High Cross-flow Velocity Research Articles

Stratified structure of membrane clogs observed in full scale cross-flow tubular ultrafiltration (UF) system: Fouling characterization and a proposed two-stage formation mechanism

Membrane fouling remains a significant challenge in wastewater treatment, hindering both efficiency and lifespan. This study reports a distinct phenomenon of stratified membrane clogging observed in a full-scale cross-flow tubular ultrafiltration (UF) system treating sludge anaerobic digestion (AD) reject water. The distinct stratified structure, comprising inner and outer layers within the cake layer, has not been previously described. This research involved characterizing the filtration performance, analyzing membrane clog composition, and proposing a two-stage formation mechanism for the stratified clogs. It was revealed that higher inorganic and lower organic content in the outer layer compared to the inner layer. Acid and alkali treatments demonstrated the effectiveness of combined cleaning strategies. A mathematical model was developed to determine the critical conditions for stratified clog formation, influenced by membrane flux and cross-flow velocity (CFV). It is proposed that outer layer forms through long-term selective deposition, while the inner layer results from short-term dewatering within limited tubular space. High CFV (>2.5 m/s) prevents inner layer formation. Critical conditions for stratification occur at a flux of 18 L/m2/h with a CFV of 0.1 m/s or 65 L/m2/h with a CFV of 0.35 m/s. This study contributes a novel understanding of stratified membrane clogging, proposing a two-stage formation mechanism and identifying critical conditions, which provides insights for effective fouling control strategies and maintenance of operational efficiency for membrane systems.

Effect of wall-impingement distance on fuel adhesion characteristics of split injection spray under cross-flow condition

In direct-injection spark ignition (DISI) engines, high-pressure fuel injectors introduce fuel into the cylinder, causing the spray to come into contact with and adhere to the cylinder wall; this is known as fuel adhesion. Over time, fuel adhesion contributes to carbon deposition on the cylinder wall, which affects its thermal conductivity and consequently diminishes engine combustion efficiency. This study investigates the influence of different impingement distances and cross-flow velocities on fuel adhesion under the triple injection strategy. The fuel adhesion propagation and side-view spray are measured using refractive index matching (RIM) and Mie scattering, respectively. The findings demonstrate that high cross-flow velocity promotes the fuel adhesion shape to be elongated strips. In the early stage, the growth rate of the fuel adhesion area increases with an increase in cross-flow velocity. In the later stage, the decrease rate in the fuel adhesion area initially increases with an increase in the cross-flow velocity; however, when the critical velocity threshold (20 m/s) is exceeded, the decrease rate in the fuel adhesion area tends to stabilize. The average fuel adhesion thickness then accordingly decreases with the increase in the cross-flow velocity and impingement distance in the later stage. In addition, the cross-flow promotes the volatilization of spray and fuel adhesion, thereby decreasing the fuel adhesion mass over time. In the context of carbon neutrality, this study underscores the importance of optimizing fuel injection conditions to reduce emissions and fuel consumption.

Crossflow-induced membrane vibration and its effects on fouling and scaling in direct contact membrane distillation

Membrane distillation (MD) is driven by vapor pressures rather than hydraulic pressures. Consequently, certain regions of the membrane may be in an unrestrained state, which potentially lead to membrane vibrations due to the turbulence caused by crossflow. However, this crossflow-induced membrane vibration has been largely overlooked in the previous studies. Here, we made the first attempt to investigate the crossflow-induced membrane vibration through the observation using optical coherence tomography (OCT), and explore the effect of this vibration on membrane fouling and scaling. The experimental results revealed that the vibration exhibited a self-excited vibration mode, with amplitudes reaching tens of micrometers. Moreover, the amplitude could be readily adjusted through changing crossflow velocity, transmembrane pressure, and the mesh size of spacer. For humic acid (HA) fouling, the membrane vibration could effectively reduce the thickness of HA cake layer, thereby stabilizing the specific flux at a constant level of 0.85, in contrast to the continuous decline of the flux under non-vibration condition. For gypsum scaling, the membrane vibration allowed the MD under low crossflow velocity (5 cm/s) to achieve comparable scaling occurrence concentration and scaling layer thickness as those under high crossflow velocity (10 cm/s). Our findings emphasized the potential of crossflow-induced membrane vibration as an effective strategy to mitigate membrane fouling and scaling in MD and other non-pressure driven membrane systems.

