CFD Simulation and Modeling of Membrane‐Assisted Separation of Organic Compounds from Wastewater

The extraction of volatile organic compounds (VOC) from water in porous hollow fibers was simulated with toluene, a hazardous material. The system to be simulated included a VOC stream and air as stripping gas, which were contacted using a porous hollow‐fiber membrane contactor. To model the process, the contactor was considered as three compartments, including shell side, porous membrane, and tube side. The model equations were derived and solved using computational fluid dynamics of momentum and mass transfer in all zones of the contactor. The profiles of concentration and pressure were obtained for the VOC in the hollow fibers.     

Simulation of ammonia removal from industrial wastewater streams by means of a hollow-fiber membrane contactor

The performance of a hollow-fiber membrane contactor in removing ammonia from aqueous solution was simulated. An unsteady state 2D mathematical model was developed to study the ammonia stripping in the hollow-fiber membrane contactor. Two sets of equations were considered for the membrane contactor and the feed tank. CFD technique was applied to solve the model equations in which concentration distribution was determined using continuity equation. Velocity field is also determined using Navier–Stokes equations for the contactor. The model was implemented in linked MATLAB–COMSOL Multiphysics. COMSOL software was applied to solve the model equations for the contactor while MATLAB software was employed to consider changes in the concentration of the feed tank. Predictions of the model were then validated against experimental data which were found to be in good agreement. The assumption of Knudsen diffusion for the transport of ammonia molecules through the membrane pores increased the accuracy of the model. The effect of different parameters including feed velocity, feed concentration and pH on the removal of ammonia was investigated. The results of simulation revealed that the developed model can be used to evaluate the effective parameters which involve in the ammonia removal by means of membrane contactors.     

Computational simulation of mass transfer in extraction of alkali metals by means of nanoporous membrane extractors

Theoretical study on extraction of alkali metals using a membrane contactor is carried out in this work. A novel model is built to simulate the flow and the concentration in a membrane extractor based on the finite element analysis. CFD model is applied by solving the 2D Navier–Stokes equations as well as mass conservation equations for steady state conditions in membrane extractors. The model predicts of the velocity field, the pressure and the concentration of alkali metal in the membrane module under laminar flow regime. The results of simulation were used for determination of concentration distribution and effect of extractant concentration on extraction efficiency and mass flux of cesium in the membrane extractor. Moreover, the simulation results for the extraction of cesium using the membrane extractor were compared with the experimental data in order to validate the proposed mass transfer model and showed great agreement.     

Modeling of Thallium Extraction in a Hollow-Fiber Membrane Contactor

A mass transfer model was developed to simulate the solvent extraction of thallium by butyl acetate through a hollow-fiber membrane contactor. The model was based on the solving conservation equations including mass and momentum transfers for thallium in both aqueous and solvent phases as well as membrane. The model equations were solved by a numerical method based on the finite element method. The simulations were conducted using experimental data obtained from literature for two different modules consisting of hydrophobic polypropylene (PP) and polyvinylidene fluoride (PVDF) fibers. The fibers had different effective membrane areas. The model findings were then verified through comparison with experimental data. The comparison showed an average deviation of 9% with the experimental data. The effect of process variables on thallium mass transfer was investigated. The results confirmed that the developed model predicts well the general behavior of thallium extraction versus process variables.     

Removal of pesticides from wool wax by continuous countercurrent dual-solvent extraction

Pesticide residues in raw wool wax were removed to below detectable levels by continuous countercurrent extraction with hexane and N,N-dimethylformamide (DMF) in a pilot-scale mixer-settler contactor. The disengagement of the phases in the settling compartments was promoted by the addition of a small amount of formic acid (3% vol/vol) to the DMF-rich feed. Empirical equations were developed to predict the effect on the pesticide partition coefficients of the wool wax concentration, the presence of small amounts of water, ethanol, and/or isopropanol in the solvents, and the temperature used in the contactor. These empirical equations were included in equations that describe the concentration of the pesticides in the different stages of the contactor and were used to develop a spreadsheet model that accurately predicted the mixer-settler’s performance. The raffinate wool wax produced by this process after conventional neutralization met all BP and USP specifications for pharmaceutical lanolin.     

