Process for removing unreacted monomers from polymerization slurries

The method of recovering unreacted mono-olefinic monomers from an acrylonitrile polymerization slurry containing 30 to 50 weight percent of polymer solids and 70 to 50 weight percent of unreacted monomers wherein the slurry is first diluted with a solvent for the polymer and then fed in series through first and second treatment zones, with each of the treatment zones having a first stage for mixing the slurry with a solvent in vapor form to vaporize some of the unreacted monomers from the slurry and form a polymer solution and a second stage for separating monomer vapors from the liquid polymer solution. Fresh solvent, in vapor form and at an elevated temperature is supplied to the mixing stage of the second treatment zone and the vaporized solvent and unreacted monomers from the separation stage of the second treatment zone are fed to the mixing stage of the first treatment zone. The preferred ratio of solvent to slurry being fed to the mixing stages is 0.8 to 1.2. The polymer solution leaving the separation stage of the second treatment zone has a very low content of unreacted monomers.

BACKGROUND OF THE INVENTION 
a. Field of the Invention 
This invention relates to methods for removing unreacted monomers from 
acrylic bulk polymerization slurries. 
b. Description of the Prior Art 
Acrylic polymers are conventionally manufactured by a suspension 
polymerization process, with the polymer solids subsequently being removed 
from the polymerization slurry by a filtration process. The polymers are 
then washed with water to remove unreacted monomers and are then dried. 
This process has the disadvantage that the polymer must be dried. 
In one prior art method of removing unreacted monomers from a 
polymerization slurry, the slurry is allowed to cascade down a baffle type 
stripping column while steam is fed upward through the column, 
counter-current to the cascading slurry. Pluggage and fouling are reduced 
in this process but contact between the slurry and the steam is not as 
good as desired, resulting in reduced monomer stripping. 
In the present invention neither water or steam is used, the slurry being 
contacted with heated solvent vapors to remove unreacted monomers. Mixers 
are used to insure good slurry solvent contact and the slurry is 
circulated through two mixing and separating stages in a particular manner 
to remove substantially all of the unreacted monomers. Polymer solutions 
from this process may be fed directly to a spinning machine without 
further treatment. 
SUMMARY OF THE INVENTION 
In the process of the present invention a bulk polymerization slurry 
containing 30 to 50 weight percent of polymer solids and 70 to 50 weight 
percent of unreacted monoolefinic monomers is diluted with a solvent to 
reduce the solids content of the diluted slurry to 20 to 25 weight 
percent. The diluted slurry is then passed through first and second 
treatment zones, each of the treatment zones having a first stage for 
mixing the slurry with solvent in vapor form and a second stage for 
separating vapors from the liquid polymer solution. A fresh solvent for 
the polymer, in vapor form and at an elevated temperature, is fed to the 
mixing stage of the second treatment zone to strip unreacted monomers from 
the polymer solution to an acceptable level. The vaporized unreacted 
monomers and that portion of the solvent which remains in vapor form are 
withdrawn from the separation stage of the second treatment zone and fed 
to the mixing stage of the first treatment zone. Some of the solvent fed 
to the mixing stages will be condensed to a liquid, while the remainder 
will be taken off as a vapor. Vaporization of the unreacted monomers and 
condensation of the solvent is such that the polymer solution leaving the 
first treatment zone will be made up of less than 5 weight percent 
unreacted monomers, 20-25 weight percent polymer and 73-78 weight percent 
solvent. The polymer solution leaving the second treatment zone will be 
made up of about 20-25 percent polymer and about 75-80 weight percent 
solvent, with there being less than about 0.1. weight percent of unreacted 
monomers remaining in the solution at this point. With an unreacted 
monomer content this low, the solution can be fed directly to a spinning 
machine without further treatment.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now in detail to the drawing there is shown a bulk polymerization 
reactor 11 in which acrylonitrile (AN) is polymerized in a conventional 
manner. Other mono-olefinic monomers may be present in the reactor and 
copolymerized with acrylonitrile. Other mono-olefinic monomers which are 
copolymerizable with acrylonitrile are well known to those skilled in the 
art. Preferably, the polymers of the present invention will contain at 
least 35 weight percent acrylonitrile. 
