Abstract (Summary)
Characterization of a membrane is necessary to predict its performance under different operating conditions and to optimize a membrane separation process. In this study a novel technique to characterize nanofiltration and reverse osmosis membranes is developed. This technique combines a membrane transport model based on irreversible thermodynamics with the description of concentration polarization to describe the separation process in the flat disk membrane cell. The irreversible thermodynamics model describes the solute transport through nanofiltration and reverse osmosis membranes by diffusion as well as convection as opposed to the solution diffusion model that describes salt transport only by diffusive flux. Due to the symmetrical circular geometry of the disk membrane cell the hydrodynamics remains uniform over the entire surface of the membrane. This helps in describing concentration polarization more accurately as compared to hollow fiber and spiral wound modules where the concentration of solute changes in an unknown way as the mass transfer boundary layer thickness develops along the flow. The permeation data obtained by varying feed flow rates, feed concentration and applied pressures were best fit to the theoretical model to obtain the three phenomenological coefficients that characterize the membrane, namely: (1) the hydraulic permeability (2) the solute permeability, and (3) the reflection coefficient of the membrane. For the nanofiltration membrane dealing with NaCl, the reflection coefficient was determined to be 0.75, which shows that the contribution of advection could be significant to total salt flux through this nanofiltration membrane. The method also enabled determining the mass transfer boundary layer thickness that was subsequently used to calculate the concentration of solute at the surface of the membrane. The intrinsic solute rejection, which is based on the concentration of salt at the membrane surface, was correlated with the total volumetric flux through the membrane that can be used to design membrane processes on a commercial scale. This technique is an improvement over the other characterization studies as the mass transfer boundary layer thickness was determined from the permeation data rather than using Sherwood number correlations from the literature that are based on heat and mass transfer correlations.
Bibliographical Information:


School:University of Cincinnati

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:nanofiltration and reverse osmosis concentration polarization membrane characterization irreversible thermodynamics permeation experiments


Date of Publication:01/01/2003

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