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We are here concerned with Bénard instabilities in a horizontal layer of a binary liquid, considering as a working example the case of an aqueous solution of ethanol with a mass fraction of 0.1. Both the solvent and the solute evaporate into air (the latter being insoluble in the liquid). The system is externally constrained by imposing fixed 'ambient' pressure, humidity, and temperature values at a certain effective transfer distance above the liquid-gas interface, while the ambient temperature is also imposed at the impermeable rigid bottom of the liquid layer. Fully transient and horizontally homogeneous solutions for the reference state, resulting from an instantaneous exposure of the liquid layer to ambient air, are first calculated. Then, the linear stability of these solutions is studied using the frozen-time approach, leading to critical (monotonic marginal stability) curves in the parameter plane spanned by the liquid layer thickness and the elapsed time after initial contact. This is achieved for different ratios of the liquid and gas thicknesses, and in particular yields critical times after which instability sets in (for given thicknesses of both phases). Conversely, the analysis also predicts a critical thickness of the liquid layer below which no instability ever occurs. The nature of such critical thickness is explained in detail in terms of mass fraction profiles in both phases, as it indeed appears that the most important mechanism for instability onset is the solutal Marangoni one. Importantly, as compared to the result obtained previously under the quasi-steady assumption in the gas phase [H. Machrafi, A. Rednikov, P. Colinet, and P. C. Dauby, Eur. Phys. J. Spec. Top. 192, 71 (2011)], it is shown that relaxing this assumption may yield essentially lower values of the critical liquid thickness, especially for large gas-to-liquid thickness ratios. A good-working analytical model is developed for the description of such delicate transient effects in the gas. The analysis reveals that the system considered in this paper is generally highly unstable, the instability setting in even for very small times and liquid thicknesses. [ABSTRACT FROM AUTHOR]

This work deals with the experimental analysis and the mathematical modelling of the CO 2 absorption in an initially quiescent aqueous solution of NaHCO 3 and Na 2 CO 3 inside a Hele-Shaw cell. This absorption, initially driven by the coupling between diffusion and chemical reactions, eventually leads to the apparition of a peculiar gravitational instability with non-monotonic dynamics, and liquid plumes generated at some distance from the gas-liquid interface (Wylock et al., 2014). Considering that this instability is triggered by a Rayleigh–Taylor like mechanism, a two-dimensional model, coupling diffusion, chemical reaction and convection, is proposed to simulate the onset and the dynamics of such an instability. It is observed that the simulated instability dynamics agree qualitatively with the experimental observations and that the order of magnitude of the onset time is well estimated. Thanks to the simulation, the interaction between the various phenomena after the instability onset is further investigated and a mechanism is proposed to explain the unusual dynamics of the studied system. It is notably shown that this dynamics is due to the particular non-monotonic liquid density variations with the depth, induced by the absorption. A criterion to obtain such type of density profile is presented. In addition, the simulation enables to assess the influence of the instability on the CO 2 absorption rate and it is observed that the generated flow pattern does not lead to a significant enhancement of the gas–liquid absorption rate. This result is of significant importance for optimizing chemisorption (e.g. for CO 2 capture or sequestration) processes. [ABSTRACT FROM AUTHOR]

Abstract: A linear stability analysis is performed for a horizontal layer of a binary liquid of which solely the solute evaporates into an inert gas, the latter being assumed to be insoluble in the liquid. In particular, a water-ethanol system in contact with air is considered, with the evaporation of water being neglected (which can be justified for a certain humidity of the air). External constraints on the system are introduced by imposing fixed “ambient” mass fraction and temperature values at a certain effective distance above the free liquid–gas interface. The temperature is the same as at the bottom of the liquid layer, where, besides, a fixed mass fraction of the solute is presumed to be maintained. Proceeding from a (quasi-)stationary reference solution, neutral (monotonic) stability curves are calculated in terms of solutal/thermal Marangoni/Rayleigh numbers as functions of the wavenumber for different values of the ratio of the gas and liquid layer thicknesses. The results are also presented in terms of the critical values of the liquid layer thickness as a function of the thickness of the gas layer. The solutal and thermal Rayleigh and Marangoni effects are compared to one another. For a water–ethanol mixture of 10wt.% ethanol, it appears that the solutal Marangoni effect is by far the most important instability mechanism. Furthermore, its global action can be described within a Pearson-like model, with an appropriately defined Biot number depending on the wavenumber. On the other hand, it is also shown that, if taken into account, water evaporation has only minor quantitative consequences upon the results for this predominant, solutal Marangoni mechanism. [Copyright &y& Elsevier]

Densityvariations induced by gas absorption in reactive aqueoussolutions often trigger buoyancy-induced motions, generally in theform of plumes monotonically sinking into the bulk liquid and enhancingthe absorption rate. Here, we contrast two types of CO2-absorbing alkaline solutions, studying their dynamics inside a verticalHele-Shaw cell by interferometry. While the first one indeed behavesas expected, the second one leads to a quite unusual oscillatory (phase-slipping)dynamics of convective plumes, which moreover does not lead to a significanttransfer enhancement. Thanks to a simplified model of momentum andspecies transport, we show that this particular dynamics is relatedto a nonmonotonic density stratification, resulting in a stagnantlayer close to the interface. Conditions for this to occur are highlightedin terms of the ratios of species’ diffusivities and theircontribution to density, a classification deemed to be useful foroptimizing chemisorption (e.g., for CO2capture or sequestration)processes. [ABSTRACT FROM AUTHOR]

Abstract: This work deals with the experimental characterization of carbon dioxide ( ) absorption in an aqueous solution of N-(2-hydroxyethyl)piperazine, by using of digital holographic interferometry. This method enables visualizing the phenomena taking place in the vicinity of the gas–liquid interface. Thanks to a calibration, the obtained results yield the time evolution of the loading field in the aqueous solution and the gas–liquid absorption rate for various operating conditions. The processing method and the obtained results are detailed. A phenomenological model, highlighting the limiting steps of the gas–liquid absorption, is proposed. A set of experiments is realized and the parameters of this model are estimated for various initial amine concentrations and initial loadings. The results are discussed and correlations are proposed for the model parameters. [Copyright &y& Elsevier]

Abstract: This work deals with the development of a new experimental tool and an original procedure to study the gas–liquid absorption process coupled with chemical reactions in the liquid phase. This absorption is realized inside a Hele-Shaw cell and digital holographic interferometry is used to visualize the formation and the development of the diffusion layer during the mass transport in the liquid phase in the vicinity of the gas–liquid interface. An image processing code is developed to extract quantitative information from the raw experimental results. The experimental results are compared to a mass transfer model in the liquid phase of the Hele-Shaw cell and some physico-chemical parameters of this model are estimated from this comparison using on a non-linear least-square fitting method. The developed procedure is used to study the gaseous CO2 absorption in NaHCO3–Na2CO3 aqueous solution. Calibration curves are determined and experiments are realized for several couples of NaHCO3–Na2CO3 initial concentrations. The estimated values of the physico-chemical parameters for each experiment are presented and they are compared to the values calculated from correlations found in the literature. A reasonable agreement is observed, which tends to show that the methodology is promising and could be applicable to other systems. [Copyright &y& Elsevier]