An-Najah Staff

Drop size distribution data for kerosene-water dispersion were obtained in 1."I.D. pipe at a range of velocities in turbulent flow for a straight horizontal pipe. U shaped pipe and an offset pipe fitting oriented horizontally and vertically (upward and downward) to the main flow. A Lightnin in line static mixer was used as a premixer and the drop size distribution was measured by a Malvern 2600 analyzer. By changing the number of internal elements from 4 to 18 the mixer produced a primary dispersion with the mean drop sizes in the range of 50-700 um for the flow rates of 20 to 84 l/minute. The Sauter mean diameter, d32, was found to decrease as the number of elements was increased until an equilibrium drop size was reached. This equilibrium drop size varied with the fluid velocity through the mixer. For a dispersion of ~0.5% kerosene in water, the correlation of drop site with energy dissipation rate, e, was found to give a reasonable agreement with Kolmogoroff’s theory with an exponent in the range of -0.47 to -0.56 for a horizontal pipe and -0.60 to -0.72 for U-shaped and offset pipe fittings. The Sauter mean diameter was also correlated against Weber number with an exponent in the range of -0.71 to -0.83 for all the linings used.

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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Silverson high shear in-line rotor–stator mixers are widely applied in industry for the manufacture of emulsion-based products but the current understanding of droplet breakage and coalescence in these devices is limited. The aim of this paper is to increase the understanding of droplet break-up mechanisms and to identify appropriate literature correlations for in-line rotor–stator mixers. Silicone oils with viscosities ranging from 9.4 to 969 mPa s were emulsified with surfactant in an in-line Silverson at rotor speeds up to 11,000 rpm and flow rates up to 5 tonnes/h. The effect of rotor speed, flow rate, dispersed phase fraction up to 50 wt%, inlet drop size and viscosity ratio on droplet size was investigated. It was found that rotor speed and dispersed phase viscosity have a significant effect on the droplet size, while flow rate, inlet droplet size, viscosity ratio and dispersed phase volume have a lesser effect. The results indicate that low viscosity droplets are broken by turbulent inertial stresses, while droplets smaller than the Kolmogorov length scale are broken by a combination of inertial and viscous stresses. It also appears that the weak dependence of drop size on flow rate enables the energy efficiency of an in-line high shear Silverson to be significantly improved by operating at as high a flow rate as possible.