Temperature-controlled enhancement of oxygen uptake from water using oxygen carrier solution

Development of an artificial gill, for the uptake of oxygen from water to air, requires an increase in oxygen transfer rate. In the present study, oxygen transfer rate was enhanced using a washed red blood cell suspension as a thermo-responsive oxygen carrier solution, which changes oxygen affinity with temperature. Oxygen dissolved in water first combined with the oxygen carrier solution at a low temperature using a membrane module. The oxygen carrier solution was then heated to release oxygen into the air using a second membrane module. The water flow rate required to sustain a human being at rest was greatly reduced by heating the oxygen carrier solution due to increase in the limit of the oxygen partial pressure of water of which can be transferred, compared with when oxygen was transferred directly from water. The required membrane surface area is 225 m², sufficient for the development of a compact artificial gill.     

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.     

The effect of shell side hydrodynamics on the performance of axial flow hollow fibre modules

Fluid flow and mass transfer experiments have been performed on axial flow hollow fibre modules of varying packing density (32 to 76%). Shell-side pressure drop was found to be proportional to (flowrate)ⁿ, where n varied from about 1.1 at high packing density to 1.5 at low packing density, for shellside Reynolds numbers < 350. Assuming an Ergun-type pressure drop relationship it was found that for packing densities < about 50% the inertial (turbulent) losses exceeded the viscous (laminar) losses. Inspection of cross-sections taken from the middle of modules revealed non-uniform fibre packing with regions of high and low packing density. The cross-sections also change along the length of the module. It is inferred that, in addition to axial flow along fibres, there is also a degree of stream splitting which provides transverse flow across fibres as fluid continuously seeks preferential paths through regions of lower packing density. The presence of transverse flow would explain the higher than expected velocity exponent. Mass transfer experiments involving the removal of oxygen from water flowing through the shell to a sweep gas in the fibre lumens produced higher than expected shell-side mass transfer coefficients. The results are correlated within ± 15% by Sh = (0.53 − 0.58φ)Re^0.53Sc^0.33. The exponent on Re is consistent with entry region conditions, caused by repeated stream splitting and transverse flow. Compared with mass transfer predicted for axial flow through a uniformly packed shell the experimental results are up to 2× higher, with the most significant enhancement at the lower packing densities. The implication of this work is that module design requires a more sophisticated approach than the traditional assumption of laminar flow through parallel axial ducts.     

Blood flow and oxygen transfer rate of an outside blood flow membrane oxygenator

We evaluated effects of the number of tied hollow fibers of an outside blood flow membrane oxygenator with cross-wound hollow fibers on the blood flow pattern and oxygen transfer rate. The number of tied hollow fibers in a bundle was varied from one to six, and the blood flow pattern was observed by X-ray computed tomography. The oxygen transfer rate and blood pressure drop were measured by in vitro experiments using bovine blood. Uniform blood flow patterns were obtained for each number of tied hollow fibers. A decrease in the number of tied hollow fibers caused more effective contact of blood with the tied hollow fibers and oxygen transfer rate was enhanced, demonstrating that single hollow fiber was the most effective. Empirical equations were obtained based on these results and optimum structure parameters of the membrane oxygenator were determined by simulation analysis. Optimum membrane surface area and axial jacket length of the oxygenator were 3.0 m² and 320 mm, respectively, at a hollow fiber outside diameter of 250 μm.     

Methane oxidation in a mixed ionic–electronic conducting ceramic hollow fibre reactor module

A reactor module, consisting of six gas-tight hollow fibre membranes made of the mixed ionic–electronic conducting perovskite, , has been tested for oxygen permeation and stability during methane oxidation in the temperature range of 540 to 960°C. Rigorous leak testing was undertaken and it was demonstrated that the module could be adequately sealed. Oxygen permeation fluxes were similar to those reported by previous workers. At higher temperatures of operation, it appeared that mass transfer limited the oxygen flux, as this flux was dependent upon the flow rates on either side of the membrane. In this way, reactant flow rates could be used to manipulate the transmembrane oxygen flux. It was found that the product distribution on the methane side was dependent upon this flux, with carbon monoxide and hydrogen production being favoured at low fluxes and carbon dioxide and water production being favoured at higher fluxes. Furthermore, at low oxygen flow rates, periodic increases in the transmembrane oxygen flux were observed. The cause of this behaviour is unclear but may be as a result of phase/stoichiometric changes associated with the membrane material.     

