Pervaporation of hydrazine hydrate: separation characteristics of membranes with hydrophilic to hydrophobic behaviour

Anhydrous hydrazine, used as rocket propellant, is an important inorganic compound and is obtained by dehydrating hydrazine hydrate. However, hydrazine forms an azeotrope with water which makes conventional separation processes energy intensive. Pervaporation may act as an alternative process; however, because of the highly alkaline nature of hydrazine, proper selection of polymer plays a vital role. Several polymeric membranes (PERVAP® 2200, 2201 and 2202, ethyl cellulose (EC), acrylonitrile butadiene styrene (ABS), polystyrene, and modified EC) were chosen to study separation characteristics. Reacting EC with phenyl isocyanate and varying its amount, modifications were carried out in order to alter the hydrophilic characteristics of the membrane to hydrophobic. 1H NMR spectra estimated degree of substitution in terms of carbamate groups. Contact angle measurements were taken to observe hydrophilic/hydrophobic characteristics. The higher contact angles of water with modified EC membranes, compared to unmodified form, indicate increase of hydrophobicity of the membranes. Further, FT-IR, XRD and positron annihilation techniques were employed to observe characteristics of one such modified ethyl cellulose membrane. Sorption studies were also carried out and sorption of both water and hydrazine hydrate in modified EC membrane were observed to be lower compared to unmodified. Pervaporation studies with all chosen polymers revealed selective diffusion of water playing major role compared to sorption. Further, apolar materials exhibited higher separation factors than polar materials. Encouraging results, in terms of higher PSI, were obtained with modified EC (262.4) and ABS (395.9) membranes.