Pervaporation dehydration of isopropanol-water systems using chitosa membranes

Abstract

Sorption and pervaporation for the dehydration of isopropanol/water mixtures using chitosan membranes were investigated. The chitosan membranes showed preferential water sorption and permeation. It was clarified that both sorption selectivity and diffusion selectivity are important to the overall permeation selectivity. Preferential sorption affects and leads to preferential permeation.

Composite chitosan membranes consisting of a dense skin layer and a porous substrate were developed for dehydration of isopropanol. At 30*C, a permeation flux of 375 g/m^2.hr and a separation factor of 348 were achieved for the dehydration of 90 wt. % isopropanol aqueous solution; a significantly larger separation factor (807) with a lower permeation flux (265 g/m^2.hr) were obtained for the dehydration of 95 wt. % isopropanol solution. It was shown that the composite membranes' performance was affected not only by the dense skin layer but also the porous substrate.

Modifications of chitosan membranes via crosslinking and polymer blending for the dehydration of isopropanol were studied. It was shown that the chitosan membranes crosslinked with diisocyanate and the chitosan/poly(vinyl alcohol) blended membranes were preferentially permeable to water. The crosslinked chitosan membranes increased the separation factors up to 1238 while the blended chitosan membranes increased the separation factors up to 1238 while the blended chitosan membranes increased the separation factors up to 1806, for the dehydration of 90 wt.% isopropanol aqueous solution. Optimal conditions and overall performances for the crosslinking and the polymer blending were determined based on the use of the pervaporation separation index (PSI).

A comparison between pervaporation (pVAP) and vapor phase pervaporation (VPVAP) for the separation of isopropanol/water mixtures using chitosan membranes was also investigated. Preliminary results showed that significantly higher separation factor could be obtained via VPVAP, but due to the common permeability-selectivity trade-off, PVAP was more effective.

A relatively new technique to determine diffusion coefficients using thin-channel inverse gas chromatography was developed. This new technique was shown to be potentially useful for determining diffusion coefficients and especially applicable to pervaporation membranes. The technique provides a simple fast and efficient alternative to the more conventional sorption and desorption methods for measuring diffusion coefficients of permeants through thin membrane films.

A polar pathway model to describe the mass transport in hydrophilic chitosan pervaporation membranes was developed. This model was based on the solution-diffusion model and incorporated hydrophilic polar pathways to describe single component permeation of water and isopropanol in the chitosan membrane. Reasonable agreements between the calculated and the experimental values of single permeabilities were obtained. The free volume model approach based on the free volume theory and thermodynamics was employed to conduct the modelling of the binary component permeation of isopropanol-water mixtures through the chitosan membranes. Reasonable estimates of the pervaporation parameters were achieved.