NL7906512A - METHOD FOR SEPARATING MONOMERIC VINYL CHLORIDE FROM AN AQUEOUS POLYMER SUSPENSION. - Google Patents

Description

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Shin-Etsu Chemical Co., Ltd., Tokyo, Japan

Method for separating monomeric vinyl chloride from an aqueous polymer suspension.

This invention relates to a process for separating and recovering unreacted monomeric vinyl chloride from a polyvinyl chloride slurry obtained in polymerization in suspension.

As is known, polyvinyl chloride is mainly prepared by polymerization of vinyl chloride in aqueous suspension. In addition, it is difficult or in principle even impossible to get the conversion completely to 100%, so that after the polymerization has ended at 80 to 90% conversion, the mixture still contains considerable amounts of unreacted vinyl chloride monomer. The separation and recovery of unreacted monomeric vinyl chloride from such aqueous suspensions is one of the major technical problems in the art, not only for economic reasons, but also for the release of vinyl chloride (suspected of being carcinogenic) from PVC objects and pollution of the environment.

In the known processes, the separation and recovery of unreacted vinyl chloride from aqueous suspensions usually takes place in several steps, depending on how much vinyl chloride can be expected in the suspension. The first step involves either (1) direct separation of the gaseous monomer in a gas container connected to the polymerization reactor, after which the polymer slurry with reduced unreacted monomer content is transferred to a buffer vessel to the polymerization reactor for the next batch. or (2) the direct transfer of the polymer suspension after the termination of the polymerization from the reactor to a so-called vent kettle where the unreacted monomer is separated out, after which the polymer suspension with reduced monomer content is transferred from the drain boiler to a buffer tank. In both cases, it is common practice to subject the polymer suspension with reduced monomer content to 790 65 12 t 2 ____ \ t- for further processing whereby the monomer content is reduced as much as possible before the suspension is dewatered and dried to the final product. is becoming.

The first of the above two processes does not succeed because the removal of monomer in the polymerization reactor takes a considerable time, so that the utilization rate of the reactor is unnecessarily low. And the more one wants to remove the unreacted vinyl chloride (for better efficiency of the further processing of the polymer slurry) the greater part of the polymerization cycle 10 will be in that removal.

The other mode of operation is free from this problem of low reactor utilization, but this advantage is negated by the fact that one now has a drain boiler which must contain the same amount of liquid and be able to withstand the same pressure as the polymerization reactor itself, so that the installation is a lot more expensive. In addition, the removal of unreacted monomeric vinyl chloride from the aqueous slurry in the vent vessel takes about the same amount of time as in the former polymerization reactor process.

This invention provides a method for removing unreacted vinyl chloride from polymer suspensions that do not have the above drawbacks.

The apparatus to be used for this purpose comprises a polymerization reactor which, after the polymerization has ended, contains the suspension, a gas-liquid separator which is connected to the polymerization reactor by a first line, and a discharge boiler which is connected to the gas-liquid by a second line. separator is connected.

The process of this invention, which is carried out with such an apparatus, involves transferring the aqueous polymer slurry from the polymerization reactor through the first line to the gas-liquid separator, the aqueous polymer slurry m the first line undergoes a pressure drop of 2 to 10 kg / cm, the pressure of the aqueous polymer suspension at the end of the gas-liquid separator being about 1 atmosphere and the residence time of the aqueous suspension in the first line at least 0, 05 seconds, causing the unreacted vinyl chloride adsorbed on the polyvinyl chloride particles to evaporate rapidly to form a mixture of 7906512 3 aqueous polymer suspension and gaseous monomeric vinyl chloride, and then the gaseous vinyl chloride in the gaseous liquid separator is separated from the aqueous polymer suspension, after which the aqueous polymer suspension with the thus reduced content of unreacted vinyl chloride is e to the drain boiler.

In this process, the unreacted monomeric vinyl chloride adsorbed on the suspended polyvinyl chloride particles during the flow of the first line is rapidly evaporated, and a mixture of an aqueous suspension and gaseous vinyl chloride is formed, which is incorporated in the gas liquid separator can be separated with great efficiency, and the gaseous vinyl chloride thus released is stored, for example, in a gas container, while the aqueous polymer suspension, which has a reduction in the content of monomer vinyl-15 which has never been achieved by the methods used so far. chloride, is passed through the second conduit to the vent vessel, and is post-processed there for more complete removal of the final residues of unreacted vinyl chloride.

