Magnetic field enhanced press-filtration

The proposed hybrid process represents a new application of a magnetic field which directly influences a classical press filtration. The new technology offers high potential in the field of magnetic pigment production and iron oxide processing as well as bio-separation with functionalized magnetic particles. Especially in the field of fine-scale particulate product systems high specific cake resistances result in slow cake building and dewatering kinetics, which leads to economic inefficiency.Experimental and theoretical investigations show that the magnetic field has strong influence on cake building. Two major effects were observed: (I) In inhomogeneous magnetic fields magnetic particles experience a magnetic force counter directed to the pressure force, that results in slow down of cake formation; (II) Interparticle magnetic forces lead to structured cake formation.This gives on one hand the possibility to uncouple fluid and magnetic particle motion to force a cake built-up in designated location of the filter chamber. The result is a big increase of the overall filtrate mass flow and therefore an improvement of filtration kinetics. On the other hand due to the particle's magnetization including the formation of an attracting north and south-pole chainlike agglomerates can be observed. This leads to a “structured” cake building and therefore higher permeability.This work will show the effect of a superposed magnetic field on press filtration of ferromagnetic iron oxide particles (Fe₃O₄) in a lab-scale filter press.     

液力过滤与液力压密脱水的理论(Ⅱ)

将滤室固定不变的板框或箱式压滤机的操作过程分成滤饼过滤阶段和液力压密脱水阶段两个过程来研究。在滤饼过滤阶段 ,应用表面过滤机理 ,采用传统的鲁思过滤基本方程和平均过滤比阻的理论进行研究。在液力压密脱水阶段 ,应用达西定律建立了液力压密基本微分方程 ,根据研究得出滤饼渗透系数与孔隙率在压密脱水阶段的线性取代关系的结论 ;滤室固定的限制条件和物料平衡条件 ,推导出液力压密的脱水方程式。分析了影响液力压密脱水的时间因素和液力压密速率的各操作参数。建立了完整的液力过滤和压密脱水理论 ,为自动压滤机的设计提供了理论指导     

液力过滤与液力压密脱水的理论(一)

将滤室固定不变的板框或箱式压滤机的操作过程分成滤饼过滤阶段和液力压密脱水阶段两个过程来研究。在滤饼过滤阶段 ,应用表面过滤机理 ,采用传统的鲁思过滤基本方程和平均过滤比阻的理论进行研究。在液力压密脱水阶段 ,应用达西定律建立了液力压密基本微分方程 ,根据研究得出滤饼渗透系数与孔隙率在压密脱水阶段的线性取代关系的结论 ;滤室固定的限制条件和物料平衡条件 ,推导出液力压密的脱水方程式。分析了影响液力压密脱水的时间因素和液力压密速率的各操作参数。建立了完整的液力过滤和压密脱水理论 ,为自动压滤机的设计提供了理论指导     

煤气化细渣陶瓷膜真空脱水试验与数值模拟

气化细渣是煤炭气化过程的废弃物,高效脱水是其资源化利用、减量化处置的必要前提。本文采用陶瓷膜真空过滤系统开展了脱水实验并对脱水过程进行了数值模拟。气化细渣料浆浓度和液下吸附时间影响滤饼厚度且滤饼厚度增加导致水分运移路径增长,使得有效脱水时间增加;滤饼脱水过程的脱水速率值呈现非线性降低趋势且滤饼水分极限值为40%,这与气化细渣物化性质有关;真空度>0.08MPa时气化细渣滤饼中“通道水”能够在约24s有效脱除。Fluent数值模拟过程选用了欧拉模型并确定了陶瓷膜滤板和气化细渣滤饼的阻力系数,脱水过程的实验值与模拟结果误差小于5%,证实了模型可靠性。模拟过程分析了气化细渣脱水过程中压力场和水分含量分布云图的演变规律,结果表明,增加脱水系统真空度、降低滤饼厚度、提高“通道水”比例以及增大气化细渣颗粒等效当量直径能够提高气化细渣脱水效率。此外,陶瓷膜真空脱水过程所得滤液洁净度高且部分指标达到了工业用水的标准。     

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.     

