D. Leclerc

Clogging of fibrous filters by solid aerosol particles Experimental and modelling study

A fibrous filter is a common cleaning device often used to remove particles from industrial gas streams. The main question which often arises concerns the evolution of the pressure drop and the filtration efficiency during the filter clogging. In the present study, the loading characteristics of HEPA filters have been studied experimentally. The increase of pressure drop and filter efficiency was measured and was linked to both the penetration profile inside the filter bed and the deposit structure observed thanks to scanning electron micrograph. We have also studied the influence of various parameters such as air velocity, particle size, aerosol concentration and filter main characteristics. A depth and surface filtration model has been developed based on the distinction between the fibres of the filter and deposited particles resulting in additional fibres inside the filter or on the filter surface. We can notice a good agreement between model and experiment. Moreover, model very well describes the transition area between depth filtration and cake filtration. This transition from one type of filtration to another is a continuous process. Model describes also the exponential decrease of penetration profile.

Modelling pressure drop in HEPA filters during dynamic filtration

A new model has been developed to predict the clogging behaviour of high-efficiency particulate air filters. The model takes into account the heterogeneous deposit of particles inside the filter medium. The filter is considered to be a series of elementary slices which are assumed to be homogeneously loaded by aerosol. Particles are assumed to form dendrites which can be considered as newly formed fibres. For each time increment and for each slice, particle collection efficiency of initial and new fibres is calculated according to the Payet model in order to give an overall prediction of the mass of aerosol deposited and the pressure drop. Model results are compared with experimental results.

Evolutions of efficiency and pressure drop of filter media during clogging and cleaning cycles

Abstract The collection and cleaning cycles of the filter media used in industrial air cleaning were analysed experimentally. Examination of the simultaneous pressure drop and filtration efficiency changes established that the efficiency of filters increases sharply during the collection phase only to fall back during cleaning. The amplitude of this reduction of efficiency is attained as the cycles progress. In contrast, the residual pressure drop rises. Our study has shown that treating the surface with thermobonding or covering the surface with a fine layer of submicronic fibres promotes detachment of the dust cake and prevents particle penetration inside the initial porous media.

Optimization of a discontinuous microfiltration-backwash process

Abstract Microfiltration membranes can be used for the dead-end filtration of suspensions of fine particles, with a periodic backwash to remove the cake and restore filtrate flux. The optimum operation of such a two-stage discontinuous operation is determined by using cake filtration theory and assuming that the backwash period depends on the geometry of the filter module. This allows identification of the conditions where such a process can be used. The case where flux restoration by backwashing is imperfect due to progressive internal fouling of the membrane is also treated and allows the periodicity of chemical cleaning to be determined as a function of the degree of fouling. Finally, mention is made of the possible extension of the theory to the case of cross-flow filtration.

Description of the clogging and cleaning cycles of filter media

The filter media used in industrial air cleaning, if they are regenerable, are subjected to clogging and cleaning cycles. Their performance is closely linked to these two operating phases. A model of the phenomena based on the assumption of patchy cleaning is presented. The aim of the study was to determine the pattern of a parameter representative of cleaning, namely the cleaned fraction, from experimental values of the residual pressure drop of a filter medium subjected to given filtration conditions. This characterisation then allows the pressure drop pattern of the filter medium over a number of cycles to be determined. A comparison was made with the experimental results obtained on a laboratory test rig. Close agreement was observed, which confirms the hypothesis of patchy cleaning in this approach to describe the filtration and cleaning phenomena of a filter medium.

Flow and diffusion of particles in a channel with one porous wall: Polarization chromatography

Abstract An analysis is made of the behaviour of a suspension of particles in flow in a rectangular channel with one porous wall, which is the configuration used in a technique for particle chromatography and for cross-flow filtration. General expressions are established for the axial and transversal velocity profiles which reduce to previously published simplified relationships when the channel is short and the membrane permeability is low. An asymptotic solution is given for the diffusion convection equation in this system. Experimental results for the chromatographic separation of latex particles and dextran macromolecules show that the diffusion regime is attained at low flow rates. At higher flow rates the experimental results correspond to a convection regime and at intermediate flow rates we obtain a double peak responses corresponding to both regimes.

Clogging of HEPA fibrous filters by solid and liquid aerosol particles: An experimental study

Abstract A fibrous filter is a common cleaning device often used to remove particles from industrial gas streams. A fundamental question concerns the evolution of the filter performance under aerosol load and the prediction of its life time. The purpose of this paper is to illuminate some of the issues involved, in presenting our experiments of solid and liquid particle filtration. The evolution of pressure drop of HEPA filters is described in both cases and the deposit structure is analysed as precisely as possible.

Filtration of solid and liquid aerosol mixtures: Pressure drop evolution and influence of solid/liquid ratio

Dust and droplet filtration is an essential process step in many industries. Although many studies have been dedicated to the mechanisms of dust filtration, little is known about the behaviour of an air filter when challenged with solid and liquid particles at the same time. This research article answers questions about the pressure drop during the filtration of a solid/liquid particle mixture, as well as on the influence of the respective solid and liquid particle concentrations.

Changes in the Performances of Filter Media During Clogging and Cleaning Cycles

The performance of two filter media used in industrial air cleaning were studied both in the initial state (new filter) and after a number of collection and pulse pressure cleaning cycles. The main difference between them is that one has anti-clogging properties and the other does not. The test aerosol is composed of alumina particles with a median volumetric diameter of 2.6 microm (MMAD=4.8 microm) generated at a concentration of 700 mg x m(-3). Filtration took place at a velocity of 2 cm x s(-1). Two parameters, namely pressure drop and efficiency, were monitored according to the collection and cleaning cycles. The comparison of the filtration efficiency of the two media and that of the corresponding industrial dust separator at the end of the cycles showed a close agreement. The separation efficiency calculated with a new medium (corresponding to initial switch-on of the installation) was low and increased very quickly during the cycles. Finally, a phenomenological model was developed to represent the increase in pressure drop of a filter medium after cleaning and was found to be in close agreement with the experimental values.

A technique for the study of the fouling of microfiltration membranes using two membranes in series

This paper describes a technique for investigating the fouling of microfiltration membranes by measuring the pressure drops across two membranes fed in series by a constant rate pump, which enables a distinction to be made between surface fouling and internal fouling of the membrane. In the case of the microfiltration of BSA solutions, the technique shows how the type of pump and the operating temperature influence membrane fouling and how protein denaturation and adsorption give rise to different types of fouling. The technique is also used to investigate the microfiltration of beer and shows how this is affected by membrane properties and the aging of beer.