Influence of Particle Shape on Filtration Processes

Abstract

The influence of particle shape on filtration processes was investigated. Two types of particles, including spherical polystyrene latex (PSL) and iron oxide, and perfect cubes of magnesium oxide, were examined. It was found that the removal efficiency of spherical particles on fibrous filters is very similar for corresponding sizes within the range of 50-300 nm, regardless of the fact that the densities of PSL and iron oxide differ by a factor of five. On the other hand, the removal efficiency of magnesium oxide cubic particles was measured, and found to be much lower than the removal efficiency for the aerodynamically similar spheres. Such disparity was ascribed to the different nature of the motion of the spherical and cubic particles along the fiber surface, following the initial collision. After touching the fiber surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could either slide or tumble. During tumbling, the area of contact between the particle and the fiber changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of the particle trajectory. The extra probability of particle bounce by the cubes was derived from the experimental data. The particle kinetic energy was proposed to be responsible for the difference in removal efficiency of particles with alternative shapes, if all other process parameters remain the same. The increase in kinetic energy is shown to favor the increase of the bounce probability.

An Overview of Airborne Nanoparticle Filtration and Thermal Rebound Theory

“…Studies show that cubic particles provide lower particle removal efficiency than do aerodynamically similar spheres. The contact areas following collisions between the filter surface and spherical particles travelling in divergent directions are the same; however, this area may be different for other particle shapes, which alter particle collection efficiency (Boskovic et al, 2005). Spherical particles may either slide or roll upon collision; however, cubic particles could either slide or tumble, increasing the probability of detachment from the surface (Boskovic et al, 2005).…”

Filtration of nanosized particles with different shape on oil coated fibres

“…It is reported that A and e ought to be determined experimentally because it is very hard to determine them theoretically. Boskovic et al (2005) showed that the effect of particle motion along the fibre should not be ignored. Their experimental results showed that polystyrene latex (PSL) particles of spherical shape in the size range of 50-300 nm are removed from the gas carrier with significantly higher efficiencies compared to cubic MgO particles in the same size range, even if their densities vary by a factor of 3.6.…”

Removal of Elongated Particle Aggregates on Fibrous Filters

“…The size distribution of individual MgO particles generated by a metal combustion was discussed in [10]. As was reported, cubic MgO particles with the side length of 35 nm represented the maximum number concentration of polydisperse aerosol produced by magnesium combustion.…”

“…A difference in filtration efficiency between spherical and nonspherical particle of the same electrical mobility diameter in the size range from 50 to 300 nm was reported by Boskovic et al [10,11]. Lower efficiency of non-spherical (cubic) particle compared to the spherical one was ascribed to some difference in rebound probability during particle motion at the fiber surface right after collision.…”

“…In both cases, air drying was performed in a 1 m long home made diffusion dryer filled with silica gel. The production of loose particles was done according to [10]. In brief, before reaching the nebulizer, the air was passed through HEPA filter to remove all extraneous aerosols entering the system and thus to ensure clean incoming air.…”