Cross-flow Velocity Research Articles

Direct numerical simulation and stability analysis of the transitional boundary layer on a marine propeller blade

It is generally assumed that the cross-flow instability exists on propeller blade boundary layers due to their similarity with the rotating disk flow. However, to the best of our knowledge, there is no experimental or numerical evidence that directly shows the existence of the cross-flow instability on propellers. In this paper, we present a direct numerical simulation of the boundary layer flow around a marine propeller blade. For the investigated case, the cross-flow velocity inside the attached laminar boundary layer is smaller than the rotating disk case, but it is large enough to lead to the cross-flow instability. The wave-numbers and wave-direction of the cross-flow vortices observed agree well with the prediction of linear stability theory. Linearized Navier–Stokes simulations show that the boundary layer is absolutely unstable in the radial direction but convectively unstable in the streamwise direction. A significant flow direction deviation between laminar and turbulent regions is also observed on the blade surface. In the separation bubble, there is an unusually large radial velocity (around 50% of the inviscid streamwise velocity). Both the flow direction deviation and the large radial velocity in the separation bubble can be explained by the relationship between Coriolis forces and circumferential velocities.

Selection of membranes and operational parameters aiming for the highest rejection of petrochemical pollutants via membrane distillation

In search of more sustainable approaches for water reclamation from petrochemical industries, Membrane Distillation (MD) has become an alternative to pressure driven membrane technologies. In the present study, the efficiency of a Direct Contact MD (DCMD) configuration with hydrophobic and oleophobic commercially available membranes was examined, aiming to reject the main petrochemical pollutants such as acetate, propionate, and phenol. The influence of the feed and distillate temperatures, cross-flow velocity and pH on the flux and quality of the obtained distillate was further studied in a systematic way. The highest overall rejection efficiency of acetate, propionate, and phenol of approximately > 97% was obtained with the oleophobic membrane at pH ~ 13. The rejection of phenol was mainly influenced by pH, as this impacts its dissociation and thus its volatility. Moreover, to calculate the rejection of the components a novel mathematical method for lab-scale experiments was developed. This method is based on (dynamic) mass balances and was shown to be superior to state-of-the-art methods, which can greatly overestimate rejection. The method is independent of the experimental time and avoids the dilution effect of the initial water in the distillate vessel. The findings underline the importance of correct calculation of rejection in MD, to obtain results valuable for practical application.

Direct membrane filtration of municipal wastewater: Linking periodical physical cleaning with fouling mechanisms

Direct membrane filtration (DMF) is a promising alternative secondary wastewater treatment process in Iceland, where biological treatment is not effective due to low strength wastewater nature and low temperature. This study aims to investigate membrane fouling mechanisms and mitigation approaches during DMF of municipal wastewater using a crossflow membrane filtration system integrated with an optical coherence tomography (OCT) imaging system. During DMF of wastewater, it was observed that intermediate pore blocking was dominant during the early stage of fouling, followed by cake filtration. Multi-filtration cycles were performed under different conditions, and the results revealed that (1) elevating flushing water temperature from 25 to 50 °C greatly reduced the intermediate pore blocking constant accompanied with a decreased physically-irreversible fouling; (2) increasing both filtration and flushing crossflow velocities did not influence the pore blocking constant, but caused a lower cake filtration constant with reducing both physically-reversible and irreversible fouling; (3) extending filtration-flushing duration interval appeared to slightly lower the pore blocking constant; (4) with extending filtration cycles, a shift of reversible fouling to irreversible fouling was noticed and associated with the compression of the tightly attached cake layer that was not readily removed by periodical flushing. A combination of periodical physical flushing with short term chemical-enhanced cleaning was employed and sustainable long-term operation of DMF was achieved. Furthermore, the foulants autopsy indicated that biofouling combined with organic/inorganic fouling influenced the cake fouling development.

