Law of substance separation on cross-flow filtration in turbulent flow

Cross-flow filtration is a filtration process for separation of a disperse phase from liquids. Suspension flows tangentially to a membrane and the filtrate is drawn off perpendicular to the direction of flow. Formation of a filter cake on the membrane is thus prevented, reduced, or its composition, modified. The principle of the separation is based on tow opposing effects: on the one hand, the particles are transported by the filtration flux to the membrane where they cause and increase in concentration; on the other hand, concentration differences are again reduced by the turbulence of the cross-flow and by Brownian motion of the particles. The two mechanisms compete with each other and depend upon particle size in different ways. An energetic comparison of the two effects yields the separation law of cross-flow filtration as a steady state solution of the Fokker-Planck equation. The separation law has an exponential form and assigns each particle size a separation probability with which it reaches the membrane. Once on the membrane the particles may form a filter cake, flow through the pores or return to the bulk flow. If the particles remain on the membrane the ranges of layer-free and cake-forming filtration can be calculated from the hydro-dynamic and geometric conditions of the cross-flow filter. Conventional cake filtration is regarded as limiting case. In continuous cross-flow filtration process a low separation probability through the filtration pressure on selection of the filter medium resistance.     

Clarification of liquids using filter aids Part I. Mechanisms of filtration

Filter aids were characterized by an effective particle diameter and a pore diameter in the filter aid cake, calculated from the specific cake resistance using the Kozeny equation. The results agreed well with those from mercury porosimetry. The transition from surface filtration to depth filtration by filter-aid cakes was studied with uniform polystyrene particles as a model impurity. The critical ratio between pore diameter and impurity diameter was between 2 and 3. At conditions of non-surface filtration, an important concentration of impurity exists in the liquid flowing through the cake causing a danger of blocking. When pre-coat and body feed were used with polystyrene particles as an impurity, the blocking appeared to occur rather easily on top of the original pre-coat and on the filter medium. The type of pre-coat and the way in which it was formed were very important. In beer filtration, which is mainly a surface filtration, a small concentration of passing impurity was able to block the pre-coat layer, when the filter aid of the body feed was coarser than that of the pre-coat.     

Pressure Slip Casting of Graded Coarse Grained Oxide Ceramics for Refractory Applications

In the present study the fabrication of graded coarse grained ceramic bodies for refractory applications was investigated using a pressure filtration cell,which uses pressurized air as the pressure medium. The pressure filtration cell was used to examine the influence of different ceramic raw materials,two dispersants,and of the particle size distribution on the filtration behavior and on the filter cake surface quality. Pressure slip casting of spinel based slips resulted in crack-free filter cakes,while alumina based slips resulted in filter cakes,which always stuck on the used PMMA filter medium. Furthermore,citric acid as an electrostatic dispersant resulted in a much lower filtration cake resistance and hence shorter filtration times than an electrosteric dispersant. A broader particle size distribution with a higher addition of the finest particle fraction caused an increase in the filter cake resistance but also yielded much better surface qualities. Finally,graded filter cakes with three layers of alumina rich magnesium aluminate spinel with a maximum grain size of 3 mm,1 mm,and 0. 5 mm,respectively,were fabricated. The computed X-ray analyzes indicated a perfect bonding between the three layers,which is a significant improvement to previous studies. However,the graded filter cakes exhibited frequently cracks at the bottom,which was probably caused by tensile stresses from the friction of the filter cakes with the pressure filtration cell. This will be further investigated in subsequent studies. The achieved results demonstrate thefeasibility to produce coarse grained oxide ceramics for refractory applications by pressure slip casting,which eventually allows the production of composites with tailored compositions,microstructure and functionality.     

The influence of conditioning and regeneration on the separation behaviour of rigid surface filters for the separation of particles from gases

Rigid ceramic filter media can be used for the separation of particles from gas streams at elevated temperatures. In order to characterize comparatively the separation behaviour of differently structured filter media over a multitude of filtration cycles, experiments were performed in a filter test rig. The filter test rig used is built in accordance with VDI guideline 3926 and equipped with a special type of optical particle counter, which has the advantage of measuring both the particle size and the particle concentration simultaneously and in situ on the clean gas side.

