Effect of Cake Compression on the Performance of Centrifugal Dewatering

The effect of cake compression on the centrifugal dewatering is investigated under various rotating speeds and different kinds of cakes. A numerical method is proposed to estimate the capillary pressure and cake permeability under various cake saturations. The most dewatering in this study are operated at the funicular state of cake. The cake permeability decreases, while the capillary pressure increases rapidly, with the decrease of cake saturation during a centrifugal dewatering. The cake saturation can also be simulated once the relations among capillary pressure, cake permeability, and cake saturation are known. The deviations between simulated saturations and experimental data for compressible talc cakes are less than 2%. The centrifugal dewatering of compressible cake includes two mechanisms, the water squeeze due to cake compression and the centrifugal drainage due to pressure difference. The role of cake compression is dominant at the initial 500 s of the dewatering period, and these two factors are almost equal in magnitude as well as in importance when the cake approaches its equilibrium saturation. The porosity and compressibility of cake play the major roles in determining the equilibrium cake saturation. The equilibrium saturation of the compressible talc cake with a lower porosity is much higher than that of incompressible Al₂O₃ cake under a fixed rotating speed. An increase in rotating speed (centrifugal effect) results in a lower equilibrium cake saturation for either compressible or incompressible cakes; however, the increase in rotating speed is more efficient for an incompressible cake in the centrifugal dewatering.     

Compression Dewatering of Particulate Suspensions and Sludge: Effect of Shear

Experiments were carried out at the lab-scale to study the effect of a shearing action in a compression dewatering process. The impact of different process parameters, such as shear rate, filter-cake thickness, and pressure, on the final dry solid fraction was analyzed. Also, materials with different degrees of compressibility have been used. For slightly and moderately compressible materials (talc and kaolin, respectively), significantly higher final dry solid fractions (up to 0.79 wt/wt with talc and 0.75 wt/wt with kaolin) were obtained in the presence of a torsion shear. This behavior may mainly be explained by a rearrangement of the solid particles within the cake induced by the radial forces. A low shear rate (around 0.7 s^?1) was generally sufficient to cause a quite significant increase in the cake dryness (a 12.8% increase was observed with 11-mm-thick talc cakes). A further increase in the shear did not lead to noticeable changes in the final dry solid fraction in the cakes. Otherwise, reducing the filter-cake thickness was beneficial for the dewatering. For talc cakes, the dry solid fraction was increased by 21% for a 3 mm thickness against 7% for a 25 mm thickness. With a highly compressible material, such as activated sludge, the filter-cake dryness did not change with the application of a shearing action. This behavior may be mainly attributed to the fact that biological sludge dewatering is markedly influenced by a low permeability skin at the cake/medium interface, which is very thin and not perturbed by shear forces, due to the cake thickness. The low rigidity of the solid matter in this particular case may also limit the reorientation of the particles.     

Consolidation dewatering and centrifugal sedimentation of flocculated activated sludge

This study investigated experimentally the consolidation dewatering and centrifugal-settling processes for activated sludge subjected to cationic polyelectrolyte flocculation. The results were reported for the dynamic response of sediment cake thickness (an index for cake compaction) under various doses of polyelectrolyte conditioning, compression-permeability cell configuration and mode of operation (batch and continuous) in a centrifugal-settling cell. The reduction in sediment thickness of sludge by consolidation and centrifugation was found to correspond mostly well with the optimal dose of polyelectrolyte based on the capillary suction time. The relaxation/rebound of cake thickness was observed in both consolidation dewatering and centrifugal dewatering with comparable compaction/relaxation time scale ratios. The equilibrium sediment consolidation ratio increases with the effective solid pressure characterized by P_m and P_s, for the consolidation dewatering and centrifugal sedimentation, respectively. The experimentally determined time scales of the cake consolidation dewatering/centrifugal sedimentation processes agree reasonably well with the theory by Landman and Russel (Phys. Fluids A 5 (1993) 550).     

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.     

Theory of Filtration of Ceramics: I, Slip Casting

The theory of colloidal filtration in slip casting of ceramics as developed by Aksay and Schilling for incompressible cakes has been extended to compressible materials. The rate of cake deposition in the mold depends upon the capillary pressure and permeability of the mold. Assuming that the capillary pressure is inversely proportional to an average diameter typifying the mold and that the permeability is directly proportional to the square of the diameter, an optimum diameter exists for production of maximum pressure drop across the cake and maximum rate of deposition.     

