Monomer removal from the slurry of vinyl chloride resin with steam by using improved plate tower

A method for removing vinyl chloride monomer rapidly from a large amount of slurry of vinyl chloride resin to give waste water separated from said slurry, exhaust gas used in the drying of separated resin and resin product which are not objectionable from the environmental sanitation or to produce such a treated slurry which can give waste water separated from said slurry, exhaust gas used in the drying of separated resin and resin product which are not objectionable from the environmental sanitation by the repetition of said method. In said method a slurry of vinyl chloride resin is caused to flow down a plate column in which flow of slurry on the trays in each step of plates is treated with steam in a combination of specified plates and specified conditions of treatment.

DESCRIPTION 
This invention relates to a method for removing vinyl chloride monomer from 
a slurry of vinyl chloride resin. More particularly it relates to a novel 
improvement in a method for removing vinyl chloride monomer from a slurry 
of vinyl chloride resin by blowing steam into said slurry flowing down in 
a plate column, and stripping off remaining monomer together with 
generated water vapour. 
A slurry of vinyl chloride resin is prepared by polymerising vinyl chloride 
monomer suspended in an aqueous medium and separating and recovering 
unpolymerized monomer after completion of the polymerization. Then the 
slurry of vinyl chloride resin has the aqueous medium separated by a 
mechanical means and separated resin is dried by a hot drying gas stream 
or various kinds of other methods to produce dried powder of vinyl 
chloride resin. 
Whereas the above-mentioned aqueous medium separated from resin, the 
exhaust of hot drying gas stream and the dried powder of vinyl chloride 
resin contain small amounts of vinyl chloride, monomer, the amount is 
regarded to be clearly injurious or questionable by reason of 
environmental sanitation. 
Various methods have been proposed in order to completely remove vinyl 
chloride monomer remaining in such discharged materials or powder of vinyl 
chloride resin or reduce its content to the extent that it is 
non-injurious from the point of environmental sanitation. A method has 
been disclosed in which wet powder of vinyl chloride resin (so-called wet 
cake) or a slurry of vinyl chloride resin is treated with steam to 
separate and vaporize the vinyl chloride monomer occluded or included in 
said powder of resin or said slurry of resin, together with the steam 
generated by said treatment (Japanese patent publication No. 1955 of 1977; 
Japanese laid-open patent application No. 50991 of 1976). However, the 
optimum ranges of concrete conditions (vinyl chloride concentration in the 
raw material resin, optimum time, temperature, etc. of the treatment) of 
these methods are relatively narrow and there are often cases where the 
removal effect of vinyl chloride monomer from said powder is insufficient 
depending upon the polymerization degree and the powder form of the vinyl 
chloride resin. 
Accordingly, we have been endeavoring to find an easy and rapid method 
which is suited for the large scale treatment of a slurry of vinyl 
chloride resin which is being produced in a large amount with various 
kinds of quality and capable of producing the above-mentioned separated 
waste water, exhaust drying gas and powder of vinyl chloride resin 
(hereinafter referred to as vinyl chloride resin product) which do not 
contain such an extent of vinyl chloride monomer which would be injurious 
from the point of environmental sanitation and found that the use of 
special trays in a plate tower and a combination thereof with specified 
conditions (temperature, pressure, retention time, etc.) in the treatment 
of slurries of vinyl chloride resin which is caused to flow down through 
in a plate tower and contacted during the course of the flow thereof on 
each tray of the plate tower with steam provides a vastly superior effect 
in stripping off remaining vinyl chloride monomer. 
As is evident from the foregoing explanation, the first object of the 
present invention is to provide a method for removing vinyl chloride 
monomer included in a slurry of vinyl chloride resin by treating said 
slurry in a large amount, rapidly and efficiently with steam. The second 
object of the present invention is to provide a treated slurry of vinyl 
chloride resin which does not allow the separated waste water of said 
slurry after said removal, gas used in drying and vinyl chloride resin 
products to contain vinyl chloride monomer in an amount injurious to 
environmental sanitation. The third object of the present invention is to 
provide a method which enables one to separate and recover the vinyl 
chloride monomer separated in the above-mentioned treatment. Other objects 
will become apparent from the description which hereinafter follows. 
The present invention resides in the following methods (1)-(9). 
(1) In the method for removing vinyl chloride monomer in the resin as well 
as in the dispersing medium by treating the aqueous dispersion of the 
powder of vinyl chloride resin flowing down through a plate tower, the 
improvement in the removal of the remaining monomer which comprises 
letting said aqueous dispersion flow down through a plate tower having a 
series of channel-like flow paths from the inlet of liquid to the outlet 
upon the trays of said plate tower; said outlet of one tray being 
connected with inlet of next tray by over-flow while maintaining liquid 
sealing between the space upon one tray to another; contacting said 
aqueous dispersion flowing through said channel-like flow path with steam 
supplied from a large number of small holes uniformly distributed all over 
the surface of the trays, having a diameter of 0.5-2 mm, the ratio of the 
total sum of the cross-sectional area of holes to cross-sectional area of 
tray being in the range of 0.04-0.0004; under the condition of a pressure 
of -0.803.about.0.5 kg/cm.sup.2 G, at a temperature of 80.degree. 
