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.     

RECOVERY OF PHENOL WITH CYANEX® 923 IN MEMBRANE EXTRACTION-STRIPPING SYSTEMS

The recovery of phenol from aqueous solutions with CYANEX® 923 was studied. Classical dispersive extraction and three membrane extraction-stripping systems (bulk liquid membranes, three-phase hollow fiber contactor and two hollow fiber modules set-up) were used. It was found that CYANEX® 923 was a convenient carrier for recovery of phenol from aqueous streams in extraction-stripping membrane processes. The problem of emulsion formation, so important in dispersive extraction, was avoided. Both mass transfer experiments in different membrane systems and measurement of the dynamic interfacial tension demonstrated importance of the interfacial phenomena occurring in the stripping stage. A blocking of this interface was observed that resulted in a decrease of phenol mass transfer.     

CFD simulation of acetone separation from an aqueous solution using supercritical fluid in a hollow-fiber membrane contactor

In the present study, separation and mass transfer of acetone using a polymeric hollow fiber membrane by a supercritical fluid as a dense solvent was simulated. The propane is used as supercritical fluid for extraction of acetone. The simulated hollow-fiber membrane contactor has three compartments: tube, porous membrane and shell. The aqueous solution and solvent pass in the lumen and shell sides, respectively. The model equations have been solved by CFD technique using a finite element as numerical method. The simulation results were compared with experimental data obtained from literature and showed great agreement with the measured values. The simulation results of acetone extraction also showed that reducing the feed rate and increasing the solvent velocity will enhance the separation of acetone.     

Computational Simulation of Mass Transfer in Molecular Separation Using Microporous Polymeric Membranes

Computational simulation was conducted to predict mass‐transfer phenomenon in the molecular separation of solute using microporous membrane contactors. Both diffusional and convectional mass‐transfer mechanisms were considered. The membrane system was a hollow‐fiber contactor in which an aqueous phase containing an organic compound was contacted with an organic phase for extractive separation of the solute. Benzoic acid was used as the solute. The main focus was on understanding the mass transfer of solute in the process. The concentration equation for the solute was solved numerically using a computational fluid dynamics approach. It was found that the model can predict the solute transport from the aqueous phase to the organic phase and can be used as a predictive model for process understanding.     

SIMULATION AND PROCESS OPTIMIZATION OF A MEMBRANE-BASED DENSE GAS EXTRACTION USING HOLLOW FIBER CONTACTORS

Supercritical fluid and membrane technology coupling is a relatively new concept applicable to solvent separation and solute extraction. In these processes a hydrophobic or hydrophilic macroporous membrane is used as a two-different-nature solutions contactor. This methodology is an alternative to conventional liquid solution supercritical fluid extraction processes, which are associated with high investment costs. In the present work, a membrane-based supercritical fluid extraction module is modeled, simulated, and optimized as an independent industrial-scale operational unit. UniSim design suite R390 software from Honeywell was used as the platform for the simulation. Acetone and ethanol literature extraction results and methanol experimental extraction results (27.6% to 14.5% with a 10 wt.% aqueous solution; 7.1% to 5.9% with a 500 ppm aqueous solution) were used for validation of the model and definition of the semi-empirical equation parameters. The generated industrial-scale system optimization, which used a modular membrane arrangement, was strongly dependent on thermodynamic, economic, and energetic variables (higher mass transfer resistance in the carbon dioxide phase increased the number of membranes needed; process feasibility was affected by the number of membrane units, carbon dioxide flow rate, and product added value; compression energy requirements affected the optimization result). The modeled system proved to be an important aid in the design, scaling, and optimization of systems that use membranes as phase contactors in liquid solution supercritical carbon dioxide extraction.     

New absorption liquids for the removal of CO2 from dilute gas streams using membrane contactors

A new absorption liquid based on amino acid salts has been studied for CO₂ removal in membrane gas-liquid contactors. Unlike conventional gas treating solvents like aqueous alkanolamines solutions, the new absorption liquid does not wet polyolefin microporous membranes. The wetting characteristics of aqueous alkanolamines and amino acid salt solutions for a hydrophobic membrane was studied by measuring the surface tension of the liquid and the breakthrough pressure of the liquid into the pores of the membrane. The dependence of the breakthrough pressure on surface tension follows the Laplace-Young equation. The performance of the new absorption liquid in the removal of CO₂ was studied in a single fiber membrane contactor over a wide range of partial pressures of CO₂ in the gas phase and amino acid salt concentrations in the liquid. A numerical model to describe the mass transfer accompanied by multiple chemical reactions occurring during the absorption of CO₂ in the liquid flowing through the hollow fiber was developed. The numerical model gives a good prediction of the CO₂ absorption flux across the membrane for the absorption of CO₂ in the aqueous amino acid salt solutions flowing through the hollow fiber.     