A polymerization slurry is withdrawn from the reactor 11 through a line 12 
into a stirred slurry tank 14. This slurry will contain about 30 to 50 
weight percent of polymer solids, with the remainder being unreacted 
monomers and very minor amounts of catalysts and other additives. A line 
13 leading from a supply 16 feeds a liquid solvent for the polymer into 
the tank 14 to dilute the polymerization slurry to a solids content of 
about 20-25 weight percent in the slurry tank. Solvents such as 
dimethylacetamide (DMAC), dimethylformamide (DMF) and dimethylsulfoxide 
(DMSO) may be used. The preferred solvent is dimethylacetamide. If 
desired, the tank 14 may be heated to raise the temperature of the diluted 
slurry. 
The diluted slurry is fed through first and second treatment zones 17 and 
18 in series. Each of the treatment zones of stripping units 17 and 18 has 
a first stage which is a mixing stage and a second stage which is a 
separating stage. The mixing stages are made up of mixers of a known type 
such as that illustrated in FIG. 2. A mixer of this type is made up of a 
length of pipe 20 having secured therein a series of helical vanes 21 
which cause the material flowing through the pipe to flow through pipe 
along a helical path which reverses direction at each of the vanes 21. The 
slurry flows radially into the mixer through an inlet tube 24, while the 
solvent vapor enters the mixer axially. 
The separating stage of each of the treatment zones 17 and 18 comprises a 
known separator, such as a cyclone separator, for separating vapors from 
liquids. 
The diluted slurry will flow first to the mixer 22 of the first treatment 
zone 17 where the slurry is mixed with a solvent in vapor form and at a 
temperature of 115.degree.-150.degree. C. to vaporize off some of the 
unreacted monomers in the slurry, with some of the solvent being 
condensed. The condensed solvent will further dissolve the polymer, with 
some being left in solid form. Most of the unreacted monomers will be 
separated from the slurry in a cyclone separator 23 making up the second 
stage of the first treatment zone and will be collected in a monomer 
recovery unit 36 for further use. The remaining polymer solution is taken 
off the bottom of the separator 23 and fed through a mixer 26 and a 
separator 28 of the second treatment zone 18. The solution fed from the 
separator 23 to the mixer 26 will consist of about 20-25 weight percent of 
polymer in solvent, with less than about 5 weight percent unreacted 
monomers. 
Fresh solvent, of the type discussed above, from a source 30 is fed axially 
into the mixer 26 to be mixed with the solution. This solvent is in vapor 
form and at a temperature of 115 to 150.degree. C. The solvent at this 
temperature will vaporize off substantially all of the remaining unreacted 
monomers in the polymer solution and these vapors will be separated from 
the polymer solution by the separator 28. The solvent will also complete 
the dissolving of the polymer. 
The polymer solution or spin dope leaving the bottom of the separator 28 is 
about 20-25 weight percent polymer dissolved in about 75-80 weight percent 
of the solvent and is carried off through a line 31 to a spinning machine 
32. The polymer solution at this point will contain less than 1.0 weight 
percent unreacted monomers and will usually contain less than about 0.1 
weight percent of unreacted monomers, so that it can be fed to the 
spinning machine 32 without further treatment. 
The vapors from the separator 28 are fed through a line 35 to the mixer 22 
of the first treatment zone 17. Vapors separated from the solution by the 
separator 23 are fed to the monomer recovery unit 36 which may be a 
distillation column of a well known type. 
The pressure in separator 23 is maintained at 50-150 mm Hg abs. The 
pressure drop (.DELTA.P) through the vapor line from the separator 28 to 
the mixer 22 and through the mixer 22 to the separator 23 is about 50-75 
mm H. The pressure in separator 28 is maintained at about 100-225 mm Hg. 
The amount of solvent vapor fed from the source 30 to the mixer 26 will be 
equal in weight to the weight of the slurry entering the mixer 22. The 
amount of solvent vapor used will preferably be 80-120 weight percent, 
based on slurry weight. Greater amounts of solvent, up to four to five 
times the amount of slurry, may be used. However, this is less economical. 
