Transient solute adsorption incorporated modeling and simulation of unstirred dead-end ultrafiltration of macromolecules: An approach based on self-consistent field theory

An unsteady state mass transfer model of unstirred dead-end ultrafiltration module has been developed in the present study. The dynamic membrane surface concentration is evaluated using a modified self-consistent field theory, which enables the development of an algorithm to incorporate the contribution of solute adsorption in the membrane surface concentration dynamics. Knowing the corrected membrane surface concentration, permeate flux and permeate side concentration is determined using osmotic pressure model and flux–rejection relation as predicted by irreversible thermodynamics. The time evolution of all the different process variables is achieved by solving two component balance equations developed at the membrane surface and in the solution phase respectively. The basic feature of the model is the incorporation of adsorbed fraction in the unsteady state membrane surface concentration. For the validation of the proposed model, experiments were conducted with Bovine Serum Albumin (BSA)/water as feed in a standard unstirred batch ultrafiltration module fitted with Polyethersulfone (PES) membrane of 30 kDa molecular weight cut-off (MWCO). The model predicted flux and the permeate concentration were found to be in good agreement with the experimental data.     

Simulation of continuous stirred rotating disk-membrane module: An approach based on surface renewal theory

A rigorous surface renewal model has been developed describing the aspects of mass transfer in a rotating disk-membrane (RDM) ultrafiltration cell. The model takes into consideration of two distribution functions of random surface elements, one with respect to their point of origin and the other related to the corresponding residence time on the membrane surface. The back transport flux and the permeate flux are evaluated at the membrane surface in order to develop a surface component balance equation. The component balance equation and a flux-rejection relationship arising from irreversible thermodynamics are solved simultaneously to develop a dynamic simulation. The simulation elucidates on permeate flux, membrane surface concentration and the permeate concentration under various operating conditions of transmembrane pressure, bulk concentration, membrane and stirrer speeds. For validation of the proposed model, experiments were conducted with bovine serum albumin (BSA)/water as feed in a standard rotating disk membrane module fitted with polyethersulfone (PES) membrane of 30 kDa molecular weight cut-off (MWCO). The model predicted flux and permeate concentration was found to be in good agreement with the experimental data, and the maximum absolute deviation for both cases was found to be well within ±5%.     

The Usage of Non‐woven Fabric Material as Separation Media in Submerged Membrane Photocatalytic Reactor for Degradation of Organic Pollutants in Water

Abstract

Photodegradation tests with continuous mode were carried out in a photocatalytic reactor containing a submerged flat sheet non‐woven fabric membrane, which was able to retain the photocatalyst. The non‐woven fabric membrane was made of polypropylene with a pore size of 2 µm. In this study, three operational parameters were investigated: light intensity, feed concentration, and feed flow rate. Experimental results showed no significant variation of permeate flux and TMP (transmembrane pressure) during the experimental period. The SS (suspended solid) of the permeate was non‐detectable. These behaviors were due to the formation of a dynamic porous cake layer on the non‐woven fabric membrane surface in our experimental condition. We also found that it was important to control the operation parameter mentioned above as to exactly when we wanted to maintain the steady pollutant concentration in the permeate at very low value.     

Unsteady-State Permeate Flux of Crossflow Microfiltration

Abstract

In this study a membrane filtration cell was installed to investigate the variation of permeate flux with filtration time under various operating conditions including crossflow velocity, pressure drop, particle concentration, membrane pore size, particle size, pH, and electrolyte concentration. The dimensions of the filtration channel in the CFMF cell were 6 cm x 0.6 cm x 0.036 cm, and the flow of the suspension in the channel was controlled under the laminar flow region. Spherical polystyrene latex particles of 0.303, 0.606, and 1.020 μm were used as the suspension particles in the experiments. The density of the particles was 1.05 g/cm³. It was found that the unsteady-state permeate flux increased with an increase in particle size, membrane pore size, or crossflow velocity, but decreased with an increase in particle concentration or electrolyte concentration in the suspension. A mathematical model based on mass balance and hydrodynamic theory was developed in this study. In addition, the effect of cake growth and particle concentration decline during experiments on the permeate flux were also considered in this model. This model predicts satisfactorily the unsteady-state permeate flux of CFMF under various operating conditions.     