Mathematical modeling of CO2 separation using different diameter hollow fiber membranes

This research presents a 2D mass-transfer simulation model using computational fluid dynamics (CFD) for separation of CO2 from a binary gas mixture of CO2/CH4 by means of polytetrafluoroethylene (PTFE) hollow fiber membrane contactor (HFMC). Governing equations with their corresponding boundary conditions are solved using COMSOL Multiphysics and the results are validated against reported experimental data. Convection and diffusion flux vectors and concentration gradient of CO2 species in the radial and axial directions of the HFMC are investigated. This study provides an opportunity to investigate the effects of gas and liquid cross flow velocities on the overall performance of membrane contacting system. The results demonstrate that increasing the liquid phase velocity improves the CO2 absorption performance of the membrane system, while increasing the gas mixture velocity deteriorates the CO2 separation of the system. The impact of hollow fiber geometry on the removal of CO2 is investigated and the results indicate that hollow fibers with smaller inner diameter provides higher effective mass-transfer area and therefore superior CO2 removal performance. Furthermore, the modeling predictions for the CO2 removal are in good agreement with the experimental data under various ratios of gas to liquid velocity. The PTFE hollow fiber membrane contacting system showed a great potential for separation of CO2 from gas mixtures.

Unravelling colloid filter cake motions in membrane cleaning procedures

The filtration performance of soft colloid suspensions suffers from the agglomeration of the colloids on the membrane surface as filter cakes. Backflushing of fluid through the membrane and cross-flow flushing across the membrane are widely used methods to temporally remove the filter cake and restore the flux through the membrane. However, the phenomena occurring during the recovery of the filtration performance are not yet fully described. In this study, we filtrate poly(N-isopropylacrylamide) microgels and analyze the filter cake in terms of its composition and its dynamic mobility during removal using on-line laser scanning confocal microscopy. First, we observe uniform cake build-up that displays highly ordered and amorphous regions in the cake layer. Second, backflushing removes the cake in coherent pieces and their sizes depend on the previous cake build-up. And third, cross-flow flushing along the cake induces a pattern of longitudinal ridges on the cake surface, which depends on the cross-flow velocity and accelerates cake removal. These observations give insight into soft colloid filter cake arrangement and reveal the cake’s unique behaviour exposed to shear-stress.

Prediction of the Limiting Flux and Its Correlation with the Reynolds Number during the Microfiltration of Skim Milk Using an Improved Model.

Limiting flux (JL) determination is a critical issue for membrane processing. This work presents a modified exponential model for JL calculation, based on a previously published version. Our research focused on skim milk microfiltrations. The processing variables studied were the crossflow velocity (CFV), membrane hydraulic diameter (dh), temperature, and concentration factor, totaling 62 experimental runs. Results showed that, by adding a new parameter called minimum transmembrane pressure, the modified model not only improved the fit of the experimental data compared to the former version (R2 > 97.00%), but also revealed the existence of a minimum transmembrane pressure required to obtain flux (J). This result is observed as a small shift to the right on J versus transmembrane pressure curves, and this shift increases with the flow velocity. This fact was reported in other investigations, but so far has gone uninvestigated. The JL predicted values were correlated with the Reynolds number (Re) for each dh tested. Results showed that for a same Re; JL increased as dh decreased; in a wide range of Re within the turbulent regime. Finally, from dimensionless correlations; a unique expression JL = f (Re, dh) was obtained; predicting satisfactorily JL (R2 = 84.11%) for the whole set of experiments

Biofouling Mitigation Approaches during Water Recovery from Fermented Broth via Forward Osmosis.

