M.C. García-Payo

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.

Separation of binary mixtures by thermostatic sweeping gas membrane distillation: II. Experimental results with aqueous formic acid solutions

Aqueous solutions of formic acid have been experimentally investigated in a modified sweeping gas membrane distillation (SGMD) configuration. A thermostated sweeping gas was used in order to enhance the mass transfer performance. A new tubular module was designed and built for this purpose. The effects of the relevant process parameters on the permeate flux and selectivity have been studied. Experiments with pure water and pure formic acid were used to estimate certain parameters in the model. From these mass transfer coefficients, the fluxes and selectivity for aqueous formic acid mixtures have been calculated using the mathematical model previously described [J. Membr. Sci., 2001, in press]. The model predictions were compared with the experimental data and a good agreement between both flux values were obtained.

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.

Separation of binary mixtures by thermostatic sweeping gas membrane distillation: I. Theory and simulations

A math. model describing a novel sweeping gas membrane distn. (SGMD) configuration is presented and the sepn. of two volatile components from binary mixts. by this membrane process is theor. investigated. SGMD configuration is modified by using a thermostated sweeping gas in order to enhance the driving forces decrease along the module. A Stefan-Maxwell-based model that includes vapor-liq. equil. and heat and mass transfer relations is used. This model that takes into account temp. and concn. polarization effects as well as temp. and concn. variation along the module length is employed to predict the flux and selectivity under the relevant operating conditions. Finally, a theor. evaluation of this particular configuration with respect to direct contact MD configuration is discussed. [on SciFinder (R)]

Membranes used in membrane distillation: preparation and characterization

Abstract Interest in the design and development of membranes for membrane distillation (MD) technology has been increasing since 2003. About 23% of the total published studies on MD since 2003 (up to 31 December 2013) are focused on membrane fabrication and modification for MD purposes. Therefore, this chapter covers the different types of materials used in membrane engineering for MD. Interesting and promising membrane designs and fabrication techniques of advanced membranes for MD are highlighted. Some novel and emerging membranes for MD are described. Techniques used to determine the most important characteristics needed for an adequate MD membrane are also summarized. Future trends and useful sources of further information are also included.

Desalination by Membrane Distillation

Desalination of seawater is the technology predominantly used to alleviate the problem of water scarcity in coastal regions. The sustainability of all desalination processes depends mainly on the reduction of energy costs, the increase of water recovery and the enhancement in recovery and reuse of the generated waste. Integrated/hybrid membrane processes have attracted much interest in the desalination field. Membrane distillation (MD) is a membrane process that can be used in combination with other processes. The objective of this chapter is to review the capability of MD in treating highly concentrated aqueous solutions derived from other desalination processes. This potential application is attracting an increasing interest due to the problem of brine discharges to the environment. Various benefits can be obtained with the utilisation of MD in integrated/hybrid membrane processes, e.g. enhanced quality of the water produced, brine concentration up to zero discharge and energy savings.

Novel and emerging membranes for water treatment by hydrostatic pressure and vapor pressure gradient membrane processes

Abstract Hydrostatic pressure (microfiltration, ultrafiltration, nanofiltration, and reverse osmosis) and vapor pressure gradient membrane processes (membrane distillation and pervaporation) have received increased attention since the 1980s for different water treatment applications. Significant progress on these processes has been achieved during the last ten years on the fabrication of novel membranes and their modification. Some related features for water treatment are highlighted. Novel flat sheet and hollow fiber membranes made with innovative materials and with improved properties suitable for specific applications are reviewed.

Novel and emerging membranes for water treatment by electric potential and concentration gradient membrane processes

Abstract Electric potential and concentration gradient membrane processes have attracted growing attention in many potential applications such as power generation, desalination, wastewater treatment, and food processing. This chapter focuses on water treatment by electrodialysis and forward osmosis. Special attention is also paid to the alternative technologies of emerging interest used for power production, reverse electrodialysis and pressure retarded osmosis, that can generate electricity from salinity gradients. Interesting recent research studies on the improvement of the performance of these processes and their advanced membranes are reported. Critical challenges, including concentration polarization, membrane fouling, reverse solute diffusion, draw solute design, etc., are also highlighted.