Method of discharging gaseous vinyl chloride monomer from a polymerization reactor

The unreacted gaseous vinyl chloride monomer still remaining in a polymerization reactor from which the polymerizate has been removed can be discharged by blowing steam into the reactor from its top so that the gaseous monomer is pushed downward and moved out from the bottom of the reactor, followed by a conventional treatment for condensation. This method of discharge serves to put the reactor open to the ambient atmosphere without the emission of the unreacted monomer and, as such, solve the problems of the workers' health and environmental pollution.

BACKGROUND OF THE INVENTION 
The present invention relates to an improvement in the process for the 
production of a polymerizate of vinyl chloride, vinylidene chloride or a 
monomer mixture mainly composed thereof in an aqueous medium in a 
polymerization reactor, by which the reactor can be opened to the ambient 
atmosphere without the emission of any unreacted residual monomer or 
monomers into the atmosphere, so that the residual monomers are completely 
recovered for re-use. 
The polymerization of vinyl chloride, vinylidene chloride or a mixture 
thereof in an aqueous medium has generally been conducted, using a 
polymerization reactor containing the aqueous medium and closed 
hermetically or gas-tight. In the continuous repetition of polymerization 
runs, the inside of the reactor is required to be inspected from time to 
time. 
When the inspection is carried out, it is a customary practice according to 
the inventors' plant that after the polymerization is over, the aqueous 
dispersion of the polymerizate is taken out of the reactor; then fresh 
air, preceded by nitrogen gas, is introduced to replace the gaseous 
monomer remaining in the same reactor, the gaseous monomer being emitted 
into the atmospheric environment; and thereupon the inspector or 
inspectors enter the reactor. Such a procedure is undesirable from the 
standpoint of atmospheric pollution because of the emission into the 
atmosphere of considerably large volumes of the unreacted gaseous monomer 
or monomers. Moreover, the procedure is disadvantaged by the fact that the 
replacement of the gaseous monomer by fresh air can not be complete 
enough, even with prolonged introduction of fresh air, to protect the 
inspectors entering the reactor from the toxicity of the unreacted gaseous 
monomer though in a very low concentration. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a novel and 
improved process free from the above-described problems encountered in the 
prior art. 
The improvement proposed by the present invention in a method for the 
production of a polymerizate of vinyl chloride monomer or a monomer 
mixture mainly composed thereof by polymerization in an aqueous medium 
contained in a polymerization reactor by the successive steps of 
equilibrating the pressure inside the reactor and a receiver tank 
connected to the reactor after completion of a polymerization reaction, 
removing most of the unreacted gaseous vinyl chloride monomer, transfering 
the aqueous dispersion of polymerizate from the reactor into the receiver 
tank, washing the interior of the polymerization reactor if necessary, and 
opening the reactor to the ambient atmosphere, comprises an additional 
step of blowing steam into the reactor from the top prior to the step of 
opening the reactor to ambient atmosphere, so that the unreacted gaseous 
monomer still remaining in the reactor is pushed downward and moved out 
from the bottom of the reactor. The gas moved out of the reactor is then 
transferred to a condenser where the unreacted monomer or monomers are 
separated to be recovered, while the condensation water produced is sent 
to a stripper where any unreacted monomer or monomers contained in the 
condensation water is separated to be recovered. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the improved process in accordance with the present invention, the 
unreacted gaseous monomer or monomers remaining in the polymerization 
reactor from which the aqueous dispersion of polymerizate has been removed 
to the receiver tank can be completely expelled and replaced with steam 
together with any other gaseous components. The thus expelled or 
discharged gaseous monomer or monomers are then separated from the steam 
by a conventional means using a condenser, and the thus separated monomer 
or monomer mixture is collected in a gas holder, while the condensation 
water produced which still contains unreacted monomer or monomers is 
treated by a stripper where the monomer or monomers are separated to be 
sent to the gas holder. 
The process of the invention ensures the complete recovery of the unreacted 
monomer or monomers thereby to avoid the undesired emission of such 
unreacted monomer or monomers into the environment when the polymerization 
reactor is opened, e.g., for inspection, to the ambient atmosphere. This 
is an essential advantage of the present invention that the problems of 
workers' health and environmental pollution arising from the presence of 
unreacted monomer may become out of consideration.

