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.     

Avoiding filter cake cracking: Influence of consolidation on desaturation characteristics

In cake filtration processes with an air-blowing step, cracking is an undesirable phenomenon as it leads to deterioration of the filtration process by highly increasing gas throughput. This leads to higher residual moisture if the pressure difference cannot be maintained and an increase in overall cost. Crack formation can be avoided by compacting the filter cake before desaturation. While this action will make the separation process applicable by highly reducing gas consumption, there are also potential negative effects. Compaction increases filter cake resistance and might therefore slow down desaturation kinetics. Therefore, the authors investigated how the filter cake characteristics governing desaturation change from the nonconsolidated to the consolidated state of the filter cake and compared these findings to the actual dewatering kinetics. The results showed that for the case where cracking could be oppressed, dewatering kinetics of the consolidated cake are actually faster than for the nonconsolidated cake, despite higher resistance of the consolidated cake. Thus, compaction is an appropriate action when dealing with filter cake cracking.     

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.     

The preparation and filtration characteristics of Dextran-MnO2 gel particles

Different kinds of Dextran-MnO₂ gel particles are prepared in different conditions, depending on variables such as the molecular weight of dextran and formation temperature. Some physical properties and filtration characteristics of these gel particles are measured and discussed. Although the mean sizes of these gel particles are very close to each other, their filtration characteristics are far different due to their mechanical strength and compressibility. A typical filtration curve of gel particles can be divided into three regions, and a retardation cake compression during the filtration can be observed from the curve. The particle formation temperature has a trivial effect on their mechanical strength. An increase in formation temperature leads to only a slight decrease in particle size. On the other hand, gel particles are formed by using dextrans with three different molecular weights - 70,000 (sample A), 500,000 (sample B) and 2,000,000 Da (sample C) - and are used in filtration experiments. The results show that the molecular weight of dextran plays a major role in determining particle mechanical strength; the sequence of particle hardness is sample B > sample A > sample C. The dynamic analysis method proposed by Hwang and Hseuh [K.J. Hwang, C.L. Hseuh, J. Membr. Sci. 214 (2003) 259] is employed to estimate the local cake properties in a filter cake, e.g., solid compressive pressure, porosity and specific filtration resistance. Because sample C has the highest compressibility, it constructs a cake with the lowest porosity and the highest specific filtration resistance; and consequently, the lowest filtration rate. Although the mechanical strength of sample B is higher than that of sample A, its wider particle size distribution results in lower cake porosity and higher filtration resistance, as well as a lower filtration rate. It could be said that the filtration rate decreases with an increase in the molecular weight of dextran.     

Rheology,Sedimentation, and Filtration of TiO2 Suspensions

It is shown that with aqueous suspensions of colloidal oxide and hydroxide particles simple rheometric and sedimentation experiments allow the determination of the optimum pH for filtration and washing. This approach is more practical than zeta potential measurements, as explained by a theoretical review and illustrated by experiments with industrial TiO₂ white pigments of varying surface chemistry. Besides, the solids content of the filter cake and its dependence on pH is correctly predicted from the rheology of the suspension, thus giving an idea of the mechanism of filter cake formation. This follows from a comparison to the results of filtration experiments. There, the influences of filter vacuum as well as of pH and solids content of the pigment feed suspension on filtration capacity and washing efficiency have been studied. The experiments also illustrate an inexpensive method of how to find the optimum set of parameters for an economical drum filter operation in the plant.     

Simulation of filter cake porosity in solid/liquid separation

A simulation of filter cake formation as a random process has been conducted. The random packing occurs after spherical particles are generated at a randomly selected coordinate above the cake. The force balance of the particles considers the driving force of filtration, skeleton forces due to virtually overlying particle layers, and adhesive forces. The factors for determining when a particle stops are defined as follows:

a.

Adhesion to the contacting particles based on the force balance.

b.

Finding a steady position if contacting three or more particles of the cake.

The calculations deliver local porosities, mean porosities and size distributions of the pores dependent on the filtration parameters. The results confirm observations made in filtration experiments.     

