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.     

Extraction and re-extraction of phenylalanine by cationic reversed micelles in hollow fibre contactors

This work reports the extraction of phenylalanine with a reversed micellar system consisting of TOMAC/hexanol/n-heptane using hydrophobic hollow fibre modules. Extraction studies were performed under different hydrodynamic conditions and mass transfer correlations for the shell and tube sides were developed. The correlations were determined using a one-step calculation method and the results obtained are in agreement with the literature for the range of Reynolds numbers studied.Based on the obtained correlations and on the resistance in series model, a transport model was developed in which the phenylalanine concentration in the feed phase can be predicted during the experimental run. For the extraction process the model developed describes satisfactorily the evolution of phenylalanine concentration with time under different hydrodynamic conditions. The re-extraction process was found to be kinetically controlled due to the higher dynamic stability of reversed micelles when contacting a stripping phase with high ionic strength. The experimental results obtained were described using a kinetic model developed.Simultaneous extraction/stripping of phenylalanine was also accomplished using two hollow fibre modules in series, using different volume phase ratios. The mass transfer process was modelled and compared with the experimental results.     

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.     

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.     

Mass and momentum transport in extra-luminal flow (ELF) membrane devices for blood oxygenation

In this paper, we report on the characterisation of transport in membrane modules for blood oxygenation where blood is circulated outside hollow fibre membranes arranged in double layer cross-laid mats at an angle with respect to the main direction of blood flow. The effect of design and operating variables on module performance was investigated with respect to oxygen transfer into water, as gaseous oxygen and water are circulated counter-currently, respectively inside the membrane lumen and through the membrane assembly.Increasing water flow rates and membrane angles enhanced oxygen transfer across the membrane and resulted in robust operation but also in high pressure drops.Module pressure drop and oxygen transfer rate were correlated to module geometry, fibre packing density, water flow rate and membrane angle with respect to the main direction of the liquid flow in non-dimensional equations that can be used by membrane module manufacturers for the design of optimal ELF blood oxygenators. The results suggest that an optimum membrane angle exists, beyond which module operation is not convenient in terms of energy.     

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.     

Extraction of Cu(II) with Acorga M5640 using hollow fibre liquid membrane

The extraction of copper from sulphuric/sulphate solutions using a hollow fibre module as contactor was studied. The aldoxime Acorga M5640 was used as an extractant. The effects on the extraction rate of the flow-rates, the concentrations of copper and extractant, pH, and the presence of Na₂SO₄ in the feed phase were investigated. The overall mass transfer coefficient for copper extraction was calculated from the experimental data and was compared with the value evaluated by the resistance in the series model. The extraction process was found to be governed by the diffusion in the boundary aqueous layer and also by the chemical reaction. The kinetic data obtained were used to simulate the extraction of copper with the pseudo-emulsion-based hollow fibre with strip dispersion technique. The accordance between the results thus calculated and the experimental data was found to be satisfactory, particularly for dilute feed solutions.     

Heat and mass transfer in membrane distillation

Equations for heat and mass transfer in membrane distillation (MD) have been developed and tested experimentally. The concept of temperature polarisation is introduced and shown to be important in the interpretation of experimental results. Vapour transport through the membranes tested is reasonably described by combined Knudsen and molecular diffusion. The significance of temperature polarisation in the design and operation of large-scale MD modules is discussed, and hollow fibre and tubular systems shown to be potentially the most effective.     

Modeling hollow fiber membrane contactors using film theory,Voronoi tessellations,and facilitation factors for systems with interface reactions

A model to evaluate and predict performance of hollow fiber membrane contactor modules is presented for the extraction of compounds with interfacial chemical reactions. The model is based on the film theory approach combined with facilitation factors. Mass transfer coefficients are calculated using published correlations and chemical reaction effects are accounted for by calculated facilitation factors. The shell side flow maldistribution due to random packing of fibers in the module bundle is estimated using Voronoi tessellations and the friction factor-Re. The computer simulation of random packing is accomplished using random sequential addition. The model is correlated with experimental data on the extraction of uranyl nitrate from an acidic aqueous medium into TBP/DIPB extractant. The model correlates experimental data very well for this system. By accounting for the non-linearity of the interfacial effects in this way the rigorous solution of a system of coupled non-linear PDEs may be avoided, but the model can still provide accurate predictive design and analysis information for the system.     

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.     

Local hydrodynamic investigation of the aeration in a submerged hollow fibre membranes cassette

The better understanding of the effective air distribution inside a membrane cassette is a particular challenge in submerged membrane bioreactor. The present study is the first one that investigates the hydrodynamics of the coarse bubbles flow inside a hollow fibre membranes cassette. The experimental investigations were carried out in a reactor equipped with commercial modules from ZENON ZeeWeed^® 500d. A bi-optical probe was used to measure the bubble size, the bubble velocity and the gas hold-up at different locations between the modules and for three different gas flow rates. These local measurements gave significant information about the lateral distribution of the air and its evolution with the height on the surface of the membrane modules, which can impact on the filtration performance and are the first step to an optimisation of the aeration system and module geometry.     

