Conventional methods of mixing powders into emulsions can be slow with a high loss of material. VMI-Rayneri, of France, argues that mixing via vacuum injection into the centre of the emulsifier is more efficient and effective
Several pharmaceutical products require the distribution of powder-like solids into a liquid or pasty phase during their manufacture.
The systems usually employed for these operations inject the powders into an agitation tank. Dispersion is often achieved using the same tool (located on the bottom of the tank) as that used to produce the liquid emulsion. The most widely used tool to perform this operation is a single or double cage rotor- stator for basic operations and to provide the desired thinness of emulsion.
Homogenisation of these mixtures is ensured by the circulation of liquids in the tank; this can be achieved by two different procedures: circulation inside the tank or recirculation using piping outside the tank.
The current practice of injecting powders involves two different methods:
1. Vacuum: evacuation of the tank allows the injection of powders during the liquid phase contained in the tank; or
2. Venturi: the start-up of the emulsifier generates a fixed flow in the recirculation piping for the injection of powders
Each system presents limitations, but the solution developed and currently being patented by VMI-Rayneri will resolve these. The innovative technology is based on the vacuum injection of powders directly into the centre of the emulsifier (see figure 2).
The main drawback of one or other of the conventional procedures is the simultaneous action of two distinct hydrodynamic states in the tank to ensure the dispersion of particles and homogenisation during liquid phases.
The fact that the vacuum injected powders do not enter the emulsifier directly increases the risk of preferential transition of powders during the liquid phase, resulting in a loss of the materials that are being dispersed. This inconvenience can also mean that more cleaning is required, caused by the gradual accumulation of powder deposits under the cover.
Injection in the centre of the emulsifier allows a greater range of vacuum levels to be applied than those offered by conventional systems, without any risk of the powders going through the liquid phase without being dispersed. This procedure allows a substantial increase in output, ensuring a higher production rate.
For the second procedure, the presence of a recirculation loop outside the tank creates zones that are difficult to access and difficult to clean, as well as offering only limited circulation output during the incorporation of powders into a viscous or very pseudo-plastic medium. This limitation makes these systems very inefficient during dispersion, homogenisation or thermal transfer procedures. In the manufacture of several products the paramount criteria is often the circulation output, not just the shearing capacity of the emulsifier.
The solution suggested by VMI-Rayneri ensures an optimal injection of powder. Since solid particles are injected directly in the centre of the rotor-stator, the quality of dispersion ensured by the emulsifier is completely independent of the circulation of fluids in the tank and linked to functions other than the dispersion itself. Each tool in the tank is, therefore, used for only one specific mixing action. The result is an increase in performance and productivity of the whole system.
The use of a rotor/stator integrated with vacuum injection is complete when used with pendulum agitation, creating an efficient flow of the mixture in the tank that is not hindered by the output generated by the emulsifier. In this manner the unit becomes a
production tool capable of ensuring multiple phase activity:
dispersion of powders,
emulsion of fatty phases, during W/O emulsion as well as O/W emulsion,
dispersion, homogenisation and thermal transfer without the need of shearing, notably for the final phases of cosmetic product processing.
VMI-Rayneri therefore offers users an enhanced, multiple-use tool that allows the same production efficiency for several different creams using the same mixer.
In addition, the ability to use a wider range of vacuum settings than offered by conventional systems allows quicker commissioning as well as increased productivity thanks to more simple, immediate and less frequent adjustments of vacuum levels.
To summarise, vacuum injection in the centre of the emulsifier, as described here, offers a superior quality of dispersion and emulsifying procedures
compared with conventional systems. Mixers equipped with this new technology become high performance production tools offering increased versatility and productivity.