CA1278639C - Vinyl chloride monomer stripping process - Google Patents

Abstract

IMPROVED VINYL CHLORIDE
MONOMER STRIPPING PROCESS
ABSTRACT OF THE INVENTION
Polyvinyl chloride homopolymer or copolymer in wet slurry form, containing high levels of residual vinyl chloride monomer, is stripped of residual monomer in a process wherein the polymer is contacted with a hot gas such as saturated steam at atmospheric pressure or above.
Low levels of residual vinyl chloride monomer can be obtained (less than 1 ppm).

Description

~Z786.'39 This invention relates to an improved vinyl chlo-ride stripping process.
Polyvinyl chloride polymer (PVC), whether a homo-polymer or a copolymer containing predominantly interpoly-merlzed units of vinyl chloride monomer, is well known to the art as a versatile plastic. PvC is made using many types of processes, including emulsion (latex), solution, suspension and bulk polymerization processes. No matter which process is employed, however, total conversion of vinyl chloride monomer to polymer is not obtained. This results in residual vinyl chloride monomer (VCM) often dissolved in or entrapped in the PVC polymer.
One method to remove the residual VCM from the polymer is to heat up the PVC to about 180F. (82C.) to volatilize the VCM and evaporate off the VCM. The process is performed under reduced pressure (vacuum) to facilitate the removal of VCM. As an example of the state of the art on stripping of VCM, a typical stripping operation would be conducted at about 170F. and 400 to 450 millimeters of mercury absolute. Significantly higher temperatures are not employed for fear of degrading the PVC. A patent recently issued to Solvay and Company (Belgium Patent 793,505, issued on June 29, 1973) discloses a technique of stripping VCM from PVC consisting of condensing steam onto PVC polymer thereby heating the PVC to above its glass transition temperature, and then applying vacuum to evaporate off the water and VCM.
The evaporation of water and VCM cools the PVC to below its glass transition temperature. Again, the stripping of the VCM is done under vacuum.

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~2786.39 In accordance with the invention an improved process of reducing the residual monomeric matter, in parti-cular VCM, content of PVC polymers comprises contacting a PVC
polymer containing unacceptably high levels of monomeric matter, in particular VCM, with a hot gas, for example, steam, at a temperature of from about 200F. ( 132C~ ) and at atmospheric pressure or above. The hot gas and monomeric matter, in particular, VCM, mixture is withdrawn and the monomeric matter, in particular, VCM, recovered for reuse.
In particular, the process may be carried out to remove residual vinyl chloride monomer present, after poly-merization, in vinyl chloride polymers that are in the form of an aqueous dispersion by contacting the dispersion with the hot gas, in accordance with the invention, and removing from the contact area a mixture comprising the hot gas and vinyl chloride monomer.
In one embodiment the process is applied to an aqueous dispersion of the polymer which dispersion is intro-duced into the upper portion of a column containing a plurality of sieve plates or perforated plates. In parti-cular the openings of the plates are effective to allow the aqueous dispersion to descend in the column. The dispersion is contacted with the hot gas, for example, steam, in the column for a period of 1 to 60 or more minutes, typically 1 to 15 minutes. In particular the total area of openings in each sieve plate or perforated plate may be 1 to 10% of the cross-sectional area of the plate.
Polyvinyl chloride homopolymers or copolymers (hereinafter referred to as PVC~ can be prepared using 3G emulsion, suspension, solution, or bulk polymerization lZ786;39 techniques known to the art. Unfortunately, no polymeri-zation technique or process converts the total amount o~
vinyl chloride monomer (hereinafter referred to as VCM) to polymer. Much of the unreacted VCM is dissolved in or entrapped by the PVC polymer. The VCM, if not removed, is later released upon further processing and/or use of the polymer. Because of recent pollution and toxicity standards proposed by the Environmental Protection Agency and set by the Occupational Safety and Health Act Board, a level of residual VCM content in PVC in the hundreds of parts per million (ppm) is unacceptably high. The PVC must be post-treated to remove the VCM to low levels (down to 10 ppm and, preferredly, down to less than 1 ppm).
A known process to remove residual VCM from PVC
polymer is to heat the polymer up to about 180F. under vacuum to release and vaporize off the VCM. It has unex-pectedly been discovered that residual VCM can be efficiently and effectively removed from PVC polymer by contacting the polymer with a hot gas at a temperature of from about 200F.
20 (93C.) to about 270F. (132C.) and at atmospheric pressure or above. Residual VCM contents of the PVC as low as 0.5 ppm have been obtained using the novel process. The PVC polymer is not degraded in the process~
Any PVC polymer, whether homopolymer or copolymer F