Tubular Membrane Filtration with A Side Stream and its Intermittent Backwash Operation

The tubular membrane filtration system is widely applied to solid-liquid separation processes. Any improvements to the filtration module would increase separation efficiency, thus reducing operating costs. In this experiment, PMMA powder with an average particle diameter of 0.8 µm was filtered by a ceramic tubular membrane with an average pore size of 0.2 µm, and the impacts of the operating variables, such as suspension concentration, the filtration pressure, and the crossflow velocity on the permeate flux were discussed. In order to understand the increased permeate flux, the proposed module is comparable to the tubular membrane filtration module, but with an additional side stream under the same filtration mass flow rate. In addition, variations of shear force on the membrane surface are analyzed by CFD simulation, and the influence of backwash operations on the permeate flux is discussed. The results show that the side stream membrane filtration increased the shear force on the membrane surface, reduced fouling on the membrane surface, and increased the permeate flux. Furthermore, a backwash operation with a side stream flow channel could effectively clean the particles deposited in the module, thus, increasing the permeate flux.     

Numerical study of BSA ultrafiltration in the limiting flux regime — Effect of variable physical properties

Ultrafiltration of BSA solutions in a parallel cell was studied by numerical methods taking into account concentration dependent properties. The Navier-Stokes equations, in the stream function-vorticity formulation, and the mass transport equation were solved simultaneously. The numerical code was validated by comparison with data from benchmark analytical solutions. The flow and concentration fields in the limiting flux regime were studied in detail and compared with the fields for constant properties. Numerical results were also compared with experimental data of the permeate velocity in the limiting flux regime published in the literature. This comparison shows that the concentration dependence of the viscosity, diffusivity and osmotic pressure are all necessary to explain experimental results published in the literature.     

Elimination of organic matter present in wastewaters from the cork industry by membrane filtration

BACKGROUND: The first stage of the cork industrial process generates great volumes of wastewater with moderate to high organic pollutant content that must be purified using different procedures, such as filtration by membranes. RESULTS: The tangential filtration of these wastewaters was studied using two different laboratory equipments. In the first one, three ultrafiltration (UF) membranes were tested, with molecular weight cut‐off (MWCO) 100 kDa and 30 kDa, and two operating modes were used: total recycling of permeate and retentate streams, and in continuous mode, without recycling both streams. In the total recycling UF experiments, the influence of the operating variables on the permeate flux was first established. The effectiveness of the different membranes was determined by evaluating the rejection coefficients for several parameters that measure the global pollutant content of the effluent. The values found for these rejection coefficients were in the following order: ellagic acid and color > absorbance at 254 nm > tannic content > COD (chemical oxygen demand). In the continuous mode experiments, the fouling mechanism for each membrane was established by fitting the experimental data to various filtration fouling models given in the literature. The operating mode in the second equipment was batch concentration, and additional experiments were carried out with an UF membrane (2 kDa), and with a NF membrane (with MWCO in the range 150–300 Da). CONCLUSIONS: The three operating modes tested provided different rejection levels of organic matter; among them, the most effective procedure tested was batch concentration mode using a NF membrane. Copyright © 2007 Society of Chemical Industry     

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.     

Modeling of the Ultrafiltration of a Dextran T500 Solution in a Tubular Membrane Module

Mathematical modeling of an ultrafiltration membrane separation process, based mainly on the transmembrane pressure (TMP), is undertaken in the present work. The main objective is the prediction of the permeate flux of a solution containing Dextran T500 through a cylindrical module. The proposed model incorporates the resistance‐in‐series model coupled with the equation describing the solute (Dextran T500) transport, as well as the continuity and Navier‐Stokes equations for solution flow modeling. The model equations are solved using finite‐volume numerical methods, with appropriate initial and boundary conditions. The effects of the TMP and the length of the membrane on the mean permeate flux were also investigated. The influence of the membrane dimensions (aspect ratio) on the relative dimensionless mean permeate flux, at different inlet TMPs and different solution concentrations, respectively, have also been considered. The variations of the TMP with the membrane length as well as the influence of the Peclet number on the solute surface concentration were also examined. The numerical results obtained are compared with experimental values reported in the literature, and in general, the agreement is satisfactory enough to encourage further refinement of the model.     

Hydrophobic mullite ceramic hollow fibre membrane (Hy-MHFM) for seawater desalination via direct contact membrane distillation (DCMD)

A low-cost hydrophobic mullite hollow fibre membrane (Hy-MHFM) fabricated via phase inversion/sintering technique followed by fluoroalkyl silane (FAS) grafting is presented in this study. The prepared CHFMs were characterized before and after the grafting step using different characterization techniques. The pore size of the CHFM surface was also determined using ImageJ software. The desalination performance of the grafted membrane was evaluated in direct contact membrane distillation (DCMD) using synthetic seawater of varying salt concentrations for 2 h at various feedwater temperatures. The outcome of the evaluations showed declines in the permeate flux of the membrane at increasing feed concentration, as well as increased flux with increased feed temperature. The long-term stability of the membrane was achieved at time 20 h, feed temperature 60 °C, and permeate temperature 10 °C, the membrane achieved a salt rejection performance of about 99.99 % and a water flux value of 22.51 kg/ m² h.     

