Migration and Deposition of Submicron Particles in Crossflow Microfiltration

Abstract

The migration and deposition of submicron particles in laminar crossflow microfiltration is simulated by integrating the Langevin equation. The effects of operating conditions on the particle trajectories are discussed. It is found that the Brownian motion of particles plays an important role in particle migration under a smaller crossflow velocity of suspension or a smaller filtration rate. Based on the simulated trajectories of particles, the transported flux of particles arriving at the membrane surface can be estimated. The particle flux increases with an increase of filtration rate and with a decrease of particle diameter; however, the effect of crossflow velocity on the particle flux is not obvious. The forces exerted on particles are analyzed to estimate the probability of particle deposition on the membrane surface. The probability of particle deposition increases with an increase of filtration rate, with a decrease of crossflow velocity, with a decrease of particle diameter, or with an increase of zeta potential on the particle surfaces. The simulated results of packing structures of particles on the membrane surface at the initial stage of filtration show that a looser packing can be found under a larger crossflow velocity, a smaller filtration rate, or a smaller diameter of filtered particles. Crossflow micro-filtration experiments are carried out to demonstrate the reliability of the proposed theory. The deviation between the predicted and experimental data of filtration rate at the initial period of filtration is less than 10% when the Reynolds number of the suspension flow ranges from 100 to 500.     

管式膜十字流微滤过程分析

分析了管式膜器中过滤悬浮液从膜管外壁向内壁渗透过滤的过程。以流体力学分析为基础 ,利用力平衡原理和多孔介质渗透定律 ,分析了该结构的过滤特性和滤饼形成特点及规律。     

Use of Chemical Species as Dynamic Membranes with Crossflow Microfiltration

ABSTRACT

The feasibility of utilizing the phenomenon of dynamic membrane formation with crossflow microfiltration in treating domestic wastewater was investigated. The primary membrane, used throughout the investigation, was made of woven polyester. Different chemical species, such as CaCO₃, FeCl₃, and NaAIO₂, were used in forming dynamic membranes on top of the primary membrane. Secondary effluent from a domestic activated sludge wastewater treatment plant was treated. A calcium carbonate dynamic membrane produced a stabilized permeate flux of 90 L/m²·h, with a permeate turbidity of 0.21 Nephelometric Turbidity Unit (NTU), at optimum conditions. Ferric chloride produced optimum results when it was mixed with tap water. A permeate flux and turbidity of 70 L/m²-h and 0.16 NTU, respectively, were obtained. Sodium aluminate produced a stabilized permeate flux of 77 L/m²·h when it was mixed with tap water during the formation of the dynamic membrane. The permeate turbidity was 0.16 NTU. The fouling mechanism of the three dynamic membranes was investigated, and empirical models were produced.     

十字流微滤过滤机理研究评述

近年来，膜分离过程中的十字流微滤机理研究在国外非常活跃，在预测过滤速率，动态滤饼的形成，提出了一些新观点和新的机理，加深了对十字流过滤通量下降和膜污染的认识和理解。本文介绍了这些理论和实验研究的成果。     

微滤过程阻力分析及过滤速率

以膜分离基本理论为基础,依据实验结果,研究微滤过程中微粒在膜材料上的吸附和沉积及其对膜透过液流动的影响,建立了吸附阻力和沉积层阻力随时间变化的关系式,就各项阻力对膜分离过程的影响情况进行了分析,并提出了减小这些阻力的途径。在分析膜分离阻力基础上,本文提出了计算微滤膜通量的数学模型,计算结果的平均偏差为５．９％,明显优于同类模型。     

旋转流强化管式膜微滤过程中分离因数研究

分离因数是影响旋转流强化管式膜微滤过程的关键操作变量之一。本文首先对管式膜分离器内分离因数的分布特征进行了研究,然后,不仅从理论上研究了分离因数对过滤过程行为的影响,而且进行了探讨分离因数对过滤性能影响的试验研究。结果表明,管式膜分离器内分离因数沿轴向从顶部到底部呈迅速减小的分布特征;分离因数的增大,可使过滤过程推动力提高,并使外旋流时过滤过程中固相颗粒不易向膜面迁移并沉积,但加剧了内旋流时过滤过程中固相颗粒向膜面迁移并沉积的趋势;在悬浮液固相浓度保持一定时,过滤通量随分离因数的增大而提高     

