The electret effect in cellulose acetate reverse osmosis membranes

The electret potentials developed by reverse osmosis electret membranes help control the undesirable deposition of charged colloidal particles on the membrane surfaces during membrane desalination. These antifouling electret membranes should help prevent the costly flux declines normally associated with deposition of colloidal iron oxides on the reverse osmosis membrane surfaces. Homocharge and heterocharge behavior of cellulose acetate membrane electrets have been studied. Asymmetric reverse osmosis membranes and dense membrane films were studied. The homocharge and heterocharge of cellulose acetate reverse osmosis electret membranes have been explained.     

Precise measurement of membrane constants of cellulose acetate membranes by direct osmosis tests

A method for the determination of membrane constants by direct osmosis tests was studied using cellulose acetate membranes. A countercurrent type osmosis cell was designed and made for this study, and a method for precise measurements of permeated water and solutes through the membrane was established. Based on the membrane constants derived from direct osmosis tests, membrane performances of cellulose acetate membranes under pressure of 40 atm were predicted. The predicted values were in good agreement with the observed values in reverse osmosis experiments and it was confirmed that membrane performances under pressure could be predicted by the direct osmosis with considerably good accuracy.     

反渗透膜的研究进展与展望

反渗透具有低能耗、高效率等突出优点,是目前应用最为广泛的分离技术之一。反渗透膜的性能是影响反渗透过程效率的决定因素,反渗透膜的研制一直是国内外膜领域的研究热点。特别是近年来,石墨烯、碳纳米管等新型材料展现出优异的水传递行为,成为新型反渗透膜材料的研究热点。本论文回顾了反渗透膜的研制发展历程,介绍了不同单体通过界面聚合反应成膜的研究进展,综述了国内外新型混合基质膜和无机分子筛反渗透膜材料及其成膜研究,最后提出了新型反渗透膜的研究方向。     

Asymmetric reverse osmosis and ultrafiltration membranes prepared from sulfonated polysulfone

The concept of solubility parameter, developed by Hansen, has been applied to the determination of solubility diagram of polysulfone and sulfonated polysulfone. Data showing the effect of various parameters such as thermodynamic quality of the solvent, casting solution composition and viscosity, solvent evaporation period and precipitation bath composition, on membrane structure and performance have been presented. By adjusting these parameters a wide variety of sulfonated polysulfone membranes useful for both ultrafiltration and reverse osmosis applications could be obtained.     

REVERSE OSMOSIS CONCENTRATION OF FLAVOR COMPONENTS IN APPLE JUICE-AND GRAPE JUICE-WATERS

Several flavor components including alcohols, aldehydes and esters were identified and quantitatively estimated, by gas chromatography, in membrane permeated apple juice- and grape juice-waters collected by reverse osmosis treatment of natural juices using cellulose acetate membranes. The flavor components present in the juice-waters so collected were then concentrated by reverse osmosis treatment of the latter at 5°C and 68.95 x 10² kPa gauge pressure (1000 psig) using aromatic polyamide membranes. It is shown that the concentrations of each flavor component in the concentrate and the permeate solutions obtained in the latter treatment are predictable from only a single set of experimental reverse osmosis data with a reference NaCl-H₂O feed solution for each membrane. This prediction technique is described, and its utility in reverse osmosis process design is illustrated.     

Asymmetric polivinylidenfluoride (pvdf) radiation grafted membranes: preparation and performance in reverse osmosis application

A new type of reverse osmosis membrane has been synthesized. Membranes were prepared starting from asymmetric PVDF films, obtained by the casting and gelation technique and modified by radiochemical grafting and sulphonation. These membranes were tested in an RO laboratory plant and their performances were determined as a function of preparative parameters.The influences of evaporation time and temperature grafting and solvents were investigated.These membranes exhibit permeabilities as high as 2000 l/m²d and sodium chloride rejections up to 70%.     