COMATIVE EXAMPLE I 
A slurry was mixed to simulate a polymerization slurry from a bulk 
polymerization process having an initial polymer solids content of 40 
weight percent and which had been diluted to a solids content of about 25 
weight percent by the addition of DMAC. This was achieved by mixing 
together 375 grams of a copolymer of 93 weight percent acrylonitrile and 7 
weight percent vinyl acetate, 562.5 grams of monomeric acrylonitrile and 
562.5 grams of DMAC. The slurry was heated to 50.degree. C. and then 
passed through a mixer of the type described above, where it was contacted 
by solvent vapor fed at an elevated temperature and at varying rates and 
then fed into a cyclone separator operating at a pressure of 100 mm Hg 
absolute. Only one mixer and one separator stage were used in this 
example. The results of three runs stage were used in this example. The 
results of three runs as shown in Table A. 
TABLE A 
______________________________________ 
Run No. 1 2 3 
______________________________________ 
DMAC vapor rate 
50 cm.sup.3 /min* 
75 cm.sup.3 /min* 
100 cm.sup.3 /min* 
DMAC vapor temp. 
110.degree. C. 
107.degree. C. 
112.degree. C. 
Slurry rate 50 cm.sup.3 /min 
50 cm.sup.3 /min 
100 cm.sup.3 /min 
Separator temp. 
105.degree. C. 
99.degree. C. 
92.degree. C. 
Polymer in 
treated slurry 
25.2 wt. % 26.5 wt. % 25.5 wt. % 
Monomer in 
treated slurry 
1.3 wt. % 1.4 wt. % 2.9 wt. % 
______________________________________ 
*amount in liquid form prior to vaporization 
COMATIVE EXAMPLE II 
Comparative Example I was repeated using a separator pressure of 150 mm Hg 
absolute. The results of three runs are shown in Table B. 
TABLE B 
______________________________________ 
Run No. 4 5 6 
______________________________________ 
DMAC vapor rate 
50 cm.sup.3 /min* 
75 cm.sup.3 /min* 
100 cm.sup.3 /min* 
______________________________________ 
DMAC vapor temp. 
113.degree. C. 
109.degree. C. 
112.degree. C. 
Slurry rate 50 cm.sup.3 /min 
50 cm.sup.3 /min 
100 cm.sup.3 /min 
Separator temp. 
103.degree. C. 
106.degree. C. 
101.degree. C. 
Polymer in 
treated slurry 
24.4 wt. % 24.7 wt. % 25.3 wt. % 
Monomer in 
treated slurry 
3.2 wt. % 2.5 wt. % 2.9 wt. % 
______________________________________ 
*amount in liquid form prior to vaporization 
In both Examples I and II it will be seen that a significant amount of 
unreacted monomer remains in the spinning solution after it has passed 
through the evaporator. These amounts, 1.3-3.2 weight percent, are too 
high for practical operation. 
EXAMPLE III 
A copolymer of 93 weight percent acrylonitrile and 7 weight percent vinyl 
acetate was produced in a conventional reactor using a known bulk 
polymerization process. Conversion was 37.7 percent and the composition of 
the slurry fed from the reactor to the tank 14 was as follows: 
______________________________________ 
Polymer solids 37.7 wt. % 
Acrylonitrile monomer 48.0 wt. % 
Vinyl acetate monomer 14.3 wt. % 
Total 100.0% 
______________________________________ 
The reactor slurry, fed at a rate of 95.6 gm/min, was mixed with DMAC, fed 
at a rate of 54.7 gm/min, in the tank 14 to form the feed slurry for the 
monomer stripping units. The slurry, which was heated to 50.degree. C. in 
the tank 14 and fed to the mixer 22 at 150.3 gm/min, had the following 
composition and feed rates: 
______________________________________ 
Polymer 36.1 gm/min 
24.0 wt. % 
Acrylonitrile monomer 
45.9 gm/min 
30.5 wt. % 
Vinyl acetate monomer 
13.6 gm/min 
9.1 wt. % 
DMAC 54.7 gm/min 
36.4 wt. % 
Total 150.3 gm/min 
100.0% 
______________________________________ 
The slurry was contacted with hot solvent vapors from the second stage 
separator 28 in the mixer 22. The polymer solution was separated from the 
DMAC/monomer vapors in a 15 cm I.D. cyclone separator. Flow rates and 
processing conditions for the first stage monomer stripping operation was 
as follows: 
Slurry feed flow rate: 150.3 gm/min 
Slurry feed temperature: 50.degree. C. 
