用于浆态床费托合成的错流过滤数学模型的研究进展

分析了浆态床费托合成错流过滤过程中滤膜污染的机理,并分别综述了三相过滤和两相过滤过程中数学模型的研究进展根据阻力不同,将模型分为堵塞阻力模型、滤饼层阻力模型和组合阻力模型.在对现有模型研究总结的基础上,通过对比和分析指出了组合阻力模型是浆态床费托合成错流过滤数学模型研究的方向.     

Modeling of fouling layer deposition in cross-flow microfiltration during tomato juice clarification

The effect of parameters, such as transmembrane pressure and axial flow rate, on membrane fouling during tomato juice clarification were studied by cross-flow microfiltration using flat sheet polyvinylidenefluoride membranes. The effect of fouling on permeate flux was modeled using a classical constant pressure dead-end filtration equation and its modified form for cross-flow filtration. The main physico-chemical properties of tomato juice were evaluated. The clarified juice was very similar to the feed except for insoluble solids and lycopene, which were concentrated in the retentate. Cake formation was identified as the main reason for flux decline. At different axial flow rates, the fouling mechanism evolves from cake filtration to an intermediate pore blocking mechanism with increasing pressure.     

Treatment of Oily Wastewaters Using the Microfiltration Process: Effect of Operating Parameters and Membrane Fouling Study

This work focuses on the treatment of oily wastewater using the cross-flow microfiltration (MF) process to determine the effect of different operating parameters such as transmembrane pressure (TMP) and cross-flow velocity on the separation performance and to study the mechanism of membrane fouling during microfiltration of oil in water emulsions. In this regard, the permeation flux and oil rejection of a polyvinylidene fluoride (PVDF) membrane in a flat-frame MF module for separation of 3000 ppm oil/water emulsions were measured. The results indicated that the permeate flux increased by an enhancement in both TMP and cross-flow velocity, while the oil rejection decreased. The analysis of variance (ANOVA) showed that the individual effect of TMP and cross-flow velocity is more important than the interactional effect of these operating parameters on the permeate flux and oil rejection. The results of fouling modeling revealed that the membrane fouling mechanism was affected by the applied TMP. The cake filtration model dominates the fouling mechanism at lower operating pressures. The fouling mechanism was changed from the cake formation to intermediate pore blocking and then to standard pore blocking as the TMP varied from 1 to 3 bar. Finally, a five-step procedure was used for cleaning the oil/water fouled membranes.     

动态膜处理污水时阻力分布及污染机理

对高岭土动态膜处理污水过程中膜污染阻力分布及膜污染机理进行了研究。通过测定和计算得知不同操作条件下各部分阻力的比例及其变化情况。动态膜过滤污水的阻力主要由动态膜膜孔堵塞阻力和膜面污染层阻力所控制。随着错流速度的增大,总阻力减小,各部分阻力的比例有所变化,但动态膜膜孔堵塞阻力和膜面污染层阻力依然占主导地位。利用传统膜过滤时有关膜污染的堵塞模型和滤饼过滤模型对动态膜处理污水时的实验数据进行分析,结果表明,动态膜过滤污水过程中,过滤初期约10 min左右,污染以膜的微孔堵塞为主,此后,膜的污染情况因操作条件不同而有所差异,影响最显著因素为错流速度,当错流速度较小时,膜的污染以膜面沉积污染物为主,符合滤饼过滤模型。适当提高错流速度有利于减小过滤阻力。     

颗粒体系错流过滤过程的数学模拟

以流体力学理论为基础,通过颗粒在水溶液中的受力分析,结合早期的阻塞过滤模型和近期的滤层过滤模型,建立了一种新的颗粒体系错流过滤过程的数学模拟方法,在该模拟过程中同时考虑了膜孔径和颗粒大小两种分布,并对具有相同平均孔径而孔分布不同的情形对膜渗透通量的影响进行了模似,经实验验证,模拟结果与实验值能够较好地吻合。     

