Separation of volatile organic compounds from nitrogen using polyurethane membranes
Description
Membranes prepared from commercially available polyurethanes were tested for their usefulness in separating organic vapors from nitrogen. Experiments were also conducted to understand the effect of polymer morphology on the swelling and diffusion of organics species in polyurethanes. Homogeneous polyurethane membranes, made from ether or ester soft segment spandex polymers, were found to be highly effective for the vapor-phase separation of organics from dry and humidified nitrogen. Organic permeabilities in the ether soft segment polyurethane membranes were greater than those measured in the polyester films, due to higher organic solubility coefficients. The greater swelling of the polyether membranes increased the nitrogen permeabilities and lowered the organic/N$\sb2$ selectivities, relative to those for polyester membranes. Also, the presence of water vapor in the organic vapor feed (up to 1.2 vol %) had no effect on transmembrane fluxes Equilibrium organic vapor solubilities and diffusion coefficients in four commercially available polyurethanes (containing the same polyether soft segments) were measured as a function of organic activity and temperature (30$\sp\circ$C-60$\sp\circ$C) using a McBain vapor uptake apparatus. For three organic penetrants (benzene, tetrachloroethylene and hexane), the Flory-Rhener sorption equation was found to fit the organic swelling data well, when two parameters in the equation were adjusted properly. The interaction parameters, $\chi$, in the Flory-Rhener equation for the three organic vapors were found to increase linearly with Hydrogen Bonding Index (HBI). The M$\rm \sb{c}$ parameter was correlated with the empirical relationship $\rm d\sb2/(\Phi\sb{HS}$$\cdot$HBI)$\sp{1/3}$, where $\Phi\sb{\rm HS}$ is the volume fraction of hard domains, and d is the long spacing distance between hard domains The free volume theory, corrected for the presence of hard segments, was found to describe well the dependence of organic penetrant diffusion coefficient on concentration and temperature in ether soft segment polyurethane membranes. Correlation equations were developed in order to relate the free volume parameters to the polymer morphology, as quantified by HBI. The benzene and tetrachloroethylene Fickian diffusion coefficients that were predicted using the correlations of the free volume parameters in combination with the Flory-Rhener parameter correlations, were within 12% of the experimentally determined values