Recent advances in the understanding of fibrous filter behaviour under solid particle load

doi:10.1016/S0015-1882(97)84316-9

Filtration & Separation

Volume 33, Issue 6, June 1996, Pages 501-506

https://doi.org/10.1016/S0015-1882(97)84316-9 Get rights and content

Abstract

Fibrous filters are a well established technology for the removal of fine particulates (<10 μm) from gas flows at high efficiency and low pressure drop. The behaviour of these materials in their clean state is well understood, but their behaviour as they become loaded with particulates has received surprisingly little attention; especially when one considers that a fibrous filter is in its clean state for only the tiniest fraction of its lifetime. At the Loughborough University of Technology a number of projects have been, and are being, undertaken where the dynamic characteris of fibrous filters are studied. The paper reports on recent advances in this field made at Loughborough, and elsewhere, and suggests some futher studies required.

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Due to growing concern about air pollution and its harmful effects on the health of the population, especially in regard to sub-micrometric particles, some studies have reported that applying an electric field to particle suspensions can improve filter performance by enhancing the deposition of particles in the filter medium. This can result in better particulate retention, which is particularly important for industrial processes such as cement production. The objective of this study was to investigate the behavior of cement particles with electrostatic charges during cake formation in fabric filters. The particles (with a d50 % of 17 μm) were generated using a dust feeder at a flow rate of 0.083 kg s⁻¹. The fiberglass filter medium was subjected to filtration tests with constant dust concentrations (9–12 g.m⁻³) and air surface velocities (6 cm.s⁻¹and 10 cm s⁻¹) until the pressure drop reached the maximum value of 400 Pa. The electrostatic precipitator utilized discharge voltages of 0, 4, 10, and 12 kV. The particles were initially passed through the electrostatic precipitator to become charged with voltages of 0, 4, 10, and 12 kV applied. The results indicated a reduction in pressure drop of up to 55 %. The study observed a change in the deposition behavior of particles on the filter medium surface and in the filter cake formation, demonstrating that the electrostatic charge improves air filtration performance, resulting in higher efficiency and cost-effectiveness.
Influence of pre-stage filter selection and face velocity on the loading characteristics of a two-stage filtration system
2019, Separation and Purification Technology
Citation Excerpt :
Furthermore, pre-filters are typically inexpensive and easy to install [6–9]. Therefore, in order to reduce particles loading on the high efficiency filter, extend their service lifetime and reduce the cost of replacing them, two-stage filtration systems which consist of a pre-filter on the pre-stage to remove large particles and a more efficient main-filter on the main-stage to capture finer particles below 1 µm have been designed and widely used in modern filtration systems [10–15]. It was found that many factors could influence the loading performance of filter, such as the filter types, the distribution of particles, filtration face velocity and so on [12,16–21].
A two-stage filtration test system was successfully developed previously, and the objective of this study is to investigate the effect of pre-filter selection and face velocity of pre-stage and main-stage on the loading characteristics of a two-stage filtration system. Three different types of pre-stage filter media with different capture efficiency, i.e., G3, G4 and M5 pre-filters, were selected as filter type variables. Three different face velocities on pre-stage, i.e., 26.3, 45.0 and 65.2 cm·s⁻¹, and three different face velocities on main-stage, i.e., 2.1, 3.6, 5.2 cm·s⁻¹ were chosen as face velocity variables. Self-defined indexing parameters were used to analyze the effect of pre-filter in this study. The experimental results indicated that although M5 pre-filter with the highest capture efficiency presents the best protection to mitigate the pressure drop growth of main-filter, the pressure drop of M5 pre-filter itself increases the fastest, resulting in the shortest effective protecting time of M5 pre-filter in front of a main-stage filter, so the overall effectiveness of M5 pre-filter fades out due to the limited protection period. On the other hand, G4 pre-filter can run for a relative longer time while maintaining the pressure drop of the whole two-stage filtration system to increase relatively slowly. G4 pre-filter achieves highest T^** value, outperforming the lowest efficient G3 pre-filter by its higher efficiency and the highest efficient M5 pre-filter by longer effective protection period. With the decrease of face velocity on either the pre-stage or the main-stage, the effectiveness of pre-filter on main-filter and the whole two-stage filtration system improves, by slowing down the pressure drop increasing rate of main-stage filter, which further contributes to extend the service lifetime of the main-filter and increase dust holding capacity of the whole filtration system. Besides, the specific resistance k₂ of the dust cake formed on main-filter were found deeply affected by the main-stage face velocity. With the face velocity on the main-stage increased, the specific cake resistance k₂ increased obviously, which implies that the dusts structure on the main-filter changed and became more compacted under higher main-stage filtration velocity. To better illustrate the relatively importance between pre-filter grade and filtration velocity selection, the analyzed performance indexing parameters were further arranged in a set of contour plots, which unveils the predominance of pre-filter grade over filtration velocity within the variation ranges tested in current study.
Particle loading characteristics of a two-stage filtration system
2019, Separation and Purification Technology
