Effect of Pore Size and Particle Size Distribution on Granular Bed Filtration and Microfiltration

Abstract

The paper reviews the effect of particle size distribution and pore size distribution on granular bed filter and crossflow microfiltration performance. The experimental results of the granular bed filter with pollen particles in suspension showed that the presence of large particles improved the filter efficiency of smaller particles in suspension. Microfiltration results with bi and tri‐modal latex suspensions showed that the permeate flux and the quality were significantly affected by the particle size and its distribution, especially when the particle size was smaller than the pore size of the membrane. The mathematical model simulation results of granular bed filtration show that media pore size distribution is an important parameter of filtration for the particle removal and pressure drop across the filter.     

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.     

颗粒粒径和膜孔径对陶瓷膜微滤微米级颗粒悬浮液的影响

通过测定颗粒悬浮液通过陶瓷微滤膜时的参透通量及污染阻力,确定了陶瓷膜处理微米级颗粒悬浮液时,颗粒粒径和膜孔径对微滤过程的影响和膜污染机理,获得了微米级颗粒悬浮液微滤过程中膜孔径的选择方法。     

HYDRODYNAMIC MODEL AND EXPERIMENTS FOR CROSSFLOW MICROFILTRATION

Crossflow microfiltration, in which a suspension is passed through a pressurized open-ended tube or channel having microporous membrane walls, is an effective means of filtering fine particles from a liquid and is finding increasing application in separations involving microbial suspensions and products. The particles, which are carried toward the walls with the filtrate cross-flow, form a thin cake layer on the membrane surface which does not accumulate substantially but is rather swept along the channel by the tangential flow of suspension. This paper presents a stratified-flow model of this phenomenon which predicts the steady-state permeation flux, and the velocity, pressure, and concentrated particle layer thickness profiles, as functions of the system parameters. In addition, the results of laboratory experiments which used a crossflow microfiltration channel with glass sides are reported. The measured steady-state thickness of the cake layer as a function of distance from the channel entrance shows good agreement with the theory, except for the case of a relatively thick layer when it is believed that a stagnant sublayer had formed beneath the flowing cake layer.     

HYDRODYNAMIC MODEL AND EXPERIMENTS FOR CROSSFLOW MICROFILTRATION

Crossflow microfiltration, in which a suspension is passed through a pressurized open-ended tube or channel having microporous membrane walls, is an effective means of filtering fine particles from a liquid and is finding increasing application in separations involving microbial suspensions and products. The particles, which are carried toward the walls with the filtrate cross-flow, form a thin cake layer on the membrane surface which does not accumulate substantially but is rather swept along the channel by the tangential flow of suspension. This paper presents a stratified-flow model of this phenomenon which predicts the steady-state permeation flux, and the velocity, pressure, and concentrated particle layer thickness profiles, as functions of the system parameters. In addition, the results of laboratory experiments which used a crossflow microfiltration channel with glass sides are reported. The measured steady-state thickness of the cake layer as a function of distance from the channel entrance shows good agreement with the theory, except for the case of a relatively thick layer when it is believed that a stagnant sublayer had formed beneath the flowing cake layer.     

Unsteady-State Permeate Flux of Crossflow Microfiltration: Effect of Particle Size Distribution

Abstract

A mathematical model based on a hydrodynamic theory and mass balance was developed for the prediction of the unsteady-state permeate flux in crossflow microfiltration under the influence of particle size distribution. Experiments were also conducted in a membrane filtration cell to verify this model. Spherical polystyrene latex particles of 0.303, 0.606, and 1.020 μm were used to make suspensions of various particle size distributions. The flow of the suspension in the channel of the filtration cell was controlled under the laminar flow region. It was found that the unsteady-state permeate flux increased as the mean particle size of the suspension was increased. Moreover, the model predicted satisfactorily the unsteady-state permeate flux under the effect of particle size distribution.     

Dynamic cross flow filtration

The dynamic cross flow filtration with the DYNO filter is a very versatile and economic high performance filtration process especially for suspensions with critical separation characteristics. Thickening, washing and clarifying of suspensions but also a classifying sieve filtration can be performed with the DYNO filter in a continuous operation with permanently high throughput rates. The principle of the dynamic high shear filtration ensures almost ideally physical conditions for the separation process. Contrary to classical cross flow filters a repeated recirculation of the suspension is not necessary for attaining the separation target. In the DYNO filter suspensions can be highly concentrated up to the flow limit in only one filtration cycle. In most cases, the concentrate is as dry as a firm filter cake and the filtrate is crystal-clear. Sieve filtration tasks for separating of coarse grain are performed with high concentration factors of up to 1000. Thus, the coarse fraction is obtained highly concentrated.     