Forward Osmosis (FO) is a promising technology that can offer sustainable solutions in the biorefinery wastewater and desalination fields, via low energy water recovery. However, microbial biomass and organic matter accumulation on membrane surfaces can hinder the water recovery and potentially lead to total membrane blockage. Biofouling development is a rather complex process and can be affected by several factors such as nutrient availability, chemical composition of the solutions, and hydrodynamic conditions. Therefore, operational parameters like cross-flow velocity and pH of the filtration solution have been proposed as effective biofouling mitigation strategies. Nevertheless, most of the studies have been conducted with the use of rather simple solutions. As a result, biofouling mitigation practices based on such studies might not be as effective when applying complex industrial mixtures. In the present study, the effect of cross-flow velocity, pH, and cell concentration of the feed solution was investigated, with the use of complex solutions during FO separation. Specifically, fermentation effluent and crude glycerol were used as a feed and draw solution, respectively, with the purpose of recirculating water by using FO alone. The effect of the abovementioned parameters on (i) ATP accumulation, (ii) organic foulant deposition, (iii) total water recovery, (iv) reverse glycerol flux, and (v) process butanol rejection has been studied. The main findings of the present study suggest that significant reduction of biofouling can be achieved as a combined effect of high-cross flow velocity and low feed solution pH. Furthermore, cell removal from the feed solution prior filtration may further assist the reduction of membrane blockage. These results may shed light on the challenging, but promising field of FO process dealing with complex industrial solutions.

Formation of electrically conductive hollow fiber membranes via crossflow deposition of carbon nanotubes – Addressing the conductivity/permeability trade-off

Electrically conductive membranes (ECMs) have significant potential for enhanced membrane separations. While most of the existing studies on ECMs have focused on flat sheet membranes, it is the hollow fiber membrane format that is the preferred format for large-scale treatment applications. While ECMs in hollow fiber format are emerging, existing approaches tend towards extremes of membrane conductivity or permeability without considering the trade-off between these metrics. We aim to understand and optimize this trade-off by studying the effect of the normal (pressure, lift) and tangential (shear) forces associated with conductive coating deposition. A suspension of functionalized single walled/double walled carbon nanotubes (SW/DWCNTs) in solution containing sodium dodecyl sulfate and polyvinyl alcohol was pressure deposited in crossflow along the active lumen surface of polyethersulfone microfiltration membranes. The impact of the crossflow deposition parameters affecting surface forces (feed pressure, crossflow velocity) were quantified according to both membrane permeability and conductivity in a 22 design-of-experiments study. Unexpectedly, the highest membrane permeability (~2900 LMH/bar) and conductivity (~670 S/m) and were obtained at the high feed pressure and high crossflow velocity condition. A Robeson-like plot was generated that demonstrates a trade-off between composite membrane permeability and conductivity for 29 CNT-coated hollow fiber membranes fabricated at various SW/DWCNT concentrations, crossflow velocities, and applied pressures. Measured conductivities (up to 2500 S/m) and permeabilities (up to 6000 LMH/bar) for the CNT-coated hollow fiber membranes were several orders of magnitude higher than previous conductive hollow fibers, indicating the substantial benefits of crossflow deposition. The electrically conductive hollow fiber membrane fabrication technique developed herein will be useful in a variety of conductive membrane applications including fouling prevention, enhanced removal of charged particles, and integrated membrane sensing capabilities.

Biofilm removal efficacy using direct electric current in cross-flow ultrafiltration processes for water treatment

Biofouling of membranes in water treatment is considered as one of the major practical problems. A novel and an efficient approach for cleaning biofilm grown on the membrane surface is proposed by applying a direct electric current (124 mA, 90 s) through platinum electrodes inside a cross-flow ultrafiltration channel. Depending on the electrochemical reactions occurring at the electrodes, either chlorine or hydrogen-producing configuration is realized by interchanging the current polarity. Baseline determination of the amount of chlorine generated and change in pH is assessed as a function of current intensity, linear cross-flow velocity, and duration of applied current. The efficiency of the proposed method is determined by investigating electrically treated biofilm through bacterial inactivation using Confocal Laser Scanning Microscopy (CLSM), bacterial cell structure changes through Scanning Electron Microscopy (SEM), and by estimating the amount of biomass removal through Optical Coherence Tomography (OCT). When a chlorine-producing electrode is placed at the inlet of the flow cell, 68% of bacterial inactivation is achieved without any modification of bacterial cell shape. Furthermore, a high and near-complete biomass removal is achieved (99%) after a subsequent forward flush of the electrically treated biofilm. However, placing a hydrogen-producing electrode at the inlet reveals a slightly lower bacterial inactivation (65%) and lower biomass removal (77%). Additional systematic experiments using individually sodium hydroxide (NaOH), sodium hypochlorite (NaOCl), or gas microbubbles enabled to elucidate the cause of biofilm removal, synergic effect of caustic agent NaOH and microbubbles.

