Continous, microwave enhanced liquid-liquid separation in solvent extraction processes
Résumé
Background During solvent extraction operations, solute transfer between the two phases is achieved by dispersing one as droplets in the other, thereby increasing surface available for mass transfer. Once solute transfer is achieved, the resulting emulsion needs to be separated into two clear phases again. Conventional solution involves letting the emulsion settle in a high residence time settler. Pollutions in the feed, or a high agitation rate, can lead to excessively stable emulsions, eventually leading to flooding of the separator. Among the methods investigated to mitigate these problems (Frising, Noïk, & Dalmazonne, 2006), microwave heat treatment is of interest, because of its intrinsic advantages (fast, tunable, potentially volumetric heating). Method Available articles in the literature typically (Binner, Robinson, Silvester, Kingman, & Lester, 2013) employ a two stage strategy: heating the emulsion first, then letting it settle, while separation time is used as a judging criterion. Here, a continuous vertical liquid-liquid settler was used, where millimeter sized drops of either phase are generated via a drilled plate inside an extraction column. Drops settle through a stagnant continuous phase, and coalesce while being exposed to 2.45GHz microwave radiation, generated by a magnetron and propagating inside a single-mode cavity jacketing the column. The organic phase used was a tri-butyl-phosphate (30%) / alkane (70%) mixture, typical of solvent extraction operations in the nuclear fuel cycle. The aqueous phase was a Potassium Nitrate solution, mimicking the high ionic content and conductivity of aqueous metal extraction phases while ensuring that no mass transfer takes place. Commercial surface-active agents were also used in minute amounts to mimic pollutions. Performed experiments involved imposing a dispersed phase throughput and reading the resulting emulsion height inside the column, first with and then without microwave irradiation. Temperatures where recorded at various points in the dispersed phase circuit, and absorbed powers were determined by incident and reflected power measurements. Results Emulsion height as a function of throughput was recorded for two diphasic systems (one aqueous continuous, the other organic continuous) Significant reduction (up to a factor 5) in emulsion height was recorded, both for oil-in-water and water-in-oil experiments, with a mean exit temperature rarely exceeding 35°C, input temperature being approximately 20°C. Without microwave power, an excessive height of emulsion is generated, incompatible with the correct operation of a real extraction apparatus. Conclusion Results obtained demonstrate the applicability of microwave demulsification to solvent extraction. Nevertheless, comparison with a conventional heating method is difficult owing to the different temperature fields obtained with each method. To better understand microwave demulsification, simulations of the temperature field inside the column, based on permittivity measurements performed earlier, will be carried out.
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