Systematic analysis and optimization of power generation in pressure retarded osmosis: Effect of multistage design

This work presents a systematic method for analysis and optimization of specific energy production (SEP) of pressure retarded osmosis (PRO) systems employing single‐stage configuration as well as multistage design with interstage hydro‐turbines. It is shown that the SEP normalized by the draw solution feed osmotic pressure increases with the number of stages as well as a dimensionless parameter . As compared to the single‐stage PRO, the multistage arrangement not only increases flux and volume gain, but also allows a stage‐dependent, progressively decreasing hydraulic pressure, both of which contribute to enhanced SEP and power density. At the thermodynamic limit where γ_tot goes to infinity, the theoretical maximum SEP by an N‐stage PRO system is , where q_tot is the ratio of the draw solution flow rate at the outlet to the inlet on the system level. For single‐stage PRO, it is no more than π₀. For infinite number of stages, the theoretical limit becomes . SEP under realistic conditions and practical constraints on multistage design are discussed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 144–152, 2018     

Robust and high performance pressure retarded osmosis hollow fiber membranes for osmotic power generation

Novel fabrication perspectives have been demonstrated to molecularly construct robust hollow fiber membrane supports for high performance thin‐film composite (TFC) pressure retarded osmosis (PRO) membranes. For the first time, we found that the desirable hollow fiber supports should possess high stretch resistance and acceptable ductility. The microstructure strength of the hollow fiber support may have more weights on overall robustness of the TFC PRO membranes than the apparent cross‐section morphology. Effectively manipulating the kinetics of phase inversion during spinning by maneuvering bore fluid chemistry, and polymer solution composition is a promising method to tailor the strength of hollow fiber supports. Prestabilization of the TFC membranes at elevated lumen pressures can significantly improve their PRO performance. The newly developed TFC PRO hollow fiber membranes exhibit a power density as high as 16.5 W/m² and a very low specific reverse salt flux (J_s/J_w) of 0.015 mol/L at a hydraulic pressure of 15 bar using synthetic seawater brine (1.0 M NaCl) as the draw solution. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1107–1119, 2014     

Progress in pressure retarded osmosis (PRO) membranes for osmotic power generation

The rapid increases in global energy consumption and greenhouse gas emissions have stimulated the exploration of renewable energy sources as alternative fuels. Osmotic pressure gradient energy released from the mixing of water streams with different salinities is an unexploited resource of renewable energy. By employing a semipermeable membrane to control the mixing process, the osmotic pressure gradient energy can be harvested in terms of electrical power via pressure retarded osmosis (PRO) without causing adverse environmental impacts. The ideal of harvesting osmotic power via PRO was proposed in the early seventies; however, the absence of effective membranes with desirable structure and performance hindered further advancement of the PRO technology. During the last few years, a significant progress in PRO technology has been achieved. Novel flat-sheet and hollow fiber polymeric membranes with desired structure, mechanical robustness and permeation characteristics have been developed for PRO applications. Membranes with a target power density of 5 W/m² to produce commercially viable PRO processes have been achieved. At this point of time, a comprehensive review is imperative in order to summarize what we have accomplished and provide insights for the development of next generation PRO membranes. After a brief introduction of the PRO process and the early PRO development using the existing RO/NF and FO membranes, this review focuses primarily on novel and the state-of-the-art PRO membranes. Furthermore, the requirements for fabricating effective PRO membranes will be discussed and future perspectives will be presented.     

Pressure retarded osmosis: From the vision of Sidney Loeb to the first prototype installation — Review

The energy released from the mixing of freshwater with saltwater is a source of renewable energy that can be harvested using pressure retarded osmosis (PRO). In PRO, water from a low salinity solution permeates through a membrane into a pressurized, high salinity solution; power is obtained by depressurizing the permeate through a hydroturbine. The combination of increased interest in renewable and sustainable sources of power production and recent progress in membrane science has led to a spike in PRO interest in the last decade. This interest culminated in the first prototype installation of PRO which opened in Norway in late 2009. Although many investigators would suggest there is still lack of theoretical and experimental investigations to ensure the success of scaled-up PRO, the Norway installation has evoked several specialized and main-stream press news articles. Whether the installation and the press it has received will also boost competitive commercialization of membranes and modules for PRO applications remains to be seen. This state-of-the-art review paper tells the unusual journey of PRO, from the pioneering days in the middle of the 20th century to the first experimental installation.     

Osmotic power — power production based on the osmotic pressure difference between waters with varying salt gradients

Statkraft engaged in the development of osmotic power technology in 1997, and is today the most prominent developer of the osmotic power technology in a global context. The commercialisation of osmotic power is dependent on large volumes of sufficiently efficient semi-permeable membranes with high flux and salt retention. In addition to developing the technological concept and critical parts of the system, such as the membrane technology, Statkraft has also identified the energy potential, the financial aspects and the environmental implications of the technology. Osmotic power stands out as a promising and yet unexploited, new renewable energy sources. Throughout the last years, Statkraft has been successful in its furtherance of the necessary membrane technology and has recently started the detailed planning and design of the first osmotic power prototype plant in which it will further verify and test the technology.     

