Enhanced water desalination performance through hierarchically-structured ceramic membranes

Developments of membrane water desalination are impeded by low water vapor flux across the membrane. We present an innovative membrane design to significantly enhance the water vapor flux. A bilayer zirconia-based membrane with a thick hierarchically-structured support and a thin functional layer is prepared using a combined freeze drying tape casting and screen printing method. The hierarchically-structured YSZ support has a porosity of 42.6%, pores of 4.5 μm or larger, and a relatively low tortuosity of 1.58 along the thickness direction. The bilayer membrane is then converted from naturally hydrophilic to hydrophobic via grafting with a fluoroalkylsilane. A water flux of 28.7 Lm⁻² h⁻¹ and a salt rejection of 99.5% are achieved by exposing the functional layer to 80 °C salt water of 2 wt.% NaCl and the support layer to 20 °C distilled water. These results are the best performing ones for ceramic membranes in direct contact membrane distillation operation.     

A thermodynamically stable La2NiO4+δ/ Gd0.1Ce0.9O1.95 bilayer oxygen transport membrane in membrane-assisted water splitting for hydrogen production

A bilayer configuration of mixed ion-electron conducting La₂NiO_4+δ and oxygen-ion conducting Gd_0.1Ce_0.9O_1.95 (LNO/GDC10) was proposed for hydrogen production by water-splitting and its properties were measured as a function of temperature, reducing gas CO content and water vapor pressure during the hydrogen production by water-splitting. The hydrogen production flux increased with increasing water vapor pressure and oxygen chemical potential to a maximum of 0.12 cm³ (STP)/min-cm² with 23.25% CO/76.75% CO₂ (40 sccm)/balance He (60 sccm) gas mixture on the oxygen-permeate side and wet N₂ (pH₂O=0.49 atm) on the oxidizing side at 900 °C. The stability of the bilayer membrane was tested in a very low oxygen partial pressure (pO₂) on the oxygen-permeate side. The presence of GDC10 on the oxygen-permeate side of the bilayer prevented the direct exposure of LNO to very low pO₂ and thus protected it from decomposition, even at pO₂≈10^?15 atm.     

Effect of water on the performance of silica-pillared layered niobate-supported copper oxide catalyst for NO reduction by CO

A 2 wt% copper catalyst supported on silica-pillared layered niobate (Cu/Si–Nb₃O₈) with a high hydrothermal stability was prepared and tested for the NO + CO reaction. Pretreatment with water vapor could improve the activity of the catalyst greatly, while the selectivity to N₂ almost kept unchanged. The addition of water vapor (2 vol%) into the reaction gas could increase the activities of the catalyst without pretreatment and those pretreated by 2 vol% and 5 vol% stream, but reduce the activity of the catalyst pretreated by up to 10 vol% water vapor. Generally, water vapor in the reaction gas slightly reduced the selectivity to N₂.     

Synthesis and characterization of alumina supported sub-nanoporous SiO2–10 wt%TiO2 membrane for nitrogen separation

Membrane of SiO₂–10TiO₂ was produced by dip coating on mesoporous-titania-coated macroporous alumina support. The amorphous SiO₂–10TiO₂ top layer with thickness of 0.9 μm was obtained as a homogenous and defect free surface. Gas permeation tests show a decrease in permeation in order of CH₄ > N₂ for the support, and N₂ > CH₄ for the multilayer membrane. The membrane delivering promised single gas separation factor for N₂/CH₄ (1.75). It was also observed that the membrane separation factor in binary gas mixture was 12–15% of the single gas separation factor, while the permeation revealed a decrease in gas mixture.     

Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3−δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3−δ layers

The oxygen permeation performance of a number of La_0.1Sr_0.9Co_0.8Fe_0.2O_3−δ (LSCF1982)-based membranes, consisting of dense LSCF1982 layer with/without porous LSCF1982 layer, was analyzed on the basis of the thickness of the dense layer and catalytic effect of the porous layer. A 0.27 mm thick dense membrane gives oxygen permeation flux ($J_{O_2}$) of 2.33 sccm min⁻¹ cm⁻² at 900 °C, which is increased to 3.55 sccm min⁻¹ cm⁻² on applying a porous layer of LSCF1982 onto the dense membrane. The membrane gives a stable flux for 300 h. The flux was further improved by reducing the thickness of the dense LSCF1982 layer and at 950 °C a flux of 4.47 sccm min⁻¹ cm⁻² is obtained with 0.012 mm thick membrane.     

Determination of gas diffusion coefficients in saturated porous media: He and CH4 diffusion in Boom Clay

Boom Clay is presently studied as the reference host formation for the disposal of high-level and long-lived radioactive waste in Belgium. In a geological repository, the production of gas is unavoidable. Gas is produced by different mechanisms: anaerobic corrosion of metals in waste and packaging, radiolysis of water and organic materials in the packages and microbial degradation of various organic wastes. Corrosion and radiolysis yield mainly hydrogen while microbial degradation leads to methane and carbon dioxide. The generated gas will dissolve in the ground water. As transport in Boom Clay is dominated by diffusion, the dissolved gas is transported away from the repository by diffusion as dissolved species. If the rate of gas generation is larger than the diffusive flux into Boom Clay, the pore water within the disposal gallery will become oversaturated and a free gas phase might form. If the gas pressure keeps increasing, free gas ingress into Boom Clay will occur, most likely through creation of new pathways. In order to make a good evaluation of the balance between gas generation and gas dissipation through engineered barriers and host formation, good estimates for the gas diffusion coefficients of the gases are needed. The currently available gas diffusion parameters for hydrogen in Boom Clay suffer from a large uncertainty, and by application of conservative values for both the source term and the gas migration term the formation of a free gas phase can presently not be excluded for some waste types.In this study a versatile method was developed to determine more precisely the gas diffusion coefficient for dissolved gases in Boom Clay. For the development of the technique, He and CH₄ were used.The proposed method is based on a through diffusion methodology and allows for two dissolved gases to diffuse through a clay sample at the same time. From the evolution of the diffusant concentration in both compartments, the apparent diffusion coefficients of dissolved He and CH₄ were obtained: 12.2 × 10^− 10 and 2.42 × 10^− 10 m²/s, with uncertainties of 10%, respectively.     

Separation of gas and organic/water mixtures by MFI type zeolite membranes synthesized in a flow system

MFI type zeolite membranes were synthesized in a recirculating flow system at 95 °C where the synthesis solution was flown over the tubular α-alumina supports. The performance of the membranes for the separation of binary gas mixtures and alcohol/water liquid mixtures was investigated. A membrane synthesized by two consecutive synthesis steps had a separation selectivity of 15 and 11 for equimolar mixtures of n-C₄H₁₀/CH₄ and n-C₄H₁₀/N₂ at 200 °C, respectively. The membrane selectively permeated large n-C₄H₁₀ over small CH₄ and N₂, suggesting that the separation is essentially adsorption-based and the membrane has few nonselective intercrystalline pores. The selectivities in the pervaporation separation of 5% ethanol/95% water mixture were 43 and 23 with permeate fluxes of 0.2 and 1.9 kg/m² h at 25 and 85 °C, respectively. The separation performance of membranes showed that MFI type membranes prepared in a recirculating flow system can be used both in the separation of gas and liquid mixtures.     

