Parameter study and optimization on filtration and resistance characteristics of granular bed filter

In this paper, experimental studies were carried out to investigate the filtration and resistance characteristics of a fixed granular bed filter (GBF) during the process of dust particles removal for hot industrial flue gas. The effects of geometric and operating parameters (including filtration superficial velocity, granules size, bed height, dust particles concentration and flue gas temperature) were examined according to the orthogonal array design method. The experimental results show that the filtration superficial velocity has the most significant effect on filtration efficiency, which is followed by granules size, bed height, flue gas temperature and dust particles concentration respectively; for pressure drop, the order of the factors are granules size, filtration superficial velocity, bed height, flue gas temperature and dust particles concentration respectively. Based on the experimental results, correlations of filtration efficiency and pressure drop with these parameters were established, which have good prediction accuracy with most of the deviations within 15%. The optimized parameters combinations of the maximum filtration efficiency and the minimum pressure drop of the GBF were gained which agree well with the experimental results. The correlations are significant for design and evaluation of GBF in practical applications.     

Observation of dust release behavior from ceramic filter elements

The dust-releasing behavior from a ceramic candle filter, which is a key technology of the hot gas cleaning system in advanced coal power generation processes such as pressurized fluidized bed combustion and integrated gasification combined cycle, is discussed based on the observation of the dust-releasing process using a high-speed video camera and the measurement of time change of pressure inside the filter. Time changes of dust-released area and geometrical characteristics and motion of released dust cake fragments were investigated by analyzing photo images from the video camera, compared with the time change and distribution of pressure inside the filter. In order to discuss the influence of porosity of the dust layer, a consolidated dust layer was prepared by clean air filtration at an elevated filtration velocity after the dust filtration. The dust-releasing process was found to be a multi-mode process, i.e. release occurred in a short time after the injection of cleaning air, followed by one or two intermittent releases afterwards. The amount of dust released by the first strike increased with tank pressure. The dust-releasing behavior was sensitive to the cake porosity: the mean size and perimeter of cake fragments increased with porosity and tank pressure for the consolidated dust layer. The initial radial velocity of cake fragment released just after the first strike increased with tank pressure and was almost independent of the filter location.     

Estimation of resistance of filter media used for Prefil Footprinter tests of liquid aluminium alloys

Abstract

During the pressure filtration of molten metals, an essential parameter is the resistance of the filter medium, which is relevant to the calculation of flow velocity, pressure loss and distribution, and drag forces. The resistance has significant influences on the filtration behaviour and structure of the residual cake of inclusions retained on the filter. The Prefil Footprinter, a portable pressure filtration instrument, is usually used to determine the quality of molten aluminium alloys. To date, however, no data on the resistance of the filter medium used have been reported. The present work is an attempt to estimate the resistance. Experiments are described and analyses of flow behaviour are presented. Values of the medium resistance of the filters are determined and reported for the first time.     

Effect of particle size on the performance of cross-flow microfiltration

The effects of particle size on the cake properties and the performance of cross-flow microfiltration are studied. A particulate sample with a wide size distribution range from submicron to micron is used in experiments. The probabilities of particle deposition are analyzed based on a force analysis. Since themajor forces in determining the particle deposition and packing in the filter cake are different for submicron and micron particles, the particle size plays an important role in the filtration performance. Cake properties, such as mass, porosity and average specific filtration resistance of the cake, are calculated theoretically and compared with experimental data. Except for the overestimation of the mean particle size for about 1 μm, the calculated results of the pseudo-steady filtration rate and cake properties under various operating conditions agree fairly well with the experimental data.     

Evaporation of suspensions to form an incompressible cake and to fill filter pores with solid particles

Equations of filtration of suspensions to form an incompressible cake of particles on the surface of the filter with simultaneous passage of a certain share of the particles from the cake to the filter’s pore space and next to the region of a filtered liquid are derived from the principles of the mechanics of multiphase media. The influence of the travel of the particles in the region of the cake and the filter on the dynamics of growth of the cake bed is investigated. An analysis of the derived dynamic filtration equations shows that allowance for the factors of travel and accumulation of particles in the cake and the filter causes their total filtration resistance, in particular the resistance in the inertial component of the filtration law, to decrease.     

