反渗透、纳滤过程的物理化学研究(Ⅰ)多孔膜的溶质脱除率方程和膜渗透通量方程

引言 溶解扩散模型、摩擦模型、Sourirajan方程和表面力孔流模型均假设稳态条件下溶质通量和溶液体积通量恒定[13].     

Flux and rejection in the ultrafiltration of colloids

Charge on colloidal particles is shown to affect the filtration process in two ways. The effect of changing the charge of the particles, such as by varying pH, is to alter the observable flux which goes through a minimum as the charge is increased. At fixed charge the flux first decreases with particle size and then increases. Charge also influences the mechanism of particle rejection. If primary particles are stable colloids, smaller than the surface pores, they may penetrate the membrane but not necessarily pass all the way through. When particle charge is low enough for flocculation to be significant, rejection will most likely occur at the membrane surface as flocculation results in particles growing to become large enough to be rejected.     

Potential of DMSO as greener solvent for PES ultra‐ and nanofiltration membrane preparation

In this article, the performance of polyethersulfone (PES) ultra‐ and nanofiltration membranes, prepared with the non‐toxic solvent dimethyl sulfoxide (DMSO), was investigated. The membranes were prepared by immersion precipitation via phase inversion. Experimental results proved that DMSO is a better alternative to N‐methyl‐2‐pyrrolidone (NMP) as solvent for PES ultrafiltration membranes as the membranes had a higher permeability and rejection of bovine serum albumin (BSA). An explanation was found based on experimental cloud point data and scanning electron microscopy images showing the morphology. The rejection of BSA and rose Bengal (RB) was proportional to the polymer concentration. On the contrary, the permeability decreased with increasing polymer concentration. For a casting thickness of 250 µm, an optimal balance between permeability and rejection of macromolecules for ultrafiltration was found at 24 wt % PES. The permeability was inversely proportional to the casting thickness, but a small decrease in rejection was observed when lowering the thickness. A good balance between permeability and rejection of RB was found, using a reference nanofiltration membrane of 28.5 wt % PES with 150 µm casting thickness. This membrane achieved a RB rejection of 95.3% and a pure water flux of 2.03 L m^?2 h^?1 bar^?1. The membrane thickness and polymer concentration did not have a clear influence on the hydrophilicity of the membranes. It can be concluded that DMSO is a benign alternative as compared to traditional solvents such as NMP and also results in better PES membrane performances. DMSO is a perfectly suitable solvent for ultrafiltration applications and has potential to be used for nanofiltration applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46494.     

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.     

Enhancement of flux and solvent stability of Matrimid® thin‐film composite membranes for organic solvent nanofiltration

The development of high flux and solvent‐stable thin‐film composite (TFC) organic solvent nanofiltration (OSN) membranes was reported. A novel cross‐linked polyimide substrate, consisting of a thin skin layer with minimum solvent transport resistance and a sponge‐like sublayer structure that could withstand membrane compaction under high‐pressure was first fabricated. Then the solvent flux was significantly enhanced without compromising the solute rejection by the coupling effects of (1) the addition of triethylamine/camphorsulfonic acid into the monomer solution, and (2) the combined post‐treatments of glycerol/sodium dodecyl sulphate immersion and dimethyl sulfoxide (DMSO) filtration. Finally, the long‐term stability of the TFC membrane in aprotic solvents such as DMSO was improved by post‐crosslink thermal annealing. The novel TFC OSN membrane developed was found to have superior rejection to tetracycline (MW: 444 g/mol) but was very permeable to alcohols such as methanol (5.12 lm^?2h^?1bar^?1) and aprotic solvents such as dimethylformamide (3.92 lm^?2h^?1bar^?1) and DMSO (3.34 lm^?2h^?1bar^?1). © 2014 American Institute of Chemical Engineers AIChE J, 60: 3623–3633, 2014     

