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continuous phase with it. If the volume of the continuousphase is limited by a thicker needle wall, the drag of thedetaching drop affects the following drops. They detachearlier and are smaller. The drag effect can be reduced bythe flow rate through the ring aperture as shown in Fig. 6.Here, at a ring velocity of vR = 0.57 m/s, the dependence ofthe average drop diameter d10 on the Weber number is aboutthe same as for the needles C and D.

Figure 6. Drop diameter d10 vs. the Weber number for the needles C and D andthe ring aperture I at a ring velocity vR = 0.57 m/s.

Fig. 7 presents the average drop diameter d10 versus theWeber number We for the ring apertures I, II, III and IV andthe needle A at a ring velocity of vR = 0.1 m/s. The variation ofthe ring diameter or the aperture area has no systematicinfluence on the drop size.

Figure 7. Drop diameter d10 vs. the Weber number for the ring apertures I, II, IIIand IV with the needle A at a ring velocity vR = 0.1 m/s.

4 Conclusions

The systematic investigation of the two-phase nozzle withthe liquid system toluene (dispersed) / water shows that thesize of the drops formed at the nozzle depends on the needlegeometry. Apparently, the needle wall thickness has a largeinfluence on the width of the drop size distribution. The outerflow of the continuous phase makes it possible to produce aconstant drop size over a wide loading range. The velocity ofthe continuous phase should be low, as a high velocity of thecontinuous and / or the dispersed phase leads to a formation ofvery small size drops.

Acknowledgement

The authors wish to thank the Deutsche Forschungs-gemeinschaft for the financial support of this work.

Received: February 27, 1998 [K 2389]

Symbols used

di [m] inner needle diameterda [m] outer needle diameterD [m] diameter of the ring apertured10 [mm] average drop diametervd [m/s] velocity of the dispersed phase in the

needlevR [m/s] velocity of the continuous phase in the

ring apertureWe [±] Weber numbers [mm] wall thicknesss10 [mm] standard deviation

Greek symbols

rd [kg/m3] density of the dispersed phases [N/m] surface tension

References

[1] Pilhofer, T.; Miller, H.-D., Photoelektrische Meûsonde zur Bestimmungder Gröûenverteilung mitteldisperser Tropfen in einem nicht mischbarenflüssigen Zweistoffsystem, Chem. Ing. Tech. 44 (1972) 5, pp. 295±300.

[2] Mersmann, A., Auslegung und Maûstabsvergröûerung von Blasen- undTropfensäulen, Chem. Ing. Tech. 49 (1977) 9, pp. 679±691.

This paper will also be published in German in Chem. Ing. Tech. 70 (1998) No. 12.

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MAO-Free Metallocene Based Catalystsin High Pressure Polymerization ofEthylene and 1-Hexene*

By Christian Götz, Gerhard Luft, Alexander Rau, andStefan Schmitz**

1 Introduction

The introduction of metallocene catalysts into existingindustrial polymerization processes is mainly hampered by the

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[*] Lecture of Dr. A. Rau given at the GVC-Jahrestagung, Sept. 22±24, 1997in Dresden.

[**] Dipl.-Ing. Ch. Götz, Prof. Dr.-Ing. G. Luft, Dr. A. Rau, Dipl.-Ing. S.Schmitz, Darmstadt University of Technology, Department of ChemicalEngineering, Petersenstr. 20, D-64287 Darmstadt, Germany.

0930-7516/98/1212-0954 $ 17.50+.50/0

costs of the cocatalyst methylalumoxane (MAO) [1]. There-fore, the activation of metallocenes by alternative cocatalystsis of great interest.

In this paper the use of a MAO-free activated metallocenedichloride in high pressure polymerizations, which is arecently developed, highly efficient technique, is described.The purpose of this work is to study the influence of thecatalyst composition on productivity as well as the effect of theco-monomer 1-hexene.

2 Experimental

2.1 Polymerization Unit and Catalyst System

The polymerization experiments were performed in acontinuously operated unit equipped with a high pressureautoclave of 100 ml capacity. In each test the pressure was 150MPa and the average residence time was 240 s (Fig. 1). Thetemperature was adjusted to 483 K by the amount of catalyst inthe feed and an electric heater. The mixture of monomers wascontinuously fed into the reactor along the stirrer. The desiredratio between ethylene/1-hexene in the feed was adjusted bymass flow controllers. Ethylene (99.8%) was taken frompressurized bottles and further purified by molecular sievesand a copper catalyst. A two stage compressor was used topressurize the ethylene, while 1-hexene was metered by amembrane pump. The catalyst solution was fed into thereactor by a syringe type pump. A constant pressure wasmaintained by an motor driven outlet valve controlled by theprocess computer. Behind the outlet valve, the pressure wasreleased to atmospheric pressure. The polymer was separatedin a melted or powder form from the unreacted monomers.When the steady state was attained within 10 min, the polymersamples were collected in different separators. The unit wasoperated by a computer, which recorded data such astemperature, pressure, and mass flow during polymerizationtests.

