An-Najah Staff

The preparation of dilute aqueous silicone oil emulsions has been investigated with particular attention to the effect of oil viscosity (0.49–350mPa s), impeller selection (equal diameter Sawtooth and pitched blade turbines) and the method of addition of the oil. Emulsification was found to be sensitive to how the oilwas added to the vessel with narrower drop size distributions and smaller Sauter mean diameters, d32, obtained when the oil was injected into the impeller region. The equilibrium values were also attained in a shorter time with the equilibrium d32 ∝We−0.6. For addition of the oil to the surface the relationship was weaker with equilibrium d32 ∝We−0.4. The viscosity group was particularly useful in describing the behaviour of equilibrium particle sizes for different viscosity oils and also for viscosity changes arising from different process temperatures. An unexpected result is that the Sawtooth impellor proved to be more energetically efficient at drop break-up producing smaller droplets than the Pitched Bade Turbine. This result is particularly interesting since the power number for the latter is larger and therefore for equivalent operating conditions should produce smaller drop sizes. We suggest that one possible reason is that the local shear rates for the Sawtooth impellor are larger. Another possible reason is that the Sawtooth geometry provides more points where the local shear rates are high. © 2008 Elsevier B.V. All rights reserved.

1. Introduction It is well-accepted that local shear, elongation and necking are very important aspects of drop formation as are the physical properties of the fluids involved. Hence a successful design depends on developing amechanistic understanding of how the equipment selection, process strategy and material properties interact to affect the resulting microstructure (e.g. particle size) and hence the performance of the products. Typically two approaches are adopted:
• Scale-up at geometric similarity and constant tip speed.
• Scale-up at equal specific power input. Scale-up on the basis of geometric similarity and constant tip speed assumes that the relevant shear that produces the limiting drop size occurs in the agitator region where the velocity gradients are the steepest. These are assumed to scale with the peripheral velocity of the impeller and the approach generally works
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