Hydrophobic Porous Membrane Research Articles

A Review of Membrane Distillation Process: Before, During and After Testing

Membrane Distillation (MD) is a promising technology for separation and purification processes. It is a thermally-driven separation process which allow only vapour molecules are to pass through a porous hydrophobic membrane. MD separation is driven by the vapour pressure difference existing between the porous hydrophobic membrane surfaces unlike normal membrane processes which operate on temperature difference. This paper focus on the expectation of MD treatment process primarily for the readers who have no idea about this membrane process A brief overview is given of MD before treatment process which includes membrane materials, membrane preparation techniques, membrane characteristics, module and configuration. Membrane performance during treatment process will be highlighted. Membrane fouling which is one of the major drawback of MD will be also discussed.

Superhydrophobic polypropylene membrane with fabricated antifouling interface for vacuum membrane distillation treating high concentration sodium/magnesium saline water

Vacuum membrane distillation (VMD) is a promising technology for high salinity wastewater treatment. One of the key issues in VMD research is improving the antifouling performance of the hydrophobic porous membrane under proper interface modification theory and approach. In this work, the polypropylene (PP) composite membrane coated with SiO2 nanoparticles and fluorinated modification was fabricated to simultaneously enhance the interface roughness and superhydrophobicity. The fabricated superhydrophobic PP membrane was then applied to treat highly concentrated NaCl (15 wt%)/MgCl2 (from 3 to 9 wt %) saline solution in VMD process under various flow conditions. The model concerning the surface roughness and surface nucleation free energy was constructed to predict the influence of membrane surface structure on potential anti-fouling and anti-wetting properties. Classical heterogeneous nucleation theory was introduced to illuminate the improved performance of developed membrane with targeted surface morphology on inhibiting the initial interfacial nucleation process. The experimental results illustrated the fabricated superhydrophobic PP membrane exhibited unique anti-fouling and anti-wetting abilities, as well as extremely stable permeating flux even under low feed rate and highly concentrated salt solution system during the continuous operation. In addition, it was also indicated that sodium ion and magnesium ion with distinct features in the structure of water-ion solution system had the potential impact on the permeate flux decline and concentration polarization effect, which was helpful for further improving the stability and durability of this series modified membrane when treating the high concentrated multiple solution system.

Fluorosilaned-TiO2/PVDF Membrane Distillation with Improved Wetting Resistance for Water Recovery from High Solid Loading Wastewater

Membrane distillation (MD) has emerged as an important technology for applications in industries such as seawater desalination and wastewater treatment due to its low energy requirement and theoretically low fouling propensity. However, the main obstacle to obtain high separating efciency in MD lies on the availability of porous hydrophobic membrane that can withstand pore wetting and membrane fouling. In this work, a dual coagulation bath method was introduced to alter the membrane morphology by increasing its porosity, surface roughness as well as polymer crystallinity. To increase the membrane hydrophobicity, membrane roughness was induced by adding TiO2 nanoparticles. However, this has brought concomitant impacts by lowering its porosity due to the pore blocking and reducing hydrophobicity due to the presence of hydroxyl group on TiO2 surface. Introduction of silanized TiO2 modifed at pH 7 gave higher contact angle (131.7±4) that could withstand the pore wetting and at the same time maintained its high permeation flux (12kg/m2.h) and excellent nutrient removal efciency of 99.65%. Consistent flux around 6 kg/m2.h for Paper Mill Sequence Batch Reactor (PMSE) could be achieved showing that the membrane wetting and fouling resistance towards solids were good. The system efciency was around 55% which was comparable to the pure water treatment process (50%). However, the membrane was not suitable to be used for treatment of the oil-rich Palm Oil Mill Efuent (POME) as the flux dropped from 6 to 2 kg/m2.h after 7 hours of operation with thermal efciency dropped to 26% due to fouling phenomena.

