Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF_ THE INVENTION
The present invention relates to a new and
improved reactor for continuously performing polymerizations
in highly viscous media, and furthermore, also relates to a
new and improved method of preparing polymers using the
aforementioned reactor for continuously performing
polymerizations in highly viscous media.
For performing heretofore known methods of
polymerization, either in substance or in a solvent to high
degrees of conversion, there were used batchwise or
continuously operated reactors with a uniform residence time
of the reaction mixture.
Stirrer containers can be used for conversions
up to 80~ in discontinuous operations. Since the viscosity
strongly increases during the reaction (cf. R.L. Zimmerman
et al, Adv. Chem. 5er. 34, 1962) different types of stirrers
are suggested depending upon the range of viscosity Icf.
C.K. Coyle et al, Can. J. Chem. Eng. 48, 1970 and V.W. Uhl,
H.P. Voznick, Chem. Eng. Prog. 56, 1960).
Insufficient intensity of stirring causes local
overheating (cf. V.W. Uhl, H.P. Voznick, Chem. Eng. ProgrO
56, 1960) and may result in marked temperature peaks during
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the course of the reactlon (cf. J.A. Biesenberger, Appl.
Pol. Symp. 26, 1975). It will be self-evident that the heat
exchange can be intensified by increasing the rotational
speed of the stirrer. Thus, the heat transfer is increased
by a factor of about 102 when the rotational speed is
increased by a factor of 103. However, the relative
eneryy input increases by a factor of about 107 (cf.
R.H.M. Simon, D.C. Chappelear, "Polymerization Reactors and
Processes'l, ACS Symp. Ser. 10~, tl979) 71). As a
consequence, regions will be rapidly attained in which net
heat no longer can be removed.
At conversions exceeding 80~ stirred containers
no longer can be employed and the mixture will have to react
to completion in molds like, for example, in filter presses
(cf. ~. Gerrens; Chem~ IngO Techn. 52, l9~0).
For preparing a polymer which is as homogeneous
as possible and which is formed by -the combination of
monomers with termination like, for example, in the case of
polystyrene, there will have to be ensured temporarily and
locally constant temperatures and concentrations. It is for
this reason that the homogeneous continuous agitator vessel
reactor renders the narrowest molecular mass distribution.
This distribution is the so-called
"Schulz-Flory-distribution". The inhomogeneities U for such
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distributions range between a value of U = 1.5 in the case
of termination by combination and a value of U = 2.0 in the
case of termination by disproportionation~ The
inhomogeneity _ is defined by the ratio o~ the weight
average ~ and the number average MN of the molecular
mass.
In batchwise operated isothermal reactors and in
continuous isothermal plug flow reactors ~ideal :Elow tubes),
there is obtained a wider distribution of molecular masses
which can be attributed to the concentration profiles.
However, if segregations occur in a continuously operated
agitator vessel the distribution of molecular masses will
further increase and will even be wider than that obtainable
in a continuous plug flow reactor (cf. H. Gerrens, Chem~
Ing. TechO 52 (1980).
Thus the industrial continuous polymerization of
styrene either in substance or in solution is per~ormed in
well-stirred continuous agitator or stirrer vessels, in
cascades of agitator vessels and in tower-type reactors. In
case that a conversion which is as high as possible is
intended a tower-type reactor suggests itself, if desired,
including pre-polymerization in a continuous agi.tator vessel
reactor as known, for example, from J.M. De sell et al,
"German Plastics Practice", publishers De Be1.1, ~ichardson~
Springfield, Mass. 1946, and from United States Paten-t Nos.
2,496,653 and 2,727,884. Cooling coils are arranged in the
towers for better heat removal. Temperature profiles can ~e
set up in the tower-type reactors by suitable supply and
removal of heat, see the aforementioned United States Patent
No. 2,727,884. When the pre-polymerization is performed to
relatively high degrees of conversion, agitator vessel
cascades can be employed, see, for example, British Patent
No. 1,175,262. Depending upon the degree of conversion and
the range of viscosity which changes in accordance therewith
there are employed different types of stirrers and reactor
configurations. The same is true for cascades of agitator
vessel reactors wi-th no reaction tower following the same,
see Canadian Patent No. 864,047. In such apparatus the
energy expense for mixing and heat transfer already is so
large at higher viscosities that the polymerization becomes
terminated at relatively low degrees of conversion.
