MIAM-CO2

Aqueous foams are a priori suitable systems for separating and capturing gas mixtures, in particular CO2 from air. They feature a large gas-liquid contact surface and high hydraulic permeability, enabling continuous supply of fresh liquid, as well as gas circulation via the global displacement of bubbles. In addition, solubility contrasts between different gas molecules (N2 and CO2, for example) ensure selectivity between chemical species. However, foam-based gas separation processes are poorly implemented. The origin of this paradox is probably geometrical: the hydraulic connection between the soap films, which represent a large part of the specific surface area of foams, and the liquid channels (at the intersection of three bubbles), which often contain most of the liquid, is ensured by a zone of very low thickness. This dimple considerably slows down hydraulic exchanges, preventing the liquid in the films from being rapidly replenished. In addition, these thin zones probably allow gas to escape rapidly from the bubbles, limiting their residence time in the foam and the resulting mass transfer. The aim of MIAM CO2 is to propose an experimental, theoretical and numerical study to i) identify maximum gas-to-liquid transfer regimes and ii) intensify this transfer by creating foams laden with microalgae, which can fix and convert CO2 into biomass in situ. As a first step, we propose to study how CO2 is distributed between the liquid and gas phases of an aqueous foam in a column geometry where the aqueous foam is formed by continuous bubbling of CO2 and the liquid is supplied by forced gravity drainage in the absence of microalgae. Instrumentation of the column with dedicated CO2 sensors will enable measurement of the transfer dynamics as well as the gas/liquid partition coefficient for different values of the control parameters (gas and liquid flow rate, bubble size, liquid volume fraction, CO2 content of the gas). In parallel, a theoretical and numerical framework will be implemented by developing both continuous equations (in the spirit of the drainage equations) and numerical modeling by writing the physical ingredients of the exchanges on a minimal foam in a COMSOL solver for comparison with experiments. Secondly, we will incorporate microalgae into the foam to study whether the transfer dynamics and partition coefficient are modified by them. Before carrying out foam-scale experiments, we will take care to find compatible surfactant/algae pairs and study how algae partition spatially at both single-film and foam scales. We hypothesize that the presence of microalgae in the films will result in thicker soap films, which will exacerbate transfer, since hydraulic exchanges between films and liquid channels will be facilitated, and excessively rapid escape of the gas of interest slowed by greater film thickness. Next, we will measure the amount of CO2 absorbed by a foam laden with microalgae and correlate it with the specific surface of the foam, particle distribution, liquid velocity and the initial CO2 concentration of the mixture. The experimental results obtained will be implemented in the theoretical framework developed previously, which will take account of the presence of microalgae, modeled as a CO2-absorbing sink in the continuous model or as objects transported passively or actively by the liquid flow and capable of absorbing CO2 in COMSOL simulations.