WO2017178323A1

WO2017178323A1 - Small scale polymerization reactor

Info

Publication number: WO2017178323A1
Application number: PCT/EP2017/058218
Authority: WO
Inventors: Tom SCHOFFELEN; Frans VISSCHER; Francesco Bertola; Nicolaas Hendrika Friederichs
Original assignee: Sabic Global Technologies B.V.
Priority date: 2016-04-12
Filing date: 2017-04-06
Publication date: 2017-10-19

Abstract

The invention relates to a reactor for the preparation of polymers, comprising: - a reactor vessel, for holding a polymer suspension, having a volume of less than 1 L and a height versus diameter ratio of at least 2; - a reactor vessel cover comprising at least one sampling line inlet for monitoring a gas phase composition; - a drive shaft having attached thereon at least one stirrer having at least three stirrer blades positioned in a lower part of the reactor vessel; - a drive being capable of driving the stirrer at a speed of 1000 rpm or less; - a baffle that is top mounted to the inside of the reactor vessel cover and comprises at least two baffle blades having elongated cross-sections and having lower portions that extend into the polymer suspension to a level above the at least one stirrer that comprises an axial impeller.

Description

SMALL SCALE POLYMERIZATION REACTOR

DESCRIPTION FIELD OF THE INVENTION

The invention relates to a small scale polymerization reactor for the preparation of polymers and a method of preparing polymers using the reactor,.

BACKGROUND

In the development of processes in the chemical industry, especially in polymerization processes, small scale reactors are used in laboratory facilities having one or more reactors in various configurations for the development of new processes and products. In case such reactors are used to develop new products, these reactors are known as product development reactors. When a development reaches a level of maturity, sufficient reliability and effect, the development will be scaled up and translated into medium and/or large scale processes using increasingly large volumes of raw materials and having appropriate yields for medium scale research and eventually economic use.

Such process development reactors have reactor vessels, usually in a range of 1 liter in volume or less. Translatability from small scale tot medium and/or large scale is a challenge, since process conditions differ between small scale and medium or large scale, Especially when these small scale reactors involve the chemical transformation of gaseous chemicals into suspended or dissolved polymers in a diluent, the volumetric gas-liquid mass transfer rates of a gas in a polymer suspension in small scale reactors can be too low.

Similarly, in the process for ethylene polymerization in a solution phase, the volumetric gas- liquid mass transfer rate of monomer gas into a viscous solution of an ethylene containing polymer in a hydrocarbon diluent, may show considerable difference between small scale processes and medium and/or large scale processes.

Especially in the field of olefin polymerizations, it is important to know the exact composition of the gaseous phase as the composition of the gaseous phase translates into a certain composition of the polymer. For instance, when polyethylene is produced with the aid of so- called Ziegler catalysts or single site catalysts, it is very well known that the molecular mass of the resulting polyolefin can be regulated by the addition of hydrogen. Thus by adjusting the ratio of hydrogen to ethylene in the gaseous phase, the molar mass of the resulting polymer can be steered. Similarly, when a copolymer is produced from ethylene and for instance an alfa-olefin like 1-butene, the incorporation of 1-butene in the resulting polymer can be adjusted by changing the ratio of 1-butene to ethylene in the gaseous phase.

There are several techniques known that can be used to measure the composition of the gaseous phase. Known analytical techniques are for instance gas-chromatography or mass- spectrometry. In these techniques, a gaseous sample from the reactor is continuously fed through a sampling line to the analytical device, The sampling line inlet has to be in the gaseous phase and the sampling line inlet is commonly integrated in the upper part of the reactor. For small scale reactors, these sampling lines are typically mounted in the cover of the reactor. The results of this on-line analysis can subsequently be used to steer the composition of the gaseous phase to a desired setpoint during an experiment. In order to have a fast response of the analysis and have a high frequency of analytical measurement, the time required to transport the gaseous sample from the reactor to the analytical device should be short. To do so, the sampling lines are of low volume, which is reached by having a small inner diameter of the sampling lines. This creates a challenge, as one has to prevent that liquid or solid components can enter into the sampling line and as a result would block the feed to the analytical device. In medium and large scale reactors that contain a gaseous phase and a suspension or solution of polymer in a diluent, the distance between the liquid level and the sampling line is relatively large because of the large scale of the reactor. However, for small scale reactors, the distance is intrinsically short due to the dimensions of such small reactors. Therefore one has to prevent that liquids or solids splash onto the upper part of the reactor and could subsequently block the feeding lines to the analytical device. Usage of a filter at the entrance of the feeding line to the analyzer often only postpones the blockage of the lines to the analyzer.

