Osmotic distillation of oily feeds

This paper describes the development of modified hydrophobic membranes for osmotic distillation (OD) which are tolerant to oily feeds. Three commercial membranes were chosen as substrates including the Celgard 2500, Millipore GVSP and the UPVP. The focus has been on using PVA coatings which were found to have an insignificant effect on flux.For concentrated sucrose solutions, flux was reduced significantly by viscosity-related concentration polarisation effects. The presence of the coating did not increase this effect, suggesting that the flux limitation mostly occurred adjacent to the feed-side of the laminate and not within the top-layer or inside the pores.The laminate membrane is a necessity for oily feeds since the uncoated membranes were promptly wetted out even for low concentration of oil (limonene) dispersion in water. The laminate membranes were all stable in oil emulsions when tested for periods up to 24 h; i.e. the membranes did not wet out during flux measurement and no visual damage nor coating detachment was observed for laminates. These observations confirm the efficacy of having coated membranes for OD of oily feeds.     

Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement

New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.     

Hydrophobic modification of poly(phthalazinone ether sulfone ketone) hollow fiber membrane for vacuum membrane distillation

New hydrophobic poly(phthalazinone ether sulfone ketone) (PPESK) hollow fiber composite membranes coated with silicone rubber and with sol–gel polytrifluoropropylsiloxane were obtained by surface-coated modification method. The effects of coating time, coating temperature and the concentration of silicone rubber solution on the vacuum membrane distillation (VMD) properties of silicone rubber coated membranes were investigated. It was found that high water permeate flux could be gotten in low temperature and low concentration of silicone rubber solution. When the coating temperature is 60 °C, the coating time is 9 h and the concentration of silicone rubber solution is 5 g L⁻¹ the water permeate flux of the silicone rubber coated membrane is 3.5 L m⁻² h⁻¹. The prepolymerization time influence the performance of polytrifluoropropylsiloxane coated membranes, and higher prepolymerization time decrease the water permeate flux of the membrane. The water permeate flux and the salt rejection was 3.7 L m⁻² h⁻¹ and 94.6%, respectively in 30 min prepolymerization period. The VMD performances of two composite membranes during long-term operation were studied, and the results indicated that the VMD performances of two composite membranes are quite stable. The salt rejection of silicone rubber coated membrane decreased from 99 to 95% and the water permeate flux fluctuated between 2.0 and 2.5 L m⁻² h⁻¹. The salt rejection of polytrifluoropropylsiloxane coated membrane decreased from 98 to 94% and the water permeate flux fluctuated in 1 L m⁻² h⁻¹ range.     

Bio‐inspired method to fabricate poly‐dopamine/reduced graphene oxide composite membranes for dyes and heavy metal ion removal

Due to the narrow layer spacing, graphene oxide (GO) composite membrane usually exhibits a relatively low water flux in the process of wastewater treatment. In this study, GO was reduced to reduced graphene oxide through a bio‐inspired method, which was functionalized modified by poly‐dopamine (PDA). Then a series of PDA/reduced graphene oxide sheet films were prepared by vacuum filtration on the surface of cellulose acetate membrane (under the pressure of −0.1 MPa). The result indicated that the novel membranes had an excellent stability owing to the cross‐link of PDA. In addition, the hydrophilicity of membrane was increased significantly after PDA modification, which presented a superior water flux than pure GO composite membrane. More importantly, as‐prepared membranes were successfully applied for the removal of dyes (including Congo red, methylene blue, and rhodamine B) and heavy mental ion (Cu(II)) from simulated wastewater. This work might provide a new method for preparation and application of GO composite membranes.     

混合导体透氧膜反应器中甲烷部分氧化反应的催化行为(英文)

应用组成为Ba0.5Sr0.5Co0.8Fe0.2O3-(的钙钛矿型混合导体陶瓷膜制成膜反应器。该膜在进行氧分离的同时具有活化甲烷氧化偶联的催化功能。随着温度升高和膜的富氧端氧分压的增大,透氧量有所增加。在空气、氦气的氧分压梯度下,850(C,膜厚度为1.5 mm时,JO2可达到1.2 mL/(cm3(min)。同时在800(C~900(C温度范围内,该膜对于甲烷转化为乙烷和乙烯一般只具有0.5%~3.5%的低转化率,而选择性可达40%~70%。在反应尾气中发现了大量的未反应的分子氧,说明过量的氧与甲烷未经催化反应的气相反应导致了C2的选择性相对较低。OCM膜反应模式情况下的透氧量与空气、氦气梯度情况下的透氧量相比只有微小增加,这与POM膜反应模式情况下透氧量大量增加显著不同。     

