Toshihiko Myojo

Removal of aerosols by bubbling through porous media

Aerosols can be filtered on fibrous filters with or without the pres ence of circulating water. The wet filtration leads to the formation of bubbles within the filter as the carrier gas passes through. This provides alternate mechanisms for the removal of aerosols. Experiments are described to investigate the effects of the irrigating fluid and to compare the efficiency of wet and dry filtration. The results indicate a marked increase in efficiency of filtration of the wet filter as compared with the dry filter.

Influence of Particle Shape on Filtration Processes

The influence of particle shape on filtration processes was investigated. Two types of particles, including spherical polystyrene latex (PSL) and iron oxide, and perfect cubes of magnesium oxide, were examined. It was found that the removal efficiency of spherical particles on fibrous filters is very similar for corresponding sizes within the range of 50–300 nm, regardless of the fact that the densities of PSL and iron oxide differ by a factor of five. On the other hand, the removal efficiency of magnesium oxide cubic particles was measured, and found to be much lower than the removal efficiency for the aerodynamically similar spheres. Such disparity was ascribed to the different nature of the motion of the spherical and cubic particles along the fiber surface, following the initial collision. After touching the fiber surface and before coming to rest, the spherical particles could either slide or roll compared to the cubic ones, which could either slide or tumble. During tumbling, the area of contact between the particle and the fiber changes significantly, thus affecting the bounce probability, whilst for the spheres, the area of contact remains the same for any point of the particle trajectory. The extra probability of particle bounce by the cubes was derived from the experimental data. The particle kinetic energy was proposed to be responsible for the difference in removal efficiency of particles with alternative shapes, if all other process parameters remain the same. The increase in kinetic energy is shown to favor the increase of the bounce probability.

Comparative study of the performance of nine filters utilized in filtration of aerosols by bubbling

A new process of air purification has been developed and studied experimentally (Agranovski et al. 1999). This process is based on passing air through a filter immersed in liquid. It leads to the formation of bubbles within the filter as the carrier gas passes through and thus provides alternate mechanisms for the removal of aerosols. It was identified (Agranovski et al. 1999) that for relatively large particles (0.3-2.7 w m) the efficiency of the filter utilized in wet filtration is higher than for dry filtration. In the current paper, experiments are described that investigate the performance of the technology for particles in the size range of 0.03 to 2.7 w m, to identify the importance of an alternative mechanism of purification (diffusion). Also, 9 different filters were employed to compare the efficiencies of the wet and dry filtration for different applications of the technology.

A simple method to determine the length distribution of fibrous aerosols

We present a new method for determining the length distribution of fibrous aerosols based on the theoretical finding that penetration of fibrous particles through a set of wire mesh screens depends only on the ratio between fiber length and the opening size of the mesh. The validity of the method is supported by both Monte Carlo simulation of particle motion through a set of mesh screens and experimental results. When a log-normal distribution is assumed for the fiber length of a fibrous aerosol, this method is capable of indicating a count median length (CML) range of 5-80 mu m using data on penetration through each set of 4 wire mesh screens: 100, 200, 325, 500, 635 mesh and 10 mu m screen. The method is less sensitive for the geometric standard deviation sigma g of fiber length distribution than for CML.

Combined Wettable/Non-wettable Filter for Mist Purification

Previous studies by Agranovski and Braddock [1] show that wettable filters are very efficient at collecting small liquid aerosols. However, a major problem associated with wettable filters is the breakthrough process associated with the destruction of film covering the fibres, and the removal of relatively large droplets by the carrier gas passing through the rear surface of the filter. A non-wettable filter is relatively efficient at collecting large droplets and discarding them down its front face [2]. These two types of filters can be combined in layers to make use of these distinctive properties. The best arrangement is where the carrier gas passes first through the wettable filter, and then through the non-wettable filter. Where there is a contact between the two filters, interface effects assist the filtration and filter system drainage. The filtration mechanisms are enhanced when the filter box is tilted so that gravity partially opposes the drag forces on the liquid in the filter. In the current paper, the results of experimental analysis of combined filtration systems are presented, and further steps towards industrial design are discussed.

Inclined Wettable Filter for Mist Purification.

Agranovski and Braddock [AIChE J. 4 (1998) 2784] investigated a process of filtration of ultra small liquid particles on wettable fibrous filters. They found that liquid droplets captured by a wettable filter spread along the fibres and create a thin liquid film covering each fibre. These films of liquid establish a self-draining, tapered equilibrium flow down the filter. The thickness of the liquid film is an important parameter altering the physical characteristics and performance of the filter. The properties of the film depend on the density, viscosity and amount of liquid present, the physical parameters, wettability and dimensions of the filter and also on the orientation and angle of inclination of the filter. The thickness of the liquid film can be increased by increasing the angle of inclination of the filter and by selecting the orientation. It leads to the possibility of increasing the efficiency of wet filtration without any increase in the amount of irrigating liquid involved. It is especially important for industries where, due to some technological or economical reasons, the amount of fresh irrigating liquid available for the process, is limited. In the current paper, the results of theoretical and experimental analysis of inclined wettable filtration systems are presented and further steps towards industrial design are discussed.