In the field of ethylene polymerization, especially the process for catalytic ethylene polymerization in a diluent phase, the volumetric gas-liquid mass transfer rate of ethylene gas into a hydrocarbon diluent, is a challenging task. For instance, it is desirable that the rate of the catalytic conversion of dissolved ethylene into polyethylene, which takes place in the diluent, does not exceed the volumetric gas-liquid mass transfer rate of ethylene from the gas into the diluent. Only when the volumetric gas-liquid mass transfer rate of ethylene into the diluent is higher than the catalytic conversion, one can fully benefit from the intrinsic performance of the catalyst, like for example productivity. Otherwise, polymerization is taking place under so-called gas-liquid mass transport limited conditions, also sometimes referred to as "starved conditions", and the observed catalyst productivity is low due to a lack of monomer in the diluent In a polymer suspension phase, the volumetric gas-liquid mass transfer rate of monomer gas in the polymer suspension of ethylene polymer in a hydrocarbon diluent, may show considerable difference between small scale processes and medium and/or large scale processes. In such medium and/or large scale ethylene polymerization processes, a value for volumetric gas-liquid mass transfer coefficient of 0,023 per second may be obtained, and a value of 0.030 can be targeted to avoid entering into starved conditions (Gas-Liquid Mass Transfer Coefficient in Stirred Tank Reactors - Archis A. Yawalkarl , Albertus B.M. Heesink2, Geert F. Versteeg2 and Vishwas G. Pangarkarl ). Translatability from small scale reactors to large scale reactors however is low in this field.

In view of the above described challenges related to the volumetric gas-liquid mass transfer rate of a gas into a diluent and the continuous analysis of the headspace composition, it is clear that the combination of these requirements puts a severe challenge in developing suitable small scale reactors with adequate translatability towards large scale reactors.

The volumetric gas-liquid mass transfer rate of materials used in the processing can be improved by various measures such as increasing stirring efficiency, reactor configuration and pressure. Each measure has its own limitation. Stirrer efficiency can be improved by for example high stirrer rotation speed and stirrer type, i.e. blades configuration, number of blades. However stirrer rotation speeds cannot be increased unlimited and the choice in stirrer types is limited.

In medium or large scale reactors, a baffle can be used for mixing improvement. However the use of a baffle in small scale polymerization reactors is uncommon. Baffles limit space inside the small reactor vessels and are prone to contamination by deposits inside the reactor vessels. High stirrer speeds however may lead to spattering and splashing of the polymer suspension, which is undesirable as this may cause plugging of the sampling line in overhead reactor vessel covers. Instrumentation within the reactor vessel, such as sensors or fluid inlets for measurements, may cause turbulent flow within the reactor vessel, however such elements generally do not provide a controlled flow profile, sufficient for improving the volumetric gas-iiquid mass transfer rate.

From EP1310296 feed lines for supplying monomer and other components for performing polymerization under experimental conditions are known, which may extend into a development reactor vessel, however such feed lines are also not known to provide additional disturbances to the polymer suspension and polymer suspension surface, to sufficiently improve the stirring and obtain the desired volumetric gas-liquid mass transfer rate. Moreover it is described that such feed lines have narrow inner diameters. Therefore it is undesirable to extend such feed lines into the polymer suspension, as the polymer particles or catalyst particles may cause plugging of the inner channel of the feed lines. In addition, at high stirrer speeds, there is a high probability of splashing of polymer particles throughout the complete inner part of the reactor, which in turn would cause eventual installed sampling lines to plug. The reactor system from EP1310296 has no integrated analytical device that measures the composition of the headspace of the reactors, making it less suitable to use for accurate product development. International published patent application WO02/081076 shows a combinatorial chemistry reactor system for the parallel processing of reaction mixtures. The system comprises a frame, a head mounted in fixed position on the frame, and a reactor block having a plurality of wells therein for containing reaction mixtures. The reactor block is movable with respect to the head between a first position in which the reactor block and head are assembled for conducting reactions in the wells and a second position in which the reactor block is removed from the head for providing access to the vessels. Gaps in the reactor block between the wells serve to thermally isolate the wells from one another. Other features relating to pressure relief, reactor block temperature control, and sensor mounting are also disclosed. The reactor has wells in which stirrers are present for stirring a reaction mixture. Also a sensing device is described which can be present in the wells, which may enhance the stirring. The reactor system from WO02/081076 has no integrated analytical device that measures the composition of the headspace of the reactors, making it less suitable to use for accurate product development. Experiments in ethylene polymerization show that presence of instrumentation such as a temperature sensor extending into the polymer suspension does not provide a sufficient volumetric gas-liquid mass transfer rate in processes for ethylene polymerization, wherein solid particles are obtained, sufficient for translatability of the small scale process to medium or large scale processes.

