Intensification of nanoparticles collection efficiency on a bubble column
Résumé
Nanometric aerosols localized in workplace environments mainly stem from two sources: on the one hand, nanoparticles which are manufactured in more and more industrial applications and on the other hand, ultrafine particles generated by different processes: fumes, cutting processes and metallization processes... This latter technique, which involves projecting (by compressed air) fine metallic particles on a surface, is a very intense source of ultrafine toxic particles. Currently, the most effective and widely used dedusting techniques in order to separate particles from a carrier fluid are fibrous media (often in the form of cartridges). The main problem with the filtration of such particles is the progressive clogging of the filter that induces a pressure drop increase over time and thus requires a regular cleaning of the media or its replacement. This problem is mostly pronounced in the case of metallization processes as agglomerates of nanoparticles tend to increase the filter clogging. Following these observations, we proposed to investigate an alternative to these classical techniques that could be applied to metallization fumes. We concentrated our choice on bubble columns which offer advantages such as little maintenance requirement due to simple construction, easy temperature control, low initial costs of installation and operation at constant pressure. An experimental study was conducted to determine the performance of a bubble column in terms of collection efficiency of nanostructured particles. The choice of a demineralized water as collection fluid was done in order to avoid the formation of secondary particles when bubbles explode at the surface of the liquid. Particle concentrations upstream and downstream of the bubble column were determined with a Scanning Mobility Particle Sizer. Results showed that collection efficiency is governed by two key parameters: the residence time and size or air bubbles. An optimization of these parameters induced an increase of bubble column performance but collection of ultrafine particles remains lower than those of fibrous media. Consequently, to become a viable alternative to fibrous media in terms of collection efficiency, this separation process may be intensify and we propose to study the influence of the presence of beads (of various sizes and natures) in the liquid.