U.S. patent application number 09/772077 was filed with the patent office on 2001-06-21 for method of stripping for latex of vinyl chloride paste resin, stripping apparatus, and vinyl chloride paste resin latex having low concentration of residual monomer.
This patent application is currently assigned to KANEKA CORPORATION. Invention is credited to Kimura, Toshihiko, Makino, Tsukasa, Yoshida, Tsuyoshi.
Application Number | 20010004666 09/772077 |
Document ID | / |
Family ID | 27334920 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004666 |
Kind Code |
A1 |
Makino, Tsukasa ; et
al. |
June 21, 2001 |
Method of stripping for latex of vinyl chloride paste resin,
stripping apparatus, and vinyl chloride paste resin latex having
low concentration of residual monomer
Abstract
A method for continuously stripping an unreacted monomer from a
vinyl chloride paste resin latex, comprising the steps of
continuously supplying the latex to a vacuum recovery vessel,
wherein the latex is boiling with generation of foam, under the
condition that the temperature of the latex to be supplied is
higher than that of the latex in the recovery vessel, withdrawing a
monomer gas generated in the recovery vessel through an exhaust
line provided with a foam separator, contacting the foam entrained
into the exhaust line with steam introduced to the exhaust line
and/or the foam separator, thereby destroying the foam, and
returning the resulting latex in the separator to the recovery
vessel. The unreacted monomer can be efficiently recovered from the
latex by a continuous operation, and defoaming can be achieved
without deteriorating the quality and without lowering the
productivity.
Inventors: |
Makino, Tsukasa;
(Akashi-shi, JP) ; Yoshida, Tsuyoshi; (Himeji-shi,
JP) ; Kimura, Toshihiko; (Takasago-shi, JP) |
Correspondence
Address: |
ARENT FOX KINTNER PLOTKIN & KAHN, PLLC
1050 Connecticut Avenue, N.W., Suite 600
Washington
DC
20036-5339
US
|
Assignee: |
KANEKA CORPORATION
|
Family ID: |
27334920 |
Appl. No.: |
09/772077 |
Filed: |
January 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09772077 |
Jan 30, 2001 |
|
|
|
09656761 |
Sep 7, 2000 |
|
|
|
Current U.S.
Class: |
528/501 ; 521/65;
528/499; 528/502C |
Current CPC
Class: |
B01J 19/1881 20130101;
C08F 6/003 20130101; C08L 27/06 20130101; B01J 2219/00103 20130101;
B01J 2219/0011 20130101; B01J 2219/00094 20130101; C08F 6/003
20130101 |
Class at
Publication: |
528/501 ;
528/502.00C; 528/499; 521/65 |
International
Class: |
C08F 006/10; C08F
006/16; C08J 011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 14, 1999 |
JP |
11-260467 |
Jan 31, 2000 |
JP |
2000-23058 |
Claims
What we claim is:
1. A method for continuously stripping an unreacted monomer from a
latex of a vinyl chloride paste resin, comprising the steps of
continuously supplying a vinyl chloride paste resin latex from a
latex feed line to a vacuum recovery vessel maintained under
reduced pressure, the temperature of said latex to be supplied
being higher than that of said latex which has been introduced to
and present in said recovery vessel, boiling said latex in said
recovery vessel under said reduced pressure, withdrawing a monomer
gas generated in said recovery vessel through an exhaust line which
is connected to the upper part of said recovery vessel and is
provided with a foam separator, bringing a foam, which has
generated in and overflowed from said recovery vessel toward said
foam separator through said exhaust line, into contact with steam
introduced to said foam separator and/or a pipe of said exhaust
line, thereby destroying said foam, returning the resulting vinyl
chloride paste resin latex in said foam separator to said recovery
vessel, and continuously withdrawing the vinyl chloride paste resin
latex from said recovery vessel.
2. The method of claim 1, wherein the pressure in said vacuum
recovery vessel is from 55 to 240 mmHg.
3. The method of claim 1, wherein said steam introduced to said
foam separator and/or a pipe of said exhaust line has a temperature
higher than that of the saturated water vapor in said recovery
vessel.
4. The method of claim 1, wherein said steam introduced to said
foam separator and/or a pipe of said exhaust line has a temperature
higher than the saturated water vapor temperature in said recovery
vessel by 10 to 50.degree. C.
5. The method of claim 1, wherein said steam to be contacted with
said overflowing foam is introduced to a pipe of said exhaust line
located between said recovery vessel and said foam separator.
6. The method of claim 1, wherein the exhaust velocity in a pipe of
said exhaust line located between said foam separator and an
exhaust pump provided to withdraw the monomer gas from said foam
separator is decreased when said foam separator is filled with the
foam.
7. The method of claim 1, wherein said latex to be supplied to said
vacuum recovery vessel is heated by a heat exchanger provided in
said latex feed line.
8. The method of claim 1, wherein said latex to be supplied to said
vacuum recovery vessel is heated at a temperature of not more than
80.degree. C. by a heat exchanger provided in said latex feed
line.
9. The method of claim 1, wherein said latex to be supplied to said
vacuum recovery vessel is heated by introducing a reduced pressure
steam to said latex feed line.
10. The method of claim 1, wherein said latex to be supplied to
said vacuum recovery vessel is heated at a temperature of not more
than 80.degree. C. by introducing steam to said latex feed
line.
11. The method of claim 1, wherein said latex in said recovery
vessel is heated by a jacket provided to said recovery vessel.
12. The method of claim 1, wherein said latex in said recovery
vessel is heated by introducing steam to said recovery vessel.
13. The method of claim 1, wherein said latex in said vacuum
recovery vessel is withdrawn therefrom, heated by contact with
steam and/or by a heat exchanger, and then returned to said
recovery vessel.