Study of the separation of yeast by microsieves: In situ 3D characterization of the cake using confocal laser scanning microscopy

In situ 3D characterization of Aquamarijn microsieves fouling was achieved using Confocal Laser Scanning Microscopy (CLSM). A filtration chamber allowing direct microscopic observation of microbial cell deposition and cake characterization, specially designed for in situ observations, was used. Fluorescent dyed Saccharomyces cerevisiae yeast suspensions were filtered through 0.8 μm and 2 μm pore diameters silicon nitride microsieves under constant flow rate. The on-line yeasts deposition was recorded and the cake construction was followed layer by layer. Based on the 3D image processing, cake properties (particle arrangement, homogeneity, thickness and porosity). The compressibility of the yeast cake was analyzed. Finally, cake removal efficiency was also studied during microsieve cleaning operation.     

Flux Improvement during Cross-flow Microfiltration of Wheat Starch Suspension using Turbulence Promoter

The objective of this study was to investigate the effects of the process variables (transmembrane pressure, flow rate, and concentration) on the permeate flux during the microfiltration of model starch suspensions, and to determine the conditions under which the use of Kenics statics mixer as a turbulence promoter is justified. A response surface methodology was used to examine the influence of the selected operating conditions on starch suspension microfiltration using a single channel ceramic membrane with 200 nm pore size. The experimental results clearly show that the improved performance of starch suspension cross-flow microfiltration can be obtained by using a Kenics static mixer, especially at lower flow rates. Compared to the operation without the turbulence promoter, the average permeate flux improvement during the filtration period ranged from 30% to 230%. As a result of the statistical analysis, the optimal conditions for starch suspension microfiltration were determined and applied to microfiltration of starch industry wastewater.     

Flocculation in laminar tube flow

The extent of orthokinetic flocculation of suspensions as a result of laminar tube flow has been calculated using Smoluchowski theory and shown to depend primarily on the tube dimensions and not on the flow rate. When a correction factor allowing for hydrodynamic resistance between approaching particles is introduced, the calculated flocculation rate can be much lower than the Smoluchowski value and depends inversely on flow rate and particle size. A further effect which has to be included is the non-uniformity of shear rate in the tube, which leads to less flocculation than expected on the basis of a uniform shear rate throughout the tube. Coiling of the tube should given an extra degree of mixing to the flowing suspension and hence a more uniform shear rate and an enhanced flocculation.Preliminary experiments using model suspensions flowing through straight and coiled tubes have shown fair agreement with these predictions.It is suggested that laminar flow through coiled tubes could provide the basis of a practical flocculation test method.     

Simulating simultaneous fines deposition under catalytic hydrodesulfurization in hydrotreating trickle beds—does bed plugging affect HDS performance?

The deposition of fine particles under chemical reaction conditions in a high pressure/temperature trickle bed reactor was analyzed theoretically using a dynamic multiphase flow deep-bed filtration model coupled with heat and species balance equations in the liquid, gas and solid (catalyst+deposit) phases. The hydrodesulfurization process in the presence of sulfided Co-Mo/γ-Al₂O₃ catalyst was considered as a case study. The deep-bed filtration model incorporates the physical effects of porosity and effective specific surface area changes due to fines deposition/detachment, gas and suspension inertial effects, and coupling effects between the filtration parameters and the interfacial momentum exchange force terms. The detachment of the fine particles from the collector surface was assumed to be induced by the colloidal forces in the case of Brownian particles or by the hydrodynamic forces in the case of non-Brownian particles. The three-phase heterogeneous model developed to simulate the trickle bed performance incorporates the concentration gradients inside the catalyst particle and solid deposit. An important finding of the work is that fine particles deposition does not influence appreciably trickle bed reactor performance. Thus, the only undesirable effect of the fine particles deposition process is the bed plugging and the increase of the resistance to two-phase flow.     

纤维束滤床过滤性能的研究

用不同尺寸悬浮颗粒进行了上向流纤维束滤床过滤性能的实验研究。结果表明,在床层下部或过滤初期,较大颗粒的去除率较高,而在床层上部或过滤后期,不同粒径颗粒的去除率无明显差异;较小颗粒过滤时产生的水头损失较大,且增加较快。     

Modulation of suspension electrical conductivity to counter fines plugging in trickle‐bed reactors

Modulation of electrical conductivity in kaolin/kerosene suspensions was examined as a means for the attenuation of fines retention in trickle‐bed reactors. The suspension stability was remarkably enhanced through ON‐OFF concentration modulation of an electrolyte‐based kerosene conductivity improver and resulted in an efficient bed‐cleaning strategy under operating conditions. Periodic additions of the conductivity improver enabled fines and deposits to gain momentarily large and similar electrical charges undoing, or impeding, multilayer deposition. The time evolution of the two‐phase pressure drop and specific deposit with/without conductivity improver was monitored, as well as the corresponding local deposition structure via electrical capacitance tomography (ECT). Electrical conductivity modulation was found to reduce the bed‐specific deposit by an order of magnitude and the bed pressure drop increment by a factor 14. ECT imaging evaluated the efficacy of this modulation strategy in mitigating deposition and in preventing filtration‐induced flow maldistribution. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

