High flux nanofiltration membranes based on interfacially polymerized polyamide barrier layer on polyacrylonitrile nanofibrous scaffolds

Electrospun polyacrylonitrile (PAN) nanofibrous scaffold was used as a mid-layer support in a new kind of high flux thin film nanofibrous composite (TFNC) membranes for nanofiltration (NF) applications. The top barrier layer was produced by interfacial polymerization of polyamides containing different ratios of piperazine and bipiperidine. The filtration performance (i.e., permeate flux and rejection) of TFNC membranes based on electrospun PAN nanofibrous scaffold was compared with those of conventional thin film composite (TFC) membranes consisting of (1) a commercial PAN ultrafiltration (UF) support with the same barrier layer coating and (2) two kinds of commercial NF membranes (i.e., NF90 and NF270 from Dow Filmtec). The nanofiltration test was carried out by using a divalent salt solution (MgSO₄, 2000 ppm) and a cross-flow filtration cell. The results indicated that TFNC membranes exhibited over 2.4 times more permeate flux than TFC membranes with the same chemical compositions, while maintaining the same rejection rate (ca. 98%). In addition, the permeate flux of hand-cast TFNC membranes was about 38% higher than commercial NF270 membrane with the similar rejection rate.     

The influence of the porous support layer of composite membranes on the separation of binary gas mixtures

Thin film composite (TFC) membranes exhibit a high flux for gas and vapor permeation and are viable for a wide range of applications. The high flux may also increase the importance of the resistance of the porous support structure depending on the application and process conditions. A comprehensive modeling approach for TFC membranes is introduced, which considers boundary layer resistances near the membrane surface, solution-diffusion through the coating, and the influence of the porous sublayer. Permeation through the support structure is described by the dusty gas model (DGM) with the support treated as a two-layered structure with a dense but porous skin and a more open substructure.The model accurately describes experimental data on TCE/nitrogen separation using a sweep gas on the permeate side very well. The main resistance towards TCE permeation through two different membranes tested is the porous support. It is shown that changes in the support morphology can greatly enhance the performance of the composite membranes. Model calculations were also performed for vacuum assisted permeation. The pressure drop across the support is considerable depending on the coating thickness. The TCE permeation is again dominated by the resistance of the support layer, which can be reduced by altering the morphological parameters of the structure.The proposed model is able to describe the performance of the composite membrane and to identify optimum process conditions for given performance characteristics. It can be used to aid in the development of membrane structures for enhanced performance.     

Multicomponent pervaporation process for volatile aroma compounds recovery from pomegranate juice

The present work describes the possibility of using pervaporation process to recover the pomegranate aroma compounds from an actual pomegranate juice and a model aroma solution. Four different chemicals representing four major kinds of aroma compounds, namely, 3-methyl butanal, isopentyl acetate, n-hexanol and α-ionone, were utilized in this work. Three POMS membranes and two PDMS membranes were tested for pervaporation and compared for their separation performance. The influence of various operating parameters such as feed flow rate, feed temperature and permeate pressure on the permeation flux and aroma compounds enrichment factor was investigated. Feed flow rate was shown to have no significant effect on both total flux and aroma enrichment factor, whereas feed temperature and permeate pressure had highly significant effects. An increase in feed temperature led to higher flux and enrichment factor. As permeate pressure increased, the flux and enrichment factor of some aroma compounds decreased. Some of the aroma compounds showed higher enrichment factor at higher permeate pressures. Finally, the activation energy of permeation and the membrane permeability for each aroma compound were determined.     

A mass transfer model for the prediction of rejection and flux during ultrafiltration of PEG-6000

A mass transfer model in case of ultrafiltration is proposed in the present study which is capable of predicting the permeate volumetric flux and rejection at different pressure, concentration and stirrer speed. The model is based on the steady state mass balance over the boundary layer, coupled with the results from irreversible thermodynamics. It first predicts the membrane surface and permeate concentrations — which are then utilized to calculate rejection. Permeate flux is then predicted using the result obtained from filtration theory. The model utilizes four parameters, namely, solvent permeability, solute permeability, reflection coefficient and specific cake resistance. These parameters along with the known values of the operating conditions and solution properties enable one to predict the flux as a function of time and rejection. The computed results are found to be in good agreement with the previously published data of Bhattacharjee and Bhattacharya during ultrafiltration of PEG-6000 by cellulose acetate membrane.     