Modeling of a membrane reactor system for crude palm oil transesterification. Part II: Transport phenomena

The mechanistic modeling of biodiesel production process in membrane reactor with the consideration of chemical reaction, phase equilibrium, and ultrafiltration is important for the membrane reactor design. In part II of this work, the chemical and phase equilibrium (CPE) model for crude palm oil transesterification reaction in the membrane reactor developed in part I is extended to an integration of CPE with modified Maxwell–Stefan model, which considers multicomponent mass transport phenomena of concentration polarization and intramembrane. A good fit of simulated permeate fluxes and apparent solute rejection to the experimental data shows that the model has a good prediction capability. Reversible fouling was found to be the major fouling and no pore plugging was observed. Simulation results verified that micelles were retained by the membrane at CPO:MEOH molar ratio of 1:24 and catalyst concentration of 0.5 wt %. However, phase inversion happened when catalyst concentration of 0.05 and 0.1 wt % were used. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1981–1996, 2015     

Arsenic removal from drinking water by a “loose” nanofiltration membrane

The removal of arsenic from water by a “loose” nanofiltration (NF) membrane was investigated. Prior to the arsenic removal studies, the loose NF membrane was characterized for molecular weight cut-off and pore size by saccharide retention measurements, and electrokinetic charge by streaming potential measurements. In addition, separation of both single salt and mixed salt electrolyte solutions was studied to investigate the ion transport properties of the membrane. Arsenic rejection experiments included variation of pH, arsenic feed concentration, and presence of background electrolyte. In general, arsenic rejection increased with increasing pH and arsenic feed concentration, and was enhanced in the presence of 0.01 M NaCl. Arsenic was removed 60–90% from synthetic feed waters containing 10, 32, 100, and 316 μg/L As(V), resulting in permeate arsenic concentrations of 4, 6, 10, and 25μg/L, respectively. The behavior of the membrane is consistent with the extended Nernst-Planck equation model predictions for an uncharged membrane where size exclusion controls ion retention. However, separation of Arsenic species was a due to a combination of size exclusion, preferential passage of more mobile ions, and charge exclusion.     

γ-Al_2O_3纳米孔膜截留镁离子机理的研究

纳米孔膜的过滤类似于反渗透和超滤,均属于压力驱动的膜过程,但其传质机理却有所不同。根据实验数据用作图的方法得出膜的真实截留率,进而求出膜面浓度。通过实验发现,细孔模型中对特征参数σ和ω的计算与实验值有一定的出入,主要是由于在细孔模型中只考虑了结构尺寸对截留率的影响,而在道南效应决定膜对盐的截留性能的情况下,截留性能主要依赖于离子和膜之间的静电相互作用,电荷因素不容忽略,因此细孔模型用于描述纳米孔膜的分离机理时,其准确性不高。     

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。     

Evaluation of commercial PTFE membranes in desalination by direct contact membrane distillation

In this study, nine flat-sheet commercially available hydrophobic PTFE membranes were used in desalination by direct contact membrane distillation and their characteristics were investigated under different operating conditions including feed temperature, feed flow rate, cold stream flow rate, and feed concentration. Membrane properties, i.e. pore size, thickness, support layer, and salt rejection were also studied. Moreover, membrane module designs including flow arrangements (co-current, counter-current and tangential) for process liquid and depth both on hot and cold sides were tested experimentally. Finally, the long-term performance of the selected membranes for direct contact membrane distillation as a stand-alone desalination process was investigated. The results indicated that increasing feed temperature, hot feed flow rate, and module depth on the cold side led to increase permeate flux. On the other hand, increasing membrane thickness and module depth on the hot side (at constant flow rate) had negative effects on the flux. The highest permeation flux and salt rejection was achieved when the membranes with a pore size of 0.22 μm were used in the cross-current follow arrangement of hot and cold streams. In addition, the requirements for support layer for a successful DCMD process has been extensively discussed.     

Comparative analysis of the processes of collagen concentration by ultrafiltration using different types of membranes

Tangential flow filtration of the collagen protein solutions with a molecular weight 12, 14, and 24 kDa is investigated using flat sheet membranes. The effects of tangential ultrafiltration (UF) on the permeate properties using two regenerated celluloses (RCs) and two polyethersulfone (PES) membranes with molecular weight cut-off (MWCO) of 5 and 10 kDa are reported. The permeate and concentrate obtained in the UF experiments are characterized from a physical–chemical point of view by determining the temperature, pH, electrical conductivity, nitrogen content, and protein concentration. In addition, the experimental data are modeled using Hermia's model. The UF experiments demonstrated that permeate flux declined with increasing molecular weight of collagen at constant concentration (1%). Regardless of the molecular weight of collagen, the rejections decrease in the following order: PES 5 kDa > RC5kDa > RC10kDa > PES10kDa. In case of membrane with higher MWCO, the clogging phenomenon is mainly due to the blockage of the internal pores of the membrane than the formation of a polarization layer. Morphologies and characteristics of the membranes are characterized using scanning electron microscopy.     