New morphological Ba0.5Sr0.5Co0.8Fe0.2O3−α hollow fibre membranes with high oxygen permeation fluxes

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?α (BSCF) hollow fibre membranes were fabricated by a combined phase inversion and sintering technique. The membranes were characterised by XRD, SEM and tested for air separation. The membrane possesses a novel morphology consisting of one dense layer and one porous layer. Oxygen permeation fluxes through the obtained hollow fibre membranes were measured in the temperature range 650–950 °C using helium sweep gas rates from 50 to 200 mL min^?1. Experimental results indicated the oxygen permeation flux through the BSCF hollow fibre membrane sintered at 1050 °C was approximately 11.46 mL min^?1 cm^?2 at 950 °C when the helium sweep rate was kept at 200 mL min^?1. The BSCF hollow fibre membrane showed a stable oxygen permeation flux of 8.60 mL min^?1 cm^?2 over the investigated period of 120 h at 900 °C.     

Application of response surface methodology for modeling and optimization of membrane distillation desalination process

In this work, response surface methodology (RSM) was applied for modeling and optimization of operating parameters for water desalination by direct contact membrane distillation (DCMD) process using polypropylene membrane (PP) with low pore size. Operating parameters including vapor pressure difference, feed flow rate, permeate flow rate and feed ionic strength were selected and the optimum parameters were determined for DCMD permeate flux. The developed model for permeate flux response was statistically validated by analysis of variance (ANOVA) which showed a high value coefficient of determination value (R² = 0.989). The obtained optimum operating parameters were found to be 0.355 × 10⁵ Pa of vapor pressure difference, feed flow rate of 73.6 L/h, and permeate flow rate of 17.1 L/h and feed ionic strength of 309 mM. Under these conditions, the permeate flux was 4.191 L/(m² h). Compared to a predicted value, the deviation was 3.9%, which confirms the validity of the model for the DCMD process desalination optimization. In terms of product water quality, the DCMD process using hydrophobic PP membrane can produce high quality of water with low electrical conductivity for all experimental runs.     

Fabrication of polyetherimide microporous membrane using supercritical CO2 technology and its application for affinity membrane matrix

Polyetherimide (PEI) microporous membranes with uniform cellular structure, high porosity, and narrow pore size distribution were formed by supercritical CO₂ (ScCO₂) phase inversion method, and the membrane was modified to be a matrix for the preparation of affinity membrane due to its low solvent residue and appropriate porous structure. The effects of ScCO₂ temperature and pressure on the morphology and pure water flux of the membrane were investigated. The membrane prepared at 24 MPa and 45 °C with a large mean cell diameter of 6.0 μm, high porosity of 73%, narrow pore size distribution and a pure water flux of 56 L/(m² h bar) was coated with chitosan to improve its hydrophilicity and coupled with Cibacron Blue F3GA (CB) as a special ligand to form an affinity membrane (PEI-coated chitosan-CB membrane). The PEI-coated chitosan-CB membrane showed a high adsorption capacity of 33.9 mg/g membrane to bovine serum albumin and was higher than most of affinity membranes. Moreover, the tensile strength of PEI-coated chitosan-CB membrane was 11.58 MPa and was much higher than those of affinity membranes. This work demonstrates that ScCO₂ phase inversion method is a potential method to prepare an affinity matrix.     

Facile synthesis of AlOx dielectrics via mist-CVD based on aqueous solutions

Aluminum oxide (AlO_x) thin films were synthesized by mist-chemical vapor deposition (mist-CVD) using aluminum acetylacetonate (Al(acac)₃) dissolved in an aqueous solvent mixture of acetone and water. Nitrogen gas was used to purge the precursor solution and growth rates between 7.5–13.3 nm/min were achieved at substrate temperatures of 250–350 °C. The AlO_x layers deposited at temperatures below 350 °C exhibit 3–5 at% residual carbon levels, however those grown at 350 °C exhibit only 1–2 at% carbon impurity. Reasonable dielectric properties were obtained in the latter, with a dielectric constant (κ) of ~ 7.0, breakdown field of ~ 9 MV/cm and relatively low leakage current density of ~ 8.3×10⁻¹⁰ A/cm².     