1~86~91 or higher enumerated polymer, can be used in the process.
Of course, the use of a PVC polymer having a low thermal stability or a melting point or softening point well below 200F. (93~C.) is not favorable. The molecular weight of the PVC polymer is not critical. Preferredly, the PVC
polymer used is porous, and excellent results have been obtained using a PVC polymer having uniform porosity.
Polymers of interpolymerized units o~ vinyl chloride monomer with copolymerizable vinylidene monomers such as vinyl bromide, vinylidene chloride, ~-olefins such as ethylene and propylene, acrylic and methacrylic acid, acrylates and methacrylates such as ethyl acrylate and methyl methacrylate, vinyl aromatics such as styrene and vinyl toluene, and the like, and mixtures of these monomers are known in the art or canbe prepared. Any or all of such PVC
polymers can contain unacceptably high levels of VCM. Hence, the novel process of this invention can be used to remove the residual VCM.
The PVC polymer can be prepared using any method or technique known to the art. Emulsion, suspension, solution, and bulk polymerization processes can be employed.
The process of the invention applies to PVC in particulate form containing residual amounts of VCM, and the polymerization method used to prepare the PVC is not critical. However, if the PVC polymer is not prepared in a polymerization that yields particles of PVC on completion, the polymer should be isolated in a particulate form before using the VCM stripping process. The actual particle size is not critical as stripping of the VCM will occur in all cases. However, stripping of the VCM is faster if the PVC particle size is under 200 microns. Typically~ the PVC polymer used in the stripping process has a residual VCM content of above 1000 ppm 1278~;.'3~
by weight of VCM in the polymer, and can contain up to lO0,000 ppm of VCM and more.
The temperature range employed in the process is from about 200~1. (93C.~ to abollt c70E. (1~2"C.~, arld more preferredly from about 212"F. (100C.) to about ~4Q'F.
(118C.). The stripping is performed at atmospheric pressure or above. A normal pressure range is from 0 psig to about 25 psig. The PVC polymer is contacted with a hot gas which serves to both heat up the PVC and act as a carrier for the VCM. The gas is preferredly an inert gas such as nitrogen or helium, and is not a gas that promotes oxidation of the polymer such as oxygen. Hot air, as it contains mostly inert gases, is useful. A preferred gas to use is saturated water vapor (saturate~d steam). Temperatures and pressures of saturated steam are well known and can be found in saturated steam tables (see Chemical Engineers' Handbook, 3rd Ed., McGraw-Hill Book Co., Inc. (1950), pages 277-278).
The use of saturated steam as the hot gas heats up the PVC
polymer, provides a positive pressure in the stripping area, and acts as a carrier for the VCM.
The PVC polymer is used in the form of a slurry of PVC polymer particles in a liquid carrier. The slurry form facilitates pumping and agitation of the particles.
The liquid carrier can be any non-solvent for PVC polymer having a relatively high boiling point (above 70C.).
Examples of such liquid carriers are ethanol, butanol, cyclohexane, water, and the like. Water is the preferred liquid carrier. The total solids content of the PVC slurry can be from a very low percent by weight of solids to a total solids content wherein the slurry can just barely be pumped. As a practical matter, the total solids of the PVC
slurry ranges froln about 5~ by weight to about 80~ by weight ~27~ 9 of PVC polymex in the slurry. The PVC polymer slurry can ~e contacted wit}- the hot gas in a variety of l~ays. The PVC
slurry and hot gas can be mixed together in a c]osed kettle, the PVC slurry and hot gas can be mixed and expelled together into a lower pressure area, or the PVC ~lurry and hot gas can be contacted with each other in a counter-current flow operation.
As an embodiment o~ the novel proce~s, the PVC
slurry can be stripped in a kettle. The PVC polymer slurry is placed in a closed vessel, which can be a polymerization vessel or a holding tank, and hot gases introduced intv the vessel. To insure good contact between the PVC and the hot gases, the hot gas is normally introduced into the bottom of the vessel. Agitation of the PVC can aid the contact.
Pressure in the vessel is atmospheric or above and normally ranges from 0 psig to about 20 psig. Temperatures in the vessel and of the PVC polymer range from about 200F. (93C.) to about 250~F. (121C.), and more preferredly, from about 200F. (93C.) to about 220F. (104C.). Contact times vary as to the capacity of the vessel, and range from about 5 minutes to 60 minutes or more. Stability of PVC at elevated temperatures is a time-temperature phenomenon. Therefore, shorter contact times should be employed as the temperature is raised. The hot gas and VCM are withdrawn from the vapor space in the vessel and the VCM recovered. The PVC is pumped from the vessel into a hold tank or directly into a drier. Residual VCM contents of down to 4 ppm and lower can be obtained. It was unexpectedly discovered that not only was residual VCM effectively and efficiently removed from the PVC polymer, butthat the PVC polymer product was not signlficantly degraded in the process. Prior to this discovery it was widely believed that any process operation lZ786~'39 that would heat PVC to above 180F. (820C.) would have a severe detrimental effect on the PVC and its subsequent stability. Under the preferred operating conditions, little or no PVC degradation was observed.
Another embodiment of the novel process is to admix the PVC polymer slurry and hot gas at the temperature and pressure range described and inject the mixture into an area (such as a kettle) of lower pressure (not a vacuum).
The process can employ a single flash stripping, recycle flash stripping wherein the hot gas and VCM are withdrawn from the vapor space in the kettle after the injection into the kettle and the PVC is then pumped back into an area for re-mixing with hot gas~ and multi-stage flash stripping wherein the PVC slurry is mixed with the hot gas and the mixture injected into a first kettle and then the process repeated in subsequent kettles. Temperatures of up to and over 250F. (121C.) were investigated using the flash stripping operation. The hot gas and PVC slurry mix was flashed to a kettle at atmospheric pressure. The PVC
slurry was used at about 35~ total solids by weight.
A preferred embodiment of the novel process is to contact the PVC polymer slurry and the hot gas in a stripping column. The hot gas used is saturated water vapor (steam).
