Abstract
The effect of scale, processing conditions, interfacial tension and viscosity of the dispersed phase on power draw and drop size distributions in three in-line Silverson rotor–stator mixers was investigated with the aim to determine the most appropriate scaling up parameter. The largest mixer was a factory scale device, whilst the smallest was a laboratory scale mixer. All the mixers were geometrically similar and were fitted with double rotors and standard double emulsor stators. 1wt.% silicone oils with viscosities of 9.4mPas and 339mPas in aqueous solutions of surfactant or ethanol were emulsified in single and multiple pass modes. The effect of rotor speed, flow rate, dispersed phase viscosity, interfacial tension and scale on drop size distributions was investigated.It was found that for all three scales, power draw is the sum of the rotor and flow contributions, with proportionality constants, PoZ and k1, that are practically scale independent. Sauter mean drop size appeared to correlate better with tip speed than energy dissipation rate. For ethanol/water solutions, mean drop size correlated well with Weber number based on interfacial tension, but for surfactant solutions effective interfacial tension gave better correlations with Weber number.
Highlights
Mixing of two or more immiscible liquids to form a stable emulsion is an important processing step in the manufacture of products such as shampoos, salad dressings, bitumen, pharmaceuticals and many others, and is commonly carried out in in-line high shear rotor–stator mixers
It was found that for all three scales, power draw is the sum of the rotor and flow contributions, with proportionality constants, PoZ and k1, that are practically scale independent
The power constants for three scales of in-line Silverson rotor–stator mixer were obtained from multi-linear regression
Summary
Mixing of two or more immiscible liquids to form a stable emulsion is an important processing step in the manufacture of products such as shampoos, salad dressings, bitumen, pharmaceuticals and many others, and is commonly carried out in in-line high shear rotor–stator mixers. Despite the widespread application of in-line rotor–stator mixers, the current understanding of their performance is still rather limited. To accurately scale-up emulsification in rotor–stator mixers it is important to understand the effect of process and formulation parameters on droplet size to predict and control the characteristic properties of multiphase products from the laboratory scale through to the manufacturing scale. The first step in scaling up of high shear mixers is to determine the power draw necessary to accomplish the required degree of emulsification in two-phase systems. The full expression for power draw in turbulent flow is given by (Baldyga et al, 2007; Cooke et al, 2008; Kowalski, 2009):
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