A new approach for modeling of RO membranes using MD-SF-PF model and CFD technique

The modified surface force-pore flow (MD-SF-PF) model is a successful and powerful model for prediction of reverse osmosis (RO) membrane separation. In the MD-SF-PF model, a suitable mass and momentum balances were made through the cylindrical micro porous of the membranes. A one-dimensional potential function in radial direction was used to attain the velocity profile. The obtained nonlinear equations were solved by orthogonal collocation method. In our model, extended MD-SF-PF (Ex-MD-SF-PF) model, the potential function of the MD-SF-PF model was developed as both radial and axial directions. Both Ex-MD-SF-PF and MD-SF-PF models were solved by finite volume and finite difference methods based on a computational fluid dynamics (CFD) technique. The profiles of velocity and concentration were achieved and thereby the values of separation factors and fluxes for both models were obtained and were compared accordingly. This approach affected only at the end of pore, mainly on the concentration profile and slightly on the velocity profile.     

Membrane potential in charged porous membranes

For charged porous membranes, the separation efficiency to charged particles and ions is affected by the electrical properties of the membrane surface. Such properties are most commonly quantified in terms of zeta-potential. In this paper, it is shown that the zeta-potential can be calculated numerically from the membrane potential. The membrane potential expression for charged capillary membranes in contact with electrolyte solutions at different concentrations is established by applying the theory of non-equilibrium thermodynamic to the membrane process and considering the space-charge model. This model uses the Nernst–Planck and Navier–Stokes equations for transport through pores, and the non-linear Poisson–Boltzmann equation, which is numerically solved, for the electrostatic condition of the fluid inside pores. The integral expressions of the phenomenological coefficients coupling the differential flow (solute relative to solvent) and the electrical current with the osmotic pressure and the electrical potential gradients are established and calculated numerically. The mobilities of anions and cations are individually specified. The variations of the membrane potential (or the apparent transport number of ions in the membrane pores) are studied as a function of different parameters: zeta-potential, pore radius, mean concentration in the membrane, ratio of external concentrations and type of ions.     

Calculation of the salt separation by negatively charged gel-filled membranes

The salt separations of negatively charged gel-filled membranes composed of poly(2-acrylamido-2-methylpropanesulfonic acid) gels anchored within a polypropylene microporous substrate have been determined experimentally and modeled theoretically. The separation of these membranes were calculated by both the Teorell, Meyer and Sievers (TMS) model and the Donnan–Steric Pore (DSP) model coupled with the extended Nernst–Planck equation. For modeling, the membrane effective thickness, effective charge density, and pore radius were either directly measured or calculated from theories without the use of fitting procedures. Good agreement between the experimental measurements and the theoretical calculations of salt separation was observed. For the theoretical calculations, the TMS model is suitable for membranes with moderate gel polymer volume fractions, while the DSP model is more suitable for membranes with high gel polymer volume fractions. Moreover, with a calculated constant effective charge density, the salt separation with different salt concentrations could be accurately predicted. The separation of various other salts could also be predicted with good accuracy.     

Modelling of the pore size distribution of ultrafiltration membranes

A dilute aqueous solution of polydisperse neutral dextrans was used to determine the sieving properties (flux and rejection) of porous polyacrylonitrile membranes. Gel ermeation chromatography was used to measure the solute mole and concentration in the permeate. From these data, rejection coefficients were calculated as a function of solute molecular size. A mathematical model was then developed to relate the flux and solute rejection to pore size distribution and the total number of pores, based upon the assumption that solute rejection was the result of purely geometric considerations. As a first approximation, a solute molecule was considered either too large to enter a membrane pore, or if it entered, its concentration in the permeate from that pore, as well as the solvent flux through the pore, were not affected. This model also considered the effects of steric hindrance and hydrodynamic lag on the convection of solute through a membrane. The shape and sharpness of pore size distributions were found to be useful in comparisons of ultrafiltration membranes.     

Pore Engineering for Covalent Organic Framework Membranes

Membrane technology is of particular significance for the sustainable development of society owing to its potential capacity to tackle the energy shortage and environmental pollution. Membrane materials are the core part of membrane technology. Researchers have always been pursuing predictable structures of advanced membrane materials, which provides a possibility to fully unlock the potential of membranes. Covalent organic frameworks(COFs), with the advantage of controllable pore microenvironment, are considered to be promising candidates to achieve this design concept. The customizable function of COF membranes through pore engineering does well in the enhancement of selective permeability performance, which offers COF membranes with great application potentials in separation and transportation fields. In this context, COF-based membranes have been developed rapidly in recent years. Herein, we present a brief overview on the strategies developed for pore engineering of COF membranes in recent years, including skeleton engineering, pore surface engineering, host-guest chemistry and membrane fabrication. Moreover, the features of transmission or separation of molecules/ions based on COF membranes and corresponding applications are also introduced. In the last part, the challenges and prospects of the development of COF membranes are discussed.     

