Transport-limited Dissolution Research Articles

Pore-scale study of dissolution-induced changes in permeability and porosity of porous media

Summary We apply a reactive transport lattice Boltzmann model developed in previous studies to study the dissolution-induced changes in permeability and porosity of two porous media at the pore scale. The permeability–porosity relationship is explored for a wide range of Peclet and Damkohler numbers. It is found that this relationship depends not only on different dissolution regimes characterized by Pe and Da, but also on the specific porous medium structure. The permeability–porosity relationship for the more geometrically complex porous medium shows much more complexity than that for the simple fractured medium. While a very small Da sets an upper bound for the permeability–porosity relationship for the simple medium, a combination of a high Da and Pe results in wormholing, and the fastest permeability increase for the complex medium. At a moderate Pe but large Da, a transition from transport-limited dissolution regime to wormholing is also observed for the complex medium.

Reactive-infiltration instabilities in rocks. Fracture dissolution

AbstractA reactive fluid dissolving the surface of a uniform fracture will trigger an instability in the dissolution front, leading to spontaneous formation of pronounced well-spaced channels in the surrounding rock matrix. Although the underlying mechanism is similar to the wormhole instability in porous rocks there are significant differences in the physics, due to the absence of a steadily propagating reaction front. In previous work we have described the geophysical implications of this instability in regard to the formation of long conduits in soluble rocks. Here we describe a more general linear stability analysis, including axial diffusion, transport-limited dissolution, nonlinear kinetics, and a finite-length system.

Neutralization of sulfuric acid solutions by calcite dissolution and the application to anoxic limestone drain design

Abstract Batch reactor (BR) experiments were conducted to measure the effect of hydrodynamics and gypsum coatings on calcite neutralization rates. A factorial array of BR experiments measured the H+ concentration change by calcite dissolution over a pH range of 1.5–3.5 and Na2SO4 concentrations of 0–1 M. The rate of H+ concentration change with time was determined by numerical differentiation of H+ concentration versus time. Regression modeling showed that for uncoated calcite, rates are only significantly affected by pH, r = - 10 - 2.32 a H + 0.76 . Whereas, for calcite coated with gypsum only time had a significant effect on calcite dissolution rates, r = −10−1.96t−0.53. Because transport-limited dissolution rates for uncoated calcite are a function of the pH and Reynolds number, a model was developed to express the effects of these two variables on the rate of H+ consumption for a solution with a Darcy velocity, q, through a porous medium with a particle radius, rp, such that r ′ = 1.08 × 10 - 3 q 0.31 r p - 0.69 m H + 0.87 . This equation was integrated via a numerical model to simulate the performance of an idealized anoxic limestone drain (ALD). This model predicts the pH and alkalinity change along the length of an ALD. The model shows that the efficiency of an ALD is greater when the Darcy velocity is low and the particle radius is small. In addition, the growth of gypsum coatings causes the rate of H+ neutralization to decline as the square root of time as they form and block the H+ transport to the calcite surface. Supersaturation with respect to gypsum, leading to coating formation, can be avoided by diluting the ALD feed solution or by replacing limestone with dolomite.

Interfacial dynamics in transport-limited dissolution

Various model problems of "transport-limited dissolution" in two dimensions are analyzed using time-dependent conformal maps. For diffusion-limited dissolution (reverse Laplacian growth), several exact solutions are discussed for the smoothing of corrugated surfaces, including the continuous analogs of "internal diffusion-limited aggregation" and "diffusion-limited erosion." A class of non-Laplacian, transport-limited dissolution processes is also considered, which raises the general question of when and where a finite solid will disappear. In a case of dissolution by advection-diffusion, a tilted ellipse maintains its shape during collapse, as its center of mass drifts obliquely away from the background fluid flow, but other initial shapes have more complicated dynamics.