C.-110.degree. C. and a retention time on the trays of 1-60 minutes. 
(2) A method according to item (1) wherein said plate tower has a 
combination of two or more than two said trays, a slurry of vinyl chloride 
resin is fed from the top of the plate tower in the liquid sealing state 
and the treated slurry is discharged from the column base also in the 
liquid sealing state, treating steam is fed to the column base and/or to 
the tray chambers formed with and between the trays and separated vinyl 
chloride monomer is discharged from the top of the plate tower or column 
together with generated steam. 
(3) A method according to item (1) or (2) in which the temperature of the 
fed slurry of vinyl chloride resin is in the range of 50.degree. C. to 
100.degree. C., the temperature of the slurry retained on the trays is in 
the range of 50.degree. C. to 110.degree. C. and the temperature of 
discharging slurry is in the range of 60.degree. to 110.degree. C. 
(4) A method according to item (1)-(3) wherein the retention time of the 
slurry of vinyl chloride resin is in the range of 1 to 90 minutes 
preferably 3 to 60 minutes, and most preferably 3 to 30 minutes. 
(5) A method for recovering vinyl chloride monomer according to item 
(1)-(4) in which a mixture of steam and vinyl chloride monomer discharged 
from the top of the plate tower is condensed by cooling followed by phase 
separation of liquid. 
(6) A method according to item (1)-(5) wherein a discharged treated slurry 
is recirculated by itself alone or together with a new slurry of vinyl 
chloride resin. 
(7) A method according to item (1)-(6) wherein the fed slurry of vinyl 
chloride resin is subjected to heat-transfer in a heat exchanger with a 
discharging treated slurry. 
(8) A method according to item (1)-(7) wherein the discharging treated 
slurry is cooled by suddenly flashing under vacuum. 
(9) A method according to item (1)-(8) wherein said vinyl chloride is a 
copolymer of vinyl chloride with other vinyl monomer containing at least 
50% by weight vinyl chloride unit. 
Explanation will be given with regard to the constitutional elements and 
the effectiveness of the present invention. 
a. Plate column 
For the treatment of a slurry of vinyl chloride with steam according to the 
method of the present invention a so-called plate column, plate tower or 
tray tower is used. A plate column has stepwise arranged plates generally 
in the upward and downward direction of the column. The trays constituting 
each step of the tray tower may include a flat plate, a tray form plate, a 
sieve plate, a perforated plate, a bubble cap plate, etc. In the method of 
the present invention, special perforated plates or trays hereinafter 
described are used. There is no limitation as to column diameter but it is 
convenient to use those having a column diameter of 0.3 to 3 m and 
particularly a diameter of 0.5 to 2 m. In case of the above-mentioned 
column diameter, the amount of slurry of vinyl chloride resin used as 1 
m.sup.3 to 300 m.sup.3 preferably 5 m.sup.3 to 100 m.sup.3 per hour per 
column for the concentration of resin of slurry of 25% by weight. There is 
no limitation as to the column height but it is convenient to use those 
having a column height 5 to 20 times preferably 5 to 15 times the column 
diameter. 
b. An aqueous dispersion of powder of vinyl chloride resin to be caused to 
flow down 
It is a slurry of vinyl chloride resin obtained generally by subjecting 
vinyl chloride monomer to suspension polymerization in an aqueous 
dispersion medium according to a conventional process and separating 
unpolymerized monomer under atmospheric pressure after completion of the 
polymerization and contains vinyl chloride monomer in an extent of less 
than 40,000 ppm to 500 ppm depending upon the polymerization condition and 
recovering method of unpolymerized monomer after polymerization. As for 
chemical composition of powder of vinyl chloride resin, there are included 
copolymers of vinyl chloride and other vinyl monomer e.g. vinyl acetate, 
alkylvinyl ether, including olefins, acrylate or methacrylate) containing 
50% by weight or more of vinyl chloride unit. An aqueous dispersion is a 
so-called slurry of vinyl chloride resin consisting of water, a suspending 
agent e.g. polyvinyl alcohol and other necessary adjunct or an aqueous 
medium consisting of water and an organic solvent and the above-mentioned 
powder of vinyl chloride resin but also includes an aqueous emulsion of 
vinyl chloride resin prepared according to an emulsion polymerization 
process so long as it is the one to which the method of the present 
invention is applicable. 