First stage separator pressure: 100 mm Hg 
First stage separator temperature: 103.degree. C. 
.DELTA.P across mixer 22: 70 mm Hg 
The solution from the first stage separator 23 was pumped to the second 
stage mixer 26 and mixed with DMAC vapor fed from the source at a rate of 
426.5 gm/min, a solvent to solution ratio of 2.84. The solution and DMAC 
vapors were thoroughly mixed in the mixer 26. The final polymer solution 
was separated from the DMAC/monomer vapors in a 15 cm I.D. cyclone 
separator. The DMAC/monomer vapors were recycled to the first stage mixer 
22. The flow rates and process conditions for the second stage were as 
follows: 
DMAC vapor flow rate: 426.5 gm/min 
DMAC vapor temperature: 135.degree.-150.degree. C. 
DMAC vapor pressure: 260 mm Hg 
Second stage separator pressure: 170 mm Hg 
Second stage separator temperature: 116.degree. C. 
.DELTA.P across second stage mixer: 90 mm Hg 
Slurry polymer content: 19.0 wt.% 
Slurry monomer content: 0.13 wt.% 
EXAMPLE IV 
A copolymer of 93 weight percent acrylonitrile and 7 weight percent vinyl 
acetate was produced in a conventional reactor using a known bulk 
polymerization process. Conversion was 39.6 percent and the composition of 
the slurry from the reactor was as follows: 
______________________________________ 
Polymer solids 39.6 wt. % 
Acrylonitrile monomer 46.2 wt. % 
Vinyl acetate monomer 14.2 wt. % 
Total 100.0% 
______________________________________ 
The reactor slurry, fed at a rate of 115.3 gm/min, was mixed with DMAC, fed 
at a rate of 62.5 gm/min, in the tank 14 to form the feed slurry for the 
monomer stripping units. The slurry, heated to 50.degree. C. in the tank 
14, was fed to the mixer 22 and had the composition and feed rates: 
______________________________________ 
Polymer solids 45.7 gm/min 
25.9 wt. % 
Acrylonitrile monomer 
52.3 gm/min 
29.6 wt. % 
Vinyl acetate monomer 
16.3 gm/min 
9.2 wt. % 
DMAC 62.3 gm/min 
35.3 wt. % 
Total 176.6 gm/min 
100.0% 
______________________________________ 
The polymer solution was separated from the DMAC/monomer vapors in a 15 cm 
I.D. cyclone separator. Flow rates and processing conditions for the first 
stage monomer stripping operation was as follows: 
Slurry feed flow rate: 176.6 gm/min 
Slurry feed temperature: 50.degree. C. 
First stage separator pressure: 100 mm Hg 
First stage separator temperature: 103.degree. C. 
.DELTA.P across first stage mixer 22: 70 mm Hg 
The solution from the first stage separator 23 was pumped to the second 
stage mixer 26 and mixed with DMAC vapor fed at a rate of 380 gm/min, or 
solvent to slurry ratio of 2.15. The final spinning dope was separated 
from the DMAC/monomer vapors in a 15 cm I.D. cyclone separator. The 
DMAC/monomer vapors were recycled to the first stage mixer 22. The flow 
rates and process conditions for the second stage were as follows: 
DMAC vapor flow rate: 380 gm/min 
DMAC vapor temperature: 135.degree.-150.degree. C. 
DMAC vapor pressure: 260 mm Hg 
Second stage separator pressure: 170 mm Hg 
Second stage separator temperature: 116.degree. C. 
