J.I. Mengual

Theory and experiments on sweeping gas membrane distillation

Abstract A theoretical model is presented that describes sweeping gas membrane distillation processes through porous hydrophobic membranes. The approach considers the case in which the liquid feed and the sweeping gas counterflow in a plate and frame membrane module. The model developed emphasises the importance of the heat fluxes in the directions parallel and perpendicular to the membrane surface and permits to obtain the temperature profiles inside the fluid phases. In order to check the model, two membranes have been studied in different experimental conditions. The influence of some relevant parameters, such as the inlet and outlet temperatures or the circulation velocities of the fluids has been studied. The theoretical predictions of the model have been applied to the obtained results and the accordance may be considered good.

Nature of flow on sweeping gas membrane distillation

Abstract The process of sweeping gas membrane distillation (SGMD), with the liquid feed and the sweeping gas counterflowing in a plate and frame membrane module, has been studied. A theoretical model, which was presented in a previous paper and permitted to obtain the temperature profiles inside the fluid phases, has been developed in order to analyse the physical nature of the transmembrane water flux. Two porous hydrophobic membranes have been studied in different experimental conditions. The influence of some relevant parameters, such as the inlet and outlet temperatures or the circulation velocities of the fluids, has been studied. The experimental results have been analysed according to the model and the conclusion is that the water transport takes place, apparently, via a combined Knudsen and molecular diffusive flow mechanism. From the temperature profiles, a local temperature polarisation coefficient may be defined. From this local value, an overall one for the whole system is then defined. The new theoretical predictions have been applied to the obtained results and the accordance may be considered good.

Modelling mass transport through a porous partition: Effect of pore size distribution.

Abstract Direct contact membrane distillation process has been studied using microporous polytetrafluoroethylene and polyvinylidene fluoride membranes. The membranes were characterized in terms of their non-wettability, pore size distribution and porosity. The mean pore sizes and pore size distributions were obtained by means of wet/dry flow method. The mean pore size and the effective porosity of the membranes were also determined from the gas permeation test. A theoretical model that considers the pore size distribution together with the gas transport mechanisms through the membrane pores was developed for this process. The contribution of each mass transport mechanism was analyzed. It was found that both membranes have pore size distributions in the Knudsen region and in the transition between Knudsen and ordinary diffusion region. The transition region was the major contribution to mass transport. The predicted water vapor permeability of the membranes were compared with the experimental ones. The effect of considering pore size distribution instead of mean pore size to predict the water vapor permeability of the membranes was investigated.

Theoretical and experimental studies on desalination using the sweeping gas membrane distillation method

Abstract Sweeping gas membrane distillation was investigated as a possible technique for desalination by using a shell-and tube polypropylene membrane module. Humid air was employed as the vapor carrier gas and the feed solution was circulated in counter-current through the lumen side of the membrane module. The effects of the process parameters, liquid feed flow rate, feed temperature, air flow rate and salt concentration on the distillate flux have been investigated. A theoretical model that considers the heat and mass transfer through microporous and hydrophobic membranes as well as the temperature and concentration polarization effects was developed and validated with the experimental data of distilled water and sat aqueous feed solutions. The distillate flux decreases with the salt concentration, but no drastic flux decline was observed with the increase of the feed electrical conductivity up to 182 mS/cm. High-purity water was obtained.

Osmotic distillation through porous hydrophobic membranes

Abstract Osmotic distillation through porous hydrophobic membranes has been studied theoretically and experimentally. In particular the influence of the unstirred diffusion boundary layer has been determined. A model has been develop which describes the contributions of solution concentration and stirring rate. The influence of these and other relevant parameters is discussed.

Possibility of nuclear desalination through various membrane distillation configurations: a comparative study

Three membrane distillation processes, direct contact membrane distillation, sweeping gas membrane distillation and vacuum membrane distillation, have been experimentally studied in a shell-and-tube capillary membrane module. Preliminary experiments were conducted using distilled water and sodium chloride aqueous solutions as feed. The effects of the operating parameters - flow rate, temperature and salt concentration - have been investigated. A theoretical analysis that considers the heat and mass transfer through microporous hydrophobic membrane has been developed. A comparative study was made between the three membrane distillation configurations. Membrane distillation can be an alternative for liquid nuclear waste treatment.

Characterization of membrane distillation membranes prepared by phase inversion

In this paper, flat membrane distillation membranes have been successfully manufactured from PVDF/DMAc and PVDF/DMF blends by using phase inversion induced by an immersion precipitation technique. The structure of the membranes is asymmetric with a porous top layer and macrovoids, as assessed by SEM. The existence of MD fluxes in these membranes is established by performing various pure water flux experiments. A maximum in the MD fluxes for a particular value of the polymer content in the casting solution from which the membrane is manufactured has been observed. The dependence of the magnitude of the fluxes on the membrane thickness is also discussed and the influence of temperature polarization evaluated.

Concentration of Bovine Serum Albumin Aqueous Solutions by Membrane Distillation

ABSTRACT The use of membrane distillation in the concentration of protein solutions is proposed. Semidilute BSA solutions (concentrations ranging between 40 and 100%), at pH 7 were employed. Transmembrane volume fluxes and evolution of the BSA concentration were measured at different conditions. Temperatures ranged between 20 and 38°C. The filtration cell was operated at laminar regime with tangential velocities of 579 and 89cm-s°. Special attention was devoted to analysis of the temperature polarization and fouling effects. The results obtained show that fouling effects are practically absent in membrane distillation operations with BSA solutions, at least for concentrations up to 10% w/w and in the range of crossflow velocities employed. This may be due to the fact that the transmembrane fluxes are below the value of the critical flux for fouling.

Membrane distillation: theory and experiments

Abstract A theoretical approach is presented that describes membrane distillation processes due to the simultaneous action (in a proactive or in a counteractive way) of temperature and concentration differences through porous hydrophobic membranes. The model developed emphasizes the importance of the boundary layers, shows the existence of a coupling term between the two thermodynamic forces acting on the system, and permits the definition and characterization of the so-called steady states. In order to check the model, two membranes have been studied in different experimental conditions. The influence of some relevant parameters, such as solution concentration, stirring rate, mean temperature and temperature difference has been considered and the theoretical predictions of the model have been applied to the obtained results. The accordance may be considered good.

Study of Asymmetric Polarization in Direct Contact Membrane Distillation

Abstract The objective of this study was to analyze the polarization phenomena in each side of a microporous hydrophobic membrane using a direct contact membrane distillation process. Experiments were conducted with distilled water and sodium chloride aqueous solutions as feeds. Different flow rates and temperatures at both membrane sides were employed. Thefeed and permeate temperature polarization coefficients as well as thefeed and permeate vapor pressure polarization coefficients were defined and evaluated. Two methods: a velocity extrapolation method and a semiempirical method that considers the heat and mass transfer empirical correlations, were used. It was proved that there is an asymmetric polarization in direct contact membrane distillation.