Microporous hollow fibers for gas absorption : II. Mass transfer across the membrane

Microporous hollow fiber modules offer a larger area per volume between gas and liquid than that commonly encountered in packed towers. This larger area can be sustained at very low flows, where packed towers will not be loaded, and at very high flows, where packed towers will flood. As a result, the modules offer the potential of faster mass transfer. This potential can be compromised by the resistance to mass transfer of the membrane itself, a resistance which is increased if the liquid wets the membrane. The results presented in this two-part series show when the advantage of the increased area is greater than the disadvantage of the membrane resistance. In this part, overall mass transfer coefficients are studied, including resistances in both liquid and membrane, and the performance of hollow fibers is compared with that of packed towers.     

Liquid-liquid extractions with microporous hollow fibers

Liquid—liquid extractions in microporous hollow fiber modules are unusually fast because of the large surface area per volume in these modules. Extractions of p-nitrophenol into amyl acetate show no membrane resistance, and hence are especially rapid. Extractions of acetic acid into methyl amyl ketone are controlled by the membrane resistance. The results show that extractions in hollow fibers will be fastest when the fibers are wet by the fluid in which the solute is more soluble. They also show when fiber modules are a sound inexpensive alternative to centrifugal extractors.     

中空纤维致密膜基吸收CO2传质过程

研究了硅橡胶/聚砜中空纤维致密膜基吸收CO2的传质机理,考察了吸收剂种类(NaOH,MEA,DEA和TEA)、NaOH浓度、吸收剂流速、吸收剂压力和气相压力对CO2传质通量及传质速率的影响.其中,用2×103mol/m3NaOH作吸收剂时,聚合物膜传质为控制步骤,其传质效率与膜自然渗透相近.     

Membrane extraction for separation of copper cations from acid solution using polypropylene hollow fibre membrane

The membrane extraction of copper ions was carried out using hydrophobic poly(propylene) (PP) hollow fiber membrane modules and kerosene solutions containing organic extractant. The influences of different extractant on the extraction yield, mass transfer performance and mass transfer mechanism were studied. Compared with 2‐ethylhexyl phosphoric acid (2EHPA) and 2‐methyl‐5‐sulpho benzaldoxime (2M5SB), di‐(2‐ethylhexyl)phosphoric acid (D2EHPA) extractant system with high distribution coefficient exhibited higher extraction yield of 99.7%. The extraction equilibrium time, the final extraction yield and the total mass transfer coefficient were independent of the flow rates of two phases. The extraction equilibrium time and the final extraction yield at different flow rates of two phases were 80 min and near 99.5%, respectively. A mass transfer model of a complexation reaction describing the overall mass transfer resistance was controlled by interfacial reactions rather than the aqueous and organic boundary layer which could explain the effect of flow rate on the final extraction yield and the total mass transfer coefficient. This model showed that the mass transfer resistance and mass transfer coefficient were independent of Cu²⁺ when copper ion concentration was more than 0.06 g/L. However, when copper concentration was less than 0.06 g/l, the mass transfer resistance increased as Cu²⁺ concentration decreased, and the mass transfer coefficient decreased as Cu²⁺ concentration decreased. Extractant entrainment in the aqueous phase and membrane fouling were investigated primarily. It was found that the solvent entrainment could reduce to 10 ppm much lower than 200 ppm of the classic liquid–liquid extraction, and that the cleaning of contaminated membranes was not complete. However, it can be still concluded from this research that the membrane extraction in PP hollow fibre with D2EHPA extractant would be an effective and promising processing means for Cu²⁺ separation from aqueous solution. Copyright © 2005 John Wiley & Sons, Ltd.     

Racemic ofloxacin separation by supported-liquid membrane extraction with two organic phases

As the relation of chirality and activities of drugs is researched deeply, people become to re-alize the clinic importance of chirality. The different enantiomers of a drug can have vastly differ-ent pharmacological activities, pharmacokinetic processes and toxicity[1,2]. The most well-docu- mented example is that of the drug substance thalidomide. Bitter lessons and scientific research promote the interest in single-enantiomer drugs, so the potential of the chiral drug market is enormous[3]. …     

Shell-side mass transfer of hollow fibre modules in osmotic distillation process

The effect of packing density of hollow fibre modules on mass transfer in the shell side of osmotic distillation process was studied. The osmotic distillation experiments were carried out with several modules of the packing densities ranging from 30.6 to 61.2%. It was found that the Reynolds number was a function of packing density and packing density affected mass transfer performance. The shell-side mass transfer coefficient increased with the brine velocity. The membrane permeability can be predicted from the experimental flux at the maximum brine velocity. The mass transfer correlation was proposed in order to determine the shell-side mass transfer coefficient in the randomly packed modules for osmotic distillation process. The empirical correlation proposed was fitted to the experimental results and it was found that the mass transfer coefficients calculated from the proposed correlation were in good agreement with those from the experimental data. Comparison of the results obtained from the proposed correlation with other correlations in the literature was discussed.     

