Conventional Filtration Theory Research Articles

Modeling pressure drop values across ultra-thin nanofiber filters with various ranges of filtration parameters under an aerodynamic slip effect

Computational fluid dynamics simulations of fibrous filters with 56 combinations of different fiber sizes, packing densities, face velocities, and thicknesses were conducted for developing models that predict pressure drops across nanofiber filters. The accuracy of the simulation method was confirmed by comparing the numerical pressure drops to the experimental data obtained for polyacrylonitrile electrospun nanofiber filters. In the simulations, an aerodynamic slip effect around the surface of the small nanofibers was considered. The results showed that, unlike in the case of conventional filtration theory, pressure drops across the thin layers of electrospun nanofiber filters are not proportional to the thickness. This might be a critical factor for obtaining precise pressure drops across the electrospun nanofiber filters with extremely thin layers. Finally, we derived the product of drag coefficient and Reynolds number as a function of packing density, Knudsen number, and ratio of thickness to fiber diameter to get the correlation equation for pressure drop prediction. The obtained equation predicted the pressure drops across the nanofiber filters with the maximum relative difference of less than 15%.

Prediction of collector contact efficiency for colloid transport in porous media using Pore-Network and Neural-Network models

Conventional colloid filtration theory (CFT) uses the single collector contact efficiency (η) to describe the mass transfer of colloids to a collector surface. However, this approach neglects the full complexity of the pore structure and flow field of real porous media. In this study, the porous medium geometry, flow field, and colloid mass transfer are quantified using a pore-network model (PNM). A database of pore scale η is established by finite-element method to train a Neural-network model (NNM). The reasonable prediction of η indicates the potential of using the developed NNMs as an alternative to correlation equations, which can free the users from repeated numerical simulation. In contrast to the prediction by conventional CFT, the value of η in the PNM occurs as a distribution, which is dependent upon the geometry parameters of the PNM. The mean value of η increases with the standard deviation of pore radius and decreases with the curvature number, but the dependency on coordination number is more complex. Upscaled values of the deposition rate coefficient (kd) corresponding to the distribution of η are calculated by the breakthrough curves by PNMs. The prediction of kd by PNM is then compared with that by CFT. Results show that kd predicted by PNM shows more significant response to velocity change, and less remarkable response to colloid density change than kd predicted by CFT. The comparison between the flow velocity distribution between PNM and CFT shows that the high-velocity region of the flow field in the porous media has been neglected in CFT, which can lead to insufficient consideration of convection. The results of this work imply that it is necessary to consider the influence of the complex pore structure of porous media on the collection of colloids.

Analysis of electrokinetic response of solid-liquid mixture during expression operation

In the expression process of slurry material using a filter-press of compression type, the process is divided into two periods: the filtration period and the consolidation period. Identification of the exact time of the culmination of filtration period is essential for operating the press effectively. This study focused on the electrokinetic phenomena during expression operation of zinc oxide slurry to obtain information on the status of the filter chamber. We have measured the time course of electrical potential difference (EPD) between both ends of expressed material during the expression process using a laboratory piston press. The absolute value of EPD increased with the progress of the filtration period, followed by the decrease during the consolidation period. It was observed that the time at which the absolute value of the EPD began to decline coincided with the time at which the filtration period ended. EPD reached the plateau value at the end of consolidation. In a consolidation of homogeneous semisolid material, the absolute value of EPD increased at the beginning of the operation, followed by the decline to the plateau value. We have extended the theoretical equation of streaming potential for a straight capillary to be applied to a tortuous flow path of the expressed material. The time course of EPD can be qualitatively explained by using the proposed equation for EPD and calculated liquid pressure distributions based on the conventional filtration and consolidation theories by considering the medium resistance.

Optimal operation of parallel dead-end filters in a continuous bio-based process

Dead-end cake filtration as one of the first unit operations deployed in biochemical production plants has been subject of academic investigation for many decades. The recurrent discrete reinitialisation events are challenging from a process control point of view, especially in a continuous downstream line. The complexity that arises when multiple units are operated in parallel seems to have received little attention. This work aims at illustrating this complexity and delineates the arising plantwide control problem. Guidelines for optimised operation are derived from general process understanding and at hand of an industrial case study. The need for a predictive model to solve multiple scheduling problems is identified, and a mathematical model based on conventional filtration theory is derived. Due to raw material variability and operational uncertainties, the predictions are found to be too imprecise for deployment. This is expected to be representative of many bio-based processes, where manual scheduling needs to be integrated effectively into plantwide control structures.

