Permeation of supercritical CO₂ through dense polymeric membranes

Supercritical carbon dioxide (scCO₂) is used in the food industry as a water-extracting drying agent. Once saturated with water, the scCO₂ needs to be regenerated. A promising way of drying scCO₂ is by using H₂O permeable membranes. Ideally, these membranes demonstrate low CO₂ permeability. Here, we investigated the CO₂ 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 CO₂ permeability (=losses) were explored. Even though the absolute CO₂ permeability depends on the intrinsic membrane material properties, its trend with increasing feed pressure is defined by the (supercritical) behavior of CO₂, 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 CO₂ 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 CO₂ 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.