Temperature drop of feed liquid during pervaporation

Heat transfer during pervaporation through a membrane module of silicone-rubber microtubes was studied for ammonia/water and ethanol/water feeds. The temperature drops of the feed mixture were measured as a function of flow rate, concentration and permeate side pressure. A model calculation with a vapor-phase driving force was compared with the data. The vapor permeability of the permeate components needed in the model were independently measured using an original measurement method with a differential transformer. The present simple model for heat and mass transfer during pervaporation proved to be applicable to the theoretical calculation for a membrane module of pervaporation to be used as a heat-transfer unit.     

Flux enhancement during dean vortex tubular membrane nanofiltration. 10. Design,construction, and system characterization

Controlled centrifugal instabilities (called Dean vortices) resulting from flow in helical tubes have been used to reduce concentration polarization and membrane fouling during nanofiltration. These vortices enhance back-migration of solute through convective flow away from the membrane–solution interface and allow for increased membrane permeation rates. Based on the theory of Dean vortex flow, a new prototype vortex generating tubular nanofiltration element was designed. Two sets of nanofiltration modules were constructed; a linear module and a new module containing hollow fibers wrapped around rods of small diameter in helical geometry. Optimization of the design is discussed with respect to the diameter and thickness of the hollow fibers. Axial pressure drop and energy consumption measurements for the helical module agreed very well with available correlations for various experimental conditions. Water permeabilities for the helical modules were similar to those of the conventional linear modules. No significant effect of pH was observed on the water permeability.     

Improvement of the performance of facilitated transport membrane process through nonuniformity

Facilitated transport process has attracted much attention because high selectivity and high permeability may be achieved. However, most research on facilitated transport process is concerned only with uniform membranes. In this paper, a model predicting the gas separation performance of a hollow-fiber module with facilitated transport membrane is developed. The influence of feed rate, operation pressure, and permeant-feed flow pattern on the module performance are analyzed and the effect of the nonuniform distribution of reaction equilibrium constant is examined. The calculated results show that the nonuniform active distribution may cause an improved module performance. Because of the passive transport characteristics of the facilitated transport process, the mass transfer driving force across the membrane has a great influence on the improvement of the module performance through a facilitated transport effect.     

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.     

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.     

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.     

The duomodule. Part 1: Hydrodynamic investigations

On-line plasma detoxification requires two circulation: the separation of the plasma from the patients blood, in a first circulation, and a plasma fractionation, in a second circulation, before the detoxified plasma returns back to the patient. These processes can be realised in one single step by use of a duofilter. The duofilter is a modular construction integrating two different hollow fibre filters in one common module housing. In this paper the liquid flow and the pressure distribution in the duofilter module system is calculated based, on the one hand, on the laws of hydrodynamics, and on the other hand, on the finite volume method, where each part in the duofilter is scrutinized separately and supplemented with a computational simulation program for liquid flow. The data from such calculations are compared with experimental investigations. The results of both, calculation methods and the experimental technique, demonstrate correspondence. On this basis the module equipment can be optimized with respect to its performance.     