陶瓷膜构型对错流微滤酵母悬浮液性能的影响

采用3种构型的陶瓷微滤膜元件对酵母悬浮液进行错流过滤实验,考察陶瓷膜元件的构型对于其错流过滤性能的影响。结果表明:减小陶瓷膜元件的通道直径可以提高料液在膜表面的剪切力,有助于提高过滤稳定通量和临界压力,在3 m/s膜面流速、0.1 MPa跨膜压差条件下,单管、19通道、55通道的稳定通量分别为96、128、196 L/(m2.h);在3 m/s膜面流速条件下,3种陶瓷膜元件的临界压力分别约为0.15、0.2、0.2 MPa。另外,减小通道直径还可以减小滤饼层的比阻,有利于降低过滤阻力;与提高膜面流速来增大过滤通量的方法相比,减小陶瓷膜的通道直径具有能耗较小的优点。     

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.     

Verfahren zur Mikrofiltration von Prozeßlösungen und ihre Anwendungen

Approaches to microfiltration of process solutions and its application. Particles and microorganisms in the site range of 0.02 to 10 μm are reliably removed by microfiltration processes. Microporous polymer membranes are suitable as filter medium. The particles are then separated mainly on the exterior surface (surface filtration). This leads to formation of a covering layer on the membrane and a relatively high hydraulic resistance. Dynamic filtration processes counteract covering layer formation. The most widely developed technique is crossflow microfiltration, in which the membrane is subject to continuous flow of the suspension. The development of large membrane units fulfils requirements for large-scale industrial application of crossflow microfiltration.     

Role of gas phase in the deposition dynamics of fine particles in trickle-bed reactors

Experiments were conducted to study the role of gas velocity in the capture of fine particles from non-aqueous suspensions circulated in co-current down-flow trickle flow reactors. The rate of filtration and pressure drop in the trickle bed were investigated using surfactant-stabilized kaolin-containing kerosene suspensions. It was determined that the filter coefficient was sensitive to liquid holdup and specific deposit. The initial collection efficiencies were compared with predictions based on existing theories. Agreement was generally not good with the exception for the limit of low superficial gas velocity. A general correlation establishing the relationship between the filtration rate and the liquid holdup in trickle beds was proposed to reconcile the experimental data with existing filtration theories.     

Slot coating flows of non‐colloidal particle suspensions

Slot coating is used in the manufacturing of functional films, which rely on specific particle microstructure to achieve the desired performance. Final structure on the coated film is strongly dependent on the suspension flow during the deposition of the coating liquid and on the subsequent drying process. Fundamental understanding on how particles are distributed in the coated layer enables optimization of the process and quality of the produced films. The complex coating flow leads to shear‐induced particle migration and non‐uniform particle distribution. We study slot coating flow of non‐colloidal suspensions by solving the mass and momentum conservation equations coupled with a particle transport equation using the Galerkin/Finite element method. The results show that particle distribution in the coating bead and in the coated layer is non‐uniform and is strongly dependent on the imposed flow rate (wet thickness). © 2016 American Institute of Chemical Engineers AIChE J, 63: 1122–1131, 2017     

Filtration Behavior of Suspensions of Uniform Polystyrene Particles in Aqueous Media

Abstract

Cake filtration experiments of suspensions of polystyrene particles, of uniform morphology, through Nuclepore membranes having uniform pores were carried out. The effects of particle properties (size and surface charge), suspension properties (particle concentration and ionic strength), and applied pressure were determined. The results were analyzed in terms of the conventional Darcy-Ruth filtration equation. Plots of resistance versus weight of solids in the cake revealed two distinct regions with a transition occurring early on in the filtration process at a cake thickness of the order of 1 mm. The initial portion has a lower average slope (specific resistance) than that of the second region. It is by only plotting this second region (i.e. ignoring the initial stages of the filtration process) that apparent negative values for the medium resistance are obtained.

The specific cake resistance obtained from the slope of the second region, which spans at least 90% of the filtration time, was correlated with particle and dispersion properties. The specific filtration resistance was essentially independent of slurry concentration and of the total applied pressure. Specific cake resistance measured at constant pressure and slurry concentration showed an inverse dependence on ionic strength.     

Energy dissipation during flow of coagulating concentrated suspensions

The energy dissipation during stationary flow is calculated for suspensions of such concentration that division into separate flocs and surrounding medium can be neglected (absence of sedimentation in the suspension at rest). This restriction removes some uncertainties in the “elastic floc” model [5]. The system is conceived as one giant flox when at rest, and to be divided by shear planes into domains when being sheared.Experiments performed on aqueous suspensions of Ca(OH)₂ (solid volume fractions = 0.25) show that the energy dissipation can be accounted for by that connected with the work required for overcoming the viscous drag experienced by particles moving within the domains. The influence of the nearby presence of other particles on the viscous drag is taken into account, but no separate term for energy dissipation by fluid flow in the flocs is necessary.