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 effect of sodium lauryl sulphate (SLES) surfactant and the operating temperature on the drop size distribution of a 350 cSt Dow Corning 200 series oil water dispersion was successfully studied. The dispersion was prepared in a standard 6 litres mixing tank at different impeller speeds. A measurement of the SLES critical micelle concentration (CMC) at 25°C was carried out. The interfacial tension of silicon oil water under various SLES concentration at a temperature range of 25 to 80°C was accomplished. Results showed that the interfacial tension of the silicon oil water decreased as the operating temperature increased and as the surfactant concentration increased. When the operating temperature was increased at the highest SLES concentration tested, a decrease of d 32 was observed. This was attributed to the possibility of hydration of the surfactant at high temperature. Same behavior was observed when measuring the drop size distribution at constant temperature but different SLES concentration. It was found that the mean drop size decreases with mixing time. Different slopes of the change of the median drop size with time were obtained for different SLES concentration. For the same concentration, the slope changes after 1 hour. The degree change of the slope is due to the change of interfacial area of the oil water as mixing time elapsed and the depletion of the surfactant concentration.

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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