Study of wet porous filtration

Abstract A growing demand for efficient air quality management calls for the development of technologies capable of meeting the stringent requirements now being applied in areas of chemical, biological and medical activities. Currently, filtration is the most effective process available for removal of fine particles from carrier gases. Purification of gaseous pollutants is associated with adsorption, absorption and incineration. In this paper we discuss a new technique for highly efficient simultaneous purification of gaseous and particulate pollutants from carrier gases, and investigate the utilization of Nuclear Magnetic Resonance (NMR) imaging for the study of the dynamic processes associated with gas–liquid flow in porous media. Our technique involves the passage of contaminated carrier gases through a porous medium submerged into a liquid, leading to the formation of narrow and tortuous pathways through the medium. The wet walls of these pathways result in outstanding purification of gaseous, liquid and solid alien additives. NMR imaging was successfully used to map the gas pathways inside the porous medium submerged into the liquid layer.

Removal of Aerosols by Bubbling through Porous Media Submerged in Organic Liquid.

Aerosols can be filtered by passing the carrier gas through a fibrous filter immersed in water (Agranovski, I. E., Myojo, T., & Braddock, R. D. (1999a). Bubble filtering through porous media. Aerosol Science and Technology, 31, 249-257. Using water as the irrigating fluid significantly increases the efficiency of filtration of particles and adds the possibility for simultaneous removal of alien gases from the carrier. Organic compounds (gaseous and particulate) form a significant proportion of pollutants in the atmosphere, and effective purification is needed for ambient air as well as for cleaning exhaust streams. Water does not have a high level of solubility for gaseous organic compounds, and alternative irrigating liquids need to be considered. Experiments were conducted using sunflower oil as the irrigating fluid. The filtration efficiencies of the oil are better than for water, for liquid di-ethyl-hexyl-sebacate particles. As the solubility of organic vapours is much higher in oil compared with the one in water, oil provides an excellent opportunity for utilizing as the irrigating liquid for high-efficient simultaneous removal of organic particles and vapours from air carrier.

Model for gas-liquid flow through wet porous medium

A method involving bubbling of air through a fibrous filter immersed in water has recently been investigated (Agranovski et al. [1]). Experimental results showed that the removal efficiency for ultra-fine aerosols by such filters was greatly increased compared to dry filters. Nuclear Magnetic Resonance (NMR) imaging was used to examine the wet filter and to determine the nature of the gas flow inside the filter (Agranovski et al. [2]). It was found that tortuous preferential pathways (or flow tubes) develop within the filter through which the air flows and the distribution of air and water inside the porous medium has been investigated. The aim of this paper is to investigate the geometry of the pathways and to make estimates of the flow velocities and particle removal efficiency in such pathways. A mathematical model of the flow of air along the preferred pathways has been developed and verified experimentally. Even for the highest realistic gas velocity the flow field was essentially laminar (Re approximate to 250). We solved Laplaces equation for stream function to map trajectories of particles and gas molecules to investigate the possibility of their removal from the carrier.

Study of the gas flow in porous media submerged in liquid layer

The collection mechanism of charged aerosols by a two-dimensional array of oppositely charged drops is conisdered. Trajectories of aerosols are computed, using a new simulation program, under conditions of gravity sedimentation of the collecting drops and aerosols. Electric forces, given in spherical coordinate system of each drop, are projected on the spherical coordinate system of the central drop. The equation of motion is solved in this central system using creeping flow approximation and the sum of all projected electric forces. Analysis of deviations of streamlines, resulting from the flow approximations, is provided. It is shown that for drop spacing larger than 10 diameters, a deviation of less than 3% is expected at the symmetry axis between the drops. The charges of the collecting drops and aerosols were set in the range below the Rayleigh limit and in accordance with constraints imposed by the charging mechanism. Collection radius, collection zone, and related efficiencies, are defined for the case of a single drop, and array of drops. In the case of an array of drops, these parameters are defined with respect to the number of rows that produce the given level of collection. Small, low inertia aerosols can be characterized by a monotonic relation between the collection radius and the number of rows. Larger and higher inertia aerosols exhibit multiple radii of collection for the same number of rows. Radius and efficiencies of collection are computed for different spacings of the collecting drops, and at different operating conditions. In this context computed data, of minimum number of rows that are required for complete collection, is displayed. Plots of the simulation program disclose the intricate mechanism whereby aerosol trajectories are progressively filtered out within the array. The results of this work show that collection of charged aerosols, by an array of oppositely charged drops, under the conditions of gravity sedimentation, can be highly efficient.