压力温度双敏性聚氨酯中空纤维膜通透性研究

以聚氨基甲酸酯(PU)为基质相,添加成孔剂及二氧化硅粒子,采用熔融纺丝法制得了具有对压力、温度刺激双重敏感的PU中空纤维膜;研究了膜的水通量随压力、温度的变化,对膜表面形貌进行了表征;并考察了中空纤维膜的截留性能。结果表明:制得的PU中空纤维膜为均质膜;膜孔径分布较窄,87.17%的孔集中在0.15～0.20μm。随着压力、温度上升或下降,膜的水通量相应增大或减小,水通量与压力、温度变化有着明显的相关性。经一次过滤,无机粒子清除率达98.52%～99.78%。     

Dispersion characteristics of magnetic particles: Surface charge, surface adsorption and acid-base interaction behavior

Electrophoretic mobility (EM) of magnetic oxide particles has been measured in aqueous as well as organic solvent media using a zeta meter and a mass transporter analyzer. EM has also been measured for the magnetic particles after treatment with surface active compounds and/or in the presence of a surfactant. The results show that the sign and magnitude of the EM values change depending on the type and polarity of the surfactants or surface-treating compounds. The sign reversal of the EM values when the medium is changed from water to organic solvent can be understood on the basis of acid-base interaction. The potential energy between particles calculated using the maximum available EM value in an organic solvent is far below the value required for dispersion stability. These results along with the calculation o magnetic potential energy show that the stability of these dispersions cannot be maintained by electrostatic potential alone. Treatment of these oxide particles with surface active compounds provide stable dispersions by a ‘stearic stabilization’ mechanism. Adsorption of surfactants such as organic phosphate compounds and binder resins such as nitrocellulose or polyurcthanes on magnetic oxide particles is due to acid-base interfacial interaction between oxide particles, organic macromolecules, and solvents. Adsorption of these macromolecules in multilayers seems to be a major cause for high dispersion viscosity and the poor magnetic properties of these dispersions on tapes. These results indicate flocculation of the particles, possibly by a bridging mechanism. Thus, surface treatment of oxide particles with proper compounds inhibits adsorption of the binder resins in multilayers, promotes better rheological properties of the formulated grinds, and improves magnetic performance of tapes coated with these grinds.     

Molecularly imprinted organic solvent nanofiltration membranes – Revealing molecular recognition and solute rejection behaviour

A new class of organic solvent nanofiltration (OSN) membranes featuring molecular recognition sites has been fabricated by a phase inversion molecular imprinting technique. Polybenzimidazole (PBI) was employed as a functional polymer for molecular imprinting for the first time. Apart from acting as a functional polymer, PBI exhibits excellent chemical and solvent stability and can be used as a nanofiltration membrane, acting both as shape-specific adsorbent and size-exclusion membrane. The molecularly imprinted organic solvent nanofiltration (MI-OSN) membranes fabricated in this study showed both nanofiltration membrane performance, and excellent molecular recognition ability. The model system comprised roxithromycin pharmaceutical, 2-aminopyrimidine building block and N,N-dimethylaminopyridine catalyst, which are retained, adsorbed and permeated through the MI-OSN membrane, respectively. The effect of both dope solution concentration and applied pressure on the molecular recognition behaviour of MI-OSN membranes has been investigated by employing Sips and Freundlich adsorption isotherms, as well as examining the physical morphology of the membranes. The rate of adsorption was investigated, revealing that the adsorption follows second-order kinetics and is not limited by diffusion. The imprinted membrane exhibited fourfold higher flux whilst maintaining the same rejection performance in comparison to the control membrane. It is shown that increasing the transmembrane pressure across the MI-OSN membrane irreversibly suppresses the molecular recognition whilst maintaining the rejection and flux performance.     