The aqueous dispersion fed to a tray of relatively higher position of a 
plate column flows downward by gravity and passes stepwise through each of 
the tray chambers in the method of the present invention. The tray 
chambers herein refered to means the spaces formed with and between trays 
or formed with the top tray and a cover over it. Incidentially though 
detailed later, since said slurry supplied to the tray constituting each 
step flows in overflow manner, it does not flow down to the trap of the 
next step until the tray (substantially the tray) to which the slurry is 
being fed is filled up. Further there is no occurrence of the depositing 
of wet powder of vinyl chloride resin upon the trays nor back mixing of 
said slurry during the retention time in the trap plate because of the 
reason hereinafter described and these points constitute the essential 
part of the invention of the present invention. 
c. Trays used in each step of plate column 
The reason for the steam treatment of slurry of vinyl chloride resin by 
using a plate column is to bring said slurry sufficiently in contact with 
steam to effect gas liquid contact and to increase thermal efficiency by 
the multiple use of steam in the chamber of each step. It is clear from 
the object of the present invention that the use of a plate column is not 
merely for extending the retention time of the slurry of vinyl chloride 
resin in a plate column. On the contrary, in order to shorten the 
retention time and to narrow the dispersion of variation in retention 
time, as a structure of plate, the one having a series of channel-like 
flow paths as shown in the attached drawing was used.

In FIGS. 1(A) and (B), 1 is a bottom plate, 2 are side plates which form 
channel-like flow path for liquid in the tray. 2' is a plate used for 
forming liquid level by overflow (weir). 2" is a side plate 2 which 
extends downward into the liquid of the tray chamber of the next lower 
step wherein the lower end part thereof is submerged in the liquid there 
to form liquid feed as well as side plate of the tray chamber of the next 
lower step. 
As shown in FIG. 1(C), the bottom plates of the trays of FIG. 1(A) are 
provided with a number of small holes 10 (0.5-2 mm in diameter d) 
uniformly distributed all over their entire surface. The ratio of the sum 
of the cross-sectional area of said small holes to the total area of the 
bottom plate (excluding the area of the part occupied by the joint of the 
side plates 2 and 2') is in the range of 0.04-0.0004 preferably 
0.02-0.002. The outer circumferential side plate 3 of the tray in FIG. 
1(B) is constructed integrally with the plates of upper and lower trays, 
but it is also possible to build these plates separately and connect them 
with the plates of trays air-tightly by flanges or the like. It is 
possible to construct as above-mentioned in such a way that the side plate 
3 form directly the shell of a plate column. In place of the tray of FIG. 
1(A) and 1(B), it is also possible to use trays having a structure shown 
in FIG. 2(A) and (B) in their cross-section. In the plane view of FIG. 
2(A), volute form of channel-like flow paths are formed in which side 
plates are not crossed with each other and flow of slurry is directed to 
the center of volute or vice verse (meanings of numerals are the same with 
those of FIG. 1). 
d. A plate column used in the method of the present invention in which a 
plurality of trays above-mentioned in (C) are fitted 
The vertical cross-sectional view is illustrated in FIG. 3. In the drawing, 
3 is the shell part of a column forming simultaneously an outer side plate 
of a channel. Said shell part 3 containing two trays 1 as one set within 
this part, and is connected above and below it by means of the upper and 
lower shell parts with flanges 4. 
Further 5 is a feed port for slurry of vinyl chloride resin which is 
connected to a feeding pipe of said slurry. 6 is a top cover and 6' is a 
discharging outlet of a mixture of steam and vinyl chloride monomer. 7 is 
a bottom chamber of column. 7' is a withdrawal outlet of treated slurry. 8 
is an inlet of blowing-in steam. 9 is a piping for washing water. 
e. Embodiment of the method of the present invention 
From the top of the column illustrated in FIG. 3, a to be treated vinyl 
chloride slurry is caused to flow in while keeping the liquid seal state. 
The liquid seal state means the arrangement for preventing gas such as 
water vapour, vinyl chloride monomer, etc. from escaping. The feed port 5 
is generally connected to a piping 12 of FIG. 4. The supplied slurry fills 
the tray of the uppermost tray chamber, fills the trays of the next step 
by over-flow and by overflowing in the same manner it reaches the tray 
immediate upon the lowermost column bottom chamber 7 and is led to a next 
step of dehydration step (FIG. 4, 23). Further steam is blown from a steam 
blowing-in port 8 fitted to the bottom chamber 7 of the column in an 
adequate amount under adequate pressure, absorbed into a slurry of vinyl 
chloride resin retained on the tray of each tray chamber from a number of 
small holes provided on the bottom plate of the tray of the lowermost tray 
chamber. It heats said slurry to generate a mixed vapour of water having a 
lower temperature than that of blowing-in steam, and vinyl chloride 
monomer. Since the inner pressure of the column bottom chamber 7 is 
maintained at a value slightly higher than the pressure of the lowermost 
tray chamber plus a liquid head of the slurry of vinyl chloride resin on 
the tray of said tray chamber, there will be no flow-down of said slurry 
or water from the above-mentioned small holes. Such a relationship of 
pressure difference is likewise maintained between each tray chamber i.e. 
the pressure of the lower tray chamber is maintained at a value higher 
than the pressure of the upper tray chamber plus a head of retained 
liquid, hence there will be no flow-down of said slurry or water from the 
small holes of the tray of each tray chamber. What makes possible the 
prevention of flow-down is not only the above-mentioned pressure 
difference but also the small diameter of the small holes in the range of 
0.5-2 mm preferably 0.7-1.5 mm which makes the counter-flow of flow-down 
slurry against ascending steam and the like substantially impossible. 