Zero solvent emission process for sulfur dioxide recovery using a membrane contactor and ionic liquids

Selective absorption into a liquid is a widespread method to separate and concentrate sulfur dioxide from gas emissions, reducing air pollution and environmental risks. Process intensification can be performed first by the substitution of the equipment (e.g. scrubbers) for a membrane device to avoid drops dragging, and second by the substitution of the absorption solvent (e.g. N,N-dimethylaniline) for ionic liquids to avoid solvent volatilization. According to this intensification, a zero solvent emission process has been developed.The ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate is used as the absorption solvent and the results are compared to the N,N-dimethylaniline results. A ceramic hollow fibre module is the membrane device where the sulfur dioxide absorption takes place. A gas stream with a typical composition of roasting effluents flows through the shell side and the absorption liquid flows counter currently by the inside of the hollow fibres. The influence of carbon dioxide in the absorption is also evaluated and the overall mass transfer coefficients are calculated. The difference between the estimated mass transfer coefficients and the experimental results for both solvents is discussed assuming partial wetting of the membrane.     

BPA transfer rate increase using molecular imprinted polyethersulfone hollow fiber membrane

Bisphenol A (BPA) imprinted polyethersulfone (PES) hollow fiber membrane was spun using a dry–wet spinning method, the membrane was then prepared as a filter with an effective area of 200 cm². The hollow fiber filter was employed to study the BPA transport behavior. The transport ability of the prepared hollow fiber membrane was measured using 100 μmol/l BPA aqueous solutions at a flow flux of 50 and 75 ml/min, respectively. The BPA transfer rate increased for the imprinted hollow fiber membranes due to the larger amount of binding sites, comparing with the non-imprinted one. In the present study, hollow fiber membrane and the molecular imprinting technique were combined for advanced separation and the data suggested that small molecules could transfer in the direction opposite to the concentration gradient due to different pH.     

Turbulent flow technique for the estimation of oxygen diffusive permeability of membranes for the oxygenation of blood and other cell suspensions

When transport-efficient membrane modules (such as those where the liquid flows outside hollow fibre membranes) or membranes with prolonged resistance to wetting are used for the oxygenation of blood or other cell suspensions, membrane contribution to the overall oxygen transfer resistance into the liquid may become significant. Thus, estimation of membrane diffusive permeability towards relevant gases (e.g., oxygen) is important to develop new membranes and to ensure reproducible commercial membrane performance.

In this paper, we report on a turbulent flow technique for the estimation of the oxygen diffusive permeability of membranes used in outside-flow oxygenators. Water is re-circulated under turbulent flow conditions in a closed-loop from a reservoir to the shell of lab-scale membrane modules. The overall oxygen transfer to water coefficient is estimated at increasing water flow rates from the time the change of dissolved oxygen tension in the stream leaving the water reservoir occurs. Oxygen diffusive permeability is estimated as the reciprocal overall transfer resistance at infinitely high water flow rates, for negligible gas-side oxygen transport resistance. The technique was used to estimate oxygen diffusive permeability of commercial Oxyphan^® polypropylene membranes for blood oxygenation and of two laboratory polypropylene membranes, the one featuring a microporous wall structure with smaller-than-standard pore size, the other featuring an outer thin, dense layer supported by a thick spongy layer. The turbulent flow technique yields oxygen diffusive permeability estimates consistent both with membrane hydraulic permeability towards gaseous nitrogen, membrane wall structure, and with values in literature obtained using a liquid reactive with oxygen, but without the complications associated with reaction and physical transport kinetic characterisation. We conclude that the turbulent flow technique is a useful tool in the development and quality control of membranes for the oxygenation of blood and other cell suspensions.     

