Optimum design of reverse osmosis system under different feed concentration and product specification

The design of various multistage RO systems under different feed concentration and product specification is presented in this work. An optimization method using the process synthesis approach to design an RO system has been developed. First, a simplified superstructure that contains all the feasible design in present desalination process has been presented. It offers extensive flexibility towards optimizing various types of RO system and thus may be used for the selection of the optimal structural and operating schemes. A pressure vessel model that takes into account the pressure drop and concentration changes in the membrane channel has also been given to simulate multi-element performance in the pressure vessel. Then the cost equation relating the capital and operating cost to the design variables, as well as the structural variables of the designed system have been introduced in the objective function. Finally the optimum design problem can be formulated as a mixed-integer nonlinear programming (MINLP) problem, which minimizes the total annualized cost. The solution to the problem includes optimal arrangement of the RO modules, pumps, energy recovery devices, the optimal operating conditions, and the optimal selection of types and number of membrane elements. The effectiveness of this design methodology has been demonstrated by solving several seawater desalination cases. Some of the trends of the optimum RO system design have been presented.     

Production of process water using integrated membrane processes

Application of ultrafiltration, nanofiltration, reverse osmosis, membrane distillation, and integrated membrane processes for the preparation of process water from natural water or industrial effluents was investigated. A two-stage reverse osmosis plant enabled almost complete removal of solutes from the feed water. High-purity water was prepared using the membrane distillation. However, during this process a rapid membrane fouling and permeate flux decline was observed when the tap water was used as a feed. The precipitation of deposit in the modules was limited by the separation of sparingly soluble salts from the feed water in the nanofiltration. The combined reverse osmosis—membrane distillation process prevented the formation of salt deposits on the membranes employed for the membrane distillation. Ultrafiltration was found to be very effective removing trace amounts of oil from the feed water. Then the ultrafiltration permeate was used for feeding of the remaining membrane modules resulting in the total removal of oil residue contamination. The ultrafiltration allowed producing process water directly from the industrial effluents containing petroleum derivatives. Presented at the 33rd International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 22–26 May 2006.     

Improving the design stresses of high density polyethylene pipes and vessels used in reverse osmosis desalination plants

Nanocomposite materials have been used on a wide scale in industrial and structural applications. The present work aims at studying the mechanical properties of high density polyethylene (HDPE) grade TR-401 hexene copolymer reinforced by montmorillonite nanoparticles (MMT), used to fabricate pipes and membranes vessels for reverse osmosis desalination plants. Different volume fractions and particle sizes of the MMT clay were used to investigate the effect of this filler on the mechanical properties of the produced composite. Mechanical properties tests were carried out and good improvements of the composite properties were obtained compared to the parent polymer. The test results showed a significant enhancement of the mechanical properties at low filler proportions. Pipe fabricated from these composites had many outstanding and desirable features. For example, by adding 4.75% MMT to the HDPE produced quality pipes and fittings with the highest design stress basis of any polyethylene. A significant increase in the modulus of elasticity observed, together with an unusual increase in the design stress, approved the HDPE/MMT composite for high pressure piping and membrane vessels used in reverse osmosis desalination plants.     

Development of robust organosilica membranes for reverse osmosis

Hybrid organically bridged silica membranes have attracted considerable attention because of their high performances in a variety of applications. Development of robust reverse osmosis (RO) membranes to withstand aggressive operating conditions is still a major challenge. Here, a new type of microporous organosilica membrane has been developed and applied in reverse osmosis. Sol-gel derived organosilica RO membranes reject isopropanol with rejection higher than 95%, demonstrating superior molecular sieving ability for neutral solutes of low molecular weight. Due to the introduction of an inherently stable hybrid network structure, the membrane withstands higher temperatures in comparison with commercial polyamide RO membranes, and is resistant to water to at least 90 °C with no obvious changes in filtration performance. Furthermore, both an accelerated chlorine-resistance test and Fourier transform infrared analysis confirm excellent chlorine stability in this material, which demonstrates promise for a new generation of chlorine-resistant RO membrane materials.     