14. A method for continuously stripping an unreacted monomer from a
latex of a vinyl chloride paste resin, comprising the steps of
heating a vinyl chloride paste resin latex, continuously supplying
said heated latex to a vacuum recovery vessel maintained under
reduced pressure, boiling said latex in said recovery vessel,
withdrawing a monomer gas generated in said recovery vessel through
an exhaust line which is connected to the upper part of said
recovery vessel and is provided with a foam separator, bringing a
foam, which has generated in and overflowed from said recovery
vessel toward said foam separator through said exhaust line, into
contact with steam introduced to said foam separator and/or a pipe
of said exhaust line, thereby destroying said foam, returning the
resulting vinyl chloride paste resin latex in said foam separator
to said recovery vessel, and continuously withdrawing the vinyl
chloride paste resin latex from said recovery vessel.
15. An apparatus for stripping an unreacted monomer from a latex of
a vinyl chloride paste resin, comprising a vacuum recovery vessel,
an exhaust line connected to said recovery vessel to take out a
monomer gas, a foam separator installed in said exhaust line, a
feed line for continuously feeding a vinyl chloride paste resin
latex to said recovery vessel, a heating means provided in said
latex feed line to heat the latex being fed, a recovery line for
continuously recovering said latex from said recovery vessel, and a
steam feed line connected to a pipe of said exhaust line and/or
said foam separator.
16. The apparatus of claim 15, wherein said steam feed line is
connected to a pipe of said exhaust line located between said
recovery vessel and said foam separator.
17. The apparatus of claim 15, wherein said heating means is a heat
exchanger.
18. The apparatus of claim 15, wherein said heating means is a
steam feed line connected to said latex feed line.
19. The apparatus of claim 15, wherein said vacuum recovery vessel
is provided with a jacket for heating the latex present
therein.
20. The apparatus of claim 15, wherein said vacuum recovery vessel
is provided with a steam feed line for heating the latex present
therein.
21. The apparatus of claim 15, wherein said vacuum recovery vessel
is provided with a latex circulation line comprising a withdrawal
line to withdraw the latex from said recovery vessel, a returning
line to return the withdrawn latex to said recovery vessel, and a
heating means to heat the circulated latex.
22. The apparatus of claim 21, wherein said heating means is a heat
exchanger.
23. The apparatus of claim 21, wherein said heating means is a
steam feed line connected to said latex circulation line.
24. A latex of a vinyl chloride paste resin containing at most 500
ppm of a residual monomer based on the weight of a solid matter of
said latex.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a continuation-in-part of application Ser. No.
09/656,761 filed on Sep. 7, 2000 pending.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method of continuous
stripping of an aqueous latex of a vinyl chloride resin suitable
for paste technique, and more particularly to a method for
continuously stripping unreacted monomer or monomers under reduced
pressure from the latex of vinyl chloride paste resin, which is
easy to foam during the stripping, while defoaming the latex
without deteriorating the product quality and without lowering the
productivity. The present invention also relates to an apparatus
suitable for the removal of unreacted monomer or monomers by
continuous vacuum stripping, and a vinyl chloride paste resin latex
having a low concentration of residual vinyl chloride monomer.
[0003] In general, in the production of vinyl chloride paste
resins, firstly a resin latex is obtained by micro-suspension
polymerization, emulsion polymerization or seed-emulsion
polymerization of vinyl chloride alone or a mixture of vinyl
chloride and other vinyl monomers copolymerizable therewith. For
obtaining the paste resin products, the thus obtained latex is then
usually spray-dried. The obtained vinyl chloride paste resin powder
is dispersed in a liquid plasticizer to form a plastisol, and the
plastisol is used in the paste technique.
[0004] The term "latex of vinyl chloride paste resin" or "vinyl
chloride paste resin latex" as used herein means an aqueous
dispersion of a vinyl chloride homopolymer or copolymer usually
having a particle size of 0.1 to 10 .mu.m prepared for the purpose
of the paste technique. The latex may contain a surfactant such as
an anionic surfactant or a non-ionic surfactant, as occasion
demands.
[0005] In general, the vinyl chloride paste resin latex contains a
large amount of unreacted monomer such as vinyl chloride.
Therefore, prior to spray-drying the latex to obtain the paste
resin powder, it is required to recover the unreacted monomer in a
high efficiency, ideally a whole amount of the unreacted monomer,
from the latex in order to reuse the monomer and in order to
suppress the release of monomer into the atmosphere from the
environmental point of view.
[0006] As a method for stripping an unreacted monomer from an
aqueous liquid containing a vinyl chloride resin are known, for
instance, a batchwise method wherein the aqueous liquid is
transferred into a pressure container such as tank and the
unreacted monomer is recovered under reduced pressure with heating,
a method as disclosed in Japanese Patent Publication Kokoku No.
53-38187 wherein steam is blown into an aqueous polymerization
reaction mixture under reduced pressure, a continuous stripping
method as disclosed in Japanese Patent Publication Kokai No.
54-8693 wherein the aqueous liquid is introduced into a perforated
plate column and is brought into counter current contact with steam
introduced from the bottom of the column. However, if these methods
are applied to latex of a vinyl chloride paste resin, a problem of
foaming of the latex is encountered. That is to say, the latex
contains a large amount of a surfactant in order to retain the
stability, whereby vigorous foaming of the latex occurs to fill the
container or column with foam and the foam is scattered into a
monomer recovery pipe. The scattered foam may cause failure or
trouble of equipment. Also, the recovery pipe may be filled with
the foam to increase the pressure loss in the pipe, thus resulting
in lowering of the recovery ability of an exhaust pump.
[0007] In order to prevent the foam generated during the monomer
recovery under reduced pressure from entering into the exhaust pipe
for the monomer recovery, a suitable defoaming agent can be added
to a vinyl chloride paste resin latex or the flow rate of the
monomer recovered can be decreased. However, the addition of
defoaming agent into the latex may introduce a problem of
deterioration in paste resin quality, since a large amount of the
defoaming agent is required in the defoaming of such a latex and
may change the physical and chemical properties of vinyl chloride
paste resins, although the quality is not adversely affected by the
addition of a small amount, e.g., several hundreds ppm, of
defoaming agent. The decreasing the flow rate of the recovered
monomer requires a long time for recovering the monomer gas, thus
resulting in lowering of the productivity.