Effects of retained natural organic matter (NOM) on NOM rejection and membrane flux decline with nanofiltration and ultrafiltration

Two natural source waters containing natural organic matter (NOM) with different physical and chemical characteristics were dead-end filtered using five types of membranes having different material and geometric properties. In this study retained dissolved organic carbon (DOC) per unit membrane area is introduced as a better parameter compared to permeate volume, time, and delivered DOC to provide a reasonable comparison of NOM rejection and flux-decline trends. Retained DOC/NOM was calculated, which influences NOM concentration polarization at the membrane interface, and transport measurements of NOM rejection and flux decline were made. Molecular weight (MW) distribution measurements (by size exclusion chromatography) were used to calculate the average MW of the NOM. This persuasively demonstrated that the nominal molecular weight cut-off (MWCO) of a membrane is not the unique predictor of rejection characteristics for NOM composites. The charge density of NOM from the source waters was measured to estimate its effects on NOM rejection and flux decline during filtration. The contact angle of the membranes was used to determine hydrophobic interactions between NOM and membrane. All filtration measurements were performed at approximately the same permeate flow rate in order to minimize artifacts from mass transfer at the membrane interface. ESNA having a nominal MWCO of 200 Daltons showed NOM rejection greater than 95% and flux decline lower than 10% under a condition of a retained DOC of 0.5 mg C/cm² for the feed source waters. The other membranes having larger membrane pores (nominal MWCOs ranging from 8,000 to 20,000 Daltons) than the ESNA showed NOM rejection ranging from 68% to 86% and flux decline ranging from 5% to 17% at the same retained DOC for the waters.     

Spherulite growth in isotactic polypropylene-based blends: Energy and morphological considerations

The kinetics of isothermal crystallization of polymer blends in which the matrix is a crystallizable polymer is considered. It is shown that depending on the difference in interfacial energies the inclusions are rejected or engulfed by the growing spherulite. Other factors influencing rejection, engulfing, and/or deformation of dispersed particles of the second polymer are the viscosity of the melt, the spherulite growth rate, and the size of dispersed particles. If the difference in interfacial energies is positive, then rejection or engulfing requires additional work to be done by the crystallization front. This dissipation of energy decreases the spherulite growth rate. It is estimated that the rejection of the second component is the most important phenomenon in the crystallization of blends. The spherulite growth rate of isotactic polypropylene in blends with low-density polyethylene and several elastomers was studied as a function of crystallization temperature and concentration. The comparison of growth rate data with morphological changes occuring during crystallization of blends studied shows very good agreement with the theoretical predictions based on energetics considerations.     

Effect of operating parameters and membrane characteristics on the permeate rate and selectivity of ultra- and microfiltration membranes in the depth filtration model

The effect of transmembrane pressure, pore depth and initial radius, inlet concentration of suspended particles, and particle-collection efficiency of pore walls on the permeate rate and selectivity of ultraand microfiltration membranes with pores of the same radius operated under standard blocking conditions is studied using the previously developed depth filtration model, which takes into account the nonuniform capture of particles over the pore depth. It is shown that the variation of permeate rate and selectivity with time in standard blocking strongly depends on the studied parameters. Simple analytical formulas for calculating the membrane permeate rate and selectivity as well as the time at which the pore diameter reaches the cutoff value (time at which the transition to cake filtration takes place) are proposed. The formulas make it possible to find the values of the filter coefficient, which accounts for the particle-collection efficiency of membrane pore walls, and the cutoff pore diameter using the experimental curve of the membrane selectivity. It is demonstrated that the use of the classical Hermia standard blocking model for describing the experimental data on membrane permeate rate can lead to high errors, which are caused by its assumption on the uniform profile of the layer of particles caught inside the pore.     

