Method for removal of residual solvent from particulate crumb made by solution polymerization

A method for removing residual solvent dissolved in non-sticky crumb particles produced by solution polymerization that have been treated substantially to remove solvent from the polymer surface. Particles are transferred into a closed storage zone under inert atmosphere conditions and are maintained for a period of time sufficient to allow diffusion of solvent from the particles as free solvent. Inert gas is then passed through the storage zone to remove the free solvent. The processes of allowing diffusion followed by purging of the free solvent is repeated as necessary.

BACKGROUND OF THE INVENTION 
This invention relates to the removal of solvent. In one of its aspects, 
this invention relates to the removal of residual solvent from particulate 
crumb produced by solution polymerization. In another of its aspects, this 
invention relates to a process for further removal of residual solvent 
from solution polymerization particles after the particles have been 
treated by powder form evaporation. In still another aspect of the 
invention, it relates to an integrated process for removal of solvent 
using inert gas which is then recycled for further use. 
The removal of residual solvent from solid polymer particles such as 
solution rubber is difficult because there is residual solvent dissolved 
in the solid polymer which must diffuse from the polymer to be removed. 
This makes the process of residual solvent removal from these solution 
polymers time dependent. Although most of the solvent can be removed from 
the surface of the polymer by various evaporative techniques, solvent will 
still remain dissolved in the polymer. Gas purging can only remove solvent 
from the particle surface and this can be done in a short period of time. 
With the realization that there must be time for the dissolved solvent to 
diffuse from the polymer particles, it has become apparent that continuous 
purging of the polymer particles would only be a waste of gas and would 
complicate recovery of solvent from the purge gas since it would be 
present in a low concentration. The present invention provides an 
intermittent procedure for purging the polymer which greatly reduces the 
volume of inert gas used and inherently enhances solvent recovery since 
the same amount of purged solvent will be contained in a greatly reduced 
volume of purge gas. 
It is therefore an object of this invention to provide a method for 
removing residual solvent dissolved in particulate crumb produced by 
solution polymerization. It is another object of this invention to provide 
a polymeric product made by solution polymerization from which dissolved, 
residual solvent has been at least partially removed. 
It is still another object of this invention to provide a method for 
recovering polymer produced by solution polymerization processes from the 
solvent of the process using a method for substantially removing solvent 
from the surface of the polymer, particularly powder form evaporation, and 
then removing residual solvent that has been dissolved in the polymer with 
an integrated process for collecting inert purge gas with recovery and 
recycle of the purge gas. 
Other aspects, objects and the various advantages of this invention will 
become apparent on reading the specification and claims of this disclosure 
in conjunction with a study of the drawing. 
SUMMARY OF THE INVENTION 
According to this invention, a method is provided for removing residual 
solvent dissolved in non-sticky crumb particles prepared by solution 
polymerization that have been treated substantially to remove solvent from 
the polymer surface. In the method, the particles containing residual 
solvent are transferred into a closed storage zone in an inert atmosphere, 
after which (1) the particles are maintained in the storage zone at a 
temperature below the softening point of the polymer for a period of time 
sufficient for diffusion of solvent from particles as free solvent and, 
subsequently, (2) an inert gas is passed through the polymer particles at 
a temperature below the softening point of the polymer in an amount 
sufficient substantially to remove the free solvent from the storage zone, 
and steps (1) and (2) are repeated until a desired level of solvent 
content is obtained in the polymer in the storage zone. 
In a further embodiment of this invention, the polymer containing residual 
solvent has been specifically treated for removal of surface solvent by 
powder form evaporation and the inert gas used for both the powder form 
evaporation and in the process of the present invention are recovered and 
treated to recover the solvent and provide inert gas of sufficiently low 
solvent content to be useful, both in the powder form evaporation process 
and in the process of this invention. 
Although the process of the present invention is designed for operation on 
the product from a powder form evaporation, it should be useful with any 
process that produces solution polymers and recovers a non-sticky crumb 
particle having a residual solvent dissolved therein. 
The powder form evaporation process is described in U.S. Pat. No. 