A comparison of filtration characterisation devices for compressible suspensions using conventional filtration theory and compressional rheology

Solid-liquid separation is carried out in many industries and plays a crucial role in overall process efficiency and product specification. The dewatering characteristics of solid-liquid mixtures, or particulate suspensions, have to be determined to compare filtration equipment performance or to design new equipment. Two theoretical frameworks used to describe solid-liquid separation, and therefore analyse experimental results, are the conventional filtration theory developed by Ruth, Tiller and Shirato and the compressional rheology framework developed by Buscall, White and Landman.In this paper, we compare the characterisation techniques that have developed over time based on these two approaches. This includes a comparison between an air-driven Nutsche Filter, a Compression-Permeability cell and a piston-driven Filtration Rig. The test material is calcium carbonate coagulated by attractive Van der Waals forces at its isoelectric point. We use three different particle sizes, leading to varying degrees of compressibility. The results show good agreement between the two theoretical frameworks for these compressible materials. Overall, this work highlights the differences and similarities between the theories and characterisation techniques alike and gives recommendations for good practice.

Modeling airborne nanoparticle filtration through a complete structure of non-woven material used in protective apparel

Airborne nanoparticles represent a new danger in occupational health. Numerous theoretical and experimental studies have been conducted on the efficiency of filtering media used for respiratory protection, but few have focused on media used for skin protective equipment. Indeed, a significant proportion of airborne nanoparticles can end up on the skin, causing local effects and eventually penetrating the human body. Following experimental data obtained with sodium chloride nanoparticles, the authors propose an empirical model to evaluate the penetration of airborne nanoparticles through medium used in disposable coveralls. This study presents an adaptation of the conventional filtration theory used for filtering media used in respirators. The authors' model is compared with Wang et al.'s and Brochot's models and demonstrates improvements in their descriptive ability. Moreover, a domain of validity of the proposed model was determined that will enable the evaluation of the efficiency of similar protective apparel material structures against airborne nanoparticles.

Erythrocyte fouling on micro-engineered membranes.

Crossflow microfiltration of plasma from blood through microsieves in a microchannel is potentially useful in many biomedical applications, including clinically as a wearable water removal device under development by the authors. We report experiments that correlate filtration rates, transmembrane pressures (TMP) and shear rates during filtration through a microscopically high channel bounded by a low intrinsic resistance photolithographically-produced porous semiconductor membrane. These experiments allowed observation of erythrocyte behavior at the filtering surface and showed how their unique deformability properties dominated filtration resistance. At low filtration rates (corresponding to low TMP), they rolled along the filter surface, but at higher filtration rates (corresponding to higher TMP), they anchored themselves to the filter membrane, forming a self-assembled, incomplete monolayer. The incompleteness of the layer was an essential feature of the monolayer's ability to support sustainable filtration. Maximum steady-state filtration flux was a function of wall shear rate, as predicted by conventional crossflow filtration theory, but, contrary to theories based on convective diffusion, showed weak dependence of filtration on erythrocyte concentration. Post-filtration scanning electron micrographs revealed significant capture and deformation of erythrocytes in all filter pores in the range 0.25 to 2 μm diameter. We report filtration rates through these filters and describe a largely unrecognized mechanism that allows stable filtration in the presence of substantial cell layers.

Filtration Properties of Nanofiber/Microfiber Mixed Filter and Prediction of its Performance

ABSTRACTThere is an increasing demand of air filters with a high collection performance, i.e., high collection efficiency and low pressure drop, for the application to indoor air cleaning. Air filters consisting of nanofibers have attracted great interests since they may have a low pressure drop because of slip flow effect and high collection efficiency due to interception. Although various nanofiber filters are available on the market, their collection performance is not as high as expected by the conventional filtration theory because non-uniform packing of fibers plays a significant role. In the present work, the collection performance of nanofiber (780 nm) and microfiber (11.2 µm) mixed filters with various mixing fractions was studied in order to maximize the quality factor of filter, qF, as a function of mixing fraction of nanofibers. The collection performance of mixed fiber filters was predicted by using theoretical equations reported by Bao et al. (1998) for bimodal distribution of fibers. As a result, it was found that the mixed fiber filters had a uniform fiber packing compared to laminated filters and that the collection efficiency was well predicted by introducing the inhomogeneity factor calculated for the filter consisting of two distinct fiber sizes. Furthermore, we found that the mixed fiber filter with the nanofiber mixing fraction of 5% in mass had the highest quality factor.

