Chapter eight - Electret filter for depth and cake filtration

Abstract

Recent developments on electrostatically charged (electret) filters made of polyvinylidene fluoride or polyvinylidene difluoride (PVDF) nanofibers are presented. The charged PVDF nanofibers are produced by electrospinning followed by charging the fibers using corona discharge. The capture of neutrally charged aerosol is by dielectrophoresis, wherein a dipole (a pair of positive and negative charges) is first induced on the incoming neutrally charged aerosol and this is followed by attraction between the opposite charges of the dipole with the charged nanofibers. PVDF nanofiber filters in both uncharged and electrostatically charged states are compared. Factoring out the filter thickness, the single-fiber efficiency for dielectrophoresis is a good means for evaluating the capture efficacy of the filter. Test results on charged nanofiber filter have shown that the single-fiber efficiency for dielectrophoresis is related to a governing parameter raised to the power of 0.4–0.5. This is close to 0.4 as suggested by the classical theory. The parameter depends on the product of the Cunningham slip factor and aerosol size raised to the quadratic power divided by the product of the face velocity and fiber diameter. The latter implies that nanofiber filter can harness electrostatic effect better than microfiber filter, which has been demonstrated both theoretically and experimentally. Also lower face velocity allows longer retention time which enhances the electrical capture, and vice versa for higher face velocity. For a given basis weight of fibers in the PVDF filter, the difference in behavior between single-module/layer versus multimodule/layer configuration is illustrated. By multilayering the electrical interference between charged nanofibers on the incoming aerosols is reduced, thus the efficiency can be restored to a high value, which is most favorable for large aerosols with stronger induced dipoles. The multimodule filter configuration also reduces pressure drop which is a great advantage. The charges in the filter can stay for over 3 months with minimum charge decay, which is an important property for charged filters.