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.     

Analysis and optimization of the influence of operating conditions in the ultrafiltration of macromolecules using a response surface methodological approach

In this work, the ultrafiltration of macromolecules was analysed using a response surface methodological approach. The behaviour of two different inorganic membranes was investigated. The membranes selected were a Carbosep M2 membrane (Orelis, France) with a molecular weight cut-off (MWCO) of 15 kDa and a Tami MSKT membrane (Tami Industries, France) with a MWCO of 5 kDa. The solute employed was polyethylene glycol of 35 kDa molecular weight. The influence of transmembrane pressure (0.1, 0.2, 0.3, 0.4 and 0.5 MPa), crossflow velocity (1, 2 and 3 m/s) and feed concentration (5, 10 and 15 g/L) on permeate flux and permeate flux decline was investigated. Analysis of variance was proved to be a useful tool to determine the effect of operating variables on both parameters. The method used demonstrated the presence of coupled effects between factors as well as squared effects that are relevant to the ultrafiltration process. The surface contours obtained from fitted models were used for the optimization of the operating conditions. The goal was to simultaneously maximize the average permeate flux and minimize the flux decline. The optimal operating conditions for the Carbosep M2 membrane were a transmembrane pressure of 0.38 MPa and a crossflow velocity of 3 m/s. The optimal operating conditions for the Tami MSKT membrane could not be determined by means of multiple response optimization due to the low accuracy of the regression model obtained for the cumulative permeate flux decline (SFD) response variable.     

Phenolic composition and antioxidant activity of the aqueous extract of bark from residues from mate tree (Ilex paraguariensis St. Hil.) bark harvesting concentrated by nanofiltration

The goal of this study was to determine the best conditions to obtain the highest total polyphenol content from mate bark aqueous extract and investigate the phenolic composition and antioxidant activity of the concentrate obtained during nanofiltration. The Response Surface was employed to determine the optimum condition for extraction of polyphenolics from mate bark aqueous extract. The extract obtained using the best conditions (a temperature of 85 ± 5 °C and with extraction time of 1.5 min) contained approximately 1.6 mg/mL of chlorogenic acid equivalent, and was subjected to nanofiltration. The total polyphenol content values in the permeate and in the concentrate collected at different volume reduction factors (VRF of 1.5–6) were different from those detected in the optimized extract (unfiltered). The concentration of phenolic compounds and antioxidant activity increased when VRF was increased. The major compounds detected in optimized mate bark extract and its concentrates (VRF 4 and 6) were chlorogenic acid and epigallocatechin gallate, which can be related to the high antioxidant activity of mate bark aqueous extract.     

Clarification of Stevia extract by ultrafiltration: Selection criteria of the membrane and effects of operating conditions

Clarification of pre-treated Stevia extract using ultrafiltration is presented in this study. Performance of four different ultrafiltration membranes, namely 5, 10, 30 and 100 kDa were investigated in terms of permeate flux and permeate quality, mainly the recovery of Stevioside in the permeate. In this regard, the 30 kDa membrane was found to be most suitable. A systematic set of experiments under steady state were conducted to analyze the effects of the operating conditions, transmembrane pressure drop and stirrer speed on the permeate flux and permeate quality. Steady state was reached in between 8 and 16 min depending on the operating conditions. Steady state was reached earlier at higher stirring speed. A simple resistance in series model was used to quantify the fouling resistance. Membrane resistance was found to be negligible compared to the fouling resistance. It was a strong function of the stirrer speed but remained almost invariant with transmembrane pressure drop. The steady state permeate flux increased with pressure drop as well as the stirrer speed. 45% average recovery of Stevioside was obtained during stirred steady state experiments at lower operating pressures (276 and 414 kPa). At higher operating pressures, recovery of Stevioside in the permeate decreased remarkably.     

Detoxification of olive mill wastewaters by zinc–aluminium layered double hydroxides