It is demonstrated that by far largest share of the particles reaches the clean gas as a result of the filter regeneration process. During the subsequent formation of the dust cake, the particle penetration is almost zero. The regeneration parameters, actually in essence only the tank pressure, possess the decisive influence on the separation behaviour. In the course of a filtration experiment, the number of particles reaching the clean gas reduces with the increasing number of filtration cycles due to the filter conditioning. The particles arriving in the clean gas during regeneration are extremely fine. The mean particle size is almost entirely independent of the regeneration conditions and the filter cycle number. The fine particulate emissions measured here are an anthropogenic source of particulate matter in ambient air. They have to be considered as relevant with respect to the new standards of ambient air quality (PM 10 and PM 2.5).

It is shown that membrane-coated ceramic filter media at identical operating conditions exhibit, as expected, a better separation and regeneration behaviour than fibrous ceramic, open-pored filter media. However, the advantages are at the expense of a 10 times higher pressure loss, which is essentially attributed to the membrane layer.     

Use of models in the design of cross-flow microfilters for the purification of protein from bio-mixtures

In order to enhance the purification efficiency of protein from binary bio-mixtures, the parameters for the design of cross-flow microfilters were examined by using models. The filtration flux, the cake thickness, the protein rejection coefficient and the protein purification flux for two different geometrical-shape microfilters, two-parallel-plate (TPP) and circular porous tube (CPT) type filters, under various cross-flow velocities and filtration pressures were calculated and are discussed here. The major factors affecting cake formation as well as the overall filtration resistance were found to be the hydrodynamic forces exerted on the depositing microbial cells. Therefore, the filtration rate increased with the increase of the cross-flow velocity and filtration pressure under a wide range of conditions. Nevertheless, two competitive effects, the cake thickness and the sweeping factor on the membrane surface, should be taken into consideration in order to evaluate the exact rejection coefficient and purification flux of proteins. The cross-flow velocity can be set at 0.7–0.9 m/s for a TPP filter to minimize the protein rejection and maximize the filtration rate and purification flux. Furthermore, the effects of the filter length, the clearance of TPP filters and the hydraulic diameter of CPT filters on the filtration rate and the protein rejection coefficient are also discussed. The filtration rate and the purification flux did not show significant variation along the cross-flow direction except in the filter inlet region; in contrast, decreasing the clearance or hydraulic diameter of filters was found to be an efficient way to enhance the performance of cross-flow microfiltration. According to the results of this study, the separation rate and purification efficiency can be improved by increasing the cross-flow velocity and using a TPP filter.     

Ansatz zur Quantifizierung der mechanischen Festigkeit eines Staubkuchens

In surface filtration for dust separation, the mechanical strength of dust cakes is of great importance for the operating behavior of the surface filter. This can be significantly influenced by various conditioning methods. A simple measurement method to quantify the strength of dust cakes does not exist at present. An approach for the experimental determination of the strength of dust cakes with measurement results comparing dust cakes with modified packing structur, generated by doing raw gas conditioning with hygroscopic salt particles, is presented and discussed.     

Selbstansaugende rotierende Sonde zur Entnahme von partikelfreien Gasproben aus Prozessgasen

A filtration system for keeping the analytics free of particles is usually required for inline characterization of particle‐laden process gasses. Additionally gas pumps may be needed for overcoming the pressure drop of filter element and filter cake. Consecutive reactions of process gas and filter cake may be disadvantageous for the quality of the gas analysis. A new probing apparatus is presented for extraction of particle‐free gas samples from particle‐laden process gasses. The probe combines the functionalities of a gas pump and a particle separator. While the gas is transported from process to analysis, particles are separated according to the principle of air classification.     

Effect of cake layer structure on colloidal fouling in reverse osmosis membranes

A series of reverse osmosis (RO) membrane filtration experiments was performed systematically in order to investigate the effects of various hydrodynamic and physicochemical operational parameters on a cake layer formation in colloidal and particulate suspensions. Bench-scale fouling experiments with a thin-film composite RO membrane were performed at various combinations of trans-membrane pressure (TMP), cross-flow velocity (CFV), particle size, pH, and ionic strength. In this study, silica particles with two different mean diameters of 0.1 and 3.0 μm were used as model colloids. Membrane filtration experiments with colloidal suspensions under various hydrodynamic operating conditions resulted that more significant permeate flux decline was observed as TMP increased and CFV decreased, which was attributed to the higher accumulative mass of particles on the membrane surface. Results of fouling experiments under various physicochemical operating conditions demonstrated that the rate of flux decline decreased significantly with an increase of the ionic strength as well as particle size, while the flux decline rate did not vary when solution pH changed. The experimentally measured cake layer thickness increased with a decrease in particle size and solution ionic strength. Furthermore, the model estimation of cake layer thickness by using a cake filtration theory based on the hydraulic resistance of membrane and cake layer was performed under various ionic strength conditions. The primary model parameters including accumulated mass and specific cake resistance were calculated from the cake layer resistance. This result indicated that the formation of cake layer could be closely related with solution water chemistry. The model estimated cake layer thickness values were in good agreement with the experimentally measured values.     

