Scale rotor-stator

On a manufacturing scale rotor stator mills have proven to be very efficient. Their use in the preparation of emulsions has been described in a wide body of literature [7]. When preparing suspoemulsions the same principles can be applied. 

Lee I, Variankaval N, Lindemann C, Starbuck C. 2004. Rotor-stator milling of APIs— emperical scale up parameters and theoretical relationships between the morphology and breakage of crystals. Am. Pharm. Rev. 7(5) 120-123. 

Scale-Up of a Rotor-Stator Unit from 6" Diameter to 18" Diameter... 

Rotor-stator Silverson Model GX25 Production scale batch Maximum rpm 3600 2400 L (low viscosity) Emulsions dispersions. 

For the toothed rotor/stator mills, there are localized areas of high shear where the most energy is imparted to the particles, and breakage is believed to occur, through a combination of shear and collision. Therefore, useful parameters that quantify mill performance will reflect these effects. Three typical parameters used in rotor-stator mill scale up are tip speed (rotation rate of rotor x rotor circumference), shear rate (tip speed/distance between rotor and stator), and shear frequency (rotation rate x number of slots on rotor x number of slots on stator).Rotor-stator types typically differ in the number of rows of teeth and the number of teeth in each row of rotor and stator, though other geometric differences are possible. When rotor-stators of different design are available for study, shear frequency appears to have... 

Rotor-stator mixers Slotted ring or impeller, along with slotted stator Either, often continuous 1 < d32 < 50 pm Sparse data for scale-up need extensive testing... 

The focus in this chapter is on small scale preparation with a mortar and a pestle, a rotor stator mixer or an ointment miU. For preparation on a larger scale pharmacies in several countries use the mixing-dispersing apparatus Stephan mixer. This mixer exists in various models and is suitable for the preparation of almost aU cutaneous preparations (see Sect. 28.6.1). 

Copredpitation Batch mode vs. continuous mode Flow rate Droplet size distribution Mixer design (shear and energy, e.g., stir bar, vortex, propeller, homogenizer, rotor-stator) Solvent to antisolvent ratio Temperature Processing times (scale dependent)... 

HaUa S, Paceka AW, Kowalskib AJ, Cookec M and, Rothmand D (2012) The effect of scale of liquid-liquid dispersion in line Silverson Rotor Stator Mixers. 14th European conference on mixing, Warszawa... 

The need for a fundamental characterization of the performance of these devices and the criteria for scale-up and design are well recognized. In response to this need, two consortia, the British Hydromechanics Research Group (BHRG) and the High Shear Mixing Research Program at the University of Maryland, have embarked upon systematic study and characterization of rotor-stator mixers. 

There are few published data for power draw and pumping capacity in either batch or in-line rotor-stator mixers. Even less is known about the velocity fields in these devices, so there is little hard evidence to support proposed mechanisms for dispersion and emulsification. As a result it is often necessary to rely on equipment vendors for scale-up rules. Although many vendors have facilities for customer trials, few have well-equipped laboratories for acquisition of basic data for performance characterization. In reality, it is difficult to know how many vendor data are available, since many consider the information to be proprietary. Until recently, there has been little academic interest in high-shear mixers. This work is only starting to appear in the open literature, and it is important for the practitioner to stay informed as a body of knowledge evolves. 

Figure 8-8 Schematic diagram of prototype IKA mixer with 12 rotor and 14 stator teeth. Inner diameter of rotor is 11.8 cm outer diameter of stator is 15.4 cm. The rotor tooth depth is 1 cm and the rotor-stator gap width is 0.5 mm. Approximately drawn to scale, except for gap width.

For turbulent flow through rotor-stator devices with teeth, the aforementioned velocity field results indicate that flow stagnation on the leading edge of the downstream stator teeth provides a major energy field for emulsification and dispersion. It is not clear from these results what role is played by flow in the shear gap. The simulations indicate that the flow in the rotor-stator gap is not a simple shear flow but is more like a classical turbulent shear flow. Use of nominal shear rate may not be useful in scale-up. 

As stated above, there are few fundamental data to allow design and scale-up of rotor-stator mixers available in the open literature. Academic activity is increasing and it is important to remain aware of new information. It is impoitant to note that there is activity in this field that may be documented in non-English publications that has not been discussed above. The results discussed above were general in nature. Additional information may be available in industry-specific publications, such as those for food processing, paints and pigments, and so on. 

There are many important process and mixer variables to consider in the process selection, scale-up, and operation of rotor-stator mixers. Key variables among them include ... 

The authors are not aware of any published fundamental studies of solid dispersion in rotor-stator mixers but have seen videos supplied by vendors that show preparation of fish-eye free solutions of hard-to-wet powders such as Carbopol, with apparently remarkable ease. Scale-up and design of these systems would be based on operational information obtained with vendor equipment. This is highly recommended. 

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