Density variations induced by gas absorption in reactive aqueous solutions often trigger buoyancy-induced motions, generally in the form of plumes monotonically sinking into the bulk liquid and enhancing the absorption rate. Here, we contrast two types of CO 2 -absorbing alkaline solutions, studying their dynamics inside a vertical Hele-Shaw cell by interferometry. While the first one indeed behaves as expected, the second one leads to a quite unusual oscillatory (phase-slipping) dynamics of convective plumes, which moreover does not lead to a significant transfer enhancement. Thanks to a simplified model of momentum and species transport, we show that this particular dynamics is related to a non-monotonic density stratification, resulting in a stagnant layer close to the interface. Conditions for this to occur are highlighted in terms of the ratios of species' diffusivities and their contribution to density, a classification deemed to be useful for optimizing chemisorption (e.g. for CO 2 capture or sequestration) processes.

We examine the motion of a liquid-air meniscus advancing into a microchannel with chemically heterogeneous walls. We consider the case where a constant flow rate is imposed, so that the mean velocity of the interface is kept constant, and study the effects of the disorder properties on the apparent contact angle for each microchannel surface. We focus here on a large diffusivity regime, where any possible advection effect is not taken into account. To this end, we make use of a phase field model that enables contact line motion by diffusive interfacial fluxes and takes into account the wetting properties of the walls. We show that in a regime of sufficiently low velocities, the contact angle suffers a hysteresis behavior which is enhanced by the disorder strength. We also show that the contact line dynamics at each surface of the microchannel may become largely coupled with each other when different wetting properties are applied at each wall, reflecting that the dynamics of the interface is dominated by nonlocal effects.

Biogas upgrading by water scrubbing followed by biomethane compression is an environmentally benign process. It may be achieved using various plant configurations characterised by various power requirements with associated effects on biomethane sustainability. Therefore, the current study has been undertaken to systematically investigate the power requirements of a range of water scrubbing options. Two groups of water scrubbing are analysed: (1) high pressure water scrubbing (HPWS) and (2) near-atmospheric pressure water scrubbing (NAPWS). A water scrubbing plant model is constructed, experimentally validated and simulated for seven upgrading plant configurations. Simulation results show that the power requirement of biogas upgrading in HPWS plants is mainly associated with biogas compression while in NAPWS plants a significant power is required for water *Manuscript Click here to view linked References pumping. Biomethane compression to 20 MPa also contributes remarkably. It isobserved that the lowest specific power requirement can be obtained for a NAPWS plant without water regeneration (0.24 kWh/Nm 3 raw biogas) but this plant requires cheap water supply, e.g. outlet water from a sewage treatment plant or river. The second is HPWS without flash (0.29 kWh/Nm 3 raw biogas). All other HPWS with flash and NAPWS with water regeneration plants have specific power requirements between 0.30 and 0.33 kWh/Nm 3 raw biogas. Biogas compression without upgrading requires about 0.29 kWh/Nm 3 raw biogas. The thermodynamic efficiency of biogas upgrading is between 2.2 and 9.8% depending on the plant configuration while biomethane compression efficiency is higher, about 55%. This result implies that the upgrading process has a remarkable potential for improvement whereas compression is very close to its thermodynamic limit. The potential for minimising energy dissipation in the state-of-the-art HPWS upgrading plant with flash by applying a rotary hydraulic pumping device is evaluated at about 0.036 kWh/Nm 3 raw biogas meaning the specific power requirement reduction of 10%.