Michael Lallemand [2] describes the performances of Ni-exchanged MCM-41 mesoporous materials as catalysts for ethylene oligomerization with a continuously stirred tank reactor (CSTR). It describes the use of a high speed agitator, rotating at speeds of 1000 rpm. A CSTR with agitator at this speed may provide effective oligomerization resulting mainly in C4, C6, C8 and CIO olefins. This is however insufficient for obtaining the desired volumetric gas- liquid mass transfer rate in processes for ethylene polymerization, wherein solid particles are obtained, sufficient for translatability of the small scale process to medium or large scale processes, In addition, the oligomerization of ethylene in such system does not lead to a large increase in the viscosity of the diluent, contrary to processes where high molar mass polymers are being formed that dissolve in the diluent and subsequently translate into a very large increase in the viscosity of the polymer- diluent mixture,

SUMMARY

It is therefore an object of the invention to obtain sufficient volumetric gas-liquid mass transfer rate to achieve translatability between small scale reactors to medium and/or large scale reactors in the field of polymerization, without plugging of sampling lines. The object is achieved in a reactor for the preparation of polymers defined by claim 1 , The reactor according to the invention comprises a reactor vessel for holding a polymer suspension, wherein the reactor has a volume of less than one liter and wherein a reactor vessel height versus a reactor vessel diameter is greater than or equal to two. The reactor further comprises a drive shaft extending into the reactor vessel having attached at least one stirrer, each stirrer has at least three, more preferably four, stirrer blades, and the stirrer blades are positioned in a lower part of the reactor vessel. The drive shaft is driven by a drive causing rotation of the stirrer, the drive is preferably capable of stirring the stirrer at a speed in a range of 1000 rpm or less. More preferably the drive is capable of driving the stirrer at a speed in a range of 800 - 1000 rpm. This keeps the stirring speed sufficiently low to prevent splashing of the polymer suspension.

The at least one stirrer is arranged for having an axial flow profile parallel to a central axis of the reactor vessel coinciding with the drive shaft. The at least one stirrer can comprise an axial impeller.

The flow profile ensures that the polymer suspension attains an axial motion in the reactor vessel near the drive shaft and another axial motion with in opposite direction near the reactor vessel wall. The flow profile allows high rate dissolution of the monomer into the polymer suspension.

The opposite axial polymer suspension flow near the drive shaft and near the reactor vessel wall causes a radial flow of polymer suspension from the center of the reactor vessel towards the reactor vessel wall or vice versa. The stirrer also causes a rotary motion of the polymer suspension in the reactor vessel around a central reactor vessel axis, coinciding with the drive shaft. The limited rotation speed of the stirrer prevents the polymer suspension to splash, thereby preventing the sampling line inlet to be plugged by the polymer suspension. Thus high rate volumetric gas-liquid mass transfer rate can be achieved whilst preventing plugging of the sampling line inlet. Having achieved sufficient monomer volumetric gas-liquid mass transfer rate, the small scale polymerization reactor allows translatabiiity of the small scale process to a medium or large scale process,

The aspect ratio of the reactor vessel, i.e. the reactor vessel height versus reactor vessel diameter is greater than or equal to two. A higher aspect ratio has various advantages, such as better transportability, relatively small foot print, and also reduced wall thickness so less wall material is necessary. Furthermore, a stirrer fitted in a reactor vessel with high aspect ratio needs less power due to its reduced radius, and will exhibit lower mixing intensity variation.

