Author: Colleen Michelle Gorey

Publisher:

Published: 2008

Total Pages: 254

ISBN-13:

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This project focused on modifying membrane surfaces to increase fouling control while improving permeability and selectivity, resulting in membranes with longer operational lives, lower cleaning frequencies, and higher efficiencies. The modifications involved the attachment of a temperature sensitive polymer to keep the membrane from fouling. The membrane modifications, which produced a self-cleaning cellulose acetate membrane, involved grafting the surface with a thermally responsive film layer of hydroxypropyl cellulose (HPC). HPC possesses a lower critical solution temperature (LCST) of approximately 43C. When attached to a surface, HPC forms a film, which collapses at temperatures above the LCST and expands when cooled to below the LCST. By keeping the film in a non-equilibrium state, via oscillating the temperature of the membrane surface, (as was observed by flux decline), fouling was reduced. Two surface modification techniques were tested and compared to the unmodified membrane - Method 1 and Method 2. Method 1 involved the attachment of a gel layer whereas Method 2 was the attachment of polymer structures on the surface of the membrane. Roughness measurements, using a wet atomic force microscopy (AFM) cell, and filtration experiments (to monitor flux declines) were performed at cold temperatures (25C), at hot temperatures (60C) and with temperature oscillations. The unmodified membrane had roughness values that were higher when hot and lower when cold, and it displayed flux declines under all temperature conditions when a humic solution was filtered. Since the humic solution was acting as a fouling agent, the decrease in flux indicated the membrane was fouling. When Method 1 was used, both roughness values and filtration experiments supported temperature activation. Wet Method 1 membranes at cold temperatures displayed an average roughness of 8.40 nm while at hot temperatures, the roughness decreased to 0.92 nm, supporting the hypothesis of the film being expanded at cold temperatures and collapsed at hot temperatures. Filtration experiments using the Method 1 membrane showed that flux measurements remained nearly constant at all temperature test conditions; however, initial flux values were significantly lower than the unmodified membrane due to the potential film formation within pores. On the other hand, the Method 2 showed higher initial flux values (not significantly different from the unmodified membrane) since the film was formed in solution then attached to the membrane, which was believed to be farther away from the membrane surface. Membrane roughness values for Method 2 were on average 6.80 nm and 5.02 nm for cold and hot temperatures. As with Method 1, Method 2 membranes displayed nearly no flux decline irrespective of temperature condition when a humic solution was filtered through. Even though temperature responses were more pronounced when Method 1 membranes were used as compared to the Method 2 membranes, the loss in initial fluxes due to pore blockage with Method 1 made Method 2 more advantageous. A Method 2 modified membrane would produce a self-cleaning membrane. The use of harsh chemicals during normal membrane cleaning would be avoided with the Method 2 modified membrane as a constant flux operation could be achieved using temperature oscillations only.

Download or read book Grafting of Stimuli-responsive Polymer Films to Ultrafiltration Membranes written by Colleen Michelle Gorey and published by . This book was released on 2008 with total page 254 pages. Available in PDF, EPUB and Kindle. Book excerpt: This project focused on modifying membrane surfaces to increase fouling control while improving permeability and selectivity, resulting in membranes with longer operational lives, lower cleaning frequencies, and higher efficiencies. The modifications involved the attachment of a temperature sensitive polymer to keep the membrane from fouling. The membrane modifications, which produced a self-cleaning cellulose acetate membrane, involved grafting the surface with a thermally responsive film layer of hydroxypropyl cellulose (HPC). HPC possesses a lower critical solution temperature (LCST) of approximately 43C. When attached to a surface, HPC forms a film, which collapses at temperatures above the LCST and expands when cooled to below the LCST. By keeping the film in a non-equilibrium state, via oscillating the temperature of the membrane surface, (as was observed by flux decline), fouling was reduced. Two surface modification techniques were tested and compared to the unmodified membrane - Method 1 and Method 2. Method 1 involved the attachment of a gel layer whereas Method 2 was the attachment of polymer structures on the surface of the membrane. Roughness measurements, using a wet atomic force microscopy (AFM) cell, and filtration experiments (to monitor flux declines) were performed at cold temperatures (25C), at hot temperatures (60C) and with temperature oscillations. The unmodified membrane had roughness values that were higher when hot and lower when cold, and it displayed flux declines under all temperature conditions when a humic solution was filtered. Since the humic solution was acting as a fouling agent, the decrease in flux indicated the membrane was fouling. When Method 1 was used, both roughness values and filtration experiments supported temperature activation. Wet Method 1 membranes at cold temperatures displayed an average roughness of 8.40 nm while at hot temperatures, the roughness decreased to 0.92 nm, supporting the hypothesis of the film being expanded at cold temperatures and collapsed at hot temperatures. Filtration experiments using the Method 1 membrane showed that flux measurements remained nearly constant at all temperature test conditions; however, initial flux values were significantly lower than the unmodified membrane due to the potential film formation within pores. On the other hand, the Method 2 showed higher initial flux values (not significantly different from the unmodified membrane) since the film was formed in solution then attached to the membrane, which was believed to be farther away from the membrane surface. Membrane roughness values for Method 2 were on average 6.80 nm and 5.02 nm for cold and hot temperatures. As with Method 1, Method 2 membranes displayed nearly no flux decline irrespective of temperature condition when a humic solution was filtered through. Even though temperature responses were more pronounced when Method 1 membranes were used as compared to the Method 2 membranes, the loss in initial fluxes due to pore blockage with Method 1 made Method 2 more advantageous. A Method 2 modified membrane would produce a self-cleaning membrane. The use of harsh chemicals during normal membrane cleaning would be avoided with the Method 2 modified membrane as a constant flux operation could be achieved using temperature oscillations only.