Exergy analysis of desalination by solar-powered membrane distillation units

There has been an increasing interest in using exergy as a potential tool for analysis and performance evaluation of desalination processes where the optimal use of energy is considered an important issue. Unlike energy, exergy is consumed or destroyed due to irreversibilies in any real process and thus provides deeper insight into process analysis. Exergy analysis method was employed to evaluate the exergy efficiency of the “compact” and “large” solardriven MD desalination units. The exergy efficiency of the compact and large units with reference to the exergy collected by the solar collector was about 0.3% and 0.5% but was 0.01% and 0.05%, respectively, when referenced to the exergy of solar irradiance. The exergy efficiency of the flat plate solar collectors in both units varied diurnally and the maxima was 6.5% ad 3% for the compact and large units, respectively. The highest exergy destruction was found to occur within the membrane distillation module.     

Analysis of a solar-powered membrane distillation system

Nowadays, in dry and rural areas, solar-powered membrane distillation (SPMD) technology is considered a feasiblemeans for the production of pure water from brackish water. Prior to the design and construction of a SPMD pilot plant, there is a need to predict its performance theoretically by means of a computational simulation program. Unlike previous approaches followed by other investigators to develop a mathematical model that can describe the components of a SPMD pilot plant, the developed mathematical model in this study is based on the fact that the SPMD process by nature is unsteady. The performance of a proposed SPMD pilot plant is then obtained by means of a numerical solution of the model with the aid of a simulation computer program. The results reveal that the proposed SPMD pilot plant has some unique features, which differ from a similar MD process operated at steady-state conditions in a laboratory. The analysis of the system has shown that heat recovery via an external heat exchanger is not only possible, but even effective, and an economical way to intensify the SPMD process. The plant productivity can be improved by increasing the heat-exchanger capacity (KA), decreasing the flow rates of both feed and permeate or otherwise by increasing the effective surface area of the membrane. The achieved enhancements in the SPMD pilot plant productivity are directly related to an improved heat recovery rate in the heat exchanger. However, further analysis reported in this paper shows that the increase in KA and membrane area should be optimized for any planned capacity in the design of a SPMD pilot plant.     

Design of an autonomous solar desalination plant using vacuum membrane distillation,the MEDINA project

The unit presented in this paper is designed to provide a high quality of potable water in remote coastal areas with bad infrastructure and without a network connection. The installation is designed completely autonomous; indeed the only source of energy is the sun.     

Seawater desalination by direct contact membrane distillation

Membrane fouling still posts as one of the major obstacles in membrane distillation (MD). This is why the MD approach still cannot successfully compete with other conventional seawater desalination methods. In this study, both the NaCl solution and real seawater are used as the feed of MD processes to investigate the differences in permeate flux, product water quality and membrane fouling. The results indicate the accumulation rate of membrane scale can be depressed by reducing the degree of polarization if NaCl solution is used as the feeding fluid, however, this kind of depression effect is not so obvious when real seawater is used as the feeding fluid. An ultrasonic cleaning technique is found to be an effective way to restore the flux rate for these MD processes and extend the life span of MD membrane.     

膜蒸馏海水淡化研究进展及发展趋势

海水淡化是解决我国水资源短缺的重要措施之一。膜蒸馏海水淡化技术可以充分利用太阳能等低品位热源，具有成本低、设备简单、操作容易、能耗低等优点，在海水及苦咸水淡化方面应用前景广阔。     

A neural network-based optimizing control system for a seawater-desalination solar-powered membrane distillation unit

Several schemes have been proposed so far for coupling desalination processes with the use of renewable energy. One of their main drawbacks, however, is the nature of the energy source that requires a discontinuous and non-stationary operation, with some control and optimization problems. In the present work, a solar powered membrane distillation system has been used for developing an optimizing control strategy. A neural network (NN) model of the system has been trained and tested using experimental data purposely collected. Afterwards, the NN model has been used for the analysis of the process performance under various operating conditions, namely distillate production versus feed flow rate, solar radiation and cold feed temperature. On this basis, a control system that optimizes the distillate production under variable operating conditions has been developed, implemented and tested.     

Economic evaluation of seawater desalination for a nuclear heating reactor with multi-effect distillation

Seawater desalination for low temperature nuclear heat reactor (NHR) has been studied for many years in China and other countries in the world, such as Morocco, and five countries in North Africa. Now, the feasibility studies of nuclear desalination demonstration plants are being performed using NHR with 200 MW thermal coupling to a multi-effect distillation (MED) plant, with a production of 160,000 m³/d (NHR-200) in Yintai City, Shandong province and Tianjin City on the coastal area in eastern China. This paper presents the economic assessment of the NHR-200 in Tianjin City, and reviews the economics of the different power sizes of NHR coupling to the MED, e.g. NHR-10, NHR-200, double NHR-200 in North Africa, the Middle East, the Red Sea and the Arabian Gulf. It is very suitable to satisfy the site of no great electric grid, drought and scarce of freshwater for the characteristics of NHR with safe, clean and low total investment.     