The reactor further comprises a reactor vessel cover, for overhead covering the top of the reactor vessel. The reactor vessel cover comprises at least one sampling line inlet accommodated in the cover for monitoring, in use of the reactor, a gas phase composition. In this way, gaseous components of the reaction mixture can be fed from the reactor to an analytical device via the sampling lines.

In an embodiment, the stirrer has four stirrer blades. This provides optimal volumetric gas- liquid mass transfer rates within the given speed range of less than 1000 rpm, and preferably 800 - 1000 rpm.

The reactor furthermore comprises a baffle that is top mounted and the baffle is mounted to a structure extending above the reactor vessel. Preferably the at least two baffle blades are mounted to a side of the reactor vessel cover that, in use of the reactor, is arranged to face the interior of the reactor vessel. In this way, the baffle can be extracted from the reactor vessel when reactor vessel and reactor cover are separated. This allows adequate cleaning of the reactor vessel and of the baffle blades, further preventing undesired deposits.

In use of the reactor, the at least two baffle blades partially extend along an inner side of an upper part of the reactor vessel with a length in a range of 40% to 70% of the reactor vessel height to a level above the at least one stirrer. This allows a stirrer to be installed underneath the lower side of the baffle blades. Furthermore, each one of the at least two baffle blades, as seen in use of the reactor in a direction perpendicular to a reactor vessel wall next to which it is arranged, has an elongated cross-section and a blade width in a range of 2% to 10% of the reactor vessel diameter. The baffle blades divert the rotational flow and also cause a radial flow of the polymer suspension from the reactor vessel wall towards the center of the reactor vessel towards the drive shaft connected to the stirrer or vice versa. The radial flow at the surface of the polymer suspension can cause an improved monomer volumetric gas-liquid mass transfer rate.

The baffle blades which partially extend into the reactor vessel prevent deposit of polymer especially in the lower part of the reactor vessel and allow space for the stirrer in the lower part of the reactor vessel In use, the baffle blades extend into the liquid phase of the polymer suspension, at least causing disturbance of the polymer suspension surface, to improve contact between liquid phase and monomer gas phase.

In an embodiment at least two baffle blades are spaced evenly along the inner side of the reactor vessel wall. This allows a more homogenous distribution of the polymer suspension within the reactor vessel.

In a preferred embodiment, the at least one stirrer is configured to' establish, in use of the reactor, a flow profile in the polymer suspension that is radially inward and axially downward. This causes the polymer suspension to be axially drawn down from the top center part of the reactor vessel to be pushed downward and radially outward towards the reactor vessel, to be subsequently flowing towards the upper part of the reactor vessel in a region of the baffle blades. From there the polymer suspension flows towards the center part of the reactor vessel. This causes the radial flow of polymer suspension towards the center of the reactor vessel by the action of the baffle blades to be reinforced.

In another embodiment of the invention, a diameter of the stirrer outer circumference is in a range of 50% to 90% of the reactor vessel diameter. In a preferred embodiment the range is 50% to 70% of the reactor vessel diameter. This provides a relatively large stirrer size relative to the reactor vessel diameter, which allows a strong axial flow within the reactor vessel at a relatively low stirrer speed.

In another embodiment of the invention a distance of at least one stirrer to a lower end of the baffle blades is at least 20% of the reactor vessel height. In another embodiment the reactor vessel has a cylindrical shape. The reactor vessel height is preferably in arrange of 80 to 200 mm. In this range the desired axial flow of polymer suspension near the reactor vessel inner wall is improved. In a preferred embodiment the reactor vessel height is 145 mm and the reactor vessel diameter is 65 mm. With these dimensions improved volumetric gas-liquid mass transfer rate of monomers is achieved.

In a further preferred embodiment of the invention the polymers comprise polyolefins. More preferably the polyolefins comprise polyethylene or polypropylene.

The invention also relates to a method of preparing polymers using a reactor as described above. The method comprises stirring a polymer suspension in the reactor vessel with the stirrer, the stirrer is driven at a speed of 1000 rpm or less. In a preferred embodiment the speed is further in a range of 800 - 1000 rpm.

BRIEF DESRIPTION OF THE DRAWINGS

Figure 1 shows a cross-section of a reactor according to an embodiment of the invention. Figure 2 shows the reactor according to Figure 1 with an improved stirring effect.

Figure 3 shows a top view of the reactor according to Figure 1 with an improved stirring effect.

Figure 4 shows a graph of catalytic monomer dissolution of a prior art process.

