Impact of pressure, salt concentration, and temperature on the convective dissolution of carbon dioxide in aqueous solutions

Investigation of Gelation Techniques for the Fabrication of Cellulose Aerogels

Because of the pronounced degradation of the environment, there has been an escalated demand for the fabrication of eco-friendly and highly efficient products derived from renewable sources. Cellulose aerogels have attracted significant interest attributable to their structural characteristics coupled with biodegradability and biocompatibility. The features of the molecular structure of cellulose allow for the use of various methods in the production of gels. For instance, the presence of hydroxyl groups on the cellulose surface allows for chemical crosslinking via etherification reactions. On the other hand, cellulose gel can be procured by modulating the solvent power of the solvent. In this study, we investigate the impact of the gelation methodology on the structural attributes of aerogels. We present methodologies for aerogel synthesis employing three distinct gelation techniques: chemical crosslinking, cryotropic gelation, and CO2-induced gelation. The outcomes encompass data derived from helium pycnometry, Fourier-transform infrared spectroscopy, nitrogen porosimetry, and scanning electron microscopy. The resultant specimens exhibited a mesoporous fibrous structure. It was discerned that specimens generated through cryotropic gelation and CO2-induced gelation manifested higher porosity (93-95%) and specific surface areas (199-413 m2/g) in contrast to those produced via chemical crosslinking (porosity 72-95% and specific surface area 25-133 m2/g). Hence, this research underscores the feasibility of producing cellulose-based aerogels with enhanced characteristics, circumventing the necessity of employing toxic cross-linking agents. The process of gel formation through chemical crosslinking enables the creation of gels with enhanced mechanical properties and a more resilient structure. Two alternative methodologies prove particularly advantageous in applications necessitating biocompatibility and high porosity. Notably, CO2-induced gelation has not been hitherto addressed in the literature as a means to produce cellulose gels. The distinctive feature of this approach resides in the ability to combine the stages of obtaining an aerogel in one apparatus.

Effect of variable solubility on reactive dissolution in partially miscible systems

When two partially miscible systems are put in contact, one phase, A, can dissolve into the other one with a given solubility. Chemical reactions in the host phase can impact this dissolution by consuming A and by generating products that impact the solubility of A. Here, we study theoretically the optimal conditions for transfer of a reactant A in a host phase containing a species B when a bimolecular A + B → C reaction generates a product C that linearly decreases the solubility of A. We have quantified numerically the influence of this variable solubility on the reaction-diffusion (RD) concentration profiles of all species in the host phase, on the temporal evolution of the position of the reaction front, and on the flux of A through the interface. We have also computed the analytical asymptotic concentration profiles, solutions at long times of the RD governing equations. For a fixed negative effect of C on the solubility of A, an increase in the initial concentration of reactant B or an increase in the diffusion rate of species B and C results in a larger flux of A and hence a larger amount of A dissolved in the host solution at a given time. However, when the influence of C on the solubility increases, the mass transfer decreases. Our results help understand to what extent a chemical reaction can optimize the reactive transfer of a solute to a host phase with application to, among other things, the geological sequestration of carbon dioxide in an aquifer.

Linear stability analysis and nonlinear simulations of convective dissolution in an inclined porous layer between impermeable surfaces

Convective dissolution can occur in porous media when a given solute dissolves in a host layer from above and increases the density of the host solution. Buoyancy-driven fingering can then develop, which increases the transfer flux of the solute. We investigate here numerically the properties of this convective dissolution when the porous host layer is inclined by an angle θ relative to the horizontal direction. We consider an incompressible flow in porous media governed by Darcy's law, driven by density gradients associated with the concentration of the dissolving solute. The model problem focuses on the case of a very long (infinite) tilted porous layer limited by two parallel impermeable surfaces. A linear stability analysis and nonlinear simulations are performed using the Boussinesq approximation. A vorticity-stream function formulation is adopted to solve the two-dimensional hydrodynamic field through the finite element method. We find that the inclination of the interface decreases the growth rate of the instability and the range of unstable wavenumbers, delaying or even suppressing the onset of the fingering instability. Moreover, it introduces a drift velocity on the perturbations, which is characterized here in both the linear stability analysis and the nonlinear simulations.

