Fundamental aspects of membrane characterization by constant volume and constant pressure techniques
Creators: name: Lashkari, Siamak
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2009
Subjects: subject: Engineering, Chemical.

description: Membrane characterization in laboratory-scale systems is an important step in the membrane development process. However, transport properties of polymeric gas separation membranes reported in the literature, such as permeability, diffusion, and solubility coefficients, are typically not reproducible. This variation is often explained on basis of differences in molecular weight, purity, crystallinity and membrane casting conditions. It is also acknowledged that different laboratories use different testing systems and different testing protocols. This thesis investigates fundamental aspects of the two basic membrane characterization methods---constant volume (CV) and constant pressure (CP) techniques. Considering CV technique, the effect of non-negligible resistance to gas accumulation was extended into the actual configurations of CV systems by studying the effect of the presence of a single and multiple resistance-free tanks downstream from the membrane. Using the concept of an asymptotic solution, the resistance to gas accumulation in the receiving volumes of gradually increasing complexity was characterized by means of a position-dependent time lag. The derived analytical solutions provide a convenient tool for assessing the resistance in existing CV systems and for the design of new CV systems. In addition, recognizing that in a slip flow regime the diffusion coefficient varies with pressure, the set of governing partial differential equations was solved numerically. Analytical and numerical solutions showed good agreement with the experimental results collected from two different configurations of the receiving volume. Moreover, using an optimization procedure in combination with the numerical method, the permeability and diffusion coefficients in membrane were estimated from the data obtained under high resistance to gas accumulation. With respect to CP technique, the phenomena of back diffusion and back permeation and their combined effect on the experimentally determined permeability coefficients were investigated. A mathematical model that allows estimation of an error arising from back diffusion and back permeation was derived from the first principles. The theoretical predictions were then compared with experimental results obtained in a specially designed, fully-automated CP system, in single gas permeation tests involving nitrogen and oxygen. The experimental results confirmed theoretically predicted trends resulting from the phenomena of back diffusion and back permeation. However, the influence of these phenomena on the experimentally determined permeability coefficients was greater than that predicted by the model. Using a CP system with sweep gas designed and built in our laboratory, a novel procedure for the evaluation of the diffusion coefficient of a single gas in membranes exposed to gas mixtures of known composition was developed. For O2/N2 and CH4/N2 systems the results indicate that the diffusion coefficients of nitrogen and oxygen in poly(phenylene oxide) membrane are enhanced in the presence of another gas, but the diffusion coefficient of methane decreases in the presence of nitrogen. This novel method is expected to become an important tool in fundamental studies on the mechanisms of gas transport in polymeric membranes.