Permeation of supercritical CO2 through dense polymeric membranes.

Graphical abstract Highlights • Pressure where CO 2 permeability reaches maximum is temperature dependent. • Feed density and not intrinsic membrane property dictate CO 2 permeability profile. • Trend of CO 2 fluid density and concentration in PDMS are alike, at high pressures. • Phase change from gaseous- to liquid-like CO 2 leads to altered permeation behavior. • Transition to liquid-like state leads to lowered CO 2 diffusion within the polymer. Abstract Supercritical carbon dioxide (scCO 2) is used in the food industry as a water-extracting drying agent. Once saturated with water, the scCO 2 needs to be regenerated. A promising way of drying scCO 2 is by using H 2 O permeable membranes. Ideally, these membranes demonstrate low CO 2 permeability. Here, we investigated the CO 2 permeability of three types of dense membranes, Nafion, Natural Rubber and PDMS, of which the latter in more detail because of its ease of handling. The experimental conditions, temperature and pressure, resulting in minimum CO 2 permeability (=losses) were explored. Even though the absolute CO 2 permeability depends on the intrinsic membrane material properties, its trend with increasing feed pressure is defined by the (supercritical) behavior of CO 2 , notably its density as a function of temperature and pressure. The data points to transitions within the supercritical regime, from the gaseous-like supercritical state to the liquid-like supercritical state, graphically visualized by the Widom line for CO 2 density. Sorption measurements with PDMS membranes confirm this behavior that follows the diffusion-solution theory. In the gaseous state, the (normalized) permeability follows the (normalized) solubility, indicating a constant CO 2 diffusivity. With increasing pressure and when entering the liquid-like (supercritical) regime, the diffusivity drops, resulting in a (normalized) permeability that starts to lag behind the (normalized) solubility. [ABSTRACT FROM AUTHOR]