Stirrers rotor-stator

When a high degree of shear on small liquid volume is desired, a rotor-stator stirrer is used (see Fig. 4). This stirrer consists of a blade or paddle stirrer enclosed by a ring of baffles. This type of stirrer thus alleviates the need for separate baffles in the vessel. This stirrer is not useful for biological reactors, but finds special use in polymeric reactors containing non-Newtonian liquids. 

Fig. 4. A rotor-stator stirrer. (Reprinted with permission from the publisher, VCH Publishers, Inc., after Zlokarnik and Judat, 1988.)...

For this purpose a fast stirrer with zig-zag blade or a colloid mill such as a rotor-stator mill are useful. After the stirring or the passage through die mill is completed the formulation needs no further processing. Compared with the suspension/emulsion method, fewer vessels are required, low viscous fluids are handled and incorporation of the thickener is more easily achieved. 

In stirrer types acting according to the rotor/stator principle, the rotor is a turbine stirrer (Fig. 1.6), or a toothed ring (as implemented in the Ultra-Turrax from IKA Janke Kunkel [227], Fig. 1.7), which is surrounded by a baffle ring as stator. In this way extremely high shear forces are realized in a small space ( wet grinding ). 

Judat [244] investigated the dispersion action of rapidly rotating stirrer types 3-vane propeller stirrer, 6-pitched-blade stirrer (a = 45°), 6-blade turbine stirrer and a rotor-stator arrangement (rotor 6-turbine stirrer, stator 24 vertical paddles whose angle could be varied between —60° and +60° with respect to the radial position). 

Fig. 6.4 shows the droplet distribution for the three stirrer types and Fig. 6.5 shows that for the rotor/stator arrangement. The particle size distributions in Fig. 6.4 are flatter for the material system (2) and hence broader than that for the material system (1). This proves that the results cannot only be described by Hi, but also cannot be correlated with the material numbers p /p and p /p either [253, 597]. It is also necessary to take the interfacial tension into consideration with Hj. It is found that fliy is dependent upon Hz, whereas crin decreases with increasing Hz. 

FIGURE 11.6 Active part of some emulsifying machines, (a) Rotor-stator type stirrer ( ultra-turrax ). (b) Colloid mill, (c) Valve of a high-pressure homogenizer. The slit width in (b) and (c) is greatly exaggerated. 

The stirrers are mainly of the flat-blade, disk, propeller, or turbine type [7.16]-[7.19]. Rotor-stator systems are also used as continuous mixers (Fig. 7.5). These machines are employed particularly if an emulsion or an emulsion-like product has to be produced (e.g., waterborne paints) [7.20]-[7.22]. 

Table 8.6. Influence of different rotor/stator systems on the steady-state droplet size distribution (32) , stirrer speed in rotations per minute dispersion time u, volume flow rate...

Dispersion of larger quantities of active substance is best performed by using a rotor-stator mixer, which cannot be executed within the suppository molding apparatus. An alternative way of dispersing is triturating the powder with molten base in a mortar and add it to the rest of the molten mass in the apparatus vessel. The blade stirrer of the molding... 

In the practice of pharmacy preparations, it is usual to try to distribute the dispersed phase more finely with a rotor-stator mixer or a simple mechanical (paddle) mixer (pourable emulsions), or, for semisolid emulsions, with a mortar and pestle or a mixing device. Magnetic stirrers are unsuitable. Impaction of air should be avoided because the liberation of air bubbles can accelerate the creaming process (in case of pourable emulsions) and promote possible oxidation of the lipid base or of an active substance. A Stephan mixer with vacuum (see Sect. 28.6.1) is a suitable device for the preparation of emulsions without inclusion of air. 

The stirred tank, of course, uses the stirrer for mixing, but the performance, even with modem designs of paddles, does little to ensure highly uniform reactions - the main point of stirring (or mixing) the fluid(s) within the pot . The most common types of mixers are in-line units and rotor stator mixers. Other mixer types considered below include variants based upon ejectors, fluidics, types using venturi aeration and ideas based upon spinning discs. 

At first, the surfactant and thickener were well mixed into the continuous phase (compositions, see Table 23.1). Then the dispersed phase was introduced slowly and dispersed using an impeller type stirrer to prepare the pre-emulsion. Afterwards a rotor-stator device (Polytron PT6000, Kinematica AG, Luzem, CH) was used for further dispersion of the pre-emulsion droplets to produce stable emulsions. Rotational speeds from 1000 to 10,000 rpm were applied within fixed constant time periods, and secondary drop size distributions were measured over time after representative sampling to achieve desired-size secondary droplets of the model emulsions. These model emulsions were stable for several days. However, the model SE was in general used at the same day of preparation for spray characterization employing air-assisted nozzles and rotary atomizers RA. 

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