Potential use of nanofiltration like-forward osmosis membranes for copper ion removal

The discharge of industrial effluent containing heavy metal ions would cause water pollution if such effluent is not properly treated. In this work, the performance of emerging nanofiltration(NF) like-forward osmosis(FO)membrane was evaluated for its efficiency to remove copper ion from water. Conventionally, copper ion is removed from aqueous solution via adsorption and/or ion-exchange method. The engineered osmosis method as proposed in this work considered four commercial NF membranes(i.e., NF90, DK, NDX and PFO) where their separation performances were accessed using synthetic water sample containing 100 mg·L~(-1) copper ion under FO and pressure retarded osmosis(PRO) orientation. The findings indicated that all membranes could achieve almost complete removal of copper regardless of membrane orientation without applying external driving force.The high removal rates were in good agreement with the outcomes of the membranes tested under pressuredriven mode at 1 MPa. The use of appropriate salts as draw solutes enabled the NF membranes to be employed in engineered osmosis process, achieving a relatively low reverse solute flux. The findings showed that the best performing membrane is PFO membrane in which it achieved N 99.4% copper rejection with very minimum reverse solute flux of 1 g·m~(-2)·h~(-1).     

Chapter 2.6 Measurements and control in reverse osmosis desalination

This chapter discusses the constraints of reverse osmosis desalination and the techniques and controls generally recommended in the design and operation of reverse osmosis systems. Adequate quality of feedwater to reverse osmosis systems is vital to obtain satisfactory service between cleanings and to obtain maximum membrane life. The controls employed to assure this quality are discussed. Reverse osmosis membranes have constraints in pH, temperature, pressure, flow and oxidation resistance which are important in the selection and operation. The reverse osmosis system and its design must take account the analysis of the saline water, the service requirements, and the operability of the system. The process engineering of a reverse osmosis system achieves these goals within the guidelines of the membrane manufacturer. This is done to safeguard the warranties which are extended by the membrane manufacturer on the performance and life of the membranes.Biological constraints of reverse osmosis systems are frequently satisfied by chlorination. However, as some membranes are sensitive even to very low concentrations of oxidant, effective dechlorination and measurement of residual oxidant is necessary.The measurements, instrumentation and controls employed to assist the operator in meeting the constraints of reverse osmosis desalination are discussed in considerable detail. The objective is to provide general guidance to the user of reverse osmosis systems.     

Recent advances in polymer and polymer composite membranes for reverse and forward osmosis processes

Semipermeable membranes are the core elements for membrane water desalination technologies such as commercial reverse osmosis (RO) process and emerging forward osmosis (FO) process. Structural and chemical properties of the semipermeable membranes determine water flux, salt rejection, fouling resistance, and chemical stability, which greatly impact energy consumption and costs in osmosis separation processes. In recent years, significant progress has been made in the development of high-performance polymer and polymer composite membranes for desalination applications. This paper reviews recent advances in different polymer-based RO and FO desalination membranes in terms of materials and strategies developed for improving properties and performances.     

Sulfonation of cross-linked asymmetric membranes based on polystyrene and divinylbenzene

Asymmetric microporous membranes based on polystyrene cross-linked with divinyl benzene were prepared. They were sulfonated with sulfonic acid and washed with solutions of gradually increasing pH to reduce environmental shock. The sulfonation was monitored by infrared spectroscopy, and changes in wettability and sulfur content were also recorded. Morphologies and reverse osmosis performance of sulfonated membranes were measured, with both water flux and salt rejection increasing after treatment. © 1994 John Wiley & Sons, Inc.     

Reverse osmosis with dynamically formed membranes from water-soluble polymers

Membranes were dynamically formed from poly(acrylic acid), poly(1-methyl-2-vinylpyridinium iodide), and methylcellulose on a Millipore filter. The effects of the conditions of membrane formation on reverse osmosis performances and the membrane structure were investigated. The primary factors that affect membrane performance were ascertained to be the kind and concentration of salts and the pH of the preparing solutions, the concentration and the polymerization degree of the membrane materials, the pore size of filters, and the pressure and circulation velocity during membrane formation. Two types of dynamically formed membranes, the membranes inside the filter and the membranes on the filter, were distinguished.     