[0008] Also, it is proposed to install a foam separator in an
exhaust line for the recovery of unreacted monomer from a vacuum
tank, whereby the foam scattered from the vacuum tank into the
exhaust line is stored to prevent the foam from entering into an
exhaust recovery pipe connected to the separator. However, since
the vinyl chloride paste resin latex has strong foamability and
foam retainability, the foam separator is filled with foam in a
short period of time and the foam may enter the recovery pipe.
[0009] Another defoaming method is also proposed in Japanese Patent
Publication Kokai No. 2000-212214 wherein a foam generated in a
vacuum recovery tank during vacuum stripping is defoamed by
bringing the foam into contact with a cooling means provided in the
space of the vacuum recovery tank or in an exhaust pipe connected
thereto. This method is effective for defoaming the vinyl chloride
paste resin latex. However, in case that the vacuum stripping is
carried out by introducing steam to the recovery tank, there arises
a problem that steam is condensed at the cooling portion to lower
the solid concentration of the latex and, therefore, the
productivity in spray drying of the latex in the subsequent step is
lowered.
[0010] Like this, known stripping methods are not satisfactory from
the viewpoint of defoaming of vinyl chloride paste resin latex or
productivity. Satisfactory stripping method and apparatus suitable
for vinyl chloride paste resin latexes have not been proposed.
[0011] It is an object of the present invention to provide a
continuous method for stripping unreacted monomer or monomers such
as vinyl chloride under reduced pressure from a vinyl chloride
paste resin latex, which is a polymerization reaction mixture and
is easy to foam during the stripping due to the presence of a
surfactant, while defoaming without deteriorating the quality of
the vinyl chloride paste resin and without lowering the
productivity.
[0012] Another object of the present invention is to provide a
vacuum stripping apparatus capable of recovering unreacted monomer
or monomers in a high efficiency by a continuous operation from a
vinyl chloride paste resin latex obtained by polymerization without
causing any trouble resulting from foaming of the latex during the
stripping.
[0013] A still another object of the present invention is to
provide an aqueous latex of a vinyl chloride paste resin having a
low concentration of residual monomer or monomers.
[0014] These and other objects of the present invention will become
apparent from the description hereinafter.
SUMMARY OF THE INVENTION
[0015] It has been found that substantially the whole amount of the
residual monomer in an aqueous latex of a vinyl chloride paste
resin can be removed by merely supplying the heated latex to a
vacuum tank, preferably in a spraying manner, so the residence time
of the supplied latex in the vacuum tank can be remarkably
shortened and, therefore, the stripping can be continuously carried
out with ease. It has also been found that the foaming problem can
be eliminated by leading foam overflowing from the vacuum tank to a
foam separator through an exhaust line connected to the vacuum tank
and bringing the foam with steam in the exhaust line and/or the
foam separator.
[0016] Thus, in accordance with the present invention, there are
provided:
[0017] (1) a method for continuously stripping an unreacted monomer
from a latex of a vinyl chloride paste resin, comprising the steps
of continuously supplying a vinyl chloride paste resin latex to a
vacuum recovery vessel maintained under reduced pressure, the
temperature of said latex to be supplied being higher than that of
said latex which has been introduced to and present in said
recovery vessel, boiling said latex in said recovery vessel under
said reduced pressure, withdrawing a monomer gas generated in said
recovery vessel through an exhaust line which is connected to the
upper part of said recovery vessel and is provided with a foam
separator, bringing a foam, which has generated in and overflowed
from said recovery vessel toward said foam separator through said
exhaust line, into contact with steam introduced to said foam
separator and/or a pipe of said exhaust line, thereby destroying
said foam, returning the resulting vinyl chloride paste resin latex
in said foam separator to said recovery vessel, and continuously
withdrawing the vinyl chloride paste resin latex from said recovery
vessel;
[0018] (2) the method of item (1) wherein the pressure in said
vacuum recovery vessel is from 55 to 240 mmHg;
[0019] (3) the method of item (1) or (2) wherein said steam
introduced to said foam separator and/or a pipe of said exhaust
line has a temperature higher than that of the saturated water
vapor in said recovery vessel;
[0020] (4) the method of item (1), (2) or (3) wherein said steam
introduced to said foam separator and/or a pipe of said exhaust
line has a temperature higher than the saturated water vapor
temperature in said recovery vessel by 10 to 50.degree. C.;
[0021] (5) the method of any one of items (1) to (4) wherein said
steam to be contacted with said overflowing foam is introduced to a
pipe of said exhaust line located between said recovery vessel and
said foam separator;
[0022] (6) the method of any one of items (1) to (5) wherein the
exhaust velocity in a pipe of said exhaust line located between
said foam separator and an exhaust pump provided to withdraw the
monomer gas from said foam separator is decreased when said foam
separator is filled with the foam;
[0023] (7) the method of any one of items (1) to (6) wherein said
latex to be supplied to said vacuum recovery vessel is heated by a
heat exchanger provided in a latex feed line;
[0024] (8) the method of any one of items (1) to (6) wherein said
latex to be supplied to said vacuum recovery vessel is heated at a
temperature of not more than 80.degree. C. by a heat exchanger
provided in a latex feed line;
[0025] (9) the method of any one of items (1) to (6) wherein said
latex to be supplied to said vacuum recovery vessel is heated by
introducing steam to a latex feed line;
[0026] (10) the method of any one of items (1) to (6) wherein said
latex supplied to said vacuum recovery vessel is heated at a
temperature of not more than 80.degree. C. by introducing steam to
a latex feed line;
[0027] (11) the method of any one of items (1) to (10) wherein said
latex in said recovery vessel is heated by a jacket provided to
said recovery vessel;
[0028] (12) the method of any one of items (1) to (11) wherein said
