Flux enhancement during dean vortex tubular membrane nanofiltration. 10. Design,construction, and system characterization

Controlled centrifugal instabilities (called Dean vortices) resulting from flow in helical tubes have been used to reduce concentration polarization and membrane fouling during nanofiltration. These vortices enhance back-migration of solute through convective flow away from the membrane–solution interface and allow for increased membrane permeation rates. Based on the theory of Dean vortex flow, a new prototype vortex generating tubular nanofiltration element was designed. Two sets of nanofiltration modules were constructed; a linear module and a new module containing hollow fibers wrapped around rods of small diameter in helical geometry. Optimization of the design is discussed with respect to the diameter and thickness of the hollow fibers. Axial pressure drop and energy consumption measurements for the helical module agreed very well with available correlations for various experimental conditions. Water permeabilities for the helical modules were similar to those of the conventional linear modules. No significant effect of pH was observed on the water permeability.     

Flux enhancement during Dean vortex microfiltration. 8. Further diagnostics

Controlled centrifugal instabilities (called Dean vortices) resulting from flow around a curved channel have been used to reduce both concentration polarization and membrane fouling during microfiltration. These vortices enhance back-migration through convective flow away from the membrane–solution interface and allow for increased membrane permeation rates.As part of an effort to further understand the opportunities and limitations of using Dean vortex microfiltration, we report here on a series of experiments in which the performance of runs in the presence of vortices are compared to that in the absence of vortices. They include changing the following variables: transmembrane pressure, particle suspension concentration, ionic strength of the solution, and the reversibility of changing flow rate (from high rates in the presence of Dean vortices to low rates in the absence of such vortices and the reverse procedure) within a particular experiment. Unexpected results were observed for the ionic strength and flow reversal cases. The flux decreased with increasing ionic strength, passed through a minimum and then increased to a constant value. In the same experiment, the flux decreased when the flow rate was reversed from a high to low and back to a high value, until the flux was not affected by changing flow rate. We explain these interesting observations by contrasting the behavior of aggregated colloids in solution and within the deposited cake layer on the membrane surface. Pressure effects on the cake layer are also considered.     

Flux decline during nanofiltration of naturally-occurring dissolved organic matter: effects of osmotic pressure,membrane permeability,and cake formation

Nanofiltration of naturally-occurring dissolved organic matter (NOM) by an aromatic polyamide membrane was measured in a crossflow bench-scale test cell and modeled using a semi-empirical osmotic pressure/cake formation model. Our objective was to examine flux decline due to NOM fouling while explicitly accounting for flux decline due to osmotic effects and changes in membrane permeability. This approach allowed quantification of the effect of ionic composition on specific NOM cake resistance, and yielded insight into flux decline due to enhanced NaCl rejection by the NOM deposit. In the absence of NOM, increasing NaCl concentration reduced salt rejection and decreased membrane permeability. Flux decline was modeled by accounting for changes in osmotic pressure with time, and by employing an effective permeability. The addition of calcium significantly reduced rejection of sodium and feed conductivity, and thus mitigated flux decline. Increasing pH from 4 (near membrane pI) to 10 increased the effective permeability but also increased NaCl rejection, which resulted in greater flux decline. The presence of NOM caused greater flux decline resulting from a combination of NOM cake resistance and increased rejection of NaCl by negatively charged NOM functional groups. Increasing NaCl concentration had little effect on the mass of NOM deposited, but significantly increased the specific resistance of the NOM cake. The effect of ionic strength on specific resistance correlated with a reduction in NOM size, estimated by separate UF permeation experiments and size exclusion chromatography analysis of UF permeate. Therefore, increased specific cake resistance is consistent with a more compact, less porous cake. Flux decline by NOM solutions showed a maximum at pH 7, where salt rejection was also a maximum. Binding of calcium reduced the ability of NOM to enhance NaCl rejection, and likely increased NOM cake resistance. Flux decline caused by NOM fouling in the presence of calcium was only significantly different than that caused by NOM in a solution of NaCl at the same ionic strength when the calcium concentration corresponded to saturation of NOM binding sites.     