SUMMARY OE THE INVENTION
. _ _
Therefore, with the foregoing in mind, it is a
primary object of the present invention to provide a new and
improved reactor for continuously performing po~ymerizations
in highly viscous media which is distinguished by its simple
structure.
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Another important object of the present
invention is directed to the provision of a new and improved
reactor for continuously performiny polymerizations in
highly viscous media in a manner which enables the
prevention of local overheating and segregation at
substantially less energy expense as compared to prior art
reactorsO
~ till a further significant object of the
present invention is directed to a new and improved
construction of a reactor for continuously performing
polymerizations in highly viscous media which permits
radical polymerization of monomers such as, for example,
styrene and to obtain a narrow molecular mass distribution.
Yet a further significant object of the present
invention is directed to a new and improved method of
preparing polymers to obtain high degrees of conversion at
narrow molecular mass distributions.
Now in order to implement these and still
further objects of the invention, wllich will become more
readily apparent as the description proceeds t the reactor of
the present development is manifested by the features that
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it comprises at least one tube or tubular~in which static
mixing elements are arranged so as to fill the cross-section
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thereof. The tube is provided with a recirculation line or
conduit equipped with a pump at least at one location
between the beginning or front end and the terminating end
or rear end of the tube.
With respect to the section preceding the
recirculation line such a reactor acts like a continuous
system working with complete back-mixing. Tf high degrees
of conversion are intended, however, the reaction rate in a
one-stage reactor working with complete back-mixing becomes
relatively small, so that the reaction volume required
therefor become relatively large.
Advantageously, therefore, the apparatus
according to the invention is designed as a multi-stage
reactor in which a pre~polymerizer is combined with a series
connected or subsequently arranged plug flow reactor.
Since, however, the reaction medium is very viscous simple
tubes without stirring cannot be employed for the reactor.
Due to the laminar velocity profile which would form in a
simple tube a very wide distribution of residence times
would be obtained with nearly complete segregation.
In polymerizing, for example, styrene, it is
significant to provide for a sufficiently high heat transfer
in order to remove the reaction heat from the highly viscous
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medium and to simultaneously provide for a short mixing time
for homogenizing the supplied reactant which has a low
viscosity with the reaction mixture. The utilization of
static mixing elements in the reactor according to the
invention solves this problem.
A number of static mixing elements which may be
used to achieve the aforementioned purposes are described,
for example, in the aforementioned publication of H.
Gerrens; Chem. Ing. Tech. 52, 1980. Constructions of mixing
elements, as known, for example, from German Patent
Publication No. 2,943,688 have proven to be particularly
suitable.
As alluded to above, the invention is not only
concerned with the aforementioned reactor aspects, but also
relates to a novel method of preparing polystyrene in which
use is made of the reactor described hereinbefore.
Generally speaking, the inventive method comprises using the
reactor in such a manner that the reactants are
pre-polymerized in the section preceeding the recirculation
line to a degree of conversion in the range of 30% to 60~
and at a recirculation ratio which is smaller than 10. The
recirculation ratio is ~efined as the amount of recirculated
reactant to the amount of supplied reactant.
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It has been recognized for the first time with
the invention that the characteristics of an ideal
continuous agitator or stirrer vessel reacto.r can be
maintained even if the recirculation ratio i5 relatively
small and the reactor is operated at high degrees of
conversion. The unexpected result was obtained that e~en at
small recirculation ratios the inhomogeneity of the polymer
corresponds to that of the theoretically calculated reactor
with ideal mixing. Accordingly, no segregation occurs with
decreasing recirculation ratio. Additionally, it was not
expected tha~ the quality or band of the product, i.e. the
molecular mass and the width of the molecular mass
distribution, was only insubstantially changed in the
consecutive or following plug flow reactor section. The
operation thus can be performed in regions which usually
have been accessible only by providing specifically
constructed agitator or stirrer vessel reactors, and the
energy which is to be supplied when the reactor is desi~ned
according to the invention is smaller ~y about one order of
magnitude. For obtaining high degrees of conversion under
especially economical conditions the recirculation of the
pol~mer may be accomplished, not from the reactor outlet or
outlet means, but from a location intermediate its length
i.e. between the inlet and outlet th_reof. A further
advantage resides in the possibility of adding further
reactants, for example, for copolymerization and for
affecting in a desired manner -the molecular mass and its
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distribution, at locations continuously locally distributed
over the reactorO The entire polymerl~ation reactor
realizes in a technically simple manner the strived for
reactor system including complete back-mixing and a series
connected plug flow reactor. It is to be noted that the
plug flow characteristic is maintained in spite of the
extreme axial viscosity gradient occurring due to the change
from 30% to 96% conversion and that there does not occur any
break-through of partially reacted medium.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be better understood and
objects other than those set forth above, will become
apparent when consideration is given to the following
detailed description thereof. Such description makes
reference to the annexed drawing wherein the single Figure
c.