Figure 1. Flow sheet of the high pressure unit.

The catalyst system was based on the metalloceneMe2Si[Ind]2ZrCl2, triisobutylaluminum, and [Me2PhNH]+

[B(C6F5)4]±.1) The activation of the metallocene dichloridewas performed as follows. Metallocene dichloride was

dissolved in toluene and mixed with 10±200 equivalents oftriisobutylaluminum. After 20 min, this solution was added toa solution of [Me2PhNH]+[B(C6F5)4]± in toluene. In each case,the ratio between borate and metallocene was 1.3 mol B /molZr.

3 Results

3.1 Influence of Aluminumalkyl on Productivity

First we studied the influence of the amount of aluminu-malkyl used in the catalyst system in the copolymerization ofethylene and 1-hexene. Maintaining the above mentionedreaction conditions constant, the ratio of [Al]/[Zr] was variedfrom 25±300, while the Zr concentration in the feed wasmaintained at 0.5 mol-ppm. The amount of 1-hexene in thefeed was adjusted to 20 mol%.

At small [Al]/[Zr] ratios, the productivity was very low,while increasing this ratio to 100 the productivity increasedsharply to 300 kg polymer per g Zr. Further increases of theamount of aluminumalkyl led only to a minimal increase ofproductivity (Fig. 2).

Figure 2. Influence of [Al]/[Zr] ratio on productivity; Ð *: 0.5 mol-ppm Zr inthe feed, Al concentration variable; ± - ± - &: 40 mol-ppm Al in the feed, Zrconcentration variable.

As known from metallocene/methylalumoxane basedcatalyst systems, aluminumalkyls can act as scavengers toprevent catalyst poisoning. From this point of view, in thisseries of experiments our polymerization system needs aminimum of 20 mol-ppm Al in the feed.

Following this assumption, we performed another series ofexperiments in which the molar ratio of [Al]/[Zr] was varied,while the Al concentration in the feed was maintained at 40mol-ppm (Fig. 2). The plot of productivity as a function of the[Al]/[Zr] ratio shows nearly the same sigmoidal shape as in thefirst series of experiments, in which the Zr feed concentrationwas kept constant. The main difference can be seen at thebeginning of the curve. With the higher concentration of Al inthe feed, there is already polymerization at small [Al]/[Zr]ratios.

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As it can be seen from Fig. 2, for reaching optimalproductivity, it is necessary to use a molar ratio of [Al]/[Zr]of more than one hundred. Therefore, it is not sufficient toadjust the minimum concentration of the scavenger in thereactor. In addition, a definite Zr concentration is to be chosenand/or the activation require a definite ratio between themetallocene dichloride and triisobutylaluminum.

3.2 Influence of the Zirconium and AluminumalkylConcentrations on Molecular Weight Distribution

Polymer molecular weight and molecular weight distribu-tion were determined by high temperature gel permeationchromatography. Similar to the results from MAO basedmetallocene systems, the weight-average molar masses (Mw)were only slightly influenced by the aluminumalkyl content(Fig. 3). These systems showed very low values for the rateconstants of the chain transfer to the aluminum compound,and the chain length of the polymer is limited by b-hydrideelimination [2].

Figure 3. Influence of zirconium and aluminumalkyl concentrations on weight-average molecular mass; Ð *: 0.5 mol-ppm Zr in the feed, Al concentrationvariable; ± - ± - &: 40 mol-ppm Al in the feed, Zr concentration variable.

However, if the molar ratio of [Al]/[Zr] was raised from 50to 100 maintaining the concentration of aluminum at 40 mol-ppm, Mw sharply increases. The corresponding Zr concentra-tions of 0.8 and 0.4 mol-ppm in the feed indicate an increase ofthe molecular weight with decreasing concentration of Zr.Accordingly, the Mw obtained in the series of experimentswith 0.5 mol-ppm Zr and varying Al content are between thevalues found with 0.8 and 0.4 mol-ppm Zr in the feed.

Within the range of the catalyst concentrations used, theconversion of the monomers varied only between 21 and 25%,so that the increase of Mw can not be attributed to a change ofthe monomer concentration in the reactor. This is confirmedby the polydispersity Mw/Mn. Narrow molecular weightdistributions with Mw/Mn ~ 2, typical for single-site catalysts,were obtained in both series of experiments with molar ratiosof [Al]/[Zr] larger than 100, respectively, with less than 0.5

mol-ppm Zr in the feed (Fig. 4). Transition metal concentra-tions of more than 0.8 mol-ppm yield polymers with broadermolecular weight distributions and lower Mw. This can beexplained by terminating reactions in which more than onecatalyst molecule is involved.