聚乙烯醇(PVA)对模拟胞外聚合物(EPS)在错流超滤中膜污染的影响

研究了在模拟胞外聚合物(EPS)溶液中添加聚乙烯醇(PVA)对错流超滤膜过滤过程的影响. 对有无添加PVA的模拟EPS溶液分别用错流平板膜进行超滤实验,用Hermia修正模型对实验数据进行拟合,分析验证膜污染机理. 结果表明,无论是否添加PVA,模拟溶液的通量-时间实验数据对滤饼堵塞模型的拟合度都最高,为0.891~0.994;添加PVA的模拟EPS溶液形成的吸附阻力和膜污染阻力均比无添加的溶液高约0.5倍,滤饼比阻比无添加的溶液高约4倍,分别为2.29′1014和9.57′1014 m-1;模拟EPS溶液添加PVA后,通量对操作压力的敏感度增加,对膜面流速的敏感度却降低.     

Effect of operating parameters on permeate flux decline caused by cake formation — a model study

A phenomenological model employing cake formation theory has been developed for describing permeate fluxdecline in cross-flow membrane filtration. In the model the physicochemical parameters, which are often difficult to estimate, were excluded. Instead, the flux decline due to cake formation caused by inorganic scaling/precipitation was related to the operating parameters for fouling prediction. The processes of solute deposition on membrane surface and its re-dissolution back to the bulk phase were modeled to estimate dynamic cake formation and permeate flux profiles. The modeled results show that the permeate flux declined rapidly at the early stage of cake formation, then gradually leveled off as time progressed, and eventually reached a steady-state “ultimate” flux when the rate of solid deposition was balanced by back dissolution. Sensitivity analyses show that an increase of cross-flow velocity from 0.06 to 0.14 m/s increased the ultimate flux from 0.016 m/h to 0.035 m/h. Membrane permeability and transmembrane pressure (400-750 kPa) affected the initial flux but not the ultimate flux. The flux decline pattern strongly depended on the specific cake resistance, which affects the time to reach steady state but not the ultimate flux. Verification of the model with data in the literature showed excellent agreement.     

Investigation of fouling mechanisms governing permeate flux in the crossflow microfiltration of beer

The local phenomenology associated with membrane fouling has been investigated experimentally at laboratory scale for the crossflow microfiltration (CMF) of beer. Two downstream membrane processes were involved: clarification and sterilisation of beer. Fouling mechanisms were interpreted and compared for two types of beer (clarified beer and rough beer), filtered through a track-etched 0.2 μm polycarbonate membrane. Flux decay was analysed by using the combination of the constant pressure blocking filtration laws with the measurement of membrane resistances arranged in series. It was found that for both types of beer the permeate flux was governed by two identical fouling mechanisms: an internal fouling of pores at the initial stages of filtration that conforms to the standard blocking model, followed by an external surface fouling conforming to the cake filtration model. It was shown that the predominant membrane resistance arises from the build-up of a loosely bound and reversible fouling layer over the membrane surface (representing more than 80% of the total filtration resistance). Macrosolutes and colloids are likely to be involved both in the progressive pore plugging and in the external fouling layer (in combination with the yeast cells cake in the case of rough beer), because of their high tendency to interact with porous material. Thus the relevance of using the so-called classical filtration laws for the investigation of fouling mechanisms in terms of total resistance of the membrane in beer CMF has been demonstrated.     

Sugar Purification from Enzymatic Rice Straw Hydrolysis Products using Cross-Flow Diafiltration

The sugars produced by enzymatic hydrolysis of rice straw are separated using cross-flow diafiltration in this study. The effects of membrane type, membrane pore size, cross-flow velocity and transmembrane pressure on the filtration flux, sugar rejection, and sugar mass flux transported to the filtrate are discussed. The filtration flux increases with increasing cross-flow velocity or transmembrane pressure. When the membrane made of mixed cellulose ester (MCE) is used, over 70% filtration resistances are caused by the membrane fouling; while the resistance due to virgin membrane is dominant when regenerated cellulose (RC) membranes are used. A force balance model is applied to relate the filtration flux and filtration resistance to operating conditions. The calculated data of filtration flux based on this model agree fairly well with experimental data. In addition, a theoretical model is used to explain the sugar transmission through the cake and membrane pores. The sugar rejection coefficient decreases with increasing cross-flow velocity because of the effect of cake reduction. This effect is more significant when the MCE membrane is used. Comparing the sugar mass flux transported into the filtrate, it is more effective for sugar purification by using 10 kD RC membrane and under higher transmembrane pressures.     

