膜蒸馏跨膜传质过程的新模型--TPKPT

以纯水为介质,用直接接触式膜蒸馏测定了材料或性能参数不同的三种孔疏水膜在不同温度下的渗透通量。根据测量结果计算出了各种膜在不同温度下的渗透系数,发现渗透系数均随着温度的升高而升高;这一结果说明Poiseuille流动在跨膜传质中起着非常重要的作用。据此提出了Knudsen扩散－Poiseuille流动两参数跨膜传质模型,即TPKPT模型。用此模型拟合实验数据,得到了三种实验用膜的模型参数。用这些模型参数计算膜在不同温度下的渗透系数,其值与实验测量值吻合较好,说明TPKPT模型能较好地描述膜蒸馏的跨膜传质过程。     

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。     

Understanding and enhancing the direct contact membrane distillation performance by modified heat transfer correlation

The sensitivity of direct contact membrane distillation to feed temperatures and feed flow rate is investigated experimentally. At very low flow rates, the process becomes heat transfer limited where the thermal energy of the aqueous feed solution is lost by the conduction resistance minimizing the distillate production. At high flow rates, the film heat transfer coefficient for both liquid solutions increases, creating a larger temperature difference across the membrane boundaries. Hence, the mass fluxes increase linearly with the water flow rate. These findings are also assessed by simulating the heat and mass transfer equations that describe the membrane distillation (MD) thermo-physical operation. It is found that the underlying physics of the MD module must faithfully capture and explain the true process behaviour. Hence, a modified correlation for the Nusselt number is developed and tested.     

疏水性PTFE微孔膜处理含Cr(Ⅲ)稀溶液的实验研究

废水中的三价铬在自然环境中容易转化为毒性更强的六价铬。控制并回收废水中的三价铬可达到节约资源和降低污染的目的,用减压膜蒸馏(VMD)分离装置,实验探讨了不同平均孔径大小的聚四氟乙烯(PTFE膜对处理含铬(Ⅲ)溶液的膜通量、截留率等影响,研究了进料浓度、进料温度对分离性能的影响。实验结果表明,对于膜孔径较小的膜,膜内的传质阻力成为主要因素,膜内的传质是VMD过程的控制步骤。     

Innovative hydrophobic/hydrophilic perfluoropolyether (PFPE)/polyvinylidene fluoride (PVDF) composite membrane for vacuum membrane distillation

Though membrane distillation (MD) has gained more and more attention in the field of desalination, the wetting phenomenon was still a non-negligible problem. In this work, a method combined dip-coating and UV in situ polymerization for preparing hydrophobic/hydrophilic perfluoropolyether (PFPE)/polyvinylidene fluoride composite membranes. This composite membrane consisted of a top thin hydrophobic coating layer and hydrophilic substrate membrane. In terms of anti-wetting properties, contact angle and liquid entry pressure of all composite membranes (except for those based on 0.45 μm) exceeded 160° and 0.3 MPa, respectively. In particular, the desalination performance was tested in vacuum membrane distillation tests by feeding 3.5% (mass) saline solution (NaCl) at 60 ℃. The composite membranes with larger support pore size and lower PFPE content had higher membrane distillation flux. And for stability tests (testing the 0.22 μm membrane coated by 5% (mass) PFPE), the highest MD flux 29.08 kg·m^-2·h^-1 and stable salt rejection (over 99.99%) during the period. Except that, the effects of coating material concentration and pore sizes of substrate membrane were also investigated for surface morphology and topography, porosity, mechanical strength and pore size characteristics. This work provided a simple and effective alternative to prepare excellent hydrophobic composite membranes for MD applications.     

Energetic Performance and Permeate Flux Investigation of Direct-Contact Membrane Distillation for Seawater Desalination

The performance of the direct-contact membrane distillation (DCMD) process in desalting Mediterranean seawater was investigated. Theoretical and experimental optimization of various operating parameters was conducted. The effects of temperature differences, feed velocity, and membrane characteristics were studied. When using commercial polyvinylidene fluoride membranes, the vapor transfer throughout the membrane pores is dominated by the Knudsen-molecular diffusion model. Maximum permeate flux was obtained when increasing temperature, feed velocity, membrane pore size, and porosity and decreasing membrane tortuosity and thickness. Thermal efficiency, gained output ratio, and specific thermal energy consumption were improved when increasing feed temperature. By application of the DCMD process to Mediterranean seawater, a high water quality was obtained.     