Experimentelle und theoretische Untersuchungen zur Filtration mit überlagerter Sedimentation in Drucknutschen

Sedimentation superimposed on industrial cake filtration leads to longer filtration times and often has a detrimental effect on subsequent process steps such as washing and demoisturing. The influence of sedimentation is seldom recognised in laboratory filtration experiments. Methods are presented for evaluation of pressure filter experiments with superimposed sedimentation which avoid the error made in the usual evaluation methods. For the case of zone sedimentation the article presents a graphical evaluation and a numerical method of modelling permitting scale up to any desired cake height. In superimposed classifying sedimentation simultaneous measurement of filtrate volume and cake height provide information about local variation in filter cake resistance. It is shown for a model system that, owing to sedimentation, the cake resistance shows a minimum at mean cake height and increases rapidly towards greater heights. The local cake resistance corelates with particle size distributions measured for layers of a horizontally cut filter cake. The method of evaluation presented permints determination of the flow resistance of the uppermost layers of a cake and hence estimation of the gas pressure necessary for demoisturing. The profiles of local filter cake resistance show that the relative cake layering is largely independent of the level of suspension filling. A scale-up model is presented for use in those cases where classifying sedimentation cannot be suppressed.     

Thermal Dewatering (TDW) to Reduce the Water Content of Sludge

Thermal dewatering describes the process whereby a heating plate and heat supply unit are incorporated into a filter press system to improve separation of water from sludge. The performance of our thermal dewatering system for both wastewater and waterworks sludge was measured and compared with mechanical dewatering in terms of water content, dewatering velocity, cake specific resistance, and energy consumption. Dewatering velocity was improved by a factor of two, cake water content was lower, and specific cake resistance was improved. However, energy consumption was higher. Thermal dewatering systems may be economical and have practical application to improving dewaterability.     

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。     

One‐dimensional model of vacuum filtration of compressible flocculated suspensions

This work details the one‐dimensional modeling of the different processes that may occur during the vacuum filtration of compressible flocculated suspensions. Depending on the operating conditions of the applied pressure and the initial solids concentration relative to the material properties of the compressive yield stress and the effective capillary pressure at the air–liquid interface, the dewatering process undergoes a combination of cake formation, consolidation, and/or desaturation. Mathematical models for these processes based on the compressional rheology approach are presented and appropriate solution methods outlined. Results using customary material properties are given for different operating conditions to illustrate the three dewatering processes. This approach lays the theoretical basis for further work understanding two‐ and three‐dimensional effects during desaturation, such as cracking and wall detachment. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Effect of entrapped air bubbles on fine coal dewatering via filtration

An experimental study was conducted to obtain detailed information concerning entrapped air bubbles in fine coal filter cakes and their effects on filtration and the dewatering process. Four size fractions of coal samples (?80 mesh, ?42 mesh, ?82 mesh and ?100 +200 mesh) were investigated. An image analyzer with the aid of quantitative stereology was used to measure the entrapped air bubbles in relation to the microstructure of the filter cake (i.e., the particle and pore size distribution in the filter cake).The results show that the size and number of air bubbles entrapped in the filter cake can be controlled by the degree of aeration/deaeration of the coal slurry prior to filtration. Both the rate of filtration and the extent of dewatering are adversely affected by the presence of air bubbles in the filter cake. These adverse effects can best be understood in terms of the alteration of cake structure caused by air bubbles. Specifically, the reduction in connectedness of pores within the cake matrix tends to impede the flow of filtrate through the filter cake.     

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.     

Effect of Hydrodynamics During Crystallization on Mechanical Dewatering of Salicylic Acid

Mechanical cake dewatering is always desired to reduce the load on thermal dewatering (drying). Any change in the upstream process such as crystallization can have a significant influence on the filtration as well as cake dewatering characteristics. The present study deals with the effect of hydrodynamics (mixing intensity) during salicylic acid crystallization on the air dewatering characteristics in the subsequent pressure filtration. The mixing conditions during crystallization were varied by using three different types of agitators (anchor impeller [AI], curved blade turbine [CBT], and bar turbine [BT]) and by varying the speed of agitation. The effect of operating pressure and dewatering time on the final moisture content of the cake was also studied.