DEM‐simulation of the magnetic field enhanced cake filtration

Using the discrete element method, we simulate numerically the cake formation and growth in magnetic field enhanced cake filtration to give further insight on the mechanisms of the structuring of the filter cake due to the interaction of magnetic, hydrodynamic, and mass forces. The motion of the discrete particles is obtained by applying the three‐dimensional Newton's equations to individual particles, allowing for both external forces (gravity, applied magnetic field) and particle–particle interactions calculated using the modified DLVO‐theory. Continuous liquid phase flow is assumed as one‐dimensional. The simulation results compare favorably with reported experimental data, 1 and can be used to delineate the regimes associated with different liquid flow and magnetic field effects that are observed experimentally. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

ABSOLUTE ON-LINE MEASUREMENT OF THE MAGNETIC MOMENT OF AEROSOL PARTICLES

We developed a method of on-line measurement of the magnetic moment of aerosol particles. For this purpose we determine the deposition of the particles in a filter consisting of magnetic nickel screens. Deposition is governed by the dipole force magnetic particles experience near the magnetized wires of the screens. The number concentration at the in-and outlet of the filter is measured by optical particle counters. The so called single fiber efficiency is derived directly from the measured data as a function of particle size, particle magnetic moment and fiber magnetization. The relationship between this dimensionless value and the magnelic moment is obtained by a computer simulation using trajectory analysis. The method is tested with equally sized iron/iron oxide particles in the size range from 40 to 170nm.     

The Effect of Particle Sedimentation on Gravity Filtration

ABSTRACT

Simulation of cake formation of mono-sized and dual-sized particles under gravitational sedimentation and filtration is presented. The dynamic analysis proposed by Lu and Hwang in 1993 is applied to examine the local cake properties formed under a falling head by considering the hindered settling effect of particles in the slurry and the variation of the pressure drop across the filter septum. Results of this study show that, at a given position in a cake, the solid compressive pressure reaches a maximum value and then decreases for a gravity filtration due to the decrease in the driving head. A cake constructed with dual-sized particles has a more compact structure than does one with mono-sized particles, and larger particles will form looser packing than will smaller ones for mono-sized particles. A dual-dispersed suspension with a lower fraction of large particles will result in the lowest cake porosity and the highest specific filtration resistance of cake. Comparison of the porosity distribution in filter cake formed by means of gravity filtration and constant head filtration shows that the porosity near the filter septum of gravity filtration has a convex behavior while that of constant head filtration has a tendency toward concavity. This discrepancy is mainly due to the change in the driving head during the filtration process. Both theoretical and experimental results show that the uniformity of particle size distributions in the filter cake will be much better when the relative settling velocity between large and fine particles is reduced.     

The Vacuum Filtration of Iron Ore Ultrafines

ABSTRACT

The dewatering of fine particles (? 0.5 + 0 mm) derived from iron ore processing is a topic of increasing importance to Australian producers. With little previous published work in this area, it is difficult to predict how these materials are going to dewater, in particular during vacuum filtration. The work described in this paper was therefore undertaken in order to help overcome this paucity of background information. The sample of iron ore that was tested responded well to vacuum filtration provided that anionic flocculants were used as a filter aid. In the presence of these polymers, cake formation times were very short (mainly below 10 seconds) and the moisture levels of 18 mm thick filter cakes were reduced to 10–11 wt% after dewatering times of only 40 seconds. By comparison, the cationic and nonionic flocculants tested were much less effective. The mathematical model devised by Wakeman shows considerable promise as a toolfor predicting the kinetics of desaturation.     

Improvement of dead-end filtration of biopolymers with pressure electrofiltration

Solid/liquid separation of biopolymers such as polysaccharides and proteins is still a problem not solved sufficiently on a technical scale. The main objective of the presented work was to set up a process strategy in order to improve the recovery of biopolymers by dead-end filtration. One aspect was the investigation of the influence of the pH value, ionic strength and pressure on the filtration kinetics of the dead-end filtration of the polysaccharide xanthan. pH value and ionic strength have an impact on the hydrodynamic radius and the zeta potential of biopolymers, and thus they have an impact on the filter cake structure and the filtration kinetics. The main focus was set on the enhancement of the filtrate flux by an electric field. This process, called pressure electrofiltration, leads to a drastic improvement of the filtration kinetics. The filtration time was thereby reduced from the range of hours down to minutes. Additionally another strategy was followed up, which aimed at an improvement of the specific filter cake resistance by changing process parameters like the pH and the ionic strength. These parameters influence the polymer-polymer and the polymer-water interactions and thus have an influence on the filter cake properties. Due to the great acceleration of the filtration kinetics the pressure electrofiltration serves as an interesting alternative to the cross-flow filtration and the precipitation with alcohol for the separation of biopolymers.     

Experimental Study of the Mechanism of Constant Pressure Cake Filtration: Clogging of Filter Media

Abstract

Failure of data taken in pilot plant filtration of liquefied coal to fit conventional analysis led to research summarized in this paper. Historically, the effect of migrating fine particles in cake filtration has been ignored in theoretical treatments. In usual development, the total resistance to flow has been broken into cake resistance R_C and medium resistance R_m. Experimentors have only measured the total resistance and have assumed that R_m remained constant. In this investigation, a filter with seven probes was empolyed to measure the individual resistances. Medium resistance is found to increase with time and mass of dry cake per unit area due to migration of fine particles into the interestices of a filter medium.     