Further, the occurrence of clogging of said small holes due to wet powder 
of vinyl chloride resin is actually non-existent or extremely small. This 
reason seems to be due to the fact that the diameter of the powder of 
vinyl chloride resin is considerably smaller than the diameter of the 
small holes in general and it is considered on one hand that in such a 
small size the bridge forming of agglomerated powder does not occur in the 
inside of the small holes. 
The slurry of vinyl chloride resin forming retained liquid in each tray 
chamber moves while keeping the flow state close to piston flow from the 
inlet toward the outlet on the tray having the channel-like flow path as 
illustrated in FIG. 1 or 2. Said "piston flow" means a flow state in which 
a fluid flows at a uniform flow rate in the cross-section of a flow path 
as if a piston moves. In the present invention, a small agitating action 
of vapour going up through the slurry on a tray as well as a frictional 
resistance of the wall make the flow state of the slurry differ from the 
piston flow. Forming of flow state close to piston flow by the slurry 
means that the polymer particles in the slurry pass each tray at 
approximately the same retention time and this is one of the important 
elements of the present invention because occurrence of large 
back-mixing-flow means partial existence of particles having extremely 
long retention time and particles that have undergone such a long heat 
career causes heat deterioration even under the heat treatment conditions 
of the present invention, resulting in ultimate injury of the quality of 
the vinyl chloride resin product. During this time said slurry is agitated 
mainly in an upward and downward direction by the blowing-in of mixed 
vapours from the above-mentioned small holes and deposit and separation of 
wet powder of vinyl chloride resin in said slurry are prevented and 
simultaneously said slurry is heated to desorb and evaporate, vinyl 
chloride monomer absorbed in the wet powder to form mixed vapour going to 
the tray chamber immediately thereupon, together with steam formed by 
reevaporation. The slurry of vinyl chloride resin which is the retained 
liquid increases its own liquid level by several to about 20% greater than 
the calculated value at the time of still standing by the liquid expansion 
caused by the blowing-in of the mixed vapour from the lower step tray 
chamber. The necessary amount of steam for keeping the state of the tray 
chambers as above-mentioned, is from 1 to 100 Kg, preferably 5-50 Kg per 1 
m.sup.3 of a slurry of vinyl chloride resin (having a standard content of 
resin of 25%) per hour (which varies depending upon the number of plates 
(step), ratio of areas of small holes and velocity of pass. As 
illustrative dimension of trays shown in FIGS. 1(A) and 1(B) and FIGS. 
2(A) and 2(B), a diameter of bottom plate 1 of 120 cm, a width of flow 
path of 17 cm, a height of side plate 2 of 65 cm and a height of side 
plate of weir 2' of 40 cm and a height of the tray chamber at the time of 
use of 70 cm can be mentioned. The ratio of the total sum of area of 
numberless small holes having a hole diameter in the range of 0.5-2 mm to 
the area of a bottom plate is, as mentioned in (C), in the range of 
0.04-0.0004, preferably 0.02-0.002. If said ratio of area is less than 
0.0004, the present invention is inoperable due to insufficiency of steam 
treatment. If it is greater than 0.04, a greater amount of steam is 
necessary and the effectiveness of removal of vinyl chloride is not 
notably increased. 
In the method of the present invention, increase of heat efficiency is made 
by utilizing steam in each of the tray chambers in multiple ways but heat 
economy can be further improved by carrying out heat transfer between the 
slurry to be fed to a plate tower and the slurry to be discharged 
therefrom. This is significant because the slurry to be fed contains a 
large amount of water having a large value of specific heat. Furthermore 
since the slurry discharged from a plate tower is of relative higher 
temperature, and the exposure of vinyl chloride resin in the temperature 
condition like that, has a danger of spoiling the qualities of said resin, 
the immediate heat transfer between discharging slurry and feeding slurry 
with a heat exchanger, means dual effect of increase of heat economy and 
prevention of quality degradation. In case of a kind of vinyl chloride 
resin having the tendency of notable quality degradation by heat it is 
possible to cool discharged slurry quickly by flashing it to the 
atmosphere of reduced pressure as shown in the Japanese laid-open patent 
application No. 64588 of 1976. 
f. Embodiment of the present invention 
(Condition of operation): 
The operation conditions in the method of the present invention will be 
explained by referring to the flow sheet of the method of the present 
invention shown in FIG. 4. 