Effect of different PVDF and additives on the properties of hollow fiber membranes contactors for CO2 separation

In the present work, different poly(vinil fluoride) (PVDF) were selected for preparing membranes, based on the fact that they are able to form polymer solutions with different viscosities. This characteristic can affect spinning, as well as, mass transfer between the polymer solution and precipitation bath; therefore, each PVDF solution can differently affect membrane formation. The effect of different additives in the polymer solutions was also investigated. Flat sheet and hollow fiber membranes were characterized by Scanning Electron Microscopy analysis, contact angle, gas permeation, porosity, and membrane gas–liquid contactor tests, aiming carbon dioxide removal. The hollow fibers prepared by the polymer which formed a less viscous solution (named PVDF-I) had a faster light transmittance decay, which started around 150 s before the more viscous solution (PVDF-II). Hollow fibers obtained using PVDF-I and propionic acid, in the polymer solution, presented the best gas–liquid contactor performance. CO₂ removal increased from 21 to 35.1%, for PVDF-II and PVDF-I, respectively, using aqueous diethanolamine solution, as absorbent liquid. In conclusion, even though PVDF-I and PVDF-II membranes were obtained by using the same spinning conditions and experimental methodology, the difference between the polymers properties certainly affected the final membrane morphology and transport properties.     

Reduction of CO2 emissions by a membrane contacting process☆

A membrane-based gas–liquid contacting process was evaluated in this work for CO₂ removal from flue gases. The absorption of CO₂ from a CO₂–N₂ mixture was investigated using a commercial hollow fiber membrane contactor and water or diethanolamine as absorbing solvents. Significant CO₂ removal (up to 75%) was achieved even with the use of pure water as absorbent. By using aqueous amine solutions and chemical absorption, mass transfer improved, and CO₂ removal was nearly complete (∼99%). A mathematical model was developed to simulate the process and it was validated with experimental data. Results show that membrane contactors are significantly more efficient and compact than conventional absorption towers for acid gas removal.     

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     

Phenol recovery with tributyl phosphate in a hollow fiber membrane contactor: Experimental and model analysis

The extraction and stripping of phenol using a solution of tributyl phosphate in kerosene in a hydrophobic polypropylene hollow fiber membrane contactor has been studied. The effect of the aqueous and the organic phase flow rates on the overall mass transfer coefficient for both extraction and stripping steps was investigated. Experimental values of the overall mass transfer coefficient were determined and compared with predicted values from the resistance in series model. Results showed that the overall mass transfer coefficients for extraction were about one order of magnitude greater than those measured during the stripping process. The experimental values were in good agreement with the predicted values for the extraction module. However, the predicted values were slightly overestimated for the stripping module. The individual mass transfer resistances were analyzed and the rate-controlling steps of mass transfer were also identified in both extraction and stripping modules. The major resistance in extraction and stripping was in the aqueous phase and in the membrane phase, respectively.     

Mass transfer correlations for membrane gas-solvent contactors undergoing carbon dioxide desorption

Membrane gas-solvent contactors are a hybrid technology combining solvent absorption with membrane gas separation, which demonstrates potential for CO₂ capture through the ability of the membrane to rigidly control the mass transfer area. Membrane contactors have been successfully demonstrated for CO₂ absorption, and there is strong research interest in using membrane contactors for the complimentary CO₂ desorption process to regenerate the solvent. However, understanding and modelling the various stages of mass transfer in the desorption process is less well-known, given the existing mass transfer correlations had been developed from absorption experiments. Hence, mass transfer correlations for membrane contactors are reviewed here, and their appropriateness for desorption analysed. This is achieved through simulating CO₂ desorption through a membrane contactor from loaded 30wt% monoethanolamine solvent to enable comparison of the correlations. It was found that the most cited correlations by Yang and Cussler were valid for shell side parallel flow, while that of Kreith and Black was viable for shell side cross flow. A limitation of all of these correlations is that they assume single phase flow on both sides of the membrane; however, the high temperature of CO₂ desorption can lead to partial solvent vaporisation and hence two phases present on one side of the membrane contactor during desorption. A mass transfer correlation is established here for two phase parallel flow on the shell side of a membrane contactor, based on experimental results for three composite and one asymmetric hollow fibre membrane contactors stripping CO₂ from loaded MEA at 105-108℃. This correlation is comparable to that reported in the literature for mass transfer in other two phase systems, but differs from the standard format for membrane contactors in terms of the exponent on the dimensionless Schmidt and Reynolds numbers.     