Transport properties and fouling issues of membranes utilized for the concentration of dairy products by air-gap membrane distillation and microfiltration

The work contains results related to the application of membrane separation processes (i.e., membrane distillation and microfiltration) in dairy products processing. The performance of titania ceramic membranes (pristine as well as hydrophobized) was studied in the microfiltration process of whey and lactose solutions. On the other hand, polymeric porous hydrophobic membranes, (polypropylene—PP and polytetrafluoroethylene—PTFE) were tested in the air-gap membrane distillation (AGMD) process in contact with whey and lactose solutions. The extended material and physiochemical characterization (including fouling issues) of membrane materials before and after the separation process was performed. Transport of lactose solution across the membranes was characterized by higher effectiveness comparing with whey solution. Lactose played an important role and reduced the interactions with membrane material. Lactose solution possessed lower adhesion ability than whey what influenced the fouling and reduction of permeate flux. Membrane distillation was found to be a process that generates high-quality water with retention > 99% while simultaneously concentrating whey or lactose solutions. Microfiltration process was characterized by a rejection of lactose in the range of 80–90% for pristine ceramic membrane and hydrophobized one, respectively. The tendency to fouling in the case of AGMD process was associated with the higher contribution of polar part (29%) of the surface free energy SFE for PTFE membrane. The lowest value of normalized flux decline was observed for PP membrane, and that was correlated with the smallest contribution of polar interaction in SFE, the lowest value of roughness (RSM), contact angle (CA), and contact angle hysteresis (HCA), determined for the membrane after its utilization in the separation process.

Economic Comparison of Crystallization Technologies for Different Chemical Products

Crystallization is an ancient unit operation that remains vital for the chemical process industry. Traditional single-effect evaporation consumes a great deal of energy, and various alternatives to this method have been proposed. In this work, the total cost of producing a fixed quantity of different solid chemicals by crystallization from water is determined for several different technologies: evaporative crystallization, membrane distillation with porous hydrophobic membranes, reverse osmosis membrane-assisted crystallization, and eutectic freeze crystallization (EFC). Among the solute properties, the solubility has the greatest effect on the cost of the process since it determines the amount of water that must be removed per unit product produced. If waste heat is available at a unit price lower than that of low-pressure steam produced using coal or natural gas, then the assumed price of waste heat also has a significant effect on the economics. The results indicate that, if feasible, reverse osmosis h...

Stable Superhydrophobic Ceramic-Based Carbon Nanotube Composite Desalination Membranes.

Membrane distillation (MD) is a promising process for the treatment of highly saline wastewaters. The central component of MD is a stable porous hydrophobic membrane with a large liquid-vapor interface for efficient water vapor transport. A key challenge for current polymeric or hydrophobically modified inorganic membranes is insufficient operating stability, resulting in some issues such as wetting, fouling, flux, and rejection decline. This study presents an overall conceptual design and application strategy for a superhydrophobic ceramic-based carbon nanotube (CNT) desalination membrane having specially designed membrane structures with unprecedented operating stability and MD performance. Superporous and superhydrophobic surface structures with CNT networks are created after quantitative regulation of in situ grown CNT. The fully covered CNT layers (FC-CNT) exhibit significantly improved thermally and superhydrophobically stable properties under an accelerated stability test. Due to the distinctive structure of the superporous surface network, providing a large liquid-vapor superhydrophobic interface and interior finger-like macrovoids, the FC-CNT membrane exhibits a stable high flux with a 99.9% rejection of Na+, outperforming existing inorganic membranes. Under simple and nondestructive electrochemically assisted direct contact MD (e-DCMD), enhanced antifouling performance is observed. The design strategy is broadly applicable to be extended toward fabrication of high performance membranes derived from other ceramic or inorganic substrates and additional applications in wastewater and gas treatment.