Effects of free volume in thin‐film composite membranes on osmotic power generation

For the first time, the effects of free volume in thin‐film composite (TFC) membranes on membrane performance for forward osmosis and pressure retarded osmosis (PRO) processes were studied in this work. To manipulate the free volume in the TFC layer, a bulky monomer (i.e., p‐xylylenediamine) was blended into the interfacial polymerization and methanol immersion was conducted to swell up the TFC layer. Results from positron annihilation lifetime spectroscopy (PALS) show that p‐xylylenediamine blending and methanol induced swelling enlarge and broaden the free volume cavity. In addition, the performance of TFC membranes consisting of different free volumes were examined in terms of water flux, reverse salt flux, and power density under high pressure PRO operations. The TFC‐B‐5 membrane (i.e., a TFC membrane made of blending monomers) with a moderate free volume shows the highest power density of 6.0 W/m² at 9 bar in comparison of TFC membranes with other free volumes. After PRO operations, it is found that the free volume of TFC layers decreases due to high pressure compression, but membrane transport properties in terms of water and salt permeability increase. Interestingly, the membrane performance in terms of resistance against high pressures and power density stay the same. A slow positron beam was used to investigate the microstructure changes of the TFC layer after PRO operations. Compaction in free volume occurs and the TFC layer becomes thinner under PRO tests but no visible defects can be observed by both scanning electronic microscopy and PALS. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4749–4761, 2013     

反渗透系统中浓差极化的影响

以实测数据为依据,论证了反渗透膜系统中浓差极化的影响,测定了保证系统稳定运行时浓差极化系数的临界值。由此为反渗透膜系统的稳定运行与优化设计提供了一个重要依据。     

影响反渗透制备脱离子水的运行因素

介绍了影响反渗透装置制备脱离子水的运行因素并进行了有针对性的分析,提出了相应的预防措施。     

反渗透系统中难溶盐极限回收率的解析分析

基于反渗透系统给水三类难溶盐饱和度的解析计算,计及了温度、pH、回收率、阻垢剂及浓差极化等因素的影响,导出了用系统给水参数解析表达的反渗透系统浓水难溶盐饱和度,得出了反渗透系统给水参数决定的系统难溶盐极限回收率．从而为反渗透系统的极限收率指标提供一个简捷而清晰的求取方式。     

Osmotic power production from seawater brine by hollow fiber membrane modules: Net power output and optimum operating conditions

This study analyzes the net energy output and optimum operating conditions for osmotic power generation from seawater brine based on the currently available hollow fiber membranes in the module scale. Factors that are influential on membrane performances, such as external concentration polarization, internal concentration polarization, salt reverse diffusion, and dilution have been taken into account. Net power density is defined and applied to characterize the efficiency of the PRO system, in terms of power production minus pumping energy, pretreatment cost and energy consumption by pressure drop in the membranes. When using 1 M NaCl as the draw solution and 10 mM NaCl as the feed, it is found that up to 7 W m^?2 net power density can be harvested by the PRO system depending on the water sources. Coupling with the existing RO plant is highly beneficial in terms of readily available high pressure source, high salinity and less or negligible pretreatment costs for the draw solution. Sources with higher salt concentrations are preferred. The optimum hydraulic pressure, module length, flow rate to membrane area ratio and feed to draw flow rate ratio have also been analyzed to maximize the net power output. In addition, implications on hollow fiber development are discussed. Fibers with high water permeability, lower structural parameter, good mechanical stability, better fouling resistance, and outer‐selective configurations are recommended for further studies. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1216–1225, 2016     

A novel ammonia—carbon dioxide forward (direct) osmosis desalination process

A novel forward (direct) osmosis (FO) desalination process is presented. The process uses an ammonium bicarbonate draw solution to extract water from a saline feed water across a semi-permeable polymeric membrane. Very large osmotic pressures generated by the highly soluble ammonium bicarbonate draw solution yield high water fluxes and can result in very high feed water recoveries. Upon moderate heating, ammonium bicarbonate decomposes into ammonia and carbon dioxide gases that can be separated and recycled as draw solutes, leaving the fresh product water. Experiments with a laboratory-scale FO unit utilizing a flat sheet cellulose tri-acetate membrane demonstrated high product water flux and relatively high salt rejection. The results further revealed that reverse osmosis (RO) membranes are not suitable for the FO process because of relatively low product water fluxes attributed to severe internal concentration polarization in the porous support and fabric layers of the RO membrane.     

