A comprehensive formalism for air-gap membrane distillation applied to the design of full-scale modules with direct solar heating

Air-gap membrane distillation (AGMD) is used to extract volatile compounds from a heated feed solution, through a porous hydrophobic membrane, into a cooled compartment, then recovered by condensation. AGMD is a promising technology for desalination and aqueous concentration, but its scale-up is limited by incomplete physical descriptions of the module physics. This work proposes a new CFD-based multiphysics framework to design AGMD full-scale plate-and-frame modules for freshwater extraction. The three physics features comprised within an AGMD module are first formalized: (i) the flow of a solution in contact with a porous membrane; (ii) gas mixture (vapor) transport through a porous membrane; (iii) vapor condensation on a (vertical) surface. They are thus combined into a consistent formalism of the AGMD module physics, with a particular focus on gas transport built upon the Maxwell-Stefan theory, which is here improved to account for medium vapor saturation. Model predictions are validated experimentally against lab-scale AGMD data for feed temperature up to 60 ◦C. The model is then employed to assess full-scale flat-sheet modules, connected in series, and enhanced with direct solar heating. Simulations reveal that system productivity is highly sensitive to configuration (single vs. multi- module; bulk solar vs. direct solar heating), with optimal productivity achieved with considerably different module compartment design and process parameters. When enhanced with direct solar heating, system optimal productivity can increase by up to 230 % compared to standard configurations. This formalism provides a robust basis for AGMD modules design and prior to their effective integration into real-world desalination system.