Mass transfer correlations in membrane extraction: Analysis of Wilson-plot methodology

Mass transfer correlations have been obtained for the past eight decades by the Wilson-plot method which has proved to be suitable for systems operating in steady-state conditions and where the only variable is the fluid velocity. In this work, this methodology is evaluated by using a membrane extraction process with a hollow-fiber membrane contactor as a case study. Taking into consideration the currently available mathematical tools, alternative methods to obtain mass transfer correlations are proposed and discussed. The proposed one-step calculation methodology proved to be a most suitable approach, leading to a drastic reduction in the errors associated with the estimated parameters. Additionally, improvements were observed when accounting for the partition coefficient variation.     

Critical parameters in a supported liquid membrane extraction technique for ionizable organic compounds with a stagnant acceptor phase

The reviews cover important critical parameters that are often optimized in a supported liquid membrane extraction technique in both flat sheet and hollow fibre designs for ionizable organic molecules. Understanding of these parameters can enable one to predict the behavior of the compound before hand and thus reduce the number of optimization experiments. Moreover, less number of experiments can be also generated using statistical techniques which are now becoming more commonly used. Supported liquid membrane extraction optimal parameters such as the conditions of the pH of the acceptor and donor phases should easily be fixed from the pKa values of the compounds. Other parameters, including the polarity of the compound can help to predict the partitioning into the membrane and the behavior of the compound. The influence of parameters such as temperature on the mass transfer in supported liquid membrane depends on the design of the module, experimental design and type of mass transfer controlling the extraction process.     

Microporous hollow fibers for gas absorption : I. Mass transfer in the liquid

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, a theory for the operation of hollow fiber membrane modules is developed, and mass transfer coefficients in the liquid phase are investigated.     

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.     

Mathematical modeling of gas separation permeators — for radial crossflow,countercurrent, and cocurrent hollow fiber membrane modules

A new approach to solve the mass transfer problem posed by the permeation process in a hollow fiber permeator is presented and analyzed. The algorithm models the separation offered for a membrane module, for given gas conditions, simulating the permeate and residue composition and the stage cut. The advantage of the ‘succession of states’ approach utilized here is the option of retroactive incorporation of more complex interactions such as permeate pressure buildup, a pressure, composition and temperature dependent permeability. The two dimensional mass transfer in a radial crossflow permeator has been qualitatively discussed in the past, but it has not been modeled in the literature. The countercurrent, cocurrent and crossflow configurations (all single dimensional mass transfer cases) for gas separation have been modeled in literature primarily by numerical integration of the differential equations over the relevant boundary conditions. Incorporation of nonlinearities such as pressure and permeability variations complicate the mathematics considerably for a single dimension, and make their solution almost impossible in two dimensions. This paper proposes an algorithm that simplifies the understanding of the problem posed, in terms of practical parameters (such as stage cut), and analyses the three flow patterns (radial crossflow, countercurrent, and cocurrent) in detail.     

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.     

Washing frozen red blood cell concentrates using hollow fibres

Today the cryopreservation of human blood products is routine. However, before reinfusion the cryoprotectant, often glycerol, has to be removed. We have designed a combined microfiltration diafiltration process using microporous hollow fibres for removing glycerol from frozen red blood cell concentrates. As the system can be closed to the atmosphere there is no possibility of infection of the “washed” blood. Thus the post-thaw shelf life of the blood may be greatly increased. The process has been optimized by minimizing both the processing time and diluent volume required. Finally a hollow fibre module capable of completing the entire washing process in 30 min has been developed. We show that such a module requires hollow fibres with an inside diameter of 200 μm. The design equations we present are generally applicable to the design of hollow fibre microfiltration systems.     

Hollow-fibre membrane module design: comparison of different curved geometries with Dean vortices

The performance of several designs of curved membrane modules with Dean vortices was compared through experiments using a colloidal bentonite suspension and cellulose acetate hollow-fibre ultrafiltration (UF) membranes. The different module geometries were: straight, helically coiled, twisted and sinusoidal, or meander-shaped. The experiments show a remarkable increase in mass transfer in curved modules as compared to conventional straight ones. Comparisons were made for modules equipped with the same hollow fibres and the same Dean number (De) for a given Reynolds number (Re). At the same Dean number, all the curved geometries gave the same limiting permeate flux. A mass transfer correlation relating limiting UF flux with the mean wall shear stress has been obtained.     

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.