(424d) Modelling and Simulation of Extensional Flow-Units in Emulsion Formation

Every day consumers are in direct contact with emulsions and dispersions. Food products, detergents and paints, for instance, do not form homogeneous mixtures but are composed of at least one component in the form of solids or droplets. Within food emulsions the product attributes relevant to the consumers are distinguishable and particular of each product category. Thus, margarine should be spreadable, mayonnaise spoonable and ice-cream creamy. All attributes of an emulsion products (e.g. mouthfeel, colour, aroma, stability, etc) are the result of the close marriage between product composition, microstructure and processing. Due to the inherent business importance of emulsion products, the improvement of current processes and synthesis of novel processing routes are key activities within FMCG companies.

Current technologies for the production of food emulsions include static mixers, stirred tanks, rotator-stator devices, and high-pressure and ultrasonic homogenisers, among others. The governing droplet break-up and coalescence mechanism in each device is highly dependent on its geometry, operation policy and the actual physico-chemical properties of the colloidal system. For instance, stirred tanks are basically driven by simple shear in the laminar regime, whereas turbulent inertial mechanism is mainly responsible for droplet break-up in high-pressure homogenisers.

Despite the sound expertise generated over the last decades regarding droplet disruption and coalescence mechanisms, the effort has been exclusively channelled to current available units. Thus, relatively small attention has been paid to explore novel unit designs. The aim of this contribution is to broaden the spectrum of current emulsification devices, by addressing the design, modelling, and simulation aspects of extensional-flow units. This type of apparatus is characterised by the dominant extensional flow of material in the turbulent regime, and from a mechanical perspective, it is realized in a converging element with high extensional stresses.

The strong feature of our model approach resides in the fully mechanistic description of the governing phenomena. Namely, a population balance equation is formulated and solved to account for the disappearance and appearance of droplets at each size class. Moreover, coalescence mechanism is included to account for the instability of newly created droplets. Additionally, our model estimates the prevailing break-up mechanism at each size class as a function of droplet diameter, the acting forces on the droplet and the exposure time in the high energy zone.

To validate our modelling approach we aim to obtain an oil-in-water emulsion with a dispersed phase volume fraction close to 0.74 and Sauter diameter ranging 4-5 ?Ým. The initial pre-emulsion is represented by a log-normal distribution of droplets. The results obtained by simulation show that at a given set of operational conditions and pre-emulsion properties the product obtained is within the desired and narrow specifications space.

A sensitivity analysis suggests that the breakage mechanism changes with the flowrate. Regardless the disperse phase volume fraction, large throughputs lead to finer dispersions, while coarser pre-emulsions lead to coarser product. The simulated data show an acceptable degree of agreement with experimental pilot plant data, stressing the practical relevance of extensional-flow units in emulsion formation.