On the stabilization mechanisms of a diffusion edge flame in a cross-flow configuration

This work focuses on the stabilization and combustion efficiency of a gaseous diffusion flame at the lip of a hole where pure methane is injected into a crossflow of air. This configuration mimics the flame base of many flares, used to burn excess gas in the oil and gas industry. In a flare, the flame base, near the lips of the fuel jet duct, is a critical zone where the flame attaches or not. When the cross flow air speed is too high or the torch lips too cold, the flame cannot attach to the lips so that methane can leak below the lifted flame front near the injector lip and avoid combustion totally. This cannot be accepted for climate change since unburnt fuel like methane has a radiation forcing larger than 20 times the value reached for CO2. Combustion efficiencies close to unity are expected for the flares of the future. Two main parameters control the anchoring of the diffusion flame on a thin lip: the air cross-flow velocity uair and the temperature of the lip or equivalently its cooling, measured here by a thermal resistance Rth. Dimensional analysis is used to construct two reduced parameters controlling stabilization and simulations are performed to populate this diagram. The resulting diagram shows how uair and Rth control flame anchoring. Large values of Rth (adiabatic lips) lead to high lips temperatures and to flames which can resist high air cross-flow velocities. For low values of Rth (cold lips), the flame lifts off for smaller values of uair. The simulations also reveal that triple flames are the flame elements controlling the flame anchoring and how these triple flames interact with the lips forming a flame topology called RS (Rim Stabilized) edge flame.

A brief review on systematic approach to polymer selection for development of capillary/hollow-fibre membrane for practical applications

Polymeric membranes are widely used for treatment of lean stream in chemical process industries. These membranes are used in different configurations such as tubular, plate & frame, disc-tube, spiral and capillary/hollow-fibre. Membrane modules with capillary/hollow-fibre configuration appears promising in ultrafiltration applications due to its relatively higher packing density, ease of backwashing, ease of cleaning and lower pressure drops since it can be operated at laminar flow regime with high cross-flow velocity. Hence, efforts are being made by researchers to make capillary/hollow-fibre membrane modules from various polymers, ranging from most hydrophilic polyacrylonitrile (PAN) to super-hydrophobic polypropylene (PP) and polytetrafluorethylene (PTFE). Here, we discuss the qualifying properties of the polymeric materials suitable to spin into capillary/hollow-fibre ultrafiltration membranes. Selection of polymers for making fibres requires in-depth knowledge of properties of base polymer and its processability/fabricability. The important properties to be considered for making capillary/hollow-fibre membrane are intrinsic structural properties of the base polymer like degree of crystallinity, tensile strength, tensile modulus, etc. The functional properties such as permeability, hydrophilicity/hydrophobicity etc. also plays role in selecting polymer for a given application. The polymer should also have appreciable dissolution in available solvents or should have degradation temperature higher than melting point so that it can be processed through appropriate membrane preparation process.

Experimental Study on the Adhesive Fuel Features of Inclined Wall-Impinging Spray at Various Injection Pressure Levels in a Cross-Flow Field