A two-stage filtration testing system, consisting of a pre-stage filter and a main-stage filter was successfully developed, which proves capable of being used to investigate the loading characteristics of a two-stage filtration system. A G4 filter media was selected as pre-stage media sample while an E11 filter media was tested as main-stage sample. Bi-modal aerosols consisting of both super-micrometer A2 coarse dusts and sub-micrometer KCl fine particles with varied mixing ratios were employed to challenge the two-stage filtration system to investigate the influence of aerosol size distributions. Two sets of self-defined performance-based indexing parameters were introduced to quantify the effectiveness of the pre-stage filter and simplify the comparison among various test conditions. It was found installing a pre-filter in front of a main-filter can improve the performances of the entire filtration system, by extending the service lifetime of the main-stage filter and increasing the dust holding capacity of the whole filtration system. Moreover, the effectiveness of pre-stage filter was found to be strongly influenced by the particles size distribution: the protection by a pre-stage filter is more pronounced for large particles and tends to fade out with increasing fraction of fine particles. The specific cake resistance K₂ of the main-stage filter was found to increase with increasing fraction of fine particles, and was higher at the presence of a pre-stage filter than the corresponding reference case without a pre-filter.
Filtration of Solid Aerosols
2017, Aerosol Filtration
During filtration, the collection of particles by the filter results in an increase in the pressure drop across the filter. For solid aerosols, this evolution, based on filtration time or the mass of the collected particles, can typically be divided into three phases. The duration of each phase is based on the filter media, the aerosol and operating conditions. Each phase is separated by more or less clearly marked transition zones.
The effect of a drainage layer on filtration performance of coalescing filters
2016, Separation and Purification Technology
Citation Excerpt :
However, when a filter is clogged with aerosol particles, its filtration behavior will change significantly. A number of works have been conducted to investigate gas-solid filtration [5–7]. In the case of gas-liquid filtration, especially for the coalescence filtration, the filtration mechanism requires further investigation, because the droplets migration process in the filter material is very complex and is affected by a variety of parameters.
The effect of a drainage layer on the filtration performance of coalescing filters was evaluated experimentally. The “jump-and-channel” model was used to analyze saturation, pressure drop and efficiency of the filters with and without the drainage layer. The results showed that jump pressure drop dominated the wet pressure drop. The steady-state pressure drop was not proportional to filter saturation which was related primarily to the channel pressure drop. A liquid film was present between coalescing layer and drainage layer at steady state. The jump pressure drop was affected by the thickness of this liquid film. A thicker liquid film produced a higher jump pressure drop. When the drainage layer was assembled outside of the coalescing layer, pressure drop and penetration changed, and both saturation and efficiency increased significantly. A filter with a polyester drainage layer had the overall best performance. Furthermore, the dynamic thickness of the liquid film can be predicted using theoretical calculations, which allows qualitative analysis of the effect of the liquid film on filtration performance.
Pressure drop and liquid transport through coalescence filter media used for oil mist filtration
2014, International Journal of Multiphase Flow
Citation Excerpt :
As long as a filter is dry, this knowledge can be used to estimate its pressure drop and efficiency of aerosol deposition, regardless of whether the arriving particles are solid or liquid. However, once aerosol has accumulated to an appreciable degree, a filter will behave very differently with respect to the two types of aerosol: While solid “dust” remains on the fibers (e.g. Kasper et al., 2009, 2010) until the filter clogs and forms a cake (Walsh, 1996), liquid accumulating in a coalescence filter does not stay where it is first deposited due to forces such as gravity of friction from the air flow. These subsequent processes of liquid redistribution and transport inside the media are complex and still poorly understood, in some regards not even qualitatively, because they depend on numerous parameters such as media structure, fiber wettability and capillary effects, fluid viscosity, as well as the interaction of the coalesced liquid with the air flow.
A phenomenological model is presented to explain the increase in pressure drop (Δp) of air filters during steady operation with oil mist. It is based on (currently) semi-quantitative conclusions obtained from measurements of liquid distribution patterns in the media associated with the transport of coalesced liquid by the flowing air. Correlation of these patterns in space and time with the evolution of the pressure drop suggests that the over-all increase in Δp (the “wet” pressure drop) is governed by two distinctly different liquid transport mechanisms:
A steep Δp jump is required to overcome the capillary exit (or entry) pressure and pump liquid into non-wettable, or out of wettable fibrous matrices. It is associated with the formation of a thin liquid film covering almost the entire front (or rear) face of the respective media. With the help of a polymerization technique to “freeze” the liquid distribution, the film is shown to be confined to the outermost surface without entering the media while the aerosol flow is on.
Liquid transport inside the media is shown to occur in multiple parallel channels spanning almost the entire thickness of a filter. The channel Δp associated with this transport mechanism increases linearly with media thickness. Wettable media form numerous fine channels which feed a liquid film on the rear face by which drainage takes place. Non-wettable media form fewer, relatively wide channels ending in large drops on the rear face, through which drainage takes place during steady operation.
Sandwiched combinations of wettable and non-wettable media show the same combination of features in their respective Δp curves. There are separate Δp jumps and channel regions for each media type. In case of a transition from wettable to non-wettable media, the combined exit and entry Δp jump takes place at the internal interface.

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Recent advances in the understanding of fibrous filter behaviour under solid particle load

Abstract

Air filtration

Experimental studies of electrically active fibrous filter loading

Part. Part. Syst. Charact.

The clogging of fibrous filters with monodisperse aerosol

The loading of fibrous filters with sub-micron particles

J. Aerosol Sci.

Experimental study of the clogging of a fibrous filter

The effect of solid aerosol on prefilter material performance

J. Aerosol Sci.

Life-loading tests on certain filter media

Chainlike formation of particle deposits in fluid-particle separation

Science

Deposition of solid particles on a collector; formulation of a new theory

AIChE J.