SHS of metal-oxide systems in a DC magnetic field: Part 1. TRXRD and thermal imaging studies of the Fe-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The effect of a 0.2 T external magnetic field on the heterogeneous combustion of a mixture of iron metal and oxide (Fe₂O₃) system with solid oxidizer (NaClO₄) was studied. Time-resolved X-ray diffraction (TRXRD) was used to study the kinetics of intermediate product formation and thermal imaging experiments to measure the reaction temperature and velocity. Metastable fcc iron phase formation was discovered as an intermediate predominantly in applied field reactions.     

Factors affecting cake resistance in non-Newtonian filtration

A methodology for evaluation of surface effects including polymer adsorption-gel formation or slip within the pores of a filter cake has been developed and applied to constant pressure filtration of calcium carbonate in aqueous hydroxyethyl cellulose (Natrosol 250G, Hercules Powder Company) slurries. The fundamental framework of non-Newtonian filtration is generalized to include the Blake, Kozeny-Ergun and Kozeny-Carman cake models and the cake resistances γ_K and α_R, in conjunction with the correction terms J_RN and J_gen, within a unified framework. Relationships are developed between the latter quantities and the effect of the bed model on the evaluations of the filtration characteristics is delineated. Significant polymer adsorption-gel formation was found to take place in the constant pressure filtration of calcium carbonate in aqueous Natrosol 250G slurries. The resultant cake porosities were larger and the effective cake porosities lower than comparable porosities for cakes deposited from water slurries. The wide variation of J_RN with the characteristic index N is attributed to the small size of the slurry particles. Polymer adsorption is shown to extend, and slip on the particle surface to reduce the variation of J_RN with N.     

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.     

A spatially resolved model for pressure filtration of edible fat slurries

A spatially resolved one dimensional pressure filtration model was developed for a slurry of edible fat crystals. The model focuses on the expression step in which a cake is compressed to force the liquid through a filter cloth. The model describes the local oil flow in the shrinking cake modeled as a porous nonlinear elastic medium existing of two phases, viz. porous aggregates and interaggregate liquid. Conservation equations lead to a set of two differential equations (vs. time and vs. a material coordinate ω) for two void ratios, which are solved numerically by exploiting a finite-difference scheme. A simulation with this model results in a spatially resolved cake composition and in the outflow velocity, both as a function of time, as well as the final solid fat contents of the cake. Simulation results for various filtration conditions are compared with experimental data collected in a pilot-plant scale filter press.     

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.     

Removal of corrosion products from viscous aqueous suspensions by magnetic filtration

The removal of micron-sized corrosion products, which may occur during industrial processing by magnetic filtration, has been theoretically and experimentally investigated. The effect of alterations in rheological properties of suspensions carrying particles on magnetic filter performance was determined. Magnetic filter dimensions were 0.03×0.04×0.09 m. The filter was filled with 6.35×10⁻³ m diameter ferromagnetic spheres. Magnetic fields ranging from B=0 to B=1.4 T perpendicular to the flow direction were applied. The suspension flow rate through the magnetic filter was 0.4 m/s. It was determined that 85% of corrosion products show magnetic properties. The magnetic filtration of suspensions composed of water, corrosion products and glycerine at different concentrations was studied. The effects of initial concentration of corrosion products, viscosity of aqueous suspension, magnetic field intensity and the number of filtration cycles on magnetic filter performance were determined. Theoretical and experimental results were in good agreement.     

热压过滤完善选煤工艺

论述了热压过滤干燥脱水机具有过滤与干燥的双重作用、脱水效率高等特点;分析了干燥压滤机内脱水峰面的形成和移动过程,以及物料中液体饱和度与时间的关系;介绍了热压过滤干燥工艺系统与装备,并通过应用实例说明其完善选煤工艺的意义。     

Flow Resistance in Filter Cakes Due to Air

Abstract

The objective of this work is to show that air bubbles can adversely effect the performance of pressure filtrations. The effect of air on liquid flow resistance is measured in a well-characterized filter cake of Lucite particles. Air is introduced into the cake by allowing air bubbles to become entrained with the particles in the slurry at the start of filtration. Filtration with entrained air bubbles is compared to filtration without air bubbles. Experimental measurements show a significant increase in flow resistance due to the presence of air in a pressurized cake filtration. The presence of air bubbles in the filter cake can result in misinterpretation of experimental data. This in turn can increase operating and experimental testing costs and complicate efforts to predict filtration performance.