In this drawing, 11 is a slurry tank for vinyl chloride resin provided 
with, though not shown, usually receiving piping, a stirring apparatus for 
preventing deposit of resin powder, and a water seal device for sealing a 
gaseous phase from the atmospheric air. Though there is no limitation for 
capacity, it is preferable to have a capacity which can sustain from one 
or more than one hours to several hours for the slurry treating capacity 
(m.sup.3 /hr) of a plate tower 14, because the operation of a plate tower 
used in the method of the present invention is preferably continued for a 
long period of time in order to avoid the time loss from reopening of 
operation after a stop to the stationary operation but receiving of slurry 
of vinyl chloride resin to the above-mentioned slurry tank 11 is mostly 
intermittent due to the intermittent nature of the operation of vinyl 
chloride polymerization. Said slurry is fed to a column top tray chamber 
of a plate column 14 after passing through a feed piping 12, a pump 13, 
and a heat exchanger 22 where said slurry is heated with discharged liquid 
and further in a preheater (not shown). Liquid temperature is in the range 
of 50.degree.-100.degree. C. preferably 60.degree.-95.degree. C. The tray 
of the column top tray chamber has the same structure as that of FIG. 1. 
The slurry on said tray is heated by a mixed vapour blown through a number 
of small holes of bottom plate of the tray chamber from immediately 
therebelow and a part of the included vinyl chloride monomer is thereby 
separated by vaporization. Said slurry flows over a weir of the tray as 
overflow, moves down into the tray of the tray chamber immediately 
therebelow and while being heated and successively elevating its liquid 
temperature, goes eventualy to the tray chamber of the lowermost step. The 
temperature of treated slurry at the column bottom chamber will be in the 
range of 70.degree. C.-110.degree. C. preferably 80.degree. C.-100.degree. 
C. by the blowing-in of steam from a blowing port 15. 
The treated slurry goes from the tray chamber of the lowermost step, 
through a discharging pipe 19, a pump 20 and the heat exchanger 22 where 
it is cooled by the feed slurry, to a receiving vessel 21 for treated 
slurry. The treated slurry is sent to a dehydrating and drying step (not 
shown) through a discharge pipe 23 but when the separation of vinyl 
chloride monomer is insufficient, it is possible to recirculate one part 
or all of the treated slurry to the slurry tank 11. On the other hand, the 
mixed steam generated successively in each tray chamber by steam blowing 
exits from the column top chamber, through a steam discharging pipe 16 to 
a condenser 17 where it condenses at a temperature higher than the 
freezing temperature of water under a superatmospheric pressure and is 
separated into water and vinyl chloride monomer. The vinyl chloride 
monomer is rectified and reused in polymerization. 
The proportion of vinyl chloride resin part in a slurry of vinyl chloride 
resin used in the method of the present invention (hereinafter often 
referred to simply as a slurry concentration), % by weight has the 
intimate relationship with the capacity of plate tower (expressed by 
m.sup.3 /hr of slurry), removal property of vinyl chloride monomer and 
operational condition. A suitable concentration range operable in the 
method of the present invention is in the range of 5-45% preferably 10-40% 
and most preferably 15-30%. If it exceeds 45%, the slurry cannot flow down 
efficiently due to poor fluidity. In the range less than 5%, treating 
capacity is exceedingly reduced but there is no notable increase of 
removal efficiency of vinyl monomer. 
The treating capacity of a plate column is controlled by retention time if 
temperature of treatment is fixed and a fixed concentration of treated 
slurry is used. The amount of liquid retention which determines retention 
time is determined fundamentally by the sum of the capacity of tray under 
the level of weir plate 2' (FIG. 1) in each tray chamber. Further, the 
retention time is influenced by the resistance of the flow path within a 
column (cross-sectional area of flow path, length of flow path, heads, 
etc.) to a certain extent but is readily known by actual measurement. It 
is easy to achieve any value of design by chemical engineering enlargement 
or reduction of scale. 
According to the actual measurement made by the inventors of the present 
invention, in the case of a 5-plate column, a retention amount of 500 l, a 
cross-sectional area of 0.01 m.sup.2 and a head of 3 m (between the bottom 
plate of the column top tray chamber and the discharge pipe at a part 
immediately above the column bottom chamber, i.e. 7' of FIG. 3) it was 
possible to let a slurry having a slurry concentration of 25% to flow down 
(retention time of 3 minutes) in an amount of 10 m.sup.3 /hr. The reason 
that the retention time can be shortened to such an extent is considered 
due to the fact that each tray chamber is sealed by liquid seal and the 
stream of each tray is brought to the state close to a stream within a 
pipe by the provision of side plates. Of course it is possible to lengthen 
the retention time to any preferable value by adjusting the feeding 
velocity of the slurry of vinyl chloride resin within a fixed plate column 
in order to increase the efficiency of steam treatment. 