Factors affecting the tensile strength of hot-gas filter dust cakes

The University of North Dakota Energy and Environmental Research Center is determining some of the fuel-, sorbent- and operations-related conditions that lead to blinding or bridging of hot-gas particle filters in pressurized fluidized bed combustion systems. Several mechanisms contribute to the stickiness of dust particles within a filter cake: electrostatic and van der Waals attractions, and mechanical, liquid and solid bridging. The magnitudes of these mechanism impacts depend on the size and composition distributions of the dust particles, cake porosity, gas composition and system temperature. High-temperature tensile strength measurements of filter dust cakes have shown that only a small amount of liquid is necessary to double the tensile strength of a cake. The amount of strength increase is related more to the wettability of the dust than to the liquid's viscosity or air-liquid surface tension. Also, if the tensile strength of a filter cake is less than approximately 50 N/m^2, the cake re-entrains after being pulsed off and can lead to blinding of the filters. If the cake strength is over approximately 300 N/m^2, it can bridge between filters, in some cases forcing them apart and causing them to break. However, the factors affecting cake strength are not independent variables and laboratory measurements have shown that the tensile strength of filter dust does not adequately explain its propensity to bridge across filters in a hot-gas filter system. A better measurement is the tensile strength divided by the density of the cake, or its specific strength. The authors have defined the cake specific strength as its critical thickness index (CTI) because it indicates the maximum thickness for which a cake would build before failing or shedding under its own weight. Laboratory measurements show that the CTI is a better indicator of the observed propensity of a particular dust to form bridges between filters than its simple tensile strength.     

Observation of the process of dust accumulation on a rigid ceramic filter surface and the mechanism of cleaning dust from the filter surface

The accumulation process of captured particles on the surface of a rigid filter element is experimentally studied by measuring the pressure drop. It is then related to the packing density of the dust layer. The process of the release of dust from filter surface is also studied through the changes in the pressure difference across the filter and the movement of the released dust after clean air is injected. As a result, for a given filtration condition, dust forms the densest layer at the initial and the loosest at the middle stage of the filtration, and forms a uniform layer at the final stage. The cleaning efficiency of the dust layer is found to depend upon the layer structure, i.e, it decreases as dust forms a denser layer, even if the accumulated mass per unit filter surface area is the same. The release velocity of dust from the surface is also found to become slower as the porosity of the layer decreases.     

Fabrication of a carbon/polytetrafluoroethylene nanoparticle-coated antistatic bag filter using air-assisted electrospraying

Fabric bag filters have been widely used as air pollution control equipment owing to their high collection efficiency. However, the low efficiency of the fabric filtration method against fine dust and the high operating cost owing to the high-pressure drop have restricted its effective application. In addition, the filling of the fabric of bag filters by the passage of gas and deposited charged dust particles results in the generation of a static electricity that can cause a fire. To solve this problem, this study aimed to develop an antistatic filter by electrospraying carbon nanoparticles. The surface properties and conductivity of the antistatic filter as a function of the mixing ratio were compared, and the particle filtration performance as a function of the coating type was confirmed by evaluating the filter filtration performance. The results revealed that among the samples that exhibited conductivity, the sample with a mixing ratio of 1:1 exhibited the highest resistance value and best hydrophobicity. In addition, the filter performance evaluation revealed that the coated filter exhibited enhanced filtration performance compared to the uncoated filter at all particle ranges.     

Influence of slip length on filtration performance of fine particles

Fine particle filtration has been known to become progressively inefficient as the filter cake builds up owing to restricted movement of liquid through the small cavities formed in the cake. In different chemical industries, this restricts higher throughput rates and also results in higher transportation costs due to increase in moisture content. This paper discusses the influence of using a surfactant, DAH (dodecylamine hydrochloride), in enhancing the filtration rate of a finely ground particles and the reduction of moisture content in the cake. The observed enhanced filtration rate has been attributed to a reduction in the resistance to liquid flow due to the increase in hydrophobicity at the particle surface. The resulting enhanced filtration rate has been modelled by superimposing a slip velocity at the boundary of the capillaries formed in the cake. The model evaluates the cake and medium resistances by incorporating a slip length into the filtration equation which varies with the concentration of hydrophobic reagent and the effective size of capillaries. The increase in filtration rate is more pronounced for finer particle slurries. Also, it has been observed that the moisture content of the filter cakes formed was reduced.     

