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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