Development of novel grafted hybrid PVA membranes using glycidyltrimethylammonium chloride for pervaporation separation of water–isopropanol mixtures

Tetraethylorthosilicate incorporated hybrid poly(vinyl alcohol) membranes were grafted with glycidyltrimethylammonium chloride (GTMAC) in different mass%. The resulting membranes were subjected to physico-chemical investigations using Fourier transform infrared (FTIR) spectroscopy, wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA) and scanning electron microscopy (SEM). The effects of grafting and feed composition on pervaporation performance of the membranes were systematically investigated. The membrane containing 30 mass% of GTMAC exhibited the highest separation selectivity of 1570 with a flux of 1.92 × 10^?2 kg/m² h at 30 °C for 10 mass% of water in the feed. The total flux and flux of water are almost overlapping each other, manifesting that these membranes could be used effectively to break the azeotropic point of water–isopropanol mixtures. From the temperature dependent diffusion and permeation values, the Arrhenius activation parameters were estimated. The activation energy values obtained for water permeation (E_pw) are two to three times lower than those of isopropanol permeation (E_pIPA), suggesting that the developed membranes have higher separation ability for water–isopropanol system. The E_p and E_D values ranged between 63.73 and 33.07, and 62.78 and 32.75 kJ/mol, respectively. The positive heat of sorption (ΔH_s) values was obtained for all the membranes, suggesting that Henry's mode of sorption is predominant in the process.     

Development of a metallic/ceramic composite for the deposition of thin-film oxygen transport membrane

Asymmetric perovskite membranes have an attractive potential in the application of O₂/N₂ gas separation for future membrane-based power plants using oxyfuel technology. In this study – a metal-supported membrane structure with a thin-film perovskite layer and porous ceramic interlayers was developed. Porous NiCoCrAlY sintered at 1225 °C in H₂ was selected as the substrate based on a sufficient permeability and corrosion resistance in co-firing conditions. According to the oxidation behaviour of NiCoCrAlY, the temperature for co-firing of the substrate and the interlayers was defined as 1100 °C for 5 h in air. Two interlayers of La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ were applied by screen printing. The top layer was deposited by magnetron sputtering with a thickness of 3.8 μm. While gas-tightness was improved considerably, significant air-leakage was still detected. In summary, the successful development of a metal-perovskite-composite is shown, which acts as a basis for further development of a gas-tight metal-supported oxygen transport membrane structure.     

High performance BaCe0.8Y0.2O3−a (BCY) hollow fibre membranes for hydrogen permeation

In this work, BaCe_0.8Y_0.2O_3−α (BCY) perovskite hollow fibre membranes were fabricated by a phase inversion and sintering method. BCY powder was prepared by the sol–gel technique using ethylenediaminetetraacetic acid (EDTA) and citric acid as the complexing agents. Gel calcination was carried out at high temperature to form the desired crystal structure. The qualified BCY hollow fibre membranes could not be achieved even the sintering was carried out at temperatures up to 1550^oC due to the poor densification behavior of the BCY material. The addition of sintering aid (1 wt% Co₂O₃) inside BCY powder as the membrane starting material significantly improved the densification process, leading to the formation of gas-tight BCY hollow fibres. The optimum sintering temperature of BCY hollow fibre membrane was 1400 °C to achieve the best mechanical strength. H₂ permeation through the BCY hollow fibre membranes was carried out between 700 and 1050 °C using 25% H₂–He mixture as feed gas and N₂ as sweep gas, respectively. For comparison purpose, the disk-shaped BCY membrane with a thickness of 1 mm was also prepared. The measured H₂ permeation flux through the BCY hollow fibres reached up to 0.38 mL cm⁻² min⁻¹ at 1050 °C strikingly contrasting to the low values of less than 0.01 mL cm⁻² min⁻¹ from the disk-shaped membrane. After the permeation test, the microstructure of BCY hollow fibre membrane was still maintained well without signals of membrane disintegration or peeling off.     