.DELTA.P across second stage mixer: 90 mm Hg 
Final solution polymer content: 17.4 wt.% 
Final solution monomer content: 0.11 wt.% 
EXAMPLE V 
A copolymer of 93 weight percent acrylonitrile and 7 weight percent vinyl 
acetate was produced in a conventional reactor using a known bulk 
polymerization process. Conversion was 40.1 percent and the composition of 
the slurry from the reactor was as follows: 
______________________________________ 
Polymer solids 40.1 wt. % 
Acrylonitrile monomer 45.8 wt. % 
Vinyl acetate monomer 14.1 wt. % 
Total 100.0% 
______________________________________ 
The reactor slurry, fed at a rate of 115.3 gm/min, was mixed with DMAC, fed 
at a rate of 62.5 gm/min, in the tank 14 to form the feed slurry for the 
monomer stripping units. The flow rates and composition fed to the mixer 
22 were as follows, the slurry having been heated to 50.degree. C. in the 
tank 14: 
______________________________________ 
Polymer 46.2 gm/min 
26.0 wt. % 
Acrylonitrile monomer 
52.8 gm/min 
29.7 wt. % 
Vinyl acetate monomer 
16.3 gm/min 
9.2 wt. % 
DMAC 62.3 gm/min 
35.1 wt. % 
Total 177.6 gm/min 
100.0% 
______________________________________ 
After passing through the mixer, the polymer solution was separated from 
the DMAC/monomer vapors in a 15 cm I.D. cyclone separator. Flow rates and 
processing conditions for the first stage monomer stripping operation was 
as follows: 
Slurry feed flow rate: 177.6 gm/min 
Slurry feed temperature: 50.degree. C. 
First stage separator pressure: 100 mg Hg 
First stage separator temperature: 103.degree. C. 
.DELTA.P across mixer 22: 70 mm Hg 
The solution from the first stage separator 23 was pumped to the second 
stage mixer 26 and mixed with DMAC vapor fed at a rate of 324.5 gm/min, a 
solvent to slurry ratio of 1.83. The solution and DMAC vapors were mixed 
in the mixer 26. The final spinning dope was separated from the 
DMAC/monomer vapors in a 15 cm I.D. cyclone separator. The DMAC/monomer 
vapors were recycled to the first stage mixer 22. The flow rates and 
process conditions for the second stage were as follows: 
DMAC vapor flow rate: 324.5 gm/min 
DMAC vapor temperature: 135.degree.-150.degree. C. 
DMAC vapor pressure: 260 mm Hg 
Second stage separator pressure: 170 mm Hg 
Second stage separator temperature: 116.degree. C. 
.DELTA.P across second stage mixer: 90 mm Hg 
Dope polymer content: 17.5 wt.% 
Dope monomer content: 0.11 wt.% 
EXAMPLE VI 
A copolymer of 93 weight percent acrylonitrile and 7 weight percent vinyl 
acetate was produced in a conventional reactor using a known bulk 
polymerization process. Conversion was 39.0 percent and the composition of 
the slurry from the reactor was as follows: 
______________________________________ 
Polymer solids 39.0 wt. % 
Acrylonitrile monomer 46.8 wt. % 
Vinyl acetate monomer 14.2 wt. % 
Total 100.0% 
______________________________________ 
The reactor slurry, fed at a rate of 95.6 gm/min, was mixed with DMAC, fed 
at a rate of 54.7 gm/min, in the tank 14 to form the feed slurry for the 
monomer stripping units. The slurry feed flow rates and compositions to 
the mixer 22 were as follows, the slurry having heated to 50.degree. C. in 
the tank 14: 
______________________________________ 
Polymer 36.1 gm/min 
24.0 wt. % 
Acrylonitrile monomer 
45.9 gm/min 
30.5 wt. % 
Vinyl acetate monomer 
13.6 gm/min 
9.1 wt. % 
DMAC 54.7 gm/min 
36.4 wt. % 
Total 150.3 gm/min 
100.0% 
______________________________________ 
After being mixed, the polymer solution was separated from the DMAC/monomer 
vapors in a 15 cm I.D. cyclone separator. Flow rates and processing 
conditions for the first stage monomer stripping operation was as follows: 
Slurry feed flow rate: 150.3 gm/min 
Slurry feed temperature: 50.degree. C. 