Effect of cake layer structure on colloidal fouling in reverse osmosis membranes

A series of reverse osmosis (RO) membrane filtration experiments was performed systematically in order to investigate the effects of various hydrodynamic and physicochemical operational parameters on a cake layer formation in colloidal and particulate suspensions. Bench-scale fouling experiments with a thin-film composite RO membrane were performed at various combinations of trans-membrane pressure (TMP), cross-flow velocity (CFV), particle size, pH, and ionic strength. In this study, silica particles with two different mean diameters of 0.1 and 3.0 μm were used as model colloids. Membrane filtration experiments with colloidal suspensions under various hydrodynamic operating conditions resulted that more significant permeate flux decline was observed as TMP increased and CFV decreased, which was attributed to the higher accumulative mass of particles on the membrane surface. Results of fouling experiments under various physicochemical operating conditions demonstrated that the rate of flux decline decreased significantly with an increase of the ionic strength as well as particle size, while the flux decline rate did not vary when solution pH changed. The experimentally measured cake layer thickness increased with a decrease in particle size and solution ionic strength. Furthermore, the model estimation of cake layer thickness by using a cake filtration theory based on the hydraulic resistance of membrane and cake layer was performed under various ionic strength conditions. The primary model parameters including accumulated mass and specific cake resistance were calculated from the cake layer resistance. This result indicated that the formation of cake layer could be closely related with solution water chemistry. The model estimated cake layer thickness values were in good agreement with the experimentally measured values.     

膜生物反应器中膜污染机理的研究

在采用膜生物反应器（MBR）处理染料废水的过程中,通过对活性污泥进行终端过滤来反映膜污染机理,着重考察了膜过滤的形式、膜通量变化和膜污染的形成。污泥的终端过滤过程严格符合沉积过滤定律;膜通量随过滤时间呈指数衰减趋势,并在几分钟内就达到相对稳定值;扫描电镜照片也证实了膜污染主要是膜面沉积层引起的。     

Measurements and evaluation of concentration distributions in filter cake formed in dead-end ultrafiltration of protein solutions

To clarify the internal structures of the filter cake formed on the membrane surface in protein ultrafiltration, a method has been developed for measuring the variations of protein concentration across the filter cake on the basis of the principle of inclined ultrafiltration, where the membrane was inclined and a large amount of filter cake was formed, and the results were compared with the calculations based on a compressible cake filtration model, which explicitly took the non-homogeneity and the compressibility of the filter cake into account. The experimental results obtained from ultrafiltration of bovine serum albumin (BSA) solutions under constant pressure conditions clearly demonstrated that the filter cake tended to have a much more compact structure at the membrane in comparison with a relatively loose condition at the surface. It was also found that the thickness of the filter cake formed on the membrane increased as the filtration progressed. Further, the effects of pH and the solute concentration in the feed solution on the structure of the filter cake have been examined experimentally. The measured concentration distributions accorded well with the calculated results based on a compressible cake filtration model. This study revealed that the dynamic deposition behaviors of the protein molecules in dead-end ultrafiltration could be accurately described by a compressible cake filtration model.     

Organic fouling of RO membranes: Investigating the correlation of RO and UF fouling resistances for predictive purposes

This study examines the characteristics of deposits from dead-end UF and cross-flow RO filtration of dilute mixtures of sodium alginate (SA) and humic acids (HA), which are representative of common foulants in RO and NF feed waters. The data are interpreted in terms of specific cake resistance, α. The presence of alginates in the mixed fouling layers, even in relatively small percentage, imparts characteristic properties to these deposits; i.e. a gel-like structure which is associated with reduced compressibility and permeability. With SA/HA mixtures, high rejection of organic species and high deposition factors are obtained in UF and RO tests, respectively. The specific resistance of mixed SA and HA deposits tends to increase with increasing SA concentration in solution. The resistances α exhibit fairly strong (power law) dependence on the pressure difference across the cake, ΔΡ_c, for both RO and UF tests. The fairly satisfactory correlation of resistance α versus ΔΡ_c data, obtained from both types of tests (for the mixtures of organic species tested at small salinity) is a significant result of this work. Based on this correlation, an approach is suggested for estimating fouling index I and initial membrane fouling rate of a specific RO process, relying on limited UF tests.     