Hierarchical structure control of MgAl2O4 porous ceramics and application to organic polymer filtration membrane

Asymmetric porous ceramic membranes typically have a very thin top layer with finer pores covered on thick porous layers with micrometer-scale pores. In this study, triple-layer asymmetric MgAl₂O₄ filtration membranes composed of (i) an Al₂O₃ support layer (circular pellet with ~10 µm pores) prepared by dry pressing, (ii) a reactively sintered MgAl₂O₄ intermediate layer (~40 µm in thickness), prepared by dip-coating with tackifier (PEG-20000), and (iii) another reactively sintered finer MgAl₂O₄ membrane layer (~20 µm in thickness), prepared by dip-coating without tackifier. Different from our previous studies for the microfiltration of submicrometer-sized particles, in this study we have challenged the ultrafiltration of water-soluble polymer molecules. Their filtration performance was investigated by removing 1 million molecular-weight polyethylene oxide (PEO) from water. The rejection rate of the triple-layer asymmetric filtration membrane to PEO was ~14%. The all-ceramic membrane in this study showed a comparable rejection rate with the reported inorganic–organic membrane, and it must be promising for excellent chemical and thermal stabilities as well as long durability.     

Preparation and properties of polyimide solvent‐resistant nanofiltration membrane obtained by a two‐step method

This study investigated the preparation of polyimide solvent‐resistant nanofiltration membranes by a two‐step method (casting the membrane first and then crosslinking by the thermal imidization method). The influences of polymer concentration, thickness of membranes, temperature of the imidization, phase inversion time and thermal imidization procedure were studied. The membranes with the highest rejection rate of Fast Green FCF (molecular weight 808.86 g mol^?1) were prepared in the following conditions: polymer concentration 13 wt%, phase inversion time 1 h, membrane thickness 150 µm and thermal imidization procedure 200 °C for 2.5 h, 250 °C for 2 h and 300 °C for 2 h in a vacuum environment; the heating rate was 5 °C min^?1. The membrane was stable in most of the solvents tested and the fluxes of some common solvents were equal to or higher than a number of commercial solvent‐resistant nanofiltration membranes. A much higher rejection of dyes in water than in methanol was observed in the filtration experiments and a new way to explain it was developed. Copyright © 2011 Society of Chemical Industry     

Studies on Cellulose Acetate/Low Cyclic Dimmer Polysulfone Blend Ultrafiltration Membranes and their Applications

Abstract

Flat sheet ultrafiltration membranes from cellulose acetate (CA)/low cyclic dimer polysulfone (LCD PSf) were prepared by a phase inversion method. N, N′‐Dimethyl formamide and different molecular weight of polyethylene glycol (PEG 200, PEG 400, and PEG 600) were used as solvent and pore‐forming additive, respectively. The membranes were characterized in terms of pure water flux, water content, porosity, membrane hydraulic resistance, and morphology. The pure water flux was found to reach the highest value of 181.82 Lm^?2h^?1 at 5 wt.% PEG of 600 molecular weight and 10 wt.% LCD PSf content in the blended solution for membrane preparation. SEM micrographs indicated that the addition of PEG into the CA/LCD PSf solution changes the inner structure of the membrane. The influence of filtration time and applied pressure on membrane permeability was examined by copper/polyethylenimine complex rejection studies. With increase in filtration time, the rejection of the copper/polyethylenimine complex decreased and the results were discussed.     

Use of models in the design of cross-flow microfilters for the purification of protein from bio-mixtures

In order to enhance the purification efficiency of protein from binary bio-mixtures, the parameters for the design of cross-flow microfilters were examined by using models. The filtration flux, the cake thickness, the protein rejection coefficient and the protein purification flux for two different geometrical-shape microfilters, two-parallel-plate (TPP) and circular porous tube (CPT) type filters, under various cross-flow velocities and filtration pressures were calculated and are discussed here. The major factors affecting cake formation as well as the overall filtration resistance were found to be the hydrodynamic forces exerted on the depositing microbial cells. Therefore, the filtration rate increased with the increase of the cross-flow velocity and filtration pressure under a wide range of conditions. Nevertheless, two competitive effects, the cake thickness and the sweeping factor on the membrane surface, should be taken into consideration in order to evaluate the exact rejection coefficient and purification flux of proteins. The cross-flow velocity can be set at 0.7–0.9 m/s for a TPP filter to minimize the protein rejection and maximize the filtration rate and purification flux. Furthermore, the effects of the filter length, the clearance of TPP filters and the hydraulic diameter of CPT filters on the filtration rate and the protein rejection coefficient are also discussed. The filtration rate and the purification flux did not show significant variation along the cross-flow direction except in the filter inlet region; in contrast, decreasing the clearance or hydraulic diameter of filters was found to be an efficient way to enhance the performance of cross-flow microfiltration. According to the results of this study, the separation rate and purification efficiency can be improved by increasing the cross-flow velocity and using a TPP filter.     