A lot of chemical engineering processes are based on chemical reactions between a gas and a component in a liquid phase. The gas has to be dissolved in the liquid phase. It is commonly admitted that the global gas-liquid mass transfer rate is controlled by phenomena occurring in layers close to the gas-liquid interface (mainly diffusion, but also convection and reactions). Therefore, a good understanding of these phenomena is required to achieve an optimization of the processes. This work deals with the development of a new experimental tool and an original procedure to study phenomena occurring during gas-liquid absorption coupled with chemical reactions in the liquid phase. The digital holographic interferometry is used to visualize the formation and the development of the diffusion boundary layer of the transferred mass in the liquid phase close to the gas-liquid interface. The gas-liquid mass absorption is realized inside a Hele-Shaw cell and a Mach-Zehnder interferometer is used to visualize the phenomena occurring in the liquid phase during this transfer. A procedure is developed to estimate the physico-chemical parameters (solubility, diffusion coefficient, kinetic constant, …) of a mass transfer model, coupling diffusion and chemical reactions, in the liquid of the Hele-Shaw cell. On the one hand, an image processing program is developed to extract quantitative information from the raw experimental results. On the other hand, a parametric estimation method, which is based on a non-linear least-square fitting, is developed to estimate the physico-chemical parameters of this mass transfer model from the experimental results. The presented method is used to study the gaseous CO2 absorption in NaHCO3-Na2CO3 aqueous solution. A mass transfer model, coupling diffusion and chemical reactions, in the liquid of the Hele-Shaw cell, is presented and solved numerically by a finite element method using the COMSOL Multiphysics software. A set of CO2 absorption experiments, using several couples of NaHCO3-Na2CO3 initial concentrations, is realized. The physico-chemicals parameters of the mass transfer model are estimated for some experiments. The obtained values are compared to the value calculated from correlations found in the literature. Using the estimated parameter values, a good agreement between the experimental and the simulated evolutions of the diffusion boundary layer is obtained. These results tend to validate the proposed procedure.

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34Aimed: 35 The main aim of this study is to improve the mechanistic understanding of soil CO 2 efflux 36 (F s ), especially its temporal variation at short-time scales, by investigating, through modeling, 37 which underlying process among CO 2 production and its transport up to the atmosphere is 38 responsible for observed intra-day variation of F s and soil CO 2 concentration [CO 2 ]. 39Methods: 40In this study, a measurement campaign of F s and vertical soil [CO 2 ] profiles was conducted in 41 a Scots Pine Forest soil in Hartheim (Germany) and used to develop a model testing several 42 hypotheses. A reference model taking into account a purely diffusive CO 2 transport and a 43 temperature-dependent CO 2 production is compared to models with a more complex 44 description of either CO 2 production or CO 2 transport. For transport, the introduction of 45 advection and the dispersion is investigated. For the production, the emergent hypothesis of 46 the phloem pressure concentration wave (PPCW) influence is tested. 47Results: 48We show that intra-day variation of F s and [CO 2 ] is better represented when the more 49 complex CO 2 production expression is taken into account compared to the more detailed 50 description of CO 2 transport. 51Conclusion: 52We conclude that focus should be placed on the potential factors affecting the CO 2 53 production, rather than on the transport process description 54This is the preprint version of a manuscript that has been published in Plant and Soil 2015