PDMS/ceramic composite membrane synthesis and evaluation of ciprofloxacin removal efficiency

The present study employs an unexplored, one-step route for remediation of ciprofloxacin, an emerging contaminant, using hydrophobically modified ceramic membranes. Hydrophobic interaction between the membrane and the target contaminant, i.e., ciprofloxacin, is the governing factor responsible for its removal. The hydrophilic surface of hollow, single channel, macroporous clay-alumina membranes was made hydrophobic using cross-linked polydimethylsiloxane. The influencing parameters—concentration of polymer, cross-linking agent, catalyst and coating time—were optimized by Taguchi analysis to yield a membrane with enhanced ciprofloxacin rejection and high permeate flux. The synthesized membrane was characterized for its contact angle, clean water permeability, degree of swelling, degree of cross-linking, X-Ray diffraction, atomic fluorescence microscopy, field emission scanning electron microscopy. Effect of various operating parameters—cross flow velocity, transmembrane pressure, filtration time, solution pH—was investigated upon removal of ciprofloxacin in cross flow membrane filtration. Maximum rejection of 99.3% was obtained by the hydrophobic membrane having contact angle of 138.5° for 5 mg/L feed solution. The stability of the membrane was judged in terms of change in ciprofloxacin rejection upon filtration for five consecutive cycles, each cycle being 180 min. The developed PDMS/ceramic composite membranes could have great prospect for long-term application in removal of emerging contaminants from water.

Effect of the operating conditions on a nanofiltration process to separate low-molecular-weight phenolic compounds from the sugars present in olive mill wastewaters

The efficiency of nanofiltration to purify the tyrosol present in the olive mill wastewaters (OMWWs) has been studied. The similar molecular weight of tyrosol and the sucrose existing in this kind of by-products restricts the discrimination between both molecules through a membrane process, but the interest of phenolic compounds to be applied in cosmetics and pharmacology greatly motivates its recovery at the highest purity possible. Thus, two different simulated OMWWs composed of tyrosol and mixtures of tyrosol and sucrose, respectively, were nanofiltered using the NF270 membrane. Three transmembrane pressures (TMPs) and three cross-flow velocities were tested. The optimum results were obtained at 0.5 m⋅s−1 and 15 bar. The rejections of the chemical oxygen demand (COD) were above 78 %, whereas phenolic compounds were barely retained. This indicates that the sugar was accurately separated from tyrosol, which was recovered in the permeate stream at a high purity.

Pilot‐scale microalgae harvesting with ceramic microfiltration modules: evaluating the effect of operational parameters and membrane configuration on filtration performance and membrane fouling