The wall-impingement phenomenon significantly impacts mixture formation, combustible performance, and pollutant release in DISI engines. However, there is insufficient knowledge regarding the behavior of fuel adhesion. Thus, here, we examine adhesive fuel features at various injection pressure levels (5 and 10 MPa) in a cross-flow field (0 to 50 m/s). The RIM optical method was employed to track the expansion and distribution of fuel adhesion. As a result, adhesive fuel features such as area, mass, thickness, and lifetime were assessed. Postprocessing image analysis reveals that fuel adhesion was consistently thinner at the edge region. With increased injection pressure, the cross flow led to a rise in the fuel-adhesion area and mass; however, small changes in pressure did not affect adhesive thickness. Adhesive thickness significantly decreased in the cross flow, indicating enhanced evaporation potential. Furthermore, lifetime prediction was conducted to quantitatively evaluate the impact of cross flow and injection pressure upon fuel adhesion, which could be calculated by examining the decreasing trend in adhesive area. Results show that the lifetime was dramatically reduced with higher cross-flow velocity, and slightly decreased with lower injection pressure. Under injection pressure of 10 MPa, the adhesive lifetime in the cross-flow field of 50 m/s was reduced by 77.5% compared with the static flow field (0 m/s). The experimental results provide corresponding guidance for low-carbon fuel utilization and emission reduction in DISI engines.

Membrane BioReactor (MBR) Activated Sludge Surrogate Alternatives Carboxymethyl Cellulose and Xanthan Gum: A Statistical Analysis and Review.

Membrane Bioreactors (MBR) combine traditional biological treatments such as Activated Sludge (AS) with a membrane-based filtration process to extract suspended and organic solids. MBR operation involves high shear rates near the membrane surface due to the high crossflow velocity, which complicates any simulation process from a hydrodynamic point of view. In this regard, the viscosity as a function of total suspended solids (TSS) plays an essential role in characterizing and modeling the behavior of activated sludge (AS). However, AS has an intransparency property that prevents experimental measurements (i.e., velocity profiles) commonly associated with optical techniques from being peformed. In light of this limitation, two polymeric compounds, carboxymethyl cellulose (CMC) and xanthan gum (XG), are considered here in order to explore the possibility of mimicking the rheological behavior of AS. These compounds are commonly used in the food industry as food thickeners, and their rheological behavior is supposedly well defined in the literature. In this work, we reviewed the viscosity behavior of these compounds through their reported flow behavior and consistency indexes. It was found that the rheological properties of these two polymers differ depending on the chemical manufacturer, rheometers, and measurement protocols involved. Different curves (shear rate vs. viscosity/shear stress) are obtained, as each device and procedure seem to modify the polymer structure. Therefore, a statistical analysis was performed based on the flow and consistency indexes using different concentrations and temperatures reported in experimental data. Several insights regarding CMC, XG, and AS performance were obtained, including a better relationship with concentration than with temperature or certain exponential-based performances, which can support further MBR design and operational decision-making.

Size-dependent transport and fouling formation of organic matters in a pilot-scale PFFO–RO hybrid system for real wastewater treatment

The forward osmosis (FO)-reverse osmosis (RO) hybrid system has been gaining considerable interest because of its wide applicability and feasibility in both wastewater treatment and seawater desalination. However, as the FO process treats wastewater in the hybrid system, fouling of the FO membrane is a crucial factor in the FO-RO hybrid system, which determines the performance of the entire hybrid system. A few studies have reported that the configuration of the PFFO module has potential advantages, such as a higher cross flow velocity, higher fouling resistance, and lower pressure drop compared to conventional spiral wound-type modules. In this study, the first to investigate the performance of the PFFO-RO hybrid system in long-term operation, the performance and fouling characteristics of the PFFO-RO hybrid system in real wastewater treatment, and seawater desalination over two months were investigated. At the end of the process, organic compounds, mainly humic substances such as humic and fulvic acid, accounted for 89.32% and 91.46% of the foulant on the PFFO and RO membranes, respectively. Moreover, organic compounds with molecular weights over 500 Da were removed by PFFO, and those over 60 Da were removed by RO. This study can provide the information about maintenance of the PFFO-RO hybrid process in wastewater treatment.