As opposed to a batch process conducted in a kettle, herein the PVC polymer, in the form of a slurry, is pumped into a stripping column at or near the top of the column. The feed rate can vary depending upon the capacity of the column, the level of residual VCM in the PVC, the particle size and porosity of the PVC polymer, and with operating temperatures and pressures. Feed rates employed in production facilities could vary from about 100 pounds of resin per hour to 20,000 pounds of resin per hour and higher. The steam is introduced 127B6.'39 into the column at or near the bottom of the column. ~ence, the PVC and steam will run counter-current to each other Pressure and temperature within the column can be controlled using any known technique including external jacketing, internal heating coils, and the use of compressed gases.
However, it is both practical and convenient to use saturated steam in a colurnn that can withstand pressure, to both heat and pressurize the PVC and the column. For example, saturated steam at a pressure of 20 psi absolute has a temperature of 2280F. (109C.), and at 25 psi absolute has a temperature of 2400F. (116C.). The amount of steam introduced into the column varies according to the feed rate of the PVC slurry and column design.
The steam and PVC polymer make contact in the stripping column. It is preferred that the stripping column be of a tray column design to help with control of flow and aid in more uniform contact. The height and width of the column and the number of trays and their spacing and design are all design variables readily calculable having knowledge of flow rates and properties of the PVC slurry.
In a preferred method of operating the stripping column process, the PVC polymer is pumped from a PVC slurry feed tank into a tray-design stripping column near the top.
Saturated steam at about 235F. (113C.) is introduced into the column near the bottom. Pressure in the column is about 22 psi absolute. Total contact time in the column varies as to feed rate and column capacity. With a 30 inch O.D.
column having 17 trays and a feed rate of abo-ut 5000 pounds of PVC per hour, total residence time in the column is from about 3 minutes to about 15 minutes. The PVC polymer exits the colurnn at the bottom of the column and is pumped to a hold tank or into a drier. The steam with the VCM monomer 1~86~g exits at the top of the column and goes into a condensate receiver where the steam is then condensed to water and the VCM recovered. PVC polymer entering the column has an average of about 20,000 ppm of residual VCM. The PVC polymer exiting the column has an average of about 10 ppm of residual VCM in the polymer. Residual VCM content in the PVC polymer of lower than 1 ppm can be obtained.
Residual VCM contents in the PVC polymer were determined by Gas Chromatograph analysis of the particulate PVC, using a preset calibration. Stability of the PVC resin both before and after the stripping operation can be determined by using comparative oven aging tests run against a control or by using a Capillary Viscometer Heat Stability Test wherein the resin is admixed with a set level of stabilizer (if desired), placed into the barrel of the capillary viscometer, heated to 210C. and slowly extruded. Darkening of the resin upon extruding indicates degradation of the PVC polymer. Comparative tests between PVC polymer not subjected to the novel stripping process and PVC stripped by the process shows little if any change in the time to develop color (darkening of the resin).
The following examples further illustrate the invention.
EXAMPLES
Experiments wherein the PVC polymer slurry and hot gas (steam) were mixed using a kettle-type apparatus showed the feasibility of using such a method to strip residual VCM from PVC polymer at temperatures above 200~F.
and pressures at atmospheric and above, without degrading the polymer. Residual VC~ contents in the PVC of less than 10 ppm can be obtained. It was believed, though, that more efficient VCM stripping could be obtained with a counter-~27~36~9 current flow of PVC slurry and hot gas where maximum diffusion of VCM would be promoted. Furthermore, less volume capacity and greater productivity would result from using a continuous stripping operation as opposed to the batch operation in a ke-ttle-type apparatus. Hence, experiments focused upon the use of a stripping colwnll apparatus to conduct the novel ~-rocess. Tllese experimellts were porlorlned il) two stages; i.e. at a bench-scale level and at a semi-works level.
At the bench-scale level, the apparatus used was a 6-inch diameter, 8-tray stripping column. The column accommodated different tray designs, and six different tray designs were evaluated (one bubble cap design and five variations of sieve plate designs having open areas ranging from 1 to about 10 percent determined by hole size and number). Liquid level on all the trays, regardless of dcsign, W.IS about .~.~) i.llCIleS for all experiment~l rU~lS. 'L'llo l'VC
slurry was used at 25~o by weight total solids in water as the liquid carrier. The PVC slurry was pumped into the column above the first tray at a feed rate of about 2 to about 45 pounds of resin per hour. The feed rate controlled residence time. ~s column capacity was about 1.4 gallons, residence time in the column, based upon the feed rates given above, ranged from about 2 minutes to about 45 minutes.
Initial residual VCM in the PVC polymer was about 1000 to 2000 ppm. Levels of residual VCM in the stripped PVC as low as 10 ppm were obtained. VCM content was determined using gas chromatograph analysis.
Experiment A
A series of runs were made in the strippingcolumn using a common sieve-tray design throughout the series. The runs were performed at about 215F. (102C.) and about 16 psi absolute. The PVC used was a suspension PVC homopolymer l~q86~9 5 having an averag~ particle si~e of about 130 microns ~nd a porosity of ahout 0.14 cc./gm. At ~eed rates of the PV~
slurry ~rom about 12 to about 44 pounds of PVC per hour, residence time varied from 2 minutes -to about 8 minutes.
Efficiency of VCM stripping was evaluated, at the different flow rates, by measuring VCM content both before entry into the column and a~ter exit. Percent removal of residual VCM
varied from about 60% to about 98% by weight (based upon the original VCM content in the PVC polymer). Higher stripping efficiency occurred at longer residence times (6 to 8 minutes) indicating that a balance between feed rate and operating conditions exists for optimum VCM removal in the stripping operation.
Experiment B
A series of runs was made in the stripping column using a common sieve-tray design and a co~non feed rate for all the runs. The PVC polymer and slurry used in Experiment A was used for these tests also. The purpose of this series of runs was to explore temperature and pressure effects upon stripping of VCM from PVC polymer. Saturated water vapor (steam) was used as the hot gas. Again, residual VCM in the PVC was measured at entry and upon exit in the column.
Results of the runs are given in the following table.