In the Figures:

100 reactor

101 reactor vessel

102 stirrer

103 drive shaft

104 stirrer blades

105 baffle, baffle blades

106 polymer suspension

107 drive

108 reactor vessel wall

109 reactor vessel cover 110 sampling line

111 reactor vessel bottom

112 reactor vessel top

113 feed line

114 outlet of feed line

201 axial flow profile

202 polymer suspension surface

301 stirrer rotation direction

302 combined rotary and radial flow

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Figure 1 shows a cross section of a reactor 100 according to an embodiment of the invention, The reactor 100 has a reactor vessel 101 , which has preferably vertically extending vessel walls 108 and a bottom. 111 suitable for holding a polymer suspension 106 of a monomer to be polymerized. The reactor vessel 101 can be mounted in a frame (not shown). The reactor 100 can be part of a system of reactors which may be cascaded or arranged in other configurations, Within the reactor vessel 101 a stirrer 102 having at least three, preferably four stirrer blades 104 is placed. The stirrer 102 is attached to a drive shaft 103 which can be driven by a drive 107. The drive 107 can be a controllable electric drive, operable in a speed range from zero to 1000 rpm and more, preferably in a range of 800 - 1000 rpm.

Along the inside of the reactor walls 108 a baffle is disposed having baffle blades 105 extending from the top of the reactor vessel 112 into the reactor vessel 101. Preferably, the baffle blades 105 have elongated cross-sections. The baffle blades can be massive but can also have a continuous outer surface enclosing a cavity. The baffle biades 105 can be attached to the reactor vessel wall 108, The baffle blades 105 are preferably attached with one of their ends to a side of the reactor vessel cover 109 that in use of the reactor 100 is arranged to face the interior of the reactor vessel 101. This allows the baffle blades 105 to be extracted from the reactor vessel e.g. for cleaning when the reactor vessel cover 109 is removed from the reactor vessel 101 that is kept stationary. It is also possible to keep the reactor vessel cover 109 stationary and remove the reactor vessel 101.. Preferably a minor spacing d2 between reactor vessel wall 108 and baffle blade 105 is observed. This is to prevent settling of polymer particles that are being formed during the polymerization process. For the reactor-baffle spacing as a rule of thumb distances in the order of reactor vessel diameter divided by 72 or 10 times polymer particle size can be used. The reactor 100 can be provided with input means such as feed lines for introducing the polymer suspension 108 into the reactor vessel 101 and/or output means for extracting the polymer suspension 106 from the reactor vessel 101. Normally the reactor 100. i.e. reactor vessel 101 can also be pressurized. The reactor 100 shown in Figures 1 and 2 has a reactor vessel cover 109 that is provided with a feed line 113 for introducing gaseous chemicals into the reactor vessel 101. Feed line 113 has an outlet 114 that preferably is arranged above the surface 202 of the polymer suspension 106.

Figure 2 shows a flow profile 201 of the polymer suspension 106 in a vertical cross section of the reactor 100. The stirrer 102 is arranged to have an axial flow profile 201. Preferably, the stirrer blades 104 cause the polymer suspension 106 to be pushed downward into the reactor vessel 101 towards the bottom 11 1. Subsequently the polymer suspension flows axially along the reactor vessel wall 108 up towards the surface 202 of the polymer suspension 108 where the polymer suspension 106 assumes a more radial motion towards the center axis of the reactor vessel 101 coinciding with the drive shaft 103. Due to suction of the stirrer blades 104 at the top side of the stirrer 102, the polymer suspension now flows in an axial downward motion along the driveshaft 103 towards the stirrer 102. The axial flow profile can be achieved using an axial impeller. Such axial impeller can for example be a propeller type stirrer 102, for example a stirrer having blades 104 with an inclination angle. The stirrer may have 2, 3 or 4 blades. An example of such a stirrer is known as a Viscoprop™ stirrer.