4,310,973, incorporated here by reference. It is stated in that patent 
that the powder form evaporation process is most readily applicable to any 
polymer solution that can yield a flowable crumb at a temperature of 
20.degree. F. to about 50.degree. F. (11.degree.-28.degree. C.) above the 
normal boiling point of the solvent. That is, the process can be used on 
any solution of a normally solid polymer which can be particulated in a 
spraying device as not unduly sticky at a temperature necessary to 
evaporate the solvent. Suitable polymers include polymers and copolymers 
of butadiene, styrene, isoprene, polyolefins, and the like. The solvents 
used will, of course, depend on the polymer to be processed. 
Most rubber solutions, meet the requirements above. The powder form 
evaporation process and the present process are especially applicable for 
the recovery of polymers from the solutions of rubbery polymers. A 
specific rubber solution that is of interest in this invention is that of 
a rubbery copolymer of 75 weight percent butadiene and 25 weight percent 
styrene in a solvent of cyclohexane. 
In the powder form evaporation process, the polymer solution is dried by 
the removal of solvent by flashing the polymer solution as it is sprayed 
into a bed of substantially dry polymer particles. The polymer particles 
coat the partially dried solution polymer droplets and the remaining 
solvent is removed, using the energy produced from a mechanical agitation 
of the bed and a countercurrent passing of a heated inert gas through the 
polymer bed. The polymer that is removed from the evaporation process is a 
particulate product that can best be described as granules or crumb, 
although some particles may approach powder in size, depending on the 
polymer. The products from the powder form evaporation process normally 
will pass to storage silos for blending, testing and subsequent 
utilization or sales. The product, if sufficiently free of solvent, is 
suitable for sale or direct use in many applications without extrusion or 
devolatilization, pelleting, or other such preparations of polymer. Since 
the polymer normally will have solvent dissolved within the particles, the 
present invention provides for an intermittent inert gas purge of the 
storage facilities to sweep out residual solvent which diffuses from the 
pores and surfaces of the granules into the void space of the storage zone 
.

Referring now to the drawing, an integrated system is presented in which 
polymer-solvent cement is fed into a powder form evaporator to produce 
polymer containing dissolved solvent which is transferred to storage zones 
and treated by the process of this invention with the nitrogen used as 
inert gas in the process of the invention being further treated for 
removal of the solvent in accordance with an embodiment of the invention 
and recycled for further use. 
In the drawing, which is a schematic representation of the process, a 
polymer-solvent cement is fed to line 1 into a powder form evaporating 
unit 3. The powder form evaporator which contains a charge of polymer 
powder which has been raised to operating temperature of about 200.degree. 
F. by mechanical agitation with agitator 5 which is driven by motor 7 
operates by surrounding the polymer-solvent cement sprayed from line 1 
into the evaporator with the fine powder agitated by agitator 5 so that a 
non-sticky agglomerated particle of polymer-solvent cement surrounded by 
polymer powder is formed. This material passes into the vertical portion 
of the evaporator in which particles that have agglomerated into slurry 
like masses are contacted with grinder 9 driven by motor 11 and broken 
into smaller particles. These smaller particles are stirred by agitator 13 
driven by motor 15 as nitrogen supply through blower 17 enters into the 
base of evaporator 3 through line 19 to provide further drying of the 
particles by picking up solvent vapor and removing it through the top of 
the evaporator and line 21 back to a nitrogen-solvent separation process 
23. In the process of drying in the evaporator unit, powdered polymer is 
continuously produced for contact with the incoming polymer-solvent cement 
so that the process constantly renews itself while solvent bearing 
nitrogen passes up to the evaporator and out line 21 and dried polymer 
particles containing a trace of solvent dissolved in the polymer particles 
are removed through line 25 to be carried by nitrogen supplied by nitrogen 
blower 27 and line 29 through jet 31 and lines 33, 35 into storage vessels 
37, 39. 
In the storage vessels, the relatively heavy particles are dropped into the 
base of the storage zone while the nitrogen exits through the top of the 
storage vessels through lines 41, 43 and 45 to be returned to the 
nitrogen-solvent separation process 23. 
It is at this point that the process of the present invention begins. In 
the usual handling of stored polymer which has either surface solvent or 
solvent that can diffuse from the polymer into the voids in the storage 
zone, a continuous purge of inert gas is passed through the particles to 
remove free solvent as it appears in the voids. In the present invention, 
the particulate material within the storage zone is held in an atmosphere 
of nitrogen for a period of time sufficient for a substantial diffusion of 
solvent from the particles as free solvent. This diffusion period extends 
for a period of about 1 to about 24 hours, preferably for a period of 
about 2 to about 12 hours and is followed by a period in which the purge 
gas--the same inert gas, here nitrogen, is supplied from blower 27 to 
lines 47, 49, 51 into the base of storage vessels 37, 39 to sweep through 
the stored particles, removing the free solvent through lines 41, 43, 45 
to the nitrogen-solvent separation operation 23. 
