A study of inertial effects in granular bed filtration

Filtration of airborne particles by a trickle granular bed: a modelling approach

The removal of airborne particles was investigated using a trickle granular bed. This filtration process maintains a constant pressure drop during particles loading, which makes it an interesting alternative for aerosol filtration. In order to establish mathematical models for the design of the process, a modelling approach that takes into account the changes of the bed characteristics due to the liquid hold up is undertaken. This approach allows predicting the pressure drop and the filtration efficiency of the trickle granular bed, based on the assumption that the bed porosity decreases and the collectors diameter increases with increasing the liquid flow rate. The model predictions are in agreement with experimental data. For particles with diameters around 100 nm the collection efficiency presents a minimum. For smaller particles the collection efficiency increases as the dominating collection mechanism is Brownian diffusion. In the micronic range the collection is governed by inertial and interception mechanisms and consequently increases with increasing the particle size.

Extended Model for Filtration in Gasoline Particulate Filters under Practical Driving Conditions

In order to reliably predict the particle number filtration of gasoline particulate filters (GPF) under practical driving conditions, an extension to established filtration models is developed. For the validation of this approach and in order to close a gap of available measurement data at high space velocity in the literature, the particle-size-resolved fresh filtration efficiency of seven different cordierite filters is determined experimentally. Moreover, the experiments on a dynamic engine test bench focus on the impact of the pore-size distribution and the filter wall thickness under steady-state as well as transient, cold-start conditions. In order to model all trends observed, a new correlation for the particle collection due to inertial deposition is proposed and embedded in a heterogeneous multiscale model framework for a GPF. The presented approach can predict all trends observed in the measurements, including a stabilization of the filtration efficiency with increasing space velocities above a certain level. A comparison of several modeling approaches reveals the partly different behaviors at varying space velocities for the here presented model as well as for established filtration models.

Experimental study of dust deposition in dynamic granular filters

The aim of the present study is to investigate the process of dust deposition and its effects on the filtration performance of dynamic granular filters. We proposed a new theoretical explanation about the model for dust cake formation and growth, especially for the compression model at higher filtration superficial velocities in dynamic granular filters. The thickness and porosity of dust cakes can be estimated by formulas. Then, the effects of cake formation and growth on filtration performance were examined. It is found that there is an optimum thickness of the cake at which the dynamic granular filter achieves excellent collection efficiency with low system resistance. Moreover, an increasing pattern of pressure drop with cake thickness under different filtration superficial velocities was observed. Experimental results show that the pressure drops across the filter system and dust cake increase exponentially with increasing filtration superficial velocity. An appropriate filtration superficial velocity should be considered to achieve optimum filtration performance in dynamic granular filters. It is expected that the results of this study could provide useful information on designing dynamic granular filters for removal of dust particulates in Integrated Gasification Combined-Cycle (IGCC) and Pressurized Fluidized-Bed Combustion (PFBC).

PDF-based heterogeneous multiscale filtration model

Motivated by modeling of gasoline particulate filters (GPFs), a probability density function (PDF) based heterogeneous multiscale filtration (HMF) model is developed to calculate filtration efficiency of clean particulate filters. A new methodology based on statistical theory and classic filtration theory is developed in the HMF model. Based on the analysis of experimental porosimetry data, a pore size probability density function is introduced to represent heterogeneity and multiscale characteristics of the porous wall. The filtration efficiency of a filter can be calculated as the sum of the contributions of individual collectors. The resulting HMF model overcomes the limitations of classic mean filtration models which rely on tuning of the mean collector size. Sensitivity analysis shows that the HMF model recovers the classical mean model when the pore size variance is very small. The HMF model is validated by fundamental filtration experimental data from different scales of filter samples. The model shows a good agreement with experimental data at various operating conditions. The effects of the microstructure of filters on filtration efficiency as well as the most penetrating particle size are correctly predicted by the model.