微滤膜法预处理周位酸有机废水

采用微滤膜预处理周位酸有机废水,探讨了废水pH值、操作压力、操作温度对微滤膜预处理效果的影响。结果表明,在废水pH值为10、操作压力为0.1 MPa、操作温度为室温的条件下,废水COD截留率达58%,色度明显减小,微滤膜片经机械清扫和反水冲洗后可恢复部分膜通量。为微滤膜法预处理周位酸有机废水的工业化提供了基础数据和理论依据。     

A New Approach to Linear Temperature Programming Calculations

Abstract

The theory of temperature programming has been reexamined. It is shown that a linear program leads to an explicit relation for the retention temperature in terms of the inverse exponential integral. Numerical examples are solved using a straight line plot of the exponential integral together with a plot based on the equivalent temperature concept. A simple explicit expression in terms of common functions for the retention temperature during linear temperature programming was deduced. This was made possible using the inverse log nonlinear program which can be made to approximate quite closely a linear program. This expression was used to explain the constancy of intervals and other phenomena encountered during the linear temperature programming of homologous series.     

错流微滤过程的无因次特征量

从近膜面微粒的受力分析出发,对错流微滤过程进行了因次分析,得到新的特征量Rem和Eum,合理地概括了重要过程参数对微粒在膜面沉积的复合作用。由基于Eum和Rem数学模型所得的预测值与实验值高度吻合,说明用Eum和Rem表征复杂的错流微滤过程的合理性。     

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。     

Crossflow Ultrafiltration of Binary Biomolecule Mixture: Analysis of Permeate Flux,Cake Resistance and Sieving Coefficient

The aim of this paper is to demonstrate the effects of hydrodynamic conditions on the permeate flux, cake resistance and sieving coefficient in a crossflow ultrafiltration process separating biomolecules of different molecular weights. A binary mixture of L‐phenylalanine (L‐phe) and lipase was ultrafiltered through a hydrophobic polyether sulphone (PES) membrane with 10 kDa molecular weight cut‐off. The changes in permeate flux, cake layer resistance and observed sieving coefficient with different transmembrane pressures (TMP) and crossflow velocities were evaluated. The effect of TMP was examined at two different velocities (0.114 m s^–1 and 0.176 m s^–1) and the effect of velocity was examined at two different TMP (20 kPa and 115 kPa) for the experimental system designed. In the initial stage of the crossflow filtration, it was determined that the TMP was more effective than the velocity. The cake layer resistance increased with increasing TMP and it decreased with increasing velocity for the high TMP value of 115 kPa. A maximum of the observed sieving coefficient was achieved with increasing velocity. An increase in TMP at low inlet velocity (0.114 m s^–1) affected the observed sieving coefficient positively.     

Microfiltration of Emulsified Oil Wastewater

Hydrophilic polyvinylidenefluoride (PVDF) membranes were employed for emulsified oil wastewater treatment. The best operating conditions for optimum flux of water, which penetrates through the membrane and rejection (prevention of oil droplets from passing though the membrane) were obtained. Concentration and pH effects on flux and rejection were determined. Increase of weight concentration caused flux decline due to the formation of a thick layer on the membrane surface. However increasing the concentration increased rejection. The pH effects were complex. By increasing the acidity of the feed, rejection was reduced.     

Effect of flocculation and/or adsorption as pretreatment on thecritical flux of crossflow microfiltration

This study aims at identifying the effect of pretreatment such as flocculation and/or adsorption on the performance of crossflow microfiltration (CFMF). CFMF performance was studied in terms of critical flux. The results indicated that: (1) flocculation alone as a pretreatment could effectively remove the large molecular weight organic matter from 30,000 to 60,000 Daltons; (2) flocculation, together with adsorption as a pretreatment to CFMF, could remove both large and small molecular weight organics; (3) flocculation as a pretreatment was significantly better than adsorption in improving the critical flux; (4) critical flux increased from 100 L/m².h to 520 L/m².h when flocculation-adsorption was applied. The critical flux in long-term experiments was the same as that of the short-term experiments. However, the critical flux slightly decreased (5%) in long-term experiments compared to short-term operation with flocculation and adsorption as a pretreatment.     