AbstractBACKGROUNDMembrane fouling is one of the major drawbacks in microalgae harvesting that prevents the widespread application of membrane filtration. This study investigates the effects of operating parameters on the filtration performance and the fouling phenomenon of a pilot‐scale microfiltration (MF) system equipped with different ceramic membranes for Chlorella vulgaris culture harvesting.RESULTSA higher permeate flux was obtained with increasing the transmembrane pressure, resulting in lower irreversible and reversible filtration resistances. On the other hand, at a higher crossflow velocity above 4 m s−1, a high amount of extracellular organic matter (EOM) released by microalgal cells led to a higher irreversible resistance which, in turn, caused severe membrane fouling and a lower filtration flux. Furthermore, the membrane configuration and pore size have also significantly affected the permeate flux. Compared to the tubular membranes, and regardless of pore sizes, the multichannel MF membrane (pore size: 0.2 μm) exhibited a higher permeate flux that resulted in lower membrane resistances due to the lower release of EOM by microalgae cells. Moreover, the tubular membrane (pore size: 0.8 μm) showed a higher flux reduction with higher pore size because of the increase in the irreversible resistance with a high amount of EOM release.CONCLUSIONThe results revealed that the amount of EOM significantly associated with filtration conditions plays an important role in membrane fouling control. Considering the membrane configurations, the multichannel ceramic MF membrane (pore size: 0.2 μm) demonstrated a higher permeate flux due to its lower fouling property. © 2020 Society of Chemical Industry (SCI)

Characterization of particle deposition during crossflow filtration as influenced by permeate flux and crossflow velocity using a microfluidic filtration system

Particle deposition during crossflow filtration is significantly influenced by the operating conditions, in particular the permeate flux and crossflow velocity. However, there is a lack of detailed knowledge about how deposit layer structures and distributions depend on operating parameters. This study uses a microfluidic visualisation filtration system to examine the influence of operating conditions on the deposition process during crossflow ultrafiltration from a microscopic perspective. Increasing the permeate flux caused an increasing amount of deposition and a thicker deposit layer. Higher crossflow velocities reduced the extent of deposition. The degree of deposition varied over a range of operating conditions due to the altered hydrodynamic forces exerted on the particles, which can be examined by the deposition probability according to an existing model. Building on this, an empirical correlation between the deposition probability and volume of deposition as a function of filtration time was developed, which gave good agreement with experimental results. The effect of solution conditions was also involved in this correlation as a interaction energies. This could be useful for predicting the dynamic deposition process during crossflow filtration over a range of operating and solution conditions.

An efficient removal of crystal violet from aqueous solution using rhamnolipid micellar solubilization followed by ultrafiltration and modeling of flux decline

Micellar‐enhanced ultrafiltration (MEUF) using rhamnolipid (RHL) biosurfactant has been shown to be an effective and promising method for removal the hazardous heavy metal ions and organic pollutants from aqueous stream. However, the potential application of rhamnolipid was not explored much in the treatment of dye wastewater. The aim of this study was to evaluate the performance of the rhamnolipid based MEUF process for the removal of crystal violet (CV) from the aqueous solution. MEUF experiments were performed in both dead-end stirred cell and cross-flow cell. Response surface methodology was used to optimize the MEUF feed conditions. Under optimal feed conditions (RHL: 175 × 10-3 kg m-3; CV: 32 × 10-3 kg m-3; NaCl: 15 kg m-3 and pH: 8.3), the maximum CV rejection efficiency was found to be 99.5%. This study also investigated the effects of the transmembrane pressure difference, cross-flow velocity and feed temperature, on the CV rejection efficiency and profiles of permeate flux and fouling resistance. A model based on resistance-in-series theory coupled with Hermia’s cake filtration model adapted to cross-flow was developed together with the expression of transition time of fouling mechanism, to analyze the flux decline behavior during operation. The experimental flux decline data were in good agreement with the model predictions. The proposed model was also found to be applicable to sodium dodecyl sulfate (SDS) based MEUF process for removal of CV. For similar CV rejection efficiency, the present study showed a 31.5% lower operating cost compared to SDS based system.

