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.     

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.     

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.     

Analyses of Compressible Suspensions for an Effective Filtration and Deliquoring

Cake filtration is frequently used in the industry to remove the majority of liquid from a suspension before the solids are dried in a thermal dryer. As the amount of liquid removed in the filtration step is an order of magnitude greater than the liquid removed during thermal drying, a slight improvement in the cake dryness from a filter can greatly reduce the duty and energy cost of the dryers. The most challenging issue in cake filtration is when a compressible material is encountered. High cake compressibility will not only slow down the filtration process, but also cause high moisture content in the final cake. Instead of pumping pressure, mechanical expression is the best deliquoring method when dealing with highly compressible cakes. Therefore, it is important for industrial users to recognize the degree of compressibility of the material being treated. In this work, a simple method is developed to estimate the cake compressibility from a single filtration run. This method provides a practical tool for engineers to understand the compressibility nature of the cake and select the right filtration/deliquoring strategy.     

Analyses of Compressible Suspensions for an Effective Filtration and Deliquoring

Cake filtration is frequently used in the industry to remove the majority of liquid from a suspension before the solids are dried in a thermal dryer. As the amount of liquid removed in the filtration step is an order of magnitude greater than the liquid removed during thermal drying, a slight improvement in the cake dryness from a filter can greatly reduce the duty and energy cost of the dryers. The most challenging issue in cake filtration is when a compressible material is encountered. High cake compressibility will not only slow down the filtration process, but also cause high moisture content in the final cake. Instead of pumping pressure, mechanical expression is the best deliquoring method when dealing with highly compressible cakes. Therefore, it is important for industrial users to recognize the degree of compressibility of the material being treated. In this work, a simple method is developed to estimate the cake compressibility from a single filtration run. This method provides a practical tool for engineers to understand the compressibility nature of the cake and select the right filtration/deliquoring strategy.     

Empirical Modelling of Cake Washing in a Pressure Filter

The focus of this article is on empirical modelling of filter cake washing. The filtration experiments introduced in this paper were conducted by using a pilot-scale (0.1 m²) filter press according to the basic principles of factorial designs. Five different variables of the filtration, pressing, cake washing, and air drying stages were considered in the tests and the examined product characteristics were the overall capacity of the filter and the purity of the cake. The results obtained from the tests were used for creating different kinds of regression models for explaining the influence of the studied variables on the success of the cake washing process. The goal of the modelling strategy for the cake washing was to determine the simplest empirical models and compare these with theoretical equations complemented with linear terms. It was found that the empirical equation could model the results more accurately than the theory-based equations could.     

Numerical modelling of filtration with and without sedimentation runs

From experimental batch runs of filtration with sedimentation above the cake formed, a procedure is proposed and applied to experimental data for obtaining the relationships between the settling solids velocity in the hindered settling zone, the local specific resistance and the effective pressure and the solids concentration. Considering the material and momentum balances in the hindered settling zone above the cake, in the cake and in the filter membrane, a simulation program was obtained that satisfactorily reproduces the experimental runs of filtration with sedimentation runs. This simulation work confirms the hypothesis considered in the theory of filtration with sedimentation runs, e.g. the fact that the characteristic lines arise tangently from the cake surface. In addition, the method developed for obtaining the different relationships considered has been demonstrated as correct. The simulation of filtration without sedimentation runs (stirring the suspension above the cake) has also been studied.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE*

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used. The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.

     

Local solidosity of microcrystalline cellulose during dead-end filtration and sedimentation

Solid–liquid separation by filtration and sedimentation are important operations used in a wide range of industries. One important characteristic of both the filtration and sedimentation processes is the solidosity of the filter cake/sediment that is formed, affecting the efficiency and design of the separation. In this study local solidosity was investigated using a γ-attenuation method during both filtration and sedimentation experiments for microcrystalline cellulose, a highly crystalline cellulose with particles of about 2–80 μm in diameter. Constitutive relationships for the solidosity were investigated using both filtration (i.e. cake build-up and expression) and sedimentation data for experiments at different pH and suspension concentrations. The sedimentation behaviour under these different conditions was also investigated. It was found that a three parameter empirical model could be used to describe the constitutive relationship between local solidosity and local solid compressible pressure for the sediment formed and the filter cake after both cake build-up and expression. This correlation worked well for the material investigated even at low solid compressible pressures.     

Comparison of different scraper designs for crossflow filtration with rotating discs and cake limitation by scrapers

Results from filtration experiments comparing the filter cake formation of different scraper designs during multiple-pass operation of the continuous pressure filter at laboratory scale are to be presented. The main interest in this investigation was to clarify the influence of different scraper designs on the filter cake growth depending on the rotation speed of the filter discs. The influence of different scraper designs on flow conditions between the filter discs and the action of the scrapers was also examined. The results of the filtration experiments showed, that the formation of the filter cake essentially depends on the rotation speed of the filter discs. The main mechanism during the formation of the filter cake of finely dispersed solids is the effect of cake thickness limitation by turbulence eddies. The dominating mechanism during the formation of the filter cake of solids with a wide spectrum will be the effect of particle segregation. In experimental studies the comparison of three different scraper designs (straight and curved scraper with different scraper profiles) was done in order to investigate the influence of the scraper shape to the formation of the filter cake at different rotation speeds and to achieve a basis for scraper design optimization. Summing up, a scraper profile with increased overflow velocities (e.g. the ‘plough 120° scraper’-profile) in a curved form (in order to improve the normal flow velocity at the inside part of the filter discs) would improve the filtration behaviour compared to the straight, flat scraper most efficiently.     