Enhanced hollow fiber membrane performance via semi-dynamic layer-by-layer polyelectrolyte inner surface deposition for nanofiltration and forward osmosis applications

The layer-by-layer (LBL) polyelectrolyte deposited membranes have drawn increasing attention in various applications due to the ease of selective layer formation and their stability and versatility. In this study, the LBL deposition was performed at the inner surface of the polyethersulfone (PES) hollow fiber substrate to form composite nanofiltration (NF) membrane. The semi-dynamic deposition procedure was adopted with the aid of syringes. The newly developed inner deposited (id-LBL) membranes were then tested in NF and forward osmosis (FO) applications and the performance were compared with outer surface deposition as well as some literature data. The id-LBL membranes could not only withstand higher operating pressure but also possess superior hardness rejection especially in high concentration mixed salt solutions (more than 95% rejection to Mg²⁺ and Ca²⁺ in a 5000 ppm total dissolved salt (TDS) mixture under 4.8 bar). As for the FO process, with only two layer deposition, the id-LBL membranes also demonstrated significant performance improvement with increased water flux (up to 70 L/m² h using 0.5 M MgCl₂ as draw solution in active layer facing draw solution configuration) and reduced salt leakage (around 0.5 g/m² h using 1 M MgCl₂ draw solution in active layer facing feed water configuration). This study suggests that for hollow fiber substrate, the inner surface is more suitable for the formation of the selective layer via LBL deposition than the outer surface.     

Corrosion behavior of porous silicon nitride ceramics in different atmospheres

The corrosion behavior of silicon nitride (Si₃N₄) ceramics with a porosity of 46% at 1200–1500 °C under different conditions including dry O₂, O₂ containing 20 vol% H₂O and Ar containing 20 vol% H₂O is compared. The results show that porous Si₃N₄ ceramics exhibit good oxidation resistance up to 1200 °C. Their corrosion behavior varies depending on the temperature and atmosphere. Water vapor can obviously affect the morphology of the reaction product and thus accelerate the corrosion rate due to its specific inward diffusion mechanism and devitrified effect at high temperature. In view of the reaction kinetics, it proceeds in a diffusion-controlled manner in dry O₂ while follows the parabolic-linear law at water-containing atmosphere. Furthermore, a new model considering both oxidation and volatilization reactions is established. These provide a baseline for expanding the application fields of non-oxide porous ceramics such as Si₃N₄ and silicon carbide (SiC) etc.     

H2S absorption at high pressure using hollow fibre membrane contactors

In this article, the removal of H₂S from natural gas at high pressure using hollow fibre membrane contactors and water as the absorbent solvent was described and validated by a 2D comprehensive mathematical model. The modelling predictions were in a good agreement with the experimental data at low pressure of 1 bar under non-wetting conditions. However, the experimental behaviour at high pressure in the range of 10–50 bar was always lower than the modelling predictions. And thus, a sensitivity analysis was carried out to study the influence of gas, liquid, membrane diffusion and solubility coefficients of H₂S at high pressure upon the modelling behaviour compared with the experimental trend. The modelling results confirmed that these parameters were still insensitive as the model predictions changed slightly with altering these coefficients. Conversely, membrane wetting was confirmed to be an important factor even for small ratios, i.e. pseudo-wetting (1–3%). The model was able to predict the absorption of H₂S at higher pressures under 3% pseudo-wetting. Although the type of membranes used in this study was highly hydrophobic (ePTFE) and thus membrane wetting was not expected, pseudo-wetting phenomenon was confirmed to be an important factor at high pressure.     

