直接接触式与气隙式膜蒸馏的比较研究

采用模拟计算和实验的方法对直接接触式膜蒸馏 (DCMD)和气隙式膜蒸馏 (AGMD)过程进行了比较研究。模拟计算及实验结果表明 ,AGMD中的气隙构成了过程的主要阻力 ,使得跨膜温差远小于膜两侧流体主体温差 ,这是两种膜蒸馏方式行为差别的主要原因所在。与AGMD相比 ,DCMD不仅膜通量水平高 ,而且膜通量对操作条件反应灵敏 ,易于实施过程的强化。而DCMD的热效率与AGMD相比差距并不太大。     

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。     

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。     

Comparison of continuous homogenous azeotropic and pressure-swing distillation for a minimum azeotropic system ethyl acetate/n-hexane separation

Continuous homogenous azeotropic distillation (CHAD) and pressure-swing distillation (PSD) are explored to separate a minimum-boiling azeotropic system of ethyl acetate and n-hexane. The CHAD process with acetone as the entrainer and the PSD process with the pressures of 0.1 MPa and 0.6 MPa in two columns are designed and simulated by Aspen Plus. The operating conditions of the two processes are optimized via a sequential modular approach to obtain the minimum total annual cost (TAC). The computational results show that the partially heat integrated pressure-swing distillation (HIPSD) has reduced in the energy cost and TAC by 40.79% and 35.94%, respectively, than the conventional PSD, and has more greatly reduced the energy cost and TAC by 62.61% and 49.26% respectively compared with the CHAD process. The comparison of CHAD process and partially HIPSD process illustrates that the partially HIPSD has more advantages in averting the product pollution, energy saving, and economy.     

A new enhancement technique on air gap membrane distillation

The most serious concern in the application of air gap membrane distillation (AGMD) configuration is the low permeate flux caused by an additional transport resistance owing to the air gap, by the temperature and concentration polarization and by the surface fouling. This paper presents an innovative design of a low-cost and high efficient membrane module with an advanced enhancement technique in an AGMD configuration, which not only yields a much improved permeate flux but also requires no additional facility for the enhancement. The new module design includes a tangent directional and rotational inlet turbulent flow of hot feed and a partial contact between the membrane and the cooling plate in a small air gap. The concrete structure of the module is introduced in detail in the paper. Using this new module the permeate flux can be obtained up to 119 kg/m²h for tap water when the temperature of the hot and cold water is 77 °C and 12 °C respectively, which is about a 2.5-fold improvement over the traditional AGMD technique at the almost the same conditions. Within the range of our experimental study, the optimum partial contact area ratio is about 75–80%. Mechanistically, the tangent and rotational inlet turbulent flow can accelerate the diffusive process of mass and heat, reduce the boundary layer thickness of temperature and concentration and wash the membrane surface so as to improve the temperature and concentration polarization near the membrane surface and to raise the efficiencies of mass and heat transfer. Because of the partial contact between the membrane and the cooling plate with a large area, the main heat transfer and permeate condensation in the gap both are carried out on the contact area, which is very different from either the common AGMD or DCMD (direct contact membrane distillation) so as to reduce the transport resistance in the gap and thus to raise the permeate flux significantly. The new enhancement technique is also applied for the desalination of 15 wt.% salt water, which shows the similar improvement in permeate flux.     

Hydrophobizing polyether sulfone membrane by sol-gel for water desalination using air gap membrane distillation

ABSTRACT

The hydrophobic polyether sulfone membranes were prepared by the sol-gel method to be applied in an air gap membrane distillation setup for desalination. The surface modifications were carried out using Trimethylsilyl chloride (TMSCl) and Methyltrimethoxysilane (MTMS) solutions. The membranes were characterized using Attenuated Total Reflection Infrared (ATR-IR) spectroscopy, Scanning Electron Microscopy (SEM), and Optical Contact Angle (OCA) methods. The effects of membrane preparation as well as operating conditions such as temperature difference, salt concentration, feed rotation speed, and cold-side temperature on membrane performance were investigated using central composite design method. It was found that feed temperature has the largest effect among the parameters on the permeation flux. The flow rate and salt rejection of the membrane in the optimum conditions were 4.47 Kg m^?2 h^?1 and 99.37%, respectively.     