Semi-coke powder filtration experiments using a dual layer granular bed filter

The separation of semi-coke powders from tars and gases during coal pyrolysis is of crucial importance to the coal chemical industry. This work describes an experimental study on semi-coke powder filtration using an experimental dual layer granular bed filter with an inner diameter of 100?mm. With an initial filtration velocity of 0.2?m/s, the dual layer granular bed filter had a filtration efficiency of 99.943% and a pressure drop across the filter of only 1456?Pa. When the initial filtration velocity was increased to 0.25?m/s, the filtration efficiency was 99.937% and the pressure drop was 1834?Pa. Our results indicated that the dual layer granular bed filter we developed and tested showed significant promise as a high-temperature dust collector in low-temperature coal pyrolysis.     

CFD-DEM analysis of internal packing structure and pressure characteristics in compressible filter cakes using a novel elastic–plastic contact model

Cake-forming filtration is a proven method for separating particles from suspensions. Most filtration models are based on the simplification of incompressible and homogeneous cake structures. However, most filter cakes are in fact unevenly compressed by e.g. the high transmembrane pressures, leading to dense structures with high flow resistance at the filtration membrane. Experimental investigations of these inhomogeneous cakes are challenging due to mostly invasive procedures after filtration has already taken place. In contrast, numerical methods can provide extensive information about fluid flow, particle separation and cake formation during filtration. However, this requires that both elastic and plastic particle deformation and forces are modeled correctly. To achieve this, the present study implemented a novel elastic–plastic DEM model that only requires measurable material parameters and therefore does not need any fitting. Subsequently, previously measured material parameters for elastic–plastic cellulose-lactose pellets (MCC) were used to investigate the packing density, fluid pressure levels and contact forces inside compressible filter cakes using CFD-DEM coupling. A comparison with incompressible and elastically compressible filter cakes showed a significant difference in the filtration behavior. Due to plastic deformation, a strong increase of the packing density when nearing the filtration membrane was found, leading to higher flow resistance for the filtration process. For cyclic filtration events, only the plastically deformed cake showed reduced height recovery in a relaxed state.     

Influence of leaks in surface filters on particulate emissions

Compliance with severe limit values of dust emissions is a main characteristic of surface filters. This characteristic is due to the high particle collection efficiency of surface filters. Beside regular operation it is necessary to consider phenomena such as a "pinhole" bypass through leaks in surface filters to ensure the above mentioned compliance with the limit values at all times. Experimental research has been carried out to observe and understand the "pinhole" bypass through leaks and the behaviour of pinholes over filtration time. To work out the influence of different filtration conditions the parameters pinhole diameter, filter face velocity and dust cake thickness were varied. The results can be explained by formulas usually used to calculate volumetric flow rates of orifice gauges. The experiments and the calculations lead to the conclusions that bigger pinholes decrease the collection efficiency and higher filter face velocities increase the collection efficiency of pinholed filter media.     

Investigation to rectangular flat pleated filter for collecting corn straw particles during pulse cleaning

This work investigated the amounts of dust residual of a rectangular flat pleated filter for collecting corn straw particles during pulse cleaning and attempted to explore the causes of incomplete cleaning. In this study, dust residual, filter’s pressure drops and static peak pressures were obtained across flat pleated filter during the pulse cleaning. The optimum parameters were obtained that the pulse electromagnetic valve size was one inch, the pulse pressure was 0.2–0.3?MPa, and the filtration velocity was 0.6?m/min for the nozzle type with 7 holes with a diameter of 7?mm (7?×?Ф7?mm). Under this condition, the experimental results show that the dust residuals were 198.4 (64%), 52.7 (17%), 58.9 (19%)?g for initial collected dust residual 310?g at top, middle and bottom areas of the filter panel, respectively. The dust residuals were major on the top area of the filter panel, especially on the gap locations between the two-adjacent pulse airflows. Meanwhile, the more pulse interval or dust concentration was increased the dust residuals of the filter panel and the pressure drops of the filter were increased. Moreover, the static peak pressure distribution can give guidance to the dust residual distribution.     