反渗透技术在纯水制备中的应用

主要介绍了反渗透制备纯水的生产工艺技术,以及生产操作中影响反渗透膜性能的主要因素。     

A trimethylamine–carbon dioxide draw solution for osmotic engines

This study evaluates the pressure retarded osmosis performance of TMA–CO₂ for potential use in osmotic heat engines. Power densities up to 18.6 W m^?2 were achievable at relatively low pressure (10 bar) using 5 M TMA–CO₂ draw solutions. Compared to NaCl control tests, the TMA–CO₂ exhibited 20% lower water flux due in large part to its larger molecular size and associated higher solution viscosity and lower diffusion coefficient. Compared to the ammonia‐carbon dioxide draw solution, water flux was comparable but reverse solute flux of TMA–CO₂ was nearly one order of magnitude lower. Larger solute size was found to create a performance tradeoff as reduced reverse solute flux improved water flux while higher viscosity and lower diffusion coefficient worsened water flux. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3369–3375, 2018     

热电厂反渗透系统污堵原因解析及处理

某热电厂保安过滤器滤芯更换频繁,一级反渗透系统污堵较快.通过检查及分析,系统预处理未进行有效杀菌导致水体内微生物滋生是系统污堵的主要原因.现场采取调节杀菌剂加药量、增设分段杀菌装置及系统整体灭活等处理措施,取得了较好的效果.     

反渗透膜的背压损坏分析及相关安全性研究

背压损坏是反渗透膜元件使用中常见的故障之一,一旦发生则会对反渗透膜造成永久性损坏,无法修复。对反渗透膜元件背压损伤的位置以及背压对膜片造成的损伤进行深入分析,同时归纳了导致反渗透系统背压产生的设计和运行因素,提出现场判别背压损伤的方法。依托工业示范装置对带背压运行的反渗透系统开关机过程进行了模拟,并通过实时数据采集对全过程进行监控分析,对其安全性进行论证。     

Multi-objective optimization of reverse osmosis networks by lexicographic optimization and augmented epsilon constraint method

This study proposes a multi-objective optimization (MOO) of reverse osmosis (RO) networks for seawater desalination. The membrane transport model takes into consideration of the longitudinal variation of the velocity, the pressure, and the concentration in the membrane modules. The RO network with three type energy recovery device options (pressure exchanger (PX), Hydraulic Turbocharger, and turbine) is introduced. Lexicographic optimization (for calculation of a more effective payoff table) and augmented ε-constraint method (to avoid inefficient Pareto solutions) are proposed to solve the MOO problem. A fuzzy decision maker is introduced to derive the most efficient solution among Pareto-optimal solutions. Firstly, different energy recovery option studies show that using PX is seen to be the most profitable option. Exergy analysis is used to evaluate the contribution of the equipments in energy degradation. Secondly, the proposed multi-objective framework simultaneously optimizes the total annualized cost (TAC) and energy consumption. With the increases of weighting for the main objective function: TAC, the most efficient solution moves to lower TAC direction. Finally, system recovery rate is added as the third objective function. It is reasonable to stay at the appropriate system recovery rather than to increase up to its limit and generating high energetic losses.     

Technical and economic analysis for the integration of small reverse osmosis desalination plants into MAST gas turbine cycles for power generation

A technical and economic analysis concerning the integration of small reverse osmosis (RO) desalination plants into mixed air steam turbine (MAST) technologies for power generation was carried out. The simulation tool used is the computer aid reverse osmosis calculations optimization algorithm. This user-friendly software takes into account the capital cost, fuel cost and operation and maintenance requirements of each candidate RO desalination plan scheme and calculates the least-cost configuration. The results indicate that the integration of a RO desalination plant into MAST gas turbines has a minor effect on the final operating cost of the power plant.     

某电厂反渗透膜系统的化学清洗

分析了某电厂反渗透系统装置运行参数的变化、反渗透膜污染物的主要组成,确定了清洗方案。通过清洗液浊度、清洗液pH、一二段压差变化等指标,确定清洗终点。该装置化学清洗后进水压力大幅降低,产水脱盐率达到设计要求,产品水的回收率基本达到设计值,取得满意的清洗效果。     

压力延滞渗透膜的研究进展

压力延滞渗透(pressure retarded osmosis,PRO)是一种利用不同浓度溶液间渗透压差进行产能的新型膜技术。膜是PRO过程的核心元件,对其产能效果有巨大影响。PRO膜需要有良好的水渗透性和较高的截留率,并具有足够的强度承受高压。首先阐述了PRO过程对膜性能的要求,然后从活性层、支撑层及物理/化学预处理3个方面归纳了当前广泛使用的非对称膜的研究进展,总结了膜材料、制备过程和预处理等对膜性能的影响。同时,归纳了对称膜及准对称膜方面的研究状况,两者可有效降低膜的内部浓差极化现象(ICP),是未来PRO膜研究的一个重要方向。     

超滤和反渗透技术在电厂中水回用中的应用

为将中水回用于电厂生产,做到节约用水、节能降耗,提出预处理+机械过滤+超滤+反渗透技术的工艺设计方案,选用PLC为控制器,组态王为上位机监控软件的自动控制技术,实现电厂中水回用系统优化控制。实际运行结果表明,该方法能有效去除中水中污染物,对COD、悬浮物、氨氮、氯离子、总硬度、电导率去除率分别达到97.96%、99.36%、98.8%、99.39%、97.42%、99.56%,控制系统运行稳定,出水水质优良,满足设计要求。