精馏-膜分离技术集成过程研究进展

精馏-膜分离集成过程与传统精馏工艺相比,因其具有明显节能降耗的经济性优势,故工业应用日渐增多,所以对其进行全面研究更有理论和实际意义。本文介绍了膜分离的机理模型,根据膜的亲水性和亲有机性对膜材料进行分类,总结了精馏-膜分离集成过程的理论研究方法,并分别从脱水和脱有机物两方面介绍了精馏-膜分离集成过程的主要实验研究及工业应用研究进展。通过对现有研究分析发现,目前该集成过程在膜材料和集成优化理论研究方面存在一定局限性,使其未能在工业上得以广泛推广应用。基于上述全面分析,提出了一定改进思路。     

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。     

空分精馏系统最佳氩馏分位置的确定

指出在同时生产高纯度的氢、氮及粗氩产品，并带粗氩塔的空气分离双级精馏系统中，双级精馏塔的上塔存在最佳抽氩馏分位置，并介绍其确定方法。     

Distillate flux enhancement in the air gap membrane distillation with inserting carbon-fiber spacers

A new design of air gap membrane distillation (AGMD) with inserting carbon-fiber spacers with various hydrodynamic angles in flow channels for eddy promoting under concurrent-flow operations was developed theoretically and experimentally. Attempts to enlarge eddy flow in aiming to reduce the temperature polarization were achieved with the inserted carbon-fiber spacers that enhance the heat and mass transfer in the AGMD system. A mathematical model considering heat and mass transfer mechanisms has been developed, and the Nusselt number was correlated with the experimental data. The effects of various operation parameters on the distillate flux enhancement were studied as compared to the modules without inserting carbon-fiber spacers (empty channels).     

Conceptual design of an extractive distillation process for the separation of azeotropic mixture of n-butanol-isobutanol-water

In many chemical processes, large amounts of wastewater containing butanol and isobutanol are produced. Given that n-butanol-isobutanol-water can form triple azeotrope, high-purity butanol cannot be recovered from the wastewater by ordinary distillation. To economically and effectively recover butanol from this kind of wastewater, 1,4-butanediol is selected as an extractant to break the formation of the azeotropes, and a doubleeffect extractive distillation process is proposed. The conceptual design of the proposed process is accomplished based on process simulation. With the proposed process, the purity of recovered butanol and water is greater than 99.99 wt%. In comparison with the conventional azeotropic distillation process, economic analysis shows that the operating cost of the proposed process is lower:when the capacity of wastewater treatment is 100 t·h^-1, the total operating cost decreases by 5.385×10⁶ USD per year, and the total annual cost of the new process decreases by 5.249×10⁶ USD per year. In addition, in the extractive distillation system, variable effects on separation purities and cost are more complex than those in the ordinary distillation system. The method and steps to optimize the key variables of the extractive distillation system are also discussed in this paper and can provide reference for similar studies.     

动态法筛选萃取精馏分离乙酸丁酯-丁醇-水共沸物的溶剂

采用基于改进UNIFAC模型的动态法对萃取精馏法分离乙酸丁酯-丁醇-水共沸物的萃取剂进行了计算机高通量筛选,根据适宜萃取剂选取原则和计算结果,经综合考虑选出较好的萃取剂为甲醇、乙醇、环氧乙烷。选取乙醇进行实验验证,测定了乙酸丁酯-丁醇-水-乙醇体系的部分汽液平衡实验数据;泡点温度tb、加入溶剂后平衡汽相组成的计算值与实验值吻合良好;以乙醇作为萃取剂具有热负荷小、毒性低和热稳定性好的优点。结果表明,采用基于改进UNIFAC模型的动态法筛选萃取精馏法的溶剂具有良好的可靠性和适用性,可以极大地节省人力、物力和财力。     

Modelling and simulation of a hybrid process (pervaporation–distillation) for the separation of azeotropic mixtures of alcohol–ether