In this case, the vent boiler no longer needs to be pressure resistant, as in the previously known processes, which provides a significant saving in construction costs, and further, the removal of monomer in the vent boiler becomes much easier thereafter by reducing the amount of vinyl chloride can be removed.

The invention is further elucidated with reference to the accompanying figure 1, which shows a flow chart of the method now proposed. The suspension polymerization of vinyl chloride or of a monomer mixture consisting mainly of vinyl chloride takes place in an aqueous suspension in the polymerization reactor 1 with stirrer 2 at a temperature between 40 ° and 70 ° C, and as soon as the pressure in the 2 2 reactor from the initial value to 4-6 kg / cm or at the most down to 2 kg / cm 30, the polymerization is terminated by adding an inhibitor, whereby 10 to 20% of the vinyl chloride used does not remain converted. The resulting suspension of polyvinyl chloride in water is then passed from reactor 1 through line 3 (either directly via valve 4 or by means of suspension pump 5) to gas-liquid separator 6. The polymer slurry flowing through line 3 to gas-liquid separator 6 undergoes a pressure drop of 2 to 10 kg / cm in the last portion of the line 3, and when the slurry arrives in the separator 6 its pressure approximately equal to that of the atmosphere, and more particularly 0 to 0.2 kg / cm higher than the pressure of the outside air.

The pressure drop thus defined within line 35 can be realized in various ways. In the first place, the last part 7 of that pipe is given a larger cross-section than the initial part, as shown in figure 1, whereby the desired pressure drop occurs. Another aid is a tap 8 halfway along that line 3 so that the aqueous suspension is discharged through this valve 8.

Of course, the widening of pipe 3 in its downstream part can also be combined with the presence of such a drain valve 8, and this is even preferred. The widened section 7 of line 3 is hereinafter referred to as the "degassing tube".

As the aqueous polymer slurry flows into the degassing tube 7, the monomeric vinyl chloride contained therein rapidly evaporates due to the significant pressure drop, and a mixture of aqueous polymer slurry and gaseous vinyl chloride flows into the gas-liquid separator 6. It is necessary that the time during which the polymer slurry is in degassing tube 7 under appropriate pressure (about 20 l atmosphere) is at least 0.05 second, and preferably at least 0.5 second, for the evaporation of monomeric vinyl chloride to be so will be completely possible.

As long as the pressure in polymerization reactor 1 is sufficiently high, the aqueous suspension contained therein is allowed to flow directly through valve 4, but if the pressure in the polymerization reactor is no longer sufficient for a pressure drop of at least 2 kg / cm, it is preferable close the valve 4 and pump the suspension with pump 5 so that 2 a pressure drop of at least 2 kg / cm is ensured before and during the flow of degassing pipe 7. It is also recommended that the degassing tube 7 be arranged substantially vertically so that the mixture of aqueous suspension and gaseous monomer flows through degassing tube 7 without problems.

Degassing tube T should be long enough to ensure a residence time of the aqueous slurry therein of at least 0.05 second and the flow rate of the mixture of slurry and gaseous monomer is best between 10 and 30 m / sec to prevent premature 790 To prevent the mixture of suspension and gaseous monomer in the degassing tube 7, which is better to take place in the gas-liquid separator itself.

The evaporation of monomeric vinyl chloride in evaporation tube 7 can be further accelerated by cooling the polymer slurry due to the evaporation heat of the vinyl chloride by passing hot water through jacket 9 surrounding the degassing tube 7. A further acceleration of the evaporation can be achieved by blowing steam via the pipe 10 with tap 11 into the bottom of the degassing tube.

The mixture of aqueous polymer slurry and evaporated vinyl chloride flowing up into the degassing tube 7 enters the gas-liquid separator 6 where the gaseous vinyl chloride is separated from the aqueous slurry. The vinyl chloride 15 thus separated is then removed to a (non-sealed) gas container, in which it is stored and reused after proper purification. On the other hand, the aqueous suspension, from which a large portion of the unreacted vinyl chloride has been removed, is passed through the second conduit 16 to blow-off boiler 12, where more unreacted monomeric vinyl chloride is removed if necessary, and from there with the aid of a suspension pump 14 to the storage tank 13. It is also possible to place equipment for removing the residual unreacted vinyl chloride in the connection of drain kettle 12 with storage boiler 13. Optionally, the drain kettle 12 is omitted and the aqueous polymer suspension flows directly from the gas-liquid separator 6 to the storage kettle 13, which then also serves as a drain kettle.