A novel approach for modeling concentration polarization in crossflow membrane filtration based on the equivalence of osmotic pressure model and filtration theory

A theoretical model for prediction of permeate flux during crossflow membrane filtration of rigid hard spherical solute particles is developed. The model utilizes the equivalence of the hydrodynamic and thermodynamic principles governing the equilibrium in a concentration polarization layer. A combination of the two approaches yields an analytical expression for the permeate flux. The model predicts the local variation of permeate flux in a filtration channel, as well as provides a simple expression for the channel-averaged flux. A criterion for the formation of a filter cake is presented and is used to predict the downstream position in the filtration channel where cake layer build-up initiates. The predictions of permeate flux using the model compare remarkably well with a detailed numerical solution of the convective diffusion equation coupled with the osmotic pressure model. Based on the model, a novel graphical technique for prediction of the local permeate flux in a crossflow filtration channel has also been presented.     

High-flux ceramic membranes with a nanomesh of metal oxide nanofibers

Traditional ceramic separation membranes, which are fabricated by applying colloidal suspensions of metal hydroxides to porous supports, tend to suffer from pinholes and cracks that seriously affect their quality. Other intrinsic problems for these membranes include dramatic losses of flux when the pore sizes are reduced to enhance selectivity and dead-end pores that make no contribution to filtration. In this work, we propose a new strategy for addressing these problems by constructing a hierarchically structured separation layer on a porous substrate using large titanate nanofibers and smaller boehmite nanofibers. The nanofibers are able to divide large voids into smaller ones without forming dead-end pores and with the minimum reduction of the total void volume. The separation layer of nanofibers has a porosity of over 70% of its volume, whereas the separation layer in conventional ceramic membranes has a porosity below 36% and inevitably includes dead-end pores that make no contribution to the flux. This radical change in membrane texture greatly enhances membrane performance. The resulting membranes were able to filter out 95.3% of 60-nm particles from a 0.01 wt % latex while maintaining a relatively high flux of between 800 and 1000 L/m2.h, under a low driving pressure (20 kPa). Such flow rates are orders of magnitude greater than those of conventional membranes with equal selectivity. Moreover, the flux was stable at approximately 800 L/m2.h with a selectivity of more than 95%, even after six repeated runs of filtration and calcination. Use of different supports, either porous glass or porous alumina, had no substantial effect on the performance of the membranes; thus, it is possible to construct the membranes from a variety of supports without compromising functionality. The Darcy equation satisfactorily describes the correlation between the filtration flux and the structural parameters of the new membranes. The assembly of nanofiber meshes to combine high flux with excellent selectivity is an exciting new direction in membrane fabrication.     

Influence of operating conditions on performance of ceramic membrane used for water treatment

The removal of natural organic matter (NOM) is a critical aspect of potable water treatment because NOM compounds are precursors of harmful disinfection by-products, hence should be removed from water intended for human consumption. Ultrafiltration using ceramic membranes can be a suitable process for removal of natural substances. Previously reported experiments were dedicated to evaluating the suitability of ultrafiltration through ceramic membrane for water treatment with a focus on the separation of natural organic matter. The effects of the membrane operating time and linear flow velocity on transport and separation properties were also examined. The experiments, using a 7-channel 300 kDa MWCO ceramic membrane, were carried out with model solutions and surface water at trans-membrane pressure of 0.2–0.5 MPa. The results revealed that a loose UF ceramic membrane can successfully eliminate natural organic matter from water. The permeability of the membrane was strongly affected by the composition of the feed stream, i.e. the permeate flux decreased with an increase in the NOM concentration. The permeate flux also decreased over the period of the operation, while this parameter did not influence the effectiveness of separation, i.e. the removal of NOM. It was observed that the increased cross-flow velocity resulted in the decrease in the membrane-fouling intensity and slightly improved the retention of contaminants.     

Pervaporation dehydration of ethanol–water mixtures with chitosan/hydroxyethylcellulose (CS/HEC) composite membranes: I. Effect of operating conditions

Composite hydrophilic pervaporation membranes were prepared from chitosan blended with hydroxyethylcellulose using cellulose acetate as a porous support. The membranes were tested for dehydration performance of ethanol–water mixtures of ethanol concentrations 70–95 wt.% in the laminar flow region, at temperatures 50–70°C and at permeate pressures of 3–30 mmHg. The composite membrane showed an improved dehydration performance compared with dense CS/HEC membrane developed earlier. The effects of operating conditions also revealed that pervaporation of low water content feed carried out at high feed flow rate and at low temperature and permeate pressure was an advantage.     

