Note: Descriptions are shown in the official language in which they were submitted.
1068047
BACKGROUND OF T~: INVENTION
It is known in the art of polymerizing mono-
meric materials, such as styrene, that control of the
release of heat of polymerization is essential to achieve
reacting conditions that will produce the desired end
product. The very nature of the material makes fluid
mixing and heat transfer difficult since heat is contin-
uously evolved while the viscosity of the fluid mass is
increasing. If proper control of temperature and other
operating conditions are not maintained, the physical
characteristics of the polymer are non-uniform and
unacceptable from a sales and utility standpoint. Bulk
or mass processes have been proposed as a means by which i`~-
the nature of the reaction may be efficiently controlled
primarily through a highly efficient agitation. Because
the polymerization reaction requires from about six -~
hours time to about twelve hours to produce a satisfactory
product, the design of a continuous flow styrene polymeri-
zation process has been difficult. Others such as
United States patents 3,206,287 - Crawford; 3,049,413 -
Sifford; 2,530,409 - Stober, et al; 2,614,910 - Allen,
et al; and 2,694,692 - Amos, et al and others, have taught
varying methods of control over the polymerizing masses
by various mechanical mixing and agitation devices.
SI~RY OF T~IE INVENTION -
This invention provides a process and apparatus
~ , '.
-2- ~
~0~8047
for continuously polymerizing styrene wherein the cooler
styrene, within a tubular series flow reactor, at the
heat exchange surface is continuously mixed with the
internally hotter styrene away from the heat exchange
surface solely by the diverted motion of the styrene
through the tubular reactor. More particularly, this
invention provides a stationary mixer within the tubular
reactor providing improved mixing and heat exchange con- -
trol of the polymerizing material. -
The process and apparatus of this invention
is particularly related to the continuous polymeri~ation
of styrene but is also applicable to polymerization of
alkyl or halogen substituted styrene and mixtures thereof. -~
It is further applicable to polymerization of other vinyl
monomers alone or in mixtures with styrene or styrene
derivatives and is also applicable to styrene within
which are dispersed particles such as unvulcanized and
unsaturated natural or synthetic rubber which is either
soluble in, or can be rendered soluble in, monomeric
styrene.
Fig. 1 is a schematic flow diagram, partially
in cross section, depicting the series flow reactor
encompassed within this invention.
Fig. 2 is a partial sectional view of a flow
mixer embodiment which is an improvement over that
disclosed in U. S. patent 3,286,992.
, ' . .' '., ~ ' '' '
1~6~047
Before explaining the present invention in
detail, it is to be understood that the invention is
not limited in its application to the details of con-
struction and arrangement of parts illustrated in the
accompanying drawings, since the invention is capable
of other embodiments and/or being practiced or carried ;~
out in various ways. Also, it is to be understood that
the phraseology or terminology employed herein is for
the purpose of description and not of limitation.
Referring now to the drawings, and in typical
usage of this invention, monomeric styrene and those
additives such as modifiers or plasticizers or rubber
as desired are pumped continuously into a tubular reactor -
generally designated by the numeral 10. Preferably the
reactor is of substantially uniform cross-section tubing
about 3 to 6 inches in diameter. The reactor is provided
with means to remove the heat of polymerization through
the tubing wall into a desired coolant. The length of
the tubular reactor is sufficient to allow for the desired
reaction time when considering the flow of styrene and the
internal volume of the tubing. The internal diameter of
the tubing in the reactor may be varied to provide for
the proper velocity of flow and rate of mixing as the
reaction progresses. In some instances, several tempera-
ture zones or stages are employed over the polymerization
process. That is, control over the heat exchange coolant
may vary in accordance with different stages of the reaction
-4-
~ , .
~6~3~47
as at 12, 14 and 16. For example 0-30% polymerization at
250F.; 30-60% at ~00F., and 60-90% at ~50F. The tubular
reactor is preferably of a design shown partially in cross
section within which are a plurality of stationary dispersion
or baffle elements 18 which are so oriented that the flow of
polymer is continually diverted from the cooler outer walls
to the inside where it is divided and diverted to achieve
continuous mixing of that which has been cooled with the
hotter styrene; the baffle elements alternately divide the
flowing monomer within the reaction tubing into two essentially
equal compartments of turning flow and thence into four
essentially equal compartments of straight flow, whereby
the flow periodically merges and separates along the length ;-~
of the tubing. In some instances the entire process may
occur within the tubular reactor portion allowing polystyrene ~;~
of quality and molecular weaght desired, to flow through the
outlet 20 for further processing to a completed product.
In other instances and in an alternate embodiment
of this invention the output of the tubular reactor including
the stationary mixers 18 therein may conclude with a final
mechanical mixing stage in one or more devices, 22 and/or
24, such as an internally agitated cylindrical reactor
wherein the agitation blades are equipped for internal
circulation of heating and/or cooling medium. Further
processing includes a vacuum devolatilization in vessel 26
. : . . .
1068047
with the polymer product removed through outlet 28.
This invention is also applicable to the
production of rubber modified styrene. In order to
achieve the desired properties of the desired styrene
product the rubber particles must be present in the
styrene matrix within a certain particle size range.