Enrichment of hydrazine from aqueous solutions by vacuum membrane distillation through microporous polystyrene membranes of enhanced hydrophobicity

Abstract Hydrazine hydrate (HH) is synthesized by different chemical routes, which yield dilute aqueous solutions that need to be concentrated for further use. Separation of hydrazine is essential for its application as a rocket fuel. Conventional routes such as distillation prove to be hazardous due to the highly inflammable and explosive nature of hydrazine. Membrane distillation (MD) is an alternative but the highly safe process for dehydration of hydrazine since water can be vapourized thermally through a porous hydrophobic membrane. In this regard, proper selection of polymer is an important factor due to high alkalinity of hydrazine. A new solvent resistant, hydrophobic and mechanically robust microporous polystyrene (PS) membrane was prepared on a macroporous polyester fabric support using phase inversion technique and crosslinked with tetraethyl orthosilicate (TEOS) for MD based dehydration of HH. SEM and FTIR studies were performed to study membrane morphology and intermolecular interactions. Microporous polystyrene membranes were successful in separating feed mixture containing 50 wt% hydrazine by exhibiting a reasonable water flux of 5.03 kg/m2 h, with water concentration in permeate being 97.18% at 75 mm Hg downstream vacuum. A two-dimensional model based on finite element method (FEM) was developed to predict the concentration profile within the membrane using computational fluid dynamics. This study proves that commercial grade hydrazine could be safely recovered from its aqueous reaction liquor by MD technique.

Novel intermittent absorption cooling system based on membrane separation process

The present work analyses an intermittent absorption cooling system using a hydrophobic porous membrane unit as desorber/condenser and powered by thermal solar energy. Experimental test runs were carried out in a membrane unit to experimentally determine the amount of refrigerant produced at different operating temperatures. With the obtained information, an intermittent absorption cooling system was modelled at a larger scale using information from real solar collectors and considering a 1 m2 membrane area. According to the experimental performance of the desorber/condenser unit, after 4 h of operation, the total amount of refrigerant produced was 14.50, 11.59 and 7.20 kg for desorber temperatures of 95.1, 85.2 and 75.1 °C, respectively. The designed solar system was composed of a 0.3 m3 storage thermal tank, and 30.2, 25.6 and 20.9 m2 of solar collector area for each desorber thermal level. According to the simulation of the absorption cooling system, evaporator temperatures of 18 and 14 °C were achieved for desorber temperatures of 75.1 and 85.2 °C, while evaporator temperatures of 17 and 12 °C were obtained at 95.1 °C. The COPs were 0.15, 0.21 and 0.26, which increased according to the increase in the desorber temperatures. Based on the clear desorber/condenser tendencies obtained from the refrigerant production, additional absorption conditions were calculated and may be useful for future designs. The lowest evaporator temperature was 6 °C when the initial LiBr concentration was 54.83% w/w, instead of 50.30% w/w at a temperature of 95.1 °C; however, the COP decreased 33%.

Implementation of osmotic membrane distillation with various hydrophobic porous membranes for concentration of sugars solutions and preservation of the quality of cactus pear juice

Hydrophobic polymeric (0.45 and 0.20 μm polytetrafluoroethylene (PTFE) and 0.10 μm polypropylene (PP)) membranes were efficiently applied for cactus pear juice and sugars solutions concentration using Osmotic Membrane Distillation (OMD) process. It is reported that the PTFE membranes gave higher permeate fluxes and higher final juice concentrations. Furthermore, the filtration of raw juice prior to OMD process, the extension of the process time and the increase of processing temperature resulted in the increase of the final juice concentration. Using 0.45 μm PTFE membrane, the final concentration of the filtered juice (FJ) was 16.4 °Brix ± 0.1 (at 20 °C, after 5 h) and 23.4 °Brix ± 0.2 (at 35 °C, after 18 h). The juice quality was assessed by determining the juice viscosity, the total phenolic content, total flavonoids content, antioxidant activity and phenolic acids content. It was revealed that the physicochemical properties were preserved in the concentrated juice.

Characterization of triple electrospun layers of PVDF for direct contact membrane distillation process

Recently, electrospinning technique was applied successfully to fabricate porous hydrophobic membranes for MD applications. In this work, a novel triple layer configuration with diameter gradient for PVDF nanofiber membranes is proposed, with the objective of to minimize mass transfer resistance and heat loss. In outer layers of these membranes, the minimum concentration of PVDF (20 wt%) was used to produce bead-free nanofibers with thinner diameters and middle layers were composed of thicker nanofibers formed at higher polymer concentrations (21.5-26 wt%). Characterization of prepared membranes was conducted by the measurement of porosity, thickness, liquid entry pressure (LEP), scanning electron microscopy (SEM), contact angle, thermal and mechanical properties. Direct contact membrane distillation performance of fabricated membranes was tested using 42 g/L NaCl as feed solution. Water permeate flux of triple layer membranes (27.8-31.5 kg/m2 h) was found to be considerably higher than that obtained from single layer membrane (15.4 kg/m2 h), indicating the proposed configuration can effectively improve evaporation efficiency.