Flux enhancement in crossflow filtration using an unsteady jet

This paper deals with the influence of a new type of unsteadiness in the flow on the permeate flux in crossflow filtration. A pneumatically controlled valve generates intermittent jets from the main flow leading to the formation of large vortices moving downstream along the tubular membrane. The experimental study was carried out by filtering a bentonite suspension through an ultrafiltration mineral membrane. Flux time measurements were taken under steady and unsteady operating conditions. The unsteadiness leads to a permeate flux more than two times higher than in the usual filtration processes.     

On the behaviour of asymmetric membranes in pervaporation

Symmetric and asymmetric membranes of the Loeb type are compared with respect to their performance in pervaporation. The experiments are carried out with water—isopropanol mixtures, employing cellulose acetate membranes of different structure, but of the same total thickness. These results are compared with calculations based on a 2-layer model for asymmetric membranes. Design criteria for optimal asymmetric membranes for pervaporation, as well as the performance characteristics for the two possible modes of installation — active layer facing the feed or facing the permeate — are discussed. Contrary to reverse osmosis, the installation of the membrane with the active layer facing the permeate proves to be superior — at least for low permeabilities of the membrane material. The interdependencies between thickness and permeability of the active layer, and porosity and thickness of the support layer are much stronger than in reverse osmosis.     

Critical flux measurement for model colloids

The conventional operating membrane of a laboratory membrane filtration process is to apply controlled transmembrane pressures to the retentate side of the membrane, with the permeate side open ended. Often the minimum transmembrane pressure available is sufficient to cause membrane fouling in a given system. A membrane rig has been built which monitors transmembrane pressure in increments of 0.001 bar and by pumping permeate at a specified rate controls the flux to be constant. The technique used allows sensitive detection of trace fouling. Under a variety of low flux conditions fouling was not observed and it was found to be useful to produce an experimentally related definition of two types of critical flux. In the first definition a `strong form' of critical flux exists if the flux of a suspension is identical to the flux of clean water at the same transmembrane pressure. In the second definition a `weak form' of the critical flux exists if the relationship between transmembrane pressure and flux is linear, but the slope of the line differs from that for clean water. This paper describes how the use of this operating mode led to the successful experimental measurements of critical fluxes for two colloidal silica suspensions, BSA solution and a baker's yeast suspension with a 50k MWCO membrane. These measurements could not be made successfully in constant-pressure mode. The paper also reports experimental evidence in support of a `strong form' of the critical flux for the filtration of X30 silica suspension. Finally, we report the effect of membrane pore size on critical flux measurements for the three types of feed fluids.     

Secondary membranes for flux optimization in membrane filtration of biologic suspensions

We employ in situ deposited secondary membranes of yeast (SMYs) to optimize permeate flux during microfiltration and ultrafiltration of protein solutions. The deposited secondary membrane was periodically removed by backflushing, and a new cake layer was deposited at the start of the next cycle. The effects of backflushing time, backflushing strength, wall shear rate, and amount of secondary membrane deposited on the permeate flux were examined. Secondary membranes were found to increase the permeate fluxin microfiltration by severalfold. Protein transmission was also enhanced owing to the presence of the secondary membrane, and the amount of protein recovered was more than twice that obtained during filtration of protein-only solutions under othewise identical conditions. In ultrafiltration, the flux enhancement owing to the secondary membrane was only 50% or less. In addition, the flux for ultrafiltration was relatively insensitive to changes in the concentration of yeast used during deposition of SMY and to the backflushing strength used to periodically remove the secondary membrane.     