Experimental verification of pressure drop models in hollow fiber membrane

A new experimental method to obtain internal pressure profiles in a hollow fiber membrane was demonstrated. The experimentally obtained internal pressure profiles were compared with the theoretically calculated ones based on Hagen–Poiseuille equations. The experimental and theoretical results agreed very well in clean water conditions only when accurate membrane permeabilities and effective internal diameters were available. New experimental methods to obtain the two parameters were demonstrated. The same experimental technique was also applied for the submerged hollow fiber membranes filtering activated sludge to find out how internal pressure profiles were changing with time. Based on the pressure profiles, evidences that indicated the local flux near membrane exit was lower than those in adjacent area were found. This observation contradicted to the filtration models based on critical flux concept. It was considered that the cake layer collapse near the membrane exit was the cause. Though there was some degree of delay in pressures detection, the method demonstrated in this study provided a great accuracy when pressure profiles did not change rapidly.     

Critical flux in NF of high molar mass polysaccharides and effluents from the paper industry

High molar mass polysaccharides (locust bean gum and karaya gum) and effluents from a mechanical pulp mill and a paper mill were nanofiltered with commercially available nanofiltration (NF) membranes. The effect of the filtration conditions on the flux (critical flux), retention, and the fouling of the membranes was studied. The experiments were conducted by increasing and decreasing the pressure and measuring the flux thus obtained.

The critical flux was observed to increase with increasing flow velocity and decreasing concentration. An increase in pH increased the electrostatic repulsion between the membrane and the dissociated (charged) components in the paper mill effluents. As a result, a higher critical flux was obtained and also the retentions of the charged substances improved. Only a weak form of the critical flux was observed with the mill effluents. The permeate flux deviated from the pure water flux even at the lowest pressure, but increased linearly with pressure until the weak form of the critical flux was exceeded. The small decrease in flux immediately after filtration was started was probably caused by the plugging of the free spaces in the membranes or by the adsorption of foulants onto the membrane surface.

In the filtrations with the high molar mass polysaccharides, a strong form of the critical flux as well as a weak form were observed. The significant irreversible fouling of the most hydrophobic membrane was due to adsorption of the model substances by hydrophobic interaction. A precleaning of the membranes with an alkaline cleaning agent improved the pure water fluxes by up to 30%, but it had only a small effect on the critical or the limiting flux. The pure water fluxes of precleaned membranes after filtration were still higher than the pure water fluxes of the untreated membranes before filtration.     

Tracer permeabilities underestimate transmembrane solute flux under a concentration gradient

The measured flux of sucrose across Cuprophan dialysis membrane in the presence of a concentration gradient is significantly greater than the flux predicted from radioactive tracer permeabilities measured in the absence of a concentration gradient. The difference can be explained if the membrane is heteroporous. Similar results are expected for other heterogeneous biological or artificial membranes.     

Thermal effects in osmotic distillation

Osmotic distillation (OD) is a concentration technique for aqueous mixtures based on porous hydrophobic membranes in contact on both sides with liquid phases at pressure lower than the pressure needed to displace the gas phase in the pores. The driving force for the water vapour diffusion through the gas phase immobilised within the membrane pores is sustained by an activity difference by using a hypertonic solution, typically concentrated brines, downstream the membrane. The mass transfer causes a cooling down of the feed and a warm up of the brine, as a consequence a temperature difference is created which reduces the effective driving force for mass transfer. This ‘thermal effect’ is investigated both theoretically and experimentally, it is shown that the effect on the flux is substantial.     

