A study of membrane processes for gas and liquid separation
by Wenjun Lu
M.Phil. Chemical Engineering
x, 149 leaves : ill. ; 30 cm
The present study consists of two parts: membrane processes for gas separation and liquid separation. For gas separation, the study was concentrated on the relationship between gas permeability and polymer structure. For liquid separation, the research focused on electroplating wastewater treatment by reverse osmosis....[ Read more ]
The present study consists of two parts: membrane processes for gas separation and liquid separation. For gas separation, the study was concentrated on the relationship between gas permeability and polymer structure. For liquid separation, the research focused on electroplating wastewater treatment by reverse osmosis.
I GAS PERMEATION The relationship between gas permeability and polymer structure was studied. Polymer materials containing polyarylether sulfone containing a cardo group (PES-C), polyarylether ketone containing a cardo group (PEK-C), poly(hydroxyether of bisphenol A) (Phenoxy) and Phenoxy/PES-C blends were compared with bisphenol A polysulfone (PSF) in terms of gas permeability coefficient, P, for oxygen, nitrogen, carbon dioxide and methane. The physical properties of these polymers such as the mean d-spacing and glass transition temperature were characterized and compared.
It was found that polymers, such as PES-C having stiff backbone and large side groups, have a higher glass transition temperature, a larger mean d-spacing, and hence, a higher gas permeability coefficient. Lower gas permeability was found for polymers with softer backbone and with hydrogen bonds between polymer chains. No simple relationship was obtained between the selectivity and the polymer structure.
An unusual phenomenon was observed for the polymer blends of Phenoxy/PES-C in which gas permeability and selectivity coefficients of the blends were found to be smaller than those of pure Phenoxy and pure PES-C with their respective minimal values reached at the composition of 40% (wt%) PES-C. The mean d-spacing values of the blends are also smaller than those of pure Phenoxy and pure PES-C. The glass transition temperatures of the Phenoxy/PES-C blends show the blends are miscible system.
II TREATMENT OF ELECTROPLATING WASTEWATER BY REVERSE OSMOSIS Wastewater containing heavy metals of cobalt, nickel, and chromium were treated by a pilot scale reverse osmosis. The effects of operating pressure, temperature, and feed flowrate on permeate flux, permeate concentration and observed rejection were studied. The study indicated reverse osmosis process could remove the heavy metals from electroplating wastewater effectively.
An optimal operating pressure was found for the treatment of cobalt and chromium containing wastewater in this study. For nickel containing wastewater, however, it is found that the nickel permeate concentration decreases with increasing operating pressure. The permeate flux always increases linearly with the increase of operating pressure. An increase in the operating temperature resulted in a slight increase of the observed rejection for cobalt and nickel. However, the observed rejection for chromium was found to decrease with the increase of temperature. The permeate flux increases with increasing temperature irrespective of the contents of the wastewater treated. A higher feed flowrate was found to give a higher heavy metal rejection due to the reduction of the concentration polarization. This advantage was found to be more pronounced at higher operating pressures for cobalt containing wastewater. The permeate flux was not affected by the change of feed flowrate as expected for the low concentration wastewater.
A mathematical model based on both solution-diffusion theory and the film diffusion theory was used to explain and to analyze the experimental data. Solute permeability constants and film diffusion coefficients of the heavy metal containing wastewater were determined. The optimum operating pressures for cobalt and chromium containing wastewater were predicted. In addition, the degree of concentration polarization can be obtained from this model in terms of the ratio of concentration at the membrane surface to the bulk concentration.