新型气隙式膜蒸馏组件脱盐过程

利用基于聚丙烯中空纤维膜和聚丙烯中空纤维换热管的新型能量回收式膜组件(AGMD-HF),以70 g·L^-1的氯化钠溶液为研究对象,考察了膜组件长度和膜孔径大小对膜组件脱盐性能的影响。为直接衡量操作条件、组件参数以及温差、浓差极化现象对传质系数的影响,引入总传质系数,并研究进料温度和膜孔径对总传质系数的影响。实验结果表明,总传质系数随着温度的升高、膜孔径的增大而增大,提高膜孔径可有效提高总传质系数,同时可有效提高通量和造水比。通量随组件长度的增大而减小,而造水比增大,因此在应用过程中可综合考虑通量和造水比以便选择合适的组件长度。     

新型中空纤维空气隙式膜蒸馏用于海水淡化

利用自制的添加隔热管状隔网并呈螺旋缠绕结构编排的中空纤维膜组件进行了空气隙式膜蒸馏(AGMD)海水淡化过程性能研究, 实验以模拟标准海水(质量分数3.5%, 总溶解性固体含量35000 mg·L^-1)为热料液进水, 考察了热料液进水温度、热料液流量、冷凝液进水温度和冷凝液流量对膜通量、造水比和热效率的影响。结果表明, 随着热料液进水温度增加, 膜通量、造水比和热效率均增加;冷凝液进水温度增加, 膜通量下降而造水比和热效率增加;热料液流量增加, 膜通量上升而造水比和热效率明显下降;冷凝液进水流量对膜蒸馏过程性能影响较小。实验过程中产水TDS始终保持在3.0 mg·L^-1以下, 相应的离子去除率高于99.99%, 膜通量、造水比和热效率最高可分别达5.87 L·m^-2·h^-1、5.37和0.943。研究表明, 引入清洁能源取代传统电加热驱动热源将进一步突出膜蒸馏技术的实际应用潜力。     

膜蒸馏脱盐过程膜润湿的研究进展

膜蒸馏（MD）适合在偏远地区小规模制备淡水,有脱盐率高、接近常压操作、可利用太阳能等优势。但MD膜润湿影响了装置的运行稳定性,是制约MD脱盐应用的重要因素之一。本文介绍了MD膜润湿的评价方法,包括测定透水压力、临界膜孔润湿深度及可视化在线监测膜润湿进程等;简述了膜污染、MD操作变量、膜蒸馏形式及不同结垢晶体各因素对膜润湿的影响;从防垢和强化通量、掌握合适的膜清洗周期、物理干预及提升MD膜性能等方面分析了抑制膜润湿的措施;并分析了一些脱盐实验时间较长的MD膜润湿情况;指出具有自清洁性的光催化膜及防垢性能更强的双疏膜和Janus膜,具有抑制MD膜润湿的巨大潜力。本文有助于对脱盐过程中MD膜润湿的预测和有效控制。     

膜蒸馏脱盐中膜污染与膜润湿的研究进展

膜蒸馏作为一种脱盐的新兴技术受到广泛关注。然而,因为实际水质组分复杂,可能含有如表面活性剂、油类物质、易结垢盐和有机溶剂等污染物,导致一般疏水膜在长时间运行情况下极易发生膜污染或者膜润湿,最终造成膜通量或截留性能降低。本文首先简述了不同种类的膜污染和膜润湿的特点及形成原因,并分析了膜污染和膜润湿之间的区别和联系。对膜蒸馏过程中膜污染和膜润湿的监测和预测手段进行了简要介绍,最后针对膜蒸馏脱盐过程,重点介绍了近几年国内外预防膜污染和膜润湿的研究进展。研究者一般从污染物与疏水膜的相互作用力着手对疏水膜进行表面改性制备全疏膜和Janus复合膜,避免污染物在膜面的吸附以及抑制表面或孔道润湿。越来越多的研究人员采用致密亲水膜的渗透汽化脱盐来从根本上避免疏水膜带来的润湿。除此之外,对进料液进行预处理也能显著延迟膜的污染和润湿,如混凝/沉淀、膜过滤、煮沸、pH调控等,还可通过改变进料方式、辅助外加磁场等措施控制膜表面局部区域的流体力学状态,减少污染物的附着。适当的膜后处理措施也能恢复膜性能。最后,文章指出了解决膜蒸馏中膜污染和膜润湿的研究方向。     