Formation and characterization of asymmetric nanofiltration membrane: Effect of shear rate and polymer concentration

In this study, we report the effects of shear rates and polymer concentrations in the formation of asymmetric nanofiltration membrane using a simple dry/wet phase inversion technique. Employing the combination of irreversible thermodynamic model, solution-diffusion model (Spiegler–Kedem equation), steric-hindrance pore (SHP) model and Teorell–Meyers (TMS) model, the transport mechanisms and membrane structural properties were determined and have been characterized for different cases of those formation parameters. The experimental and modeling showed very promising results in terms of membrane performance with interesting structural details. The optimum shear rate (critical shear rate) was found to be at about 203.20 s⁻¹ and the best polymer concentration toward the formation of high performance nanofiltration membrane is in the range of 19.60–23.10%. The modeling results suggested that the pore radius of the membranes produced lies within the range of pore radius of 29 commercial available membranes. This study also proposed that the electrolytes transport through nanofiltration membrane was dominated by a convection factor which accounted approximately 30% more than a diffusion factor. This study also indicated that shear rate and polymer concentration were found to affect the membrane performance and structural properties by providing, to a certain extent, an oriented membrane skin layer which in turn exhibiting an improvement in membrane separation ability.     

Dielectric analysis of nanofiltration membrane in electrolyte solutions: influences of electrolyte concentration and species on membrane permeation

Dielectric properties of a nanofiltration membrane immersed in dilute aqueous electrolyte solutions were measured, and frequency dependence of capacitance and conductance of the systems was analyzed, based on the interfacial polarization theory, giving values of permittivity and conductivity of the membrane and the solutions. Permittivity, epsilon m, of the membrane slightly decreased whereas conductivity, km, of the membrane increased with increasing electrolyte concentration, as a result of entrance of ions into the membrane. The ratio of membrane/solution conductivity, km/kw, also depended on the electrolyte concentration, showing that distribution of ions in the membrane and in solutions follow Donnan equilibrium, due to the presence of negative fixed charges in the membrane. New expressions were derived from Donnan equilibrium principle to explain this phenomenon, and negative fixed charge concentration ce of the membrane was obtained; thus the Donnan potential, DeltaPhi Don, of the membrane in solutions at various concentrations could be calculated. The new expressions could be expected to be usable to analyze ion permeation property through membrane.     

Flux improvement by Dean vortices: ultrafiltration of colloidal suspensions and macromolecular solutions

Coiled and straight hollow-fibre modules have been built and tested; the permeate flux obtained in ultrafiltration with these two geometries is compared for two feeds: a colloidal bentonite suspension and a dextran solution. In the case of colloidal suspensions, the secondary flows induced by the coiled geometry allow fouling to be reduced and the permeate flux is multiplied by a factor of up to 2. An empirical relationship is proposed to express the limiting flux of permeate as a function of both the velocity and some geometrical parameters of the coiled modules. Analogous results are obtained during the ultrafiltration of dextran. It is also shown that under certain conditions almost no deposit was formed; the permeate flux under these conditions is three times higher for coiled modules than for straight ones. For a given energy expenditure and ultrafiltration process, the gain in permeate flux can reach a factor of 1.8.     

Study on a novel polyester composite nanofiltration membrane by interfacial polymerization of triethanolamine (TEOA) and trimesoyl chloride (TMC): I. Preparation, characterization and nanofiltration properties test of membrane

A novel thin-film composite (TFC) membrane for nanofiltration (NF) was developed by the interfacial polymerization of triethanolamine (TEOA) and trimesoyl chloride (TMC) on the polysulfone (PSf) supporting membrane. The active surface of the membrane was characterized by using FT-IR, XPS and SEM. The performance of TFC membrane was optimized by studying the preparation parameters, such as the reaction time of polymerization, pH of aqueous phase and the concentration of reactive monomers. It is found that the membrane performance is related to the changes of the monomer content in the aqueous phase rather than in the organic phase. Furthermore, the nanofiltration properties of the TFC membrane were tested by examining the separating performance of various salts at 0.6 MPa operating pressure. The rejection to different salt solutions decreased as per the order of Na₂SO₄ (82.2%), MgSO₄ (76.5%), NaCl (42.2%) and MgCl₂ (23%). Also, streaming potential tests indicated that isoelectric point of the TFC membrane is between pH 4 and 5. Moreover, the investigation of the flux for NaCl solution at different pH showed that the polyester NF composite membrane is also particularly suitable for treating acidic feeds: the flux increased from 8.4 to 11.5 L/m² h when pH of the feed decreased from 9 to 3. Additionally, the TFC membrane exhibits good long-term stability.     