Disposal of olive mill waste waters (OMWW) represents a serious problem due to its elevated toxicity and pollution risk for environmental and aquatic bodies, which is mainly related to its polyphenol content. The use of layered double hydroxides (LDHs) or anionic clays for OMWW detoxification may represent an effective remediation process. In the current study, OMWW was treated with cold methanol to obtain a dark humic acid-like precipitate made of high molecular weight organic compounds named polymerin and a supernatant made of low molecular weight organics named OMWW-S. After precipitation with methanol, phenol content decreased from 2.14 g L^?1 to 0.80 and 1.20 g L^?1, in polymerin and OMWW-S, respectively. The chemical oxygen demand of OMWW-S decreased by about 30% with respect to raw OMWW, from 62.2 to 40.2 g L^?1. A phenolic concentrate, named OMWW-E was also obtained by ethyl acetate extraction of OMWW-S at pH 2.0. LDHs of zinc and aluminium (LDH) were obtained at room temperature and after calcination at 450 °C (LDH-450). An organo-mineral complex of LDH with polymerin (LDH-Pol) was also synthesised. Phenols from OMWW-E were sorbed preferentially on three matrices according to the following order: LDH-450 > LDH > LDH-Pol, with percent sorption of up to 73% for LDH-450. Repeated cycles of sorption of OMWW-S onto LDH-450 decreased phenol concentration by about 90%. Use of remediated water in a phytotoxicity bioassay revealed scant inhibition of germination and coleoptile elongation rate in tomato seeds and seedlings. Thus, OMWW-S after sorption onto LDH-450 could represent a low cost source of irrigation water for arid soil of the Mediterranean region.     

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.     

Fabrication and testing of a planar microstructured concept module with integrated palladium membranes

A planar microstructured hydrogen separation module has been fabricated to study the hydrogen permeation through free-standing palladium-based membranes (Pd, PdCu and PdAg) with minimal influence by concentration polarization.The membranes were laser-welded directly between two face-to-face arranged stainless-steel sheets with 10 microchannels each (width × depth × length of the channels: 500 μm × 300 μm × 2 cm).Single gas hydrogen and mixed gas permeation experiments (H₂/N₂) were conducted between 300 and 400 °C. The permeabilities and activation energies of the membranes in this temperature range were calculated.The 12.5 μm thick membrane was successfully tested up to 650 kPa, indicating that the microchannel plates provide a good mechanical support even for very thin membranes. However, settling of the film into the microchannels on the permeate side was observed due to the overpressure on the retentate side suggesting even finer channels and eventually the use of an additional porous support for very high differential pressure.The concentration polarization effects in the membrane module were evaluated in terms of the film effectiveness factor η which is a familiar concept from heterogeneous catalysis. It could be shown that the microchannel configuration effectively decreases concentration polarization.     

Hydrophilic modification of polyacrylonitrile membranes by oxyfluorination

The surface of polyacrylonitrile (PAN) membranes was modified by oxyfluorination with various conditions to improve its wettability. The membranes were characterized in terms of morphology, structure, hydrophilicity, and membrane performance. The properties and functional groups on the surface of PAN membranes were investigated by contact angle, SEM, ATR-IR and XPS. And permeability of PAN membranes was compared by permeating pure water flux through membrane surface under 100, 150 and 200 kPa pressure. Oxyfluorination introduced oxygen contained functional groups such as the carboxylic acid groups that help increment of wettability on the surface of PAN membrane. Water flux of oxyfluorinated PAN UF membrane increased 20% at pure water permeation pressure 200 kPa compared to that of untreated PAN UF membrane.     

Pressure-assisted self-assembly technique for fabricating composite membranes consisting of highly ordered selective laminate layers of amphiphilic graphene oxide

We prepare highly ordered flexible layers of graphene oxide (GO) on modified polyacrylonitrile substrates by the pressure-assisted self-assembly technique. This composite membrane shows excellent performance during the pervaporation separation of a 70 wt.% isopropyl alcohol (IPA)/water mixture: 99.5 wt% water in permeate and 2047 g m⁻² h⁻¹ permeation flux. Despite the specific GO deposition increase from 4.3 to 43.3 × 10⁻⁵ g cm⁻² (ninefold layer thickness growth), its effect on the permeation flux is not significant, as manifested by only a little decrease in the flux. At 70 °C feed temperature, the permeate water concentration remains 99.5 wt% and the permeation flux reaches 4137 g m⁻² h⁻¹. The high selectivity may be due to the dense GO film consisting of highly ordered and packed laminates, allowing water but inhibiting IPA molecules to pass through. GO is demonstrated to be amphiphilic: water molecules adsorb first at the hydrophilic edge (hydroxides) and then rapidly diffuse through the hydrophobic core (mainly carbon), forming a water passage channel that promotes high permeation flux. When water molecules permeate through the GO layers, they accumulate and form a monolayer structure that pushes the successive layers away from each other, leading to widening of the d-spacing.     