Continous filtration: Calculation of cake impurity and liquid yield

Formulae are derived, concerning the separation of solids from liquid by a continuous filtration procedure, consisting of the filtration proper and the washing of the filter cake by a suitable wash solvent.In the derivation of the formulae it has been assumed that the mixture of solids and liquid to be filtered is prediluted with a low-viscosity liquid in order to make possible or to speed up filtration. However, the formulae can easily be adapted to cases where no predilution is used.The effect of the process variables on cake impurity and liquid yield is shown. Some numerical examples have been tabulated and plotted for practical purposes.The beneficial effect of filtrate recirculation on the degree of separation, already known in filtration practice, has been worked out quantitatively in formulae, tables and graphs.On application of the formulae to the dewaxing of lubricating oils with the aid of butanone-benzole mixtures, used both as diluent and as wash solvent, good agreement with practical results was obtained.     

The Permeability Distribution (PD) Method for Filter Media Characterization

Cake filtration is frequently used for the removal of particulate solids from fluids in industrial processes. The build up of a filter cake is usually accompanied by a decrease in overall permeability of the filter leading to an increased pressure drop over the filter medium. For an incompressible filter cake that builds up on a homogeneous filter cloth (surface filtration mode), a linear pressure drop profile is expected over time. However, occasionally experiments show curved pressure drop profiles. Whereas pressure drop profiles with increasing slope are generally ascribed to cake compression and/or depth filtration, pressure drop profiles with decreasing slopes are only ascribed to inhomogeneities in the filter. Such inhomogeneities can arise due to filter cake patches and/or an inhomogeneous filter cloth itself. In this work a method is proposed that transforms the pressure drop profile of a filter into a permeability distribution (PD) of the filter medium, thus accounting for possible inhomogeneities of the medium. The determination of the PD is looked at as an inverse problem of an integral transformation. The method is applied to experimental filter pressure drop data of laboratory scale jet-pulse cleaned bag filter plants. It is found that even clean filter media can exhibit a significant permeability profile.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE *

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used. The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.     

PRESSURIZED ELECTROOSMOTIC DEWATERING IN A FILTER CYCLE

This paper describes the role of pressurized electroosmotic dewatering (EOD) in a filter cycle. A laboratory-scale filter-press was used to filter a highly conductive silica suspension under constant pressure, followed by washing and mechanical precompression of the filter cake and finally by pressurized EOD at constant electric current. The influence of filter cycle parameters (filtration and washing pressure and duration, mechanical pressure, and electric field intensity) on the final cake dryness and energy consumption was studied. Electrodes of different materials, forms, and surfaces were used.

The optimal conditions of each filter cycle operation were found to minimize the energy consumption during EOD and maximize the cake dryness. With mechanical pressure and electric field intensity increasing, the total energy consumption increased, but the specific energy consumption (per kg of expressed water) decreased, and the final filter cake was dryer. The pressurized EOD also used less energy than thermal drying.     

Wiederbefeuchtung bei der Pressfiltration

During press filtration, the filter cake experiences a plastic deformation. However, a filter cake expansion may occur once the pressing forces are released, which is confirmed by experiments with an aqueous coal suspension. Rewetting of the cake occurs when a liquid film is present during cake swelling, which deteriorates the cake dryness significantly. However, this can be prevented when the liquid film is successfully removed, and lower pressures suffice for mechanical cake drying.     

Experimental and analytical modeling of the filtration mechanisms of a paper stack candle filter

A candle filter consisting of a paper stack is an adequate device to clarify the grinding oil employed in the industrial post-processing of hard metal because of its cleaning efficiency and its regenerative capability by means of backwashing. The complex particle deposition mechanisms occurring in this device have not yet been investigated. Filtration experiments with a suitable particle-oil suspension were performed in a pilot filter and in a laboratory filter in order to investigate and model the mechanisms taking place in the process. The effect of operating parameters such as particle concentration and pressure difference on filter performance was evaluated. The results show that more than one filtration mechanism takes place simultaneously. While some depth filtration occurs at the beginning of the lifetime of a candle filter, blocking and cake filtration are the major mechanisms responsible for the filter clogging. Although blocking and cake filtration occur, to some extent, simultaneously, the cake filtration is the mechanism relevant for the modeling and scale-up of the long term filtration process.     

Fe_3Al金属滤芯特性及在U-gas煤气化装置中的应用

为了解决U-gas粉煤气化工艺飞灰过滤器装置中陶瓷滤芯的断裂问题,分析了陶瓷滤芯和金属滤芯的性能差异,在装置中安装了国产改进2050型Fe_3Al金属滤芯并投运试验,研究了投运后合成气及飞灰特性、滤芯的微观形貌、过滤效率、流量-压差曲线、滤饼渗透性等特性。结果表明,在与陶瓷滤芯相同过滤精度条件下,Fe_3Al金属滤芯具有孔隙率高、渗透性高、压溃强度大等优势。投运结果表明,Fe_3Al滤芯投运3个月以来无断裂故障,在相近工况条件下,Fe_3Al滤芯运行平衡压差相比陶瓷滤芯下降了50%,且水洗塔悬浮物固含量由1 000~2 000 mg/L下降至100~500 mg/L。     

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.