Temperature PressurePercent Removal (C.) (mm. of Hg)of Residual VCM
Residence time of 4 minutes (7 300 20 91~ 630 55 109 1100 97.5 Residence time of 12 minutes 109 1100 over 99 110 1150 over 99 ~zq86~9 The data shows the unexpected result that much more effective and efficient removal of residual VCM is obtained at conditions of both higher temperature and pressure. Prior to this discovery, it was generally believeA str-ipping of ~CM under reduced pressure (vacuum) was requir~d f`or et'fectivc renloval of' residllal morlomer i~
the polymer. Operating conditions of from about 200~.
(93C.) to about 270~, (132C,) at pressures from atmospheric to about 40 psi absolute were evaluated, Within this range, little or no degradation of the PVC polymer was observed, Semi-Work Stage Based upon favorable results obtained in the bench-scale stripping column experiments, tests were scheduled to evaluate the feasibility of using the method on a larger scale. Again, a stripping column apparatus was chosen in which to conduct and evaluate the novel process, The column used was a 30 inch diameter stripping column having 18 trays (sieve type), Capacity of the column was limited by its flood point which was about 50 gal,/minute of liquid feed, The PVC slurry was used at about 30% total solids by weight in water. PVC polymer feed rates were from about 1700 pounds per hour to about 660o pounds per hour, Steam feed rate ranged from about 1000 to about 4500 pounds~hour, Residence times in the column varied from about 1 minute to about 10 minutes (based on a column holdup of about 98 gallons), Extensive tests were conducted at various PVC polymer slurry and/or steam feed rates and different operating temperatures and pressures, EXAMPLE I
A PVC homopolymer used as a 30% by weight slurry in water was fed to the stripping column which was operating at 212F. (lOO~C.) and atmospheric pressure, The polymer lZ786.'39 has an average particle size OI about 100 microns and a porosity of about 0.12 cc./gm. Resul-ts of the tests show that 80% to 90% of the residual VCM was removed :from the PVC p o lym~ :r .