Simulation of membrane distillation modules for desalination by developing user's model on Aspen Plus platform

This paper presents a simulation study of direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD) for desalination. Simulation models are built on Aspen Plus^® platform as user defined unit operations for these two types of modules, respectively. Large scale modules for practical industrial applications are simulated and studied for the effects of design and operation variables, as well as the importance of heat and mass transfers of each phase. For each type of modules with heat recovery design, the response surface method (RSM) is applied to develop the performance-variables quadratic model, followed by the multivariable optimization. Optimal designs can realize separation efficiencies, defined as the ratio of water produced to the feed, of 8.2% and 5.8% for DCMD and AGMD, respectively.     

疏水石墨烯膜的制备及其用于膜蒸馏脱盐的研究进展

膜蒸馏技术由于理论截盐率高、操作条件温和及对盐浓度灵敏度低等优势,在脱盐领域中展现出巨大潜力。近年来,人们开始关注石墨烯材料在膜蒸馏脱盐领域中的应用。本文首先概述膜蒸馏技术的基本原理及常用膜材料;接着介绍石墨烯的疏水性质和疏水石墨烯膜的制备;再详细综述石墨烯混合基质膜、石墨烯复合膜及石墨烯纯膜这三类疏水石墨烯膜在膜蒸馏脱盐中的应用;最后总结疏水石墨烯用于膜蒸馏脱盐面临的主要挑战及未来研究方向。     

Comparison of solar thermal technologies for applications in seawater desalination

This paper deals with a global analysis of the use of solar energy in seawater distillation under Spanish climatic conditions. Static solar technologies as well as one-axis sun tracking were compared. Different temperature ranges of the thermal energy supply required for a desalination process were considered. At each temperature range, suitable solar collectors were compared in some aspects as: (1) fresh water production from a given desalination plant; (2) attainable fresh water production if a heat pump is coupled to the solar desalination system; (3) area of solar collector required for equivalent energy production. Results showed that direct steam generation (DSG) parabolic troughs are a promising technology for solar-assisted seawater desalination.     

中空纤维空气隙式膜蒸馏海水淡化过程的性能模拟与优化

利用响应曲面法(RSM),以模拟标准海水(质量分数3.5%)为进水对中空纤维空气隙式膜蒸馏(AGMD-HF)海水淡化过程的影响因子和膜通量指标进行了模拟优化。通过面向中心复合设计法(CCD)实现了基于热料液进水温度、冷凝液进水温度和料液流量的实验优化设计,并建立了响应值与影响因子之间的二次多项式回归模型。方差分析(ANOVA)、RSM分析及实验响应值与预测值的对比验证了该模型对影响因子和膜通量模拟优化的可信度。进一步地,通过期望函数的引入确定了各影响因子最佳水平,并利用太阳能加热驱动过程实验进行验证。结果表明,ANOVA的决定系数R²达到0.986,p值则低于0.0001;实验膜通量与预测值平均误差仅为6.95%,产水电导率始终保持在10 μS·cm^-1以下,脱盐率稳定在99.99%以上;最佳影响因子水平分别为83.5℃、13.2℃和60.2 L·h^-1,在此条件下太阳能加热驱动过程膜通量达到6.47 L·m^-2·h^-1。该实验不仅为潜在可行的规模放大过程提供了可参照的操作参数,而且表明将太阳能引入AGMD-HF海水淡化过程具有很强的实际应用潜力。     

焦耳效应强化气隙式膜蒸馏脱盐过程的实验研究

以膜蒸馏海水淡化为背景,通过对自制导电炭膜通入直流电产生焦耳效应,开展焦耳效应强化膜蒸馏氯化钠水溶液脱盐过程实验研究。结果表明,制备的煤基炭膜在100℃内具有良好的结构稳定性;在实验范围内,炭膜对氯化钠的截留率在99.96%以上;焦耳效应对膜蒸馏渗透通量的提高率最高可达60%,且低温时焦耳效应对膜蒸馏的强化效果更好、电转换效率更高;引入的焦耳热除用于水分汽化外,还可提高料液温度、改善温度极化、增加传质推动力;焦耳效应同时对膜蒸馏过程传质系数产生影响,在进料温度为50~80℃时,受努森扩散和分子扩散影响,电流为1A时传质系数值将减小,电流为3A和5A时传质系数值将增大;膜蒸馏过程引入焦耳效应不会产生氧化还原反应和额外膜结垢;在炭膜中引入电流不会破坏炭膜及其表面PDMS层的结构。本文研究内容丰富了膜蒸馏海水淡化过程的强化方法,也可为焦耳效应影响膜蒸馏过程的模拟研究及工业应用提供依据。     