Figure 4. Influence of the zirconium and aluminumalkyl concentrations onpolydispersity; Ð*: 0.5 mol-ppm Zr in the feed, Al concentration variable; ± - ±- &: 40 mol-ppm Al in the feed, Zr concentration variable.

Like the Mw, also polydispersity Mw/Mn of the obtainedpolymers is only slightly influenced by the Al concentration.With between 20 and 100 mol-ppm Al in the feed, thepolydispersities are narrow, having values between 1.7 and 2.1.Below 20 mol-ppm Al, a slight increase of the polydispersitywas found.

3.3 Influence of the Co-monomer Concentration onProductivity and Co-monomer Incorporation

To investigate the ability of the catalyst system forcopolymerization, the 1-hexene content in the feed was variedfrom 0±70 mol%, while the molar ratio of [Al]/[Zr] wasadjusted to 200 (Fig. 5). In ethylene homopolymerization, 400

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Figure 5. Influence of 1-hexene concentration on productivity.

kg of polymer per g Zr was obtained. With 5 mol% 1-hexene inthe feed, a sharp increase in productivity was observed.Further increase of the co-monomer content led to a decreaseof productivity. With 70 mol% 1-hexene, less than 100 kg ofpolymer per g Zr was obtained.

The increase of productivity by addition of a small amountof 1-hexene to the feed could be explained by the rateenhancement effect. It is known from low pressure ethylene/a-olefin copolymerization with soluble and insoluble catalyststhat the co-monomer can cause an acceleration in the ethylenepolymerization rate. In low pressure copolymerization cata-lyzed by metallocenes, the ethylene polymerization rate issometimes more than doubled [3]. In this work, such a co-monomer effect was observed for the first time in ametallocene catalyzed high pressure polymerization. At a[1-hexene]/[ethylene] ratio of 0.5 in the reactor, the accelera-tion factor was 1.8. A higher content of 1-hexene in the reactorcaused a decrease of the polymerization rate of ethylene(Fig. 6). In low pressure polymerization, the systemMe2Si[Ind]2ZrCl2/MAO showed a similar ethylene polymer-ization rate course with an acceleration factor of 2.6 at a [1-hexene]/[ethylene] ratio of 1 [4]. In our polymerizationsystem, the inhibitory effect of the co-monomer is found atlower ratios. This could be due to the nearly solvent-freeconditions in high pressure processes, which cause a higher co-monomer concentration in the reactor.

Figure 6. Influence of [1-hexene]/[ethylene] ratio on the polymerization rate ofethylene.

The incorporation of 1-hexene into the polymers wasdetermined by means of 13C NMR spectroscopy [5] and isoutlined in a copolymerization diagram (Fig. 7). The diagramclearly shows the preferential incorporation of ethylene intothe copolymer and an ideal copolymerization behavior. Usingthe method of Fineman-Ross, the copolymerization parame-ters were determined to be r1 = 28 and r2 = 0.05 with r1*r2 ~ 1.These values are quite similar to the values of r1=63 andr2=0.02 found in ethylene/1-hexene high pressure copolymer-ization using the catalyst system Me2Si[IndH4]2ZrCl2/MAO[6].

Figure 7. Copolymerization diagram; M1 = ethylene in the polymer; M2 = 1-hexene in the polymer; m1 = ethylene in the reactor; m2 = 1-hexene in thereactor.

4 Outlook

The reported results show that ternary catalyst systems suchas Me2Si[Ind]2ZrCl2, triisobutylaluminum, and [Me2PhNH]+

[B(C6F5)4]± could replace metallocene/MAO-based catalystsin high temperature high pressure polymerization. To gain abetter insight into polymerizations with such catalyst systems,further investigation of the polymerization process and theactivation method are on the way.

Acknowledgments

The authors greatly acknowledge financial support ofBundesministerium für Bildung und Forschung and BASFAG.

