The effect of supercritical CO₂ on the permeation of dissolved water through PDMS membranes

Water vapor permeation under supercritical carbon dioxide (scCO₂) conditions through dense polydimethylsiloxane (PDMS) was investigated up to pressure of 185 bars to evaluate the regenerability of scCO₂ as desiccant to dehydrate fresh products that are prone to product deterioration during conventional drying. This study experimentally examined the impact of concentration polarization on the H₂O vapor permeation through dense PDMS membranes in the presence of sub- and supercritical CO₂. The results were compared to a system containing N₂ instead of CO₂. For the CO₂ system, the residual mass transfer resistance, which excludes the membrane layer resistance, decreased down to zero with increasing feed pressure, at 90 bar. This is the result of the convergence of the H₂O contents of the feed bulk and permeate, which leads to a change of the main H₂O transport mechanism within the feed boundary layer from diffusion to convection. Here the H₂O and CO₂ molecules are transported with comparable speed towards the membrane surface. For the system with N₂, the opposite trend was found, due to the maintained significant difference in transport speed between H₂O and N₂ even at elevated pressures. Consequently, the water vapor transport rate through the PDMS membrane is governed by the type of matrix fluid (CO₂ or N₂).