During the production of refined NaHCO 3 by the Solvay process, CO 2 has to be absorbed in an aqueous solution of NaHCO 3 and Na 2 CO 3 . The absorbed CO 2 diffuses into the liquid phase and is consumed by chemical reactions, which can globally be written as follows : CO 2 +CO 3 = +H 2 O 2 HCO 3 -A full description of the reaction scheme and a mathematical modeling of this reaction-diffusion process was developed in a previous work (Wylock et al., 2008). Now, as the CO 2 absorption is the limiting step in the whole process, a better understanding of this interfacial transfer is sought in order to optimize the refined NaHCO 3 production by the Solvay process.To this end, CO 2 absorption in an initially quiescent aqueous solution of NaHCO 3 and Na 2 CO 3 is studied in a Hele-Shaw cell. Using a Mach-Zehnder interferometer (as shown in figure 1), refractive index variations in the liquid are followed during the absorption. Experiments show that chemo-hydrodynamical instabilities appear in the liquid phase below the interface. When the gas flow of CO 2 is initiated, a boundary layer with a different refractive index is formed in the liquid, which grows continuously over the 2 hours of testing. After some time, this boundary layer eventually becomes unstable and a highly dynamical variation of the refractive index is observed in the liquid. In figure 2, this onset of instability is shown.Since a linear dependency of the refractive index on the density of the solution was measured independently, one can translate, in a first approximation (neglecting the influence of temperature variations), the observed refractive index variations into liquid density variations.In this case, one can interpret the observed variations as follows : there is a less dense zone close to the interface and a zone which is denser than the bulk solution below this zone. This initial density stratification can be explained by the different diffusivities (Vas Bhat et al., 2000) of the involved ions in the diffusion reaction process and the different density contribution of the HCO 3 -and CO 3 = ions. Now, it is clear that this boundary layer structure could become unstable due to the presence of heavier liquid layer above the bulk, which could explain the observed instability. In order to examine this possibility, we rely upon the Navier-Stokes-Darcy model of a flow in a Hele-Shaw cell, and formally investigate what the linear growth rate of perturbations superimposed upon experimentally observed density stratifications would be in the absence of diffusion and chemical reactions. Such an approach cannot account for an instability threshold. Yet it gives an insight into the time and length scales associated with the Rayleigh-Taylor instability for the system under consideration. In fact, it turns out to yield the same order of magnitude of the instability wavelength as observed in the experiment. The computed growth rates yield faster time scales than those for a typical "cycle" observed in the experiment, which does not exclude that the Rayleigh...

A key step of the refined sodium bicarbonate (BIR ) production by the Solvay process consists in the gas-liquid mass transfer of carbonic gas CO 2 from a gaseous mixture of CO 2 and air to a sodium carbonate Na 2 CO 3 and bicarbonate NaHCO 3 brine. This transfer takes place in 20-m high and 2.5-m wide bubble columns (the BIR columns). This large size induces a large contact time between gas and liquid, in order to increase the amount of transferred CO 2 . Nevertheless, the gas phase leaving the columns contains an important quantity of CO 2 (only half of the CO 2 is transferred). It causes a huge CO 2 emission to the atmosphere (the equivalent of 150 Smart driving at 100 km/h, for a single column). More, the CO 2 is produced by lime calcination. This process requires a large amount of energy. It represents the major part of the energy consumption to produce BIR. In the past decades, several optimizations of the BIR process were performed by an empiric approach. There are today some limits to this approach for applications requiring high levels of purity and a well-defined granulometry. Moreover, the ecologic pressure becomes more and more constraining for the greenhouse effect gas emission. Accordingly, Solvay is seeking for a more fundamental approach. Currently, several studies are realized in the chemical engineering department at the ULB. The final goal is to create a complete model of a BIR bubble column, taking into account all the phenomena taking place in it. This model will be used in order to optimize the CO 2 transfer rate in the column. As a direct consequence of this optimization, the CO 2 emission and the energy consumption of the BIR production will hopefully be reduced.The objective of the work presented in this paper is to develop a model of the mean CO 2 flux density, expressed by unit of time and interfacial area, in a BIR bubble column, that will be integrated in the future complete model of a BIR column. No solid phase is considered in this paper.The CO 2 transfer rate from a bubble of gas to the brine is controlled by the physico-chemical phenomena occurring in the thin layer of liquid near the interface. After the CO 2 absorption in this liquid layer, the CO 2 is transferred by diffusion and is involved in several chemical reactions. These reactions modify (accelerate) the gas-liquid mass transfer rate.The liquid phase flow around the bubbles, rising up in the columns, influences the mass transfer rate too. The scale of this phenomenon is different from the physico-chemical phenomena. A multiscale approach is then followed.A mathematical modelling of the coupling between mass transfer and chemical reaction is first developed. The equations of the model are solved numerically, using COMSOL Multiphysics. In order to validate experimentally this model, a Mach-Zehnder interferometer is set up. The scale-up of this model to a bubble, in a second time, is carried out by completing the model of the coupling with a representation of the liquid phase flow around a bubble. It is called the bubble-li...