Selection of microporous hydrophobic membranes for use in gas/liquid contactors: An experimental approach

Contactors equipped with hydrophobic porous membranes were widely tested and operated in different separation processes. New applications, e.g. concentration of fruit juice or air conditioning, require a reliable separation of the two contacting phases at a higher level as for common applications in order to avoid product pollution by the liquid absorbent. The application of coated membranes as a liquid tight semipermeable barrier offers the best prerequisites in order to meet this requirement. Because the dense layer of such membranes acts as an additive transport resistance its thickness has to be as thin as possible which requires porous support membranes with the porosity profile of ultrafiltration membranes. Are such support membranes suitable for the application in gas/liquid contactors? In order to find an answer to this question at first a model gas/liquid contactor arrangement was installed and experimentally proved. Applying this arrangement the water vapor permeability of membrane types with ultrafiltration separation characteristics was determined. At the model development a well characterized hollow fiber with microfiltration properties was used. The results show that neglecting the temperature polarization would result in underestimated membrane permeability values. Hence the temperature polarization arising during the absorption process was determined experimentally. By taking the temperature polarization into consideration the calculated membrane permeabilities were in good accordance to literature data. Using this simple model different structured poly(ether imide) hollow fiber membranes with ultrafiltration separation profile were characterized with respect to the water vapor permeability. The results document that these ultrafiltration membranes provide equivalent or even distinctly higher permeabilities than commercial microfiltration membranes. The better performance is related to the asymmetric structure of these membranes. On basis of the obtained results it can be concluded that membranes with ultrafiltration separation profile - which are coatable with a thin coating layer - should be suitable for the preparation of effective contactor membranes without risk of remarkable losses in performance in comparison with uncoated microfiltration membranes. Furthermore the morphology of these membrane types can be tailored to special contactor applications, which require high heat transfer (e.g. osmotic distillation) or low heat transfer (e.g. membrane distillation).