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• Water vapor membrane permeation under supercritical carbon dioxide conditions. • Pressures up to 185 bar for mixed H 2 O/CO 2 and H 2 O/N 2 feeds in PDMS membranes. • Water vapor transport through the membrane determined by matrix fluid (CO 2 or N 2). • H 2 O and CO 2 transported at comparable rate towards the membrane surface. • For the H 2 O/CO 2 mixture, mass transfer resistance decreases to zero at high pressure. Water vapor permeation under supercritical carbon dioxide (scCO 2) conditions through dense polydimethylsiloxane (PDMS) was investigated up to pressure of 185 bars to evaluate the regenerability of scCO 2 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 H 2 O vapor permeation through dense PDMS membranes in the presence of sub- and supercritical CO 2. The results were compared to a system containing N 2 instead of CO 2. For the CO 2 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 H 2 O contents of the feed bulk and permeate, which leads to a change of the main H 2 O transport mechanism within the feed boundary layer from diffusion to convection. Here the H 2 O and CO 2 molecules are transported with comparable speed towards the membrane surface. For the system with N 2 , the opposite trend was found, due to the maintained significant difference in transport speed between H 2 O and N 2 even at elevated pressures. Consequently, the water vapor transport rate through the PDMS membrane is governed by the type of matrix fluid (CO 2 or N 2). [ABSTRACT FROM AUTHOR]

• There is an ideal skin-layer thickness at which the total drying costs are minimal. • Concentration polarization in feed leads to max. 150 times larger membrane areas. • Rising feed temperature is the best method to reduce the required membrane area. • Using a membrane-based system instead of absorption-based system cuts costs by 50%. Supercritical CO 2 (scCO 2), used in the food industry as a water extraction agent, requires dehydration units for regeneration. The present study assesses the economics of membrane-based dehydration of scCO 2. In contrast to earlier studies, the contribution, next to the membrane, also the contributions of the mass transfer resistances of the feed and permeate boundary are included, which have a dominant effect on the final process design and economics. In addition, our work also extrapolates the process to industrial scale evaluating different configurations and process conditions. Specifically, the contribution of the membrane and membrane unit costs is discussed in more detail. Including the mass transfer resistances of feed and permeate boundary layer reduces the water flux across the membrane up to a factor 150, implying a larger required membrane surface area for a given water removal rate, and thus higher costs. Using a SPEEK-based membrane, the total drying costs, normalized for the amount of water removed, minimize around a skin layer thickness of 1 μm, i.e., not too thin to permeate and thus spill too much CO 2 and not too thick to hamper the H 2 O flux. Because the feed boundary layer dominates water transport, conditions that minimize its thickness reduce total costs. A reduction of the feed boundary layer dominance can be achieved by adjusting channel height, cross-flow velocity and the density and viscosity of scCO 2 , the latter two by increasing the operational temperature from 45 to 65 °C (at 130 bar). Compared to the benchmark zeolite process currently available, the membrane-based process for drying scCO 2 outlined and optimized in the present study results in a 50% saving of total drying costs. These savings can be achieved by using a dense polymeric membrane with a H 2 O permeability of at least 10,000 Barrer and a CO 2 permeability of at most 10 Barrer. [ABSTRACT FROM AUTHOR]

Highlights • Feed boundary layer is mass transfer limiting for water transport. • Extend of concentration polarization correlates with fluid density at feed side. • H 2 O over CO 2 flux ratio changes with variation of skin-layer thickness. • Skin-layer material with lowest CO 2 permeability determines material of choice. Abstract Continuous drying processes using supercritical CO 2 (scCO 2) as a water extraction agent require 24/7 operational dehydration units for scCO 2 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 scCO 2 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 H 2 O permeability and H 2 O over CO 2 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 H 2 O transport, thus H 2 O 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 CO 2 fluxes. As a consequence, material selection is predominantly based on the ability to reject CO 2. Optimization of process conditions other than membrane material is briefly discussed. [ABSTRACT FROM AUTHOR]