Dehumidification and humidification of air by surface-soaked liquid membrane module with triethylene glycol

A novel liquid membrane system, a surface-soaked liquid membrane, with triethylene glycol (TEG) on the hydrophilic-treated surface of the hydrophobic microporous membrane was developed and used for the dehumidification and humidification of air. The selectivity of the TEG liquid membrane for water vapor with respect to air was over 2000, which was derived from the selective absorption of the TEG liquid. A flat-type liquid membrane module with a dual membrane surface was designed, of which the TEG liquid membrane thickness was 18 μm and the permeation area was 0.13 m². The liquid membrane humidifier and dehumidifier consisted of the membrane module and a vacuum pump. As a dehumidifier, the membrane system recovered water vapor at 4.1 g/h from 70%RH room air at 298 K. For use as a humidifier, the air flow was effectively humidified by the permeated water vapor through the membrane module. The effects of the air humidity and sweep air flow rate were studied and discussed. Simple model calculations based on the permeability of the water vapor well predicted the experimental results.     

Performance and scaling effects in a multilayer microfluidic extracorporeal lung oxygenation device

Microfluidic fabrication technologies are emerging as viable platforms for extracorporeal lung assist devices and oxygenators for cardiac surgical support and critical care medicine, based in part on their ability to more closely mimic the architecture of the human vasculature than existing technologies. In comparison with current hollow fiber oxygenator technologies, microfluidic systems have more physiologically-representative blood flow paths, smaller cross section blood conduits and thinner gas transfer membranes. These features can enable smaller device sizes and a reduced blood volume in the oxygenator, enhanced gas transfer efficiencies, and may also reduce the tendency for clotting in the system. Several critical issues need to be addressed in order to advance this technology from its current state and implement it in an organ-scale device for clinical use. Here we report on the design, fabrication and characterization of multilayer microfluidic oxygenators, investigating scaling effects associated with fluid mechanical resistance, oxygen transfer efficiencies, and other parameters in multilayer devices. Important parameters such as the fluidic resistance of interconnects are shown to become more predominant as devices are scaled towards many layers, while other effects such as membrane distensibility become less significant. The present study also probes the relationship between blood channel depth and membrane thickness on oxygen transfer, as well as the rate of oxygen transfer on the number of layers in the device. These results contribute to our understanding of the complexity involved in designing three-dimensional microfluidic oxygenators for clinical applications.     

Membrane extraction through cross-flow rectangular modules

The mass transfer for membrane extraction through a cross-flow parallel-plate module has been studied both theoretically and experimentally. Theoretical analysis of mass transfer in cross-flow membrane extractors was analogous to heat transfer in cross-flow heat exchangers with the assumption that the concentration variation in the cross-sections of flow channel was negligible. Experiments were carried out with the use of membrane sheet made of microporous polypropylene coated with polytetrafluoroethylene as a permeable barrier to extract acetic acid from aqueous solution by methyl isobutyl ketone. Theoretical predictions are in agreement with the experimental results. The simpler but still precise equation for predicting the total mass transfer rate might be more powerful than the exact solution obtained in the previous work to overcome the mathematical difficulties in device design.     

Ionic Liquid–Liquid Extraction and Supported Liquid Membrane Analysis of Lipophilic Wood Extractives from Dissolving-Grade Pulp

In this study, the extraction of lipophilic wood extractives from dissolving pulp samples using ionic liquid–liquid extraction and a two phase hollow fibre supported liquid membrane was investigated. Ionic liquids are capable of dissolving a range of organic and polymeric compounds and are biodegradable, with a negligible vapour pressure. Pulp samples were dissolved in a suitable amount of molten 1-butyl-3-methylimidazolium chloride to give 5 % cellulose solution. Pure cellulose was regenerated by adding water and filtered off. The ionic liquid-aqueous filtrate was first extracted for lipophilic extractives using liquid–liquid extraction. Then, a two phase hollow fibre supported liquid membrane extraction of lipophilic extractives was performed to extract the derivatized compounds prior to analysis by gas chromatography mass spectrometry. The operational parameters of this sample preparation approach were optimised using sterols and fatty acid methyl esters. The variation of enrichment factors and extraction efficiency with respect to liquid membrane, extraction time, stirring speed and sample pH were observed and used to get the optimal parameters. The approach was used in the analysis of oxygen bleached dissolving pulp samples in which main compounds identified were fatty acids, sterols, fatty alcohols, steroid hydrocarbons and ketones. These compounds were similar to those obtained using molecular solvent extraction method, which indicated the absence of chemical reaction between extractives and ionic liquid used.