Modification of polyacrylonitrile membranes via plasma treatment followed by polydimethylsiloxane coating for recovery of ethyl acetate from aqueous solution through vacuum membrane distillation

Abstract For the recovery of ethyl acetate (EtAc) from aqueous solution via vacuum membrane distillation (VMD), hydrophobic porous membranes with high and stable separation performance were prepared by carbon tetrafluoride (CF4) plasma treatment on polyacrylonitrile (PAN) membrane followed by polydimethylsiloxane (PDMS) coating. The effects of plasma treatment conditions and PDMS contents on the morphology, surface porosity and separation performance of modified membranes were investigated. It was found that both the two modification operations increased the hydrophobicity of the PAN membrane. CF4 plasma treatment increased the hydrophobicity (the contact angle increased from 42° to 124° measured by 2.5 wt% EtAc solution), surface mean pore size (from 24.6 nm to 150.5 nm) and porosity (from 7% to 32.3%) of hydrophilic PAN membrane, suggesting that the CF4-modified membranes (PAN-C membranes) with high hydrophobicity could be used in VMD. Optimized PDMS coating on the PAN-C membranes (PAN-C-P membranes) further narrowed the pore size distribution but retained porous and hydrophobic surface. The PAN-C membranes were much less efficient and stable in separating EtAc from water comparing with the PAN-C-P ones. For instance, for the separation of 1 wt% EtAc/water mixture, the best VMD performance in this work was achieved by the PAN-C3-P3 membrane with a separation factor of 70 and a total flux of 7.85 kg/m2 h (PSI = 542). Moreover, the PAN-C3-P3 membrane showed a very stable separation performance than that of the PAN-C3 membrane during a 90 h intermittent VMD operation, indicating a very promising application prospect for EtAc recycling.

Octadecyl‐silica—PVDF membrane of superior MD desalination performance

ABSTRACTDespite its widespread industrial and residential uses for production of potable water, the reverse osmosis (RO) desalination process has some drawbacks by discharging harmful concentrated saline water as reject stream. A hydrophobic porous membrane can treat such environmentally unfriendly RO reject stream via Membrane Distillation (MD) process. Here, we describe preparation of superior polyvinylidenefluoride (PVDF) membrane modified with superhydrophobic silica nanoparticles for desalination application. Superhydrophobicity (contact angle of 151°) of silica nanoparticles of 7 nm sizes was achieved by reaction of the silica particles with octadecyltrichlorosilane in toluene to form SiOSi links with C18 alkyl chain. A homogeneous polymer dope mixture containing a desired amount of modified silica colloids suspended in toluene was used for the membrane preparation. The PVDF membrane with optimal silica content exhibited excellent flux with >99% salt rejection efficiency when used for MD at room temperature from the saline water feed of 3.5 wt % NaCl. The prepared hydrophobic PVDF membrane has the potential for MD application in treating the RO reject stream and other aqueous industrial effluents. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46043.

CO2 Absorption with NaOH Solution through the Porous PVDF Hollow Fibre Membrane Contactor

In this study, porous hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were fabricated via a wet phase inversion process. In order to improve the phase inversion rate and provide porous membranes, 4 wt.% lithium chloride (LiCl) was used in the spinning dope. The prepared membrane morphology was studied using field emission scanning electron microscopy (FESEM). Chemical CO2absorption by NaOH solution (1M) was conducted through the PVDF hollow fiber membrane contactor. The effect of the main operating condition such as absorbent temperature, CO2 pressure and absorbent flow rate on the performance of CO2 absorption was investigated. From FESEM examination, the membrane possesses an almost sponge–like structure with ultra thin skin layer. Results of CO2absorption test showed that by increasing the absorbent flow rate the CO2 flux increased which confirmed the existence of liquid side mass transfer resistance. It was found that by increasing the absorbent temperature the CO2 flux considerably improved. Meanwhile, the effect of CO2 pressure on the absorption rate was insignificant. Therefore, it can be concluded that by applying a porous hydrophobic membrane with improved structure and optimizing the operating conditions, high CO2 removal efficiency can be achieved through gas–liquid membrane contactors