latex in said recovery vessel is heated by introducing steam to
said recovery vessel;
[0029] (13) the method of any one of items (1) to (12) wherein said
latex in said vacuum recovery vessel is withdrawn therefrom, heated
by contact with steam and/or a heat exchanger and then returned to
said recovery vessel;
[0030] (14) a method for continuously stripping an unreacted
monomer from a latex of a vinyl chloride paste resin, comprising
the steps of heating a vinyl chloride paste resin latex,
continuously supplying said heated latex to a vacuum recovery
vessel maintained under reduced pressure, boiling said latex in
said recovery vessel, withdrawing a monomer gas generated in said
recovery vessel through an exhaust line which is connected to the
upper part of said recovery vessel and is provided with a foam
separator, bringing a foam, which has generated in and overflowed
from said recovery vessel toward said foam separator through said
exhaust line, into contact with steam introduced to said foam
separator and/or a pipe of said exhaust line, thereby destroying
said foam, returning the resulting vinyl chloride paste resin latex
in said foam separator to said recovery vessel, and continuously
withdrawing the vinyl chloride paste resin latex from said recovery
vessel;
[0031] (15) an apparatus for stripping an unreacted monomer from a
latex of a vinyl chloride paste resin, comprising a vacuum recovery
vessel, an exhaust line connected to said recovery vessel to take
out a monomer gas, a foam separator installed in said exhaust line,
a feed line for continuously feeding a vinyl chloride paste resin
latex to said recovery vessel, a heating means provided in said
latex feed line to heat the latex being fed, a recovery line for
continuously recovering said latex from said recovery vessel, and a
steam feed line connected to a pipe of said exhaust line and/or
said foam separator;
[0032] (16) the apparatus of item (15) wherein said steam feed line
is connected to a pipe of said exhaust line located between said
recovery vessel and said foam separator;
[0033] (17) the apparatus of item (15) wherein said heating means
is a heat exchanger;
[0034] (18) the apparatus of item (15) wherein said heating means
is a steam feed line connected to said latex feed line;
[0035] (19) the apparatus of any one of items (15) to (18) wherein
said vacuum recovery vessel is provided with a jacket for heating
the latex present therein;
[0036] (20) the apparatus of any one of items (15) to (19) wherein
said vacuum recovery vessel is provided with a steam feed line for
heating the latex present therein;
[0037] (21) the apparatus of any one of items (15) to (20) wherein
said vacuum recovery vessel is provided with a latex circulation
line comprising a withdrawal line to withdraw the latex from said
recovery vessel, a returning line to return the withdrawn latex to
said recovery vessel, and a heating means to heat the circulated
latex;
[0038] (22) the apparatus of item (21) wherein said heating means
is a heat exchanger;
[0039] (23) the apparatus of item (21) wherein said heating means
is a steam feed line connected to said latex circulation line;
and
[0040] (24) a latex of a vinyl chloride paste resin containing at
most 500 ppm of a residual monomer based on the weight of a solid
matter of said latex.
BRIEF DESCRIPTION OF DRAWING
[0041] FIG. 1 is a schematic view illustrating the stripping method
and apparatus of the present invention suitable for recovery of
unreacted monomer or monomers from a vinyl chloride paste resin
latex obtained by emulsion polymerization or the like.
DETAILED DESCRIPTION
[0042] The present invention will be more particularly described
with reference to the accompanying drawing, but it is to be
understood that the present invention is not limited thereto.
[0043] FIG. 1 is a schematic view showing an example of an
apparatus used for practicing the present invention. In the method
and apparatus according to the present invention, a vinyl chloride
paste resin latex obtained by a polymerization is continuously
supplied into vacuum recovery vessel 1 wherein unreacted monomer
such as vinyl chloride monomer remaining in the latex is removed,
and the resulting monomer gas is recovered from vacuum exhaust line
5 located at an upper part of the recovery vessel 1 through foam
separator 2 installed in the exhaust line 5. Foam generates during
stripping the monomer from the latex in the recovery vessel 1 owing
to generation of steam and a gas of the unreacted monomer from the
latex. Since the foam enters into the exhaust line 5 and then into
the foam separator 2, it is defoamed by contacting with steam in
the exhaust line 5 and/or the foam separator 2. The vinyl chloride
paste resin latex formed as a result of the defoaming which has run
down in the separator 2 is preferably returned to the recovery
vessel 1 without being stayed in the separator 2.
[0044] Vacuum recovery vessel 1 is not limited to a specific
vessel. Any containers or vessels can be used as the recovery
vessel 1 so long as they are resistant to reduction of the inner
pressure to several mmHg. For example, a tank which has been
generally used in a step for recovering the unreacted monomer in
the production of vinyl chloride paste resins, can be used.
[0045] The exhaust line 5 is located at the gaseous phase portion
of the recovery vessel 1, namely above the liquid surface of the
latex in the vessel 1. The exhaust line 5 is usually installed at
the top of the vessel 1. The exhaust line 5 is provided with a
vacuum pump which is for example a water sealed vacuum pump or a
vacuum oil rotary pump, but is not limited so long as a
predetermined reduced pressure is achieved.
[0046] The latex is supplied to the recovery vessel 1 through latex
feed line 3 connected to an upper part of the vessel 1. The latex
may be directly supplied to the vessel 1 from a polymerization
apparatus, or may be supplied from a storage tank after once
transferring the latex from a polymerization apparatus to the
storage tank, or may be supplied to vessel 1 after decreasing the
unreacted monomer concentration by batchwise stripping of the latex
in the polymerization apparatus or the storage tank.
[0047] Preferably the latex is introduced into the recovery vessel
1 so as to be sprayed or sprinkled, whereby the residual monomer in
the latex is instantaneously stripped. Thus, the stripping
apparatus of the present invention may have a means for spraying
the fed latex at the end of the latex feed line so as to spray the
latex in the vacuum recovery vessel.