Build-up and relaxation of membrane fouling deposits produced during crossflow ultrafiltration of casein micelle dispersions at 12 °C and 42 °C probed by in situ SAXS

Skim milk filtration is performed either at low or high temperature. However, there is still a lack of knowledge concerning the influence of temperature on membrane fouling. We used in situ small-angle X-ray scattering (SAXS) to study external membrane fouling during crossflow ultrafiltration of milk protein (casein micelle) dispersions at 12 °C and 42 °C. Casein micelle concentration distribution was measured in the concentration polarization layer and the deposit in a three-step filtration experiment that consisted of (i) fouling development step (with applied pressure and average cross-flow velocity fixed at 110 kPa and 3.1 cm s−1, respectively), (ii) pressure relaxation step (with pressure reduced to 10 kPa), and (iii) deposit erosion step (with crossflow velocity increased to 15.6 cm s−1).Despite a higher average filtrate flux obtained at 42 °C, filtration at 42 °C resulted in the formation of a thicker and more concentrated deposit than filtration at 12 °C. At both temperatures studied, subsequent pressure relaxation resulted in deposit swelling, which was more intense in the less-compressed external part of the deposit. Deposit swelling rate was significantly higher at 42 °C than at 12 °C. The swelled deposit obtained at 42 °C was more eroded by the crossflow compared to the poorly swelled deposit obtained at 12°. This suggests that deposit formation and compression were more reversible at 42 °C than at 12 °C.

Direct numerical simulation of transitional boundary layers on a horizontal axis wind turbine blade

In boundary layer flow around rotating machines, a radial (or cross-flow) velocity exists due to Coriolis and centrifugal forces. This velocity component can be of great importance for laminar-turbulent transition. A series of direct numerical simulations (DNS) are performed to study the boundary layer flow transition on a rotating Horizontal Axis Wind Turbine blade. To quantify the effect of blade rotation, results are compared with that from airfoil DNS, where the section is taken from 3D blades and does not rotate. It is shown that the rotation gives rise to a small radial velocity and slightly modifies the shape of unstable waves. However, the transition location and mechanism of 3D blade boundary layer flow resemble 2D flow for the investigated case.

Temporal Variations of a Natural Hydrocarbon Seep Using a Deep-Sea Camera System

AbstractTwo video time-lapse cameras (VTLCs) were deployed by a remotely operated underwater vehicle (ROV) to observe the temporal and spatial variability of a natural hydrocarbon seep at 1180 m depth in the Green Canyon 600 lease block, Gulf of Mexico. The VTLCs were positioned approximately 60 and 90 cm away from the vent, each recording 15 s video bursts at 30 frames per second, illuminated by a 2000 lumen (lm) LED lamp. One camera functioned for 2 weeks; the second camera recorded 568 video bursts at 6 h intervals from 3 September 2017 to 2 February 2018 (153 days). Over the campaign period, seepage from three vents along a 10 cm cluster shifted toward a new fault line with up to nine intermittent individual vents shifting along 20 cm. We developed a semisupervised algorithm using Mathematica and ImageJ routines to resolve the rise velocity and size of individual bubbles. The algorithm was applied to the last 30 frames of each video burst. Bubble characteristics were also analyzed in the videos recorded by the ROV camera. Processing VTLC records yielded a bubble size distribution comparable (5% deviation) to the ROV camera, while the rise velocities were found to be 12% smaller than the ROV data. Hydrocarbon flux estimated from VTLC data was also compared favorably (2% difference) with synoptic physical collections of hydrocarbons into an ROV-held funnel. The long-term measurements indicate that bubble rise velocity was weakly correlated to the discharge rate as well as to the cross-flow velocity.

Where is the best site for loading nanoparticles in a membrane? To achieve a high flux and cephalexin separation simultaneously