During the polymerization process, the correct positioning of the baffle blades relative to the liquid phase surface 202 is preferably to be maintained throughout the polymerization process, as the liquid phase level may vary during the process. In use, the baffle blades 105 are lowered sufficiently deep into reactor vessel 101 , or the polymer suspension surface level relative to the baffle blades 105 when filling the reactor vessel 101 is chosen such that lower portions of the baffle blades 105 extend into the polymer suspension. Figure 3 shows a top view of the reactor vessel 101 where in operation the drive shaft 103 has for example clockwise rotation 301. The reactor 100 is equipped with a baffle 105 having two blades which are positioned opposite to each other in the reactor vessel 101. The polymer suspension shows a rotary flow profile as a consequence of the rotary motion 301 of the stirrer at the bottom 1 11 of the reactor vessel 101 , the polymer suspension will also exhibit a rotary motion component in the same clockwise direction 301 near the reactor vessel wall 108, The baffle blades 105 cause the polymer suspension that rotary flows near the reactor vessel wall 108 to radially flow towards the drive shaft 103, thereby combining the rotary and axial flow profile, see reference numeral 302 in fig, 3.

Figure 4 shows a prior art graph of catalytic monomer consumption. The graph shows that after less than 10 minutes a maximum consumption is attained. Gradually in time the consumption rate will drop below. The consumption rate or volumetric gas-liquid mass transfer rate is expressed using the volumetric gas-liquid mass transfer coefficient value 'k_La\ A minimum k_La of 0,03 per second ensures that mass transfer limitations are unlikely to occur, EXPERIMENTS

Experiments were performed to determine an optimal combination of stirrer design and stirrer speed where for various combinations the volumetric gas-liquid mass transfer rates were measured. Experiments of ethylene dissolution were performed in a reactor, i.e. a small scale reactor having a diameter less than 8.5 cm, volume less than 1000 ml. Pressures may be applied up to at least 50 bar. In the various experiments the volumetric gas-liquid mass transfer rate i.e. volumetric gas-liquid mass transfer coefficient of ethylene was established in a hexane suspension present in the reactor (Michael Lallemand, "Continuous stirred tank reactor for ethylene oligomerization catalyzed by NiMCM-41 ", CHEMICAL ENGINEERING, ELSEVIER SEQUOIA, LAUSANNE, CH) .

Dimensions of reactor vessel, baffle and stirrer used in the experiments:

-Reactor vessel: Cylindrical, 145 mm high, 85 mm diameter

-Baffle: 2 blades, top mounted, 70-95 mm length, 5 mm baffle width w, 1 mm thickness baffle.

-Stirrer A: shaft 5 mm, total stirrer height 40 mm, helical shape blade

-Stirrer I: shaft 5 mm, total stirrer height 40 mm, blade 12.5 mm, opening blade to shaft 5 mm.

-Stirrer II: shaft 5 mm, total stirrer height 40 mm, blade 12.5 mm, opening blade to shaft 5 mm.

-Stirrer III: shaft 5 mm, total stirrer height 40 mm, disc 30 mm, blade 10 mm.

In a first experiment comparative a helical stirrer, Stirrer A was used with increasing rotation stirrer speed. In general the volumetric gas-liquid mass transfer rate of monomer into the polymer suspension increases with increasing stirring speed. The stirrer rotation speed was increased in steps from 400 rpm to 1000 rpm. Further process conditions are summarized below:

Temperature: 85°C

Liquid diluent: Hexanes

Liquid diluent volume: 300 mL

Total pressure (start): 10 bar

Stirrer speed: variable e.g. 800 rpm

Only ethylene is used to further pressurize to a total pressure of 10 bar.

Experiments have been performed under closed conditions (no flow to GC),

The results are shown in Table 1 below.

Table 1 Table 1 shows that for Stirrer A the coefficient increases to 0,037 1/s. The targeted value of 0,03 1/s is obtained, however a rotation speed of above 500 rpm is not feasible, since at these stirrer speeds, splashing of liquids leads to undestred deposits on the reactor cover and subsequently plugging of the sampling lines for the analytical device. In a second experiment various stirrer designs were tested at a fixed stirrer speed of 800 rpm. The first stirrer which was used was Stirrer A. The second stirrer (Stirrer I) was a propeller type stirrer with double blades, the third stirrer (Stirrer II) was a propeller type stirrer with four blades, and the fourth stirrer (Stirrer III) was a turbine type stirrer having multiple vertical blades (Rushton Turbine), Table 2 below shows the transfer coefficient of this condition for the various stirrer types.

Table 2 Stirrer 11 shows a transfer coefficient in the order of 0.030 1/s at 800 rpm. This just reaches the targeted value, however the goal is to advance as far as possible beyond the targeted value. A third experiment was conducted where a baffle was introduced in combination with stirrer design (II) at various stirrer speeds.