Crossflow microfiltration of oily water

Two α-alumina ceramic membranes (0.2 and 0.8 μm pore sizes) and a surface-modified polyacrylonitrile membrane (0.1 μm pore size) were tested with an oily water, containing various concentrations (250–1000 ppm) of heavy crude oil droplets of 1–10 μm diameter. Significant fouling and flux decline were observed. Typical final flux values (at the end of experiments with 2 h of filtration) for membranes at 250 ppm oil in the feed are ≈30–40 kg m⁻² h⁻¹. Increased oil concentrations in the feed decreased the final flux, whereas the crossflow rate, transmembrane pressure, and temperature appeared to have relatively little effect on the final flux. In all cases, the permeate was of very high quality, containing <6 ppm total hydrocarbons. The addition of suspended solids increased the final membrane flux by one order of magnitude. It is thought that the suspended solids adsorb the oil, break up the oil layer, and act as a dynamic or secondary membrane which reduces fouling of the underlying primary membrane. Resistance models were used to characterize the type of fouling that occurs. Both the 0.2 μm and the 0.8 μm ceramic membranes appeared to exhibit internal fouling followed by external fouling, whereas external fouling characterized the behavior of the 0.1 μm polymer membrane from the beginning of filtration. Examination of the external fouling layer showed a very thin hydrophobic oil layer adsorbed to the membrane surface. This oil layer made the membrane surface hydrophobic, as demonstrated by increased water-contact angles. The oil layer proved resistant to removal by hydrodynamic (shear) methods. By extracting the oil layer with tetrachloroethylene, followed by IR analysis, its average thickness at the end of a 2 h experiment under typical conditions was determined to be 60 μm for the 0.2 μm ceramic membrane and 30 μm for the 0.1 μm polymer membrane. These measured amounts of oil associated with the membrane at the end of the experiments are in good agreement with those determined from a simple mass balance, in which it is assumed that all of the oil associated with the permeate collected is retained on or in the membrane, indicating that the tangential flow did not sweep the rejected oil layer to the filter exit.     

Role of backpulsing in fouling minimization in crossflow filtration with ceramic membranes

Effect of backpulsing on crossflow filtration of different process streams was studied. Laboratory scale experiments were conducted with synthetic electroplating wastewater containing Cr(OH)₃ suspension. Porous ceramic membranes of various pore sizes (0.05–5.0 μm) were evaluated. Filtration experiments with and without backpulsing show that backpulsing is effective in minimizing membrane fouling. Up to five-fold increase in steady-state permeate flux and 100% flux recovery were observed. Theoretical aspects are reviewed to develop a better understanding of the critical parameters associated with high-pressure backpulsing.Pilot and commercial scale operating results on several industrial applications, such as yeast filtration, process slurry filtration and oily wastewater filtration are presented. Data analysis shows the critical importance of backpulsing in reducing long-term membrane fouling while allowing the realization of high product recovery. Optimization of process parameters with backpulsing typically results in higher flux and reduces the total capital cost required to achieve the desired production rate.     

Experimental and modeling study of oxygen permeation modes for asymmetric mixed-conducting membranes

An asymmetric mixed-conducting membrane consists of a thin dense layer and a porous support, and its application has drawn considerable attention, because it is expected to have a more promising potential in the practical application compared with the symmetric membrane. However, with the introduction of support in the asymmetric membrane, two possible permeation modes are produced. One mode is that oxygen permeates from the support to the thin dense layer (designated as SD mode). The other is in the direction from the thin dense layer to the support (designated as DS mode). Thus, from the viewpoint of choosing an appropriate oxygen permeation mode to make better use of the membrane, it is necessary to study the oxygen flux in these two modes. In this paper, their effects on the oxygen flux of asymmetric membranes were investigated from the experiment and the model. The modeling results showed a good agreement with the experimental data. Our study demonstrates that when the asymmetric membrane adopts the SD mode, it is beneficial for the membrane to obtain higher oxygen permeation flux.     

Use of polyelectrolyte multilayer systems for membrane modification

Membranes with designed surface and filtration properties were prepared by the adsorption of polyelectrolyte multilayer systems on membrane surfaces using the layer-by-layer electrostatic self assembly (ESA) technique. Microfiltration membranes with a first polyelectrolyte layer grafted onto the surface showed excellent stability during filtration process. Although a twofold higher permeate flux was observed for a three-layer polyelectrolyte complex membrane compared to a just grafted one the protein retention did not change remarkably. Additionally, a reduced protein adsorption was detected for repulsive electrostatic forces between the substrate and the protein under applied conditions. Pervaporation membranes with an anionically functionalized polyamide-6 support or Nafion^®-117 support and a dense separating layer consisting of poly(acrylic acid) and poly(ethylenimine) were prepared. Those membranes were used to separate aqueous organic mixtures. Six double layers were sufficient to obtain membranes with high water permselectivity. Membranes with similar properties but a lower number of deposited layers were obtained, when the adsorption process was carried out at 80°C.     

