The identification of microscale oil structures formed from deposited oil droplets on the filter front face of a coalescence filter medium is essential to understand the initial state of the coalescence filtration process. Using µ-CT imaging and a deep learning tool for segmentation, this work presents a novel approach to visualize and identify deposited oil structures as oil droplets on fibers or oil sails between adjacent fibers of different sizes, shapes and orientations. Furthermore, the local and global porosity, saturation and fiber ratios of different fiber material of the oleophilic filter medium was compared and evaluated. Especially the local and global porosity of the filter material showed great accordance. Local and global saturation as well as the fiber ratios on local and global scale had noticeable differences which can mainly be attributed to the small field of view of the µ-CT scan (350 µm on 250 µm) or the minimal resolution of approximately 1 µm. Finally, fiber diameters of the investigated filter material were analyzed, showing a good agreement with the manufacturer’s specifications. The analytical approach to visualize and analyze the deposited oil structures was the main emphasis of this work.

Additive manufacturing processes are gaining increasing attention in various gas cleaning applications since new process optimizations can be achieved (e.g. higher particle separation efficiency, lower process energy consumption and less entrainment of secondary aerosol into the clean gas). When processing metals by grinding, brushing and polishing, submicron metal particles as well as airborne liquid droplets (e.g. oil droplets) may occur. These contaminants in the process air are undesired and need to be separated from the gas stream. Hence, new technologies are aiming to separate these submicron particles at a high particle separation efficiency and a comparatively low pressure drop. By utilizing additive manufacturing processes in gas cleaning applications, novel 3D printed nozzle geometries for innovative wet separators can be applied. Using those nozzles as pre-separator, the service life of downstream filter elements can be extended. Regarding the separation of airborne oil droplets from a gas stream, highly efficient coalescence filters are required. However, entrainment of secondary aerosols into the clean gas may ruin the overall filtration efficiency. Therefore, novel 3D printed support structures on the filter downstream side are developed in order to reduce this secondary aerosol, by applying e.g. drainage channels on the filter rear side to accelerate the drainage of oil. In this contribution the implementation of additive manufactured parts in gas cleaning applications, exemplary for an innovative 3D printed wet-scrubber nozzle and novel 3D printed support structures for oil mist filters, are presented. Keywords additive manufacturing gas cleaning lattice boltzmann method oil mist filtration support structures droplet separation

Fibrous nonwoven coalescence filters are commonly utilized in gas-cleaning processes to separate liquid droplets from a gas stream, e.g., oil mists. These filters are mainly composed of micro glass fibers and in some cases, small amounts of synthetic fibers. The shape of the deposited oil on filter fibers of the filter media depends on several factors, including the oil saturation, wettability, roughness, diameter of the fibers and fiber arrangement. The oil deposits can take the form of, e.g., axially symmetric barrel-shaped droplets or larger structures, such as oil sails between adjacent fibers. Understanding the initial state of the coalescence filtration process and the impact of the deposited oil structures on the separation efficiency requires characterizing these structures. X-ray microtomography (µ-CT) and artificial intelligence tools for segmentation can be utilized to visualize, identify and analyze deposited oil structures in the micrometer region. To quantify and compare oil structures formed at three distinct filtration velocities (10, 25 and 40 cm s${}^{-1}$) commonly utilized in industrial applications and one defined oil saturation of oleophilic coalescence filter media, applying X-ray microtomography is the main emphasis of this work. The results indicate that there is no significant influence of the filtration velocity on the local saturation (determined via µ-CT), the number- and volume-based fractions of the identified deposited oil structures on or between adjacent fibers as well as the droplet concentrations and distributions of deposited oil droplets. It is assumed that the structure of the deposited oil formed by coalescence in the filter medium is dominated by the wetting properties of the fibers (surface tension and surface energy) and the saturation, independent of the filtration velocity.