The effect of electrostatic forces on filtration efficiency of granular filters

The effect of electrostatic forces on the granular filtration of nanoaerosol NaCl particles in the range of 10nm to 100nm was investigated experimentally in this study. The test granular filters were made of 2mm homogeneous glass beads at three media thicknesses (25, 76 and 127mm), and they were tested at three air flow rates (27, 45, and 65lpm). The filtration efficiencies were measured for neutralized and charged NaCl nanoparticles. The corresponding difference was considered as the filtration efficiency attributed to the electrostatic attraction between the charged NaCl particles and the glass granules. Results showed that the electrostatics played a great role in nanoaerosol filtration, which is different from conventional filtration theories. Its contribution to filtration efficiency increased with the size of the nanoparticles to a level of 30% or so. Results also showed a positive correlation between the separation efficiency due to electrostatic forces and the residence time of the air flow. The correlation is relatively strong (between 0.6 and 0.9) for particles in the range of 20–100nm. However, it is weak, although positive, for sub-20nm particles.

Air filtration in the free molecular flow regime: a review of high-efficiency particulate air filters based on carbon nanotubes.

Air filtration in the free molecular flow (FMF) regime is important and challenging because a higher filtration efficiency and lower pressure drop are obtained when the fiber diameter is smaller than the gas mean free path in the FMF regime. In previous studies, FMF conditions have been obtained by increasing the gas mean free path through reducing the pressure and increasing the temperature. In the case of carbon nanotubes (CNTs) with nanoscale diameters, it is possible to filtrate in the FMF regime under normal conditions. This paper reviews recent progress in theoretical and experimental studies of air filtration in the FMF regime. Typical structure models of high-efficiency particulate (HEPA) air filters based on CNTs are introduced. The pressure drop in air filters operated in the FMF regime is less than that predicted by the conventional air filtration theory. The thinnest HEPA filters fabricated from single-walled CNT films have an extremely low pressure drop. CNT air filters with a gradient nanostructure are shown to give a much better filtration performance in dynamic filtration. CNT air filters with a hierarchical structure and an agglomerated CNT fluidized bed air filter are also introduced. Finally, the challenges and opportunities for the application of CNTs in air filtration are discussed.

Colloid transport in saturated porous media: Elimination of attachment efficiency in a new colloid transport model

Key Points This colloid transport model eliminates the empirical attachment efficiency. Introduce a new paradigm for colloid re‐entrainment behavior. Model parameters related to magnitude of secondary energy minimum

Sensitivity of the transport and retention of stabilized silver nanoparticles to physicochemical factors

Saturated sand-packed column experiments were conducted to investigate the influence of physicochemical factors on the transport and retention of surfactant stabilized silver nanoparticles (AgNPs). The normalized concentration in breakthrough curves (BTCs) of AgNPs increased with a decrease in solution ionic strength (IS), and an increase in water velocity, sand grain size, and input concentration (Co). In contrast to conventional filtration theory, retention profiles (RPs) for AgNPs exhibited uniform, nonmonotonic, or hyperexponential shapes that were sensitive to physicochemical conditions. The experimental BTCs and RPs with uniform or hyperexponential shape were well described using a numerical model that considers time- and depth-dependent retention. The simulated maximum retained concentration on the solid phase (Smax) and the retention rate coefficient (k1) increased with IS and as the grain size and/or Co decreased. The RPs were more hyperexponential in finer textured sand and at lower Co because of their higher values of Smax. Conversely, RPs were nonmonotonic or uniform at higher Co and in coarser sand that had lower values of Smax, and tended to exhibit higher peak concentrations in the RPs at lower velocities and at higher solution IS. These observations indicate that uniform and nonmonotonic RPs occurred under conditions when Smax was approaching filled conditions. Nonmonotonic RPs had peak concentrations at greater distances in the presence of excess amounts of surfactant, suggesting that competition between AgNPs and surfactant diminished Smax close to the column inlet. The sensitivity of the nonmonotonic RPs to IS and velocity in coarser textured sand indicates that AgNPs were partially interacting in a secondary minimum. However, elimination of the secondary minimum only produced recovery of a small portion (<10%) of the retained AgNPs. These results imply that AgNPs were largely irreversibly interacting in a primary minimum associated with microscopic heterogeneity.