新型放射性气溶胶取样过滤膜的研制

制备了用于放射性气溶胶取样监测的增强微孔滤膜,讨论了铸膜液配方、制膜工艺参数和添加剂对膜结构和性能的影响.实验结果表明,以PVP为致孔添加剂,以面密度较低的无纺布为支撑层,控制铸膜液浓度和成膜工艺,可得到表面收集特性好,过滤效率高的PVDF双功能层增强微孔滤膜.该增强微孔滤膜取样阻力小,流量大,强度高,配合人工核素气溶胶快速监测装置,可进行高氡浓度环境下α放射性气溶胶的实时连续监测.     

Influence of pulse-jet cleaning pressure on performance of compact dust collector with pleated filter operated in clean-on-time mode

To operate a bag filter continuously, pulse-jet cleaning of dust particles from the filter medium is commonly required, and the pulse-jet pressure significantly affects the filter performance. In this study, the accumulation structure of residual dust particles inside and on the surface of a filter medium at different pulse-jet pressures was investigated by constructing a simple model, and the influence of the dust structure on the filter performance was clarified. Using a simple model, we determined the effective ratio of filtration area β, which represents the ratio of the filterable area to the total filtration area, the true resistance coefficient due to the primary dust layer ζ_p’ thinly deposited on the filter surface, and the true resistance coefficient inside the filter media itself ζ_f’_. The effective ratio of filtration area β decreased with operation time for all pulse-jet pressures; however, it maintained a high value when the pulse-jet pressure was high. The validity of β analyzed by the model was verified using two different methods, and the results showed good agreement, indicating that the model is effective in identifying real conditions. The true resistance coefficient due to the primary dust layer ζ_p’ decreased as the pulse-jet pressure increased; however, the true resistance coefficient inside the filter media itself ζ_f’ was the highest at 0.5 MPa. In addition, the dust collection efficiency was different at each pulse-jet pressure, which was considered to be caused by the difference in the dust particle accumulation structure.     

EXPERIMENTAL STUDY OF DUST FILTRATION IN SURFACE-TYPE NONWOVENS

ABSTRACT

Surface type nonwovens are widely used in industrial dust control. Recently, they have been utilized in some engine air filtration applications as automotive filters, heavy-duty engine self-cleaning filters or safety filters. Because of their mechanical strength and regenerative ability they are a perfect material for applications where filter replacement is a problem. On the other hand, the random distribution of fibers and needle punching may result in pinhole formation during dust loading, especially at high aerosol velocities. As a result, the seepage mechanism is common in applications involving fine solid aerosols.

In the inertia dominated region, the collection efficiency of particles depends on the adhesion probability. When particle momentum increases, the efficiency decreases. In general, there is no agreement between filtration theory and experiment when the Stokes number is greater than one.

Filter efficiency increases with dust loading when the filter medium is a good dust cake supporter. In this case, dust reentrainment, causing seepage, may occur at high aerosol velocities and pressure drops. In contrast, reentrainment in nonwovens can take place even at lower aerosol velocities and dust loadings. It is difficult to predict conditions favorable for dust reentrainment and pinhole formation. This process depends on media geometry, dust particle size distribution, and aerosol flow parameters.

This paper discusses filter performance of surface-type nonwovens exposed to polydisperse dusts. Filter efficiency and pressure drop are discussed as functions of aerosol velocity, dust loading, and dust particle size distribution.     

EXPERIMENTAL STUDY OF DUST FILTRATION IN SURFACE-TYPE NONWOVENS

Surface type nonwovens are widely used in industrial dust control. Recently, they have been utilized in some engine air filtration applications as automotive filters, heavy-duty engine self-cleaning filters or safety filters. Because of their mechanical strength and regenerative ability they are a perfect material for applications where filter replacement is a problem. On the other hand, the random distribution of fibers and needle punching may result in pinhole formation during dust loading, especially at high aerosol velocities. As a result, the seepage mechanism is common in applications involving fine solid aerosols.