[0048] The recovery vessel 1 is evacuated. The pressure inside the
vessel 1 is adjusted so that water of the latex in the vessel 1 is
boiled, that is to say, to a pressure not higher than the water
vapor pressure at the temperature of the latex present in the
vessel 1. From the viewpoint of removing the unreacted monomer
without lowering the mechanical stability of the vinyl chloride
paste resin latex and without impairing the quality of the paste
resin, it is preferable that the temperature of the latex in the
vessel 1 is not more than 80.degree. C., especially less than
80.degree. C., more especially not more than 78.degree. C.
[0049] It has been found that the amount of unreacted monomer
removed from the vinyl chloride paste resin latex has a close
relation to the amount of water boiled and evaporated from the
latex in the vessel 1, and the larger the amount of evaporated
water, the larger the amount of removal of the monomer.
Accordingly, the higher the temperature of the latex to be supplied
to the recovery vessel 1 within the range not higher than
80.degree. C., the more preferred. The latex to be supplied to the
recovery vessel 1 is maintained at a temperature higher than that
of the latex which has been introduced to and is boiling in the
recovery vessel 1 maintained under reduced pressure. Preferably,
the temperature of the latex to be supplied to the vessel 1 is not
lower than 50.degree. C. The inner pressure of the recovery vessel
1 is preferably from 45 to 360 mmHg, more preferably from 55 to 240
mmHg.
[0050] In the present invention, the vinyl chloride paste resin
latex is heated prior to introducing the latex into vacuum recovery
vessel 1. Preferably, the latex is heated just before introducing
into the vessel 1 by heat exchanger 12 located in the latex feed
line 3 or by introducing steam to the latex feed line 3 from steam
feed line 13 connected to the line 3. The amount of heat exchange
by heat exchanger 12 or the amount of steam fed is set so that the
temperature of the latex to be fed may be elevated within the range
of preferably not higher than 80.degree. C. Preferably the latex to
be fed is heated at a temperature of 50 to 80.degree. C.,
especially 60 to 80.degree. C. The latex may be supplied to the
recovery vessel 1 from a latex feed tank maintained at a high
temperature.
[0051] The latex supplied into the recovery vessel 1 may be heated,
for example, by jacket 15 provided around the recovery vessel 1, or
by introducing steam into the latex in the vessel 1 from steam feed
line 18 connected to the bottom of the vessel 1, or by heating the
latex circulated in latex circulation line 20 wherein the latex is
withdrawn from the vessel 1 by pump 9, heated by heat exchanger 21
provided in the line 20 and/or steam introduced to the line 20
through steam introducing line 22, and then returned to the vessel
1. In case of heating the latex by the jacket 15, the temperature
of hot water circulated in the jacket through jacket hot water
lines 16 and 17 is not limited so long as it is higher than the
inside temperature of the recovery vessel 1, but is preferably from
60 to 80.degree. C. The amount of steam introduced into the
recovery vessel 1 is not particularly limited so long as the
temperature in the vessel 1 is controllable under a predetermined
reduced pressure. In case of heating the latex by circulation, the
amount of latex to be circulated and the amount of heating the
latex are not particularly limited so long as the temperature in
the vessel 1 is controllable under a predetermined reduced
pressure. The latex in the recovery vessel 1 is preferably
maintained at a temperature of not more than 80.degree. C. If the
latex is heated to a temperature higher than 80.degree. C., the
latex may be coagulated.
[0052] The latex in the vessel 1 may be agitated by agitating vane
19. The latex in the vessel 1 is recovered from a lower part, e.g.,
bottom, of the vessel 1 through latex recovery line 4.
[0053] In the present invention, foam of the latex which has
overflowed into the vacuum exhaust line 5 during the stripping is
destroyed by contacting with steam in a pipe of the line 5 and/or a
foam separator 2. The temperature of steam is not particularly
limited so long as it is not lower than the saturated water vapor
temperature in the vacuum recovery vessel 1 so that steam can be
introduced without condensation into the pipe of the line 5 and/or
the foam separator 2 through defoaming steam feed line 6. The steam
is maintained preferably at a temperature higher than the saturated
water vapor temperature in the vacuum recovery vessel 1 and, under
consideration of heat stability of the latex, more preferably at a
temperature higher than the saturated water vapor temperature in
the recovery vessel 1 by 10 to 50.degree. C.
[0054] Foam separator 2 is installed in the vacuum exhaust line 5.
As the separator 2 can be used any containers so long as they are
resistant to pressure reduction to several mmHg. The volume of the
separator 2 is not limited so long as it can temporarily receive
latex foam and latex partly scattered from the recovery vessel 1.
The volume of the separator 2 is usually from {fraction (1/20)}to
1/5of the volume of the recovery vessel 1.
[0055] Since it is desirable that the pressure loss between foam
separator 2 and vacuum recovery vessel 1 is small, it is preferable
to install the foam separator 2 at a location near the recovery
vessel 1. It is also preferable that the diameter of a pipe of the
vacuum exhaust line 5 located between the foam separator 2 and the
recovery vessel 1 is large.
[0056] Preferably, the vinyl chloride paste resin latex that has
run down in foam separator 2 is returned to vacuum recovery vessel
1 without being stayed or stored in the separator 2, since the
latex may be boiled again to generate foam if it is allowed to
stand in the separator 2. For the purpose of returning, the bottom
of the separator 2 is preferably in the shape of 2:1 semi elliptic,
more preferably in the conical shape, although any shape capable of
withdrawing the latex from the separator 2 is acceptable.