Development of energy-saving spinning membrane systemand negatively charged ultrafiltration membranes for recovering oil from waste machine cutting fluid

Membranes with hydrophobic surfaces have higher tendency for protein adsorption and bacteria attachment.As a result, these membranes foul rapidly in cross-flow filtration processes. Changing the membrane surface properties can slow down the membrane fouling process. For difficult membrane separation processes like oilwater emulsion separation, changing membrane properties alone cannot slow down the membrane fouling process. The ordinary cross-flow filtration system cannot be successfully employed for this kind of separation, and the spinning membrane disc system could be considered. The conventional spinning membrane disc system however is not energy efficient due to the centrifugal force acting against the permeate flow; this reduces the effective filtration pressure during the separation operation. Efforts were undertaken to develop a group of negatively charged ultrafiltration membranes, prepared from polyacrylonitrile-vinyl acetate-sodium p-sulfophenyl methallyl ether (CP-24) with polyacrylonitrile-vinyl acetate (CP-16), to be used in an energy-saving design of spinning membrane disc separation system. Our experimental results clearly demonstrate the energy saving benefit of our design; at filtration pressure of 276 kPa and at membrane disc spinning velocity of 1,000 rpm without sacrificing the oil rejection (>98% for 1,000 ppm oil-in-water) by our membrane, the permeate velocity was increased as high as 132% by our energy-saving system over conventional spinning membrane disc separation system.     

An Investigation of the Mechanism of Critical Flux in Membrane Filtration Using Electron Microscopy

The critical fluxes for colloidal latex suspensions of 1.0 µm and 0.1 µm and their mixture were measured. The similar fluxes were observed for latex suspensions and pure water. Electron microscopy was used as a tool for critical flux mechanism investigation.For all latex suspensions tested, there is no evidence of cake formation for flux below the critical value. A small deposition of the particles may form on the membrane surface which can be established during the intermittent blocking of the pores. This evidence supports the model of intermittent obstruction and blocking of pores. Another possibility is that the local fluxes in parts of the membrane may be higher than the critical value. Above the critical flux deposition of latex beads or formation of a cake is evident. The deposits grow and the cake thickened during time. This is usually accompanied by a drop in delivered flux.The results of working below critical flux suggest a possible membrane-feed interaction which is not a fouling cake layer. One possibility is that of a concentrated polarised layer with intermittent pore blockage or obstruction. A surface interaction effect also plays a role. The particle content of the feed could influence flow through the pores and may involve intermittent obstruction of pores without permanent fouling.The general picture of pumping permeate operation below the critical flux is non-formation of a stagnant cake layer. However a flowing cake layer might be established.     

混凝耦合多孔陶瓷膜净化河水简

Membrane filtration technology combined with coagulation is widely used to purify river water. In this study, micro filtration （MF） and ultrafiltration （UF） ceramic membranes were combined with coagulation to treat local river water located at Xinghua, Jiangsu province, China. The operation parameters, fouling mechanism and pilot-scale tests were investigated. The results show that the pore size of membrane has small effect on the pseudo-steady flux for dead-end filtration, and the increase of flux in MF process is more than that in UF process for cross-flow filtration with the same increase of cross-flow velocity. The membrane pore size has little influence on the water quality. The analysis on membrane fouling mechanism shows that the cake filtrationhas significant in fluence on the pseudo-steady flux and water quality for the membrane with pore size of 50, 200 and 500 nm. For the membrane with pore size of 200 nm and backwashing employed in our pilot study, a constant flux of 150 L-m^-2-h^-1 was reached during stable operation, with the removal efficiency of turbidity, total organic carbon （TOC） and UV254 higher than 99%, 45% and 48%, respectively. The study demonstrates that coagulation-porous ceramic membrane hybrid process is a reliable method for river water purification.     