浓盐溶液的膜蒸馏机理研究

考察了高浓度氯化钠溶液在直接接触式膜蒸馏操作中的热侧温度，冷侧温度，流速和料液浓度对膜渗透通量的影响。发现膜渗透通量随氯化钠溶液浓度的高而降低。当氯化钠溶液浓度约为25％时，膜渗透通量急剧下降。氯化钠溶液达到饱和时，膜渗透通量逐渐趋于稳定。研究认为此现象是由于膜面上有NaCl结晶形成而导致的。     

真空膜蒸馏过程膜参数模拟计算

为了研究膜材料对透过通量的影响,文章建立了真空膜蒸馏过程的微观传热传质模型,并利用该模型对文献中的实验数据进行了模拟,得到了比较满意的结果。进而对不同条件下膜的长度、厚度、内径以及曲折因子、传质系数进行了模拟计算,并讨论了不同膜参数对膜透过通量的影响,为真空膜蒸馏过程中膜材料的选取及合理利用提供一定的理论依据。     

多孔陶瓷膜烟气水分回收理论与模型研究

化石燃料燃烧烟气中含有大量水分和潜热,高湿度烟气的直接排放造成极大的资源浪费和环境问题。多孔陶瓷膜是目前烟气水热回收最有前景的技术之一,其水分回收热力学和动力学的定量描述是该技术发展和装置设计的关键所在。分析了水分在多孔陶瓷膜表面及内部传质机理,基于Kelvin理论建立了水分在陶瓷膜内毛细凝聚热力学模型,并选取典型烟气温/湿度条件,得出不同工况下陶瓷膜发生毛细凝聚的临界孔径、凝聚水量及工作孔体积占比;进而基于毛细凝聚的表面传质和孔道输运Hagen-Poiseuille方程建立了陶瓷膜水分传质动力学模型,对典型温/湿度工况下回收水通量进行了计算,结果表明,多孔陶瓷膜的毛细凝聚效应对烟气水分回收的优越性十分明显,其表面回水通量远远大于冷凝法的水通量,孔径越小,表面水通量越高,但及时将孔道内的液态水输运到陶瓷膜另一侧需要的压差也越大,本文计算条件下,膜孔径为20.0 nm的陶瓷膜较为适宜。     

Effect of ionic liquid on the structure and desalination performance of PVDF-PTFE electrospun membrane

The relatively large pore size of electrospun membranes might limit their application for direct contact membrane distillation (DCMD). Incorporation of ionic liquid is a potential approach to decrease the pore size of electrospun membranes, which was attributed to the increased conductivity of electrospinning solution. In this study, a novel nanofibrous membrane based on the blends of poly(vinylidene fluoride) (PVDF), polytetrafluoroethylene (PTFE) and ionic liquid (BMIMPF₆) was fabricated and applied for the DCMD. The effects of the BMIMPF₆ on the morphology, pore size and DCMD performance of the PVDF-PTFE nanofibrous membrane were investigated. Compared with neat (PVDF-PTFE) membranes (average pore size: 0.93 μm), the incorporation of BMIMPF₆ resulted in a smaller mean pore diameter (0.58 μm). The liquid entry pressure value of the modified composite membrane also increased from 62.75 kPa (neat) to 83 kPa, due to the decreased pore size. The composite membrane exhibited a longer lifespan (about 26 h) than neat membrane during long-term DCMD process, which makes this composite membrane a promising candidate for DCMD application. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48467.     