The separation potential of pervaporation : Part 2. Process design and economics

Pervaporation is a relatively complex process compared to other membrane processes like reverse osmosis for two reasons:— the process is sensitive to pressure losses at the permeate side— the evaporation enthalpy has to be transferred to the membrane surface (permeate side).Selectivity and flux can decrease markedly in case of hindered permeate flow. This is demonstrated by numerical design calculations of hollow-fiber modules. The calculations indicate that optimal fiber dimensions of hollow-fiber pervaporation modules should be much larger than those employed in RO and gas permeation modules.In principle, several alternatives exist for the supply of the evaporation enthalpy. The most economical solution seems to be to draw this energy from the liquid and to maintain the operating temperature level by a combination of modules and heat-exchangers in series. An alternative is the sweeping of the permeate by a partially condensing, and with respect to the permeate immiscible, carrier vapor. Since the latent heat of the carrier vapor can be utilized only partially, this concept will not be economically competitive.Pervaporation has a wide range of possible applicatio for this reasons it is impossible to discuss the economics of pervaporation in general. In this paper the separation of benzene/cyclohexane, an azeotropic system with similar vapor pressures of the components, has been chosen as an example. But even such a limited discussion reveals tendencies which seem to be generally valid:1.Pervaporation processes consisting of several stages (cascade) cannot compete with conventional separation processes like extractive distillation.2. Hybrid processes like a combination of extractive distillation, pervaporation are very promising, especially in cases where high product purities required     

Development of novel backwash cleaning technique for reverse osmosis in reclamation of secondary effluent

The objective of the study was to further develop a novel cleaning technique for reverse osmosis in reclamation of municipal secondary effluent. This technique is a new backwash method via direct osmosis (DO) by intermittent injection of the high salinity (HS) solution without stopping of high pressure pump and it is environment and membrane friendly technique. In the study, DO-HS trials were carried out with a UF-RO pilot system which was operated on site with the secondary treated effluent as the raw feed. Different operating conditions for DO-HS treatment in the actual process were investigated. It was found that the operation for implementation of the DO-HS cleaning technique developed was easy. For the first time, the actual profiles of HS concentration, DO backwash flow rate, brine flow rate and permeate pressure during DO-HS treatment have been demonstrated. It was observed that turbidity of the brine stream during DO-HS treatment at 3 NTU was 5 times higher than that before DO-HS treatment. The results from this study have confirmed the previous hypothesis with DO-HS treatment that there would be a strong driving force for DO backwash to lift and sweep the foulants from the membrane surface which would be carried over to the brine. The optimal plant operating conditions in terms of RO feed flow rate, HS concentration and HS injection time are ready for the DO-HS method to be adopted and validated in a long-term continuous plant operation.     

Neural network approach for modeling the performance of reverse osmosis membrane desalting

A neural network-based modeling approach with back-propagation and support vector regression algorithms was investigated as a mean of developing data-driven models for forecasting reverse osmosis (RO) plant performance and for potential use for operational diagnostics. The concept of plant “short-term memory” time-interval was introduced to capture the time-variability of plant performance since both a state of the plant model and standard time-series analyses for both flux decline and salt passage did not result in realistic predictive horizons for practical purposes. Past information of normalized permeate flux and salt passage were introduced as unique input variables along with process operating parameters to capture short-term plant performance variability. Sequential models, where the time-variation within each forecasting time-interval was also taken as input information, and marching forecasting models, where target values were predicted at fixed future times from past plant information, were developed. Models were trained, with normalized permeate flux and salt passage, for various model architectures, memory time-intervals and forecasting times using both back-propagation and support vector regression approaches. State of the plant models (without forecasting) were able to describe the relatively small permeate flux variations but were unable to capture salt passage trends (for any present time condition) since unsteady state phenomena could not be properly described without plant memory information. Forecasting of plant performance, with both sequential and marching models, yielded good predictive accuracy for short-term memory time-intervals in the range of 8–24 h for permeate flux and salt passage for forecasting times up to 24 h. Current work is ongoing to extend the approach for longer time scales and to incorporate data-driven forecasting models of RO plant into control strategies and process diagnostics.     