Polyphosphates used for membrane scaling inhibition during water desalination by membrane distillation

The desalination of surface water (lake) was performed using direct contact membrane distillation. The membrane distillation process was carried out at 358 K. As a consequence of water heating the CaCO₃ deposit formed on the membrane surfaces, which resulted in a decrease in module efficiency. The polyphosphate antiscalant was used for restriction of carbonate deposition. In order to increase the scaling potential during the desalination process, the water was additionally enriched with bicarbonates (feed alkalinity 3.1 mmol HCO₃^–/dm³ and 4.5 mmol HCO₃^–/dm³). The membrane distillation with and without antiscalant was carried out to evaluate the scale inhibition effect. Various solution compositions (2–20 ppm) of the commercial polyphosphate based antiscalant (destined for reverse osmosis) and laboratory-grade sodium polyphosphate was used. SEM–EDS was used to investigate the chemical composition and morphology of the precipitate formed on the membrane surface. It was found that the formation of CaCO₃ crystallites was almost eliminated as a result of using antiscalant. However, a thin layer of amorphous deposits on the membrane surface was observed. As a results, a decline of the permeate flux was still observed. The initial module efficiency was restored by periodical rinsing of the membranes with diluted HCl solutions. The application of antiscalant minimized the penetration of deposit into the pores, and a high permeate flux was maintained over a period of 260 h of performed investigations when periodical rinsing with HCl solution was used.     

Fabrication and modification of cellulose acetate based mixed matrix membrane: Gas separation and physical properties

[Cellulose acetate (CA)-blend-multi walled carbon nano tubes (MWCNTs)] mixed matrix membranes (MMMs), [CA/polyethylene glycol (PEG)/MWCNTs] and [CA/styrene butadiene rubber (SBR)/MWCNTs] blend MMMs were prepared by solution casting method for gas separation applications using Tetrahydrofuran (THF) as solvent. Both raw-MWCNTs (R-MWCNTs) and functionalized carboxylic-MWCNTs (C-MWCNTs) were used in membrane preparation. The MWCNTs loading ratio and pressure effects on the gas separation performance of prepared membranes were investigated for pure He, N₂, CH₄ and CO₂ gases. Results indicated that utilizing C-MWCNT instead of R-MWCNTs in membrane fabrication has better performance and (CO₂/CH₄) and (CO₂/N₂) selectivity reached to 21.81 and 13.74 from 13.41 and 9.33 at 0.65 wt% of MWCNTs loading respectively. The effects of PEG and SBR on the gas transport performance and mechanical properties were also investigated. The highest CO₂/CH₄ selectivity at 2 bar pressure was reached to 53.98 for [CA/PEG/C-MWCNT] and 43.91 for [CA/SBR/C-MWCNT] blend MMMs at 0.5 wt% and 2 wt% MWCNTs loading ratio respectively. Moreover, increase of feed pressure led to membrane gas permeability and gas pair selectivity improvement for almost all prepared membranes. The mechanical properties analysis exhibited tensile modules improvement with increasing MWCNTs loading ratio and utilizing polymer blending.     

Effects of hypochlorite exposure on flux through polyethersulphone ultrafiltration membranes

Polyethersulphone ultrafiltration membranes with a nominal molecular weight cut off of 10 kDa were degraded in solutions of sodium hypochlorite over a range of pH values at 55 °C to achieve exposure measured in ppm-days of chlorine exposure. The degraded membranes were tested, using an ÄKTAcrossflow? system, for clean water flux, demineralised whey flux and protein rejection. The water fluxes for three membranes (new, 10,000 ppm-day pH 12, and 10,000 ppm-day pH 9) were found to be about 100, 200 and 400 L m^?2 h^?1, respectively with cross flow at 1 bar transmembrane pressure. However whey fluxes were about 23, 5, and 6 L m^?2 h^?1 for the same three membranes. Size exclusion chromatography of the permeates showed significant permeation of α-lactalbumin and β-lactoglobulin through membranes degraded at pH 9 for 20,000 ppm-days, while almost no permeation was found for degradation at pH 12.These results show that hypochlorite degradation affected fluxes by at least two mechanisms. It was likely that membrane pitting increased the pore size causing increased water flux and reduced protein rejection. However hypochlorite also seemed to alter the membrane surface properties, causing the protein to form a less permeable layer that reduced the flux of whey.