The slurry treated according to the method of the present invention can 
reduce its concentration of vinyl chloride monomer easily to less than 10 
ppm in case of the use of the one having a concentration of 10,000 ppm, or 
lower even if the raw material vinyl chloride resin of the slurry is of a 
lower molecular weight kind from which the removal of monomer is most 
difficult. When the treated slurry is dehydrated according to a well-known 
method, vinyl chloride monomer in separated water is undetectable (less 
than 1 ppm). The concentration of vinyl chloride monomer in the exhaust of 
hot gas drying applied to the resin by a well-known method after the 
above-mentioned dehydration is less than 2 ppm. 
Further, water washing of said exhaust gas can easily make the amount of 
vinyl chloride monomer therein non-detectable. 
Further the concentration of the vinyl chloride monomer in the product of 
vinyl chloride resin thus dehydrated and dried is less than 2 ppm or 
non-detectable. Thus there is no problem of environmental sanitation in 
the molding and processing of vinyl chloride resin. 
When a raw material slurry of vinyl chloride resin used in the method of 
the present invention has a concentration which is greater than 10,000 ppm 
and in the neighbourhood of 50,000 ppm, by using such a raw material 
slurry after being mixed with a part of already treated slurry or by using 
two of the plate towers of the present invention in succession or 
separately, it is possible to reduce the concentration of said vinyl 
chloride monomer down to a range of 2,000 ppm to about 200 ppm in the 
first tower or in the first time treatment. These treatments can be 
carried out at a temperature of said slurry in the range of 
70.degree.-80.degree. C. in order to avoid the cumulation of heat 
hysteresis exceeding 80.degree. C. of vinyl chloride resin powder and in 
most cases such a treatment will be sufficient. 
The present invention will be illustrated by way of specific examples which 
follow hereinafter. 
EXAMPLE 1 
By using a plate tower (5 plates) constructed as in FIG. 3, having trays of 
the shape of FIG. 1 (diameter of small holes of 1 mm and ratio of area of 
0.2%), a steam treatment of slurry of vinyl chloride resin having a resin 
concentration of 25% by weight, and a vinyl chloride monomer concentration 
of 8000 ppm (mean degree of polymerization P of 700) was carried out 
according to the process of FIG. 4. 
The plate tower used had a volume of 375 l per one tray, a distance of one 
tray of 70 cm, a height of weir within the tray of 35 cm and a width of 
flow path of 17 cm. Said slurry (specific gravity of 1.05) preheated at 
80.degree. C. in a heat-exchanger and further in a preheater (not shown) 
of FIG. 4 was fed to the column top tray chamber of a plate tower 14 of 
this Fig. at a speed of 9.52 m.sup.3 /hr. The steam having a temperature 
of 110.degree. C. was blown from a blowing-in pipe 15 into the column base 
chamber at a speed of 375 Kg per hour (retention time of 10 minutes). Said 
slurry heated up to 110.degree. C. by flowing down successively through 
the trays in the inside of the plate tower, was received in a 
slurry-receiving vessel 21 by taking out from the lowermost tray of the 
plate column 14 and passing through an outlet pipe 19, and a pump 20 while 
being cooled down to a temperature of 50.degree. C. by a heat exchanger 
22. The concentration of vinyl chloride monomer in this slurry was 5 ppm 
(relative to the vinyl chloride resin portion). The mixed vapour of water 
and vinyl chloride monomer, generated in the column top chamber of the 
plate column 14 is introduced into a condenser 17 under a superatmospheric 
pressure (a compressor is not shown) and the condensed water and monomer 
were separated in a decanter connected to the outlet pipe 18. 
Thus 45 Kg/hr of condensed water and 19.99 Kg/hr of vinyl chloride monomer 
were recovered. 
On the other hand the above-mentioned treated slurry was continuously 
dehydrated by centrifugal dehydration but no vinyl chloride monomer was 
detected in the discharged water. Further dehydrated vinyl chloride resin 
was dried in an air stream according to a conventional method whereby the 
concentration of vinyl chloride monomer in the exhaust air was 0.1 ppm. 
The vinyl chloride monomer in the dried vinyl chloride resin was 1 ppm. 
COMATIVE EXAMPLE 1 
A process of example 1 was repeated except that an apparatus of FIG. 5 
provided with a vertical 5 m.sup.3 tank, a slurry feed inlet 31, a slurry 
discharge outlet 32, a steam blowing port 33, and a steam withdrawal port 
34, was used. However, the amount of retained slurry was 1.6 m.sup.3. The 
concentration of monomer of treated slurry was 640 ppm relative to vinyl 
chloride resin portion. 
By allowing the water in the withdrawn mixed steam to condense (45 Kg/hr) 
under atmospheric pressure, 18.4 Kg/hr of vinyl chloride monomer could be 
recovered. In the waste water of the centrifugal dehydration of treated 
slurry, there was included 2 ppm of vinyl chloride monomer and the 
concentration of vinyl chloride monomer in the exhaust of air flow drying 
of dehydrated product was 19.4 ppm. 
