Feed Flow Direction Research Articles

Mathematical Modeling of NaCl Scaling Development in Long-Distance Membrane Distillation for Improved Scaling Control.

Membrane distillation is a novel membrane-based separation technology with the potential to produce pure water from high-salinity brine. It couples transport behaviors along the membrane and across the membrane. The brine in the feed is gradually concentrated due to the permeate flux across the membrane, which is a significant factor in initiating the scaling behavior on the membrane surface along the feed flow direction. It is of great interest to investigate and estimate the development of scaling on the membrane surface. This work specifically focuses on a long-distance membrane distillation process with a sodium chloride solution as the feed. A modeling approach has been developed to estimate the sodium chloride scaling development on the membrane surface along the flow direction. A set of experiments was conducted to validate the results. Based on mathematical simplification and analytical fitting, a simplified model was summarized to predict the initiating position of sodium chloride scaling on the membrane, which is meaningful for scaling control in industrial-scale applications of membrane distillation.

Spatio-temporal progression and influencing mechanism of local wetting in membrane distillation

Membrane wetting is the bottleneck of membrane distillation (MD) in real applications. However, rare information is attainable on the spatial inhomogeneity and temporal instability of local wetting behavior along the membrane. In current study, the influences of operating conditions (i.e., feed temperature, velocity, and concentration) and membrane hydrophobicity on the spatio-temporal progression of local wetting along the membrane were evaluated using a non-invasive and online monitoring technique – ultrasonic time-domain reflectometry (UTDR). UTDR measurements could successfully distinguish local wetting dynamics and were validated by the off-line characterization of the wetting layer thickness via a scanning electron microscopy with energy-dispersive x-ray spectroscopy. Observations showed that the local wetting depth decreases along the feed flow direction. This wetting heterogeneity would be impaired with increasing surfactant concentration, feed temperature, and crossflow velocity. However, enhancing membrane hydrophobicity obviously reduced the rate of local wetting but hardly affected its nonuniform distribution. The wetting heterogeneous propagation at different locations is dominated by the nonuniform mass and heat transfer characteristics along the membrane based on the computational fluid dynamics (CFD) simulation. Localized wetting prevention near the module inlet is therefore critical to ensure stable MD operation.

A novel braided structure with enhanced feed flow state for hollow fiber air-gap membrane distillation performance improvement

The feed flow state affects temperature polarization (TP), concentration polarization (CP), and membrane fouling during the membrane distillation process. A novel braided structure for AGMD was proposed in this paper, in which hollow fiber membranes and heat exchange tubes are braided together. The braided structure can alter feed flow direction and reduces the membrane's contact area. Theoretical calculations show that the braided structure reduces the critical Reynolds number for the feed's unstable flow, and improves the feed flow state by forming Dean vortices. Comparative experiments of the braided module and parallel module were carried out. The results show that the braided module performs better than the parallel module. The braided module has a higher flux (J) and gained output ratio (GOR) when treating the tap water with significant TP; a lower decline rate in J and GOR besides a smaller concentration polarization coefficient (CPC) when treating 3.5 wt% NaCl aqueous solution; a slower formation of membrane fouling and a lower reduction in J and GOR when treating a humic acid/calcium carbonate mixed solution. It is proved that the braided structure can improve the performance of AGMD by enhancing the feed flow state to mitigate the TP, CP, and membrane fouling.

Engineering omniphobic corrugated membranes for scaling mitigation in membrane distillation

Membrane scaling is unavoidable when treating highly concentrated and even supersaturated brine in the continuous membrane distillation (MD) process. Omniphobic membranes with higher nucleation energy barriers could partially mitigate the membrane scaling. Herein, we fabricated omniphobic corrugated membranes to further alleviate membrane scaling via the synergistic effects of omniphobicity and corrugation on the membrane surface. The scaling behaviors of omniphobic flat membrane (#PVDF-FF), omniphobic corrugated membrane (#PVDF-CF) at parallel (PL) and perpendicular (PD) modes were investigated. Different modes represented the relative position between the feed flow direction and the pattern of corrugated membrane. The results showed that the modified corrugated membranes had water contact angles over 150° and anti-wettability against low surface tension liquids. Compared to #PVDF-FF, #PVDF-CF at the PL mode exhibited a better slippery property while the PD-mode #PVDF-CF had higher slippery resistance. The membrane omniphobicity was confirmed to be effective against membrane scaling when treating a 3.5 wt% NaCl feed solution. However, the scaling resistance of #PVDF-CF at both PL and PD modes was much better than that of #PVDF-FF when 25 wt% NaCl or 20 mM supersaturated gypsum solutions were used as feeds. The better anti-scaling properties of PL-mode #PVDF-CF should be attributed to a higher flow rate and shorter residence time of feed flow parallel to the corrugated patterns, as well as the alleviated concentration polarization. The turbulent effect induced by the perpendicular feed flow on the corrugated patterns and the consequent alleviation of concentration polarization endowed the PD-mode #PVDF-CF with the best scaling resistance.