Mathematical modeling and numerical simulation of ammonia removal from wastewaters using membrane contactors

This study investigates simulation of ammonia transport through membrane contactors. The system studied involves feed solution of NH₃, a dilute solution of sulfuric acid as solvent and a membrane contactor. The model considers coupling between equations of motion and convection-diffusion. Finite element method was applied for numerical calculations. The effect of different parameters on the removal of ammonia was investigated. The simulation results revealed that increasing feed velocity decreases ammonia removal in the contactor. The modeling findings also showed that the developed model is capable to evaluate the effective parameters which involve in the ammonia removal by means of contactors.     

Control of oxygen concentration in water using a hollow fiber membrane contactor

A novel theoretical analysis was performed to regulate the oxygen concentration in water using a membrane contactor composed of nonporous hollow fibers. The governing ordinary differential equations were derived for the countercurrent flow of the feed water and the feed gas in a membrane contactor. The governing equations were regarded as a two point boundary value problem. The nonlinear ordinary differential equations were simultaneously solved using a finite difference method. The computer program was coded in Fortran language using the Compaq Visual Fortran Software. It was found that the concentration of oxygen dissolved in water increases from 28.9 to 64.3 ppm as the area of the membrane increases from 1.24 to 3.73 m² at the given typical operating condition: the flow rate of the feed gas is kept to be 1.0 L/min; its pressure is maintained to be 4 atm; the flow rate of the water is 15 L/min. It is observed that the concentration of oxygen increases from 48.2 to 56.2 ppm as the concentration of the feed gas increases from 0.75 to 0.95 mole fraction. As the flow rate of the water increases from 15 to 25 L/min, the concentration of oxygen decreases from 56.2 to 38.6 ppm with a constant membrane area of 3.11 m².     

Modeling of water transport through nanopores of membranes in direct‐contact membrane distillation process

In this research, computational fluid dynamic simulation of direct‐contact membrane distillation process is carried out. The process consists of a nanoporous flat‐sheet membrane as contactor, a saline liquid feed, and permeate phase. The flow pattern is considered to be counter‐current. The aim of the process is to produce pure water from saline water. Heat and mass transfer equations are derived and solved for water by finite element analysis. The simulations are then validated by comparing simulation predictions with the experimental data reported in literature. Good agreement was obtained which confirmed the validity of simulation procedure in simulation of water desalination using membrane distillation. Diffusion of water vapor through pores of membrane is simulated by combination of the Knudsen and Poiseuille flow. POLYM. ENG. SCI., 54:660–666, 2014. © 2013 Society of Plastics Engineers     

Hollow fiber membrane contactor as a gas-liquid model contactor

Microporous hollow fiber gas-liquid membrane contactors have a fixed and well-defined gas-liquid interfacial area. The liquid flow through the hollow fiber is laminar, thus the liquid side hydrodynamics are well known. This allows the accurate calculation of the fiber side physical mass transfer coefficient from first principles. Moreover, in the case of gas-liquid membrane contactor, the gas-liquid exposure time can be varied easily and independently without disturbing the gas-liquid interfacial area. These features of the hollow fiber membrane contactor make it very suitable as a gas-liquid model contactor and offer numerous advantages over the conventional model contactors. The applicability and the limitations of this novel model contactor for the determination of physico-chemical properties of non-reactive and reactive gas-liquid systems are investigated in the present work. Absorption of CO₂ into water and into aqueous NaOH solutions are chosen as model systems to determine the physico-chemical properties for non-reactive and reactive conditions, respectively. The experimental findings for these systems show that a hollow fiber membrane contactor can be used successfully as a model contactor for the determination of various gas-liquid physico-chemical properties. Moreover, since the membrane contactor facilitates indirect contact between the two phases, the application of hollow fiber model contactor can possibly be extended to liquid-liquid systems and/or heterogeneous catalyzed gas-liquid systems.     