In the inertia dominated region, the collection efficiency of particles depends on the adhesion probability. When particle momentum increases, the efficiency decreases. In general, there is no agreement between filtration theory and experiment when the Stokes number is greater than one.

Filter efficiency increases with dust loading when the filter medium is a good dust cake supporter. In this case, dust reentrainment, causing seepage, may occur at high aerosol velocities and pressure drops. In contrast, reentrainment in nonwovens can take place even at lower aerosol velocities and dust loadings. It is difficult to predict conditions favorable for dust reentrainment and pinhole formation. This process depends on media geometry, dust particle size distribution, and aerosol flow parameters.

This paper discusses filter performance of surface-type nonwovens exposed to polydisperse dusts. Filter efficiency and pressure drop are discussed as functions of aerosol velocity, dust loading, and dust particle size distribution.     

Analysis of the performance of a sintered stainless steel wire mesh filter for cryogenic liquid purification

As a traditional and mature solid-liquid separating technique, filtration has been adopted in cryogenic liquid purification system owing to its low energy cost, simplified system, long lifetime and high purifying efficiency. Whereas, few data or result of filtration performance at low temperature is put forward in literatures, most of which are related to room and high temperatures applications. Furthermore, as an excellent cryogenic material, stainless steel medium is suitable for cryogenic liquid filtration, which is also seldom reported. In this paper, we propose a filtration system for purifying CO₂ particles from liquid nitrogen using a sintered stainless steel wire mesh filter with a nominal filtration degree of 0.5 μm, and characterize the separation behavior of this kind of filter medium at cryogenic temperature. Experimental results show that the whole filtration process can be divided into three stages with sufficient particle concentration in the feed slurry according to the evolution of pressure drop. Differences between surface filtration and depth filtration are demonstrated, and the influence of growth of filter cake is characterized. Pressure drop increases with increasing feed slurry flow and CO₂ concentration, and evolution tendency of filtration efficiency is affected by its calculation method. By comparison with the filtration at room and high temperatures, the effects of temperature on the fluid and media are emphasized.     

A new indirect method of measuring the contents of solid inclusions in liquid aluminium alloys

The introduction of derivative methods and the classification of three flow states (i.e. initial transient, steady and terminal transient stages) provide new approaches to understand the filtration behaviors of liquid aluminum alloys during the Prefil Footprinter tests. The effective-ness of the filtration equations of incompressible cake mode can be well identified over some steady stages during the course of the filtration tests. Based on these new findings a new indirect method of measuring the contents of solid inclusions in liquid aluminum alloys is devel-oped using the Prefil Footprinter tests. However, the benchmarks of specific cake resistance should be made for commercial aluminum alloy melts before this method becomes feasible in laboratories and industries. It is found that for a given volume concentration of solid inclusions, lighter inclusions usually lead to higher specific cake resistance, and the heavier solid inclusions reduce the specific cake resistance. For higher quality liquid metal, usually with lower lumped parameter (σ α), the types of solid inclusions have less influence on specific cake resistance.     

Structure optimization of granular bed filter for industrial flue gas filtration containing coagulative particles: An experimental and numerical study

A numerical model for the flow and filtration characteristics of industrial flue gas in granular bed filter (GBF) was established and the local filtration efficiency for different granule layers was investigated. Numerical validation results show that the GBF structure with large size granules at the inlet region and small size granules at the outlet region can effectively improve the filtration performance of GBF and the underlying mechanism was revealed. Then an experimental system was built to validate the suitability of the optimized GBF structure for the filtration of industrial flue gas with coagulative particles. The experimental results show that the optimized GBF structure is also suitable and its superiority is more significant with the increase of filtration time. The results show that the pressure drop and filtration efficiency of the experimental system increase with the increase of dust particles concentration. The existing of coagulative particles is conducive to the growth of smaller size dust particles, the pressure drop and filtration efficiency increase significantly. In addition, the pressure drop and filtration efficiency decrease with the increase of cooling rate. The results of this study are expected to be useful for the design and optimization of industrial flue gas purification and waste heat recovery.