多孔陶瓷膜烟气水分回收理论与模型研究

化石燃料燃烧烟气中含有大量水分和潜热,高湿度烟气的直接排放造成极大的资源浪费和环境问题。多孔陶瓷膜是目前烟气水热回收最有前景的技术之一,其水分回收热力学和动力学的定量描述是该技术发展和装置设计的关键所在。分析了水分在多孔陶瓷膜表面及内部传质机理,基于Kelvin理论建立了水分在陶瓷膜内毛细凝聚热力学模型,并选取典型烟气温/湿度条件,得出不同工况下陶瓷膜发生毛细凝聚的临界孔径、凝聚水量及工作孔体积占比;进而基于毛细凝聚的表面传质和孔道输运Hagen-Poiseuille方程建立了陶瓷膜水分传质动力学模型,对典型温/湿度工况下回收水通量进行了计算,结果表明,多孔陶瓷膜的毛细凝聚效应对烟气水分回收的优越性十分明显,其表面回水通量远远大于冷凝法的水通量,孔径越小,表面水通量越高,但及时将孔道内的液态水输运到陶瓷膜另一侧需要的压差也越大,本文计算条件下,膜孔径为20.0 nm的陶瓷膜较为适宜。     

吸收膜蒸馏的传热传质过程

由于吸收膜蒸馏系统的进料液和吸收液的温度非常接近甚至相同,其传热过程无相变热损失,有可能实现能耗较低的膜蒸馏过程,本文尝试开展吸收膜蒸馏法海水淡化研究。首先分别以葡萄糖水溶液和氯化钙水溶液为吸收液进行吸收膜蒸馏海水淡化实验,结果表明氯化钙吸收液时的膜通量明显高于葡萄糖吸收液时的膜通量。其次建立了吸收膜蒸馏过程的质量和热量传递模型,并对模型进行了计算,模型计算值与实验值非常接近。最后对葡萄糖水溶液和氯化钙水溶液分别作吸收液时对传质系数和极化现象的影响进行了计算分析,结果显示,氯化钙水溶液作吸收液时传质系数约为葡萄糖水溶液的1.7倍;且极化现象造成的蒸气压力差减小的值为后者的1/4左右。     

Study on a new air-gap membrane distillation module for desalination

A new air-gap membrane distillation (AGMD) module for desalination with internal latent-heat-recovery which consisted of parallel hollow fiber membranes and heat exchange hollow fibers was successfully developed. The influences of feed flow rate, feed temperature and feed initial concentration on AGMD process were investigated. The vapor pressure polarization coefficient (η) was introduced to measure the reduction in the effective driving force for mass transfer with regard to the driving force imposed. Among all AGMD experiments, the maximum water vapor permeate flux (J_D) of 5.30 kg/m² h and the gained output ratio (GOR) of 5.70 were obtained. A theoretical model based on the mass and energy balances of the hot feed side was established to calculate the temperature and the local water vapor permeate flux distributions along the hollow fiber membrane, which showed that the temperature drop and local water vapor permeate flux drop were much larger at the upper part than those at the lower part of the membrane module in the hot feed side.     

CFD simulation on membrane distillation of NaCl solution

A computational fluid dynamics (CFD) simulation that coupled an established heat and mass transfer model was carried out for the air-gap membrane distillation (AGMD) of NaCl solution to predict mass and heat behaviors of the process. The effects of temperature and flowrate on fluxes were first simulated and compared with available experimental data to verify the approach. The profiles of temperature, temperature polarization factor, and mass flux adjacent to the tubular carbon membrane surface were then examined under different feed Reynolds number in the computational domain. Results show that the temperature polarization phenomena can be reduced, and mass flux can be enhanced with increase in the feed Reynolds number.     

Effects of embedding functionalized multi-walled carbon nanotubes and alumina on the direct contact poly(vinylidene fluoride-co-hexafluoropropylene) membrane distillation performance

