Selective Transport of Sulfur Dioxide through Polymer Membranes

The purpose of this research was to identify and characterize polymeric materials which are selective to SO2 for possible applications in the monitoring or control of SO2 emissions from stationary sources. Preliminary studies indicated that polyacrylate and cellulose triacetate were very selective to SO2. The SO2 solubility of polyacrylate was 0.215 lb/lb (25°C, 1 atm), comparable to the SO2 solubilities of sulfolane (tetrahydrothiophene-1,1-dioxide) and polyethylene glycol, two excellent SO2 solvents. To achieve the high SO2 transport rates required for a membrane separation process, two techniques were investigated for preparing ultrathin films of these polymers. Polyacrylate films were cast on mercury, and cellulose triacetate films were formed on glass plates which were withdrawn at constant velocity from dilute polymer solutions. The glass plate technique was also used to make composite films consisting of a 0.2-0.5 μm layer of polyacrylate over a 0.002-0.005 μm layer of cellulose triacetate. All the films were placed on microporous filter materials for support. The performance of the membranes was initially determined by measuring their permeability to pure N2, CO2, and SO2. The N2 and CO2 permeabilities were nearly independent of pressure, but the SO2 permeability increased rapidly with increasing pressure. The SO2 permeation data were correlated with two- and three-parameter exponential models. The three-parameter model had the same functional form as the free volume theory and gave a better fit to the data than the two-parameter model. The SO2 permeability was affected by previous conditioning of the membranes and time-dependent processes occurring in the polymer. There was evidence that some SO2 molecules became immobilized in clusters within the membranes. The composite membranes were also tested with binary mixtures of N2 and SO2 at 10 atm upstream pressure with a steady-state flow system. A typical membrane produced an enriched permeant stream of 2% SO2 from 0.1% SO2 upstream, 35% from 2%, and over 90% from 10%. N2 permeation was enhanced by the presence of SO2, but the presence of N2 inhibited SO2 permeation at high SO2 partial pressures. Interactions between N2 and SO2 made it difficult to accurately predict the permeation rates of the mixtures from data for the pure gases. The composite membranes were superior in performance to the single-layer membranes and those developed by others for SO2 separations; however, they were unable to meet the stringent design goals for combustion and smelter gas cleaning. The unique properties of the composite membranes do make them attractive for instrumentation and other specialty applications.