Filtration Behavior of Slurries with Varying Compressibilities

Abstract

A novel filtration apparatus allows simultaneous measurements of filtrate volume, hydraulic pressure and cake thickness using slurry volumes on the order of 100 cm³ Differences in interparticle interactions were studied by varying the barium chloride concentration of 0.38-μm polystyrene latex and filtering at pressures between 2 and 100 psi. Cakes formed from these slurries are highly compressible for concentrations between 0.01M and 0.10M, moderately compressible for the 0.005M concentration, and incompressible for the 0.001M concentration. Plots of filtrate volume versus cake thickness were linear for the incompressible cakes, whereas the compressible cakes showed significant deviations, which were pressure dependent. The pressure distribution for the incompressible cake was found to be essentially linear as predicted from the resistance plots assuming constant α and ε. For the highly compressible cakes, most of the pressure drop appears to occur near the cake/medium Interface with only small changes occurring at the top of thp cakp.     

Removal of Moisture from the Ultra Fine Particles Using Both High Centrifugal Force and Air Pressure

Abstract

A newly developed centrifugal batch dewatering unit was developed and used to remove the moisture contents of ultra fine particles (e.g., kaolin clay, silica, talc, and precipitated calcium carbonate). In the present method, air pressure was applied into the vessel of a conventional centrifuge to improve the efficiency of the dewatering. The experimental results showed that the moisture reductions of the samples could be enhanced more than 50% depending on dewatering conditions (i.e., G-force, spin time, applied pressure, cake thickness, particle size, surface hydrophobicity, and particle agglomeration). As a result, when the new centrifuge modification is used for the fine particle dewatering, cost and environmental concerns of thermal dryers could be considerably reduced and the handleability of the filter cakes could be significantly increased.     

The influence of temporal cake moisture content on a discontinuous washing process in the centrifugal field

In solid/liquid separation processes, filter cake washing is an essential step in improving the quality of particulate products by elimination of impurities. During cake washing and dewatering, the cake saturation changes depending on the flow conditions and it cannot always be measured and controlled accurately. This article deals with investigations on the influence of the initial and temporal cake saturation on washing PVC and silica sand particles in the centrifugal field. It was found, that high initial saturation levels and high maximum saturations during the washing process had a positive impact in inducing a high hydrostatic pressure for advective flow and enabling a homogeneous distribution of the wash water inside the cake. This was achieved by increasing the wash water flux and/or decreasing the g‐factor. A good method to obtain low final impurity quantity is the combination of washing at a low g‐factor and dewatering at a high one. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Porosity distribution in highly compressible cake: Experimental and theoretical verification of the dense skin

The rate of filtration and the water content of cake are influenced by the existence of a dense skin in a highly compressible cake. The phenomenon of the dense skin has been rarely studied, and its existence has not been verified experimentally. In this study, the porosity variation in a very compressible cake is measured by using a new experimental apparatus, and with this the existence of dense skin has been established experimentally. ‘Unified theory on solid-liquid separation’, a recently developed theory, is utilized for calculating the porosity variation in a very compressible cake.     

Experimental investigation into compression properties of integrated coal gasification combined cycle fly ashes on a ceramic filter

The compression properties of IGCC (integrated coal gasification combined cycle) fly ash cake on a ceramic filter were carefully investigated under well-controlled conditions. Overall cake porosity and pressure drop of dust cake of three different particles of geometric mean diameters of 3.7, 6.2, and 12.1 Μm, and dynamic shape factors of 1.37, 1.57 and 1.65, respectively, were investigated, at face velocities of 0.02-0.06 m/s. Overall cake porosity was strongly dependent on face velocity, mass load, and particle size. The expressions for overall cake porosity, considering the compression effect, and pressure drop across the dust cake were developed with good agreement with experimental results.     

Conditioning for Municipal Sludge Dewatering. From Filtration Compression Cell Tests to Belt Press

Belt presses are commonly used to achieve sewage sludge dewatering. As this process essentially works by squeezing, it was interesting to better understand how the sludge conditioning affects cake expression. Experiments were carried out with conditioned municipal activated sludge on both a filtration compression cell (FCC) and a laboratory-scale belt press. Two cationic polyelectrolytes with a high charge density (80%) but different molecular weights were used. A significant impact of the conditioning was observed for FCC tests. The higher molecular weight polyelectrolyte led to much shorter filtration phases than with the medium molecular weight one. However, it produced wetter filter cakes. It also led to more elastic filter cakes but less susceptibility to creep. Higher final dryness was observed for sludge conditioned by the medium molecular weight polymer. Experiments carried out on the belt press device did not exhibit such impact. Consequently, the FCC test is not suitable to analyze the sludge dewatering by belt press, although it enables characterization of the expression phase. Indeed, some additional features involved in belt press, such as shear forces or progressive pressurization of the cake, are not characterized by classical FCC tests.     