Purification of Minute Virus of Mice using high performance tangential flow filtration

Membrane technology has proven to be a mainstay separation technology over the past two decades. Some major advantages of membrane technology are application without the addition of chemicals and a comparatively low energy use. With its current applications, membrane technology has been widely used in biotechnology processes. Cell harvesting and virus purification/removal are important processes in many downstream purifications of biopharmaceutical products. For this project, ultrafiltration (UF) for virus purification from cell culture broth was used. Recently, it has been demonstrated that UF is a powerful tool for purification of other viruses such as Aedes aegypti and virus-like particles. More precisely, high performance tangential flow filtration (HPTFF) will be used, which was first introduced by Robert van Reis in 1997. To date HPTFF has been used in other projects, as for protein concentration, purification, and buffer exchange as a single unit operation. The virus used in this study was the parvovirus Minute Virus of Mice (MVM); characterized by an average diameter of 22-26 nm and icosahedral symmetry. Experiments were conducted with 300, 100 and 50 kDa Sartorius membranes. Results obtained indicate that using the 50 or 100 kDa membrane, viral particles get excluded, whereas the 300 kDa membrane allows the passage of the virus particles into the permeate. In HPTFF mode the permeate flux decline of the 300 kDa ultrafiltration membrane is much greater than for the other membranes used. One possible explanation for this decay could have to do with the virus particles' access to the membrane pores (gradual pore narrowing). Additionally the permeate flux and level of protein rejection as well, are strongly affected by the cell culture medium.     

Poly(vinylidene fluoride-co-hexafluoro propylene) membranes prepared via thermally induced phase separation and application in direct contact membrane distillation

A non-toxic and environmentally safe diluent, acetyl tributyl citrate, was employed to prepare poly(vinylidene fluoride)-co-hexafluoropropylene membranes via thermally induced phase separation. Effects of the polymer concentration on the phase diagram, membrane morphology, hydrophobicity, pore size, porosity and mechanical properties (tensile stress and elongation at break) were investigated. The results showed that the pore size and porosity tended to decrease with increasing polymer concentration, whereas the contact angle, liquid entry pressure and mechanical properties showed the opposite trend. In direct contact membrane distillation operation with 3.5 wt-% sodium chloride solution as the feed solution, the prepared membranes performed high salt rejection (>99.9%). Furthermore, the prepared membranes retained excellent performance in long-term stability tests regarding the permeate flux and salt rejection.     

Steady State Permeate Flux Estimation in Cross-Flow Ultrafiltration of Protein Solution

In the membrane separation process, the cross-flow configuration in which the fluid flows parallel to the membrane is widely utilized. Due to the shear stress exerted by the tangential feed flow, the accumulation of the retained species in the membrane is reduced, and the nearly steady state operation can be attained. The determination of steady state permeate flux is significant in the design and optimization problem. Several mechanisms of transport phenomena have been proposed to estimate the steady state permeate flux such as concentration polarization and Brownian diffusion, shear-induced diffusion, inertial lift, and surface transport. Another approach is using dimensional analysis to give the correlation equation with the operating condition instead of a deep focus on mechanism. In this study, we apply the model proposed in our previous study to predict the steady state permeate flux from the experimental data. After that, a new method using dimensional analysis is also developed to predict the steady state permeate flux from the operating conditions such as the trans-membrane pressure, the feed flow rate, and the feed volume fraction in a wide range. The correlation equation provides a good estimation of the experimental results.     

陶瓷膜净化溶剂油的实验研究

研究了平均孔径0.2 mm的陶瓷膜对含杂质溶剂油的微滤过程,选用不加水和加0.5%(w)水2种料液,考察了操作时间、跨膜压差、错流速度、温度和铝粉含量对膜通量及铝粉截留率的影响,研究了反冲操作、浓缩和污染膜清洗过程. 结果表明,不加水较加水料液的膜通量明显增大;随操作时间延长,膜通量下降至稳定,铝粉截留率迅速增大至100%;跨膜压差增大或温度升高使稳定通量增大;错流速度增大,稳定通量先升高之后不变;铝粉含量越高,膜通量越低. 适宜的操作参数为跨膜压差0.16 MPa、错流流速3.9 m/s和温度40℃. 反冲操作能有效提高膜通量;浓缩过程中膜通量快速下降至平缓阶段再较快降低,净化溶剂油澄清透明;采用0.15%(w)洗洁精和0.25%(w)硝酸清洗可使通量恢复到新膜通量的94.9%.