Formation of emulsions in a screw loop reactor

Abstract

The following article concerns emulsification reaction in a continuous screw loop reactor. The influence of hydrodynamics on the emulsification process as well as the influence of some specific substances were examined. We produced oil/water model emulsions under different test conditions and we determined the correlations between the mixing behavior of the reactor and the characteristic features of the obtained emulsion. Special consideration was given to the droplet size and its distribution. Variable operating conditions of the reactor are the residence time and the rotational speed of the screw, both of which have a decisive influence on the recycle factor and the number of passages through the rotor/stator system. In addition we determined the dependence of the obtained droplet size distribution on the viscosity of the continuous and dispersed phase, on the interfacial surface tension and on the phase proportions. The influence of the specific type of emulsifying agent and its concentration has been examined as well. The positive effect of the mixing and of the recirculation on the emulsification in the screw loop reactor was particularly evident with low emulsifier concentrations and with emulsifiers with slow adsorption kinetics. The established theories for emulsion formation (Kolmogoroff, Taylor) were used for a quantitative description of the experimental results. The screw loop reactor was compared to an agitated tank reactor and a toothed-ring dispersing machine (Ultra Turrax) based on the comminution energy which is produced per unit volume of emulsion. The agitated tank reactor produced only coarsely dispersed emulsions through the entire rotational speed range, whereas the screw loop reactor and the Ultra Turrax, particularly at high rotational speeds and, therefore, high power densities, showed a similar emulsification and produced much finer dispersed emulsions.Emulsions are used by industry in many different fields and in our daily life. Emulsions are mainly used in cosmetic products, plant protection, and by the foodstuff, washing and detergent industries. They also play an important role in the textile and plastics industries and for the extraction of mineral oil.On account of the many areas of application, there are a lot of different demands on user-specific emulsions with respect to the fineness, homogeneity, stability, and rheological features, which can often only be fulfilled by improvements on equipment and processes.

Droplet break-up by in-line Silverson rotor–stator mixer

“…The span increases linearly with dispersed phase viscosity, and is well correlated with an R 2 of 0.949. At 9.4 mPa s the span averaged at 1.08 over a range of rotor speeds, rising to 3.18 at 969 mPa s. A broadening of the drop size distribution was also found by Ludwig et al (1997) for increasing m d . The width of the distribution increases greatly as the size of the largest droplet increases and there is also a growing formation of smaller droplets seen with increasing oil viscosity (Fig.…”

“…Ludwig et al (1997) found the drop size to be slightly higher for higher flow rates, while Thapar (2004) obtained smaller drop sizes at higher flow rates; however, both effects were diminished at higher rotor speeds. Thapar (2004) 3.4.…”

“…14a) as transitional flow, and in some areas of the mixing head where the flow has little time to develop, laminar flow breakage may begin to have an impact on droplet break-up. Ludwig et al (1997) also found drop size to reduce when continuous phase viscosity was increased from 1 to 200 mPa s. Drop size remained roughly constant up to a viscosity of around 10 mPa s, before decreasing as m c is reduced further. Khopkar et al (2009) also found droplet size to decrease with viscosity ratio for a given energy density.…”

“…For a screw loop reactor, Ludwig et al (1997) found higher drop sizes for an increase in volume fraction, up to 50%. However, at the highest rotational speed investigated corresponding to a tip 3.5.…”

“…Hence, subsequent turbulent fluctuations will act upon a drop which becomes steadily more deformed and more prone to forming smaller drops (Arai et al, 1977). Wang and Calabrese (1986) examined silicone oil viscosities of 1 to 1000 mPa s, dispersed in water using a Rushton turbine and obtained a plot similar to Arai et al (1977) where the relationship between drop size and dispersed phase viscosity is constant o10 mPa s, rising to a gradient of 0.75, before levelling off at 1000 mPa s. Ludwig et al (1997) investigated viscosities of 32 to 190 mPa s for a flow rate of 50 l/h and produced similar results to Arai et al (1977), but with an increase in drop size above 50 mPa s and a plateau at $200 mPa s. The data from this study is consistent with previous work in that drop size increases as viscosity increases, but above a critical value of viscosity, drop size tends to become independent of the dispersed phase viscosity. If the Sauter mean diameter is correlated with either of the drop size correlations which account for the effect of dispersed phase viscosity, Eqs.…”