Nuclear magnetic resonance microscopy studies of membrane biofouling

There is a substantial need for novel measurement techniques that enable non-invasive spatially resolved observation of biofouling in nanofiltration (NF) and reverse osmosis (RO) membrane modules. Such measurements will enhance our understanding of the key design and operational parameters influencing biofilm fouling. In this study we demonstrate the first application of nuclear magnetic resonance microscopy (NMR) to a spiral wound reverse osmosis (RO) membrane module. The presented NMR protocols allow the extraction of the evolution with biofouling of (i) the spatial biofilm distribution in the membrane module, (ii) the spatially resolved velocity field and (iii) displacement propagators, which are distributions of molecular displacement of a passive tracer (in our case, water) in the membrane. From these measurements, the effective membrane surface area is quantified. Despite the opaque nature of membrane design, NMR microscopy is shown to be able to provide a non-invasive quantitative measurement of RO membrane biofouling and its impact on hydrodynamics and mass transport. Minimal biofilm growth is observed to have a substantial impact on flow field homogeneity.     

Comparison of membrane processes with distillation for alcohol/water separation

The overall objectives of this study were to summarize and evaluate the performance of currently available membranes for purification of fermentation alcohol and to compare the economics of membrane processes with a modern-day energy-efficient distillation scheme.Literature survey showed that very little work had been done on the development of membrane processes for alcohol concentration.Based on laboratory work, it was found that the present-day thin-film composite desalination membranes can be used for partial concentration of beer solution to about 20 to 30% alcohol concentration. The water permeation coefficient for these membranes in reverse osmosis with 7.6% alcohol feed at 60 atm was about 10 kg/m²-day-atm (2 lb/ft²-day-atm).Due to the high osmotic pressures of ethanol/water mixtures, reverse osmosis can be used only for the initial concentration of beer solution and for the final dehydration of 95% alcohol to produce 199 proof alcohol. Thus, a distillation unit would have to be used for the intermediate concentration of alcohol solution. Membrane concentration schemes using distillation for intermediate concentration were prepared for comparison with a conventional distillation process. Based on preliminary analysis it was concluded that while the capital cost of the membrane-augmented distillation schemes can be significantly than that of the conventional system, the annualized cost of these schemes will be approximately equal to that for distillation. The capital and the annualized costs of the membrane process for the final dehydration of alcohol can be significantly lower than those for the conventional dehydration still.     

Molecular dynamics simulations of osmosis and reverse osmosis in solutions

Computer simulation studies using the method of molecular dynamics have been carried out to investigate osmosis and reverse osmosis in solutions separated by semi-permeable membranes. The method has been used to study the dynamic approach to equilibrium in such systems from their initial nonequilibrium state. In addition density profiles of both the solute and solvent molecules have been investigated, especially near the walls for adsorption effects. Finally the diffusion coefficients and osmotic pressure have also been measured.Our results show both osmosis and reverse osmosis, as well as a smooth transition between the two when either the solution concentration is changed, or the density (pressure) difference between the solvent and solution compartments is varied. We believe this new method can be used to improve our understanding of these two important phenomena at the molecular level.     