In the purge operation, the temperature is not critical as long as the 
softening point of the polymer is not exceeded. For most polymers, 
temperatures from ambient to about 200.degree. F. are satisfactory with 
the higher temperatures within this range promoting more rapid diffusion. 
The volume of the purged gas used for individual purges is in a range of 
about 1 to about 20 volumes of gas (measured at storage silo conditions) 
per volume of polymer bed. A range of about 2 to about 4 volumes of gas 
per volume of polymer bed is, however, more preferred. The purge pressure 
is preferably slightly above atmospheric to facilitate transport of the 
effluent gas through the storage zone. The purge velocity is not important 
as long as transport velocity for the particles is not exceeded. Pressure 
drop across the storage bed is most desirably kept less than about 5 psi 
(34.5 KPA). With the parameters given above, the purge time from the 
average sized storage zone will generally be less than 1 minute per cycle 
(the time is a function of the purge gas volume, superficial gas velocity, 
and the bed height). The purge cycle, i.e., to maintain the particles in a 
quiescent state for a period of time followed by a burst purge, can be 
repeated as desired to provide a polymer having a solvent content within a 
given range. Products having attained this range can be removed through 
lines 53, 55 for transport or sale. 
As an illustration of embodiments of the invention, note that the passage 
of nitrogen from blower 17, line 19 through the evaporator 3 and line 21 
into nitrogen-solvent separation process 23 will normally be carried on 
continuously with removed solvent exiting the separation through line 57 
and nitrogen, at least reduced sufficiently in solvent content to be 
effective in the powder form evaporator system passed through line 59 with 
addition of make-up nitrogen through line 61 as necessary, line 63 and 
line 65 back to blower 17 for recirculation. During the same time period, 
but occurring at intervals, nitrogen supplied through line 59, 63 and 67 
is passed through blower 27 and lines 47, 49, 51 as purge gas throught 
storage zones 37, 39 and lines 41, 43, 45 back to the nitrogen-solvent 
separation process 23. If the amount of solvent picked up by the purge 
process is sufficiently small that the nitrogen can be used effectively 
for more than one pass through the storage zone, the purge gas with 
solvent passing through line 45 can be diverted through line 69 around the 
nitrogen-solvent separation process 23 to be returned directly through 
line 67 to the blower 27. This prevents unnecessary loading of the 
nitrogen-solvent separation process 23 and is a further improvement of 
this invention over a normally purged storage zone which with a continuous 
purge of gas carrying a small volume of solvent would unduly load the 
nitrogen-solvent separation process as compared to a short blast of purged 
gas containing a relatively larger amount of solvent which in either case 
would be in addition to the continuous amount passing through the powder 
form evaporator. 
To illustrate the effectiveness of the process of this invention, the 
following experimental example is offered. A 24 inch diameter powder form 
evaporator was operated to dry a butadiene-styrene solution rubber at a 
solids rate of 297 pounds per hour. Polymer solution was flashed from 15.1 
to 23.1 weight percent and fed into a 4 feet deep bed of agitated polymer 
particles. The bed temperature was 224.degree. F. at the top and 
199.degree. F. at the bottom. Polymer residence time was approximately 80 
minutes. Nitrogen circulating rate was 20 scfm (standard cubic 
feet/minute) and the nitrogen make-up was 2-3 scfm. A 5 gallon bucket of 
dry product was collected and stored overnight under a blanket of 
nitrogen. The polymer was purged with a 30 second blast of nitrogen. After 
a delay of about 12 hours, the polymer was purged with the second 30 
second blast of nitrogen. Analyses in the following table show the result. 
TABLE I 
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Collected 
Polymer Polymer 
Polymer After First 
After Second 
Sample Purge Purge 
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Cyclohexane 
5009 ppm 4660 ppm 4202 ppm 
n-Hexane 385 ppm 396 ppm 391 ppm 
Total 5394 5056 4593 
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These data indicate a substantial reduction of solvent content of the 
polymer after each short period.