(128a) Filtration of Nanoparticles: Evolution of Cake-Structure and Pressure Build-up

Filtering a nanoparticle-laden gas through a porous substrate results in the formation of a homogeneous and highly porous film (Andersen et al., 2002). The physical properties of that film are determined by the Peclet number (Pe), the ratio of the convective to the diffusive transport. Porous ceramic films have several applications such as in membrane filters (Andersen et al., 2002), catalysis (Thybo et al., 2004), fuel cells (Chakraborty et al., 2005), and gas sensors (Mädler et al., 2006).

The detailed accumulation of particle deposits and the build-up of pressure-drop during filtration of monodisperse nanoparticles (diameter d_p = 50 nm) through capillaries (1 ? 4 m radius) was investigated by Langevin dynamics (LD) (Meakin, 1983), at Pe = 0.01 ? 10. Three distinct deposition regimes were identified. First capillary deposition: particles deposited primarily inside the substrate capillary. Second capillary clogging: the deposited particles near the inlet rim of the capillary shaded off incoming particles, which led to clogging. Third cake growth: filtration occurred on top of already deposited particles, leading to the formation of a cake. Whereas the solid volume fraction profile (_sd) through the clog was anisotropic, it was constant in the cake (_sd,c). This, along with clogging time (t_cl), allowed for the use of classic cake filtration theory in the calculation of the pressure-drop build-up.

Until the time of clogging, when perfect filtration had not yet commenced, 23 - 37 % of the total mass filtered had penetrated through the capillary at Pe = 1 ? 10. At low Pe, the structure of these deposits was fractal-like, while at higher Pe they formed a conical-dome upstream of the capillary inlet. The pressure-drop through the deposits formed at high Pe (ballistic deposition) therefore increased significantly in comparison to those formed at lower Pe. Therefore, as the significant part of the pressure-drop increase occurred during cake growth, the clogging time and the asymptotic cake solid volume fraction shown in Figure 1 are two important parameters for design and operation of filtration units.

Figure 1: Constant solid volume fraction (_sd,c) and clogging time for filtration of 50 nm particles at a concentration of 10¹⁴ #/m³ in a 2 m (radius) capillary as a function of the Pe number and comparison with literature (Rodríguez-Peréz et al. (2005)) for flat surface deposition.

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