Membrane contactors for process intensification of gas absorption into physical solvents: Impact of dean vortices

Membrane contactors are considered as one of the most promising intensification technologies for gas-liquid absorption processes. For chemical absorption systems, such as flue gas CO2 absorption in amine solutions, the mass transfer resistance is significantly located in the membrane due to the accelerating effect exerted by the reaction in the liquid. For physical absorption systems however, such as CO2 absorption in water for biogas purification, most of the resistance is located within the liquid phase. Process intensification requires an increase of the gas-liquid interfacial area (provided by the membrane) together with increased mass transfer performances in the rate limiting phase, namely the liquid. In the liquid phase, a boundary layer is formed, in which the solute concentration is higher than at the bulk, thus decreasing the driving force for the transfer. This study investigates the use of Dean vortices as a tool to increase mass transfer performance in membrane contactors. These vortices, generated by a helical hollow fiber, are applied to CO2 absorption in water using dense polymeric membranes. A straight hollow fiber with water flowing in the lumen is compared to different helical shaped hollow fibers. Firstly, Dean vortices, calculated by a CFD approach, are successfully compared to the velocity maps experimentally obtained by MRI, a non-invasive spectroscopy. Hydrodynamic simulations are thus validated, offering parametric studies and design optimization perspectives. The experimental results for the mass transfer performances of a hollow fiber under different liquid velocity conditions are further compared to simulations. A parametric study, combining CFD and mass transfer simulations, shows that Dean vortices enable an increase of the mass transfer coefficient by a factor of 3, with a moderate increment in pressure drop or energy expenditure, when compared to the straight fiber. Lastly, the best helical geometry, with maximal CO2 mass transfer at the expense of minimal energy requirement is investigated. For a given application, it is shown that a large process intensification effect is attainable; with the same energy requirement, a 2-fold decrease in the surface area is achieved. The potential of the concept for practical hollow fiber modules design and applications are finally discussed.