Use of axial membrane vibrations to enhance mass transfer in a hollow tube oxygenator

Membrane-oxygenator performance is limited by the mass-transfer resistance on the blood side. The most successful techniques thus far for enhancing oxygenator performance have employed liquid-side pressure pulsations. However, this technique is limited since it causes the least relative motion near the membrane. In this study we explore the use of axial vibrations of a membrane tube bundle to increase oxygen transfer to the intralumenal liquid flow. An analytical solution is first developed for the hydrodynamics of laminar flow through a sinusoidally vibrated straight cylindrical tube. This indicates that the effect of the tube vibrations is characterized by a dimensionless velocity and frequency. A novel oxygenator is designed that permits vibrating a parallel membrane hollow tube bundle without directly pulsing the intralumenal liquid flow. An embodiment of this design employing 41 silicone rubber tubes is used to study the oxygenation of water. A tuned response is observed in that the maximum enhancement in mass transfer for a fixed dimensionless vibration velocity occurs at a specific dimensionless frequency. These experiments demonstrate that axial membrane vibrations can increase the mass-transfer coefficient by at least a factor of 2.65. Even greater enhancement may be possible for systems characterized by larger Schmidt or Graetz numbers for which diffusive mass transfer is more limiting. Employing membrane vibrations may offer the additional advantage of minimizing fouling in blood oxygenator as well as other applications.     

Bromine recovery with hollow fiber gas membranes

Gas membranes supported by microporous hollow fibers have been used to concentrate bromine from a variety of brines similar to seawater. The bromine transport is governed by diffusion in the brine, and hence is almost independent of membrane properties except the surface area per volume. In some cases, this type of membrane can be an alternative to packed towers, simultaneously carrying out both absorption and stripping.     

铈(Ⅳ)与RE(Ⅲ)的中空纤维膜基萃取分离

将中空纤维膜用于膜基萃取分离是７０年代开发的新分离技术,对金属离子的萃取分离［１～４］、有机有害物质的提取［５～７］及生物发酵产品和生物活性物质的提纯［８］均有报道．由于固体膜的相分离作用,中空纤维膜基萃取可实现非分散性分离操作,避免了液相萃取中液泛...     

Diffusive transport across unconfined hollow fiber membranes

The mass transfer characteristics of gas permeable, hollow fiber membranes in a liquid jet mixed reactor are studied. A membrane module, operated in the sealed-end mode, was pressurized with oxygen at the base of the fibers and centered within a submerged jet discharge. Unlike conventional membrane module designs, this configuration did not have the hollow fibers enclosed within a tubular shell. The membranes were unconfined and free to move within the generated flow field. This design is especially well suited for use in waters containing high solid concentrations. The membranes have a greater degree of freedom for movement and are therefore less likely to become fouled due to solids being lodged within the fiber bundle. Mass transfer rates were measured over a practical range of physical and process parameters. A mass transfer correlation for the unconfined configuration is presented and the transfer performance of this configuration is compared with conventional membrane contactor designs.     

Modelling and simulation of integrated membrane processes for recovery of Cr(VI) with Aliquat 336

Extraction processes which employ porous membranes as a barrier between aqueous and organic phases provide a versatile means for selective removal and enrichment of solutes from aqueous streams. They require relatively little maintenance, their energy consumption is very low and the organic liquid losses are negligible if the pressure is controlled properly. The modelling and simulation of a complete plant for the removal and recovery of Cr(VI) with Aliquat 336 using hollow fibre modules have been studied. Both single and dual function membrane modules have been analyzed. Simulated concentration profiles through the module were obtained by solving mass transfer balances corresponding to all the species involved in the process.     

A review on hollow fiber membrane module towards high separation efficiency: Process modeling in fouling perspective

Hollow fiber microfiltration (MF) and ultrafiltration (UF) membrane processes have been extensively used in water purification and biotechnology. However, complicated filtration hydrodynamics wield a negative influence on fouling mitigation and stability of hollow fiber MF/UF membrane processes. Thus, establishing a mathematical model to understand the membrane processes is essential to guide the optimization of module configurations and to alleviate membrane fouling. Here, we present a comprehensive overview of the hollow fiber MF/UF membrane filtration models developed from different theories. The existing models primarily focus on membrane fouling but rarely on the interactions between the membrane fouling and local filtration hydrodynamics. Therefore, more simplified conceptual models and integrated reduced models need to be built to represent the real filtration behaviors of hollow fiber membranes. Future analyses considering practical requirements including complicated local hydrodynamics and nonuniform membrane properties are suggested to meet the accurate prediction of membrane filtration performance in practical application. This review will inspire the development of high-efficiency hollow fiber membrane modules.     