Further the vinyl chloride portion in the dried vinyl chloride resin was 30 
ppm. 
When the above-mentioned result is compared with that of example 1, there 
is a difference of 128 times more vinyl chloride monomer remaining in the 
product of vinyl chloride resin even if the same kind and the same amount 
of slurry of vinyl chloride resin is treated with the same amount of steam 
for the same retention time. Vinyl chloride monomer was detected in the 
separated discharged water of the treated slurry, the exhaust of air flow 
drying contained 194 times the amount of monomer and the monomer content 
in the dried vinyl chloride resin was 30 times as high as that of the 
above-mentioned example 1. Thus the superior effect of example 1 is 
evident. 
EXAMPLE 2 
By using the same plate tower as in example 1 a slurry of vinyl chloride 
resin (mean degree of polymerization P=700) having a resin concentration 
of 25% by weight, a vinyl chloride monomer concentration of 40,000 ppm was 
treated with steam according to the process of FIG. 4. Said slurry 
preheated at 80.degree. C. in a heat exchanger 22 and further in a 
preheater (not shown), was fed to the column top chamber of the plate 
tower 14 of FIG. 4 at a rate of 9.52 m.sup.3 /hr (10 %/hr) and the steam 
was blown at a temperature of 110.degree. C. from a blowing-in pipe 15 to 
the column bottom chamber at a rate of 375 Kg/hr (retention time 10 
minutes). The slurry which flowed down successively in the plate tower and 
was heated at a temperature of 105.degree. C. was received in a slurry 
receiving vessel 21 after being discharged from the lowest tray of the 
plate column 14 and passed through a discharge pipe 19 and a pump 20. 
Meanwhile, it was cooled down to 50.degree. C. by way of a heat exchanger 
22. The concentration of vinyl chloride monomer in said slurry (relative 
to vinyl chloride resin part) was 200 ppm. The mixed vapour of water and 
vinyl chloride monomer generated in the column top tray chamber of the 
plate column 14 (having a pressure of 0.25 Kg/cm.sup.2 G) was charged into 
a condenser 17 under a superatmospheric pressure. Condensed water and 
monomer were separated in a decanter connected to an outlet pipe 18. Thus 
45 Kg/hr of condensed water and 95 Kg/hr of vinyl chloride were recovered. 
The above-mentioned treated slurry was subjected to the same treatment as 
above-mentioned in a plate tower of FIG. 4 (by using 375 Kg of steam) and 
the monomer concentration in the retreated slurry thus obtained was 1 ppm. 
Further from the column top chamber after passing through a condenser 45 
Kg/hr of condensed water and 0.0475 Kg/hr of vinyl chloride monomer were 
recovered. 
Further the slurry after the above-mentioned treatment was dehydrated by a 
centrifugal dehydrator and dehydrated vinyl chloride resin was subjected 
to air flow drying whereby the concentration of vinyl chloride monomer in 
the exhaust air was 0.03 ppm and the vinyl chloride monomer content in the 
dried vinyl chloride resin was not detectable (less than 1 ppm). 
COMATIVE EXAMPLE 2 
The process of example 2 was repeated except that an apparatus of FIG. 5 
which is same with that of comparative example 1 was used (i.e. retention 
amount: 1.6 m.sup.3, retention time: 10 minutes). The concentration of 
monomer of the treated slurry was 2,500 ppm. The mixed vapour was caused 
to condense under a pressure and water (45 Kg/hr) and vinyl chloride (94 
Kg/hr) were recovered. The above-mentioned treated slurry was subjected to 
the retreatment as the above-mentioned method by using an apparatus of 
FIG. 5 and the concentration of vinyl chloride monomer in the retreated 
slurry was 210 ppm. Further, from the column top chamber and through a 
condenser, 375 Kg/hr of condensed water and 5.7 Kg/hr of vinyl chloride 
monomer could be recovered. When the dehydrated resin was dried in an air 
flow drier according to a conventional method, the concentration of vinyl 
chloride monomer in the exhaust air was 5.4 ppm and the vinyl chloride 
monomer in the dried vinyl chloride resin was 10 ppm. 
If the above-mentioned result is compared with that of example 2, there was 
a difference of 12.5 times in the concentration of vinyl chloride monomer 
of the slurry after the first step treatment even when the same kind and 
the same amount of the slurry of vinyl chloride is treated with the same 
amount of steam for the same retention time. Similarily there was a 
difference of 10 times in the concentration of the slurry after the second 
treatment. Thus it is clear that the effectiveness of the method of the 
above-mentioned example 2 is superior. 