A comparison of hydrodynamic methods for mitigating particle fouling in submerged membrane filtration

Hydrodynamic methods are used for mitigating particle fouling and for enhancing the filtrate flux in submerged membrane filtration. In the comparison membrane blocking-cake formation filtration system, the effects of filtration pressure, aeration intensity, backwash duration and stepwise increasing pressure on the filtration resistances and filtration flux are measured and discussed. Aeration is helpful for reducing particle deposition on the membrane surface, while stepwise increasing pressure can mainly mitigate internal fouling of the membrane. Periodic backwash can significantly reduce both the resistance caused by the membrane internal fouling and by cake formation; consequently, it can effectively recover the filtrate flux. In contrast, increasing the pressure in constant pressure filtration leads the flux to be decreased due to more severe membrane blockage. According to the comparison of the long-term flux and the received filtrate volume, among these hydrodynamic methods, the periodic backwash with longer duration is the optimal strategy for the filtration.     

Modeling of Particle Fouling and Membrane Blocking in Submerged Membrane Filtration

Abstract

The models of particle fouling and membrane blocking in a submerged membrane filtration are developed in this study. The effects of operating conditions, such as aeration intensity (air flow rate) and filtration pressure, on the filtration flux, membrane blocking, and cake formation are discussed thoroughly. The experimental results show that the filtration resistances due to cake formation and membrane blocking play significant roles in determining the overall filtration resistance, but the latter one is more dominant. An increase in aeration intensity leads the filtration flux to increase due to the reduction of cake formation on the membrane surface. However, a higher filtration pressure causes more severe membrane internal blocking and then to lower filtration flux. The cake properties and the filtration resistance due to membrane blocking are analyzed and can be regressed to empirical functions of filtration pressure. A force balance model for particle deposition on the membrane surface is also derived. In order to estimate the shear stress acting on the membrane surface, the diameter, shape, and rising velocity of air bubbles are analyzed based on hydrodynamics. Once the model coefficients are obtained, the pseudo‐steady filtration flux under various conditions can be estimated by the proposed model and the basic filtration equation. The calculated results agree fairly well with the available experimental data.     

The application of fluid dynamic gauging in the investigation of synthetic membrane fouling phenomena

In order to improve the performance of membrane processes there is a need to understand the basic phenomena occurring at the membrane and in the bulk fluid. Fluid dynamic gauging (FDG) is a technique that has been used previously to measure the thickness and strength characteristics of a fouling layer on solid surfaces. Here, its application has been extended to investigate the nature of fouling deposition on membranes in dead-end and cross-flow microfiltration. Two modes of FDG measurement are demonstrated, wherein either (i) the mass flow rate of liquid withdrawn through the gauge or (ii) the pressure drop across the nozzle, are fixed. In dead-end filtration, the deposition of ballotini on a polymeric membrane is monitored quantitatively via measurements of thickness and the permeate flux. FDG has also been integrated with imaging techniques to study the deposition and removal processes. In cross-flow, FDG proved to be able to monitor the growth of fouling layers during molasses processing.     

磁絮凝膜过滤工艺中附加磁场强化清洗

基于磁强化絮凝膜过滤(MEFMF)工艺中磁絮体和含磁滤饼层的特性,设计了在线(on-line)和离线(off-line)磁强化清洗(MEC)工艺.在磁场和曝气剪切的协同作用下,含磁滤饼层脱离膜纤维表面,膜通量恢复率较常规物理清洗(RC)明显提高.在离线磁强化清洗时,设计反洗装置中心的磁感应强度为6 mT,曝气强度为500 L·m^-2·min^-1,控制清洗时间为5 min,维持反洗压力0.04 MPa,可达到最佳的膜清洗效果,通量恢复率达97%以上.在外加磁场强化清洗过程中,滤饼层中的磁种发生磁化作用,滤饼层表现出微弱的宏观磁性,在磁场的作用下向磁极运动,使得膜通量恢复率明显提高.此外,在MEFMF工艺中采用间歇磁强化清洗,可以更加有效地去除引起不可逆膜污染的胶体和有机物,降低膜污染速率,减缓膜污染.