Experimental Investigation of CO2-Induced Silica Gel as the Water Blocking Grout Effect of Aquifer Ions

This study aimed to prevent water flow in microcracks and simultaneously achieve CO₂ capture during grouting (CCG). Using sodium silicate (SS) as the primary material, the microcracks were grouted by a two-step approach. The low-initial-viscosity (5 mPa s) SS was first saturated within the microcracks followed by CO₂ injection at 2 MPa. Through CO₂ dissolution, silica gel was developed and tolerated a hydraulic pressure of up to 5.5 MPa. The effects of aquifer ions (Na⁺, Ca²⁺, Mg²⁺, HCO₃ ^-, and SO₄ ^2-) were equally evaluated at harsh conditions, and it was found that the strength of the silica gel was reduced, which was caused by salting out, low CO₂ solubility, and precipitation. As a result, the hydraulic pressure was reduced to as low as 3 MPa. After 210 days, 16% of the silica gels (without ion inclusion) were reversible to the liquid phase, where a similar effect was found in the cases of Na⁺ and Mg²⁺ ions. The degradation increased with more Ca²⁺ ions (up to 55%) and decreased with more HCO₃ ^- and SO₄ ^2- ions. Microcracks grouted with CCG extended the CO₂ utilization in grouting application. Combined with the effect of dissolved ions, the proposed approach is feasible in the field implementation for underground engineering under water bodies.

Scalings of Rayleigh-Taylor Instability at Large Viscosity Contrasts in Porous Media

The scalings of the Rayleigh-Taylor instability are studied numerically for porous media flows when the denser fluid lying on top of the less dense one is also much more viscous. We show that, above a critical value of the viscosity ratio M, a symmetry breaking of the buoyancy-driven fingers is observed as they extend much further downward than upward. The asymmetry ratio scales as M^{1/2} while the asymptotic flux across the initial contact line, quantifying the mixing between the two fluids, scales as M^{-1/2}. A new fingering mechanism induced by large viscosity contrasts is identified and shows good agreement with experimentally observed dynamics.

Interface Fingering Instability Triggered by a Density-Coupled Oscillatory Chemical Reaction via Precipitation

A density fingering hydrodynamic instability is triggered by a chemical reaction at the interface between two fluids. The density instability is controlled by the density gradient between both solutions, while the excitability of the bubble-free Belousov-Zhabotinsky-1,4-cyclohexanedione (BZ-CHD) oscillatory chemical reaction controls the importance of the chemistry in the system. Both parameters are thoroughly analyzed, and the mechanism underlying the instability is unveiled. The experimental observations lead us to modify the existing and accepted models for the BZ-CHD reaction within this context. The important role played by precipitation is considered in this context and included into the model. The modified kinetic model once coupled with fluid dynamics along with the precipitation mechanism was able to reproduce the experimental observations.

Reactive convective-dissolution in a porous medium: stability and nonlinear dynamics

We investigate the effects of a dissolution reaction, A(aq) + B(s) → C(aq), on the gravitational instability and nonlinear dynamic behaviour of a diffusive boundary layer in a porous medium. Our linear stability and numerical results reveal that, unexpectedly, even when the density contribution of the soluble product C is smaller than that of the dissolved solute A, the chemical reaction can destabilize the layer and accelerate the onset of convection. However, for a very light product, the reaction stabilizes the layer. We show that these widely disparate characteristics of the reactive-diffusive layer are outcomes of the nonlinear competition between two reaction effects, the destabilizing sharpening of the solute concentration gradient and associated increase in the solute diffusive flux, and the stabilizing replacement of the solute by a less dense product near the interface.

Enhanced steady-state dissolution flux in reactive convective dissolution

Chemical reactions can accelerate, slow down or even be at the very origin of the development of dissolution-driven convection in partially miscible stratifications when they impact the density profile in the host fluid phase. We numerically analyze the dynamics of this reactive convective dissolution in the fully developed non-linear regime for a phase A dissolving into a host layer containing a dissolved reactant B. We show for a general A + B → C reaction in solution, that the dynamics vary with the Rayleigh numbers of the chemical species, i.e. with the nature of the chemicals in the host phase. Depending on whether the reaction slows down, accelerates or is at the origin of the development of convection, the spatial distributions of species A, B or C, the dissolution flux and the reaction rate are different. We show that chemical reactions can enhance the steady-state flux as they consume A and can induce more intense convection than in the non-reactive case. This result is important in the context of CO₂ geological sequestration where quantifying the storage rate of CO₂ dissolving into the host oil or aqueous phase is crucial to assess the efficiency and the safety of the project.