聚N-乙烯基吡咯烷酮对癸二酸-二苯并-18-冠-6共聚物薄膜性能的影响

分子量较高的癸二酸 二苯并 １８ 冠 ６共聚物可以制成具有一定韧性的薄膜,但该薄膜的透过性不好,当加入少量的水溶性聚合物聚乙烯吡咯烷酮（ＰＶＰ）后,薄膜的离子络合速度大幅度加快,并保持了对Ｋ＋较高选择性．同时研究了合金膜中ＰＶＰ的溶出量与温度及时间关系,以及合金膜的动态力学行为     

Synthesis and modeling of composite poly (styrene-co-acrylonitrile) membrane for the separation of chromic acid

In this work, we have prepared a composite styrene acrylonitrile (SAN) membrane on a ceramic clay plate by coating a prepolymer solution prepared using a dual initiator system. This membrane is chemically modified by gas phase nitration followed by amination and quaternization to make it charged and has been characterized by FTIR, SEM, contact angle measurements, AFM, water content, water permeability measurements and molecular weight cut-off experiments. The membrane has been further characterized using chromic acid rejection (real and observed) at different pressures, feed concentrations and pH. The modified membrane is found to possess a real rejection of above 90% with high water flux at low pressure drop.     

Nanostructured‐silver‐coated polyetheretherketone tube for online in‐tube solid‐phase microextraction coupled with high‐performance liquid chromatography

Polyetheretherketone tube is a better substrate for in‐tube solid‐phase microextraction than fused‐silica capillary and metal tube because of its resistance to high pressure and good flexibility. It was modified with a nanostructured silver coating, and characterized by scanning electron microscopy and energy dispersive X‐ray spectroscopy. It was connected into high‐performance liquid chromatography equipment to build the online analysis system by replacing the sample loop of a six‐port injection valve. To get the highest extraction capacity, the preparation conditions of the coating was investigated. Important extraction conditions including length of tube, sample volume, and desorption time were optimized using eight polycyclic aromatic hydrocarbons as model analytes. The tube exhibits excellent extraction efficiency toward them, with enrichment factors from 52 to 363. The online analysis method provides good linearity (0.5–100 or 1.0–100 μg/L) and low detection limits (0.15–0.30 μg/L). It has been used to determine polycyclic aromatic hydrocarbons in water samples, with relative recoveries in the range of 92.3–120%. The tube showed highest extraction ability for polycyclic aromatic hydrocarbons, higher extraction ability for hydrophobic phthalates and anilines, and almost no extraction ability for low hydrophobic phenols, due to the possible extraction mechanism including hydrophobic and electron‐rich element‐metal interactions.     

The application of polypropylene membranes for production of fresh water from brines by membrane distillation

Scaling and wettability of hydrophobic membranes were studied during the membrane distillation applied for the production of fresh water from the concentrated salt solutions. The studies were performed with the use of membrane modules in which the capillary membranes from polypropylene were assembled. A saline ground water containing several sparingly soluble salts was used as a feeding solution. The presence of such compounds caused an intensive surface and internal scaling. Due to the scaling, a partial wetting of the membrane walls and the permeation of salts into distillate were observed. These phenomena were eliminated for the membranes with thicker walls when the amount of deposit was limited by a periodic rinsing of the module with water. During this study, the feed was concentrated up to the supersaturation state, which caused a salt crystallization on the membrane surface, and as a consequence, the permeate flux was reduced to zero. In this case, the internal scaling can be limited using the capillary membranes with a net covering their surface.     

Oxygen and water vapor permeability of fully substituted long chain cellulose esters (LCCE)

Fully-substituted cellulose esters with acyl substituents ranging in size from C2 to C18 were synthesized using the acyl chloride method. Films were prepared from the purified esters by either solvent-casting or compression-molding at elevated temperatures. Oxygen and water vapor permeability was determined under different conditions of pressure and moisture. The relationship between cellulose ester structure and barrier properties was examined. The results revealed linear relationships between water vapor and oxygen permeabilities and molar ester substituent volume as well as several structural factors relating to polymer polarity and hydrophobicity, such as aliphatic (methylene) content, solubility parameter, and contact angle. Films from long chain cellulose esters (LCCE) with acyl substituents in the size range between C8 and C18 were found to represent effective barriers to water vapor transport while their obstruction to the transfer of oxygen remained low. It was concluded that the hydrophobic nature of LCCEs is responsible for the control of water vapor transport, and that spatial factors dominate the transfer of oxygen.     