     

CFD modeling for flow and mass transfer in spacer-obstructed membrane feed channels

The effect of spacer geometry on fluid dynamics and mass transfer in feed channels of spiral wound membranes has been investigated. Three-dimensional computational fluid dynamics (CFD) simulations reveal significant influence of spacer geometric parameters such as filament spacing, thickness and flow attack angle on wall shear rates and mass transfer coefficients. The spacers with filaments in axial and transverse direction induce higher shear stresses at the top membrane surface when compared to the bottom; the mass transfer rates are almost equal. The distribution of mass transfer coefficients become uniform when the spacing between axial filaments is increased or transverse filament thickness is decreased. For spacers with filaments inclined to the channel axis, the flow structure depends on spacing and flow attack angle. The fluid follows a zigzag path when spacing is greater while it begins to line-up with the filaments when spacing is reduced or flow attack angle is increased. The flow when aligned with the filaments increases the wall shear stress but confines the region of higher mass transfer coefficient values to a narrower portion. The zigzag flow movement increases these values on a major portion of membrane surface which enhances the mass transfer rates.     

中空纤维膜萃取分离混合稀土中的钍

通过中空纤维膜逆流萃取，研究了伯胺Ｎ１９２３对Ｔｈ４＋和ＲＥ３＋的萃取分离过程。测定了水相料液硫酸浓度、水相与油相流量对传质系数的影响，并对包头矿硫酸分解浸出液进行中空纤维膜萃取实验。结果表明，Ｔｈ４＋的传质系数受水相流量影响，与酸度及油相流量无关，总传质速率受水相临界层传质步骤控制。ＲＥ３＋的传质系数不受水相流量影响，油相流量影响很小，但受硫酸浓度的影响，总传质速率受萃取反应速率控制。根据传质速率的不同，对包头矿硫酸浸出液进行萃取分离，在８ｈ内Ｔｈ４＋可基本萃取完全，而ＲＥ３＋及Ｆｅ３＋基本不被萃取，故可在密封条件下分离钍。     

热致相分离聚偏氟乙烯膜接触器法高压吸收CO2过程及数学模拟

研究了膜接触器法高压吸收混合气中CO2的过程,考察水作为吸收剂时,操作压力、气体和吸收剂流量对聚偏氟乙烯(PVDF)中空纤维膜脱除CO2效果的影响.通过物理传质模型得出气相、膜相和液相的传质方程式,构建了二维数学模型,并结合边界层条件和多物理场耦合分析软件(COMSOL MULTIPHYSICS)对膜接触器法高压物理吸收CO2的过程进行了模拟预测.结果表明,吸收过程中膜的润湿情况显著影响CO2传质效果;在数学模型中引入润湿率,可以较准确预测CO2的物理吸收效果.     

Lung assist device technology with physiologic blood flow developed on a tissue engineered scaffold platform

There is no technology available to support failing lung function for patients outside the hospital. An implantable lung assist device would augment lung function as a bridge to transplant or possible destination therapy. Utilizing biomimetic design principles, a microfluidic vascular network was developed for blood inflow from the pulmonary artery and blood return to the left atrium. Computational fluid dynamics analysis was used to optimize blood flow within the vascular network. A micro milled variable depth mold with 3D features was created to achieve both physiologic blood flow and shear stress. Gas exchange occurs across a thin silicone membrane between the vascular network and adjacent alveolar chamber with flowing oxygen. The device had a surface area of 23.1 cm(2) and respiratory membrane thickness of 8.7 ± 1.2 μm. Carbon dioxide transfer within the device was 156 ml min(-1) m(-2) and the oxygen transfer was 34 ml min(-1) m(-2). A lung assist device based on tissue engineering architecture achieves gas exchange comparable to hollow fiber oxygenators yet does so while maintaining physiologic blood flow. This device may be scaled up to create an implantable ambulatory lung assist device.