Electrophoretic separation method for membrane pore‐forming proteins in multilayer lipid membranes

In this paper, we report on a novel electrophoretic separation and analysis method for membrane pore‐forming proteins in multilayer lipid membranes (MLMs) in order to overcome the problems related to current separation and analysis methods of membrane proteins, and to obtain a high‐performance separation method on the basis of specific properties of the lipid membranes. We constructed MLMs, and subsequently characterized membrane pore‐forming protein behavior in MLMs. Through the use of these MLMs, we were able to successfully separate and analyze membrane pore‐forming proteins in MLMs. To the best of our knowledge, this research is the first example of membrane pore‐forming protein separation in lipid membranes. Our method can be expected to be applied for the separation and analysis of other membrane proteins including intrinsic membrane proteins and to result in high‐performance by utilizing the specific properties of lipid membranes.     

Morphology of microporous poly(vinylidene fluoride) membranes studied by gas permeation and scanning electron microscopy

Microporous poly(vinylidene fluoride) (PVDF) membranes with asymmetric pore structure were prepared by a wet phase inversion process. The polymer was precipitated from a casting solution when immersed in a cold water (gelation) bath. The casting solution was, in most cases, composed of polymer, solvent, and nonsolvent. In this solvent-nonsolvent system, the solvents used were triethylphosphate (TEP) and dimethylsulfoxide (DMSO), and the nonsolvents used were glycerol and ethanol. Mean pore sizes and effective porosity of the microporous membranes were calculated using the gas permeation method. They were studied as a function of evaporation time of wet nascent film, polymer molecular weight, concentration of polymer, and concentration of nonsolvent. The morphology of the membranes was examined by scanning electron microscopy (SEM).     

Fabrication of Porous Polyimide Membrane with Through-Hole via Multiple Solvent Displacement Method

Porous polyimide (PI) membranes are widely used in separation processes because of their excellent thermal and mechanical properties. However, the applications of porous PI membranes are limited in the nanofiltration range. In this study, porous PI membranes with through-holes have been successfully fabricated by the novel multiple solvent displacement method. This new method requires only a porous polyamic acid (PAA) membrane, which was prepared by immersing PAA film in N-methylpyrrolidoneebk; (NMP) prior to immersing it in a mixed solvent consisting of NMP and a poor solvent, followed by immersion only in poor solvent. The pore size, morphology, porosity, and air permeability demonstrated that the fabricated PI membranes had a uniformly porous structure with through-holes over their surface. This new method enabled control of pore size (3–11 μm) by selecting a suitable poor solvent. This multiple solvent displacement method is highly versatile and promising for the fabrication of porous PI membranes.     

Facile Fabrication of a Continuous ZIF-67 Membrane for Efficient Azeotropic Organic Solvent Mixture Separation

Azeotropic organic solvent mixture separation is common in the chemical industry but extremely difficult. Zeolitic imidazolate framework-67 (ZIF-67) shows great potential in organic solvent mixture separation due to its rigid micropores and excellent stability. However, due to the fast nucleation rate, it is a great challenge to prepare continuous ZIF-67 membrane layers with ultrathin thickness. In this study, a hydroxy salt layer with high inducible activity was synthesized as a precursor on different porous substrates to prepare ZIF-67 membranes at room temperature. The precursor layer enables an intact ZIF-67 membrane with an ultrathin thickness of 176±12 nm. The experimental and simulation results confirmed that the size sieving through the pore windows and the preferential adsorption of polar solvent molecules provide the ZIF-67 membrane an unprecedented separation performance such as high separation factors and fluxes, for four types of azeotropic organic solvent mixtures.     

Comparison of methods for characterizing microporous membranes for plasma separation

To design membranes suitable for therapeutic use, the relationship between membrane structure and permeability needs to be studied. In this work, the solute permeability of small tubular membranes for plasma separation was determined by using radioisotope-labeled solutes. Through analysis of data on solute and pure water permeability and on water content, by means of a tortuous pore model that we have proposed, we can obtain pore diameter, surface porosity and tortuosity. Membrane structure was also analyzed by mercury intrusion and scanning electron microscopy, and the results were compared with each other. The mercury intrusion method is unsuitable for the structural analysis of polymer membranes because of the damage and/or expansion resulting from highly elevated pressure. The tortuous pore model is recommended for the elucidation of membrane structure.     