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The PVC homopolymer slurry used in Example I was fed into the column at a feed rate of 15 gpm (2500 l'~/hr. of PVC). Steam feed rate was 1500 ~-/hour, and residence time was about 6.5 minutes. Operating conditions were 215F.
(102C.) and about 15 psi absolute. Initial VCM content of the PVC polymer was 2460 ppm, and final residual VCM
content after stripping was 330 ppm, indicating a removal of 86% by weight of VCM.
EXAMPLE III
The PVC homopolymer slurry used in the previous examples was used in a series of runs at varying temperature, pressure, PVC slurry feed rate, and steam feed rate. Results of the runs are given in the following table. In all cases, until run No. 5 where the PVC slurry feed rate was approach-ing 40 gpm, over 90% of the VCM was removed.

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EXAMPLE IV
PvC ho~opolymer stripped at various temperatures and pressures was checked for its stability using the ! Capillary Viscometer Heat Stability Test. A sample of the same type o~ PVC homopolymer, which was not stripped using the novel process, was run for comparative purposes. The data ahows that the stripping process had little or no effect on the heat stability o~ the PVC polymer.

Stripping Conditions Min. to Develop Color Temperature, F Pressure, psia Control Stripped Sample EXAMPLE V
A series of runs were made using a PVC homopolymer having a porosity of about 0.15 cc./gm. and an average particle size of about 130 microns. All the runs were made at 217F. (103~C.) and 15 to 16 psi absolute. The PVC
polymer slurry feed rates and steam feed rates were varied from 10 gpm (1650 pounds per hour of PVC) to 40 gpm (6600 pounds per hour of PVC)~ and steæm feed rates were set at from 1500 to 4400 pounds per hour. Residence time varied inversely as the PVC slurry feed rate and ranged from 2.5 minutes to 10 minutes. Percent removal of residual VCM
monomer varied from 86~ to 99.8~ by weight. At a condition f 25 gpm PVC slurry feed rate (~100 pounds per hour o~
PVC) and 2000 pounds per hour of steam feed rate, a residual VCM content of 3 ppm was obtained from a PVC
polymer having an initial VCM content of 1500 ppm.
EXAMPLE VI
Using the same PVC homopolymer as in Example V, and operating at 235F. (112C.) and 23 psi absolute using a slurry feed rate of 25 gpm and a steam feed rate of 2000 #/hour, the VCM content of the polymer was reduced from 3020 pprr, to 25 ppm, a 99.2 percent VCM rer~oval.
kXA~L~ VII
rr'he e~)erimental series of runs made in ~ample V
were essen-tlal~y repeated but for the use of a PVC homo-polymer rtc-;in ilaving a porosity of about 0.25 cc./gm. and an avera~e par-tic:Le size of about 130 microns. The PVC iS
more porous than those used in Example I (0.12 cc./gm ) and Example V (0.15 cc./gm.). Particle sizes of the three ~ypes of PVC hornopolymers are roughly equal, being about 100 to 130 ~nicrons on the average. In a range of temperature from 215~F. (102C.) i,o 236F. (113C.), pressures from atmospheric to 23 psi absolute, PVC slurry feed rates of from 15 ~pm to 35 gpm (2500 to 5800 pounds per hour of PVC)g and steam feed rates Of from 1500 to 2500 pounds per hour, residual VCM
content in the FVC polymer, after stripping was below 1 ppm in all runs bUt for one out Of fourteen runs. In that one run, the percent removal of VCM was still 99 percen-t by weight.

Claims (23)

1. A process for removing residual vinyl chloride monomer from a polyvinyl chloride polymer comprising contact-ing polyvinyl chloride polymer in particulate form with a gas at a temperature of from about 200°F. to about 270°F. at atmospheric pressure or above and removing from the contact area a mixture comprising the gas and vinyl chloride monomer.