Analysis of Filtration Characteristics for Compressible Polycrystalline Particles by Partial Least Squares Regression

Crystal size and morphology have been varied by changing the initial supersaturation ratio and the temperature in reactive crystallization experiments. The influence of the chord length distribution, average cake porosity, and filtration pressure difference on the average cake resistance of polycrystalline particles of an industrially produced aromatic amine has been investigated by means of partial least squares (PLS) regression and sensitivity analysis. Analysis of the results has disclosed that wider chord length distributions as well as lower values of the measured average porosity lead to higher values for the average cake resistance. However, PLS regression and sensitivity analysis have identified the applied pressure difference itself as the most significant parameter influencing the magnitude of the cake resistance. This unexpected behavior is accounted for by compression of the filter cake occurring predominantly in small layers above the filter cloth characteristic for highly compressible cakes.     

A rigorous cake filtration theory

The currently accepted concepts of the parabolic filtration correlation stipulate that the filtration rate decreases inversely as the flow resistance which is proportional to the extent of the filter cake; that non-parabolic behavior is characteristic of only compressible cakes which exhibit an average resistance which is not constant but depends on slurry concentration, filtrate rate and applied pressure; that the septum resistance is separate from, but additive to, the cake resistance and is obtained from the intercept of the V_F^? vs V_F line; that the average porosity of compressible cakes decreases during a filtration which causes the exit flow rate to exceed the entrance flow rate; that internal variations can only be obtained via compression-permeability simulations; and that the local resistance decreases monotonically from the septum to the cake surface.A filtration theory based on the rigorous, multiphase equations of change finds, contrary to the concepts listed above, that the least permeable part of the cake at the cake-septum interface (K₀) controls the filtrate rate and a decreasing pressure gradient across this minimum K₀ causes the filtrate rate to decrease; that deviations from parabolic behavior are independent of cake compressibility (incompressible cakes are rare, if they exist at all) but can be expected when ?_α*, s, -ΔP_c, K₀ and (dP*/dξ)|_ξ=0 are not constant; that the septum resistance is intrinsically included in K₀ which appears in the slope and not the intercept of V_F^?1 vs V_F; that the average porosity is constant and the exit flow rate exceeds the entrance flow rate due to a difference between the surface porosity and the average porosity; that internal variations can be obtained directly from filtration data alone and do not require a compression-permeability simulation; and that the local filtration resistance can exhibit a minimum as a result of using a non-linear transformation from cake length to cake mass.     

The role of P<Subscript>i</Subscript>, P<Subscript>o</Subscript>, and P<Subscript>f</Subscript> in constitutive equations and new boundary conditions in cake filtration

The constitutive equations proposed by Tiller and Shirato were analyzed and a new constitutive equation originating from the sediment thickness was proposed. A new boundary condition of the filter cake based on the solid compressive pressure of the first solid layer,p _f , was also proposed. Accurate average specific cake resistances at various pressures and the thickness of cake were calculated with the new constitutive equation and boundary conditions. The influence ofp _f on the cake thickness and average porosity was studied theoretically. Using three constitutive equations, it was proved that the compressibility n obtained from filtration results instead of CPC (compression-permeability cell) of very compressible cake could not have an exact value.     

Investigation of the fundamental assumptions relating compression-permeability data with filtration