[0057] Line 11 for returning the latex from foam separator 2 to
vacuum recovery vessel 1 is preferably connected to the bottom of
the separator 2. A thicker pipe is preferred for the line 11. As
the position to which the latex is returned, any position of the
recovery vessel 1 is acceptable if the returned latex does not
directly scatter into the vacuum exhaust line 5. The latex is
returned to the vessel 1 preferably at a location below the liquid
surface of the latex in the vessel 1, more preferably to the bottom
of the vessel 1. As to the manner of returning the latex to the
vessel 1, pump 10 for returning the latex may be installed in the
latex returning line 11 and the latex may be forcibly returned by
the pump 10, or the height, direction and distance between the foam
separator 2 and the liquid level of the latex in the vessel 1 may
be suitably adjusted to return the latex by a pressure difference.
In the latter case, the latex returning pump 10 installed in the
returning line 11 may be used as assistant.
[0058] The method as explained above is effective for continuously
stripping unreacted monomer from a vinyl chloride paste resin
latex. When foam scatters from the foam separator 2 into vacuum
exhaust line 8 connected to a vacuum pump for exhaust because of
vigorous foam generation, it is possible to reduce the pumping
speed to prevent the foam from scattering into the exhaust line 8,
while promoting discharge of the latex from the foam separator 2 to
return the latex to the recovery vessel 1. In that case, it is
preferable to increase the pumping speed again to return normal
operation when most of the foam and latex in the separator 2 have
been returned to the recovery vessel 1. Although scattering of foam
toward the vacuum pump through the exhaust line 8 generally compels
to stop the operation of the apparatus in order to avoid trouble of
the apparatus, decrease in ability of the vacuum pump and the like,
the labor and time for stopping and starting up the apparatus can
be saved by the above operation since it is not needed to stop the
operation of the apparatus.
[0059] A defoaming agent may be added to a latex to be treated
through line 14, and/or may be added to foam separator 2 through
line 7 connected to the top of separator 2, as occasion
demands.
[0060] By the method for stripping unreacted monomer from a vinyl
chloride paste resin latex and the stripping apparatus according to
the present invention, defoaming can be achieved without
deteriorating the quality of the vinyl chloride paste resin and
without lowering the productivity and vacuum stripping can be
continuously performed. Since the latex is supplied to a vacuum
recovery vessel, wherein the latex that stays therein is subjected
to a boiling treatment, under such a condition that the temperature
of the latex to be supplied is higher than the boiling treatment
temperature in the vessel, the unreacted monomer can be recovered
in a short period of time in a high efficiency. Also, the apparatus
can be miniaturized. The production of vinyl chloride paste resins
from the latex obtained by the method of the present invention or
by using the apparatus of the present invention is favorable from
an environmental point of view, since the amount of unreacted
monomer discharged when drying the latex to obtain the paste resin
therefrom can be hold down.
[0061] The present invention is more specifically described and
explained by means of the following examples, but it is to be
understood that the present invention is not limited to these
examples.
[0062] In the Examples and Comparative Examples, measurement and
evaluation were made according to the following methods.
Operability of Vacuum Stripping Treatment (Defoaming)
[0063] The operability was determined by the presence or absence of
foam scattering from a foam separator into an exhaust line
connected to a vacuum pump for exhaust, which was observed through
a site glass provided in the exhaust line located between the foam
separator and the vacuum pump. The state of filling with foam in
the foam separator was also observed through a site glass provided
in an upper part of the side wall of the foam separator. The
operability was evaluated according to the following criteria.
[0064] .largecircle.: Foam in the foam separator is destroyed and
the stripping can be continuously operated without foam flowing
from the separator into the exhaust line connected to vacuum
pump.
[0065] .DELTA.: The foam separator may be sometimes filled with
foam, but by reducing the pumping speed for exhaust at that time,
the stripping can be continuously operated without overflow of foam
from the separator into the exhaust line connected to vacuum
pump.
[0066] .times.: The foam separator is filled with foam and the foam
overflows from the foam separator into the exhaust line connected
to vacuum pump.
Concentration of Residual Vinyl Chloride Monomer in PVC Paste Resin
Latex
[0067] The concentration of residual vinyl chloride monomer in a
polyvinyl chloride paste resin latex was measured according to a
known method using a gas chromatograph by weighing out 2 g (on dry
basis) of the paste resin latex, dissolving it in 40 ml of
tetrahydrofuran with stirring, injecting 0.5 ml of the obtained
solution into a gas chromatograph (model GC14-14A made by Shimadzu
Corporation), and determining the concentration of residual vinyl
chloride monomer according to a hydrogen ion detecting method. The
measured vinyl chloride monomer concentration was calculated to a
value per weight of the solid matter of the paste resin latex. The
calculated value is shown as the concentration of residual vinyl
chloride monomer in polyvinyl chloride paste resin latex.
Quality of PVC Paste Resin
[0068] A latex after stripping treatment was subjected to wet
sieving through a 100 mesh wire net having an opening diameter of
150 m, and the quality of polyvinyl chloride paste resin was
evaluated by the proportion of coarse particles having a size of
not less than 100 meshes based on the dry weight of the resin. It
is generally recognized that the proportion of coarse particles
having a size of not less than 100 meshes which would introduce a
problem in quality, is 0.5% or more.
EXAMPLE 1
[0069] A 1.2 m.sup.3 dispersing tank equipped with a stirrer was
charged with 390 kg of deionized water. Thereto were then added an
aqueous solution of an emulsifier prepared by dissolving 2.7 kg of
sodium lauryl sulfate and 2.7 kg of cetyl alcohol in 60 kg of
deionized water at 80.degree. C. and cooled to room temperature and
a polymerization initiator solution of 207 g of
.alpha.,.alpha.'-azobisisobutyrovarelonitr- ile (ABVN) dissolved in
0.5 liter of toluene. After evacuating the tank for 10 minutes, 450
kg of vinyl chloride monomer was charged into the tank and was
uniformly dispersed with stirring for 60 minutes while circulating
the mixture through a centrifugal pump having an ability of 10
m.sup.3/hour. The resulting uniform dispersion was then transferred
by a pump from the dispersing tank to a 1.2 m.sup.3 stainless steel
polymerization reactor equipped with a stirrer. The inner pressure
of the reactor was then raised with nitrogen gas by 1 kg/cm.sup.2
and the temperature was elevated to 50.degree. C. with stirring to
perform the polymerization. When the inner pressure dropped to 6.5
kg/cm.sup.2G, the temperature was elevated to 70.degree. C. and the
reaction mixture was further kept at that temperature for 30
minutes. The unreacted vinyl monomer was then removed under reduced
pressure in the polymerization reactor to give a polyvinyl chloride
paste resin latex having a solid concentration of 45% by
weight.