The gas-liquid separator 6 is not limited to a particular type of device, but any known type of device, both operating with inertia of mass, with a filter, with centrifugal forces, or whatever. Below, cyclones (which utilize centrifugal forces) are recommended for their simple structure, low cost, ease of cleaning, and high efficiency.

Sometimes it is advisable to have a defoamer 15 between the gas-liquid separator 6 and the gas container, namely if the evaporated vinyl chloride with the aqueous suspension forms a foam 790 65 12 t 6, the additional amount of suspension that is there is then passed via line 18 to the discharge boiler 12. The defoaming device 15 can also be built according to various principles, for example with a deflection plate on which the foam impacts at high speed, with rotating blades, or else an device in which the foam by centrifugal forces or by heating or by ultrasonic vibrations or the like is destroyed.

As is understood from the above, the method according to the invention is advantageous for a more complete removal of unreacted vinyl chloride from polyvinyl chloride suspensions, since this method can be applied without any significant modification of the existing equipment, so that now the transfer of the aqueous polymer slurry to the drain kettle and the removal of the monomer can be done simultaneously and in a short time and with high efficiency. Furthermore, the auxiliaries to be used for the practice of this invention are very simple and there is absolutely no risk of equipment clogging due to polymer deposition therein. In addition, the rapid evaporation between the polymerization reactor and the discharge kettle gives the polymer particles a well-developed porous structure, so that the synthetic resin obtained by the process according to the invention has an excellent quality and the objects made therefrom show much less "fish eyes".

The invention is now further illustrated by the following non-limiting examples.

Example I

An apparatus according to the flow chart of figure 1 was built up, and in the 1100 liter capacity polymerization reactor 1 with stirrer 2, 500 kg of pure water, 250 g of partially saponified polyvinyl acetate, 75 g of dimethylvalero-nitrile and 250 kg of monomer vinyl chloride, and this vinyl chloride

polymerized by heating with stirring to 57 ° C

2 at a pressure of 8.8 kg / cm. The polymerization was terminated when the pressure within the reactor had dropped to 5.5 kg / cm, giving an aqueous polyvinyl chloride slurry containing 5.02 wt% unreacted vinyl chloride 35 monomer.

The aqueous suspension thus obtained was discharged from 790 65 12 7 reactor 1 through line 3 with an inner diameter of 25 mm and valve 4; suspension pump 5 was not turned on. The last part of pipe 3, or the degassing tube 7, had an inner diameter of 25 mm, was essentially vertical, and was about 4.0 meters long. 90 ° C water was passed through jacket 9 so that the slurry in degassing tube 7 could not cool.

The flow rate of the aqueous suspension from polymerization reactor 1 was adjusted to 20 L / min with valve 8. and this flow rate was kept constant until the pressure in the polymerization reactor 2 dropped 10 to 2 kg / cm. The unreacted vinyl chloride present in the suspension evaporated rapidly after passing valve 8 in degassing tube 7. The liquid flow rate when leaving the 4 meter degassing tube was about 12.6 m / sec., So that the residence time therein was at least 0.3 seconds.

The gas-liquid detector was a standard cyclone with an inner diameter of 200 mm. At the exit of this cyclone, the pressure was approximately equal to that of the atmosphere. The polymer suspension with reduced monomer content drained therefrom was fed via line 16 to outlet vessel 12. The vinyl chloride content of the liquid in vent kettle 12 was about 1.08% by weight, meaning a 78.5% reduction of the unreacted monomer initially present.

The gas exiting the cyclone 6 contained very little foam, indicating that the liquid-gas separation in this cyclone was highly complete.

Example II (for comparison)

The experiment of Example I was repeated, except that there was still a valve at the end of the degassing tube 7, and the flow rate of the aqueous polymer slurry from reactor 1 was adjusted to 20 l / sec. with valves 4 and 8 both fully open.