Investigation of interfacial crystallization fouling behaviors and membrane re-functionalization based on a long-distance membrane distillation module

Membrane distillation technique can be applicable for the treatment of highly concentrated aqueous solutions. Crystallization and fouling on the membrane surface are important issues during a membrane distillation process, which significantly affect its separation performance. Crystallization precipitation is a gradual process along the normal direction of the membrane and the feed flow direction. It is not easy to observe the gradual behavior in a relatively small module with a short flow distance in common lab scale modules. In this work, a long-distance membrane distillation module (~2 m in length) was designed and set up to investigate the gradual evolutionary process of crystallization fouling on the porous membrane. A practical model was proposed to estimate the approximate position of crystallization fouling in a long-distance membrane distillation process based on multiple factors including operating parameters and intrinsic membrane properties. A set of proof-of-principle experiments were proceeded to observe and characterize the crystallization fouling behaviors on the membrane surface and inside the pores under various conditions. Furthermore, to re-functionalize the membrane distillation system with the membrane fully covered by salt crystals, flushing the system with pure water was applied. The performance of the re-functionalized system was also investigated.

3D-printed surface-patterned ceramic membrane with enhanced performance in crossflow filtration

Appropriate surface patterns on polymer membranes have been demonstrated to have a strong effect on improving the permeability and anti-fouling ability. Herein, for the first time we have developed a surface-patterned alumina ceramic membrane with gradient porous structure using a novel 3D-printing technology. The well-controllable surface-patterned alumina membrane was fabricated by use of 3D-printable ceramic ink through a layer-by-layer coating on a porous substrate, after which the designed patterns were made on the membrane surface by 3D printing the same ink. The 3D-printed line-patterned membrane showed a notable increase in steady flux and a significantly enhanced antifouling ability, especially when the printed lines were perpendicular to the feed flow direction, as evaluated by our experimental and computational fluid dynamics simulation results. Fouling mechanisms were similar between different feed flow directions and evolved from the pore constriction or intermediate pore blocking towards cake filtration. The distance and height of the 3D-printed lines was also revealed to have influences on the fouling performance. Together with the advantages of easily designed structures and a rapid design-to-fabrication process using 3D-printing technology, this study provides an effective new approach to fabricate the advanced ceramic membranes with fouling mitigation for wastewater treatment.

Co-current crossflow microfiltration in a microchannel.

Steady state crossflow microfiltration (CMF) is an important and often necessary means of particle separation and concentration for both industrial and biomedical processes. The factors controlling the performance of CMF have been extensively reviewed. A major factor is transmembrane pressure (TMP). Because microchannels have small height, they tend to have high pressure gradients in the feed-flow direction. In the extreme, these gradients may even reverse the pressure across the membrane (inciting backflow). It is therefore desirable to compensate for the effect of feed-flow on the TMP, aiming at constant transmembrane pressure (cTMP) at a value which maximizes filtrate flux. This is especially critical during filtration of deformable particles (e.g. erythrocytes) through low intrinsic resistance membranes. Filtration flux is generally taken to be directly proportional to TMP, with pressure drop along the channel decreasing in the flow direction. A co-current flow of filtrate in a suitably designed filtrate collecting channel is shown to allow the TMP to remain constant and permit the sieving surface to perform optimally, permitting up to twice as much filtration over that of a naïve configuration. Manipulation of the filtrate channel may be even more beneficial if it prevents backflow that might otherwise occur at the end of a sufficiently long channel. Experiments with erythrocyte suspensions, reported here, validate these concepts.