Modeling and simulation of mass transfer in near-critical extraction using a hollow fiber membrane contactor

In this study is presented a general methodology to predict the performance of a continuous near-critical fluid extraction process to remove compounds from aqueous solutions using a hollow fiber membrane contactor. The stabilization of the gas-liquid interface in the membrane porosity and a high surface area to contact both phases represent some of the advantages that hollow fiber contactors offer over conventional contactor devices for the extraction of compounds from liquid feeds.A mathematical model has been developed integrating a resistances-in-series mass transfer system that takes into account boundary layers, membrane porosity and thermodynamic considerations with mass balances of the membrane contactor. Simulation algorithms were easily implemented with low calculation requirements.The system studied in this work is a membrane based extractor of ethanol and acetone from aqueous solutions using near-critical CO₂. Predictions of extraction percentages obtained by simulations have been compared to the experimental values reported by Bothun et al. [2003a. Compressed solvents for the extraction of fermentation products within a hollow fiber membrane contactor. Journal of Supercritical Fluids 25, 119-134]. Simulations of extraction percentage of ethanol and acetone show an average difference of 36.3% and 6.75% with the experimental data, respectively. More accurate predictions of the extraction of acetone could be explained by a better estimation of the transport properties in the aqueous phase that controls the extraction of this solute.When the model was validated, the effect of the configuration and the operating parameters was studied and local mass transfer resistances were evaluated. The proposed approach allows the evaluation of the relevance of membrane hydrophobicity for extraction in solutions under different thermodynamic conditions. This original methodology based on well-known phenomenological equations represents a general approach which could be applied in other processes using membrane contactors with different configurations.     

Geometry effect on membrane absorption for CO2 capture. Part I: A hybrid modeling approach

Membrane absorption (MA) has a great prospect for CO₂ capture. In MA modeling, conventional one-dimensional (1D)- and two-dimensional (2D)-models make simplification of membrane contactor (MC) geometry. Geometry simplification allows an easy process modeling and numerical solution, however, is only reasonable for particular MCs. Here, efforts are underway to quantify the geometry effect on the MA-CO₂ performance. First, we proposed a rigorous 3D model without geometry simplification for simulating the MA-CO₂ process in real MCs and then validated it with experimental data. More importantly, we highlighted a preferable hybrid model in which two correction factors were introduced to a 2D model to make the simulation results approximately equal to the 3D simulation values. The correction factors were correlated with dimensionless fluid dynamic parameters for characterizing the geometry effects on flowing fluids. Such hybrid modeling contributes to characterizing the influence of geometry on the MA-CO₂ performance and improving computation accuracy-efficiency combinations.     

Numerical simulation studies on heat and mass transfer using vacuum membrane distillation

A general 2D mathematical model was developed to simulate the purification of water from volatile organic compounds (VOCs) via vacuum membrane distillation (VMD) process in hollow fiber membrane contactors. The model was developed for hydrophobic membrane material conditions, taking into consideration axial and radial diffusion in the tube, membrane and compartments of the contactor and was simplified to the two‐dimensional structure with a single porous membrane wall. The simulation has studied the mass and heat transfer of VMD system in the porous media, in which aqueous volatile organic solution was considered as an incompressible and steady fluid. Effect of the downstream pressure on the removal of 1, 1, 1‐trichloroethane (TCA) was studied to validation of simulation results with experimental data that it was obtained from literature. The temperature, Reynolds number, and total mass flux (convective and diffusive) distribution of TCA are determined in the membrane module. POLYM. ENG. SCI., 54:2553–2559, 2014. © 2013 Society of Plastics Engineers     

