Effect of filtration velocity and dust concentration on cake formation and filter operation in a pilot scale jet pulsed bag filter

Bag filters are used for the removal of fine solid particles from process gases. Thus, understanding the filter cake build up and its properties is a subject of interest. The filter cakes properties may depend on many factors like, for example, filtration velocity and dust concentration. The effect of dust concentration and filtration velocity on filtration time, specific cake resistance and mean cake density is investigated in a pilot scale jet pulsed bag filter. An in situ optical system is used to measure cake thickness distributions on the filter surface. Additionally, the operation is simulated using a one-dimensional model and results are compared with experiments. The experimental results indicate that cake density and specific resistance increase with increasing velocity at constant dust concentration. The effect of dust concentration on filter cake density and specific resistance is small.     

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.     

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.     

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.     

Microscopic observation of filter cake formation

Particle collisions and filter cake collapse are observed through a microscope during the formation of filter cakes. Cake filtration is widely applied throughout industry for fluid-particle separations. Models and theories have been developed to describe and predict filter cake performance, but there is little literature on direct observation of particle packing and cake formation. A filter assembly the size of a microscope slide is placed on the microscope such that the slurry flow and cake formation is across the field of view. The cake formation is recorded on video tape. Photographs of the video image show particles in the slurry colliding with and sometimes adhering to other particles when forming the filter cake. The photographs also show the cake structure collapse as the cake compacts or compresses.     

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 SURFACTANT ADDITION ON THE MICROSTRUCTURE OF COAL FILTER CAKES

Currently, water removal from fine coal fractions is affected by vacuum filtration of a coal slurry followed by thermal drying. Due to high operating costs and the potentially hazardous nature of the drying operation, alternate methods are sought.

This work investigated the change of filter cake microstructure resulting from one such method, namely, surfactant addition to the coal slurry. By impregnating a coal filter cake with an epoxy resin, micrographic analysis of the cake structure could be made by using an image analyzer with the aid of quantitative stereology. This analysis provided a particle and pore size distribution of the filter cake which is fundamental to the understanding of the dewatering mechanism.

Three surfactants were investigated in this work: non-ionic Triton X-114, anionic Aerosol-OT, and cationic dodecyl pyrindinium chloride. The behavior of Triton X-114 and DPC was similar; they both enhanced dewatering by reducing the amount of particle segregation while increasing the pore size of the cake. Conversely, Aerosol-OT, while exhibiting enhanced dewatering characteristics, did not significantly alter the filter cake structure. At low concentrations all three surfactants exhibited little change in particle and pore size distributions when compared to a filter cake formed at the same conditions without surfactant.

Additionally, a linear relationship was shown to exist between the volume-surface mean diameters of the particles and that of the pores. Similar relationships are also presented for the geometric mean diameter and the geometric deviation. The best correlation between particles and pores was obtained from coal cakes formed with Aerosol-OT.     

Development of a novel slurry condensation method by applying dispersant instead of flocculant

A novel filtration technique has been developed by adding the dispersant instead of the flocculant used in conventional solid–liquid separation technique. Slurries were prepared by controlling the pH values in order to control the dispersion and flocculation degree of particles. The prepared slurries were condensed by gravitational settling condensation or by pressure filtration condensation. On the gravitational settling condensation for the flocculated slurry, the cake was formed on the filter media and then the filtration could not be continued unless the formed cake was excreted by a mechanical hand-scraping device. In addition, the volumetric concentration of the condensate obtained from the flocculated slurry was at most 15 vol.%. On the other hand, the condensate obtained from the dispersed slurry still possessed fluidity when the concentration reached almost 40 vol.%, therefore we could collect the condensate without any hand-scraping devices. Furthermore, cake formation did not occur for the dispersed slurry, suggesting that the continuous filtration condensation is possible.     

Effect of nanoparticle behaviour on mud cake buildup for directional and horizontal wells: mathematical modelling and experimental study

The present study examined the effect of nanoparticle size and geometry on filter cake buildup during dynamic filtration using experimental and modeling approaches. A dynamic filtration setup was proposed and designed to test cross-flow circulation against a synthetic core at a constant differential pressure. The proposed mathematical model considers the critical deposition boundary of the porous media for particles along with drag, lift, friction, buoyancy, permeate, and electrostatic forces. The comprehensive model response allowed investigation of the aggregation and shape of nanoparticles at different cross-flow inclinations. The drag and lift force coefficients and moment of inertia of the nanoparticles were determined using cluster structures of nanoparticles identified using dynamic light scattering (DLS) and fractal theory. The deposition and erosion intensity index of all solid particles were experimentally determined and used in the model. Electronic microscope images of the mud cake surface confirmed the proposed cluster structure of the aggregated particles. The mud cake thickness measured by dynamic filtration tests and the nanoparticle-bentonite weight ratio acquired by DLS showed good agreement with the mathematical model results. The proposed approach allows selection of the optimum shape and size of the nano-solid particles as additives to minimize mud cake buildup along the borehole.     

