Influence of deposited liquid structures on the separation efficiency of thin oleophilic fiber layers of mist filters

Abstract

The presence of oil mists constitutes a significant hazard to both, human health and the environment. In industrial applications, the effective separation of these mists typically relies on the utilization of multi-layer nonwoven coalescence filters. The underlying mechanisms governing droplet deposition, liquid transport within the filter matrix, and the evolution of the overall differential pressure have been extensively documented in the existing literature. However, limited and contradictory information is currently available regarding the impact of deposited liquid structures under varying saturation levels on the separation efficiency of coalescence filters.In this study, the influence of an increased liquid saturation at different filtration velocities on the fractional and total separation efficiency of thin porous non-woven fiber layers comparable to the first fiber layers of the first filter layer of commercially utilized coalescence filters are evaluated. As the oil load increases to a saturation of S≤ 0.4, the total separation efficiency (number-based) decreases, for all investigated filtration velocities. In addition, a shift of the most penetrating particle size (MPPS) to smaller droplet sizes and a decrease of the MPPS efficiency is observed. This decrease is attributed to a higher interstitial velocity resulting from the accumulation of liquid within the filter matrix, leading to a lower fractional separation efficiency in the diffusion regime. However, at saturation levels S≥ 0.6, the separation efficiency (meaning both, fractional and total separation efficiency) begins to increase due to the formation of partial oil films, leading to a blockage of void space. These films significantly enhance inertial separation. At the highest investigated saturations (S=0.8), the highest separation efficiencies due to a nearly complete coverage of the filter layers by an oil film are observed. The film formation is confirmed through backlit images of the oil-loaded filter material. Furthermore, an increase of the MPPS efficiency for a saturation of S=0.8 with increasing filtration velocity due to increased diffusional separation is recorded, which cannot yet conclusively answered by the authors. Additionally, the differential pressure across the fiber sheets is presented, revealing a more pronounced influence of the oil film on the differential pressure at increasing filtration velocities.