The influence of intake flow and coolant temperature on gasoline spray morphology during early-injection DISI engine operation

Multi-hole gasoline injectors operating at conditions spanning throttled early-intake stroke operation produce spray plumes that either remained separated or merge and collapse due to flash boiling. Flash boiling occurs due to the sudden expansion of gas bubbles in the liquid fuel at high fuel temperature and low ambient pressure. This study records high-speed images of spray-morphology changes due to in-cylinder flow, thereby revealing operating conditions that do and do not affect the self-induced morphology observed in quiescent vessels. Specifically, in a central-injection, four-valve, high-tumble engine, where the thermodynamic state and in-cylinder cross flow are dynamic. Motivated by cold start and hot restart operation, the fuel pressure, coolant temperature, in-cylinder air pressure, and engine rpm were systematically varied over relevant operating conditions, which bracketed the range from non- to flash-boiling sprays. The results reveal the operating conditions at which the in-cylinder cross flow disrupts the spray morphology as well as the extent of the disruption. At 650 rpm, the spray morphology was similar to that observed in quiescent vessels at nominally equivalent fuel temperature and in-cylinder pressure, indicating that the spray’s self-induced entrainment flow dominated the in-cylinder flow. However, for fuel temperature and ambient pressure near the transition between non- and flash-boiling, the intake cross flow at higher engine speed (1950 rpm) significantly disrupted the spray morphology. The high cross-flow velocity appears to induce plume merging and collapse, whereas none was evident at low rpm (650 rpm). This study led to the postulate that the spray merging and collapse are governed by the rate of atomization near the nozzle exit, presumed to be controlled by either or both aerodynamic atomization and flash-boiling intensity. It would then follow that spray modeling in CFD requires atomization models that blend the effects of both physical processes.

Impact of Particle Shape and Surface Group on Membrane Fouling.

Membrane fouling remains one of the most critical drawbacks in membrane filtration processes. Although the effect of various operating parameters—such as flow velocity, concentration, and foulant size—are well-studied, the impact of particle shape is not well understood. To bridge this gap, this study investigated the effect of polystyrene particle sphericity (sphere, peanut and pear) on external membrane fouling, along with the effect of particle charge (unmodified, carboxylated, and aminated). The results indicate that the non-spherical particles produce higher critical fluxes than the spherical particles (i.e., respectively 24% and 13% higher for peanut and pear), which is caused by the looser packing in the cake due to the varied particle orientations. Although higher crossflow velocities diminished the differences in the critical flux values among the particles of different surface charges, the differences among the particle shapes remained distinct. In dead-end filtration, non-spherical particles also produced lower flux declines. The shear-induced diffusion model predicts all five particle types well. The Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO (XDLVO) models were used to quantify the interaction energies, and the latter agreed with the relative critical flux trends of all of the PS particles. As for the flux decline trends, both the DLVO and XDLVO results are in good agreement.

Industrial application of ceramic ultrafiltration membrane in cold-rolling emulsion wastewater treatment

A commercial ZrO2/α-Al2O3 ceramic ultrafiltration (UF) membrane system with a capacity of 24 m3 h−1 and cut off of 20 kDa was used to treat cold-rolling emulsion wastewater (CREW) from an iron and steel industry in Central China. The ceramic membrane exhibited high removal efficiency for oil and chemical oxygen demand (COD), with an average removal rate of 99.88% and 98.41%, respectively, which were better than other reported state-of-the-art ceramic membranes. In order to solve the problems of membrane fouling and low permeation flux in the treatment of CREW, the control strategies were proposed. The results showed that the membrane fouling can be mitigated by controlling the feed concentration in circulation tank, increasing the cross-flow velocity, and proper chemical cleaning. The membrane fouling mechanism revealed that the experimental data were well described by the combined model of cake filtration and pore blocking at low cross-flow velocity (4.0–4.3 m s−1). At high cross-flow velocity (5.2 m s−1), the pore blocking models fit the data well. After optimizing the process operation parameters, the continuous operation time of the UF membrane was prolonged by 1.5–2 times, the cleaning times were reduced by 40%. The total operating cost of this system is about $3.01/m3, which is cheaper than the conventional treatment process. In addition, the service life of the ceramic membrane tube was greatly extended to more than 8 years, thus saving the cost of replacement. These results indicate that the process is an economical and efficient method in the treatment of CREW.