Physical model structure capillary porous media based oriented fiberfill

It is shown that the prediction of the kinetics of impregnation process oriented and woven fibrous fillers epoxy binder solution is based primarily on an analysis of the structure of fibrous fillers. In this case, the capillary structure of fibrous fillers should include the most important factors that affect the speed and completeness of the impregnation process, after impregnating fluid flow in these materials is a process, which is mainly governed by laws of capillarity and viscosity. It is shown that in general the capillary structure of any fibrous material consists of material microstructure fiber from which it is made, as well as macro-fiber material, the diameter of the capillaries of the two structures is a completely different order. Are analyzed that the penetration depth of the liquid in the impregnation of capillaries in individual fibers, especially for those short periods during which there is a treatment, there can be significant. Therefore infinitely more practical importance is the penetration of impregnating fluid in the space between the fibers (interfiber space or penetration) or filaments of fibrous material. Describes an effective approach to the selection of an adequate physical model of capillary-porous bodies in the technology of construction of the oriented fiber composites. This physical model is then used for the determination of refined mathematical model of the process of impregnation of fibrous fillers epoxy bonding using conventional filtration theory of laminar flow of viscous non-Newtonian fluid which is not compressed.

КінетикА процесУ просочуваннЯ волокнистиХ напоВнюВачіВ композИціямИ епоксидниХ полімеріВ

Are analyzed the predictive kinetic equation impregnation process fibrous fillers epoxy binder solutions using conventional filtration theory for laminar flow of viscous non-Newtonian fluid, which is not compressed, given the structural characteristics of fibrous fillers as a capillary-porous body, namely, porosity, specific internal surface, effectively (equivalent) radius of the capillary. refined kinetic equation of the longitudinal or transverse impregnation of continuous fiber filler obtained for the most efficient from a technological point of view, cross-sectional shape of fibrous fillers (belt material: fiberglass, glass fiber, bundles of rectangular cross-section) because of its structural and geometric parameters and the parameters of the impregnation process, namely : tension force fibrous fillers, rheology liquid epoxy adhesive and pressure impregnation. The resulting equation to predict the main kinetic parameters of the impregnation process (time and leakage rate, lift height binder), as well as design parameters for the impregnation equipment (dimensions impregnating bath). The directions of the intensification of the process of impregnation.

Software for Empirical Modeling of Test Filtrations

Filtration is a widely used unit operation in chemical engineering. Conventional filtration theories are difficult to use when the system to be modeled contains all of the stages and features that are present in a complete solid/liquid separation process. Furthermore, most of the filtration theories require experimental work to be performed in order to obtain critical parameters required by the theoretical models. The statistical design of experiments provides the necessary tools for recognizing the effects of variables. It also helps to perform experimental work more economically. Design of experiments is a prerequisite for creating empirical models that can describe how the measured response is related to the changes in the values of the variable. A software package was developed that provides a filtration practitioner with experimental designs and calculates the parameters for linear regression models, along with the graphical representation of the responses. The developed software consists of two software modules. These modules are LTDoE and LTRead. The LTDoE module is used to create experimental designs for different filter types. The LTRead-module is used to read the experimental data gathered from the experiments, to analyze the data and to create models for each of the measured responses. The software provides both numerical data in the form of model equations and visual representations showing how the selected variables affect the responses. This approach to the study of cake filtration processes, as presented, has been shown to have good practical value when making filtration tests.

Lessons Learned from Bacterial Transport Research at the South Oyster Site

This paper provides a review of bacterial transport experiments conducted by a multiinvestigator, multiinstitution, multidisciplinary team of researchers under the auspices of the U.S. Department of Energy (DOE). The experiments were conducted during the time period 1999-2001 at a field site near the town of Oyster, Virginia known as the South Oyster Site, and included four major experimental campaigns aimed at understanding and quantifying bacterial transport in the subsurface environment. Several key elements of the research are discussed here: (1) quantification of bacterial transport in physically, chemically, and biologically heterogeneous aquifers, (2) evaluation of the efficacy of conventional colloid filtration theory, (3) scale effects in bacterial transport, (4) development of new methods for microbial enumeration and screening for low adhesion strains, (5) application of novel hydrogeophysical techniques for aquifer characterization, and (6) experiences regarding management of a large field research effort. Lessons learned are summarized in each of these areas. The body of literature resulting from South Oyster Site research has been widely cited and continues to influence research into the controls exerted by aquifer heterogeneity on reactive transport (including microbial transport). It also served as a model (and provided valuable experience) for subsequent and ongoing highly-instrumented field research efforts conducted by DOE-sponsored investigators.