Vinyl chloride monomer or a monomer mixture mainly composed of vinyl 
chloride is suspension- or emulsion-polymerized in an aqueous medium 
contained in a polymerization reactor 1 which is closed hermetically 
during the polymerization reaction concerned. After completion of the 
polymerization reaction, most of unreacted gaseous monomer or monomers 
staying in the reactor are recovered, and sent to a gas holder 5, and the 
aqueous dispersion of polymerizate formed in the reactor 1 is transferred 
to a receiver tank 3 located below through a discharge valve 2. Then the 
pressure inside polymerization reactor 1 and receiver tank 3 was 
equilibrated. This equilibration of the pressure is intended to facilitate 
the smooth transfer of the aqueous dispersion of polymerizate from the 
reactor to the receiver tank, and carried out by opening valves 4 and 6 on 
the piping conduit connecting the gaseous phases of the reactor and the 
receiver tank, and then opening a valve 2 to allow the aqueous dispersion 
to flow down into the receiver tank by gravity or by pumping means, while 
simultaneously the gas staying in receiver tank 3 is pushed and moved into 
reactor 1 and gas holder 5 through the piping. 
It is recommended that, following the discharge of the aqueous dispersion 
of polymerizate from polymerization reactor 1 to receiver tank 3, the 
reactor walls are washed with a pressurized stream of water ejected from 
an automatic cleaner (not shown in the drawing) installed in the reactor 
to clean the wall surfaces of any aqueous dispersion or medium adhering 
thereto. 
The aqueous dispersion of the polymerizate which has been transferred to 
receiver tank 3 is then dehydrated and dried, followed by usual processes 
to produce the finished product of polyvinyl chloride. When the 
polymerization reactor has been made empty and filled with fresh air, it 
is ready for next run of polymerization. 
In accordance with the process of the present invention, the gases 
remaining in the polymerization reactor are swept and replaced with steam 
after every or several repeated polymerization runs, prior to the opening 
of the polymerization reactor to the ambient atmosphere, followed by the 
usual steps. 
The replacement of the monomer and other gases with steam within 
polymerization reactor 1 is carried out by closing discharge valve 2 and 
equilibration valve 6, opening valve 10, and by supplying steam into 
reactor 1 through valve 9. The steam thus supplied works to gradually push 
the layer of the unreacted gaseous vinyl chloride monomer downward. In 
this case, the steam does not mix up with the gaseous monomer, since the 
density of the vinyl chloride monomer is more than 3 times that of steam 
under the same pressure and at the same temperature. The downward gaseous 
monomer and steam are then passed through valve 10 into condenser 11 where 
the steam is condensed to form condensation water which is to be sent to 
drain tank 8, while the gaseous monomer is made free of the steam. The 
freed gaseous monomer is then sent to gas holder 5 by means of blower 12 
through valve 13. 
The velocity of steam blown into the polymerization reactor should be 
carefully controlled. A too high velocity results in accelerating the 
mixing of the steam with the gaseous monomer, naturally leading to a 
remarkable reduction in the efficiency of replacement of the gases with 
steam. Thus, the velocity of blowing of steam is preferably 150 m/second 
or lower as determined at the blowing nozzle, while the rate of steam 
introduction is preferably 300 kg/hour or less per square meter of the 
horizontal cross sectional area of the polymerization reactor, which is of 
the vertical type. 
The condensation water collected in drain tank 8 is sent by pump 14 to 
stripper 15 where it is stripped of the monomer dissolved therein. The 
vinyl chloride monomer vaporized in stripper 15 is sent to gas holder 5 by 
blower 12 through valve 7, while the water almost completely freed of the 
monomer, say, as small as 1 p.p.m. or less, is discharged out of stripper 
15. The monomer collected and recovered in gas holder 5 may be recycled, 
after having been purified if necessary by suitable means, e.g. 
distillation. 