Design and economic analysis of membrane-assisted crystallization processes

Abstract Reverse-osmosis (RO) and porous hydrophobic membranes have attracted attention as an alternative to conventional evaporation for crystallization processes, the latter being recommended especially if low-grade waste heat is available to provide the enthalpy of vaporization. In this work, these two types of membrane-assisted processes are designed and costs are compared with those of conventional single or multiple-effect vacuum evaporation processes. When waste heat is not available, RO processes are shown to be the least expensive because the pumping costs, while significant, are more than offset by the fact that it is not necessary to provide the enthalpy of vaporization of the solvent. When waste heat is available, a conventional evaporation process is favorable because it can also be powered by the waste heat and does not require the capital cost for the membranes.

Energy Efficiency and Performance Limiting Effects in Thermo-Osmotic Energy Conversion from Low-Grade Heat.

Low-grade heat energy from sources below 100 °C is available in massive quantities around the world, but cannot be converted to electricity effectively using existing technologies due to variability in the heat output and the small temperature difference between the source and environment. The recently developed thermo-osmotic energy conversion (TOEC) process has the potential to harvest energy from low-grade heat sources by using a temperature difference to create a pressurized liquid flux across a membrane, which can be converted to mechanical work via a turbine. In this study, we perform the first analysis of energy efficiency and the expected performance of the TOEC technology, focusing on systems utilizing hydrophobic porous vapor-gap membranes and water as a working fluid. We begin by developing a framework to analyze realistic mass and heat transport in the process, probing the impact of various membrane parameters and system operating conditions. Our analysis reveals that an optimized system can achieve heat-to-electricity energy conversion efficiencies up to 4.1% (34% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and an operating pressure of 5 MPa (50 bar). Lower energy efficiencies, however, will occur in systems operating with high power densities (>5 W/m2) and with finite-sized heat exchangers. We identify that the most important membrane properties for achieving high performance are an asymmetric pore structure, high pressure resistance, a high porosity, and a thickness of 30 to 100 μm. We also quantify the benefits in performance from utilizing deaerated water streams, strong hydrodynamic mixing in the membrane module, and high heat exchanger efficiencies. Overall, our study demonstrates the promise of full-scale TOEC systems to extract energy from low-grade heat and identifies key factors for performance optimization moving forward.

Enzyme Carbonic Anhydrase Accelerated CO2 Absorption in Membrane Contactor

Membrane contactors offer a promising alternative to conventional CO2 absorption processes using columns. In a membrane contactor the advantages of absorption technology and membrane technology are combined as direct contact of the solution and gas feed stream is avoided by membrane barrier. In this study, the possibility of employing the enzyme carbonic anhydrase (CA) for the acceleration of CO2 reaction in MDEA and MEA solution in combination with the use of a membrane contactor was investigated in a lab scale module. The membranes employed in this study were microporous and specifically chosen to have both hydrophobic (bulk) and hydrophilic (surface) properties in order to avoid wetting of solution and reduce fouling by the enzymes simultaneously. By adding the enzyme carbonic anhydrase (CA), a significant improvement of CO2 absorption rate was observed in MDEA solution while a negative effect was observed in MEA solution. Meanwhile the porous hydrophobic membranes were coated with a highly selective poly(ionic liquids) layer increasing the affinity of CO2 towards the interfacial area and hence also the driving force. The concept may initially appear counter intuitive, as the dense membrane layer introduces an added resistance, however the active membrane material gave promising results and was observed to accelerate CO2 transport in MDEA solution. The combination of both enzyme and PILs resulted in synergies, which significantly improved CO2 absorption in MDEA solution.