溶剂对相分离法制备聚偏氟乙烯微孔膜性质的影响

研究了用相转换法制备聚偏氟乙烯（PVDF）微孔膜时溶剂对成膜性质的影响.用浊点法测定了二甲基亚砜、N,N-二甲基甲酰胺、N,N-二甲基乙酰胺、N-甲基吡咯烷酮、磷酸三甲酯等五种溶剂配制的质量分数为wPVDF=0.12的铸膜液在30℃时的相分离点,显微镜拍照法测定了这些铸膜液与水接触时相分离前沿推进速率,泡点法测定了膜孔径,并测定了气体通量.结果表明,二甲基亚砜、磷酸三甲酯、N,N-二甲基乙酰胺是适于制作聚偏氟乙烯微孔膜的溶剂.     

MICROPOROUS PVDF-HFP-BASED POLYMER MEMBRANES FORMED FROM SUPERCRITICAL CO2 INDUCED PHASE SEPARATION

Microporous poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP)membranes following supercritical CO_2 induced phase separation process were prepared using four solvents.The solid electrolytes of PVDF-HFP were formed by microporous PVDF-HFP membranes filled and swollen by a liquid electrolyte.The effect of the solvents on the morphology and structure,electrolyte absorptions and lithium ionic conductivity of the activated membranes were investigated.It was approved that all the membrane had the simi...     

溶剂浓度对PVDF相转换膜大孔结构的影响

提出决定大孔能否发展的初始分相点处溶剂浓度临界点的概念 ,认为初始分相点处较高的溶剂浓度有利于大孔的发展 ,溶剂浓度低于一定的界限后 ,大孔停止发展 ,转为海绵状结构 .实验考察了不同凝胶液组成下制得的PVDF中空膜的结构 ,建立了相应的传质模型 ,模拟不同制膜条件下初生态膜内的组成分布情况 ,根据初始分相点处溶剂浓度临界点的概念 ,预测膜的形态结构 .模拟结果与相应制膜条件下的电镜照片有很好的对应关系 ,证明了上述大孔形成机理的正确性 .     

热致相分离技术制备聚氨酯多孔膜的条件控制

采用自制的模具 ,利用热致相分离 (TIPS)的原理制备了聚氨酯 (PU)多孔膜 ,并重点研究了聚合物浓度对多孔膜的表面形貌、孔度大小、孔隙率和透湿率的影响 .在不同的聚合物浓度条件下制备的PU多孔膜的共同特征是底面 (与成膜平台接触面 )光滑平整 ,孔洞尺寸较小 ,为纳米级 ;而表面 (与空气接触的自由面 )的形貌结构较为复杂 ,但都有明显的孔洞出现 ,且孔洞的尺度大于底面 ,在微米级以上 .聚氨酯 1,4 二氧六环 (DO)形成的是上临界共溶温度 (UCST)体系 ,在发生相分离后底面与表面粗化时间的不同是导致形貌结构差异的主要原因 .改变冷台温度或调整DO H2 O的比例也会对PU多孔膜的孔度大小和形貌结构产生明显的影响     

Phase equilibria in ternary aqueous mixtures of 1,3-butanediol with 2-ethyl-1-hexanol at T = (298.2, 303.2 and 308.2) K

Experimental tie-line data for ternary system of (water + 1,3-butanediol (1,3-BD) + 2-ethyl-1-hexanol (2EH)) were determined at T = (298.2, 303.2 and 308.2) K under atmospheric conditions. This ternary system exhibits type-1 behavior of LLE. The experimental ternary LLE data were correlated using the NRTL model, and the binary interaction parameters were obtained. The average root-mean-square deviation between the observed and calculated mole fractions was 1.38%. Distribution coefficient and separation factor were measured to evaluate the extracting capability of the solvent. The separation factor values for the solvent used in this work were then compared with literature values obtained in our previous works for other butanediols.     