In the blowing of steam into the polymerization reactor, the condensation 
of the steam at the reactor walls can effectively be prevented by passing 
hot water through the jackets surrounding the reactor. The replacement of 
the gases in the reactor can naturally be made more and more perfect by 
extending the time of steam blowing, or increasing the total volume of 
steam blown. Usually, a total volume of steam blown equivalent to 3 or 
more times the volume of the reactor taking several to several tens of 
minutes in blowing is sufficient to decrease the concentration of the 
monomer in the reactor to a value lower than 5 p.p,m., which is understood 
to be the statutory upper limit of pollution for working environment. 
The present invention will further be illustrated by the following 
examples. 
EXAMPLE 1 
The usual suspension polymerization of vinyl chloride in an aqueous medium 
was undertaken in a polymerization reactor of a 130 m.sup.3 capacity, 
using 45 tons of vinyl chloride dispersed in 60 tons of water in the 
presence of hydroxypropylmethylcellulose and polyvinyl alcohol as the 
dispersing agents and 2-ethylhexylperoxydicarbonate as the polymerization 
initiator. After completion of the polymerization in about 90% monomer 
conversion, unreacted monomer was recovered by vaporization and 
transferred into a gas holder. Then the pressure inside the reactor and 
the receiver tank was equilibrated by opening the valves and the aqueous 
dispersion of polymerizate was transferred from the reactor to the 
receiver tank as described hereinbefore. Thereupon steam was blown into 
the reactor from the top under the pressure of 0.5 kg/cm.sup.2 absolute at 
the nozzle, while simultaneously unreacted gaseous monomer still remaining 
in the reactor was dischared from the bottom of the reactor. The rates of 
blowing of steam in Kg/hour per square meter of the horizontal cross 
sectional area of the reactor and the linear velocity of steam blowing at 
the nozzle in m/second were varied as indicated in Table I. 
Tests were conducted with the varied velocity and rates of steam blowing 
for 10 minutes, and at the end of such steam blowing the concentration of 
vinyl chloride monomer in the gas remaining in the reactor and the amount 
of steam consumption in each test were determined, with the results as 
shown in Table I. 
TABLE I 
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Rate of Velocity Monomer con- 
Steam con- 
Test steam of steam centration, 
sumption, 
No. blowing blowing p.p.m. kg 
______________________________________ 
1 120 23 8.5 280 
2 200 38 3.1 468 
3 280 53 1.5 655 
4 350 67 25.0 817 
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The steam thus blown into the reactor was discharged from its bottom 
accompanied by the gaseous vinyl chloride monomer, and sent to the 
condenser, wherefrom the condensation water formed was transferred to the 
drain tank, and the gaseous monomer freed from the steam was collected in 
the gas holder. 
The condensation water contained 500 to 600 p.p.m. of vinyl chloride 
monomer according to the above tests. 
The condensation water collected in the drain tank was moved to the 
stripper by pumping means, wherefrom the vinyl chloride monomer stripped 
off the water was discharged into the gas holder. 
The resultant water was analyzed for monomer content to find that the 
monomer content was below the detectable limit by gas chromatography. 
EXAMPLE 2 
The same procedure as in Example 1 was repeated except that the pressure of 
the steam was 0.15 kg/cm.sup.2 absolute at the nozzle. The results are 
summarized in Table II. 
TABLE II 
______________________________________ 
Rate of Velocity Monomer con- 
Steam con- 
Test steam of steam centration, 
sumption, 
No. blowing blowing p.p.m. kg 
______________________________________ 
5 90 50 2.6 207 
6 160 89 1.2 368 
7 250 139 4.3 575 
8 360 200 24.0 828 
______________________________________ 
As has been described herein, the essential advantage of the present 
invention is that unreacted vinyl chloride monomer remaining in the 
polymerization reactor when opened to outside atmosphere, which according 
to the prior art would amount to 0.20 to 0.25 kg per 100 kg of the 
polyvinyl chloride product and would be emitted into the atmospheric air, 
can be recovered almost completely without emission into the atmosphere. 
Taking one instance, a polyvinyl chloride manufacturing plant having a 
capacity of 100,000 tons per year can recover as much as 200 to 250 tons 
of unreacted vinyl chloride monomer in a year, when it is assumed that the 
polymerization reactor is opened to outside atmosphere at the end of every 
polymerization run for purposes of inspection. From this point of view, 
the method of the present invention advantageously contributes to 
improvement in unit of monomer used as well as to the preservation of 
working and natural environments.