THE DISPERSION PROCESS

The dispersion process: wetting, mechanical size reduction, product stabilisation​

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1. The dispersion process

The most frequent application of high speed dispersion is to incorporate extremely fine solid particles into fluids, to produce colloidal suspensions.

​The steps in the dispersion process are: 

• the wetting of the surface of the solid particles by the fluid components of the millbase.
• the mechanical breakdown of associated particles leading to smaller particles (agglomerates and aggregates).
• the smaller particles generated during the dispersion are stabilised, preventing renewed association (flocculation).

The container with the liquid component is moved into position under the dissolver and the dissolver disc is lowered down into the liquid.

Vertical Divider

The powder component is added into the liquid phase with a low speed, wetted and premixed

Vertical Divider

The premixed product is dispersed with the dissolver disc. If the process is carried out correctly, the product will be put in a rolling, turbulence free movement.

Vertical Divider

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2. The doughnut effect

The dispersion process is characterised by the rolling flow pattern known as the doughnut effect.

The doughnut like flow pattern indicates the maximum mechanical power possible is being transferred into the millbase and that all agglomerates will eventually be agitated.
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The most effective dispersion takes place in concentrated systems where the impeller tip speed is 18-26 m/sec.

The dispersion process is typically completed after 20 minutes.
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3. Optimising the dispersion process

The following can be examined to optimise the dispersion process:

• Doughnut effect should be maintained throughout.
• The impeller should be no smaller than 1/3 of the container diameter.  
• When dispersing, high energy transfer into the millbase will lead to an increase in temperature. To avoid thermally sensitive ingredients and flow characteristics being affected, a water cooled vessel can be used.
• Increasing/decreasing the amount of millbase in the container may achieve better flow characteristics.
• Shaft speed should be maximised whilst not losing the doughnut effect to optimise mechanical power input.
• If flocculation takes place after dispersion, additives can be added to prevent this and provide an improved dispersion.
• The impeller diameter as well as height from the base of the container can be varied to achieve the required rolling flow pattern in the millbase.     

4. Scale-up of the dispersion process

Dispersion results can be scaled up to production size machines. When scaling up, two factors should be considered: rate at which agglomerates are transport into the zones of shear and mechanical power transferred to the millbase.

The mechanical power limits the quality of the dispersion, whereas the rate of transportation of agglomerates determines the time required to reach optimum dispersion.

Most effective shearing conditions for deagglomeration take place at the tip of the dissolver disc, therefore tip speed is to be considered a key parameter.

The distance the agglomerates must cover to the reach the disc is important to consider when using larger equipment. Exact correlation with larger dissolvers will also naturally depend upon comparable temperature conditions.