Isabel M. Coelhoso

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.

Transport mechanisms and modelling in liquid membrane contactors

Abstract This paper discusses the use of liquid membrane contactors for extraction of fermentation and pharmaceutical products using different types of carriers. It intends to emphasise the importance of understanding the transport mechanisms involved in liquid membrane extraction with different carriers and also to discuss relevant aspects of the mathematical modelling involved in these extraction processes. Using the extraction of organic acids, namely amino acids, as a case study it is shown how the supramolecular organisation of the extractant determines the solute transport mechanisms involved. Additionally, the resolution of racemic mixtures using chiral carriers is also discussed. The modelling work analyses two different aspects of extraction using membrane contactors with microporous membranes: (i) the importance of using a correct description of solute partition between the feed and the extractant phase (use of a variable partition description versus constant partition); (ii) the correct development of mass transfer correlations in hollow fibre contactors. For the development of mass transfer correlations the calculation method proposed by Wilson has been universally used. Given the currently available mathematical tools, that enable the analytical manipulation of equations and fittings with complex expressions, a new calculation methodology is discussed.

Membrane-based solvent extraction and stripping of lactate in hollow-fibre contactors

This work reports lactate extraction with Aliquat 336, an ion-exchange carrier, using membrane contactors with hydrophobic microporous membranes. The overall mass-transfer coefficients were evaluated by either assuming a constant distribution coefficient or using the equilibrium equation. The mass-transfer coefficients calculated using the equilibrium relationship between Cf∗ and CO, proved to be more rigorous, since the variation of the distribution coefficient with solute concentration is accounted for throughout the extraction process. Assuming the resistance in series model, the membrane resistance was identified as the limiting step on the mass-transfer process. In order to increase the membrane mass-transfer coefficient, it is necessary to increase the diffusion coefficient of the lactate-amine complex; this can be achieved by reducing the viscosity of the organic phase either through a temperature increase or reduction of the carrier concentration. Using a plate-and-frame module with a well-defined hydrodynamic characterization of both phases, the mass-transfer coefficients can be rigorously evaluated; these were compared with the values obtained with hollow-fibre modules. Simultaneous extraction and stripping of lactate was accomplished using two hollow-fibre modules in series. This configuration assures that saturation of the carrier does not occur, as it is permanently regenerated; furthermore, the carrier concentration can be reduced while still maintaining the mass-transfer rate, thus decreasing the associated operating costs of the process.

Mass transfer in osmotic evaporation: effect of process parameters

The influence of relevant parameters on the osmotic evaporation process, namely temperature, stirring rate and osmotic agent’s nature and concentration, is evaluated. The water flux is expressed as a function of the Reynolds number ( Re), characterising the hydrodynamic conditions, and as a function of the water activity difference, which is the driving force of the process. It is our intention to emphasise the need of a systematic presentation of the results, in order to compare different conditions. Mass transfer is described by a series resistance model: the membrane resistance and the boundary layer resistance in both sides of the membrane. The contribution of the boundary layer resistance is determined for two osmotic agents (calcium chloride and glycerol), and a correlation for the mass transfer coefficient, Sh = 1.63Re 0.56 Sc 0.33 , was obtained. Using as osmotic agents calcium chloride, sodium chloride and glycerol, it was noticed that, the flux increases with the osmotic solution concentration, and is not affected by the osmotic agent’s nature. The water flux increases with temperature according to an Arrhenius type equation and this increase is mainly due to an increase of the water vapour pressure.

Polysaccharide-Based Membranes in Food Packaging Applications

Plastic packaging is essential nowadays. However, the huge environmental problem caused by landfill disposal of non-biodegradable polymers in the end of life has to be minimized and preferentially eliminated. The solution may rely on the use of biopolymers, in particular polysaccharides. These macromolecules with film-forming properties are able to produce attracting biodegradable materials, possibly applicable in food packaging. Despite all advantages of using polysaccharides obtained from different sources, some drawbacks, mostly related to their low resistance to water, mechanical performance and price, have hindered their wider use and commercialization. Nevertheless, with increasing attention and research on this field, it has been possible to trace some strategies to overcome the problems and recognize solutions. This review summarizes some of the most used polysaccharides in food packaging applications.

Removal of valeric acid from wastewaters by membrane contactors

Membrane contactors, providing a non-dispersive extraction technique, were used for the removal of valeric (n-pentanoic) acid from synthetic aqueous solutions simulating an industrial wastewater from polymer manufacturing. Amberlite LA-2 (secondary amine) in toluene was chosen as the extraction system. Equilibrium conditions were determined and mechanistically modelled for different extractant concentrations allowing the further calculation of mass transfer coefficients. The influence of the hydrodynamics of both the aqueous and organic phases on the overall mass transfer coefficient, calculated through two proposed methods, was studied. The integration of extraction and backextraction was also carried out, allowing a further acid removal with lower extractant concentrations.