Liquid-phase microextraction with porous hollow fibers, a miniaturized and highly flexible format for liquid-liquid extraction

Since 1999, substantial research has been devoted to the development of liquid-phase microextraction (LPME) based on porous hollow fibers. With this technology, target analytes are extracted from aqueous samples, through a thin supported liquid membrane (SLM) sustained in the pores in the wall of a porous hollow fiber, and further into a microL volume of acceptor solution placed inside the lumen of the hollow fiber. After extraction, the acceptor solution is directly subjected to a final chemical analysis by liquid chromatography (HPLC), gas chromatography (GC), capillary electrophoresis (CE), or mass spectrometry (MS). In this review, LPME will be discussed with focus on extraction principles, historical development, fundamental theory, and performance. Also, major applications have been compiled, and recent forefront developments will be discussed.     

Electrodialysis and Its Application in the Chemical Process Industry

Abstract

Hollow fiber membrane gas separation modules are used predominantly in membrane gas separation processes. The performance of these modules is often captured by simple flow and mass transfer models. However, as processes are pushed to the limits of their commercial viability, non‐ideal performance is often observed. Potential sources of this non‐ideality due to non‐uniform flows through the module are summarized and their effect on module performance evaluated.     

Experimental and theoretical study on propylene absorption by using PVDF hollow fiber membrane contactors with various membrane structures

Five kinds of asymmetric poly(vinylidene fluoride) (PVDF) hollow fiber membranes with considerable different porosities at the inner and outer surfaces of the membrane were prepared via thermally induced phase separation (TIPS) method and applied for propylene absorption as gas–liquid membrane contactors. A commercial microporous poly(tetrafluoroethylene) (PTFE) hollow fiber membrane was also used as a highly hydrophobic membrane. Experiments on the absorption of pure propylene into silver nitrate solutions were performed and the effects of membrane structure, inner diameter, silver nitrate concentration and absorbent liquid flow rate were investigated at 298 K. PVDF membranes prepared by using nitrogen as bore fluid had lower inner surface porosity than the membranes prepared with solvent as bore fluid. Except the membrane with a skin layer at the outer surface, propylene absorption flux was inversely proportional to the inner diameter of the hollow fiber membrane, and propylene absorption rate per fiber was almost the same. Propylene flux increased with increasing the silver nitrate concentration and also with increasing the absorbent flow rate.A mathematical model for pure propylene absorption in a membrane contactor, which assumes that the membrane resistance is negligibly small and the total membrane area is effective for gas absorption, was proposed to simulate propylene absorption rates. Experimental results were satisfactorily simulated by the model except for the membrane having a skin layer. The model also suggested that propylene is absorbed in silver nitrate solutions accompanied by the instantaneous reversible reaction. This paper may be the first experimental and theoretical study on propylene absorption in membrane contactors.     

Mass transfer performance for hollow fibre modules with shell-side axial feed flow: using an engineering approach to develop a framework

For several membrane separation processes, hollow fibre modules are either an already established or a promising type of module. Based on the analogy between mass and heat transfer, an engineering approach is proposed to estimate the shell-side mass transfer coefficient for axial flow in hollow fibre modules with due allowance for the void fraction. The approach enables one to take the entrance effects of the hydrodynamic and concentration profile into account. The trends obtained by this generalised approach are similar to those of empirical correlations found in the literature over a wide range of Reynolds numbers and module packing densities. The empirical correlations differ significantly one from the other. The differences between the mass transfer coefficients obtained by the empirical correlations compared to those obtained following the approach proposed in this study are discussed. The different effects influencing mass transfer in hollow fibre modules are identified and discussed as a function of void fraction. Further, an approach to reflect the influence of maldistribution on mass transfer performance is provided.     

Mathematical modeling of gas–liquid membrane contactors using random distribution of fibers

This work presents a mathematical model for gas absorption in microporous hollow fiber membrane contactors by using a random distribution of fibers. The chemical absorption of carbon dioxide into aqueous amine solutions and sulfur dioxide into water were simulated by this model. The nonlinear mathematical expressions of the component material balance for the liquid, membrane, and gas were solved simultaneously by using a numerical method. The results from the model were compared with four sets of different experimental data in the literature. In addition, the contactors were modeled based on the assumption of regular arrangement of fibers in the shell side by using Happel's free surface as well as plug flow models. The plug flow model was employed to compare the various available equations in the literature for the shell side mass transfer coefficient. The results indicate that the channeling of gas in the shell side decreases the efficiency of contactor significantly. It was found that the random distribution of fibers is a suitable method to simulate the commercial modules. The results also indicate that, the regular Happel's free surface model and the plug flow model are more suitable for handmade modules. The influence of shell side channeling on the contactor performance were investigated in different fiber packing densities, and in various gas and liquid flow rates.