Tube Transportation of Highly Concentrated Emulsions

Abstract

Tube transportation of highly concentrated emulsions is an important technological process in mining works. Emulsions used for this particular type of application are so-called “liquid explosives” — highly concentrated dispersions of aqueous droplets in a continuous oil phase. The concentration of droplets reaches 96w. %. The width of the inter-phase layers in such a multi-phase system is of the order of nano-level. The length of tube transportation in a real manufacturing process can be of the order of several miles. Hence, the design of the transportation line is of primary technical interest. The practical calculations are based upon comprehensive studies of the rheological properties of highly concentrated emulsions, including an understanding of the role of droplet size, concentration of disperse phase, temperature and time effects (stability of emulsions). Direct measurements were carried out in a wide shear rate range. The results of the measurements indicated that the emulsions under study are rheopectic liquids (viscosity increases over time at a constant shear rate). Their steady flow curve is typical for a visco-plastic medium and is well fitted by the Hershel-Bulkley model. The yield stress is of the order of several tens Pa. The choice of a rheological model is however not crucial for application, since transportation in real technological regimes takes place at high flow rates where the power-type model of flow curves dominates. Systematic studies demonstrated that wall slip is absent over the entire range of the shear stresses under study. This type of rheological behavior was then used for tube transportation design. A more careful examination (based on rheological as well as direct optical observations) also showed that inflation could be observed on the flow curve. It was proven that this type of rheological behavior is related to the two-step mechanism of the flow of a multi-phase liquid. Measurements of normal stresses in shear flows are in accordance with this model of flow. Aqueous droplets in the emulsions under study are super-cooled water solutions of nitrate salts, with the concentration of the latter being of the order of 85%. This system is thermodynamically unstable. The study of time effects (“aging”) showed that slow crystallization in dispersed droplets takes place. This leads to the evolution of the rheological properties of emulsions that can be treated as an emulsion-to-suspension transition. The work was carried out in the Flow Process Research Center, Engineering Faculty, Cape Peninsula University of Technology, Cape Town, Republic of South Africa.