Flat-sheet membranes were fabricated by incorporating alumina (Al₂O₃) and functionalized multiwalled carbon nanotubes (MWCNTs; MWCNTs-COOH) in PVDF-co-HFP membrane via the phase-inversion method for application in membrane distillation (MD) application. Scanning electron microscopy and atomic force microscopy were performed on the resulting membranes to investigate the effects of functionalized MWCNTs. The results revealed that the embedding of functionalized MWCNTs led to a significant modification of the membrane characteristics, including the structural morphology, thickness, roughness, porosity, pore size, and pore size distribution. The effects of operational parameters such as the hot feed solution temperature (47–67?°C), feed flow rate (0.35–0.55?L/min), and feed concentration (0–100?g/L) on the performance of the fabricated membrane were tested using the DCMD system. The experimental results demonstrated that the permeate flux was enhanced by approximately 32.43% by using functionalized MWCNTs, reaching a value of 16.35?kg/m² h at 35?g/L feed concentration, 67?°C hot feed temperature, and 0.55?L/min feed flow rate, at the constant temperature of 20?°C and 0.35?L/min flow rate. The functionalized MWCNTs embedded within the membrane successfully modified the interactions between water and the membrane to improve the water vapor transport while inhibiting salt penetration into the pores.     

低温膜蒸馏及其过程模拟

采用聚丙烯中空纤维膜对纯水介质膜蒸馏、冷侧盐水循环有温差膜蒸馏及等温渗透膜蒸馏进行了实验研究,建立了描述膜蒸馏过程的传质及传热数学模型,以实验数据为基础对模型中的参数进行了回归并对数学模型进行了计算机数值求解,计算结果与实验数据吻合较好     

Air gap membrane distillation on the different types of membrane

Seawater desalination through the air gap membrane distillation (AGMD) process shows merit for its ambient operational conditions and low energy consumption. In this paper nine types of commercially available membranes were characterized to understand the membranes more comprehensively. The density, porosity, mean pore radius, liquid entry pressure (LEP), and contact angle (CA) of the membranes were determined. AGMD experiments were performed for the membranes to investigate the membrane difference on permeation flux and salt rejection. The effects of operating parameters such as temperature, flow rate, and NaCl concentration were studied. The 0.22 μm pore size PTFE membrane showed excellent performance for its higher permeability and higher hydrophobicity than other membranes. The mass transfer coefficients for three types of PTFE membranes were calculated from the results of the experiments.     

过程参数对采用多孔陶瓷超滤膜回收烟气中余热和水性能的影响

将孔径为20 nm的陶瓷膜组装制成膜冷凝器,在水蒸气-空气形成的模拟体系中,采用去离子水作为冷却介质,开展了传递膜冷凝技术在烟气除湿和工业余热综合应用方面的研究。考察了空气流量、冷却水流量、进气温度和冷却水温度对陶瓷内膜和外膜过程通量的影响,并比较了两者水热回收性能。结果表明,过程通量均随进气流量和进气温度的增大而增加。随着冷却水流量的增大,过程通量也不断增加,但是冷却水流量达到一定值后,过程通量基本不再变化。冷却水温度对过程水通量的影响较小,但是热通量对冷却水温度的改变较敏感。冷却水流量的变化对陶瓷外膜的过程通量影响更加显著,表明陶瓷外膜水热回收过程更易受流体边界层的影响。在各实验工况范围内,陶瓷内膜和外膜分别具有更高的热通量和水通量,采用陶瓷膜过程的水通量和热通量最高分别可达到23.1 kg·m^-2·h^-1和47.5 MJ·m^-2·h^-1。随着传递膜冷凝技术开发和研究的不断深入,该技术在除湿和工业余热综合应用领域有着广阔的发展空间,将为我国节水、节能以及环境保护等领域的发展提供新的解决思路。     

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。     

Experimental Study of the Effect of Membrane Porosity on Membrane Absorption Process

Abstract

Five kinds of expanded polytetrafluoroethylene (ePTFE) flat membranes with different porosity and micro‐pore size were chosen to carry out the unsteady state gas absorption experiments. The influence of the membrane porosity and micro‐pore size on the membrane absorption process was investigated. Experimental results showed that the membrane porosity had an impact on the membrane absorption process, and the degree of this impact depended on the absorption system and the membrane pore size. For a rapid mass transfer process, the porosity affected the mass transfer more significantly, while for the slow mass transfer process the porosity almost did not affect the mass transfer. The modeling analysis showed that the porosity affected the concentration profile of the solute near the membrane surface on the liquid side, which led to the reduction of the mass transfer area of the membrane. Hence, the apparent effect on the mass transfer of the membrane absorption process.