Recovery of phenolic compounds from orange press liquor by nanofiltration

This research was undertaken in order to evaluate the potential of a nanofiltration (NF) process for the separation and concentration of phenolic compounds from press liquors obtained by pigmented orange peels. Four different spiral-wound NF membranes, characterised by different molecular weight cut-off (MWCO) (250, 300, 400 and 1000 Da) and polymeric material (polyamide, polypiperazine amide and polyethersulphone), were investigated. The rejection of the investigated membranes towards anthocyanins, flavonoids and sugars was evaluated in order to identify a suitable membrane to separate phenolic compounds from sugars. The performance of the investigated NF membranes was also evaluated in terms of permeate flux and antifouling performance.The obtained results indicated a reduction of the average rejection towards sugars by increasing the MWCO of the selected membranes, while the rejection towards anthocyanins remained higher than 89% for all the NF membranes investigated. The NFPES10 membrane showed the lowest average rejection towards sugar compounds and high rejections towards anthocyanins (89.2%) and flavonoids (70%). Permeate flux values at lower transmembrane pressures were also favourably high compared to the other NF membranes.     

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.     

Evaluation of commercial PTFE membranes in desalination by direct contact membrane distillation

In this study, nine flat-sheet commercially available hydrophobic PTFE membranes were used in desalination by direct contact membrane distillation and their characteristics were investigated under different operating conditions including feed temperature, feed flow rate, cold stream flow rate, and feed concentration. Membrane properties, i.e. pore size, thickness, support layer, and salt rejection were also studied. Moreover, membrane module designs including flow arrangements (co-current, counter-current and tangential) for process liquid and depth both on hot and cold sides were tested experimentally. Finally, the long-term performance of the selected membranes for direct contact membrane distillation as a stand-alone desalination process was investigated. The results indicated that increasing feed temperature, hot feed flow rate, and module depth on the cold side led to increase permeate flux. On the other hand, increasing membrane thickness and module depth on the hot side (at constant flow rate) had negative effects on the flux. The highest permeation flux and salt rejection was achieved when the membranes with a pore size of 0.22 μm were used in the cross-current follow arrangement of hot and cold streams. In addition, the requirements for support layer for a successful DCMD process has been extensively discussed.     

Experimental investigation and mathematical modeling of oxygen permeation through dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite-type ceramic membranes

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) perovskite powder was synthesized via EDTA/citrate complexation method. BSCF membranes were formed by pressing powder at 400 MPa and sintering at 1100 °C for 10 h. XRD patterns showed that a high pure powder with cubic structure was obtained. SEM micrographs revealed that the membranes are dense with large grains. Effects of temperature, feed and permeate side oxygen partial pressures, flow rates and membrane thickness on oxygen permeation flux were studied experimentally. A Nernst–Planck based mathematical model, including surface exchange kinetics and bulk diffusion, was developed to predict oxygen permeation flux. Considering non-elementary surface reactions and introducing system hydrodynamics into the model resulted in an excellent agreement (RMSD = 0.0617, AAD = 0.0487 and R² = 0.985) between predicted and measured fluxes. The results showed that oxygen permeation flux increases with temperature, feed side oxygen partial pressure and flow rates, however decreases with permeate side oxygen partial pressure and membrane thickness. Contribution of feed side surface exchange reactions, bulk diffusion and permeate side surface exchange reactions resistances in the total resistance are in the range of 8–32%, 10–81% and 11–59%, respectively. Permeation rate-limiting step was determined using the membrane dimensionless characteristic thickness.     

Modification of a hollow fiber membrane and its three-dimensional analysis of surface pores and internal structure for a water reclamation system

A modified hollow fiber membrane with organic solvents was used for a membrane separation reactor to configure a water reclamation system. Changes in the surface and inner pores of the modified follow fiber membrane were analyzed three dimensionally. The results from the operation of the membrane separation reactor with MLSS of 7200 mg/l for 120 days were compared with those from pure water. Monitoring changes in permeate flux and separation efficiency, we made an effort to predict the possibility of back wash and the breakthrough point. During the initial operation, contamination of membrane surface was increased gradually without changes in inner pores whereas a long-term operation exhibited a decrease in inner pores and a change in microfibril, suggesting that there would be a rare possibility for backwashing. As the suction pressure was raised from 1 atm to greater than 2 atm due to the increased membrane surface contamination with 40 days of operation, the permeate flux and suction pressure were required to be continuously monitored. The results of more than 100 days of operation suggested that backwashing was not possible due to fouling.     

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.     