A Granular Bed for Use in a Nanoparticle Respiratory Deposition Sampler

A granular bed was designed to collect nanoparticles as an alternative to nylon mesh screens for use in a nanoparticle respiratory deposition (NRD) sampler. The granular bed consisted of five layers in series: a coarse mesh, a large-bead layer, a small-bead layer, a second large-bead layer, and a second coarse mesh. The bed was designed to primarily collect particles in the small-bead layer, with the coarse mesh and large-bead layers designed to hold the collection layer in position. The collection efficiency of the granular bed was measured for varying depths of the small-bead layer and for test particles with different shape (cuboid, salt particles; and fractal, and stainless steel and welding particles). Experimental measurements of collection efficiency were compared to estimates of efficiency from theory and to the nanoparticulate matter (NPM) criterion, which was established to reflect the total deposition in the human respiratory system for particles smaller than 300 nm. The shape of the collection efficiency curve for the granular bed was similar to the NPM criterion in these experiments. The collection efficiency increased with increasing depth of the small-bead layer: the particle size associated with 50% collection efficiency, d50, for salt particles was 25 nm for a depth of 2.2 mm, 35 nm for 3.2 mm, and 45 nm for 4.3 mm. The best-fit to the NPM criterion was found for the bed with a small-bead layer of 3.2 mm. Compared to cubic salt particles, the collection efficiency was higher for fractal-shaped particles larger than 50 nm, presumably due to increased interception.

A correlation for the collector efficiency of Brownian particles in clean-bed filtration in sphere packings by a Lattice-Boltzmann method

In this paper, we develop a new correlation for the clean-bed filter coefficient (lambda0) for Brownian particles, for which diffusion is the main deposition mechanism. The correlation is based on numerical Lattice-Boltzmann (LB) simulations in random packings of spheres of uniform diameter. We use LB methods to solve the Navier-Stokes equation for flow and then the advection-diffusion equation for particle transport. We determine a correlation for an "equivalent" single-collector diffusion efficiency, etaD, so that we can compare our predictions to "true" single-collector correlations stemming from unit-cell modeling approaches. We compared our new correlation to experiments on the filtration of latex particles. For small particle diameters, 50 nm < dp < 300 nm, when gravity and interception are negligible, our correlation for etaD predicts measurements better than unit-cell correlations, which overestimate etaD. The good agreement suggests that the representation of three-dimensional transport pathways in porous media plays an important role when modeling transport and deposition of Brownian particles. To model larger particles, for which gravity and interception are important too, we build a correlation for the overall single-collector efficiency eta0 by adding corresponding etaG and nI terms from unit-cell correlations to our etaD model. The resulting correlation predicts experiments with latex particles of dp > 300 nm well.

Filtration of dust in a circulating granular bed filter with conical louver plates (CGBF-CLPs)

A novel circulating granular bed filter with conical louver plates (CGBF-CLPs) was designed to remove dust particulates from the flue gas stream of a coal power plant. The purpose of this investigation was to evaluate the performance of the CGBF-CLPs. Dust collection efficiency and pressure drop data were analyzed to determine better operating conditions. The effect of solid mass flow rate, collector particle size and dust/collector particles separator types on the dust collection efficiency and pressure drop in the CGBF-CLPs were investigated in this study. The solid mass flow rate (B) varied from 15.59+/-0.44 to 20.36+/-0.68 g s(-1) and the initial average collector particle sizes were 1500 and 795 microm, respectively. Two types of separators, a cyclone and an inertial one, for separating the dust and collector particles were used in the CGBF-CLPs system. An Air Personal Sampler (SKC PCXR8) was used to determine the inlet and outlet dust concentrations. A differential pressure transmitter and data acquisition system were used to measure the pressure drop. Experimental results showed that the highest dust collection efficiency was 99.59% when the solid mass flow rate was 17.08+/-0.48 g s(-1) and the initial average collector particle size was 795 microm with the cyclone type separator. The results showed that the attrition fines of the original collector particles returning to the granular bed filter (GBF) reduced bed voidage. This phenomenon significantly increased the dust collection efficiency in the CGBF-CLPs. As a consequence, a bigger bed voidage creates a lower dust collection efficiency in the GBF.