~-~ shows a schematic longitudinal section o ~ reac-tor
constructed according to the invention.
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DET~ILED DES~RIPTION OF T~IE PREFERR~D EMBODIMENTS
__ _ _
Describing now the drawing, it is -to be
understood that only enough of the construction of the
reactor has been shown as needed for those skilled in the
art to readily understand the underlying principles and
concepts of the present development, while simplifying the
showing of the drawing. Turning attention now specifically
to the single Figure of such drawing there has been shown in
schematic illustration a longitudinal section through a
polymerization reactor 1 which may serve, for example, ~or
preparing polystyrene. In the embodiment shown the reactor
1 comprises a tube la, however, the reactor 1 may also
comprise a number of tubes or tube structures arranged in
parallel relationship to one another.
The reactor 1 comprises three sections I, II and
III in which mixing elements 2 are arranged. The mixing
elements 2 are preferably static mixing elements comprising
cross-wise mounted webs, generally indicated by reference
character 2a, which form an angle with the tube axis. The
webs 2a are arranged in two groups and are essentially
parallel to each other within each of the two groupsO The
groups are arranged such that the webs of one group cross
the webs of the other groupO Successive static mixing
elements 2 are pivoted relative to each other about the axis
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o the tube by an angle which, preferably, amounts to 90.
At the inlet location or inlet means of the reactor 1 there
is connected a recirculation line or conduit IV at one end
thereof and which is connected at its other end with the tube
la of the reactor 1 intermediate the sections I and Il, in
other words at a location intermediate or between the front
end defining_the inlet or inlet means and the rear end
definin~ the outlet or outlet means as shown in the drawing.
The recirculation line or conduit IV is provided with a
conveying pump 3.
Each of the reactor sections I, II and III of
the tube or tube structure la forming the reactor 1 is
surrounded by a related double jacket 4, 5, 6 through which
flows a suitable heating medium or cooling medium depending
upon the reaction conditions during operation.
Finally, a supply line or conduit 7 containing a
metering pump 8 opens into the tube la of the reactor 1
intermediate the sections II and III which form a plug flow
reactor. An additional component like, for exampley an
~o initiator like a peroxide or a monomer or an additive like,
for example, oil or a pigment may be supplied through the
supply line 7. The monomer may be the same as the monomer
introduced into the section I through the line or conduit 9
by means of a metering pump 10, i.e. styrene, or may be
different therefromO
To obtain a different product in, ~or example, a
different polymerization reaction, another monomer may be
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supplied to the reactor 1 by a line or conduit 11 c~ontaining
a metering pump 12 and the other monomer either may be
supplied in a premixed or in a separate condition. The
section I and the recirculation line IV form a
pre-polymerizer in which, for example, during the production
of polystyrene the conversion is in the range of 30~ to 60
and the ratio of recirculated styrene quantity to the
styrene quantity continuously supplied through line 9
preferably assumes values less than 10.
10On starting the operation of the reactor 1 the
pre-polymerizer is closed by using a valve 13 or equivalent
shutoff element arranged in the reactor 1 and the
pre-polymerization product is circulated until the desired
degree of conversion is obtained.
Subsequently the plug flow reactor comprising
the tube sections II and III is placed into operation by
opening the valve 13.
The reaction product then is removed from the
reactor 1 at the outlet 14 at high degrees of conversion.
20~s already previously mentioned the mixing
elements 2 provide for a homogeneous intermixing and for a
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uniform reaction temperature throughout the entire
cross-section of the tube la in the reactor 1.
