CA1199750A

CA1199750A - Reactor for continuously performing polymerizations in highly viscous media and method of preparing polystyrene

Info

Publication number: CA1199750A
Application number: CA000427817A
Authority: CA
Inventors: Albert Renken; Felix Streiff; Nguyen K. Tien
Original assignee: Gebrueder Sulzer AG
Current assignee: Sulzer AG
Priority date: 1982-06-03
Filing date: 1983-05-10
Publication date: 1986-01-21
Also published as: JPH0113726B2; EP0096201A1; JPS591515A; CH649230A5; EP0096201B1; DE3372442D1

Abstract

INVENTORS: ALBERT RENKEN, NGUYEN KHAC TIEN, FELIX STREIFF

INVENTION: REACTOR FOR CONTINUOUSLY PERFORMING POLYMERIZATIONS
IN HIGHLY VISCOUS MEDIA AND METHOD OF PREPARING
POLYSTYRENE

ABSTRACT OF THE DISCLOSURE

The reactor for continuously perforning polymerizations in highly viscous media to obtain, for example, polystyrene, comprises a tube or tube structure which is subdivided to form individual sections. Static mixing elements are arranged within the tube or tube structure so as to fill the cross-section thereof. A
pre-polymerizer is formed by a first tube section and a recirculation line provided with a pump which is connected to the tube or tube structure between the first section and the second section thereof and to the reactor inlet, while the second and third sections of the tube or tube structure form a plug flow reactor. The reactor is distinguished by its simple structure; local overheating and segregation are prevented therein at small energy expense by means of the static mixing elements.

Description

7~i~

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

- 2 ~

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 7S~

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.

-- 5 ~

7~

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 sfr~G~,r~
it comprises at least one tube or tubular~in which static mixing elements are arranged so as to fill the cross-section ~19975~

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 ~L~99750 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.

,r' 7~

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 _ g _ 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.

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 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 3g75~;

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 97S~`

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.

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.

75~

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 7S~

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.

75~1 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.

5~

.. _ _ . . _ . . ~ ._~
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 _ . _ __ _ . _ _ 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.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A reactor for continuously performing polymerizations in highly viscous media, said reactor comprising:
at least one tube defining a tube cross-section and having a front end defining an inlet means and a rear end defining an outlet means;
static mixing elements arranged in said tube and filling said cross-section thereof;
said at least one tube successively defining between said inlet means and said outlet means a pre-polymerization section followed by a plug flow reactor section;
recirculation means including a pump;
said recirculation means being provided only for the pre-polymerization section; and said recirculation means being arranged at least at one location intermediate said inlet means and said outlet means of said at least one tube to provide recirculation of said highly viscous media only at said re-polymerization section.

2. The reactor as defined in claim 1, further including:

a number of said tubes arranged substantially in parallelism to each other.

3. The reactor as defined in claim 2, wherein:
each said tube is provided with an individual inlet for reactants to be reacted in the reactor.

4. The reactor as defined in claim 1, further including:
a jacket provided at said tube; and said jacket comprising cooled jacket sections.

5. The reactor as defined in claim 1, further including:
a jacket provided at said tube; and said jacket comprising heated jacket sections.

6. The reactor as defined in claim 1, further including:
supply means for supplying an additional polymerization component; and said supply means being connected to said tube at at least one location intermediate said front end defining said inlet means and said rear end defining said outlet means of said tube.

7. The reactor as defined in claim 1, wherein:
said tube defines a tube axis;
each said static mixing element arranged in said tube of said reactor comprises cross-wise arranged webs forming an angle with said tube axis;
said webs being arranged in at least two groups;
the webs of each said group being oriented in a substantially parallel direction and said webs of one of said at least two groups crossing said webs of the other one of said at least two groups; and consecutive ones of said static mixing elements being pivoted relative to each other about said tube axis.

8. The reactor as defined in claim 7, wherein:
said consecutive static mixing elements are pivoted relative to each other through an angle of substantially 90°
about said tube axis.

9. The reactor as defined in claim 1, wherein:
said at least one tube is devoid of any gas outlet.

10. The reactor as defined in claim 1, wherein:

said recirculation means has a first end defining an inlet connected with said at least one location intermediate said inlet means and said outlet means of said at least one tube; and said recirculation means having a second end defining an outlet which flow communicates with said inlet means of said at least one tube.

11. A method of preparing polymers from monomers in a reactor for continuously performing polymerizations in highly viscous media, which reactor comprises at least one tube defining a cross-section and having a front end and a rear end, static mixing elements arranged in said tube and filling said cross-section thereof, recirculation means including a recirculation line containing a pump, and said recirculation means being arranged at least at one location intermediate said front end and said rear end of said tube, said method comprising the steps of:
pre-polymerizing the monomer in a section preceding the recirculation line to a degree of conversion in the range of 30% to 60%; and wherein such prepolymerization of the monomer is accomplished at a recirculation ratio, i.e. the ratio of the amounts of recirculated reactants to the amounts of supplied reactants, which is smaller than 10.

12. The method as defined in claim 11, further including the step of:
infeeding said monomer near to said front end of said tube.

13. The method as defined in claim 12, further including the step of:
infeeding styrene as said monomer which undergoes bulk polymerization.

14. The method as defined in claim 12, further including the step of:
infeeding a monomer containing reactant mixture.

15. The method as defined in claim 12, further including the step of:
infeeding an additional polymerization component at a location intermediate said front end and said rear end of said tube.

16. The method as defined in claim 15, further including the step of:
infeeding said additional polymerization component at a location downstream of said recirculation means.