Investigation of the local oil distribution on oleophilic mist filters applying X-ray micro-computed tomography

Nonwoven coalescence filter media are often used in real-world applications to separate droplet aerosols from a gas stream. While mechanisms like droplet deposition, liquid transport and the evolution of the pressure drop are well understood, information on separated liquid structures formed during filtration is scarce. However, this information is important to understand the initial state of the coalescence filtration process, more specifically, the transition from individual deposited droplets to larger oil structures and subsequently the effect of these oil structures on the separation efficiency. In this work, deposited oil structures in the micrometer region of an oleophilic filter medium of different oil loading stages (saturations) are presented for the first time by utilizing micro-computed tomography (mu-CT) and several post-processing steps. The local (determined via mu-CT) and global (determined gravimetrically) porosities as well as local and global saturations are compared and evaluated. While local (Phi(local) = 0.9575) and global (Phi(global) = 0.9588) porosities are in good accordance, significant deviations in the saturation are observed. The local saturation fluctuates significantly at the highest investigated oil loading stage. Especially at high global saturations, local areas are completely saturated with oil. Other areas are weakly saturated, which are particularly relevant for the gas flow to pass through the filter media. Furthermore, a novel routine to distinguish between different coalesced oil objects (e.g. oil droplets or oil sails) is developed and presented. It is shown, that the majority of the deposited oil (> 80%) is stored in oil sails between adjacent fibers. Oil droplets surrounding fibers are found to be growing in size with increasing oil loading time. In addition, the volume fraction of the deposited oil in the direction of filter thickness shows, that most of the oil is deposited on the upstream side at the first 80% of the filter.

Automated summary

Nonwoven coalescence filter media are commonly used to separate droplet aerosols from gas streams. This study presents the deposited oil structures in the micrometer region of an oleophilic filter medium at different oil loading stages using micro-computed tomography. The local and global porosities as well as saturations are compared and evaluated, revealing significant deviations in the saturation. A new method to distinguish between different coalesced oil objects is developed. The majority of the deposited oil is stored in oil sails between adjacent fibers, and the size of oil droplets surrounding fibers increases with increasing oil loading time.