The results are shown in table 3 below:

Table 3

Table 3 shows that Stirrer II and baffle combination obtains at 800 rpm a transfer coefficient of 0.041 1/s. This rate is sufficiently above the targeted value of 0,030. The embodiments described above are intended as examples only, not limiting the scope of protection as set out in the claims.

Claims

1. Reactor (100) for the preparation of polymers, comprising:

- a reactor vessel (101 ) for holding a polymer suspension (106), wherein the reactor vessel has a volume of less than 1 L, and wherein a reactor vessel height (hi ) versus a reactor vessel diameter (d1 ) ratio is greater than or equal to 2;

- a reactor vessel cover (109), for overhead covering the top of the reactor vessel, comprising at least one sampling line inlet (110) accommodated in the cover for monitoring, in use of the reactor (100), a gas phase composition;

- a drive shaft (103) that, in use of the reactor (100), extends into the reactor vessel (101 ) via the reactor vessel cover (109), the drive shaft (103) having attached thereon at least one stirrer (102), each stirrer (102) having at least three stirrer blades (104), the stirrer blades (104) being positioned in a tower part of the reactor vessel (101 );

- a drive (107) connected to the drive shaft (103) for rotating the stirrer (102), the drive (107) being capable of driving the stirrer (102) at a speed of 1000 rpm or less;

- a baffle that is top mounted to a side of the reactor vessel cover (109) that, in use of the reactor (100), is arranged to face the interior of the reactor vessel (101 ), the baffle comprising at least two baffle blades (105) that in use of the reactor (100) partially extend into the reactor vessel (101 ) along an inner side of an upper part of the reactor vessel (101 ) with a length (h2) in a range of 40% to 70% of the reactor vessel height (hi ) to a level above the at least one stirrer (102), wherein each one of the at least two baffle blades, as seen in use of the reactor (100) in a direction perpendicular to a reactor vessel wall (108) next to which it is arranged, has an elongated cross-section and a blade width (w) in a range of 2% to 10% of the reactor vessel diameter (d1 ), wherein lower portions of the respective at least two baffle blades, in use of the reactor (100), extend into the polymer suspension (106); and

- wherein the at least one stirrer (102) comprises an axial impeller.

2. Reactor (100) according to claim 1 , wherein the drive (107) is capable of driving the stirrer (102) at a speed in a range of 800 - 1000 rpm.

3. Reactor (100) according to claim 1 or claim 2, wherein the stirrer (102) has four stirrer blades (104).

4. Reactor (100) according to any one of the preceding claims, wherein the at least two baffles blades (105) are spaced evenly along the inner side of the reactor vessel (101).

5. Reactor (100) according to any one of the preceding claims, wherein the reactor vessel cover (109) is provided with at least one feed line (1 13) for introducing gaseous chemicals into the reactor vessel (101 ), the feed line having an outlet (114) that, in use of the reactor (100), is arranged above a surface (202) of the polymer suspension (106),

6. Reactor (100) according to any one of the preceding claims, wherein the at least one stirrer (102) is configured to establish, in use of the reactor (100), a flow profile in the polymer suspension (106) that is radially inward and axialiy downward.

7. Reactor (100) according to any one of the preceding claims, wherein a diameter of the stirrer (102) outer circumference is in a range of 50% to 90% of the reactor vessel diameter (d1 ).

8. Reactor (100) according to claim 7, wherein the range is 50 to 70% of the reactor vessel diameter (d1 ).

9. Reactor (100) according to any one of the preceding claims, wherein a distance of the at least one stirrer (102) to a lower end of the baffle blades (105) is at least 20% of the reactor vessel height (hi).

10. Reactor (100) according to any one of the preceding claims, wherein the reactor vessel (101 ) has a cylindrical shape,

11. Reactor (100) according to claim 12, wherein the reactor vessel height, (hi ) is 145 mm and the reactor vessel diameter (d1) is 85 mm.

1 2. Method of preparing polymers using a reactor (100) according to any one of the preceding claims, comprising:

- stirring a polymer suspension in the reactor vessel (101 ) with the stirrer (102);

- driving the stirrer (102) at a speed of 1000 rpm or less.

1 3. Method according to claim 14, wherein the stirrer speed is in a range of 800 - 1000 rpm.