Effects of polyethylene glycol on the structure and filtration performance of thin-film PA-Psf composite forward osmosis membranes

In forward osmosis, internal concentration polarization is related to the properties (e.g., hydrophilicity, porosity, structural resistant) of membrane support layer. In this work, polyethylene glycol with a low molecular weight of 400 Da was introduced as a support layer additive during the fabrication of thin-film polyamide-polysulfone composite forward osmosis membranes. The forward osmosis performances including water flux and reverse salt flux of the membranes were tested in the mode of AL-FS where the membrane active layer faced toward feed solution. Results showed that the addition of polyethylene glycol would reduce internal concentration polarization and improve membrane performance in forward osmosis by means of enhancing membrane hydrophilicity and changing pore morphologies of membrane support layer. The membrane prepared with 6 wt.% polyethylene glycol was found to exhibit the highest water flux of 47.4 Lm^?2h^?1 with a reverse salt flux of 7.6 gm^?2h^?1 when using DI water and 2.0 M NaCl as the feed and the draw solution, respectively, indicating an optimal polyethylene glycol dosage of 6 wt.% in this work.     

Preparation of reverse osmosis membranes by plasma polymerization of organic compounds

The plasma polymerization of organic compounds was used to prepare a composite reverse osmosis membrane which consists of an ultrathin semipermeable membrane formed by plasma polymerization of an organic compound or compounds and a porous substrate. Many nitrogen-containing compounds (aromatic amines, heteroaromatic compounds, aliphatic amines, and nitriles) were found to yield excellent reverse osmosis membranes by plasma polymerization directly onto porous substrates such as Millipore filters, porous polysulfone filters, and porous glass tubes. Factors involved in the preparation of reverse osmosis membranes by plasma polymerization were investigated and discussed. The plasma polymerized membranes have the following unique features: (1) very stable performance independent of salt concentration and applied pressure (practically no water flux decline was observed with many membranes): (2) salt rejection and water flux both increase with time in the initial stage of reverse osmosis (consequently, the performance of the membrane improves with time of operation); (3) very high salt rejection (over 99%) with high water flux (up to 38 gfd) can be obtained with 3.5% NaCl at 1500 psi (membranes perform equally well under conditions of sea water conversion and brakish water treatment).     

Structure of porous cellulose acetate membranes and a method for improving their performance in reverse osmosis

Experimental data support the hypothesis that the surface layer of the asymmetric Loeb-Sourirajan type porous cellulose acetate membranes has a heterogeneous microporous structure. A general method is proposed for improving the performance of the above membranes in reverse osmosis, by which product rates are increased without decreasing solute separation. The method consists in pumping pure water past the back side of the membrane under just enough pressure for a sufficiently prolonged period of time; after such pretreatment, the membrane is used in the reverse osmosis experiments in the normal manner with the surface layer facing the feed solution. Back-pressure treatment at 400 psig for 85 hr on preshrunk and normally pressure-treated membranes increases the product rate by over 20% without decreasing solute separation in reverse osmosis experiments at 600 psig with the use of 0.5 wt-% NaCl–H₂O feed solutions; with a different sequence of back-pressure treatment, similar results have been obtained in reverse osmosis experiments at 1500 psig also. The compaction effect of a normal membrane and that of a back pressure treated membrane are the same during continuous reverse osmosis operation under 600 psig; the effects of back-pressure treatment on a normal membrane and a compacted membrane are also the same. The pure water permeability data obtained in cyclic experiments show that the smaller pores on the surface layer are opened more than the bigger ones during the back side operation. The probable structural changes taking place in the film during back-pressure treatment are discussed.     

Crosslinked poly(hydroxyethyl methacrylate) membranes for desalination by reverse osmosis

A series of crosslinked hydroxyethyl methacrylate (HEMA) membranes for reverse osmosis desalination has been prepared. The crosslinkers used were trimethylol propane trimethacrylate (TPT) or ethylene glycol dimethacrylate (EGD). Membranes were synthesized by polymerizing the monomers as a thin homogeneous film. In addition to reverse osmosis tests, the membranes were also characterized by osmosis experiments and sorption measurements. The reverse osmosis water flux (1500 psi applied pressure, 4% NaCl brine, pH = 5) for these membranes decreases from 0.6 gallonmil/ft²-day (GMFD) to 0.055 GMFD and salt rejection increase from 78% to a maximum of 94% as the amount of TPT is increased from 0 to 11 mole-%. Water contents decrease from 42% to 15% over the same range of crosslinker, but the preferential sorption of water to salt does not vary. Thus, rises in reverse-osmosis semipermeability were found to result from changes in water–salt diffusivity ratios. The mechanism of permselectivity has been interpreted in terms of parallel diffusive fluxes across the membrane of primary H-bonded water and secondary water plus salt ions.     

