Mass transfer studies on the dehydration of supercritical carbon dioxide using dense polymeric membranes

Continuous drying processes using supercritical CO₂ (scCO₂) as a water extraction agent require 24/7 operational dehydration units for scCO₂ regeneration. Dehydration units using dense polymeric membranes are considered a cost effective, sustainable alternative to the current zeolite-based units. The focus of previous studies on the membrane-based dehydration of scCO₂ was always on the membrane itself whereas boundary layer effects, e.g., concentration polarization, were not taken into account. To quantify the boundary layer effects, simulations were performed using three different membrane materials: SPEEK, Nafion^® 117, and PEBAX^® 1074. Process conditions during the simulations ranged from 8.0 to 18.0 MPa and 40 to 100 °C. Even though the three types of membranes examined differ in their H₂O permeability and H₂O over CO₂ selectivity, in all cases 80% of the total mass-transfer resistance can be assigned to concentration polarization effects, making it the dominant parameter for water transport. Despite high but differing intrinsic water permeabilities of all three membranes materials, the H₂O transport, thus H₂O flux through the membrane is significantly reduced by concentration polarization down to similar levels. This makes it necessary to use larger membrane areas, that result in higher CO₂ fluxes. As a consequence, material selection is predominantly based on the ability to reject CO₂. Optimization of process conditions other than membrane material is briefly discussed.