This work reports the modelling and simulation of a hybrid process, based on the combination of distillation and pervaporation, for the separation of azeotropic mixtures of alcohol–ether. After having selected the separation of methanol‐2‐metoxi‐2,2‐dimethyl ethane (ETHER) as a motivating example the mathematical modelling of the distillation column was achieved and used together with a mass transfer model previously reported for the pervaporation operation in order to simulate the behaviour of the hybrid process for different compositions of the feed stream (case 1: 3.2 wt% methanol, 55.4 wt% C₄, 41.4 wt% ETHER, and case 2: 5.2 wt% methanol, 42 wt% C₄, 52.8 wt% ETHER). Simulation tasks were carried out with the process modelling system gPROMS and the results of alternative process configurations that result from the relative location of the separation technologies have been compared on the basis of the required membrane area. Finally, the design of the pervaporation unit including the overall processing costs is reported. Copyright © 2001 Society of Chemical Industry     

Simulation of binary gas separation with asymmetric hollow fibre membranes and case studies of air separation

A mathematical model for high‐flux asymmetric hollow fibre membrane was developed considering the effect of permeate pressure build‐up inside the fibre bore. A new solution technique was developed to solve the model equations, which constitute a boundary value problem. The ordinary differential equations were solved as an initial value problem in two successive steps using the Gear's BDF method. The technique is advantageous since it requires minimum computational time and effort with improved solution stability, and the computational complexity does not multiply as the number of components increases. The model predictions and the robustness of the numerical technique were validated with experimental data for several membrane systems with different flow configurations. The model and the solution technique were applied to evaluate the separation characteristics of air using representative membranes with different configurations, including single‐stage, single‐stage with permeate recycle, single‐stage with retentate recycle, air blending, and two stages in series. The study demonstrates that the new solution technique can conveniently handle the high‐flux hollow fibre membrane problems with different module configurations. © 2011 Canadian Society for Chemical Engineering     

MINLP optimization of a heterogeneous azeotropic distillation process: Separation of ethanol and water with cyclohexane as an entrainer

Heterogeneous distillation processes are widely used in industry for the separation of azeotropic and close-boiling mixtures. This paper addresses the optimization of a heterogeneous distillation process for the separation of an azeotropic ethanol/water mixture using cyclohexane as an entrainer. Starting from a given process superstructure a MINLP problem is set up to consider continuous as well as discrete decision variables such as the feed locations and the number of stages of the distillation columns. A modified Generalized Benders Decomposition algorithm to account for non-convexities of the model equations solves the MINLP problem. The algorithm can be attached via Visual Basic for Applications (VBA) to any commercial process simulator with NLP and VBA capabilities. Various optimization runs show that the algorithm is easily applicable and returns solutions independent of the initial values.     

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.     

MOFs基膜材料的研究现状及其在H2/CH4分离中的应用

氢气的生产、分离和储存已经成为世界绿色能源经济的重要组成部分。通过膜分离法从工业副产物中提纯氢气,不但操作简便,且显著降低了分离的能耗,是一种有前景的分离技术。金属有机框架（metal-organic frameworks,MOFs）因具有晶态、有序、明确的多孔结构和较大的比表面积,被认为是理想的气体分离膜材料。本文以MOFs基分离膜为研究对象,对比综述了MOFs膜的常规制备技术,总结了水热/溶剂热法、界面合成法、二次生长法和浇铸法的合成机理及应用。简述了在H₂/CH₄分离方面MOFs膜的设计原理及应用。针对MOFs膜当前存在的柔性、孔径、晶界结构、稳定性等问题,重点介绍了对制备方法与改良和对薄膜的后修饰策略,以期实现对MOFs膜性能的调控。最后,指出了目前该技术存在的难以大规模生产、分离性能不足的缺点,开发低成本的大规模生产方法同时提高薄膜的分离性能将会是未来MOFs膜实现工业应用的关键。     

An overview of the mathematical modelling of liquid membrane separation processes in hollow fibre contactors

Liquid membranes have traditionally been employed for liquid/liquid mass transfer and have found applications in industrial, biomedical and analytical fields as well as in hydrometallurgical processes, wastewater treatment and remediation of polluted groundwater. However, in spite of the known advantages of liquid membranes, there are few examples of industrial application. The development of reliable mathematical models and design parameters (mass transport coefficients and equilibrium or kinetic parameters associated with the interfacial reactions) is a necessary step for design, cost estimation, process optimisation and scale‐up. This work reports an overview of the different approaches that have been proposed in the literature to the mathematical modelling of liquid membrane separation processes in hollow fibre contactors providing, at the same time, a useful guideline to characterise the mass transport phenomena and a tool for the optimal design and intensification of separation processes. Copyright © 2009 Society of Chemical Industry