FORMATION OF FILTER CAKES WITH PARTICLE PENETRATION AT THE FILTER SEPTUM

The mass of solid particles entering the formation is an important factor in industrial cake filtration operations. Predictions of the concentration at the filter septum require the ability to predict the mass transport of solid particles under variety of conditions.

This study analyzes cake formation, including particle penetration at the filter septum. In addition to the total instantaneous mass balance equation, mass balance equations for captured and suspended particles and the fluid phase are averaged along the cake thickness taking into account conditions at the surface and the septum. Capture mechanisms, such as surface straining, and internal cake erosion and particle capture are included in the analysis.

The results are ordinary differential equations in terms of thickness, concentration of suspended particles in the filtrate, average particle concentration, average porosity, and such operational parameters, as slurry concentration, injection rate, and volumetric solid fraction.

To test tbe validity of the analysis presented here, the numerical results are compared to results for a simplified case. The conclusions from the sensitivity analysis conducted in this study agree with earlier conclusions. Results show that the concentration of suspended particles in the filtrate increases rapidly and then decreases gradually until it reaches zero after 13 hours. This yields that after 13 hours we have a clear filtrate.     

Granular bed filtration of high temperature biomass gasification gas

High temperature cleaning of producer gas from biomass gasification has been investigated with a granular filter. Field tests were performed for several hours on a single filter element at about 550 °C. The results show cake filtration on the granular material and indicate good filtration of the biomass gasification producer gas. The relatively low pressure drop over the filter during filtration is comparable to those of bag filters. The granular filter can operate with high filtration velocities compared to bag filters and maintain high efficiency and a low residual pressure.     

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.     

Numerical simulation of particulate cake formation in cross-flow microfiltration: Effects of attractive forces

We present a two-dimensional simulation model to explore cake formation in cross-flow filtration. The model uses the lattice Boltzmann method (LBM) for fluid computation and the discrete element method (DEM) for particle computation; they were fully coupled with the smoothed profile method. We verified our model by simulating filtration under different transmembrane pressures. We then investigated the effects of attractive forces and particle concentration on the cake formation mechanism. Generally, as the attractive interaction and particle concentration increased, the particles formed a cake layer with a looser body and rough surface, due to the decrease in the mobility of the particles in contact with the cake surface. It is concluded that the effects of particle concentration are affected by the different conditions of attractive interactions between the particles.     

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.     

Determination of local filtration properties at constant pressure

The present work includes experimental and theoretical studies of the characterization of filter cakes. A filter test apparatus, in which the local pressure profile can be determined during the build up of the filter cake was developed. Efforts were made to minimize the influence of settling and the so called wall-effects related to the pressure measurements. Also a method to calculate local filtration properties (specific filtration resistance and porosity) was developed. In the method, conventional empirical equations describing the relation between the local filtration properties and the local pressure were used. The basic idea is to direct the calculation in such a way that the obtained numerical values of the parameters in the empirical equations reflects the local conditions. The developed method was found to be superior compared to a method where only average data on the filtration properties was used to calculate the corresponding parameters. Finally, the method was found to be stable as well as reliable.     

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.     

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.     

电气石颗粒/聚苯硫醚针刺毡复合过滤材料的过滤性能

为提高工业用过滤材料对细颗粒物的捕集效率,以袋式除尘用聚苯硫醚（PPS）针刺毡为基材,聚氨酯热熔胶膜为黏合层,通过溶液沉淀法将具有自发极化特性的电气石（TM）颗粒覆于基材表面,经热压处理制备了含不同纯度、不同含量、不同颗粒粒度的TM颗粒/PPS针刺毡复合过滤材料;利用SEM研究了TM对微细粒子的吸附情况,利用滤料性能测试装置研究了TM颗粒/PPS针刺毡的过滤性能,结果表明：附着TM颗粒后,TM颗粒/PPS针刺毡对亚微米粉尘过滤效率明显提高,TM纯度越高效果越好,纯度为87.16%时,滤料对0.3~1 μm粒子过滤效率提高幅度≥13.35%;最优附着浓度为5 mg·cm^-2时,用于综合评价滤料过滤效率与阻力的滤料品质因数Q_F值最高;TM颗粒粒径越小,过滤效率提升效果越明显,TM颗粒粒径18~38 μm时,对0.3~1 μm粒子过滤效率提高幅度≥7.25%。TM颗粒/PPS针刺毡复合滤料较传统针刺毡滤料过滤性能明显增强。     

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.     

Cycle Optimization Involving the Use of Filter Aids

A method for optimizing the quantity of filter aid powders used In constant rate pumping operations In plate and frame, recessed plate, and leaf filters Is presented. In contrast to the usual assumption that there Is a fixed fraction of filter aid admix which yields the best results, we demonstrate that the optimum amount of filter aid varies with cake thickness, washing and dead times, and use of a membrane for expression or simply pressing out residual slurry In a filter press at the end of filtration. The specific problem treated in this paper relates to the adaptation of a constant rate pump to an existing filter equipped with cake squeezing membranes. The separation is limited by a specified pressure, and the maximum cake thickness Is fixed by the filter geometry.

The optimum fraction of filter aid rises as cake thickness Increases.