Self-cleaning performance of in-situ ultrasound generated by quartz-based piezoelectric membrane

• A self-cleaning membrane coupled with in-situ ultrasound function was developed. • The membrane is composed of an anti-fouling layer and a piezoelectric support. • The internal ultrasound induced shear force to slow down the particle settling. • Foulants can be removed by in-situ ultrasound at relatively low power intensity. Membrane separation technology is playing a prominent role in treatment of suspension wastewater, but its efficiency is limited by membrane fouling. Here, a self-cleaning piezoelectric ceramic membrane (PCM) coupled with in-situ ultrasound functionality was developed. The PCM consists of a porous piezoelectric quartz (ɑ-SiO 2 ) support and a ZrO 2 microfiltration (MF) layer. The MF layer serves as a separation membrane, and it possesses a smooth and negatively charged surface, which is beneficial to weaken the interaction between membrane and foulants. The ɑ-SiO 2 support provides mechanical strength to the membrane and can generate internal ultrasound to mitigate fouling. In MF process, particulate foulants are inevitably deposited on the membrane surface, forming a cake whose permeance resistance is 3.9 times of pristine membrane. With the assistance of internal ultrasound, the thickness of cake decreased from 64.2 to 48.4 μm, and porosity increased from 39.5% to 42.5% due to the formation of shear force. Higher cross-flow velocity and more vigorous ultrasound intensity are helpful to strengthen turbulence, which can improve stationary membrane flux by 41.7%. The fouling removal efficiency of internal ultrasound is superior to the outer one, and the economic evaluation indicates that the cost of water production can be saved in the presence of ultrasound.

Effects of alternating elliptical chamber on jet impingement heat transfer in vane leading edge under different cross-flow conditions

ABSTRACTThis paper proposes an alternating elliptical impingement chamber in the leading edge of a gas turbine to restrain the cross flow and enhance the heat transfer, and investigates the detailed flow and heat transfer characteristics. The chamber consists of straight sections and transition sections. Numerical simulations are performed by solving the three-dimensional (3D) steady Reynolds-Averaged Navier–Stokes (RANS) equations with the Shear Stress Transport (SST) k– $\omega$ turbulence model. The influences of alternating the cross section on the impingement flow and heat transfer of the chamber are studied by comparison with a smooth semi-elliptical impingement chamber at a cross-flow Velocity Ratio (VR) of 0.2 and Temperature Ratio (TR) of 1.00 in the primary study. Then, the effects of the cross-flow VR and TR are further investigated. The results reveal that, in the semi-elliptical impingement chamber, the impingement jet is deflected by the cross flow and the heat transfer performance is degraded. However, in the alternating elliptical chamber, the cross flow is transformed to a pair of longitudinal vortices, and the flow direction at the centre of the cross section is parallel to the impingement jet, thus improving the jet penetration ability and enhancing the impingement heat transfer. In addition, the heat transfer in the semi-elliptical chamber degrades rapidly away from the stagnation region, while the longitudinal vortices enhance the heat transfer further, making the heat transfer coefficient distribution more uniform. The Nusselt number decreases with increase of VR and TR for both the semi-elliptical chamber and the alternating elliptical chamber. The alternating elliptical chamber enhances the heat transfer and moves the stagnation point up for all VR and TR, and the heat transfer enhancement is more obvious at high cross-flow velocity ratio.

Optimal cleaning strategy to alleviate fouling in membrane distillation process to treat anaerobic digestate

This paper deals with the membrane fouling issue in the Direct Contact Membrane Distillation (DCMD) process treating a wasted sludge from an anaerobic digestion process. The main objective is to define an optimal cleaning strategy to alleviate fouling. Using a lab scale DCMD process, a cleaning strategy based on DI water flushing followed by 0.2% sodium hypochlorite (NaOCl) and 3% citric acid (C6H8O7) cleaning was tested with different cleaning frequencies and various chemical cleaning durations at different cross-flow velocities. To avoid severe fouling, the optimal cross-flow velocity was found at 0.18 m/s (0.8 L/min). Moreover, even if higher cross-flow velocity allows higher flux, it could increase fouling risks. For a better membrane regeneration and process productivity, a cleaning of 60 min duration for each chemical cleaning applied every two days was defined as the optimal cleaning strategy. Such conditions allowed the preservation of 75.5% of the initial flux after 96 h of operation. Furthermore, the effect on membrane flux regeneration of DI water flushing, sodium hypochlorite, and citric acid cleaning registered were, 31.52%, 11.95% and 20.65%, respectively. This study revealed that in the MD process treating real wastewater both external and internal fouling are responsible of permeate flux decline due to the accumulation of organic and inorganic matter on the membrane surface as well as within the pores.