The crystal properties (crystal size and size distribution) were found to vary with the mixing intensity, which further influenced the cake dewatering kinetics as well as the residual moisture content. An AI, which is a laminar flow impeller, produced crystals with a wide size distribution and higher mean particle size, which resulted in cake with high porosity and hence higher moisture content. The high porosity (as well as high cake permeability) caused early air breakthrough, which resulted in ineffective dewatering of cake. Therefore, in this case the residual moisture in cake was found to be higher (27%) even at higher dewatering pressure (1.5 bar gauge) and longer dewatering time (90 s). A BT creates high turbulence during mixing and produced crystals with a relatively narrow size distribution and lower mean particle size, which provided low-porosity cakes. Such cakes could be efficiently dewatered and the final cake moisture content was found to decrease to about 15%, a significant improvement in the filterability of the cake. The dewatering data were modeled according to the correlation between irreducible cake saturation and capillary number for predicting the cake dewatering characteristics (residual moisture as well as dewatering kinetics) and the results were compared with the experimental data.     

硅藻土-木桨纤维为助滤剂强化过滤过程的研究(英文)

In this paper, a study to enhance the filtration for solid/liquid materials difficult to be filtered, such as highly viscous, highly compactible or gel like materials, is presented. Filter aids diatomaceous earth and wood pulp cellulose are used to enhance the filtration by improving filter cake structure and properties in the filtration of a biological health product and a highly viscous chemical fiber polymer melt product. The property of solid/liquid systems, filtration at different flow rates, specific cake resistance, cake wetness, filtration rate, filtrate turbidity for filter aid selection and evaluation, and operation optimization are investigated. The results are successfully applied to industrial process, and can be used as a reference for similar filtration applications.     

Dewatering of coal plant tailings: Flocculation followed by filtration

A sustainable alternative to tailings dam disposal of coal refuse is mechanical dewatering of tailings, which provides fast production of dry solids and water reuse. In this study, flocculation followed by filtration of coal plant tailings, a new concept in tailings dewatering is investigated in detail. This paper focuses on the effect of preconditioning tailings with varying flocculants and dosages on filtration kinetics and the resultant moisture content of the filter cake. The results show that the cationic flocculant, MAGNAFLOC LT 425, requires a high dosage to produce a low moisture content filter cake and clear filtrate. Optimal sized flocs were produced with the anionic flocculant, MAGNAFLOC 5250, even though the particles are negatively charged. The kinetics of the filtration was dependent on the composition of process water as indicated by supporting sedimentation tests. The concentration of divalent alkali earth metals such as Ca²⁺ and Mg²⁺ allow for large floc growth by a bridging mechanism, which involves binding of the polymer and the negativity charged particle. Filtration and settling curves at this dosage were also supported by filter cake analysis using Darcy plots. It was found that the large floc size significantly increases the permeability of the filter cake. Floc size measurements and fractal dimension showed that while the large flocs were produced with anionic flocculant, the flocs produced with the cationic flocculant were small and weak. The results indicate that the optimum dosage and flocculant type for effective and efficient filtration of coal plant tailings is approximately 350 g/t of anionic flocculant at a 35% solids content and 40 kPa filtration pressure.     

Statistical evaluation of hyperbaric filtration for fine coal dewatering

A statistical design of parametric study of pressure filtration for fine coal dewatering is presented. The effects of five major parameters of the dewatering, i.e. applied pressure, filtration time, cake thickness, solids concentration and slurry pH, on cake moisture reduction and air consumption were investigated. The study was conducted starting with two level factorial experiments to identify the most significant parameters, and concluding with response surface methodologies to establish an optimum operating condition for the dewatering of fine coal. It was observed that applied pressure, cake thickness and filtration time were identified to be the key operating variables for reduction of filter cake moisture as well as air consumption. With the key parameters, an optimum condition for the dewatering was determined to be an applied pressure of 93 psi with a cake thickness of 2.5 cm and a filtration time of 4.8 minutes for the laboratory filtration system. At these optimum conditions the filter cake containing about 22 percent moisture by weight and consuming air by 4.1 m³/(m²·min·kg) on dry solid basis was obtained.     