The content of the aqueous polymer slurry in vent kettle 12 of unreacted vinyl chloride was now 1.69% by weight, which means a removal of only 66.3% of the monomeric vinyl chloride initially present. Furthermore, the gas coming out of cyclone 6 contained a lot of foam, so that a defoamer 15 was indispensable to separate the rest of the aqueous suspension from this stream; this went via line 18 to exhaust tank 12.

790 65 12

Example III

i- 8

The experiment of Example I was repeated, except that a 7 meter long degassing tube with an internal diameter of 25 mm was now used, so that the residence time of the mixture of aqueous suspension and gaseous vinyl chloride (under approximately atmospheric pressure) was at least 0. , Was 53 seconds.

The aqueous suspension entering venting kettle 12 still contained 0.98% by weight of monomeric vinyl chloride, which means an removal of 82.2% of the vinyl chloride initially present.

Example IV

An apparatus according to the flow chart of Figure 1 was used, with a polymerization reactor of 40 contents, and polymerized herein in the same manner as in Example 1. The polymerization was terminated when the pressure had dropped to 5.5 kg / cm, giving an aqueous suspension with 5.38 wt% unreacted vinyl chloride. The degassing tube 7 in this case was 20 meters long and had a diameter of 150 mm.

From reactor 1, the aqueous polymer slurry was vented at a flow rate of 1100 µm / min, and passed through valve 8 into degassing tube 7, flowing up to a standard gas cyclone 6 of 550 mm internal diameter. Water at 90 ° C was passed through jacket 9 to prevent cooling of the degassing mixture.

The flow rate of the gas and water mixture in degassing tube 7 was 27 m / sec (at least at its end, where the degassing tube turned into cyclone 6). The residence time of the polymer slurry in degassing tube 7 was thus calculated to be at least 0.74 seconds.

The separation of gas and liquid in cyclone 6 was very complete, so that very little foam was included, and the aqueous slurry entering drain kettle 12 contained only 0.76 wt% unreacted vinyl chloride, removing 85.8 means% of the initially present amount thereof.

Example V

The test of Example I was repeated, except that now at the bottom of degassing tube 7 steam of 147 ° C was blown through line 10 with tap 11 at tap 11, with a flow rate of 43 kg / hour. The flow rate 790 65 12 "9 of the gas and liquid mixture in the degassing tube 7 was about 15.1 m / sec.

The aqueous polymer slurry in vent kettle 20 contained 0.66 wt% vinyl chloride monomer, meaning removal of 87.7% of the amount originally present.

790 65 12

Claims (3)

1, Method for separating and recovering unreacted vinyl chloride from an aqueous polymer suspension obtained by polymerizing vinyl chloride or a mixture of monomers 5 consisting mainly of vinyl chloride in water, characterized in that (a) the aqueous polymer suspension from the polymerization reactor is transferred through a first conduit to a gas-liquid separator, the aqueous polymer slurry undergoing a pressure drop of 2 to 10 kg / cm in this first conduit and the pressure on the aqueous polymer slurry the end of the gas-liquid separator is about 1 atmosphere and the residence time of the aqueous polymer slurry in the first line is at least 0.05 seconds, whereby the unreacted vinyl chloride adsorbed on the polyvinyl chloride particles quickly evaporates and a mixture of aqueous polymer suspension and gaseous monomeric vinyl chloride is formed, (b) the gaseous vinyl chloride in the gas - liquid separator is separated from the aqueous polymer suspension with reduced monomeric vinyl chloride content, (c) the separated monomeric vinyl chloride is collected in a gas container, and (d) the aqueous polymeric suspension with reduced unreacted monomeric vinyl chloride content is discharged to a drain boiler through a second line. 2. Process according to claim 1, characterized in that the aqueous polymer slurry is discharged from the polymerization reactor so rapidly that the liquid in the first line has a velocity of at least 10 m / sec. at least at the end of this conduit where it transitions into the gas-liquid separator. Method according to claim 1 or 2, characterized in that the cooling of the degassing aqueous suspension in the first pipe is prevented by means of a jacket around said first pipe through which a hot liquid is allowed to flow. 4. A method according to any one of the preceding claims, characterized in that the evaporation of unreacted monomeric vinyl chloride from the aqueous polymer suspension is accelerated by blowing steam into the first 790 6512 conduit. 7906512