Effects of Solvent Mixtures on Dispersion of Lanthanum-Modified Lead Zirconate Titanate Tape Casting Slurries

The effects of xylenes/ethanol solvent mixtures on lanthanum-modified lead zirconate titanate (PLZT) nonaqueous suspensions were investigated by measuring sediment height, viscosity, and adsorption of dispersant. Well-dispersed colloidal suspensions were obtained in xylene-rich solvents with a minimum amount of menhaden fish oil as a dispersant. It was also shown that adsorption of dispersant on particle surfaces strongly depends on selection of solvent(s). The ratio of solvent mixture affects PLZT solids content of unfired tapes, in which using insufficient as well as excess dispersant could decrease the packing density of particles. Solubility of other organic additives, such as binder, was also considered when selecting a xylene/ethanol ratio.     

Unsteady-State Permeate Flux of Crossflow Microfiltration

Abstract

In this study a membrane filtration cell was installed to investigate the variation of permeate flux with filtration time under various operating conditions including crossflow velocity, pressure drop, particle concentration, membrane pore size, particle size, pH, and electrolyte concentration. The dimensions of the filtration channel in the CFMF cell were 6 cm x 0.6 cm x 0.036 cm, and the flow of the suspension in the channel was controlled under the laminar flow region. Spherical polystyrene latex particles of 0.303, 0.606, and 1.020 μm were used as the suspension particles in the experiments. The density of the particles was 1.05 g/cm³. It was found that the unsteady-state permeate flux increased with an increase in particle size, membrane pore size, or crossflow velocity, but decreased with an increase in particle concentration or electrolyte concentration in the suspension. A mathematical model based on mass balance and hydrodynamic theory was developed in this study. In addition, the effect of cake growth and particle concentration decline during experiments on the permeate flux were also considered in this model. This model predicts satisfactorily the unsteady-state permeate flux of CFMF under various operating conditions.     

Low strength graywater characterization and treatmentby direct membrane filtration

The potential of direct membrane filtration on either ultrafiltration (30,200 and 400 kDa MWCO) or nanofiltration (200 Da MWCO) membranes was studied as a method of treatment of low load graywater for on site reuse. The graywater collected from a sports center's public showers and characterized for its chemical composition and physical properties fitted well to a low strength type (29.8 mg/l TSS and 170.3 mg/l as COD). Particle distribution analysis showed that colloidal size particles are the dominant fraction in terms of number distribution, while the much fewer, larger particles make up most of the particle volume (mean particle diameter0.1 μm). When treated by ultrafiltration, COD and turbidity concentration of permeates complied with established restrictions (45-70% and 92–97% reduction range, respectively), however BOD values were still above the requirements in all cases. Permeate produced by nanofiltration was of high quality with high rejection of soluble organic matter (>90%) and ionic species (50%). It can be concluded that direct dense-membrane filtration is a favorable candidate for efficient treatment of graywater for unrestricted reuse. MWCO optimization still needs to be done in order to achieve better economics at an acceptable quality of permeate produced.     

反渗透过程溶质脱除率方程

<正>溶解扩散理论、摩擦模型和表面力-孔流模型假设稳态时膜中溶质通量恒定“-”。本文分 析了该假设存在的理论依据,并基于膜的吸附-扩散模型’‘’建立了反渗透过程溶质脱除率方 程。