Experimental analysis of density fingering instability modified by precipitation

We analyze the effect of precipitate formation on the development of density induced hydrodynamic instabilities. In this case, the precipitate is BaCO₃, obtained by reaction of CO₂ with aqueous BaCl₂. CO₂(g) dissolution increases the local density of the aqueous phase, triggering Rayleigh-Taylor instabilities and BaCO₃ formation. It was observed that at first the precipitate was formed at the finger front. As the particles became bigger, they began to fall down from the front. These particles were used as tracers using PIV technique to visualize the particle streamlines and to obtain the velocity of that movement. This falling produced a downward flow that might increase the mixing zone. Contrary to expectations, it was observed that the finger length decreased, indicating that for the mixing zone development, the consumption of CO₂ to form the precipitate is more important than the downward flow. The mixing zone length was recovered by increasing the availability of the reactant (higher CO₂ partial pressure), compensating the CO₂ used for BaCO₃ formation. Mixing zone development rates reached constant values at shorter times when the precipitate is absent than when it is present. An analysis of the nonlinear regime with and without the precipitate is performed.

Chemical control of dissolution-driven convection in partially miscible systems: nonlinear simulations and experiments

Numerical simulations combined with experimental results from two laboratory-scale model systems show how to control convective dissolution by chemical reactions.

Reactive-convective dissolution in a porous medium: the storage of carbon dioxide in saline aquifers

We quantify the destabilising effect of a first-order chemical reaction on the fingering instability of a diffusive boundary layer in a porous medium. Using scaling, we show that the dynamics of such a reactive boundary layer is fully determined by two dimensionless groups: Da/Ra², which measures the timescale for convection compared to those for reaction and diffusion; and β_C/β_A, which reflects the density change induced by the product relative to that of the diffusing solute. Linear stability and numerical results for β_C/β_A in the range 0-10 and Da/Ra² in the range 0-0.01 are presented. It is shown that the chemical reaction increases the growth rate of a transverse perturbation and favours large wavenumbers compared to the inert system. Higher β_C/β_A and Da/Ra² not only accelerate the onset of convection, but crucially also double the transport of the solute compared to the inert system. Application of our findings to the storage of carbon dioxide in carbonate saline aquifers reveals that chemical equilibrium curtails this increase of CO₂ flux to 50%.

Chemo-hydrodynamic patterns in porous media

Chemical reactions can interplay with hydrodynamic flows to generate chemo-hydrodynamic instabilities affecting the spatio-temporal evolution of the concentration of the chemicals. We review here such instabilities for porous media flows. We describe the influence of chemical reactions on viscous fingering, buoyancy-driven fingering in miscible systems, convective dissolution as well as precipitation patterns. Implications for environmental systems are discussed.This article is part of the themed issue 'Energy and the subsurface'.

Density profiles around A+B→C reaction-diffusion fronts in partially miscible systems: A general classification

Various spatial density profiles can develop in partially miscible stratifications when a phase A dissolves with a finite solubility into a host phase containing a dissolved reactant B. We investigate theoretically the impact of an A+B→C reaction on such density profiles in the host phase and classify them in a parameter space spanned by the ratios of relative contributions to density and diffusion coefficients of the chemical species. While the density profile is either monotonically increasing or decreasing in the nonreactive case, reactions combined with differential diffusivity can create eight different types of density profiles featuring up to two extrema in density, at the reaction front or below it. We use this framework to predict various possible hydrodynamic instability scenarios inducing buoyancy-driven convection around such reaction fronts when they propagate parallel to the gravity field.

Chemical control of dissolution-driven convection in partially miscible systems: theoretical classification

Dissolution-driven convection occurs in the host phase of a partially miscible system when a buoyantly unstable density stratification develops upon dissolution. Reactions can impact such convection by changing the composition and thus the density of the host phase. Here we study the influence of A + B → C reactions on such convective dissolution when A is the dissolving species and B a reactant initially in solution in the host phase. We perform a linear stability analysis of related reaction-diffusion density profiles to compare the growth rate of the instability in the reactive case to its non reactive counterpart when all species diffuse at the same rate. We classify the stabilizing or destabilizing influence of reactions on the buoyancy-driven convection in a parameter space spanned by the solutal Rayleigh numbers RA,B,C of chemical species A, B, C and by the ratio β of initial concentrations of the reactants. For RA > 0, the non reactive dissolution of A in the host phase is buoyantly unstable. In that case, we show that the reaction is enhancing convection provided C is sufficiently denser than B. Increasing the ratio β of initial reactant concentrations increases the effect of chemistry but does not significantly impact the stabilizing/destabilizing classification. When the non reactive case is buoyantly stable (RA≤ 0), reactions can create in time an unstable density stratification and trigger convection if RC > RB. Our theoretical approach allows classifying previous results in a unifying picture and developing strategies for the chemical control of convective dissolution.