Effect of contact angle on equilibrium saturation during mist filtration in nonwoven and foam media

Coalescence filtration using knitted, foam or nonwoven fibrous media is the widely applied in the industry for separation and recovery of liquid mists that are generated in many processes, including lubricated machining, compressors and engine crankcases. The dynamics of multi-component/ phase fluid transport and its interaction with the filter surface at the pore-scale and the contact angle dynamics are expected to significantly influence performance and efficiency of filtration processes. Most mist filters currently used in the industry are hydro- or oleophillic in nature, even though it is expected that philic filters will develop higher steady state saturation levels compared to phobic media. For hydro- or oleo-phobic media, it can be expected that once steady operation is reached, the total level of saturation in the filter media will be significantly lower, thus providing advantages in lower pressure drops and better overall performance. However, phobic media may possibly result in greater levels of aerosol re-entrainment and a greater potential for clogging by (other) solid particles since parts of the filter may not be fully wetted. ... mehrYet, the literature on the influence of contact angles and its dynamics on mist filtration characteristics and transport of collected fluid in the filter (including drainage and re-entrainment) is sparse. Practical difficulties in the measurement of contact angles on micro-/nano-fibres have also been a consistent dilatory factor in accurate quantification of the wetting characteristics of filter media. Computational fluid dynamics (CFD) simulations at the pore-scale with interface capturing techniques offer a unique advantage in enabling parameters such as contact angles and other fluid properties to be precisely defined - and hence ideal for parametric characterization of the filter performance. With the aim of isolating the influence of contact angles on the transport of collected liquid in a filter medium, in the present study, CFD simulations are carried out where the filter is pre-saturated with Diethylhexyl Sebacate (DEHS) which is flushed with air (at constant flow rate). Two structurally different types filter media, nonwoven and foam, with similar properties such as fibre (or element of foam) diameter, packing density and size are used for the present study to compare the influence of contact angles across types of filters. The present simulations reveal that there can be a specific contact angle (between 60 Grad and 120 Grad) for both the types of filters (and operating conditions) considered where the saturation in the medium can drop close to zero resulting in a possibly self-cleaning mist filter.