聚合物膜催化接触器研究进展

对聚合物膜催化接触器的特点、所用催化膜材料及其制备方法进行归纳总结。与传统催化反应器相比,聚合物膜催化接触器具有结构紧凑,工艺及操作流程简单,催化活性、反应速率及转化率高等特点。所用聚合物膜材料分子主链和侧链具有丰富的功能基团,以便引入活性纳米粒子或基团,得到高活性的催化膜。聚合物催化膜制备方法主要有杂化法、浸渍法及化学接枝法。扩散过程为聚合物膜接触器催化反应过程的控制步骤,整个过程包括内扩散和外扩散两个步骤。文中最后提出聚合物膜催化接触器应用中存在的问题,即催化效率和使用寿命有待进一步提高;新型聚合物催化膜材料亟需开发,膜污染和催化膜失活问题亟待解决;膜催化反应动力学模型的建立和研究等关键性、基础性问题还需要更加深入地研究。     

Membrane contactor for CO2 absorption applying amino-acid salt solutions

A novel composite solution based on amino-acid salt as a CO₂ absorbent was proposed. Coupling process of membrane contactor and the composite solution was investigated. The performance of the coupling was experimentally compared between the single and composite solution. Overall mass transfer coefficients were determined. Effects of various factors such as flow rates and operation temperatures on mass transfer of membrane contactor were studied. Comparison of prediction for overall mass transfer coefficients using a resistance in series model with experimental values was performed. Results show that, performance of the composite solution is evidently better than that of the single solution. The overall mass transfer coefficient with the composite solution is much higher than that with the single solution. Higher operation temperature can enhance mass transfer of membrane contactor. Operation parameters such as flow rates can promote mass transfer, but the promotion is limited. Enhancement of mass transfer relies essentially on chemical reaction. Model values are in good agreement with experimental ones.     

减压多效膜蒸馏过程试验研究

针对膜蒸馏(MD)过程能耗高、蒸汽冷凝耗水量大的问题,首次设计了减压多效膜蒸馏过程(MEMD)。其特征是在减压膜蒸馏(VMD)过程中设立特殊的多效蒸发区。其中的膜组件同时具有蒸汽的换热降温与原料液的升温蒸发双重作用,从而实现VMD过程蒸发潜热的高效回收利用。试验研究了主蒸发区膜组件面积、多效蒸发区组件管程的进液流量、多效蒸发区组件长度等参数对MEMD过程性能的影响。当主蒸发区膜组件面积为0.10 m2、多效蒸发区组件长度为868 mm、管程进液体积流量为4.0 L/h时,系统的当量膜通量最大(34.8 kg/(m2.h)),额外冷却水用量仅为传统VMD过程的30.8%(每L产水消耗17.2 L冷却水);增加多效蒸发区的组件长度,能显著提高蒸汽相变热回收率,但不能提高系统的当量膜通量。     

Sulfur dioxide non‐dispersive absorption in N,N‐dimethylaniline using a ceramic membrane contactor

BACKGROUND: Removal of sulfur dioxide from gas emissions by selective absorption is a common method to separate and concentrate sulfur dioxide and to reduce air pollution and environmental risks. N,N‐dimethylaniline is an organic solvent used in some industrial applications for its sulfur dioxide affinity, leading to a regenerative process. However, the use of scrubbers and equipment in which direct contact between gas and liquid takes place leads to solvent losses due to evaporation and drops dragging. RESULTS: In this work, an innovative procedure based on non‐dispersive absorption in a ceramic hollow fibre membrane contactor was studied in order to avoid drops dragging. The absorption efficiency ranged between 40 and 50%, showing the technical viability of the process. The sulfur dioxide flux through the membrane has a linear relationship with the concentration of SO₂ in the gas stream and an overall mass transfer coefficient K_overall = (1.10 ± 0.11) × 10^?5 m s^?1 has been obtained. CONCLUSIONS: The mass transfer behaviour of a ceramic hollow fibre membrane contactor for sulfur dioxide non‐dispersive absorption in N,N‐dimethylaniline has been studied. The main resistance is found to be the ceramic membrane and the effective diffusivity has been inferred. The mass transfer model and parameters allow the evaluation of equipment design for technical applications. Copyright © 2008 Society of Chemical Industry     

On the implementation of membrane models in computational fluid dynamics calculations of polymer electrolyte membrane fuel cells