用于反渗透复合膜的海绵状结构支撑膜制备研究

通过调节铸膜液中聚砜浓度和非溶剂含量,浸没沉淀法制备海绵状结构的支撑膜,并在支撑膜上界面聚合制备聚酰胺反渗透复合膜。分别对支撑膜及反渗透复合膜的结构和性能进行表征,考察聚砜浓度对支撑膜结构和性能的影响,以及不同结构支撑膜对反渗透复合膜结构和性能的影响。结果显示,随着聚砜浓度的增加,支撑膜表面孔径和孔隙率下降,断面结构变致密,耐压性增强。在不同支撑膜上制备的反渗透复合膜具有不同的通量和脱盐率。综合考虑支撑膜及反渗透复合膜的性能,以聚砜浓度为15%制备的海绵状结构支撑膜更适于作为制备反渗透复合膜的支撑层。     

循环水排污水中残余阻垢剂对反渗透膜性能的影响

循环水排污水中残余的阻垢剂会导致其水质的变化,从而影响反渗透膜性能。本文以循环水中常用的阻垢剂聚天冬氨酸(PASP)、羟基亚乙基=膦酸(HEDP)和氨基三亚甲基膦酸(ATMP)为研究对象,首先考察了它们的阻垢性能,然后在此基础上,通过静态浸泡试验和动态试验考察了它们的存在对反渗透膜性能的影响。研究结果表明,PASP、HEDP和ATMP中,PASP的阻垢性能最优,阻垢率高达84.21%,三者均会对反渗透膜的表面结构、组成成分、膜通量以及脱盐率产生一定的影响。当PASP、HEDP和ATMP的浓度分别为50mg/L、10mg/L和30mg/L时,在反渗透系统连续运行10h后,膜通量分别下降5.53%、4.89%和9.09%,小于空白时的18.95%;此外,脱盐率有不同程度的提高。     

Studies on effect of irradiation on semipermeable membranes

The effect of γ irradiation on reverse osmosis (RO) semipermeable membranes has been studied in order to evaluate their performance under the radiation environment arising in the processing of various streams in nuclear industry by reverse osmosis process. Both cellulosic and noncellulosic membranes in dry as well as wet conditions were used. A Co⁶⁰ source was used for γ doses from kilorads to megarads. The transport properties, namely, salt retention and water flux of membranes determined before and after irradiation, suggested deterioration in the membrane properties due to irradiation. The tensile strength and viscosity of membrane polymer also suggested membrane degradation. Differential scanning calorimetry was taken to look into any structural changes in the membrane polymer as a result of irradiation. IR spectra and X-ray of membrane polymer was also undertaken to understand the changes on the molecular level.     

Study of reverse osmosis separation and permeation rate for sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) membranes of different ion exchange capacities

The reverse osmosis performance of thin-film composite membranes prepared from sulfonated polyphenylene oxide polymer with different ion-exchange capacities was studied using various electrolyte solutes. The effect of the solvent used for the preparation of the polymer solution for surface coating was also studied. It was found that the preparation of thin-film composite membranes with high selectivities and high fluxes was possible by adjusting properly the ion-exchange capacity and the solvent. It was also found that the membrane performance was governed primarily by the ion-exchange reaction between the solute cation and the proton in SO₃H and the Donnan equilibrium. © 1995 John Wiley & Sons, Inc.     

纳滤膜物理性能的研究

纳滤技术是从反渗透技术中分离出来的一种膜分离技术,是超低压反渗透技术的延续和发展分支。纳滤膜是荷电膜,能进行电性相互作用,具有敏锐的分子截留区,对不同物质能有目的地提纯或去除。通过研究膜电位与无机溶液溶度、有机溶液浓度、pH值的关系来确定膜的性能。