First stage separator pressure: 150 mm Hg 
First stage separator temperature: 103.degree. C. 
.DELTA.P across mixer 22: 70 mm Hg 
The solution from the first stage separator 23 was pumped to the second 
stage mixer 26 and mixed with DMAC vapor fed at a rate of 300.7 gm/min, a 
solvent to slurry ratio of 2.0. The final spinning dope was separated from 
the DMAC/monomer vapors in a 15 cm I.D. cyclone separator, with the 
DMAC/monomer vapors being recycled to the first stage mixer 22. The flow 
rates and process conditions for the second stage were as follows: 
DMAC vapor flow rate: 300.7 gm/min 
DMAC vapor temperature: 150.degree. C. 
DMAC vapor pressure: 260 mm Hg 
Second stage separator pressure: 170 mm Hg 
Second stage separator temperature: 116.degree. C. 
.DELTA.P across second stage mixer: 90 mm Hg 
Dope polymer content: 21.5 wt.% 
Dope monomer content: 0.18 wt.% 
EXAMPLE VII 
A bulk polymerization reaction was carried out in the reactor 11 to produce 
a copolymer of 93% acrylonitrile and 7% vinyl acetate in a well known 
manner. The resulting polymerization slurry was diluted with 
dimethylacetamide from the supply 16 to produce in the tank 14 a slurry 
having 24.8 weight percent solids, 40.6 weight percent unreacted monomers 
and 33.7 weight percent of dimethylacetamide. This slurry, at a 
temperature of about 50.degree. C., was fed at a rate of 40.8 kg per hour 
into the mixer 22. Vaporized solvent containing a small amount of 
unreacted monomers was fed from the separator 28 through line 35 into the 
mixer 22 at a rate of 358.1 kg per hour, so that the ratio of solvent to 
slurry fed into the mixer 22 was 0.88. 
The mixture from the mixer 22 was fed into the separator 23, with the 
vapors from the separator 23 being forwarded to the monomer recovery unit 
and the remaining solution being fed to the mixer 26. The solution being 
fed to the mixer 26 was fed at a rate of 400.6 kg per hour and contained 
25.31 weight percent of solids and 1.92 weight percent of unreacted 
monomers in solvent. Fresh dimethylacetamide from the supply 30 was fed 
into the mixer 26 at a rate of 457.8 kg per hour, so that the ratio of 
solvent to solution being fed to the mixer 26 was 1.14. The ratio of 
solvent fed to the mixer 26 to the slurry fed to the mixer 22 was 1.28. 
The mixture leaving the mixer 26 was fed through line 27 into the separator 
28, with the vapors from the separator 28 being fed back to the first 
stage mixer 22 and the liquid portion being carried off through line 31 to 
the spinning machine 32. The solution passing through the line 31 
contained 24.39 weight percent solids and 0.13 weight percent unreacted 
monomers in dimethylacetamide. The amount of solution passing through the 
line 31 was 409 kg per hour. 
A number of other runs were made under the conditions of Example VII. The 
amount of unreacted monomers passing through the line 31 ranged from 0.12 
weight percent up to 0.26 weight percent, with the average being 0.165 
weight percent. 
The ratio of solvent to slurry or solution entering the mixers 22 and 26 is 
preferably about 0.8 to 1.2 In other words, for a given amount of slurry 
entering the mixer 22 or solution entering 26, 80 to 120 percent of that 
amount will, preferably, be the amount of solvent fed to the mixer. 
However, amounts of solvent up to 3 to 4 times the amount of slurry or 
solution can be used, as other examples will show. 
For the purpose of determining flow rate ratios, it is preferably to 
measure the flow rate of solvent from the source 30 to the mixer 26 and 
compare that to the flow rate of slurry being fed from the tank 14 to the 
mixer 22. It is not necessary to measure solvent input to the mixer 22 or 
solution input to the mixer 26, since these values will be controlled by 
solvent input to the mixer 26 and slurry input to the mixer 22. The ratio 
of solvent fed from the source 30 to the mixer 26 to the slurry fed from 
the tank 14 to the mixer 22 should be about 0.8 to 4. It is preferably 
about 0.8 to 1.2.