In this research, with the aim of achieving a high flux and rejection of cephalexin antibiotic simultaneously, separate modifications were performed on the membrane top layer (selective layer) and substrate layer (support layer). Fe3O4 (Fe) and ZrO2 (Zr) nanoparticles were synthesized for modifying the PAN support layer, while Fe3O4/ZrO2 (FZ) nanoparticles were synthesized for modifying the PA top layer. In this regard, five types of nanocomposite membranes were synthesized: In order to regulate the porosity of the support layer, PAFe2@PAN and PAZr2@PAN membranes were synthesized which showed more pores number compared to PAPAN membrane. In order to enhance the surface hydrophilicity, FZ2@PAPAN membrane was synthesized, which showed around 16% lower contact angle and 58% greater roughness compared to PAPAN membrane. In order to compensate for the roughness developed on the membrane surface, concurrent modifications were performed on the top and support layers; the FZ2@PAFe2@PAN and FZ2@PAZr2@PAN membranes were synthesized, which separated cephalexin by 91% and 95.8%. Comparison of the performance of the synthesized membrane with that of commercial FILMTEC (NF270-2450) membrane showed that the cephalexin rejection (95 ± 1) and flux recovery (99 ± 1) of FZ2@PAZr2@PAN are almost the same as cephalexin rejection (98 ± 1) and flux recovery (96 ± 1) of commercial membrane. From permeation point of view, FZ2@PAZr2@PAN membrane at transmembrane pressure of 4 bar and cross flow velocity of 0.5 m/s had a water flux of 49 L/m2.h and cephalexin flux of 38 L/m2.h, while NF270-2450 at the same condition had a water flux of 42 L/m2.h and cephalexin flux of 25 L/m2.h.

Simulation study on cutting transport in a horizontal well with hydraulic pulsed jet technology

The hydraulic pulse jet is a high-efficiency drilling technology to improve the efficiency of bottom-hole cuttings removal by enhancing the down-hole flow of cuttings. In this paper, an Eulerian-Eulerian two-fluid model on the basis of the kinetic theory of granular flow is applied to simulate the flow behavior of cuttings and drilling fluid in a horizontal well with hydraulic pulsed jet technology. The influences of pulse jet velocity, amplitude, and frequency on hydrodynamic characteristics are analyzed. Increasing the pulse jet velocity can generate higher pressure and crossflow velocity in the bottom-hole area, quickly carry the cuttings out of the bottom-hole area, and form the cuttings bed near the outlet. The improvement of the amplitude of the pulsed jet can form a remarkable and deep jet penetration in the bottom-hole area, which has a positive effect on the cuttings migration at the bottom-hole area and destruction of the cuttings bed in the annulus. For a given average jet velocity in this paper, the optimal value of amplitude is a half of the average jet velocity. The pulse jet frequency has little effect on cuttings migration in the bottom-hole area, while a higher pulse jet frequency can effectively reduce the height of the cuttings bed in the annulus.

Critical flux of colloidal foulant in microfiltration: Effect of organic solvent

As membrane technology becomes more popular for separation and purification in the aqueous phase, the advantages have increasingly attracted attention for organic solvent filtration applications. However, although the knowledge base on the inevitable membrane fouling in water is rich by now, the understanding for organic solvents is limited. Accordingly, this study was targeted at providing insights on the fouling behavior of the same colloidal foulant (namely, silica) and microfiltration membrane (namely, Anopore) in four different solvents (namely, DI water, ethanol, hexane and formamide). The direct observation through the membrane (DOTM) technique was used to characterize critical flux (Jcrit), which affirmed the distinctly different values in the four solvents at various Reynolds numbers. The shear-induced diffusion model predicted well for water and formamide only, indicating the model has to be enhanced to account for solvent effects. To quantify the interfacial interactions, the Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended (XDLVO) models were used. Both models agreed in that silica-membrane and silica-silica interactions were the most attractive in hexane, which underlies the immediate membrane fouling and extensive clumping, respectively. For the other three solvents, the relative DLVO interaction energies agreed with the relative Jcrit values at fixed Re, indicating the greater relevance of the DLVO (rather than XDLVO) model and Re (rather than crossflow velocity). Because the greatest silica-silica repulsion in ethanol was mis-predicted by both DLVO and XDLVO models, the solvation film effect was additionally harnessed to provide an explanation. The understanding gained from this study on membrane fouling in organic solvents are expected to be beneficial in the design and operation of such emerging membrane filtration systems.