微滤膜聚并破乳的机理研究

A study on the membrane coalescence demulsification was carried out with four working systems of water/n-butyl alcohol, water/n-octanol, water/30% TBP(in kerosene) and water/kerosene. The membranes made of polytetrafluoroethylene (PTFE) with 1.0μm pore size were used. The results indicated that the excellent demulsification efficiency for emulsions with various oil contents was obtained. A conductivity probe was used to study the demulsification mechanism. An electrode probe was designed and used to determine the oil content near the membrane surface. The obtained data showed that the oil content in the permeated stream was much higher than that in the feed emulsion. A physical mechanism to explain the membrane demulsification was put forward.     

油水分离中污染的Al2O3陶瓷微滤膜清洗方法研究

针对在油水分离过程中,α-Al2O3陶瓷徽滤膜受污染而明显降低其分离性能的现象,采用酸碱分别清洗、酸碱交替清洗和煅烧后酸洗方法对污染膜管进行清洗,考察了清洗前后膜的纯水通量变化规律.结果表明,采用600℃煅烧,再用0.5 mol·1-1 HNO3清洗,后用清水清洗的方法,可使污染膜的纯水通量恢复到原始膜通量的87％,通过对膜的显微结构分析发现,膜表面及孔内的污染物得到有效地去除.     

Particle Deposition in Laminar Crossflow Filtration of Power Law Slurry

Abstract

A theoretical model for predicting the probability of particle deposition in crossflow filtration of power law slurry is developed. The model is based on the critical angle of friction between depositing particles, which can be estimated by analyzing the forces exerted on the particles. The binding force between the particles due to polymer adsorption plays an important role in the particle deposition. The smaller the flow behavior index of the slurry is, the larger the binding force and the higher the probability of particle deposition will be. The effects of operating conditions such as the crossflow velocity of the slurry and the filtration rate on the probability of particle deposition are also discussed in depth. The calculated values of the probability of particle deposition agree fairly well with the experimental data. A program is designed to simulate the packing structure and the porosity at the cake surface. The porosity increases not only with the increase of the crossflow velocity, but also with the increase of the flow behavior index of the power law slurry.     

EFFECTS OF AXIAL PRESSURE DROP ON THE LENGTH-AVERAGED PERMEATE FLUX IN CROSSFLOW MICROFILTRATION

A hydrodynamic model is presented which predicts the dependence of the steady-state, length-averaged permeate flux on the longitudinal pressure drop during crossflow microfiltration in both flat slit and tubular channels. The integral approach described by Romero and Davis (1988) has been extended to predict the axial variation of the transmembrane pressure drop along with that of permeate flux and cake layer thickness. The mechanism of shear-induced diffusion is employed in the analysis, which is restricted to particles with diameters of approximately 0.5-20μm (Davis, 1992), but the procedures may be extended to other particle transport mechanisms. The model predictions have been compared to the corresponding values calculated using a constant transmembrane pressure drop set equal to the arithmetic mean of those at the channel entrance and exit. The simulation results show the axial pressure drop to have the most significant effect on the average, steady-state permeate flux predictions for long, tubular channels with small critical lengths, operating under membrane resistance limited conditions and low transmembrane pressures. Neglecting the axial pressure drop, under typical operating conditions, results in as much as a 50% overestimation of the length-averaged, steady-state permeate flux.     

陶瓷微滤膜制备水包油型乳液的研究

乳液制备一直是化工领域中的一个重要的研究课题,本文选择水－正丁醇实验体系,以十二烷基碘酸钠为乳化剂,分别采用０．２μｍ和０．８μｍ的陶瓷膜为分散介质制备水包油型乳液。实验研究了压力、连续相流量等因素对乳液粒径大小、分布和分散相通量的影响。结果表明,用微滤膜可以制得分布均匀的乳状液。微滤膜的孔径较小时,连续相流量和膜两侧压差对于乳液粒径和粒径分布没有明显的影响;当膜孔径增大后,乳液滴的直径分布变宽,     

利用离心力场作用强化微滤

综述了目前利用离心力场作用强化膜微滤的几种实施方法:旋转管式动态膜微滤,旋转流强化膜微滤和旋转盘式动态膜微滤;详细讨论了各法的代表结构、基本原理及国内外的实验研究成果;评述了各法的优缺点、发展状况和应用前景。