Effects of membrane hydrophilicity on the removal of a trihalomethane via micellar-enhanced ultrafiltration process

Micellar-enhanced ultrafiltration (MEUF) process was explored for obtaining pure water from an aqueous solution containing small amount of trihalomethanes (THMs). A homologous series of polyethylene glycol alkylether was used as nonionic surfactant. To understand effects of membrane hydrophilicity on the performance of MEUF process, membranes for the ultrafiltration were prepared from polysulfone blends containing various amount of a hydrophilic copolymer, poly(1-vinylpyrrolidone-co-acrylonitrile) (P(VP-AN)). An increase in the permeate flux was observed with an increase of the membrane hydrophilicity. The performance of MEUF process in removing THM and surfactant was shown to depend on the membrane characteristics, surfactant characteristics, and operating pressure. The rejections of THM and surfactant were increased with increasing hydrophobicity of surfactant and hydrophilicity of membrane. The rejections of THM examined with hydrophilic membranes were increased with increasing operating pressure, while those examined with hydrophobic membranes were decreased with increasing operating pressure. THM included in water could be removed up to 99% via MEUF process. The performance of MEUF examined with hydrophilic membranes could be explained with the rejection of micelles containing THM, while that examined with hydrophobic membranes could be explained with hydrophobic interactions between surfactant and membrane materials.     

Alginate-coated microporous PTFE membranes for use in the osmotic distillation of oily feeds

Osmotic distillation (OD) has two main advantages over thermally driven concentration processes attributable to its ambient temperature operation. These are maintenance of the integrity of thermally labile components and minimisation of the loss of volatile flavour/fragrance components. However, a major disadvantage of osmotic distillation is the potential for wet-out of the hydrophobic membrane when fouled by surface-active agents such as citrus oils. In this work, sodium alginate hydrogel coatings were applied to PTFE membranes for protection against wet-out. The coating technique developed for this purpose resulted in a 10-fold increase in adhesion strength over that achievable by simple casting. This was effected by increased intrusion of the coating solution meniscus into the porous PTFE structure by surface tension adjustment with ethanol, precipitation of sodium alginate by the selective removal of water, and finally alginate crosslinking. Precipitation occurred both on the surface and in the void spaces between the PTFE fibres, thereby providing better anchorage for the coating. The coating decreased the overall OD mass transfer coefficient by less than 5%. OD flux measurements using coated membranes with 0.2, 0.4 and 0.8 wt.% orange oil–water mixtures over a period of 300 min indicated that the coating was successful in protecting the membrane against wet-out. An uncoated membrane was immediately wet out by a 0.2 wt.% orange oil–water OD feed. In a separate trial, a coated membrane retained its integrity after contact with a 1.2 wt.% oil–water mixture for 72 h.     

Microfiltration for separation of green algae from water

Cross-flow microfiltration was used for separation of green algae, Chlorella sp., from freshwater. The transmembrane pressure (TMP) was adjusted at 40, 50 and 60 kPa, respectively. The cross-flow velocity was set at 0.43 m/s for laminar flow and 0.84 m/s for turbulent flow, respectively. The results showed that flux increased as TMP increased from 40 to 50 kPa. But drastic flux decline was observed when operating at TMP of 60 kPa. Raising cross-flow velocity increased the initial flux of MF under TMP of 60 kPa. Nevertheless, implementing turbulent cross-flow did not improve the drastic flux decline under the highest TMP. Preozonation increased the dissolved organic carbon, decreased algal viability and made the size of algal cells smaller. It also increased dissolved polysaccharide that derived from extracellular organic matter (EOM). Different effects of preozonation on flux behavior of MF were observed when utilizing hydrophobic and hydrophilic membrane. Generally speaking, preozonation improved performance of microfiltration by reducing cake compressibility and the biomass loading when both membranes were used. However, dissolved polysaccharide released during preozonation was adsorbed onto the hydrophobic membrane. Consequently, fouling resistance of the hydrophobic membrane became higher. These arguments were verified by classification of hydrodynamic resistances.     