A model analysis on the reasons for unstable operation of jet-pulsed filters

Various physically based models are available to describe an uneven distribution of the filter cake on the filter area of jet-pulsed bag filters. Such an uneven distribution is intrinsically present, when only segments of the filter are cleaned at a time but not the entire filter. Moreover, also patchy cleaning causes an uneven filter cake loading, since only a fraction of a filter cake is removed by a jet pulse while the other fraction remains basically intact on the filter cloth. Unstable filter operation can be defined by a continuous or periodic reduction of the filtration time per cleaning pulse. The operation of a jet-pulsed filter was mathematically simulated and, by systematically altering model parameters, unstable operation was obtained. Three situations were investigated: a continuous increase of the filter cake resistance parameter, a continuous increase of the filter cloth resistance parameter, and a particular cake detachment function where, after a filter cake survived some filter cycles, it can hardly be removed. The transient pressure difference simulation results reveal characteristic patterns: Taking normal stable operation as a reference, an increase of the filter cake resistance leads to shorter filtration and somewhat longer cleaning intervals (i.e., more cleaning pulses). An increase of the filter cloth resistance causes longer filtration and also considerably more cleaning pulses. Deficient filter cleaning gives shorter filtration periods and extremely long cleaning intervals. A comparison between model simulations and pilot plant results shows that there, the experimentally observed unstable operation can most likely be attributed to problems with cake detachment. Hence appropriate measures for avoiding unstable operation were successfully introduced.     

Vibration-enhanced compaction of filter cakes and its influence on filter cake cracking

Filter cake cracking is a common problem in cake filtration that can be avoided by compacting the filter cake prior to air-blowing. Here, we present a new method for compaction by applying vibration in the form of oscillatory shear. The shrinking and cracking behavior of the model material is analyzed by combining laser displacement measurements and image analysis. Then, the compaction results are presented and discussed regarding the process time and energy consumption. Finally, we show how compaction by oscillatory shear influences the cracking behavior of the filter cake. Significant reduction in cracking could be achieved.     

The Effect of Flocculants and Surfactants on the Filtration Dewatering of Iron Ore Fines

Abstract

The filtration and separation characteristics of iron ore fines have been investigated. The experimental work included: characterization, evaluation of suitable flocculants to enhance settling and dewatering rate, determination of optimum dosages of flocculants, and influence of surfactants on nitration dewatering of both flocculated and unflocculated iron ore fines by vacuum filtration. The results showed that 1) the settling rate can be enhanced many-fold (from 2.52 to 90 m/h) by a suitable flocculant, 2) the residual filter cake moisture content can be reduced from 18.2% without reagents to 12.6% with suitable surfactant dewatering aids while the filtration rate can be enhanced from 4.8 to 97.2 L/h with suitable dewatering filter aids, and 3) the specific cake resistance to filtration can be brought down from 8.6 × 10⁹ to 1.0 × 10⁹ m/kg by using a surfactant sodium petroleum sulfonate at a concentration of 1.47 × 10^?3 kg/t. The economics of using flocculant filter aids and surfactant dewatering aids before thermal drying is described and discussed.     

Numerical simulation of soot filtration and combustion within diesel particulate filters

The numerous benefits offered by diesel engines, compared to gasoline ones, are balanced by a drawback of increasing concern, namely soot emissions. Nowadays, soot emissions can be reduced by physically trapping the particles within on-board diesel particulate filters (DPF). The filter gets progressively loaded by filtering the soot laden flue gases, thus causing an increasing pressure drop, until regeneration takes place. The aim of this work is to develop a fully predictive three-dimensional mathematical model able to accurately describe the soot deposition process into the filter, the consequent gradual modification of the properties of the filter itself (i.e. permeability and porosity), the formation of a soot filtration cake, and the final regeneration step. The commercial computational fluid dynamics (CFD) code Fluent 6.2.16, based on a finite-volume numerical scheme, is used to simulate the gas and particulate flow fields in the DPF, whereas particle filtration sub-models and regeneration kinetics are implemented through user-defined-subroutines (UDS).Model predictions highlight uneven soot deposition profiles in the first steps of the filtration process; however, the very high resistance to the gas flow of the readily formed cake layer determines the evolution into an almost constant layer of soot particles. The ignition of the loaded soot was simulated under different operating conditions, and two regeneration strategies were investigated: a “mild regeneration” at low temperature and oxygen concentration, that operated a spatially homogeneous ignition of the deposited soot, and a “fast regeneration”, with an uneven soot combustion along the axial coordinate of the filter, due to strong temperature gradients inside the filter itself. These findings are supported by comparison and validation with experimental data.