Multi-staged granular beds applied to the filtration of ultrafine particles: An optimization of collector diameters

Granular beds exhibit interesting performances, in terms of ultrafine particle collection efficiency, hot gas cleaning, low cost and robustness. Nevertheless, some authors highlighted that ultrafine particle deposit is essentially located in the first layers of the granular bed. Thus, only a small depth is useful thereby reducing both the operation time between two uncloggings and the process yield. Our improvement approach consists in reducing the pressure drop increase while maintaining high collection efficiency by using a granular bed presenting a collector diameter gradient. Performances of this improved granular bed and of a conventional one were compared during their clogging with particles generated by thermal spraying. Since the collection efficiency is a decreasing function of the collector size, this improved configuration permits to firstly clog the downstream layers of the granular bed. Although there is a 20% decrease of the initial efficiency for this new type of bed compared to the conventional one, this gap is rapidly reduced during the clogging process. At the same time, the results highlight a lower pressure drop increase of the modified granular bed. A predictive clogging model was also developed in order to find an optimized configuration of collector diameter gradient to ensure a contribution of the whole granular bed depth.