Organic solvent nanofiltration in asymmetric hydrogenation: enhancement of enantioselectivity and catalyst stability by ionic liquids

This communication describes the enhancement of the enantioselectivity and the stability of Ru-BINAP with the ionic liquid trihexyl(tetradecyl)phosphonium chloride (CyPhos101), and the use of organic solvent nanofiltration for the efficient separation of the catalyst and ionic liquid from the hydrogenation product, followed by simultaneous recycling of the catalyst and ionic liquid.     

Electrochemical characterization of an asymmetric nanofiltration membrane with NaCl and KCl solutions: influence of membrane asymmetry on transport parameters

Electrochemical characterization of a nanofiltration asymmetric membrane was carried out by measuring membrane potential, salt diffusion, and electrical parameters (membrane electrical resistance and capacitance) with the membrane in contact with NaCl and KCl solutions at different concentrations (10(-3)< or =c(M)< or =5 x 10(-2)). From these experiments characteristic parameters such as the effective concentration of charge in the membrane, ionic transport numbers, and salt and ionic permeabilities across the membrane were determined. Membrane electrical resistance and capacitance were obtained from impedance spectroscopy (IS) measurements by using equivalent circuits as models. This technique allows the determination of the electrical contribution associated with each sublayer; then, assuming that the dense sublayer behaves as a plane capacitor, its thickness can be estimated from the capacitance value. The influence of membrane asymmetry on transport parameters have been studied by carrying out measurements for the two opposite external conditions. Results show that membrane asymmetry strongly affects membrane potential, which is attributed to the Donnan exclusion when the solutions in contact with the dense layer have concentrations lower than the membrane fixed charge (X(ef) approximately -0.004 M), but for the reversal experimental condition (high concentration in contact with the membrane dense sublayer) the membrane potential is practically similar to the solution diffusion potential. The comparison of results obtained for both electrolytes agrees with the higher conductivity of KCl solutions. On the other hand, the influence of diffusion layers at the membrane/solution interfaces in salt permeation was also studied by measuring salt diffusion at a given NaCl concentration gradient but at five different solutions stirring rates.     

Surfactant fouling of nanofiltration membranes: measurements and mechanisms.

Fouling of nanofiltration membranes is studied during filtration of aqueous surfactant solutions under different conditions. To this purpose, four typical nanofiltration membranes (Desal51HL, NF270, NTR7450 and NFPES10) and three typical surfactants (nonionic neodol, anionic SDBS and cationic cetrimide) are selected. Fouling is studied as a function of the surfactant concentration, with and without addition of an electrolyte (NaCl), at different pH and when filtering a mixture of surfactants. Adsorption experiments and hydrophobicity measurements (to study the orientation of the surfactants on the membrane surface) are also performed under the different conditions. The least membrane fouling is found for the anionic surfactant SDBS, while for the cationic surfactant cetrimide very low relative fluxes are observed. Neodol shows an intermediate degree of fouling. Both hydrophobic and electrostatic interactions (in the case of ionic surfactants) between the membrane surface and the surfactant explain the degree of adsorption and hence fouling, as membrane fouling is correlated with the amount of adsorbed surfactant. The difference between cetrimide and SDBS becomes especially visible when changing the pH: increasing the pH leads not only to an opposite orientation of the adsorbed surfactants, but also to an opposite trend in adsorbed amount and membrane fouling. This study permits selection of an optimal nanofiltration membrane to recycle wastewater containing surfactants in the carwash industry. The optimal choice would be a hydrophilic membrane with a low molecular weight cut-off and a small negative surface charge at neutral pH. Cationic surfactants in the wastewater should also be avoided as much as possible.     

