The Ultrafiltration of Salt-Polyelectrolyte Solutions

Citation

Thompson, Ansel Frederick, Jr. (1968) The Ultrafiltration of Salt-Polyelectrolyte Solutions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/NAJK-3T94. https://resolver.caltech.edu/CaltechETD:etd-10042002-160148

Abstract

The ultrafiltration of sodium chloride - sodium polyacrylate solutions was studied using a membrane completely impermeable to the polyelectrolyte but permeable to water and simple salts. The membrane was a synthetic, hydrated polymer membrane with an ionic cross-linked structure. The activity of sodium chloride in the filtration cell was measured using a sodium-sensitive glass electrode/silver-silver chloride electrode pair. The continuous flow filtration cell used contained a stirring blade that was used to eliminate concentration polarization at the membrane surface. The effect of stirring speed was observed. The membrane salt-rejection and permeability characteristics were determined using distilled water and NaCl solutions. Independent salt activity measurements in NaCl-NaPA solutions were also made.

The sodium chloride activity data were compared with the simple additivity rule or dissociation theory for the activity of simple salts in polyelectrolyte solutions and with theory based upon the cylindrical electrostatic model. Agreement with the electrostatic model was good; expressions based upon the simple dissociation theory underestimated the activity.

Concentration polarization effects during filtration were eliminated only in the case of the lower polyelectrolyte concentrations studied and at relatively low filtration rates using high stirring speeds. Under these conditions, the filtrate flow rate and salt concentration were accurately predicted by flux equations typical of the thermodynamics of irreversible processes. At the solution-membrane interface, where the polyelectrolyte separation takes place, equilibration of the salt leads to an effective concentration equal to the equilibrium salt concentration; thus, the filtrate salt concentration is significantly greater than the salt concentration in the salt-polyelectrolyte solution being filtered. Results of experiments involving concentration polarization were compared with a simple boundary-layer model with only fair success.

The motivation for the research was the possibility of developing a desalination process upon the Donnan membrane equilibrium. The process is described and its performance is evaluated for various operating conditions. Economic considerations indicate only limited use for the process.

Thesis Files