Biomass assisted microfiltration of chromium(VI) using Baker's yeast by ceramic membrane prepared from low cost raw materials

This work deals with the preparation of ceramic microfiltration membrane from inexpensive raw materials such as kaolin, quartz, calcium carbonate by uniaxial dry compaction method. The prepared green membrane was initially dried at 100 °C for 24 h, 200 °C for 24 h and finally sintered at 900 °C for 6 h. The properties of the membrane such as porosity, flexural strength, chemical stability and hydraulic permeability were investigated. The fabricated membrane possessed an average pore diameter of 1.32 μm, porosity of 30% and flexural strength of 34 MPa. Furthermore, the chemical stability of the membrane was found to be excellent. Eventually, the separation performance of the membrane in terms of flux and removal of chromium(VI) ion using baker's yeast biomass as a function of applied pressure, pH, metal ion concentration and biomass dosage was also studied. The removal of Cr(VI) was found to be strongly dependent on the initial pH of the solution. At lower pH, the metal solution shows higher removal due to higher binding of the metal ion with biomass. It was also observed that the removal of Cr(VI) ion increases with increasing the biomass concentration and decreases with increasing the metal ion concentration. The removal of Cr(VI) was found to be independent of the applied pressure. The maximum removal of Cr(VI) was found to be 94% with the permeate flux of 2.07 × 10^-5 (m³/m² s) for a metal solution concentration of 100 mg/L.     

Non-fluorinated hybrid composite membranes based on polyethylene glycol functionalized polyhedral oligomeric silsesquioxane [PPOSS] and sulfonated poly(ether ether ketone) [SPEEK] for fuel cell applications

Novel hybrid composite membranes were prepared by blending poly(ethylene glycol) functionalized polyhedral oligomeric silsesquioxane [PPOSS] as nanofiller in varying concentration ranging from 1 to 5% (w/w) into sulfonated poly(ether ether ketone) [SPEEK] with degree of sulfonation ～55% for proton exchange membrane fuel cells [PEMFCs]. The effect of incorporation of PPOSS into SPEEK matrix was investigated in terms of thermomechanical and morphological properties, water uptake and proton conductivity of SPEEK. All the composite membranes were thermally and mechanically stable up to 250 °C. Transmission electron microscopy (TEM) revealed that the smallest particle size (～100 nm) of PPOSS was found for SPEEK membranes containing 2% (w/w) PPOSS where as agglomeration (～300 nm) was observed at higher loadings of PPOSS. The proton conductivity was found to be dependent on the morphology and was independent of the amount of water present in the membranes. At 100 °C and 100% RH, the highest proton conductivity (47 mS/cm compared 34 mS/cm for neat SPEEK i.e. an increase of ～51%) was recorded at 2% (w/w) PPOSS contents followed by a decrease on further addition of PPOSS.The water uptake of composite membranes increased with concentration of PPOSS while maintaining their hydrolytic stability at 100 °C for more than 24 h.     

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.     

Influence of cold stabilisation and chill membrane filtration on volatile compounds of apricot brandy

The aim of this research was to evaluate the influence of cold stabilisation treatment and chill membrane filtration on apricot brandy's stability and its volatile compounds. Cold stabilisation treatment included exposure of the brandy to the temperature of ?1 °C during 24 h. Membranes with pore sizes of 200, 450 and 800 nm were used in this study. The content of fatty acid esters (ethyl palmitate, ethyl laurate), the main causes of chill haze in strong alcoholic drinks, was efficiently reduced by all tested membranes. Content of alcohols, aldehydes and terpenes (except nerol) was not significantly influenced by applied treatments. All the brandy samples were stable after the re-exposure to lower temperatures. The sample filtered through the 800 nm pore size membrane showed the best sensory characteristics, most similar to the control sample.     

阻垢剂对海水淡化膜蒸馏试验的影响

采用直接接触式膜蒸馏技术进行海水淡化试验研究。在海水温度为55℃,循环水温度为20℃的条件下,考察了不同阻垢剂用量对膜蒸馏海水淡化的影响、工艺连续运行过程中膜通量和产水电导率随时间的变化情况,确定了膜蒸馏过程稳定运行的最优浓缩倍率。结果表明,阻垢剂的加入可明显提高淡水的产水率,产水率可达到85%以上,减少了高盐度海水浓缩液的排放,膜通量稳定,产水水质好,其电导率不超过10μS/cm,膜蒸馏海水淡化具有一定的技术可行性和可操作性。