Dielectric characterization of a nanofiltration membrane in electrolyte solutions: its double-layer structure and ion permeation

Dielectric spectroscopy (DS) was applied to a nanofiltration (NF) membrane to detect its double-layer structure and ion permeation. Dielectric measurements were carried out on the systems composed of the NF membrane NTR7450 and dilute solutions of eight electrolytes, LiCl, NaCl, KCl, NH(4)Cl, MgCl(2), CaCl(2), BaCl(2), and CuCl(2). Two relaxations were observed in the frequency range from 40 Hz to 4 MHz for each system. On the basis of characteristics of the dielectric spectra and the Maxwell-Wagner interfacial polarization theory, the low-frequency relaxation was attributed to inhomogeneity of the membrane structure itself, whereas the high-frequency relaxation was attributed to interfacial polarization between the membrane and the solution. A multiphase dielectric model previously developed by one of the authors and co-workers was adopted to present systems to analyze the dielectric spectra, and electric parameters, i.e., capacitance and conductance, of the two layers composing the membrane were obtained. The electric properties estimated for the two layers were different and changed with the environment in a different manner. Further analyses suggest that the two layers had a different separation mechanism due to their difference in materials, looseness, and fixed charge content. The fixed charge density of one layer was estimated, and the ion permeation difficulties in both layers was compared. This research revealed that DS was by far an effective method to obtain detailed electric parameters about the inner multilayer structure of the NF membrane and to elucidate separation mechanisms of each layer.     

Evaluation of fine structure of tubular zeolite NaA membrane by FTIR-ATR and FIB-TEM.

A zeolite NaA (LTA) membrane supported by an alumina porous support tube for pervaporation (PV) dehydration of ethanol was characterized by transmission electron microscopy (TEM) using a focused ion beam (FIB) thin-layer specimen preparation technique and by Fourier transform infrared attenuated total reflectance method (FTIR-ATR) using a diamond prism as the waveguide. FIB-TEM clearly presented cross-section images up to about 15 microm depth from the membrane surface. FTIR-ATR monitored the Si-O asymmetric stretching vibration spectrum. The Si-O spectrum was compared with the TEM image and their relationships were discussed. By combining the two methods, we could study the thickness of surface LTA crystals, the grain boundary, the LTA/alumina interface structure and the crystallinity and density of materials inside of the alumina porous support. Consequently, fine structure changes of the LTA membrane corresponding to the hydrothermal synthesis condition could be sensitively detected.     

Flux decline during cross flow membrane filtration of electrolytic solution in presence of charged nano-colloids: a simple electrokinetic model

An electrokinetic transport based approach for quantification of reversible flux decline due to the concentration polarization of an electrolyte solution in presence of charged colloids is presented. The model envisions the electrolyte transport across a charged cake or gel layer as transport of ions through charged cylindrical capillaries. This model is coupled with the standard theory of concentration polarization during cross flow membrane filtration. The analysis is carried out entirely in terms of generalized, non-dimensional variables. A dimensionless group termed as the scaled gel layer resistance evolves from the analysis, which accounts for the electrical properties of the charged nano-colloids and the electrolyte solution. A parametric study is performed to elucidate the coupled influence of mass transfer, membrane resistance, gel resistance, and electrical properties of the gel-electrolyte polarized layer. The effects of these parameters are examined on the filtration performance through the model equations.     

Human proteome enhancement: high-recovery method and improved two-dimensional map of colostral fat globule membrane proteins.

The human milk fat globule membrane protein composition is still largely unknown, although it counts for 2-4% of the total milk protein content and contains several important biologically active components. The aim of this work was to create a two-dimensional electrophoresis (2-DE) map of the human milk fat globule membrane proteins, both integral and membrane-associated, and to identify and characterize as many protein components as possible. A new protocol for the solubilization and extraction of the human milk fat globule membrane proteins with a double extraction procedure is presented, and the results compared with the extraction methods reported in the literature. The proteins were separated, in the first dimension, by isoelectric focusing (IEF) in the pH range 3-10 on strips of 13 cm length and, in the second dimension, by Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) on 11.5% T homogeneous gels. A reproducible 2-DE map of integral and membrane-associated proteins was obtained and the first 23 spots, representing the major components, were identified by matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometric analysis and/or by amino acid sequencing.     