关于过滤理论与滤饼可压缩性的探讨——第二部分 滤饼的可压缩性及比阻

从料浆沉降和过滤成饼两个角度同时研究过滤速率和成饼速率以及滤饼的渗透率和可压缩性,提出了更为全面的滤饼比阻关系式,证明滤饼渗透率首先和料浆的浓度及沉降速度有关,因此和料浆的预处理及料浆的化学环境密切相关。明确了滤饼都是可压缩的,其可压缩性分为两大类：滤饼中颗粒的位移、迁移和团聚颗粒的变形,由于它们的被压缩的机理不同,所以和有关因素的相关关系也不同,分析和论述了主要的关系。全文分为两大部分,相对独立     

关于过滤理论与滤饼可压缩性的探讨——第一部分表层过滤的特性和沉降的密切关系

从料浆沉降和过滤成饼两个角度同时研究过滤速率和成饼速率以及滤饼的渗透率和可压缩性,提出了更为全面的滤饼比阻关系式。证明滤饼渗透率首先和料浆的浓度及沉降速度有关,因此和料浆的预处理及料浆的化学环境密切相关。明确了滤饼都是可压缩的,其可压缩性分为两大类：滤饼中颗粒的位移、迁移和团聚颗粒的变形,由于它们的被压缩的机理不同,所以和有关因素的相关关系也不同,分析和论述了主要的关系。全文分为两大部分,相对独立。但本文不涉及滤饼的压榨     

Cross‐Flow Microfiltration of Fine Particles Suspended in Polymeric Aqueous Solution

Abstract

The filtration characteristics of cross‐flow microfiltration of fine particles suspended in polymeric aqueous solution are studied. Polymethylmethacrylate (PMMA) submicron particles are suspended in polyacrylamide (PAA) aqueous solution to prepare the suspensions used in experiments. Effects of operating conditions, such as cross‐flow velocity, filtration pressure, and PAA concentration, on the filtration flux and the cake properties are discussed. The results show that an increase in cross‐flow velocity or filtration pressure causes the filtration flux to be higher, but the filtration flux decreases with an increase in PAA concentration. Since the flow behavior indices of three prepared suspensions are almost the same, the average specific filtration resistance of cakes under various cross‐flow velocities and PAA concentrations remain almost constant; and then the cake mass plays a major role in determining the filtration resistance and the filtration flux. A force balance model is derived for particle deposition on the membrane surface. Once the empirical coefficients are obtained from experimental data, the filtration flux at pseudo‐steady state can be predicted accurately.     

Cross-Flow Microfiltration of Bacillus Subtilis Broths under Various Culture Times

Bacillus subtilis broths under different culture times are filtered in a cross-flow microfilter. The operating condition effects, such as cross-flow velocity, transmembrane pressure, and broth culture time, on the filtration flux, cake properties, and extracellular polymeric substances (EPS) transmissions are discussed thoroughly. The culture broths contain B. subtilis cells and EPS which is characterized as polysaccharides (hydrocarbons) and proteins. An increase in broth culture time leads to higher concentrations of cells, soluble and extractable EPS. The total protein to polysaccharide concentration ratio in the broths is ca 0.2. However, the soluble polysaccharide concentration is 10-fold higher than that of soluble proteins. The filtration flux increases with increasing cross-flow velocity or transmembrane pressure. However, the impact of cross-flow velocity is more significant. The filter cake resistance formed by B. subtilis cells and EPS flocs plays the most important role in determining the overall filtration resistance. The mass and average specific filtration resistance of cake can be estimated using a force balance model and empirical equations. The cake structure and thickness are analyzed using SEM. A thicker and more compact cake may be formed under longer broth culture time. Most soluble polysaccharide and protein molecules have the opportunity to penetrate through the cake and membrane into the filtrate because the solute transmissions are measured as high as 0.75–1.0. The influences of operating conditions on the polysaccharide and protein transmissions are negligible. Therefore, to enhance filtration flux by increasing transmembrane pressure or cross-flow velocity is beneficial to improve separation efficiency, especially by increasing cross-flow velocity.     