Proton NMR Relaxometry Study of Nafion Membranes Modified with Ionic Liquid Cations

Proton nuclear magnetic resonance (NMR) relaxometry was used to study the ionic mobility and levels of confinement within Nafion membranes modified by incorporation of selected ionic liquid (IL) cations. These studies were performed aiming at understanding the effect of using different types of ionic liquid cations, and their degree of incorporation, in the values of the spin-lattice relaxation times (T(1)) obtained at different values of frequency and thus detect the influence of confinement level on the ions mobility. The frequency dependence of the proton spin-lattice relaxation rate, R(1) = 1/T(1), for the modified Nafion/IL cation membranes was compared with that obtained for an unmodified Nafion membrane, allowing for distinguishing different contributions of the motions of the molecules depending on the frequency tested. The experimental R(1) results were analyzed in terms of models that consider the sum of the most effective relaxation contributions, to estimate the translational self-diffusion coefficient of the moving molecular species in the modified membranes. The stability of these membranes with temperature in terms of the spin-lattice relaxation was compared with results obtained by thermogravimetric analysis.

Development and characterization of bilayer films of FucoPol and chitosan

Bilayer films of FucoPol and chitosan were prepared and characterized in terms of optical, morphologic, hygroscopic, mechanical and barrier properties, to evaluate their potential application in food packaging. Bilayer films have shown dense and homogeneous layers, and presented enhanced properties when comparing to monolayer FucoPol films. Though, a high swelling degree in contact with liquid water (263.3%) and a high water vapour permeability (0.75×10(-11)mol/msPa), typical of polysaccharide films, was still observed. However, they presented a low permeability to O2 and CO2 (0.47×10(-16)molm/m(2)sPa and 5.8×10(-16)molm/m(2)sPa, respectively). Tensile tests revealed a flexible and resistant film with an elongation at break of 38% and an elastic modulus of 137MPa. The studied properties, in particular the excellent barrier to gases, impart these bilayer films potential to be used in packaging of low moisture content products, as well as in multilayered hydrophobic/hydrophilic/hydrophobic barriers for food products with a broader range of water content.

Using membrane contactors for fruit juice concentration

Abstract Osmotic evaporation is a membrane process that has fascinating advantages over conventional processes for the production of fruit juice concentrates since it has the ability to concentrate solutes to very high levels with minimal thermal or mechanical damage. Our purpose was to compare and analyse the results obtained in the concentration of a sucrose solution used as model juice. Two different membrane contactors with different specific areas and number and dimensions of fibres/tubes were used. The hydrodynamic conditions that maximize the water flux were previously determined by using a series resistance model. For a determined range of Reynolds number, the boundary layer mass transfer resistances can be neglected and there is only the contribution of the membrane resistance. The concentration of a sucrose solution from 12 ° Brix to 60 ° Brix, used as a model fruit juice, was carried out. The water flux decreased due to the progressive reduction of the driving force. The overall mass transfer coefficient, K p , remained constant throughout the experiment, except for one of the contactors used where there was a reduction due to the concentration polarization caused by the increase of the sucrose solutions viscosity.

Supported ionic liquid membranes for removal of dioxins from high-temperature vapor streams.

Dioxins and dioxin-like chemicals are predominantly produced by thermal processes such as incineration and combustion at concentrations in the range of 10-100 ng of I-TEQ/kg (I-TEQ = international toxic equivalents). In this work, a new approach for the removal of dioxins from high-temperature vapor streams using facilitated supported ionic liquid membranes (SILMs) is proposed. The use of ceramic membranes containing specific ionic liquids, with extremely low volatility, for dioxin removal from incineration sources is proposed owing to their stability at very high temperatures. Supported liquid membranes were prepared by successfully immobilizing the ionic liquids tri-C(8)-C(10)-alkylmethylammonium dicyanamide ([Aliquat][DCA]) and 1-n-octyl-3-methylimidazolium dicyanamide ([Omim][DCA]) inside the porous structure of ceramic membranes. The porous inorganic membranes tested were made of titanium oxide (TiO(2)), with a nominal pore size of 30 nm, and aluminum oxide (Al(2)O(3)), with a nominal pore size of 100 nm. The ionic liquids were characterized, and the membrane performance was assessed for the removal of dioxins. Different materials (membrane pore size, type of ionic liquid, and dioxin) and different operating conditions (temperature and flow rate) were tested to evaluate the efficiency of SILMs for dioxin removal. All membranes prepared were stable at temperatures up to 200 °C. Experiments with model incineration gas were also carried out, and the results obtained validate the potential of using ceramic membranes with immobilized ionic liquids for the removal of dioxins from high-temperature vapor sources.