Near critical and supercritical fluid extraction of Cu(II) from aqueous solutions using a hollow fiber contactor

The aim of this work is the experimental and theoretical characterization of the mass transfer in a membrane-based dense gas extraction of metal ions from aqueous solutions using a hollow fiber contactor. Extractions of Cu(II) were conducted in a single fiber membrane contactor operating under steady state conditions. Aqueous CuSO₄ solutions were treated using a CO₂ phase containing 1,1,1-trifluoro-2,4-pentanedione (TFA) or 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (HFA) at 40 °C and pressures ranged between 70 and 90 bar.Experiments show that the use of dense CO₂ as extraction solvent of Cu(II) ions reaches extraction efficiencies valued up to 98.7%. Simultaneously, a mass transfer model was proposed correlating an effective rate function of the complex formation at the aqueous-CO₂ interface.The highest extraction efficiencies were observed at higher pressures and lower pH values, which could confirm that a high content of protons is required to facilitate and stabilize the formation complex by means of keto-enol tautomerism.This work represents the first step in order to propose a novel intensified operation, which could be applied for high valued metals or hazardous materials.     

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.     

膜接触器中乳化液膜脱除Ni2+的研究

研究了以D2EHPA/煤油/HC1形成的微乳液体系在中空纤维膜接触器中萃取NiCl2水溶液中Ni2的过程,考察了D2EHPA和Ni2+浓度、水溶液酸度、两相流速对溶液中Ni2+的萃取率和传质性能的影响.结果表明,用D2EHPA/煤油/HCl形成的微乳液体系能有效萃取NiCl2水溶液中的微量Ni2+,增大微乳液中载体D2EHPA浓度和料液pH都能提高Ni2的萃取率和过程的总质系数,增加料液流速能显著提高Ni2+的萃取率和过程的总质系数,表明过程的传质阻力主要在水溶液相.     

Carbon dioxide stripping in ceramic hollow fibre membrane contactors

Ceramic hollow fibre membrane contactors have been applied to carbon dioxide stripping from monoethanolamine (MEA) solution at high temperature where most polymeric membranes would fail to operate. The experimental results show that the membrane contactors are immune from hydrodynamic problems, such as flooding and loading, since the gas and liquid phases can totally be separated by the hollow fibre membranes. The height of transfer unit (HTU) of the contactor was determined to be as low as 15 cm and is dependent on the fluid velocities. The mass transfer coefficients were theoretically predicted and found to be within a reasonable deviation. The mass transfer resistance in the liquid phase was found to be the majority of the total resistance.     

ABSORPTION OF CO2 INTO AQUEOUS SOLUTIONS OF STERICALLY HINDERED METHYL AMINOETHANOL USING A HYDROPHOBIC MICROPOROUS HOLLOW FIBER CONTAINED CONTACTOR

The absorption of dilute CO₂ into aqueous solutions of sterically hindered 2-methyl aminoethanol (MAE) and the desorption of CO₂ from CO₂-loaded MAE solutions into N₂ stream were investigated separately for the various combinations of operational variables, using a hydrophobic microporous hollow fiber (polytetrafluoroethylene, PTFE) contained gas-liquid contactor with aqueous solutions of MAE as liquid media in the shell side at 30°C. The absorption of CO₂ in this contactor is governed by resistance in the liquid and hollow fiber phases. The resistance to diffusion in the hollow fiber phase amounts to 76-80% of the total resistance. Nevertheless, the absorption rates of CO₂ into aqueous MAE solutions in this contactor are higher than those into aqueous solutions of sterically hindered 2-amino-2-methyl-1-propanol (AMP) in the stirred tank with a plane unbroken gas-liquid interface. The process of desorption of CO₂ from CO₂-loaded MAE solutions can be regarded as being controlled by diffusion and chemical reaction in both the stagnant film of the liquid phase and the liquid-filled pore of the hollow fiber phase under the slow or intermediate reaction regime. Both absorption and desorption rates under the simultaneous absorption-desorption operation in a single unit tend to approach the respective constant values as process time elapses. The total absorption rate here seems to be almost balanced with the total desorpion rate at the constant mass transfer rate periods.     