Membrane modification to avoid wettability changes due to protein adsorption in an emulsion/membrane bioreactor

This study addresses problems encountered with an emulsion/membrane bioreactor. In this reactor, enzyme- (lipase) catalyzed hydrolysis in an emulsion was combined with two in-line separation steps. One is carried out with a hydrophilic membrane, to separate the water phase, the other with a hydrophobic membrane, to separate the oil phase. In the absence of enzyme, sunflower oil/water emulsions with an oil fraction between 0.3 and 0.7 could be separated with both membranes operating simultaneously. However, two problems arose with emulsions containing lipase. First, the flux through both the hydrophilic and the hydrophobic membranes decreased with exposure to the enzyme. Second, the hydrophobic membrane showed a loss of selectivity demonstrated by permeation of both the oil phase and the water phase through the hydrophobic membrane at low transmembrane pressure. These phenomena can be explained by protein (i.e. lipase) adsorption to the polymer surface within the pores of the membrane. It was proven that lipase was present at the hydrophilic membrane and that this, in part, explains the flux decrease of the hydrophilic membrane. To prevent the observed loss of selectivity with exposure to protein, the hydrophobic polypropylene membrane (Enka) was modified with block copolymers of propylene oxide (PO) and ethylene oxide (EO). These block copolymers act as a steric hindrance for proteins that come near the surface. The modification was successful: After 10 days of continuous operation the minimum transmembrane pressure at which water could permeate through an F 108-modified membrane was 0.5 bar, the same value as that observed in the beginning of the experiment. This indicates that loss of selectivity due to protein adsorption is prevented by the modification of the membrane.     

PEG-coated reverse osmosis membranes: Desalination properties and fouling resistance

This study focuses on the use of surface-coated reverse osmosis (RO) membranes to reduce membrane fouling in produced water purification. A series of crosslinked PEG-based hydrogels were synthesized using poly(ethylene glycol) diacrylate as the crosslinker and poly(ethylene glycol) acrylate, 2-hydroxyethyl acrylate, or acrylic acid as comonomers. The hydrogels were highly water permeable, with water permeabilities ranging from 10.0 to 17.8 (L μm)/(m² h bar). The hydrogels were applied to a commercial RO membrane (AG brackish water RO membrane from GE Water and Process Technologies). The water flux of coated membranes and a series-resistance model were used to estimate coating thickness; the coatings were approximately 2 μm thick. NaCl rejection for both uncoated and coated membranes was 99.0% or greater, and coating the membranes appeared to increase salt rejection, in contrast to predictions from the series-resistance model. Zeta potential measurements showed a small reduction in the negative charge of coated membranes relative to uncoated RO membranes. Model oil/water emulsions were used to probe membrane fouling. Emulsions were prepared with either a cationic or an anionic surfactant. Surfactant charge played a significant role in membrane fouling even in the absence of oil. A cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), caused a strong decline in water flux while an anionic surfactant, sodium dodecyl sulfate (SDS), resulted in little or no flux decline. In the presence of DTAB, the AG RO membrane water flux immediately dropped to 30% of its initial value, but in the presence of SDS, its water flux gradually decreased to 74% of its initial value after 24 h. DTAB-fouled membranes had lower salt rejection than membranes not exposed to DTAB. In contrast, SDS-fouled membranes had higher salt rejection than membranes not exposed to SDS, with rejection values increasing, in some cases, from 99.0 to 99.8% or higher. In both surfactant tests, coated membranes exhibited less flux decline than uncoated AG RO membranes. Additionally, coated membranes experienced little fouling in the presence of an oil/water emulsion prepared from DTAB and n-decane. For example, after 24 h the water flux of the AG RO membrane fell to 26% of its initial value, while the water flux of a PEGDA-coated AG RO membrane was 73% of its initial value.     

Hydrophilic modification of polypropylene ultrafiltration membrane by air‐assisted polydopamine coating

Coating by a mussel inspired polydopamine (PDA) is a simple and promising strategy to modify the hydrophilicity of polymer membrane surfaces. In this work, PDA coating was used to modify polypropylene (PP) ultrafiltration hollow fiber membrane. PDA coating parameters, ie, solution concentration and coating time were varied, and the effect of those parameters on membrane morphology, porosity, water contact angle, and pure water flux was investigated. In addition, air‐assisted PDA coating process was also conducted by channelling the air through PP membrane to avoid pore blocking and prevent water flux decline. The results showed that PDA coating successfully improved the hydrophilicity of PP membrane indicated by the decrease of water contact angle from 110° to 67° after coated by 3 g/L of PDA solution for 3 hours. The addition of air permeation on membrane lumen also increased pure water flux up to 511.2 L/m².h, a 270% increase from unmodified PP membrane. It might be associated to the pore blocking prevention that has been proven by SEM image and the membrane porosity that was increased about 4%.