Behavior of bubble plume in shear-thinning crossflowing liquids

The behavior of bubble plume in crossflows was investigated by experimental and theoretical methods. The high-speed photography method was applied to analyze the visualization of bubble plume in tap water and carboxyl methyl cellulose (CMC) aqueous solution. The influence of crossflow velocity, superficial gas velocity and rheology of liquid phase on the inclination trajectory of bubble plume was studied. The results indicated that the trajectories of bubble plumes in tap water and CMC aqueous solutions approximated linearly, while the inclination characteristic of the bubble plume in CMC aqueous solutions were significantly different from those in tap water due to the effect of the rheological properties. The inclination angle of bubble plume in CMC aqueous solution was one-third more than that in tap water at high crossflow velocity. A correlation was developed via dimensional analysis to predict the inclination trajectories of bubble plumes in Newtonian and shear-thinning non-Newtonian fluid system. Considering the non-Newtonian rheology, the modified integral model of bubble plume was applied for theoretical calculation of centroids of bubble plume in crossflow.

A cascade microfiltration and reverse osmosis approach for energy efficient concentration of skim milk

Abstract To improve the efficiency of water removal from skim milk, a cascade membrane process of microfiltration and reverse osmosis (RO) was developed whereby skim milk was concentrated to 18% dry matter (DM) by RO at either 15 or 50 °C. The average flux of the RO process at 50 °C was 89% higher than that observed at 15 °C, linked to altered membrane surface fouling behaviour due to lower viscosity, higher cross-flow velocity and increased diffusivity of the solvent phase. In corollary, a ~57% energy reduction per unit volume of water removed was observed when the RO process was operated at 50 °C. Evaluation of the physicochemical properties of control (9% DM content skim milk) and RO retentates post-heating (at 80, 90 and120 °C) and post-evaporation (to 42% DM) demonstrated a clear relationship between heating at elevated DM contents and solution viscosity, an effect that was compounded at higher heating temperatures.

Harvesting of Microalgae Biomass Using Ceramic Microfiltration at High Cross-Flow Velocity.

This study aimed to investigate the harvesting of microalgae by microfiltration (MF) on a ceramic membrane at relatively high cross-flow velocity (CFV) of interest for commercial processes. Pilot-scale harvesting was conducted with algal suspensions (Chlorella vulgaris and Tisochrysis lutea (T-Iso)) and algal supernatants (Porphyridium cruentum) to assess the effect of feedstock characteristics and understand flux decline mechanisms. In total recycle mode (C. vulgaris, 1g/L), high steady-state permeation flux around 200L/m2/h was achieved. Total filtration resistance was mainly due to cake resistance (Rc, 57%) and pore adsorption and blocking (Ra, 40%). The process hydrodynamic conditions seemed to have relatively little effect on Chlorella cell integrity. In concentration mode, average permeate flux decreased from 441 to 73L/m2/h with increasing feed concentration (C. vulgaris, 0.25-1g/L); the contribution of Rc decreased (82 to 57%), while that of Ra rose (7 to 40%). With T-Iso suspensions and P. cruentum supernatants at 1g/L, average permeate flux was 59 and 49L/m2/h, respectively, with predominance of Rc and Ra, respectively. Distinct fouling mechanisms were inferred to explain the superior filterability of C. vulgaris. The results show that ceramic membrane MF at relatively high CFV could be a suitable option for harvesting certain microalgae including C. vulgaris.

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.

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)