M.A. Izquierdo-Gil

Air gap membrane distillation of aqueous alcohol solutions

Abstract Aqueous solutions of alcohol (ethanol, methanol or isopropanol) have been experimentally investigated in air gap membrane distillation (AGMD), for a wide range of operating conditions. The effects of the relevant process parameters on the permeate flux have been studied. On the basis of a temperature polarisation model — which takes into account the mass and heat transfers across the hydrophobic membrane — the equivalent film heat transfer coefficient and the overall membrane mass transfer coefficient can be obtained from the experimental data. Also the alcohol and water membrane transfer coefficients have been obtained assuming the validity of Graham’s diffusion law for multicomponent mixtures. From these coefficients the temperature and composition in the liquid–vapour interfaces are evaluated, taking into account the temperature polarisation and concentration polarisation models. Finally, the effect of the Reynolds number on the permeate flux has been discussed using the temperature polarisation model and the heat transfer correlation given by Sieder and Tate.

Air gap membrane distillation of sucrose aqueous solutions

In this paper results obtained with air gap membrane distillation (AGMD) using sucrose aqueous solutions are shown. The role of the relevant process parameters has been investigated experimentally (the flow rate through the cell, the feed initial concentration, the type of membrane, the air gap thickness, etc.). Equations have been proposed to estimate the intermediate temperatures for the air gap configuration. The fluxes given by different gas stagnant film diffusion models showed good agreement with the experimental results over the entire range of temperatures studied. Also a model which accounts for the thermal diffusion phenomenon was used. From the fits of the experimental flux data to the theoretical equations, the diffusion coefficient of the water vapour–air mixture, DAB, and the thermal diffusion coefficient, KT (only in the last case), were obtained and the results were analysed. For the DAB coefficient higher values than the tabulated ones have been obtained, although of the same order of magnitude, and still higher when the thermal diffusion is considered.

Factors affecting flux and ethanol separation performance in vacuum membrane distillation (VMD)

Abstract Membrane distillation (MD) has a great potential as a concentration process for temperature labile liquids such as fruit juices, etc. Besides water, also aroma compounds will permeate through the membrane depending on their volatility and how the MD process is operated on the permeate side. In this paper, an experimental and theoretical investigation of the influence of concentration polarisation and temperature polarisation on the flux and selectivity of binary aqueous mixtures of ethanol is presented for vacuum membrane distillation (VMD) processes. Experimental results include changes of the following parameters: nature of solutions, membrane material and pore size, feed temperature, recirculation flow rate. One method was proposed in order to evaluate the concentration polarisation effects from the fit of the experimental data. General models taking into account Knudsen and viscous flows were proposed, but viscous contribution resulted to be negligible under our operating conditions. Therefore, theoretical fluxes were estimated using Knudsen model and a good agreement between them and the experimental ones was found.

Gas permeation and direct contact membrane distillation experiments and their analysis using different models

Abstract Gas permeation experiments using helium, air and argon and direct contact membrane distillation (DCMD) experiments using distilled water are reported. From gas permeation experiments the characteristic parameters of the Knudsen and Poiseuille transport mechanisms were determined. Such parameters were extrapolated in order to obtain the values corresponding to water vapour and these were used to estimate theoretical fluxes in DCMD processes employing two different models, one proposed by Schofield et al. (with some improvements) and another one according to the “Dusty-Gas” literature. In both models, the different transport mechanisms: ordinary diffusion, Knudsen flow and Poiseuille flow were taken into account. A good agreement between the experimental fluxes and their theoretical predictions was found. A comparison between both models was also carried out. It was proved that in both models the viscous flow could be neglected under our experimental conditions.

Experimental estimation of equilibrium and transport properties of sulfonated cation-exchange membranes with different morphologies

Solvent uptake, hydraulic and electroosmotic permeabilities, true cation transport number, effective fixed charge concentration, and limiting current values have been determined in aqueous LiCl solutions for three commercial cation-exchange membranes with different morphologies and similar electric properties. The differences found in the equilibrium and transport properties of the membranes have been analyzed on the basis of their different structures. The experimental results show that the membrane morphology has an influence on the effect that the presence of an electrolyte has in the solvent uptake and in the liquid permeation. Differences have also been found in the polarization concentration effects, and on the loss of the membrane selectivity with the increase of the electrolyte concentration.

Methanol-Water Solution Transport in Nafion Membranes with Different Cationic Forms

Measurements of liquid transport were made with a Nafion membrane at different cationic forms. The experimental data are used to estimate the alcohol permeability when the membrane is separating water and water-methanol solutions. The obtained permeability coefficient values were useful for analyzing the influence of the substituted cations on the transport process in the membranes. In the present article, the permeability coefficient of methanol in Nafion substituted by Na+, K+, Cs+, Mg2+, Ca2+, Ba2+, Fe2+, Fe3+, and Al3+ were reported at different methanol concentration values. The analysis of the results revealed that, in general, for ions with the same period in the periodic table, the alcohol permeability decreases with increasing the valence. In contrast, when ions with the same valence are compared, the alcohol permeability decreases when the atomic mass increases, with the exception of the Mg2+. As a general trend, similar alcohol permeability variation with the concentration is observed for all the cationic forms of the membrane. There is an initial increase in the permeability, and, when the methanol concentration in the solutions is about 60%, the permeability decreases with the alcohol concentration. However, in the case of trivalent ions, the methanol permeability decreases with the methanol concentration.