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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The preparation of concentrated aqueous silicone oil emulsions has been investigated with particular attention to the effect of the dispersed-phase volume fraction ϕ
from 0.01 to 0.5 for a wide range of oil viscosities (50 to 1000 cSt). Oil was added on the top surface of a 6-L vessel. Drop size distribution and Sauter mean diameter, d32, measurements were carried out over 24 h mixing time. Emulsification was found to be relatively sensitive to the oil phase viscosity, ld, for the same
ϕ yielding a narrower drop size distribution for low oil viscosity (50 cSt) and a wider drop size distribution for the highly viscous oil (1000 cSt). For the same
, increasing ld resulted in increasing d32. The equilibrium d32 was found to be well correlated to the viscosity number by d32 D
0
026  V0
204 i for
= 0.5. For the same oil viscosity, d32 was found to increase with increasing
. A multiregression of d32 with both
and Vi for various silicone oil viscosity grades was successfully correlated by d32
9
6
0
069V0
216 i with a regression coefficient (R2) of 0.975. This shows a very weak dependence of the equilibrium d32 on
. Keywords: Dispersed-phase volume fraction, Drop size distribution, Liquid-liquid dispersion, Silicone oil, Surfactant Received: January 22, 2009; revised: March 23, 2009; accepted: April 27, 2009 DOI: 10.1002/ceat.200900038 1 Introduction Liquid–liquid dispersion is one of the most complex of all mixing operations. Agitating two immiscible liquids results in the dispersion of one phase in the other in the form of small droplets with drop size distributions whose characteristics depend on the equipment and the operating conditions. It is virtually impossible to make dispersions of uniform drop size, because of the wide range of properties and flow conditions. The knowledge of the resulting drop size distribution characteristics or, more exactly, the evolution of this distribution with changes of external energy input is of major importance. A large amount of work can be found in the literature concerning the prediction of drop size distributions in turbulent liquid-liquid dispersions in stirred vessels. Most of them use the concept of a turbulent energy cascade to predict the maximum stable diameter,