Design and optimisation of a simulated moving bed unit: role of deviations from equilibrium theory

The design of a simulated moving bed involves thermodynamic, kinetic and hydrodynamic aspects and requires the optimisation of several variables: plant design variables, such as the column length and diameter, and operating variables, among them four independent flow-rates, the feed concentration and the switch time. In this work we develop an algorithm to design both the unit and its operating conditions, with an overall view on equilibrium properties, efficiency and hydrodynamics, using a simple equilibrium stage model. In this way we determine the parameters leading to the highest possible productivity for a given separation, only requiring the knowledge of the equilibrium isotherms, the Van Deemter equation and a correlation for pressure drop. The algorithm has been used to investigate the effect on the separation performance of some parameters, such as particle size and required product purity, which are not considered by equilibrium theory. The results have been compared with the predictions of equilibrium theory and the observed deviations have been put in evidence and discussed.     

分离液体的膜

本文概述了7种以高分子为膜材料、用于混合液体分离的膜分离过程,即离子交换膜与电渗析、反渗透、超滤、微孔过滤、膜萃取、渗透汽化和膜蒸馏。对其原理、高分子膜材料、应用和发展趋向作了简要介绍;并阐述了我国膜分离发展的现状和展望。     

羟丙基醋酸纤维素反渗透膜

研究了羟丙基醋酸纤维素反渗透膜的制膜工艺和膜性能。实验表明,以甘油—正丙醇或磷酸为添加剂,蒸发时间30—60s,预热处理温度70℃,时间3—5min,可得到在2MPa操作压力下,氯化钠脱除率95—98％,水通量1.0～2.1ml/cm~2·h的反渗透膜。羟丙基醋酸纤维素膜具有一定的耐热性,其使用温度上限比醋酸纤维素膜至少提高了10℃以上。羟丙基醋酸纤维素的溶解性能与醋酸纤维素相似。X射线衍射和热重分析显示羟丙基醋酸纤维素具有与醋酸纤维素相似的聚集态结构。     

Permeate flux prediction in apple juice concentration by reverse osmosis

This work studies apple juice concentration by reverse osmosis (RO) using polyamide tubular membranes at different operating conditions. Permeate flux has been predicted by using the solution-diffusion model combined with the film model. Good agreement was found between the experimental values and the ones calculated by using both models. Pressure was found to be the most important variable controlling the process. Physico-chemical analyses were made in order to evaluate the quality of concentrated apple juice.     

Boron reduction performance of reverse osmosis seawater desalination process

In recent years, seawater desalination systems using reverse osmosis (RO) membranes have been constructed to settle the lack of drinking water. RO desalination membranes have high rejection for most of solutes in seawater. Japanese drinking water standards for the water quality of the permeate can be achieved except for boron. Therefore, the boron rejection needs to be considered in the design of the RO process and during the operation of the plant. Luckily, there is a simple and easy method to estimate boron concentration.In this paper, we report measured boron permeabilities and their relation to salt permeabilities using cross-linked polyamide membranes. Chemical degradation of the membranes affected these permeabilities to different degrees. Boron concentrations in the permeate were then calculated using a computer program that was based on the boron permeabilities calculated from the measured salt permeabilities. Results obtained were compared with actual data taken at a RO plant of Toray Industries, Inc., Ehime. The model data fitted the experimental result, well. It was also found that a relationship existed in the permeate between salt and boron concentrations and that the boron concentration can be obtained from measurement of the salt concentration.     

Aroma Profile and Chemical Composition of Reverse Osmosis and Nanofiltration Concentrates of Red Wine Cabernet Sauvignon

Wine aroma represents one of the main properties that determines the consumer acceptance of the wine. It is different for each wine variety and depends on a large number of various chemical compounds. The aim of this study was to prepare red wine concentrates with enriched aroma compounds and chemical composition. For that purpose, Cabernet Sauvignon red wine variety was concentrated by reverse osmosis (RO) and nanofiltration (NF) processes under different operating conditions. Different pressures (2.5, 3.5, 4.5 and 5.5 MPa) and temperature regimes (with and without cooling) were applied on Alfa Laval LabUnit M20 equipped with six composite polyamide RO98pHt M20 or NF M20 membranes. Higher pressure increased the retention of sugars, SO₂, total and volatile acids and ethanol, but the temperature increment had opposite effect. Both membranes were permeable for water, ethanol, acetic acid, 4-ethylphenol and 4-ethylguaiacol and their concentration decreased after wine filtration. RO98pHt membranes retained higher concentrations of total aroma compounds than NF membranes, but both processes, reverse osmosis and nanofiltration, resulted in retentates with different aroma profiles comparing to the initial wine. The retention of individual compounds depended on several factors (chemical structure, stability, polarity, applied processing parameters, etc.).     