[0070] The obtained latex had an unreacted monomer concentration of
4,000 ppm and a proportion of coarse particle having a size of not
less than 100 meshes of 0.01%.
[0071] The obtained PVC paste resin latex was then charged in a
feed tank and kept at 60.degree. C. The latex was continuously fed
at a rate of 20 liters/hour to a vacuum recovery vessel kept at 90
mmHg through a latex feed line located at an upper part of the
recovery vessel. The pipe of the latex feed line had a diameter of
0.01 m, and the flow rate of the latex fed was about 0.07 m/second.
The amount of the latex in the recovery vessel was controlled to 10
liters, and the latex was continuously taken out of a latex
recovery line located at a lower part of the recovery vessel at a
rate of 20 liters/hour. The average residence time of the latex in
the recovery vessel was 0.5 hour. The latex in the recovery vessel
was heated by a jacket which was located around the recovery vessel
and through which hot water of 60.degree. C. was passed at a rate
of 120 liters/hour. As the vacuum recovery vessel was used a
pressure vessel having a diameter of 0.22 m and a height of 0.44 m,
the top and bottom of which were in a 2:1 semi elliptic shape.
[0072] The vacuum recovery vessel was connected by an exhaust line
pipe having an inner diameter of 0.04 m and a length of 0.30 m to a
foam separator which was a pressure vessel having a diameter of
0.10 m and a height of 0.20 m, the top and bottom of which were in
a 2:1 semi elliptic shape. During the stripping treatment, steam of
80.degree. C. was fed at a rate of 0.8 kg/hour to the exhaust line
pipe located between the recovery vessel and the foam separator
from a steam feed line connected to the exhaust line pipe to
destroy foam which entered the exhaust line pipe from the recovery
vessel. The resulting latex which was formed as a result of
defoaming and had run down to the bottom of the foam separator was
discharged by a tube pump so as not to stay at the bottom of the
separator and was returned to the vacuum recovery vessel.
[0073] The recovery of the unreacted monomer stripped from the
latex was performed from a vacuum exhaust line located at the top
of the foam separator. No foam which scattered from the foam
separator to the vacuum exhaust line was observed. Thus, continuous
stripping treatment was possible.
[0074] The unreacted monomer concentration of the latex
continuously recovered by a tube pump through the latex recovery
line was 400 ppm based on the solid matter of the recovered latex.
Also, the proportion of coarse particles having a size of not less
than 100 meshes in the treated latex was 0.01% and was the same as
that of the latex prior to the stripping treatment.
[0075] The results are shown in Table 1.
EXAMPLE 2
[0076] The stripping treatment of the latex was carried out in the
same manner as in Example 1 except that the temperature of steam
introduced for defoaming to the steam feed line connected to the
exhaust line located between the recovery vessel and the foam
separator was 120.degree. C.
[0077] No foam which scattered from the foam separator to the
vacuum exhaust line connected to an exhaust vacuum pump was
observed. Thus, continuous stripping treatment was possible.
[0078] The unreacted monomer concentration of the latex
continuously recovered by a tube pump through the latex recovery
line was 400 ppm based on the solid matter of the recovered latex.
Also, the proportion of coarse particles having a size of not less
than 100 meshes in the treated latex was 0.04% that was on the
level causing no problem in quality.
[0079] The results are shown in Table 1.
COMPARATIVE EXAMPLE 1
[0080] The stripping treatment of the latex was carried out in the
same manner as in Example 1 except that steam was not introduced to
the exhaust line located between the vacuum recovery vessel and the
foam separator.
[0081] Foam vigorously scattered from the recovery vessel to the
foam separator and overflowed from the separator to be entrained to
the vacuum exhaust line connected to an exhaust vacuum pump. Since
the vacuum pump would be damaged, the operation was stopped.
[0082] The results are shown in Table 1.
COMPARATIVE EXAMPLE 2
[0083] The stripping treatment of the latex was carried out in the
same manner as in Example 1 except that steam was not introduced to
the exhaust line located between the vacuum recovery vessel and the
foam separator and a silicone defoaming agent was continuously
introduced from the steam feed line to the exhaust line in an
amount of 100 ppm based on the solid matter of the latex.
[0084] Foam vigorously scattered from the recovery vessel to the
foam separator and overflowed from the separator to be entrained to
the vacuum exhaust line connected to an exhaust vacuum pump. Since
the vacuum pump would be damaged, the operation was stopped.
[0085] The results are shown in Table 1.
COMPARATIVE EXAMPLE 3
[0086] The stripping treatment of the latex was carried out in the
same manner as in Example 1 except that steam was not introduced to
the exhaust line located between the vacuum recovery vessel and the
foam separator, and a silicone defoaming agent was continuously
introduced to the latex feed line from a feed pipe installed
therein and to the exhaust line from the steam feed pipe installed
therein in amounts of 50 ppm and 50 ppm (100 ppm in total) based on
the solid matter of the latex.
[0087] Foam vigorously scattered from the recovery vessel to the
foam separator and overflowed from the separator to be entrained to
the vacuum exhaust line connected to an exhaust vacuum pump. Since
the vacuum pump would be damaged, the operation was stopped.
[0088] The results are shown in Table 1.
