Direct Contact Membrane Distillation Experiments Research Articles

Effect of salt concentration during the treatment of humic acid solutions by membrane distillation

Membrane distillation was performed for the treatment of humic acid solutions containing sodium chloride and calcium chloride at concentrations typical of those found in natural waters. The experiments were performed with microporous polytetrafluoroethylene and polyvinylidene fluoride membranes. Direct contact membrane distillation experiments were carried out initially for pure water as feed to determine the water vapor permeability of each membrane. Sodium chloride and calcium chloride aqueous feed solutions were then used to determine the permeate flux and solute rejection of the membranes prior to humic acid tests. The effect of the salts concentration was investigated. Compared to the pressure-driven membrane separation processes, higher rejection factors with very low permeate flux decline were observed in direct contact membrane distillation application when humic acid containing salts were considered. These results suggest that membrane distillation is an attractive method for the treatment of humic acid solutions.

Effects of net-type spacers on heat and mass transfer in direct contact membrane distillation and comparison with ultrafiltration studies

The study of heat and mass transfer enhancement in direct contact membrane distillation (DCMD) by the net-type spacers was intended to develop the heat transfer correlation for the spacer-filled channels. DCMD experiments performed with 20 spacers different in voidages and hydrodynamic angles were used to determine the spacer performance in heat and mass transfer enhancement. The effect of high mass transfer rates on heat and mass transfer coefficients was also investigated to explain the experimental results compared with ultrafiltration (UF) with the spacers. The results showed that spacers enhanced mass fluxes up to 60% and increased heat transfer coefficients by approximately two times over the empty channels. The optimum spacer geometry was found at the voidage and hydrodynamic angle of 0.6 and 90°, respectively. The heat transfer correlation for spacer-filled channel was obtained. The analysis of high mass transfer rate convection showed that the effect of high mass transfer rates on mass transfer coefficients was remarkable for ultrafiltration but was insignificant for membrane distillation.

Effect of pore size distribution and air flux on mass transport in direct contact membrane distillation

The concept of mass transfer regions within the membranes was introduced to study the mass transport in membrane distillation processes. Mass transfer model for direct contact membrane distillation (DCMD) was derived to examine the influence of pore size distribution and air fluxes on water vapor fluxes across the membranes. The pore size distributions of the membranes were determined by field emission scanning electron microscopy (FESEM) and the image analysis program. DCMD experiments with pure water were carried out under laminar and turbulent flow conditions so as to compare the experimental results with the predictions. The calculation results showed that Knudsen and transition regions were found in the membranes studied, while the transition region was the major contribution to mass transport. The model including the effect of pore size distribution and air fluxes predicted water fluxes with the average discrepancy 5% of the experimental results. The mass transfer analysis indicated that the influence of pore size distribution and air fluxes on water fluxes was insignificant. Therefore, the mass transfer model with the assumptions of air trapped in membrane pores and single pore size is adequate to describe mass transport in DCMD. The concept of mass transfer regions was also applied to analyze the effect of pore size distribution on flux in vacuum membrane distillation and gas permeation.

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

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.

Theoretical and experimental studies on desalination using membrane distillation

Pure water direct contact membrane distillation experiments were carried out. A model based on a dusty-gas model of gas transport through porous media showed good agreement with the experimental results over the entire range of feed temperatures and recirculation rates studied. The model was also used to predict performance of salt solutions desalination. Fluxes similar to those typically obtained with reverse osmosis were predicted.

Study of membrane distillation using channel spacers

The permeation flux of water vapor in direct contact membrane distillation experiments has been studied. Permeation experiments with a PTFE membrane were conducted by using a flat membrane module with channels for hot and cold water created by both separators. We have studied two basic separator configurations: open flow separators and screen separators. The experimental results show as the permeate flux increases, the temperature and the recirculation rate increase. The use of screen separators also increases the flux. These results are explained by the equations for heat and mass transfer. These equations allow to calculate the heat transfer coefficients h in the liquid films adjacent to the membrane. The h values suggest the appearance of turbulences when screen separators are used. Finally the temperature polarization coefficients are evaluated for different operational conditions.

Membrane distillation. II. Direct contact MD

Pure water direct contact membrane distillation (DCMD) experiments were used to measure the permeability parameter associated with the molecular diffusion in membrane distillation (MD). The fluxes given by a recently reported MD model, which is based on the dusty-gas model of gas transport through porous media, showed good agreement with the experimental results over the entire range of feed temperatures studied. The model was also capable of predicting flux as a function of the difference between bulk feed and permeate temperatures for the limiting case in which only molecular diffusion contributes to flow. The DCMD experiments were performed in this work with a new laboratory-scale module that does not require a support for flat-sheet membranes. The resulting DCMD fluxes were two to three times higher than those reported in the literature for either DCMD or reverse osmosis. The MD model was also used to predict the performance of DCMD desalination, and the results were compared to those of reverse osmosis, in terms of both water production rates and NaCl rejection.