反渗透膜微结构的调控及海水脱硼性能的提升

通过引入聚乙烯亚胺(PEI)链与对叠氮苯甲酸(ABA)分子对薄层芳香聚酰胺复合反渗透膜(TFC)进行接枝改性, 采用傅里叶衰减全反射红外光谱(ATR-FTIR)和X射线光电子能谱(XPS)分析了反渗透膜活性分离层的化学组成和结构, 用静态水接触角仪与Zeta电位仪测试了反渗透膜表面的亲疏水性和电荷性质, 并利用扫描电子显微镜(SEM)及原子力显微镜(AFM)观察其表面形貌, 测试了反渗透膜在苦咸水与海水条件下的分离性能. 实验结果表明, 使用PEI与ABA对反渗透膜改性后, 提升了其分离层的致密度, 使硼渗透通过反渗透膜时的传质阻力变大, 从而将改性反渗透膜(TFC-PEI-ABA)对硼的截留率提升至90.45%, 达到了世界卫生组织对水质的要求.     

Biomimetic Approaches for Membrane Technologies

Membrane technology is the dominant process in water treatment. However, the operation cost of membranes cannot be decreased unless the amount of fouling, the “Achilles heel” of membranes, and energy consumed are cut. The high energy requirements in commercial nanofiltration, reverse osmosis and forward osmosis technologies lead researchers to develop new membrane designs having high flux values with high salt rejection values. The purpose of this review is to present the inadequacies of the membrane processes by considering studies related to fouling and energy minimization. In this respect, lipid bilayers, block copolymers, aquaporin Z proteins and aligned carbon nanotubes can be the base to build biomimetic membranes. Such studies are summarized due to their remarkable properties in fouling control. Furthermore, the review describes the membrane design strategies and points the limitations hindering commercialization. Additionally, it is hoped that this review will trigger further needed studies.     

苦咸水反渗透淡化中膜面结垢预测及防垢进展

苦咸水反渗透（BWRO）中的防垢过程,首先取决于给水水质,而根据水质条件和垢在膜面的形成机理采取相应的防垢措施是非常重要的。 显然,有效地管控膜面无机结垢及抑制膜面污染需要开展无机结垢趋势的预测、防垢措施和非破坏性无机垢监控等方面的技术研究。 一系列传统和新兴的分析技术,包括摩尔比率法、直接目测法和光谱法等已应用于BWRO过程中膜面防垢研究。 本文详细综述了该过程中无机结垢趋势的预测、防垢方法和非破坏性无机垢监控技术等方面的研究进展。 此外,针对目前的研究方向提出了建议。     

Membranes for power generation by pressure-retarded osmosis

A model has been developed for obtaining the projected performance of membranes in pressure-retarded osmosis (PRO) from direct osmosis and reverse osmosis measurements. The model shows that concentration polarization within the porous substrate of the membrane markedly lowers the water flux under PRO conditions. The model has been used along with experimental data obtained with a variety of reverse osmosis membranes to project PRO performance with several water—brine sources. Some literature data on PRO have been similarly examined. Based on these results and a simple economic analysis we conclude that membranes with significantly improved performance will be needed if PRO is to become an economically feasible method for power generation using seawater—fresh water as the salinity gradient resource. However, the economics of a brine/fresh water system appear competitive with conventional power generation technologies.