Fabrication of low thermal conductivity yttrium silicate ceramic flat membrane for membrane distillation

This paper reports on the successful fabrication of γ-Y₂Si₂O₇ membranes with low thermal conductivity, which is an important membrane property for achieving high performance in membrane distillation process. Single-phase γ-Y₂Si₂O₇ powder was first synthesized by calcination of SiO₂ and Y₂O₃ powders, with 3 wt% LiYO₂ as a sintering aid. The membrane was produced by tape-casting of a suspension of this powder. After sintering at 1300 °C for 4 h, a flat membrane was obtained, which had a thickness of 0.5 mm, 49% porosity, 0.9 μm pore diameter, and low thermal conductivity of 0.497 W/m⋅K at 32 °C, and 0.528 W/m⋅K at 100 °C. The obtained membrane presented hydrophobic features (water contact angle was 132°) after surface modification, which resulted in formation of a strongly adhered robust hydrophobic SiNCO nanoparticle layer on its surface. The resultant hydrophobic membrane was tested in water desalination experiments using a sweeping gas membrane distillation (SGMD) device. High water flux of 10.07 L⋅ m⁻²⋅ h⁻¹ was achieved for a 20 wt% NaCl feeding solution and a temperature at the feed of 90 °C. Stable water flux and rejection rates were recorded in long-term experiments (>400 h).     

Desalination by membrane distillation adopting a hydrophilic membrane

Direct contact membrane distillation (MD) by means of composite membranes with a PVA/PEG hydrophilic layeron a hydrophobic PVDF substrate has been developed for desalination. The effects of brine temperature, salt concentration, running time and the addition of ethanol on the flux of composite membranes have been investigated. Results showed that the flux of the composite membrane did not deteriorate by adopting an additional hydrophilic membrane although durability was obviously improved. More than 99% of the separation coefficient in one run was achieved with the conductivity of the produced fresh water in the range of 6-10 μs/cm. The flux of the composite membrane retained 91% flux of substrate at 70°C, being 23.7 kg/h·m². When the brine temperature rose to 70°C, the composite membrane showed a declined concentration polarisation, with a smaller C_mb/C_b (3.89) than that of the substrate (5.79). Although the flux decreased with the increase of brine concentration, it retained 64% flux of pure water at brine solution containing 20% NaCl and was kept almost constant until 25% NaCl. In the continuous running experiments, there was no obvious drop of flux, even after adding 25% ethanol to the brine and running overnight. It is expected that adopting a hydrophilic layer can prohibit the wetting problem that faced traditional MD with hydrophobic membranes.     

Characterization of membrane distillation membranes prepared by phase inversion

In this paper, flat membrane distillation membranes have been successfully manufactured from PVDF/DMAc and PVDF/DMF blends by using phase inversion induced by an immersion precipitation technique. The structure of the membranes is asymmetric with a porous top layer and macrovoids, as assessed by SEM. The existence of MD fluxes in these membranes is established by performing various pure water flux experiments. A maximum in the MD fluxes for a particular value of the polymer content in the casting solution from which the membrane is manufactured has been observed. The dependence of the magnitude of the fluxes on the membrane thickness is also discussed and the influence of temperature polarization evaluated.     

Fabrication and characterization of hydrophobic PVDF hollow fiber membranes for desalination through direct contact membrane distillation

The mixture of inorganic salt LiCl and soluble polymer polyethylene glycol (PEG) 1500 as non-solvent additive was introduced to fabricate hydrophobic hollow fiber membrane of polyvinylidene fluoride (PVDF) by phase inversion process, using N,N-dimethylacetamide (DMAc) as solvent and tap water as the coagulation medium. Compared with other three membranes from PVDF/DMAc, PVDF/DMAc/LiCl and PVDF/DMAc/PEG 1500 dope solution, it can be observed obviously by scanning electron microscope (SEM) that the membrane spun from PVDF/DMAc/LiCl/PEG 1500 dope had longer finger-like cavities, ultra-thin skins, narrow pore size distribution and porous network sponge-like structure owing to the synergistic effect of LiCl and PEG 1500. Besides, the membrane also exhibited high porosity and good hydrophobicity. During the desalination process of 3.5 wt% sodium chloride solution through direct contact membrane distillation (DCMD), the permeate flux achieved 40.5 kg/m² h and the rejection of NaCl maintained 99.99% with the feed solution at 81.8 °C and the cold distillate water at 20.0 °C, this performance is comparable or even higher than most of the previous reports. Furthermore, a 200 h continuously desalination experiment showed that the membrane had stable permeate flux and solute rejection, indicating that the as-spun PVDF hollow fiber membrane may be of great potential to be utilized in the DCMD process.