Transport through zeolite filled polymeric membranes

In this paper the effect of zeolite particles incorporated in rubbery polymers on the pervaporation properties of membranes made from these polymers is discussed. Pervaporation of methanol/toluene mixtures was carried out with membranes prepared from the toluene selective polymer EPDM and the methanol selective polymers Viton and Estane 5707. From the results of the pervaporation experiments it could be concluded that the addition of the hydrophilic zeolite NaX as well as the hydrophobic zeolite silicalite-1 leads to an increase in methanol flux and a decrease in toluene flux through the membranes. Pervaporation experiments with bi-layer membranes consisting of an unfilled polymer layer filled with zeolite particles demonstrated that the effect of addition of particles depends on their position in the membrane. Furthermore, the component flux through the membranes as a function of the volume fraction of zeolite is modelled with existing theories describing the permeability of heterogeneous materials. The results show that the apparent permeability of the dispersed phase is lower than the intrinsic permeability of the dispersed phase when the flux through the particle is restricted by the polymer phase. This phenomenon was confirmed by numerical simulation of the transport in the membrane through a plane parallel to the transport direction. The simulations are carried out for an unfilled membrane, a membrane filled with an impermeable particle, a rubber particle and with a particle which shows Langmuir sorption behaviour. The reason for the discrepancy between the apparent permeability and the intrinsic permeability is that the apparent permeability of the zeolite phase is calculated by dividing the flux with the driving force over the entire membrane which is larger than that over the particle. In case of numerical simulation the concentration in every position in the plane is known and therefore the intrinsic permeability of the filler can be calculated on basis of the actual driving force. This treatment results in a permeability which is correct over several orders of magnitude.     

Beer clarification by microfiltration — product quality control and fractionation of particles and macromolecules

Beer clarification by microfiltration demands a finely balanced retention of colloidal particulates (yeast cells, chill haze flocs, etc.) and transmission of soluble macromolecules including carbohydrates, proteins, flavour, and colour compounds which give the “whole some” quality of a beer. The required porous transmission of these macromolecular species led to an unavoidable, complex and dynamic in-pore membrane fouling in terms of fouling constituents, formation, structure and kinetics, which are the main obstacles in obtaining an economically viable flux and consistency in permeate quality.This experimental study was carried out with the aims of understanding the dynamic inter-relation between flux, fouling and system selectivity during a cross-flow beer microfiltration process so that an effective operating strategy for flux optimisation could be formulated in conjunction with the parallel objective of good product (permeate) quality control. Tubular ceramic membranes (Ceramem) with nominal pore diameters of 0.2, 0.5, and 1.3 μm were used. Simultaneous measurement of flux and permeate qualities, such as specific gravity and chill haze level enabled identification of the effect of anti-fouling techniques, such as backflushing on transmission of essential beer components and on the filtered beer quality. The experimental evidence lead to an understanding that the drastic flux enhancement achieved by employing backflushing at reversed membrane morphology was associated with enhanced solute transmission which could, without careful control, upset a balanced transmission of essential beer components and the retention of unwanted “chill haze” components. Further operating parameters and varying system configurations were investigated over their effect on both flux performance and system selectivity. These include membrane pore size, filtration temperature, and the addition of an amorphous silica particles as coagulation agent for hydrophilic proteins.     

Resistance analysis for enhanced wastewater membrane filtration

This study investigated enhancement techniques for synthetic wastewater filtration in a membrane bioreactor (MBR) at mixed liquor suspended solids concentrations (MLSS) of 12–18 g/L. Air sparging (AS), backflushing (BF) and a combined application of both (AS + BF) were applied to increase permeate flux compared to the conventional application (NON). Scanning electron microscope (SEM) measurements of cake thickness served for evaluating cleaning effectiveness and as input data for some of the model calculations. AS + BF showed the lowest overall resistance, and thus the highest permeate yield, for about 2 weeks of observation. The contribution of fouling resistance, cake resistance and membrane resistance to the overall resistance was evaluated, based on experimental data. Air sparging significantly lowered cake thickness and consequently cake resistance. The experimental cake resistance and the model resistances were compared. A model based on the measured cake thickness and literature values for the specific surface area proved most successful. Finally, a relationship between the backflush resistance and the permeate flow resistance was observed.