Effect of PAC Addition on the Physicochemical Characteristics of Bio-Cake in a Membrane Bioreactor

The effect of powdered activated carbon (PAC) addition on the architecture and cohesion strength of bio-cake in a membrane bioreactor (MBR) was investigated. Two reactors, a conventional MBR and a membrane-coupled biological activated carbon reactor (MBR ac ), were run in parallel. The addition of PAC led to a substantial increase in membrane permeability. Based on the conventional filtration theory, microbial floc size (d), bio-cake porosity (ε), and total attached biomass (TAB) were determined to find the key mechanism for the enhanced permeability. Unexpectedly the addition of PAC did not significantly change either the microbial floc size (d) or the porosity (ε) of the bio-cake. It decreased, however, not only the concentration of extra-cellular polymeric substances (EPS) in the bulk phase but also the TAB on the membrane. Using a separate batch cohesion test, it was revealed that the cohesion strength between microorganisms in the bio-cake in the MBR ac was weaker than that in the MBR. The addition of PAC led to the reduction of EPS, which act as glue-like materials, which in turn weakened the cohesion strength of the microorganisms. This resulted in a lower amount of TAB, i.e., a smaller amount of the bio-cake on the membrane, which ultimately gave rise to the enhanced permeability in the MBR ac .

Respiratory protection against airborne nanoparticles: a review

As a precautionary measure, it is often recommended that workers take steps to reduce their exposure to airborne nanoparticles through the use of respiratory protective devices. The purpose of this study was to provide a review and analysis of the research literature and current recommendations on respirators used for protection against nanoparticles. Key research findings were that studies with particles as small as 4 nm have shown that conventional single-fiber filtration theory can be used to describe the filtration performance of respirators and that the most penetrating particle size for respirators equipped with commonly used electrostatic filter media is in the range of 30-100 nm. Future research needs include human laboratory and workplace protection factor studies to measure the respirator total inward leakage of nanoparticles. Industrial hygienists and safety professionals should continue to use traditional respirator selection guidance for workers exposed to nanoparticles.

Colloid transport in unsaturated porous media: The role of water content and ionic strength on particle straining

Packed column and mathematical modeling studies were conducted to explore the influence of water saturation, pore-water ionic strength, and grain size on the transport of latex microspheres (1.1 μm) in porous media. Experiments were carried out under chemically unfavorable conditions for colloid attachment to both solid–water interfaces (SWI) and air–water interfaces (AWI) using negatively charged and hydrophilic colloids and modifying the solution chemistry with a bicarbonate buffer to pH 10. Interaction energy calculations and complementary batch experiments were conducted and demonstrated that partitioning of colloids to the SWI and AWI was insignificant across the range of the ionic strengths considered. The breakthrough curve and final deposition profile were measured in each experiment indicating colloid retention was highly dependent on the suspension ionic strength, water content, and sand grain size. In contrast to conventional filtration theory, most colloids were found deposited close to the column inlet, and hyper-exponential deposition profiles were observed. A mathematical model, accounting for time- and depth-dependent straining, produced a reasonably good fit for both the breakthrough curves and final deposition profiles. Experimental and modeling results suggest that straining — the retention of colloids in low velocity regions of porous media such as grain junctions — was the primary mechanism of colloid retention under both saturated and unsaturated conditions. The extent of stagnant regions of flow within the pore structure is enhanced with decreasing water content, leading to a greater amount of retention. Ionic strength also contributes to straining, because the number of colloids that are held in the secondary energy minimum increases with ionic strength. These weakly associated colloids are prone to be translated to stagnation regions formed at grain–grain junctions, the solid–water–air triple point, and dead-end pores and then becoming trapped.

Analysis of cake filtration data—A critical assessment of conventional filtration theory

AbstractExperimental results of constant‐pressure cake filtration of aqueous suspensions of four kinds of particles: CaCO3, Kaolin, Kromasil and TiO2 is presented. The data reported consist mainly of filtration performance results, that is, the cumulative filtrate volume and cake thickness as functions of time, which were obtained using a newly developed multifunction test cell. Based on these data, the medium resistance and the average cake specific resistance at various operating pressure were evaluated and corroborated with the results obtained from the compression‐permeability (C‐P) cell measurements. The filtration results were also compared with predictions from the solutions of the appropriate volume‐averaged equations of continuity using different pl − ps relationships. Generally speaking, with the appropriate pl − ps relationship, agreement between experiments and prediction was satisfactory. © 2006 American Institute of Chemical Engineers AIChE J, 2006