Abstract
Emulsification experiments have been carried out on a pilot-scale Model ACIP2 Sonolator liquid whistle device by examining the change in droplet size distributions of silicone oil in water emulsions, using SLES as a surfactant, before and after processing. The process variables considered were mass flow rate, pressure drop across Sonolator, oil viscosity (from 10 to 10,000 cSt), oil concentration (0.5–10 wt%), surfactant concentration (0.00003–0.5 wt%) and orifice size. All experiments were carried out in the turbulent flow regime. The oil phase was added as either a pure phase or as a pre-emulsion stabilised using SLES. The oil was injected just before the blade or mixed at a T-junction prior to the Sonolator; the pre-emulsion was exclusively introduced via the latter method. The resultant droplet size distributions were obtained from offline sampling using laser diffraction. The most significant parameters found to influence the drop size were found to be pressure drop, dispersed phase viscosity and surfactant (SLES) concentration, which formed the basis for an empirical power law correlation. Indices in this correlation were compared to findings in the literature for other emulsification devices, and to those predicted from the theories of droplet breakage in turbulent inertial flow. Despite an expected regime change from turbulent inertial to turbulent viscous break-up being common in the literature as the dispersed phase viscosity is increased, this phenomenon was not observed in the experimental data obtained, suggesting breakage in an intermediate regime.
Highlights
The Sauter mean diameter (d32) was the key statistic examined alongside the drop size distributions (DSDs), since the emulsion properties of interest to industry often depend upon the interfacial area exposed by the droplets, which is directly related to d32
The distributions obtained at 0.100 kgÁsÀ1 were shifted further to the left than the corresponding 0.033 kgÁsÀ1 distributions, which indicated a further reduction in droplet sizes when processing with higher flow rates
Hall et al (2011), who used an identical preparation protocol for the silicone oil in water emulsions used here, show that the outlet drop size from an inline high shear rotor-stator mixer is insensitive to the inlet droplet size distribution of the coarse pre-emulsion used; the drop size decrease is an order of magnitude or more in their experiments
Summary
Droplet cohesive force changes from interfacial tension driven to a viscous resistance to deformation Davies included this effect by modifying the critical Weber number during breakage to have two terms, one for interfacial tension (r) and an extra term for dispersed phase viscosity (lD) scaled by a constant determined from experiment (b) and the size of local velocity fluctuations (V0), giving: dmax. Emulsification has been characterised for many other fluid mixing devices, most recent examples include ultrasonic emulsification (Lin and Chen, 2006); six vaned rheometer (Baravian et al, 2007); narrow gap homogenizers (Vankova et al, 2007); valve homogenizers or HPH (Tesch et al, 2003); batch rotor stator devices (Calabrese et al, 2000; Padron, 2005) and inline rotor stator devices (Hall et al, 2011, 2013) All of these generate turbulence which, depending on the exact flow conditions, may break droplets in a similar way to the Sonolator. This paper ascertains which of the variables have a significant effect on droplet size and an empirical correlation is developed which is compared with theoretical predictions for relevant breakage regimes
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