2. A process of Claim 1 wherein the pressure in the contact area is from about 15 psi absolute to about 40 psi absolute.

3. A process of Claim 2 wherein the temperature in the contact area is from about 212°F. to about 240°F.

4. A process of Claim 3 wherein the gas used in contact with the polyvinyl chloride polymer is saturated steam.

5. A process of Claim 4 wherein the polyvinyl chloride polymer is used in the form of a slurry of particles in a liquid carrier which is a non-solvent for the polymer.

6. A process of Claim 5 wherein the weight percent total solids of the slurry is from about 5 percent to about 80 percent, and the liquid carrier is water.

7. A process of Claim 6 wherein the polyvinyl chloride polymer is a polyvinyl chloride homopolymer.

8. A process of Claim 7 wherein the polyvinyl chloride homopolymer has a porosity of at least about 0.1 cc./gram.

9. A process of Claim 8 wherein polyvinyl chloride homopolymer having an average particle of below 200 microns and a porosity of about 0.25 cc./gram is introduced into a stripping column near the top of the column in the form of a slurry of particles in water having an about 30 percent by weight total solids, saturated steam is introduced into a stripping column near the bottom of the column, and contact between the particles and saturated steam is made at a temperature of from about 215°F. to about 240°F. at a pressure from about 15 psi absolute to about 23 psi absolute for a residence time of about 1 to about 10 minutes and a mixture comprising steam and vinyl chloride monomer is removed from the stripping column near the top of the column.

10. A process for removing residual vinyl chloride monomer from an aqueous vinyl chloride polymer dispersion down to a residual monomer content of less than 50 ppm comprising contacting the aqueous vinyl chloride polymer dispersion with steam at a temperature of from 93°C. to 100°C. for 15 to 60 minutes.

11. A process of claim 10, wherein the vinyl chloride polymer is a polyvinyl chloride homopolymer.

12. A process for removing residual vinyl chloride present, after polymerization, in vinyl chloride polymers that are in the form of an aqueous dispersion comprising contacting the aqueous vinyl chloride polymer dispersion with steam at about atmospheric pressure at a temperature of from about 93°C. to about 132°C. and removing from the contact area a mixture comprising the steam and vinyl chloride monomer.

13. A process according to claim 12, wherein the temperature in the contact area does not exceed 116 C.

14. A process according to claim 12, wherein the vinyl chloride polymer is a polyvinyl chloride homopolymer.

15. A process according to claims 12, 13 or 14, wherein the steam is introduced at the base of the treatment vessel.

16. A process for removing monomeric matter from an aqueous dispersion of a polymer containing at least 50 weight percent of polymerized vinyl chloride, which comprises introducing the dispersion into the upper portion of a column provided with sieve plates and contacting the dispersion therein for a period of about 1 minute up to 15 minutes and under a pressure of about 760 up to 1200 mm Hg with hot steam at about 100 up to 132°C. flowing counter-currently with respect to the dispersion; removing the polymer dispersion so treated from the column base portion; and condensing a vaporous matter mixture issuing at the heat of the column so as to recover an aqueous phase and the monomeric matter.

17. The process as claimed in claim 16, wherein the aqueous dispersion contains about 10 up to 60 weight percent of solid matter.

18. The process as claimed in claim 16, wherein the aqueous dispersion contains about 0.2 up to .5 weight percent of vinyl chloride.

19. A method for removing unreacted residual vinyl chloride from an aqueous dispersion of a vinyl chloride polymerizate which comprises (a) feeding the aqueous dis-persion to the top of a plate column comprising a plurality of perforated plates, each perforated plate having openings effective to allow the aqueous dispersion to descend in the plate column through the openings in the perforated plates, (b) blowing steam into the bottom of the plate column so as to bring the descending aqueous dispersion into counter-current contact with the steam and (c) maintaining a temperature inside the plate column in the range of from 90 to 105°C. and a pressure inside the plate column at approxi-mately the saturated vapor pressure of water at that temperature.

20. The method as claimed in claim 19, wherein the aqueous dispersion to be fed to the plate column is subjected to pre-heating to a temperature in the range from 70 to 90°C.

21. The method as claimed in claim 19, wherein the aqueous dispersion contains 20 to 50 weight percent of solid matter.

22. A method according to claim 19, 20 or 21, wherein the total area of the openings in each perforated plate is 1 to 10% of the cross-sectional area of the plate.

23