This research project aims at working out a method for describing the dispersibility of an agglomerated material in stirred vessels and to apply this method to the improvement of the redispersing properties. It is furthermore intended to establish a connection between the results from laboratory tests of the instant/dispersing properties and the large-scale dispersion of the product.
In the first year of the project, laboratory equipment for dispersing powders in stirred vessels was set up, and experiments were performed with agglomerated and milled skim milk powder (Nestle) in 10 1 and 50 1 vessels.
This report contains information required for an experimental set up of an agitated vessel at laboratory scale that enables the engineer to gain data suitable for the design of large-scale-vessels.
Mixing performance of the stirring system was investigated with and without baffles. Appropriate laboratory tests for describing the instant properties of the powder were also carried out, and a new testing method (dynamic wetting test) was developed.
The stirred-vessel experiment constructed permits investigation of the combined wetting/dispersing behaviour of powdered products under stirrer action. The results of the experiments indicated that there is an optimal point of operation (regarding specific power and feed rate), at which the total required stirring energy is minimal. The final degree of dispersion for skim milk powder depends upon the volume-specific stirring power.
It is difficult to establish a relation between the laboratory tests (static and dynamic wetting test) and the dispersing experiments in an agitated vessel. Since wetting tests do not take stirring power into account, they can not describe the processes taking place in the liquid. It is possible anyway, to derive some trends from these tests. The results of the laboratory tests with lecithin, for example, are in good agreement with the experiments conducted using the stirred vessel. The maximal possible feed rate per surface area, short of layer formation, could be determined using the dynamic wetting test. This feed rate was somewhat higher than the rate found in experiments with the agitated 10 1 vessel, the difference being caused by the uneven distribution of the powder across the liquid surface in the stirred vessel.
Use of surfactants improves the combined wetting/dispersing behaviour of powdered products, both in the wetting test and in the stirred vessel. In this case, lower values of specific power and energy are required. It has been shown that, preferably, a premix of surfactant and powder should be used.
The results gained so far enable the design of large-scale agitated vessels for milk powder and similar products.