Mikroporöse Filtermedien für die Kuchenfiltration

Microporous filters media for cake filtration. Improved knowledge about filtration conditions, capillary phenomena in filter cakes and in filter media, as well as the properties of membranes permit the use of microporous membranes in cake-forming filtration. This allows removal of moisture without passage of gas and yields a particle-free filtrate. Apart from metallic and ceramic membranes, polymeric membranes prove highly suitable as microporous filter media since they permit utilization of the entire range of filter equipment. The present article considers first the fundamentals of cake-forming filtration, then surveys the various membrane systems, and finally discusses process-engineering aspects of filtration with microporous filter media.     

Investigations on the State of Water in a Flocculated Filter Cake

Abstract

The vacuum filtration of hematite slurries and flocculated hematite slurries with anionic, cationic, and nonionic polyacrylamides was investigated. Test results show that the flocculated filter cake generally contains higher residual water than the unflocculated cake. Measurements of flocculants adsorption on a hematite surface, zeta potential, and contact angle of the mineral surface were carried out; the filter cakes were analyzed with a microscope and image analyzer; and the structure of the filter cakes was studied from the viewpoint of fractal theory. The existence state of residual water in flocculated filter cake was discussed. It was pointed out why a flocculated filter cake generally has a higher residual moisture content.     

Clarification of liquids using filter aids Part III. Cake Resistance in surface filtration

The specific cake resistance of mixed cakes was studied in filter-aid filtration using pre-coat and body feed. The influence of the relative amounts (weight ratio c/a) and the properties of the filter aid and the impurity on the specific cake resistance (r) was determined. An exponential relationship between r and c/a was derived, based on the blocking behaviour of a filter cloth. This relationship was confirmed in the filtration of protein flocs from turbid beer, and yeast, iron hydroxide flocs and monosize polystyrene particles from water. Based on this law, an optimum body-feed dosage can be predicted in order to obtain a minimum cake resistance. In beer filtration special attention has been paid to the compressibility of the mixed cakes.     

Filtration in coal liquefaction. Influence of filtration conditions in non-hydrogenated systems

A series of experiments has been carried out to study the effects of filtration conditions upon the rate of filtration of non-hydrogenated coal digests. The results show the dependence of cake resistivity on both the filtration temperature and pressure. Filter cakes were found to be compressible, resulting in smaller increases in rate with increasing pressure than with incompressible cakes. The filtration temperature determines the packing of residual solids in the cake which in turn affects the cake resistivity. An empirical relation has been derived between filtration temperature and resistivity. With increasing temperature there is an increase in filtration rate due to the reduced viscosity, but a reduction owing to a higher packing density of solids in the filter cake.     

High-pressure filtration—a chance for the downstream processing of biological dispersions

High-pressure filtration is a common practice in solid-liquid separation. A pressure up to 150 bar is used first for short-time drainage and then to press the filter cake with the help of an elastomer diaphragm.After the principal investigation of this concept and measurements to separate biological dispersions, its use in several processes is proposed. For suspensions of cells a filter cake with a high proportion of dry residue can be obtained within a very short time. The optimal regulation of this process on an industrial scale can be obtained by evaluation of laboratory experiments. For separation procedures with biomass and fermentation liquid the high pressure can be used to shorten the filtration time. Bacterial dispersions may be separated effectively with a cross-flow filtration apparatus which is to be developed for work at high pressure.The filtration experiments were performed on bakers' yeast (leaven) and Escherichia coli (E. coli) bacteria as model substances with a laboratory filter press apparatus. The filtration velocity and the specific filtration resistance are reported. Furthermore, the processing costs in relation to the proportion of dry residue were calculated as a function of pressure. The minimum cost corresponds to the economical operation range. Though this process is unjustifiably considered as relatively expensive, it is possible that, worked at its best, it will be able to compete with other mechanical separation processes.     