ABSORPTION OF CO2 INTO AQUEOUS SOLUTIONS OF STERICALLY HINDERED METHYL AMINOETHANOL USING A HYDROPHOBIC MICROPOROUS HOLLOW FIBER CONTAINED CONTACTOR

The absorption of dilute CO₂ into aqueous solutions of sterically hindered 2-methyl aminoethanol (MAE) and the desorption of CO₂ from CO₂-loaded MAE solutions into N₂ stream were investigated separately for the various combinations of operational variables, using a hydrophobic microporous hollow fiber (polytetrafluoroethylene, PTFE) contained gas-liquid contactor with aqueous solutions of MAE as liquid media in the shell side at 30°C. The absorption of CO₂ in this contactor is governed by resistance in the liquid and hollow fiber phases. The resistance to diffusion in the hollow fiber phase amounts to 76–80% of the total resistance. Nevertheless, the absorption rates of CO₂ into aqueous MAE solutions in this contactor are higher than those into aqueous solutions of sterically hindered 2-amino-2-methyl-1-propanol (AMP) in the stirred tank with a plane unbroken gas-liquid interface. The process of desorption of CO₂ from CO₂-loaded MAE solutions can be regarded as being controlled by diffusion and chemical reaction in both the stagnant film of the liquid phase and the liquid-filled pore of the hollow fiber phase under the slow or intermediate reaction regime. Both absorption and desorption rates under the simultaneous absorption-desorption operation in a single unit tend to approach the respective constant values as process time elapses. The total absorption rate here seems to be almost balanced with the total desorpion rate at the constant mass transfer rate periods.     

Effects of baffles on separation of aqueous ethanol solution with hollow fibers

The effects of baffles on the operation and mass-transfer characteristics in novel hollow fiber membrane contactor used in distillation were investigated. Hollow fiber membranes, coated with a 7 μm polydimethyl-siloxane, were operated as structural packing in the separation of ethanol-water solutions. The parallel flow mode was chosen for separation due to the stronger driving force of the concentration difference, in which liquid flows through the lumens of the fibers and vapor flows countercurrent-wise outside the fibers. Two baffles were installed on the shell side of the membrane contactors to enhance separation, which had a round shape with a semi-lunar hole. The results show that both baffled and unbaffled membrane contactors gave better, more productive separations than traditional packing in distillation, such as the excellent Sulzer Gauze BX structured packing. The baffled membrane contactors performed better than unbaffled ones, especially at high vapor velocities. The minimal HTU of membrane contactor with baffles could reach as low as 4.5 cm, and almost all the contactors could work well above the limit where flooding normally occurs in conventional cases. Theoretical analysis predicted that baffles helped membrane module to obtain a higher masstransfer coefficient and a smaller mass-transfer resistance. Finally, theoretical mass-transfer coefficient and experimental value were compared as well as the contribution of each individual mass-transfer coefficients among liquid, gas and membrane.     

膜接触器及其相关过程在废水处理中的应用

膜接触器是一种通过膜作为两相之间的分离界面而实现相间传质的新型杂化膜过程,具体应用形式包括膜蒸馏、膜萃取、膜吸收、膜结构填料等.膜接触器使用微孔中空纤维膜将两流体分隔开,膜孔为两流体之间提供传质的场所.与传统接触分离器相比,新兴的膜接触器拥有分离效率高、工作范围宽、两相流速可单独控制以及结构紧凑等诸多独特的优点.文章着重于膜接触器及其相关过程在废水处理领域的最新研究成果和进展,具体分析比较了上述几种膜接触器的结构、工作原理和操作特点,充分展示了膜接触器在废水处理以及化工、医药、食品等领域特种分离中的广阔应用前景.     

Hollow‐fiber dispersion‐free extraction and stripping of Pb(II) in the presence of Cd(II) using D2EHPA under recirculating operation mode

Lead(II) and cadmium(II) solvent extraction (SX) with D2EHPA dissolved in heptane and kerosene from aqueous chloride media was studied. Extraction reaction stoichiometries and extraction equilibrium constants were evaluated. Application of a dispersion‐free membrane‐based extraction and stripping technique with two hollow‐fiber contactors under recirculating operation mode allowed a quantitative (>97%) removal of lead from the aqueous solution in the extraction module with a separation factor of 19 and a 62% recovery in the back‐extraction module with a separation factor of 6.3 after 7 h under optimum conditions. Mass‐transfer coefficients for the system were evaluated. Optimal conditions for transport and separation are reported and compared using SX and the membrane‐based technique. Copyright © 2004 Society of Chemical Industry