The implementation of phenomenological membrane models within computational fluid dynamics (CFD) codes requires coupling of the conservation equation for water content within the membrane to the conservation equations for species mass outside the membrane. It is common practice to treat water and current transport within the membrane as one-dimensional (1D), i.e., normal to the membrane surface only. The purpose of this study is to investigate the accuracy and efficiency of various strategies of implementing a phenomenological membrane model within the framework of a two-dimensional (2D) CFD code. Springer's membrane model was compared against two other models available from the literature, and the accuracy of each model was assessed by comparing predicted results against experimental data. Results appear to indicate that the Springer model and the Nguyen and White model over-predict the drying of the membrane, while the Fuller and Newman model provides the best match with experimental data. Following these studies, three strategies for implementation of the membrane model were investigated: (1) 2D transport in membrane, (2) 1D transport in membrane and (3) 1D transport with approximate transport properties. Fuller and Newman's membrane model was used for these studies. The results obtained using the three approaches were found to be within 4% of each other, while there was no significant difference in the computational time required by the three models, indicating that an analytical 1D transport model for the membrane that uses approximate properties is adequate for describing transport through it.     

陶瓷膜接触器化学吸收氮氧化物的传质过程与阻力分析

针对选择性催化还原技术(SCR)存在装置大、运行费用高、催化剂中毒失活等问题,将平均孔径为100 nm的Al₂O₃陶瓷膜进行疏水改性并组装成膜接触器,以NaClO₂水溶液为吸收液,开展陶瓷膜接触器在烟气脱硝领域的应用研究。考察了陶瓷膜接触器在化学吸收脱硝中的稳定性,以及气体流量、吸收液浓度、吸收液流量、吸收液pH等因素对NO脱除率和传质通量的影响,基于阻力串联模型,建立总传质系数方程。研究表明,陶瓷膜接触器在连续600 min运行过程中,NO的脱除效率及传质通量分别稳定在99%和0.038 mol·m^-2·h^-1左右。进气流量的增加会促进NO的吸收,吸收液pH=3时具有最高的氧化吸收性能,同时提高吸收液的浓度会增强NO的脱除效果。NO的传质过程同时受气、液、膜三相阻力控制,传质阻力分析结果表明,可以通过增加气体流速减小气相阻力,增加吸收液浓度同时降低pH减小液相阻力。本研究在烟气脱硝领域具有良好的应用前景。     

A continuous‐effect membrane distillation process based on hollow fiber AGMD module with internal latent‐heat recovery

A continuous‐effect membrane distillation (CEMD) process was developed by equipping air gap membrane distillation (AGMD)‐based and strictly‐parallel hollow fiber module with internal heat recovery. Its performance was indicated by flux, performance ratio and evaporation efficiency. Two kinds of CEMD modules made from different membrane fibers were tested. A face‐centered central composite experimental design was conducted to investigate the influences of operating variables including cold‐feed temperature, hot‐feed temperature, and feed‐in flow rate on the performance. Within the studied experimental range, the maximum PR of 13.8 was obtained. A theoretical model based on governing transport equations was established to predict the process performance, and the model described the experimental data fairly well. In light of the model, possible ways to further increase PR were predicted. The dilute aqueous sugar solution was successfully concentrated 12‐fold to a final concentration of about 20 wt % by using CEMD process with a final PR of 8.2. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1278–1297, 2013     

Simulation of Membrane Distillation for Purifying Water Containing 1,1,1‐Trichloroethane

Vacuum membrane distillation is modeled for the purification of water containing organic matter. The separation medium is a hollow‐fiber membrane contactor that is simplified to a two‐dimensional structure with a single porous membrane wall. The model considers the transport phenomena of a vacuum membrane distillation system in porous media, in which the aqueous volatile organic solution was considered as an incompressible and steady fluid. The numerical simulation of the two‐dimensional model of vacuum membrane distillation for an aqueous solution of 1,1,1‐trichloroethane was established under steady state. The effects of the bulk feed temperature and the feed flow rate on the percentage of 1,1,1‐trichloroethane removal from an aqueous solution are discussed.