EXAMPLE 3 
By using the same plate tower as that of example 1, the treatment of slurry 
of vinyl chloride resin (mean degree of polymerization P=700) having a 
resin concentration of 24% by weight and vinyl chloride monomer 
concentration of 200 ppm (note: the slurry obtained by the first step 
steam treatment according to the method of example 2 of the present 
invention) was carried out according to the process of FIG. 4. The 
principal conditions and result are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Treatment of slurry of vinyl chloride resin with steam 
Condition* Result 
Specific and 
monomer 
resin 
amount 
monomer 
monomer ppm 
recovered 
Comparative 
ppm in 
wt. % in 
of steam 
ppm in 
in dried 
monomer 
examples 
slurry 
slurry 
Kg/hr 
slurry 
product** 
Kg/hr 
__________________________________________________________________________ 
Specific 
200 25 190 2 ND 0.495 
example 3 
Specific 
8,000 
40 388 5 1 34 
example 4 
Specific 
8,000 
25 375 10 3 20.0 
example 5.sup..DELTA. 
Specific 
40,000 
25 375 10 ND 100 
example 6 
Comparative 
2,000 
25 375 70 20 61 
example 3 
Comparative 
8,000 
40 388 650 35 31 
example 4 
Comparative 
8,000 
25 375 750 40 18 
example 5.sup..DELTA. 
__________________________________________________________________________ 
Note: 
*Common condition of treatment: slurry inlet temperature 80.degree. C. 
amount of feed 10 m.sup.3 /hr 
.sup..DELTA. Vinyl chloridevinyl acetate copolymer (resin containing 5% b 
weight of vinyl acetate) 
**Dry condition: inlet temperature, 110.degree. C.; air in an amount 3-4 
times the dried product by weight was used. 
As is evident from this Table, even in the case of a slurry of vinyl 
chloride resin containing a relatively small amount of monomer which has 
been subjected to the steam treatment of the present invention one time, 
it is clear that effective separation and recovery of monomer is possible 
according to the method of the present invention. 
COMATIVE EXAMPLE 3 
By using the apparatus of FIG. 5 which is the same with that of comparative 
example 1, the treatment of a slurry of vinyl chloride resin (mean degree 
of polymerization P=700) containing 24% by weight of vinyl chloride resin 
and 2000 ppm of vinyl chloride monomer was carried out according to the 
process of example 3. Principal conditions and results are shown in the 
Table 1. As evident from this table, even in case of a slurry of vinyl 
chloride resin which does not have such a high vinyl chloride monomer 
concentration as in example 1 and comparative example 1 (where the vinyl 
chloride monomer concentration is 40,000 ppm), it is clear that the 
blowing of steam into a slurry tank does not provide sufficient separation 
effectiveness for vinyl chloride monomer. 
EXAMPLE 4 
By using a plate tower the same as that of example 1, a treatment of slurry 
of vinyl chloride resin (mean degree of polymerization P=700) having a 
vinyl chloride resin concentration of 40% by weight and a vinyl chloride 
monomer concentration of 8000 ppm was carried out according to the process 
of FIG. 4. The result of this treatment is shown in Table 1. As is evident 
from this table, the effect of steam treatment according to a plate tower 
of the present invention is excellent in case of high concentration of 
resin in a to be treated slurry. 
COMATIVE EXAMPLE 4 
The process of example 4 was repeated except that an apparatus of FIG. 5 
which is the same with that of comparative example 1 was used. The result 
is shown in Table 1. As evident from this table, the removal effect of 
vinyl chloride monomer is remarkably poor even when the same slurry of 
vinyl chloride resin is used for the same retention time and the same 
temperature and the same amount of steam are used in the treatment. 
EXAMPLE 5 
By using the same plate tower as that of example 1, a steam treatment of a 
slurry of vinyl chloride resin (mean degree of polymerization P=800) of 
vinyl chloride-vinyl acetate copolymer containing 5% by weight of vinyl 
acetate, having a resin concentration of 25% by weight and a vinyl 
chloride monomer concentration of 8000 ppm was carried out according to 
the process of FIG. 4. The result is shown in Table 1. As evident from the 
table, even in case of copolymer of vinyl chloride resin, the 
effectiveness of steam treatment according to a plate tower of the present 
invention is excellent. 
COMATIVE EXAMPLE 5 
The treatment of example 5 was repeated except that an apparatus of FIG. 5 
which is the same as that of comparative example 1 was used. The result is 
shown in Table 1. It is evident that the removal effect of vinyl chloride 
monomer is inferior to that of example 5. 
EXAMPLE 6 
By using a plate tower the same as that of example 1, a treatment of a 
slurry of vinyl chloride resin (mean degree of polymerization P=1300) 
having a vinyl chloride resin concentration of 25% by weight and a vinyl 
chloride monomer concentration of 40,000 ppm was carried out according to 
the process of FIG. 4. The result is shown in Table 1. 
As evident from the table, even in case of high concentration of vinyl 
chloride monomer in the to-be-treated slurry, the separation of vinyl 
chloride monomer is easier with the increase of mean degree of 
polymerization of vinyl chloride resin.