Direct osmotic concentration of tomato juice in tubular membrane – module configuration. II. The effect of using clarified tomato juice on the process performance

A study was carried out by using a new tubular direct osmosis module, constructed by PCI UK and equipped with a novel AFC99 membrane 400 μm thick, to investigate the effect of clarifying tomato juice on the rate of direct osmotic concentration. Under virtually the same operating conditions five respective clarifications of juice were tried including full, unclarified, tomato juice. The process performance was calculated in terms of permeation flux. In all the experimental runs the osmotic medium was sodium chloride brine (initial concentration approx. 23% NaCl). A remarkable increase of permeation flux (over 100%) was observed shifting from unclarified to the clarified tomato juice. Clarification was carried out by passing the juice through 35 μm mesh and also by using membranes of molecular weight cut-off 200 000, 100 000 and 25 000 Daltons (Da), respectively, in order to obtain the juice ultrafiltrates. It is also worth mentioning that the flux value obtained with 25 000 Daltons (Da) ultrafiltrate appeared to be considerably higher than values reported in experiments carried out by other researchers, where unclarified juice was used, despite the disadvantage of a far thicker membrane being used in the present investigation. This specific finding discloses great potential in using a combined low temperature and pressure ultrafiltration and direct osmosis process to produce tomato concentrates.     

Temperature and concentration effects on electrolyte transport across porous thin-film composite nanofiltration membranes: Pore transport mechanisms and energetics of permeation

The influence of temperature and concentration on nanofilter charge density and electrolyte pore transport mechanisms is reported. Crossflow filtration experiments were performed to measure transport of several electrolytes (NaCl, NaNO3, NaClO4, CaCl2, MgCl2, and MgSO4) across two commercially available thin-film composite nanofiltration membranes in the range 5-41 degrees C. Experiments were also performed with selected salts in the range 1-50 meq/L to quantify concentration effects. Three different approaches, irreversible thermodynamics, extended Nernst-Planck formulation, and theory of rate processes, were employed to interpret retentions of these symmetric and asymmetric electrolytes at varying temperature and concentration. Increasing feed water temperature slightly increased electrolyte reflection coefficients and only weakly increased permeability compared with neutral solutes. Electromigration and convection tended to counteract each other at high fluxes explaining the weak temperature dependence of the reflection coefficient. Changes in membrane surface charge density with temperature were attributed to increased adsorption of electrolytes on the polymer constituting the active layer. Activation energy of permeation for charged solutes was primarily determined by the Donnan potential at the membrane-feed water interface. Electrolyte permeation was shown to be an enthalpy-driven process that resulted in small entropy changes. Increasing sorption capacity with temperature and low sorption energies indicated that co-ion sorption on polymeric membranes was an endothermic physicosorption process, which appears to determine temperature dependence of electrolyte permeation at increased feed concentrations.     

Study on the thin-film composite nanofiltration membrane for the removal of sulfate from concentrated salt aqueous: Preparation and performance

Thin-film composite (TFC) nanofiltration (NF) membrane was prepared through the interfacial polymerization between piperazine (PIP) and trimesoyl chloride (TMC) on the polysulphone support membrane. The chemical structure of membrane surface was studied by attenuated total reflectance infrared (ATR-IR) and X-ray photoelectronic spectroscopy (XPS). Parametric studies were conducted by varying reaction time, curing temperature, curing time and additives in PIP solution for obtaining the optimum polymerization conditions. Systematic performance studies were conducted with different feed solutions, feed concentrations, feed pHs, operating temperatures and pressures. Continuous and comparative tests were also conducted to determine the performance stability and separation efficiency of the thin-film composite NF membrane prepared. High performance thin-film composite NF membrane for the selective sulfate removal from concentrated sodium chloride aqueous with the water permeability coefficient of 75 L/(m² h MPa) could be prepared under specific conditions. Experimental results on concentrated mixed solution of NaCl and Na₂SO₄ demonstrated that the NF membrane developed could be successfully used for the removal of sodium sulfate from the concentrated brine of chloralkali industry with high permeate flux, selectivity and performance stability.