not prevent the formation of incrustations in the form of large lumps, and premature operations (i.e. generally before 15% conversion) do not afford any advantage inasmuch as the reduced free volume available with the usual industrial batches does not permit the required rate of flow of water to be maintained.

The subsequent operation with water under a high pressure is necessary to prevent the slight residual incrustation from growing thicker as the subsequent batches are fed in: on the other hand, the flushing with water alone, as carried out during the entire reaction time, is capable of permanently preventing the deposit of the polymer but only with batches having a low filling coefficient, slow stirring and the like, that is, in polymerization runs which are of low industrial performance. 44300 The following Examples illustrate the need for the simultaneous adoption of the two measures.

EXAMPLE 1 (Comparative) A 25 cubic metre capacity autoclave equipped with a stirrer and Pfaudler anti-splash plates, was filled for 70% of its volume with mixture of water and vinyl chloride in a ratio of 1:1, and the mixture was stirred at a speed of 120 rpm. The filling could have been as high as 90%.

The ceiling of the autoclave was washed during the whole reaction time with water at a rate of 1,000 litres per hour under a pressure of 20 kilograms/square centimetre.

On completion of the reaction, the ceiling was perfectly clean and remained so during subsequent similar operations when the washing was effected. However, the stirring rate was slow at 120 rpm, and industrially unattractive.

EXAMPLE 2 Part A (Comparative) The same autoclave as described in Example 1 was filled to 90% of its volume with a 1:1 mixture of water and an 85/15 mixture of vinyl chloride and vinyl acetate. When, for the sake of comparison, the ceiling was washed with water under a pressure of 25 kilograms/square centimetre, while the conversion rate was from 20% to 40%, it was found at the end of the reaction that 3 kilograms of deposits were evenly distributed over the ceiling.

PART B (Illustrative) When, however, in a different test, the ceiling was washed as described above in Part A while the conversion was from 20% to 40% and the ceiling was also washed at the end of the reaction with water under a pressure of 300 kilograms/square cm., using orientatable nozzles, the autoclave ceiling was prefectly clean and no problems were experienced as to the unloading and the tightness of the bottom valve of the autoclave.

EXAMPLE 3 (Comparative) 90% of the volume of the same autoclave as described in Example 1, was filled with a 1:1 mixture of water and vinyl chloride.

The ceiling was washed with water under a pressure of 30 kilograms/ sq. cm. while the conversion rate was in the range of 40% to 65%; kilograms of deposit in the form of coarse lumps were collected at the end of the reaction.

Washing during subsequent batch loads with water under a pressure of 350 kilograms/sq. cm. removed the deposits completely, but it did considerably extend the unloading times as a manual operation was required for removing the polymer which clogged the bottom valve.

The washing technique in accordance with the present invention, can be applied to autoclaves of any size in the industry and to all the formulations which are known in the industrial practice, including those adopted in the recently patented anti-soiling procedures. 43 0.9 The polymerization to which the present invention is particularly applicable is that of vinyl chloride, either alone or in admixture with at least one other monomer generally present in an amount of 20% or less. The comonomer can be selected from, for example, vinylidene chloride, vinyl acetate, vinyl butyrate, methyl acrylate and methacrylate, butyl acrylate and methacrylate, isooctyl acrylate and methacrylate, diethyl and dipropyl maleates and fumarates, styrene, ethylene, propylene, butylene, vinyl ethyl ether, allyl acetate, diallyl phthalate, diallyl maleate, acrylonitrile and methacrylonitrile.

When reducing the present invention to practice, it is necessary to bear in mind a few expedients which aim at exploiting to the utmost the effects of the flushing and washing water without prejudicing the correct performance of the reaction.

For example, fixed installations projecting into the interior of the autoclave could be preferential points of polymer incrustations build-up which can considerably reduce the efficiency of the flushing and washing treatment.

Retractable installations should thus be preferred, for instance of the type illustrated in the accompanying drawing which shows a section through part of an autoclave provided with a retractable water-injecting means. Shown in the drawing is a hollow shaft 1 which can be projected through the autoclave ceiling 2; the shaft 1 is provided with nozzles 3, which are properly sized and orientated, when the means has been introduced in the interior of the autoclave. 43 0 9 When out of operation, a sealing packing 4 between the shaft 1 and a sleeve 5, and the outline of the bottom end of the sleeve ensure a tight seal.

In addition, the shaft 1 can undergo rotary movement, to enable 5 the entire circumference of the autoclave to be hit by the washing water.

The size of the orientation of the nozzles are a function of the operation and thus of the properties of the water (for flushing under a pressure equal to or greater than 15 kgs/sq.cm,; and washing under a pressure equal to or greater than 100 kgs/sq. cm.).

The number and the position of the water-injecting means are a function of the internal arrangement of the autoclave, especially the anti-splash plates, cooling pipes, stirrer shaft, thermocouples and all the component parts which could interfere with the water jets.

Claims (8)

1. A process for polymerizing at least one monomer in a reaction vessel in which a gas phase exists in the upper region of the vessel, in which process during at least part of the reaction, the internal wall defining the upper region of the vessel is continuously washed with water under a relatively low pressure, the washing with water under low pressure commencing before 25% conversion of the monomer(s) has occurred; and, when the reaction is complete, subjecting the internal wall defining the upper region of the reactor to the action of water under a relatively high pressure in order to remove any residual deposits from that region.

2. A process according to claim 1, wherein the monomer(s) being polymerized comprise(s) vinyl chloride.

3. A process according to claim 1 or 2, wherein the washing with water under a low pressure is started when the conversion is at or after 15% and below 25% and is continued until at least 40% conversion is attained, with a pressure of at least 15 kilograms/sq.cm.

4. A process according to claim 3, wherein the water under low pressure is used at a rate of at least 1000 litres per hour.

5. A process according to claim 3 or 4, wherein the water under high pressure is at a pressure of at least 100 kilograms/sq.cm.

6. A process according to any preceding claim, wherein the water under low and high pressures is introduced with a system of orientatable nozzles which can be introduced into or withdrawn from the interior of the reaction vessel. 4 43 0 9

7. A process according to claim 6, wherein the orientatable nozzles are substantially as illustrated in, or described with reference to, the accompanying drawing.

8. A process according to claim 1, substantially as described in Part B of Example 2.