Assessment of air-gap membrane distillation with hydrophobic porous membranes utilized for damaged paintings humidification

Decay of paintings, i.e. cracking and lifting of the picture layers is caused mainly by harmful influence of light, temperature and fluctuations in humidity. This paper describes the application of various hydrophobic porous membranes in the paintings' humidification which is a very important element of the treatment of paintings of canvas and other historical artefacts. Five hydrophobic porous membranes made of polypropylene (PP), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), possessing different porosity (60–85%) and average pore size (0.14−0.55µm) were investigated. Morphology and porosity of membranes were evaluated using SEM and modified bubble point method. Surface properties of membranes were determined by contact angle, surface free energy, and AFM analysis. Subsequently, membranes were assessed in the humidification of the artificially aged paintings. Results showed that all examined hydrophobic porous membranes can be used for the conservation of destroyed paintings. The process was dependent on the time and membrane pore size. It was also found that the membrane thickness has an impact on the efficiency of the humidity gain.

Switchable hydrophilicity solvent membrane-based microextraction: HPLC-FLD determination of fluoroquinolones in shrimps

A switchable hydrophilicity solvent membrane-based microextraction (SHS-MME) strategy for simple and highly-available sample pretreatment of complex matrices has been proposed. The SHS-MME procedure based on extraction of target analytes from an aqueous sample into a porous hydrophobic membrane impregnated with a switchable hydrophilicity solvent (SHS) followed by SHS ionization and back-extraction of the analytes in alkaline acceptor solution. The medium-chain fatty acids were investigated as SHS for the SHS-MME of fluoroquinolones (fleroxacin, lomefloxacin, norfloxacin and ofloxacin). The SHS-MME was successfully coupled with a HPLC-FLD for the determination of fluoroquinolones in shrimp samples with no further sample pretreatment. The calibration graphs were linear over the concentration ranges of 3–1500 μg L−1 for ofloxacin, 10–1000 μg L−1 for norfloxacin, 15–1500 μg L−1 for lomefloxacin and fleroxacin. The limits of detection, calculated from a blank test based on 3σ, were 1 μg L−1 for ofloxacin, 3 μg L−1 for norfloxacin and 5 μg L−1 for lomefloxacin and fleroxacin. The SHS ionization can be considered as new potential fields in analytical application of membrane-based liquid phase microextraction.

An ultrathin suspended hydrophobic porous membrane for high-efficiency water desalination

An ultrathin highly fluorinated porous membrane was designed for a large production of desalted water at very low energy consumption. Imprinting water droplets were used through a low thermally conductive tetra-fluoroethylene (TFE)/2,2,4-trifluoro-5-tri-fluorometoxy-1,3-dioxol (TIT) (HYFLON AD 60) solution and the generated porous nanofilm was suspended onto a polyethersulfone (PES) honeycomb texture. The very tiny fluorinated thickness together with a large number of small-shaped pores provided the membrane for enhanced anti-wetting surface properties, extremely reduced resistance to water vapor transfer and outstanding thermal efficiency. Fine materials structure-transport relations let the membrane reach unusual productivity-efficiency trade-off during water desalination via thermally driven membrane distillation. The exceptional performance affords this novel nanostructured membrane to catalyze the accomplishment of new-concept water desalination processes.

Experimental study on vacuum membrane distillation based on brine desalination by PVDF

Based on polyvinylidene fluoride (PVDF) hollow fiber hydrophobic porous membrane, vacuum membrane distillation desalination test bench was built to study the effects of operating parameters such as temperature of feed solution, flow rate of feed solution, concentration of feed solution and vacuum degree on the performance of vacuum membrane distillation process. Experimental results showed that with an increase in the vacuum degree and temperature of the flux of membrane is increased. While with an increase in the concentration of feed solution of the flux of membrane is decreased and retention rate were essentially the same, with an increase in the flow rate of feed solution of he flux of membrane changed slowly (increase slightly). When temperature of the feed solution was 88°C, the flow rate of feed solution was 270cm/min, the vacuum degree was 0.081MPa, the concentration of feed solution was 5%, the flux of membrane was 14.1kg/ (m2.h), the retention rate was 99.8%, the electrical conductivity of fresh water was maintained at 12us/cm.