Prediction of sustainable electricity generation in microbial fuel cell by neural network: Effect of anode angle with respect to flow direction

This study aimed to investigate for the first time the effect of anode inclination on electricity generation integrated with biodegradation of organic substrate in a mediator-less microbial fuel cell continuously fueled with actual dairy wastewater. The influence of anode inclination was investigated at angles 0, 45 and 90° with respect to the flow direction on the MFC performance with respect to power generation and COD removal, alternatively at 1 and 2mL/min wastewater flow rate. Results revealed that maximum power generation of 486 and 369mW/m2 and COD removal efficiencies of 92 and 89% were observed when the anode was positioned perpendicularly with the flow direction at steady state conditions using wastewater flow rates of 1 and 2mL/min, respectively at external resistance of 40Ω. Lower COD removal and power generation were observed for MFCs designed with anodes positioned at 0° and 45° with respect to the feed flow direction. A three-layer artificial neural network (ANN) model was investigated in this study to predict the efficiency of the MFC in regard to power generation. Results of prediction indicated a good fitting between actual and predicted data with a high correlation coefficient (R2) of 0.99889 with negligible mean square error.

Modeling of flat-sheet and spiral-wound nanofiltration configurations and its application in seawater nanofiltration

The Donnan Steric Pore Model with dielectric exclusion (DSPM-DE) is implemented over flat-sheet and spiral-wound leaves to develop a comprehensive model for nanofiltration modules. This model allows the user to gain insight into the physics of the nanofiltration process by allowing one to adjust and investigate effects of membrane charge, pore radius, and other membrane characteristics. The study shows how operating conditions such as feed flow rate and pressure affect the recovery ratio and solute rejection across the membrane. A comparison is made between the results for the flat-sheet and spiral-wound configurations. The comparison showed that for the spiral-wound leaf, the maximum values of transmembrane pressure, flux and velocity occur at the feed entrance (near the permeate exit), and the lowest value of these quantities are at the diametrically opposite corner. This is in contrast to the flat-sheet leaf, where all the quantities vary only in the feed flow direction. However it is found that the extent of variation of these quantities along the permeate flow direction in the spiral-wound membrane is negligibly small in most cases. Also, for identical geometries and operating conditions, the flat-sheet and spiral-wound configurations give similar results. Thus the computationally expensive and complex spiral-wound model can be replaced by the flat-sheet model for a variety of purposes. In addition, the model was utilized to predict the performance of a seawater nanofiltration system which has been validated with the data obtained from a large-scale seawater desalination plant, thereby establishing a reliable model for desalination using nanofiltration.

Study of the Effect of Spacer Orientation and Shape in Membrane Feed Channel using CFD Modelling

Computational fluids dynamics (CFD) modelling has been carried out for a spacer-filled membrane channel using ANSYS FLUENT 14.0. The effect of spacer angles relative to the feed flow direction and different spacer shape combinations on velocity magnitude, wall shear stress, pressure drop and power number were investigated. From the results, spacer angle of 63.565° is the best orientation as it can generate the highest shear stress and reasonably lower power number compared to other spacer angles. Although the combinations of spacer shape did not significantly improve the average wall shear stress, it helped in reducing the pressure drop of the channel. The combination of triangular and circular spacers provided lower Power number, and hence lower energy consumption was required compared to pure triangular spacer. The current results indicated that a combination of triangular and circular spacers can be employed to offer better saving in energy consumption.

Methane autothermal reforming in a reverse flow reactor on Rh/Al2O3 catalyst

Autothermal reforming of methane was investigated in a Reverse Flow Reactor with commercial Rh/Al2O3 catalyst in pellets. The process was carried out in a catalytic fixed bed reactor and switching of feed flow direction was obtained through four electrovalves synchronized in pair. Temperature profile of the catalyst bed was measured by fast IR thermography and product composition was measured with a continuous gas analyzer.Reactor performance and catalyst temperature profile are presented as a function of feed direction switching period, water to methane inlet ratio and total inlet flow rate. Results are compared to steady state operation.

Non-invasive measurement of membrane scaling and cleaning in spiral-wound reverse osmosis modules by ultrasonic time-domain reflectometry with sound intensity calculation

This study describes the ultrasonic time-domain reflectometry (UTDR) with a sound intensity calculation for monitoring membrane scaling and cleaning in a 4-inch spiral-wound reverse osmosis module. Three focused transducers with the frequency of 2.25 MHz were located equidistantly along the feed flow direction. The scaling experiments were carried out with 2.0 g/L calcium sulfate for 90 h fouling and the cleaning processes consisted of three phases: pure water flushing, immersion cleaning and acid-washing. According to this study, the ultrasonic response signals and the corresponding sound intensity gradually decreased to minimum with the fouling and deposition of the foulants on the membrane and then increased with the formation of fouling layer. The initial induction time in the change of sound intensity became shorter and the sound intensity became lower along the fluid direction during the fouling process. It was found that the CaSO 4 scaling first precipitated near the downstream owing to the increased concentration polarization. Furthermore, the permeate flux and the sound intensity all recovered step by step with the cleaning processes. The results obtained by independent methods such as SEM and weight measurements were in good agreement with the observations by ultrasonic technology. Overall, this study confirms that the UTDR technique with a sound intensity calculation and modeling is capable to evaluate the membrane scaling and cleaning in spiral-wound modules.