Otherwise, the impact strength of the styrene is greatly
affected. This invention achieves the dispersion of the
rubber in the styrene matrix by pumping monomeric styrene
as shown in the illustration within which is dissolved
rubber. The internal diversion and dividing baffles
within the tubular reaction reactor agitate the styrene
as it pa~ses through the reactor during its polymerizing
process and helps maintain the rubber suspended and dis-
persed as it tends to precipitate from the styrene.
A further understanding of this invention may
be achieved by the following example:
A blend of feedstock is prepared consisting of
approximately 98% styrene monomer, 2% white mineral oil,
and a small amount of dodecyl mercaptan. The blended
feedstock is pumped into the reactor system at a rate
o~ approximately 1500 lbs. per hour, using a high pressure
feed pump. The feed first passes through a steam heated
heat exchanger in which its temperature is increased to
approximately 240 F. From this exchanger the feed enters
the first flow-mixed tubular exchange section. This ~ -
-6- -~ ~
.. .. . . . .
1068~:)47
:~
exchanger section consists of a continuous coil of 3"
schedule 40 pipe having a total overall length of 1,000
ft. and is cooled externally by a circulating oil bath.
The pipe contains stationary internal mixing elements ~ -
through its entire length except in the return bends.
These elements keep the reaction mix in lateral flow ~-
agitation during the entire progress through the pipe.
Residence time in the 3" pipe coil is approximately 2
hours, and the temperature of the reaction mixture
averages approximately 2500 F. At the exit from this
pipe coil, approximately 25% of the styrene monomer is
converted into polystyrene. From the ~" pipe coil, the
reaction mixture passes to a second pipe coil exchange
section made of 4" diameter schedule 40 pipe with an
overall length of approximately 1,100 ft. It is cooled
externally by a circulating oil bath. This coil also
contains the stationary flow mixing elements designed to -~
keep the reaction mixture in constant lateral flow agitation.
Residence time in this 4" diameter pipe coil is approxi-
mately 4 hours, and at the exit from this coil, the reaction
mixture is approximately 50% converted into polystyrene. -
Temperature in this coil averages approximately ~00 F.
From the 4" diameter pipe coil, the reaction mixture
passes to a 6" diameter pipe coil having an overall length
,;
of approximately 500 ft. It is cooled externally by a
circulating oil bath. This coil is also filled with the
stationary flow mixing elements, designed to keep the
reaction mixture in agitation. Residence time in the 6"
10~8047
diameter pipe coil is approximately 4 hours, and the
reaction mixture leaving this coil is approximately
75% converted into polystyrene. The average temperature
in this reactor is approximately 350 F. From the 6"
diameter pipe coil, the reaction mixture passes to a
cylindrical reactor containing an internal mixer powered
by an external power source. The blades of the mixer are
equipped with means for being cooled and/or heated by a
heating or cooling medium circulated through the hollow
core of the mixer blades. The reaction mixture passing
through the cylindrical reactor is held at a temperature
of around ~50 F. during a portion of the reactor and
then a heating medium is used to increase the temperature
to approximately 450 F. Residence time in this reactor
is approximately 2 hours. The reaction mixture, at this
point, is approximately 90% or more polystyrene. The
reaction mixture is then discharged from the mixed reactor
at approximately 450 F. into a devolatilizing vessel in
which there is a vapor space held under vacuum and in
which the molten polystyrene is spread into a thin film
to allow for release of un-polymerized styrene and other
volatiles. From the vapor space of the devolatilization
vessel unreacted styrene and any other volatiles present ~ ;
are drawn through a condensor in which the volatile
materials are condensed and discharged from the process
for either recycle or reprocessing. The vacuum on the ~-~
-8-
lV686:~47
devolatilization vessel is maintained by a vacuum pump.
The molten polystyrene, now substantially 100% polymer
after removal of the volatile material, is accumulated ~
in the bottom of the devolatilization vessel from which -
it is pumped with a gear pump designed for pumping high
viscosity materials through an extrusion dye in which
it is formed into continuous strands. The strands are
cooled in a water bath and chopped into pellets.
"Essentially vinyl monomer" as used herein is
defined as not only the named ingredients alone but in
mixture with other materials such as plasticizers,
modifiers, diluents or reaction additives to vary or
change the characteristics of the vinyl polymer or to
reduce viscosity of the polymerizing reactants.
Improved mixing of fluids is accomplished with
the apparatus of Fig. 2 and is not limited to polymeri- ~;
zation reactions but includes all mixing or intermixing ~;?
of fluids. Internally of the flow tubing an essentially
integral longitudinal member defines sequential and
alternate sections A and B. Section A divides the
basically unidirectional flow into two essentially equal
compartments which turn the flow because of the curved
sheet-like portion 18. Section B, which may be equal to,
less than, or longer than A, comprises intersec~ing ~ -
straight members which divides the flow into essentially
four compartments which preferably are of equal flow area.
~9~
~ .
1~;8~47
It is found that a construction according to this
; mixing device provides improved mixing, improved heat
exchange if desired, and strength of the integral internal .,
device which must withstand the forces of fluid flow.
-10- '