EXAMPLE 3
[0089] The stripping treatment of the latex was carried out in the
same manner as in Example 1 except that the temperature of steam
introduced for defoaming to the steam feed line connected to the
exhaust line located between the recovery vessel and the foam
separator was 55.degree. C.
[0090] The foam separator was sometimes ready to be filled with
foam during the stripping. At that time, the pumping speed was
decreased by throttling a valve provided in the vacuum exhaust line
located between the separator and the vacuum pump, and after
confirming that the foam filled in the separator was destroyed and
the resulting latex was well discharged, the pumping speed was
returned by throttling back the valve. As a result of such an
operation, there was no scattering of foam from the foam separator
to the vacuum exhaust line connected to an exhaust vacuum pump, and
continuous stripping treatment was possible.
[0091] The unreacted monomer concentration of the latex
continuously recovered by a tube pump through the latex recovery
line was 400 ppm based on the solid matter of the recovered latex.
Also, the proportion of coarse particles having a size of not less
than 100 meshes in the treated latex was 0.01% and was the same as
that of the latex prior to the stripping treatment.
[0092] The results are shown in Table 1.
EXAMPLE 4
[0093] The stripping treatment of the latex was carried out in the
same manner as in Example 1 except that the latex fed from the feed
tank kept at 60.degree. C. was heated to 65.degree. by a heat
exchanger installed in the latex feed line before introducing the
latex into the vacuum recovery vessel.
[0094] No foam which scattered from the foam separator to the
vacuum exhaust line connected to an exhaust vacuum pump was
observed. Thus, continuous stripping treatment was possible.
[0095] The unreacted monomer concentration of the latex
continuously recovered by a tube pump through the latex recovery
line was 240 ppm based on the solid matter of the recovered latex.
Also, the proportion of coarse particles having a size of not less
than 100 meshes in the treated latex was 0.01% and was the same as
that of the latex prior to the stripping treatment.
[0096] The results are shown in Table 1.
EXAMPLE 5
[0097] The stripping treatment of the latex was carried out in the
same manner as in Example 1 except that the latex fed from the feed
tank kept at 60.degree. C. was heated to 65.degree. by bringing
into contact with steam in the latex feed line before introducing
the latex into the vacuum recovery vessel. The steam was introduced
into the latex feed line pipe through a steam feed line pipe
connected thereto in such an amount that the ratio of steam to the
latex was 1:20 by weight.
[0098] No foam which scattered from the foam separator to the
vacuum exhaust line connected to an exhaust vacuum pump was
observed. Thus, continuous stripping treatment was possible.
[0099] The unreacted monomer concentration of the latex
continuously recovered by a tube pump through the latex recovery
line was 240 ppm based on the solid matter of the recovered latex.
Also, the proportion of coarse particles having a size of not less
than 100 meshes in the treated latex was 0.01% and was the same as
that of the latex prior to the stripping treatment.
[0100] The results are shown in Table 1.
EXAMPLE 6
[0101] The stripping treatment of the latex was carried out in the
same manner as in Example 1 except that the latex fed from the feed
tank kept at 60.degree. C. was heated to 70.degree. by bringing
into contact with steam in the latex feed line before introducing
the latex into the vacuum recovery vessel. The steam was introduced
into the latex feed line pipe through a steam feed line pipe
connected thereto in such an amount that the ratio of steam to the
latex was 3:40 by weight.
[0102] No foam which scattered from the foam separator to the
vacuum exhaust line connected to an exhaust vacuum pump was
observed. Thus, continuous stripping treatment was possible.
[0103] The unreacted monomer concentration of the latex
continuously recovered by a tube pump through the latex recovery
line was 160 ppm based on the solid matter of the recovered latex.
Also, the proportion of coarse particles having a size of not less
than 100 meshes in the treated latex was 0.01% and was the same as
that of the latex prior to the stripping treatment.
[0104] The results are shown in Table 1.
EXAMPLE 7
[0105] The stripping treatment of the latex was carried out in the
same manner as in Example 5 except that the latex staying in the
vacuum recovery vessel was brought into contact with steam by
introducing the steam into the recovery vessel at a rate of 1.0
kg/hour from a steam feed line installed at the bottom of the
recovery vessel.
[0106] No foam which scattered from the foam separator to the
vacuum exhaust line connected to an exhaust vacuum pump was
observed. Thus, continuous stripping treatment was possible.
[0107] The unreacted monomer concentration of the latex
continuously recovered by a tube pump through the latex recovery
line was 120 ppm based on the solid matter of the recovered latex.
Also, the proportion of coarse particles having a size of not less
than 100 meshes in the treated latex was 0.01% and was the same as
that of the latex prior to the stripping treatment.
[0108] The results are shown in Table 1.
1 TABLE 1 Treating Conditions Temperature Temperature of Residual
monomer Amount of of latex in Temperature steam fed for
concentration of coarse recovery vessel of fed latex defoaming
Defoaming treated latex particles Ex. 1 50.degree. C. 60.degree. C.
80.degree. C. .largecircle. 400 ppm 0.01% Ex. 2 50.degree. C.
60.degree. C. 120.degree. C. .largecircle. 400 ppm 0.04% Ex. 3
50.degree. C. 60.degree. C. 55.degree. C. .DELTA. 400 ppm 0.01% Ex.
4 50.degree. C. 65.degree. C. 80.degree. C. .largecircle. 240 ppm
0.01% Ex. 5 50.degree. C. 65.degree. C. 80.degree. C. .largecircle.
240 ppm 0.01% Ex. 6 50.degree. C. 70.degree. C. 80.degree. C.
.largecircle. 160 ppm 0.01% Ex. 7 50.degree. C. 65.degree. C.
80.degree. C. .largecircle. 120 ppm 0.01% Com. Ex. 1 50.degree. C.
60.degree. C. -- X -- -- Com. Ex. 2 50.degree. C. 60.degree. C. --
X -- -- Com. Ex. 3 50.degree. C. 60.degree. C. -- X -- --
* * * * *