Temperature evolution on Rh/Al 2O 3 catalyst during partial oxidation of methane in a reverse flow reactor

Catalytic partial oxidation of methane was investigated in a reverse flow reactor with commercial Rh/Al 2O 3 catalyst in pellets. The process is carried out in a catalytic fixed bed reactor and switching of feed flow direction is obtained through four electrovalves synchronized in pairs. Temperature profile along the catalyst bed was measured by fast IR thermography and product composition was measured with a continuous gas analyzer. Feed direction switching time, water to methane ratio and inert section length were investigated as process parameters. Data of catalyst bed temperature evolution during the flow cycle are presented, discussed and related to reactor performance as a function of reverse flow switching period. The effect of water addition to the reacting mixture on the dynamics of catalyst bed temperature evolution is also presented.

Ultrasound, gravimetric, and SEM studies of inorganic fouling in spiral-wound membrane modules

Ultrasonic time-domain reflectometry (UTDR) is combined with permeate-flux, gravimetric, and scanning electron microscopy (SEM) measurements for studying calcium-sulfate dihydrate fouling in a commercial spiral-wound reverse osmosis membrane module. The UTDR amplitude response is shown to be more sensitive than permeate-flux decline for detecting the early stages of fouling because it is a local rather than integral measurement. Hence, appropriately located acoustic transducers can be used to detect the onset of fouling well before it impairs module performance so that appropriate remediation can be undertaken. The UTDR arrival-time response in principle can be used to determine the fouling layer thickness. However, this study indicates that arrival-time measurements in a spiral-wound module can be compromised by movement of the membrane envelope layers as is the case for the Koch 2521 spiral-wound module used in this study. The estimated module expansion of 210 µm inferred from the UTDR measurements exceeds the thickness of the fouling layer deposits estimated from the gravimetric measurements. The gravimetric studies reveal a 6 to 10-fold increase in the thickness of the fouling layer deposits in the feed-flow direction owing to concentration polarization. A particularly interesting result of the gravimetric studies is a 16 to 27-fold increase in the thickness of the fouling layer deposits in the permeate-flow direction presumably owing to expansion of the outer relative to the inner feed channels. The SEM studies indicate that fouling is initiated along the feed-spacer mesh presumably due to dead-flow regions behind mesh elements transverse to the flow. Overall, the present study demonstrates that when UTDR is combined with other independent measurement techniques, it can provide significant insights regarding fundamental fouling and membrane module behavior.

Modeling of spiral wound pervaporation modules with application to the separation of styrene/ethylbenzene mixtures

A mathematical model is derived to simulate the performance of spiral wound membrane modules for the pervaporative separation of binary liquid mixtures. Permeation through the polymer membrane is described by a detailed solution–diffusion model. Flory–Huggins theory is used to predict solubility of the penetrants, while the composition and temperature dependence of the diffusion coefficients are described by phenomenological relations. Unknown parameters of the solution–diffusion model are determined from sorption and flux data using nonlinear least-squares estimation. Standard correlations are used to estimate the mass transfer resistance due to the liquid boundary layer on the feed side of the module. The solution–diffusion model is coupled to differential mass, momentum and energy balances on the feed and permeate sides of the module to predict separation performance. The resulting model is two-dimensional as the feed-side and permeate-side stream properties vary in both the feed and permeate flow directions. Required input data to the model includes the feed flow rate, composition and temperature, the outlet permeate pressure, the membrane properties and the module dimensions. A numerical solution technique based on the use of the shooting method in the permeate flow direction and numerical integration in the feed flow direction is proposed. The model yields predictions of the feed-side and permeate-side stream properties as a function of both spatial coordinates. The separation of styrene and ethylbenzene with a polyurethane membrane is used to illustrate the modeling approach. A single module and a 10-module system with interstage heating are simulated to demonstrate potential uses of the pervaporation model.