Using breath figure patterns on structured substrates for the preparation of hierarchically structured microsieves

Microsieves are advanced filtration membranes characterized by a uniform pore size, a high pore density, and a thickness smaller than the pore diameter. The uniform pore size provides a high selectivity; the small thickness gives rise to a high flux and allows efficient removal of any filter cake by backflushing. However, microsieves are sensitive to mechanical stress. Thus, they need either an external macroporous support or a hierarchical structure that provides an integrated supportive structure. We prepare microsieves with a hierarchical pore structure by creating breath figure patterns within layers of solutions of polymers in a volatile solvent that are spread out on top of structured supports. For the formation of breath figure patterns, the volatile solvent is evaporated in a moist atmosphere. This cools the surface to such an extent that dew droplets form on the thin film, partially penetrate into the layer, and create a concave imprint in the final solid polymer layer. This procedure is usually done on flat surfaces; in our case the spreading of the polymer solution is done on a support decorated with protrusions. In this procedure, the dew droplets touch the protrusions of the structured support before the polymer solution vitrifies. At the same time, the trenches of the structured substrate are filled with polymer much deeper than the penetration depth of the dew droplets. After the separation of the vitrified layer from the substrate, we obtain thin polymer membranes with a hierarchical structure consisting of an ultrathin active separation layer with submicrometer pores and a supporting layer with larger pores.     

热致相分离法制备聚苯硫醚微孔膜

采用热致相分离法,以己内酰胺为溶剂,制备得到了聚苯硫醚微孔膜并对薄膜性能表征.聚苯硫醚-己内酰胺体系制膜的优点之一是溶剂己内酰胺是水溶性的,可以采用纯水作为后处理的萃取剂.选择了合适的浓度,利用压制成型法制备聚苯硫醚平板膜;研究了体系冷却时的相行为,并考察了降温速率、聚合物浓度等因素对微孔形态与薄膜性能的影响.研究表明,聚苯硫醚-己内酰胺体系以固液分相为主,萃取后形成球晶状的微孔结构.降温速率对薄膜的微孔形态、孔径以及连通性有重要影响;当体系以较低降温速率冷却时,多孔形态为枝叶状,形成了更多的开孔结构并获得了更大的孔径,这是获得高通量微孔膜的主要原因.通过控制降温速率可以制备纯水通量大于100 L/m2h,孔径约4～5μm且连通性良好的聚苯硫醚微孔膜;研究了聚合物浓度的影响,薄膜的纯水通量随着聚合物浓度的增大而减小,并且当聚苯硫醚浓度>50 wt%时,由于大于临界浓度而失去渗透性.     

Nanoporous thermosetting polymers

Potential applications of nanoporous thermosetting polymers include polyelectrolytes in fuel cells, separation membranes, adsorption media, and sensors. Design of nanoporous polymers for such applications entails controlling permeability by tailoring pore size, structure, and interface chemistry. Nanoporous thermosetting polymers are often synthesized via free radical mechanisms using solvents that phase separate during polymerization. In this work, a novel technique for the synthesis of nanoporous thermosets is presented that is based on the reactive encapsulation of an inert solvent using step-growth cross-linking polymerization without micro/macroscopic phase separation. The criteria for selecting such a monomer-polymer-solvent system are discussed based on FTIR analysis, observed micro/macroscopic phase separation, and thermodynamics of swelling. Investigation of resulting network pore structures by scanning electron microscopy (SEM) and small-angle X-ray scattering following extraction and supercritical drying using carbon dioxide showed that nanoporous polymeric materials with pore sizes ranging from 1 to 50 nm can be synthesized by varying the solvent content. The differences in the porous morphology of these materials compared to more common free radically polymerized analogues that exhibit phase separation were evident from SEM imaging. Furthermore, it was demonstrated that the chemical activity of the nanoporous materials obtained by our method could be tailored by grafting appropriate functional groups at the pore interface.     

MXene: A two-dimensional material in selective water separation via pervaporation

MXene, well-identified as Ti₃C₂T_X, belongs to the family of two-dimensional (2D) materials, which have been currently explored in various applications. Very recently, such materials have been pointed out as potential nanomaterials for advanced solute separations when introduced in membranes, such as ion separation, gas separation, nanofiltration, chiral molecular separation, and solvent separation. This latter separation, generally named Pervaporation (PV), is identified as a highly selective technology for water separations. To date, few pieces of research have been released but providing interesting insights into several solvent (including water) separations. Hence, this brief review aims to analyze and discuss the latest advances for utilizing MXenes for PV membranes. Particular emphasis has been devoted to the relevant outcomes in the field, along with the strategies followed by researchers to tailor membranes. Based on the current findings, the perspectives in the field are also stated.