Received: March 6, 1998 [K 2386]

Symbols used

Mw [g/mol] weight-average molar massMn [g/mol] number-average molar massMw/Mn [±] polydispersity

Abbreviations

Me2Si[Ind]2ZrCl2 Dimethylsilylbis(indenyl)-zirconium dichloride

[Me2PhNH]+[B(C6F5)4]± Dimethylaniliniumtetrakis-(pentafluorophenyl)borate

Me2Si[IndH4]2ZrCl2 Dimethylsilylbis-(tetrahydroindenyl)zirconiumdichloride

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References

[1] Chemische Rundschau (1997) 22, p. 4.[2] Chien, J. C. W.; Wang B. W., J. Polym. Science: Part A: Polym, Chem. 28

(1990) pp. 15±38.[3] Koivumäki, J., Acta Polytechnica Scandinavica, Chemical Technology

Series No. 227, Helsinki 1995.[4] Herfert, N.; Fink, G., Polym. Mater. Sci. Eng. 67 (1992) pp. 31±32.[5] Randall, J. C., Rev. Macromol. Chem. Phys. C29 (1989) pp. 201±317.[6] Bergemann, C.; Cropp, R.; Luft, G., J. Molec. Cataly. A: Chem. 105 (1996)

pp. 87±91.

This paper was also published in German in Chem. Ing. Tech. 70 (1998) No. 11.

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Development of a Continuous SynthesisMethod for Radical Polymerization ofGel Beads with a Narrow Particle SizeRange*

By Holger Wack, Hans-Jürgen Gross, and Wilhelm Althaus**

1 Introduction

Gels are dimensionally stable disperse materials with a highcontent of liquid and are made off at least two components [1].In most cases, one component is a solid, e.g., a polymer, andthe dispersant is a liquid. Some gels are able to swell in liquidsand if water is used as the solution medium, the gels are calledhydrophilic. Gels based on N-isopropylacrylamide (NIPA)show an interesting thermodynamic behavior. There is adecrease in solution content of the polymer skeleton ofpoly(NIPA) gels with increasing temperature. This behaviorstands in contrast to the behavior of many other polymers.

The macromolecular transition between the state of highsolution content (hydrophilic properties) and the state of lowsolution content (hydrophobic properties) depends on themicrostructure of the polymer matrix. The transition pointfrom high to low solution content is significantly determinedby the co-monomers which have been used for the synthesis.An important property of the poly(NIPA) gels is thereversibility of the phase transition. This is the reason why ±in combination with the three-dimensional pore structure ±for this kind of gels many fields of application are possible.

In relation to the development of a new process for wastewater treatment, which is based on the described reversiblephase transition, a new synthesis method for monodispersehydrophilic gel beads was to be developed. The boundaryconditions of the new process were determined by thedemands on scalability and the high purity of the gel beads.

2 Concept

The gel synthesis method is based on a radical polymeriza-tion mechanism. A general overview of possible synthesismethods for the described systems is given by Echte, Elias, andVollmert [2±4]. As an initiator, the redox system ammoniumpersulfate and sodium metabisulfite was used. N-isopropyla-crylamide was used as the monomer and N,N¢-methylenebi-sacrylamide as the cross-linker. All components were dis-solved in aqueous solution.

First the gel polymerization was realized using a masspolymerization technique. However, this method can only beused for small amounts of gel. For the production of large gelamounts this method is not practicable because of the reactionheat and costly further preparation steps after the synthesis.

Based on the mechanism of inverse bead polymerization, asynthesis method, which can be used for customized produc-tion of large amounts of gel, was developed (Fig. 1).

Figure 1. Schematic description of the developed synthesis method.

The aqueous monomer and initiator solution are mixed in amixing chamber. Then droplets are fed by a nozzle system intoa temperature-controlled reaction column, containing an inertoil phase. The polymerization reaction takes place within thecreated reaction solution beads during their sedimentationinside the reaction column. The hold-up time of the beads canbe controlled and adjusted. After polymerization, the gelbeads are mechanically separated from the oil phase. Oil,which adheres to the surface of the polymerized gel beads, canbe removed during a washing process. The oil and washingphase can be fed back after reprocessing.

Basic data for the reactor lay-out were obtained bydynamic-mechanical measurements of the cross-linking reac-tion in dependence of the reaction time using a shearcontrolled rotation viscometer (Bohlin CS 10). For theanalysis of the cross-linking reaction, the shear modulus G¢,which is proportional to the value of elastic energy that isstored in the gel, was used. The shear modulus G¢ increaseswith increasing cross-linking density and reaches a constantvalue after final conversion. In Fig. 2, time-dependent curvesof the shear modulus, referring to the gel at final conversion,

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[*] Poster presentation by H. Wack at the GVC-Jahrestagung, September 24±26, 1997 in Dresden.

[**] Dipl.-Ing. H. Wack, Dipl.-Ing. H.-J. Gross, Dr.-Ing. W. Althaus,Fraunhofer-Institut für Umwelt-, Sicherheits- und Energietechnik UM-SICHT, Osterfelder Strasse 3, D-46047 Oberhausen, Germany.

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