Magnetic field enhanced cake filtration of superparamagnetic PVAc-particles

The combination of two classical separation methods, cake filtration and magnetic field driven separation, for superparamagnetic nanocomposites results in positive synergetic effects and in extension of the field of application of the cake filtration process. In inhomogeneous magnetic fields magnetic particles experience a magnetic force. Experimental results show that two different effects of the magnetic field influence the cake building process. A special configuration of the magnet system leads to a slow down of the cake built-up. Due to changes of the structure the cake itself has a higher permeability. The result is an increase of the overall filtrate mass flow and therefore an improvement of filtration kinetics.This new process could be applied to the emerging field of biotechnology, especially in the so called downstream processing.Since the different components (protein, DNA, etc.) of, e.g. a fermentation broth are very small in size and have similar physical properties, the extraction of the target component out of this mixture is achieved only with high effort and expenses in a multi-step process. With the use of tailor-made magnetic adsorbent particles this process chain can be reduced considerably. The surface of these magnetic beads is manipulated in a way that only the target component is adsorbed selectively. Therefore the surface functionalization has to be concerted with the target component as well as the other side components to avoid the adsorption of one of those. The following separation of the target bio-product out of the remaining mixture is then accomplished due to the magnetic properties of the adsorbent particles.This paper discusses in detail the results of magnetic filtration experiments of non-functionalized particles. Such results are required for further development of this process for industrial scale bio-production. In this work the acceleration of cake filtration due to the above mentioned magnetic field effects is shown. Based on that a theoretical approach is suggested that describes both effects individually providing reference of their interaction.     

Flux enhancement and cake formation in air-sparged cross-flow microfiltration

The flux enhancement in cross-flow microfiltration of submicron particles by sparged air-bubble is studied. The effects of operating conditions, such as air-bubble velocity, suspension velocity and filtration pressure, on the cake properties and filtration flux are discussed thoroughly. The results show that the pseudo-steady filtration flux increases as the air-bubble velocity and filtration pressure increase. The sparged air-bubble can significantly improve filtration flux, but the flux enhancement is more remarkable in the lower air-bubble velocity region. A gas–liquid two-phase flow model is adopted for estimating the shear stress acting on the membrane surface under various operating conditions. The cake mass can be significantly reduced by increasing the shear stress acting on the membrane surface. However, the SEM analysis illustrates that the particle packing structure becomes more compact as the air-bubble velocity increases. This results in a slightly higher average specific cake filtration resistance under higher air-bubble velocity. Consequently, a minimum flux occurs at the critical shear stress, e.g., τ_w = 1.1 N/m² in this study, when these effects are both taken into consideration. As the shear stress increases by increasing the suspension or gas-bubble velocity, the filtration flux decreases in the low shear stress region but, on the contrary, quickly increases in the high shear stress region. Furthermore, a force balance model is derived for understanding the particle deposition on the membrane surface. The relationship among filtration flux, shear stress and overall filtration resistance is obtained and verified by experimental data.     

Performance of fibrous filters during nanoparticle cake formation

Fibrous filters are highly efficient in removing micrometer particles, but their performance in the nanometer particle range is still little known. The aim of this study was to evaluate pressure drop and collection efficiency during nanoparticles cake formation using commercial fibrous filters. The filter media used were High Efficiency Particulate Air (HEPA) and polyester filters. The aerosols were generated by a commercial inhaler using a 5 g/L solution of NaCl and the particles produced were in the size range from 6 to 800 nm, with a peak at around 40 nm. A superficial velocity (v_s) of 0.06 m/s was employed. During the filtration, the maximum pressure drop established was ?P ₌ ?P_f +980Pa, where ?P_f is the initial pressure drop of the filter. The collection efficiency was determined for a clean filter and for intermediate pressure drops. The filtration curves obtained showed that the HEPA filter provided greater surface filtration, compared to the polyester filter. Comparison of the collection efficiencies for clean filters revealed that the HEPA filter was highly efficient, even in the absence of cake, while the polyester filter showed initial collection efficiencies of between 20 and 40% for particles in the size range from 100 nm to 1000 nm. However, after formation of the filter cake, the collection efficiencies of both filters were almost 100% during the final stage of filtration. This shows that the fibrous filter can be applied in several industrial processes with highly efficient nanoparticle separation, after the formation of a thin layer cake filtration.