A recently reported investigation indicates several inaccuracies in the methodology of compression-permeability (C-P) testing which suggest that previously reported agreement between C-P and filtration data may be fortuitous. Until now, there has been no separate and direct confirmation of each of the two assumptions necessary to obtain a unique correspondence between C-P and filtration data. The first assumption that the specific filtration resistance is a function solely of cumulative-drag-stress is generally accepted. Direct proof requires that the parabolic filtration equation, which is derived primarily on this assumption, describes both incompressible and compressible cake behavior. Most materials produce compressible cakes and “a priori” screening to find an incompressible cake requires identification of a material (Geon) that produces a cake with a linear axial pressure distribution. Results show that the parabolic filtration equation fits both types of cake behavior but an equation based on constant filtration resistance describes only incompressible cake behavior. To engineering accuracy and for dilute slurries, the assumption is verified. The second assumption that the cumulative-drag-stress equals the cake pressure drop is a macroscopic force balance and experimental verification requires a filter chamber designed to measure both of these quantities. A theoretical development, based on integral averaging, and experimental results both indicate that the ratio of cumulative-drag-stress to cake pressure drop correlates extremely well with cake porosity. A unique one-to-one correspondence between C-P and filtration data is not possible without “a priori” knowledge of filter cake porosity. Previously reported agreement between C-P and filtration data can probably be attributed to the L/D dependence of C-P specific filtration resistances. Consequently, C-P data can be used as a research tool to simulate filtration data but predicted filtration times based solely on C-P data can be in considerable error.     

Conditioning for Municipal Sludge Dewatering. From Filtration Compression Cell Tests to Belt Press

Belt presses are commonly used to achieve sewage sludge dewatering. As this process essentially works by squeezing, it was interesting to better understand how the sludge conditioning affects cake expression. Experiments were carried out with conditioned municipal activated sludge on both a filtration compression cell (FCC) and a laboratory-scale belt press. Two cationic polyelectrolytes with a high charge density (80%) but different molecular weights were used. A significant impact of the conditioning was observed for FCC tests. The higher molecular weight polyelectrolyte led to much shorter filtration phases than with the medium molecular weight one. However, it produced wetter filter cakes. It also led to more elastic filter cakes but less susceptibility to creep. Higher final dryness was observed for sludge conditioned by the medium molecular weight polymer. Experiments carried out on the belt press device did not exhibit such impact. Consequently, the FCC test is not suitable to analyze the sludge dewatering by belt press, although it enables characterization of the expression phase. Indeed, some additional features involved in belt press, such as shear forces or progressive pressurization of the cake, are not characterized by classical FCC tests.     

Data Analysis and Modeling of Constant Pressure Batch Dewatering of Fine Particle Suspensions

The problems of data analysis and modeling of experimental constant pressure batch dewatering of materials forming compressible cakes are considered. Dewatering in these materials is typically completed in two stages, viz. cake formation and cake consolidation. A data representation method especially useful for determining the transition point between the filtration and consolidation stages, as well as for comparing accuracy of model predictions, is illustrated. It is shown that dewatering occurs via one of three qualitatively different pathways. A simplified model for engineering analysis of the process is presented. A time-invariant spatially uniform volume fraction of solids approximation is invoked in the cake formation stage. A time-dependent spatially uniform volume fraction of solids assumption is made in the cake consolidation stage. The two models contain four model parameters and have a common physical basis in Darcy's law. Interrelationships between key process parameters are determined and employed to predict the temporal evolution of dewatering in the cake consolidation stage as well as the end point of dewatering.     

Decreasing Filter Cake Resistance by Using Packing Structures

Packed beds used in absorption columns are evaluated to determine whether they can also be beneficial for cake‐forming filtrations. To assess this, model systems are characterized and separated by using a dead‐end filter cell. Filtrations are conducted with different packings; the filtrate amount over time and resulting turbidity are evaluated. Packings increase the filter cake resistance and the separation time of the cakes formed with approximately incompressible solids. However, they exhibit a positive effect on the filtration of a more complex, compressible substance; the process is not only accelerated, but also the quality of the obtained filtrate is not compromised. These results demonstrate potential in the use of packed beds for the filtration of complex biogenic suspensions.     

Theory of Filtration of Ceramics: II, Slip Casting on Radial Surfaces

The theory of filtration of slip casting of incompressible beds of ceramic materials in planar molds is extended to deposition on internal and external cylindrical surfaces. Formulas are developed for (a) calculating the variation of the liquid pressure in both cake and mold as a function of the radius and (b) determining the time to produce a given thickness of a consolidated body. Assuming that the mold permeability K _m and capillary suction P _cap are related by an inverse parabolic relation, K _m P _cap ²= J , there is a mold with a specific permeability that produces a maximum rate of deposition from the slip. The theory presented in this paper strictly applies only to incompressible cakes and should be used cautiously with moderately compactible beds. For highly flocculated slurries yielding highly compressible bodies, the equations would not be expected to yield reliable results. Radial geometry leads to mathematical complexities not encountered in analyses of planar molds.     

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.