In the following discussion numerical examples
are given for the production of polystyrene in a reactor
designed according to the in~ention. While styrene has been
used as the monomer undergoing bulk polymerization in the
specific examples given below, reactant mixtures containiny
styrene or other polymerizable monomers also can be used.
EXAMPLE 1
A reactor as shown in the drawing is used for
polymerizing styrene undergoing bulk polymerization.
The entire reactor is filled with static mixing
elements of the type described in German Patent Publication
No. 2,943,688 as already mentioned before.
The reactor 1 may be heated or cooled by using
the double heat exchange jackets 4, 5 and 6 in order to
adjust the temperatures in the reactor in accordance with
the desired degree of conversion.
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Section I and recirculation line IV form a
circulation section which constitutes a pre-polymerizer.
Section II and section III form a tubular reactor section
having the characteristics of a plug flow reactor.
This first example serves to illustrate the
effect of the recirculation ratio on the quality of the
product. In respect thereto the operation of the
circulation section is exclusively considered.
Reaction Conditions
.
Temperature in sections I and IV T = 158C
Supply of monomer V0 = 4 l/h
Recirculation ratio R = VR/V0 = 50
(V0 and VR are entered in the drawing)
Result
Degree of Conversion XB = 60
(see drawing~
Molecular Mass (g/mol)
Weight average ~ = 178,000
Number average MN = 93~000
Inhomo~eneity U - 1.9
Viscosity of reaction mixture ~ = approx7 10 Pas
Energy supply per kg polymer
in the circulation section p = 0.0].7 kWh/kg
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EXAMPLE 2
In the second example the recirculation ratio R has
been changed to study the effects thereof on product quality.
Recirculation ratio R = 10
Result
Degree of Conversion XB = 60
Molecular Mass (g/mol)
Weight average MW = 180,000
Nun~er average MN = 93~000
Inhomogeneity U = 1.9
Viscosity of reaction mixture n = approx. 10 Pas
Energy supply per kg polymer
in circulation section p = 0.00073 kWh/kg
As shown by the example the recirculation ratio
surprisingly may be reduced to 10 without the polymer obtained
being changed thereby.
The energy supply p per kilogram of polymer is
thereby reduced by a factor of about 25.
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Examples 2, 3, 4
A reactor as illustra-ted in the drawing and as
described by the embodiment hereinbefore discussed is used~
In the examples 2, 3, 4, and in contrast to the
Example 1, the sections II and III which constitute the tubular
reactor sections having the characteristics of a plug flow
reactor are additionally operative and the total conversion is
correspondingly higher than i~ Example 1.
The results obtained in Examples 2, 3 and 4 are
listed in the following Table.
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.. _ _ . . _ . . ~ ._~
Reac-tion Condi-tionsEx. 2 ¦ Ex. 3 ¦ Ex. 4
~ _ . .. ___ __ _
Temperature T (C)
in Section I ancl IV 135 135 135
in Section II 130 145 145
in Section III 140 170 180
infeed of monomer
V0 (l/h) 2 2 2
, _ _
Results
Result after Sections
I and IV __ _ __ _
Conversion XB (%)48 48 ~ 48
Molecular Mass (~/mol)
____________.____ _____
~eight average366,000366,000 366,000
Number average191,000191,000 lg1,ooo
Inhomogeneity U1.92 1.92 1.92
~.~ .
Result after
Section III
____________
Conversion Xs (%)80 90 93
tc~e drawin~)
Mol _ular M~ s (cl/mol)
Weight average330,000263,000 251,000
Number average168,000114,000 97,000
Inhomogeneity U1.96 2.3 2.6
Energy supply p
per kg polymer
~kWh/kg~ 0.00064 0.00056 0.00054
_ . _ __ _ . _ _
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It will be evident from these three Examples and
their results as given in the Table that the product quality is
not substantially changed in the plug flow section provided
that the reaction temperature therein is approximately the same
as in the circulation section.
The energy supply per kg polymer, however, is
smaller for the reactor including the plug flow section as
compared to the sole circulation reactor, see for comparison
the Example l; the obtainable conversion in such a combined
reactor almost amounts to 100%.
In these three examples, therefore, the combination
of a circulation section with a plug flow section proves to be
particularly advantageous.
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