In situ AFM study of the dissolution and recrystallization behaviour of polished and stressed calcite surfaces

We investigated the dissolution behaviour of polished calcite surfaces in situ using a fluid-cell atomic force microscope. Polished calcite surfaces enabled us to study the effects of applied surface stress and crystallographic orientation on calcite dissolution pattern formation. Thin-sections of Iceland spar single-crystals polished either parallel or with a 5° miscut angle to { 10 1 ¯ 4 } cleavage planes were studied. Compressive surface stresses of up to 50 MPa were applied to some of the thin-section samples by means of elastic concave bending. Experiments were carried out in semi-stagnant deionized water under mainly transport limited dissolution conditions. Samples polished parallel to { 10 1 ¯ 4 } cleavage planes dissolved by the formation of etch-pits originating from polishing defects. The dissolution behaviour of 5° miscut surfaces was relatively unaffected by polishing defects, since no etch-pits developed in these samples. Dissolution of the miscut samples led to stepped or rippled surface patterns on the nanometer scale that coarsened during the first 30–40 min of the experiments. Possible reasons for the pattern-coarsening were: (i) progressive bunching of retreating dissolution steps and (ii) surface energy driven recrystallization (Ostwald ripening) under transport limited dissolution conditions. A flat polished miscut surface in calcite may recrystallize into a hill-and-valley structure in a (near-)saturated solution so as to lower its total surface free energy in spite of a larger surface area. No clear effect of applied stress on dissolution pattern formation has been observed.

A mechanistic study of griseofulvin dissolution into surfactant solutions in laminar flow conditions

The in vivo dissolution of many poorly soluble drugs is enhanced by the action of surfactants secreted into the upper gastrointestinal (GI) tract. These substances may act by solubilizing individual drug molecules into two separate liquid phases: the free aqueous phase and a micellar phase in which the drug is incorporated into a complex of two or more surfactant molecules. This complex process, micellar solubilization, was the subject of this in vitro study, wherein griseofulvin (gris) dissolution was observed in flowing surfactant solutions. Aqueous solutions of sodium dodecyl sulfate (SDS), an anionic surfactant, were pumped over a gris tablet embedded in a laminar flow device to simulate flow in the human upper GI tract. SDS solutions were well above the critical micellar concentration (cmc approximately 6-7 mM), and flow rates ranged from 4 to 7 mL/min. Gris solubility in premicellar (4 mM), near-micellar (6 mM), and micellar (>6 mM) SDS solutions was also determined. The measured solubility of gris increased linearly with SDS concentrations above the cmc. Drug solubility in SDS concentrations below the cmc was also higher than that in water. Gris diffusion coefficients were measured using pulsed-field gradient NMR spectroscopy. To determine the controlling mechanism for surfactant-enhanced dissolution, a mathematical model was developed. The model solution, an equation for drug dissolution rate, was compared with experimental data to demonstrate that drug transport away from the solid surface is the slow step in the process. Measured gris diffusion coefficients and solubility values were used as constants in the mathematical model solution and were combined to calculate an effective gris diffusion coefficient. Using these experimentally determined properties, model-calculated dissolution rates were within 7% of the measured values. As hypothesized, dissolution rates were found to be directly proportional to the transport properties of the system (effective drug diffusion coefficient and fluid flow rate) as well as to the drug solubility. To further verify transport-limited dissolution, the measured dissolution rates were found to be proportional to the surrounding medium flow rate to the 1/3 power, as predicted by the model dissolution rate equation.

A Mechanistic Study of Griseofulvin Dissolution into Surfactant Solutions under Laminar Flow Conditions

□ The in vivo dissolution of many poorly soluble drugs is enhanced by the action of surfactants secreted into the upper gastrointestinal (GI) tract. These substances may act by solubilizing individual drug molecules into two separate liquid phases: the free aqueous phase and a micellar phase in which the drug is incorporated into a complex of two or more surfactant molecules. This complex process, micellar solubilization, was the subject of this in vitro study, wherein griseofulvin (gris) dissolution was observed in flowing surfactant solutions. Aqueous solutions of sodium dodecyl sulfate (SDS), an anionic surfactant, were pumped over a gris tablet embedded in a laminar flow device to simulate flow in the human upper GI tract. SDS solutions were well above the critical micellar concentration (cmc≈6-7mM), and flow rates ranged from 4 to 7mL/min. Gris solubility in premicellar (4mM), near-micellar (6mM), and micellar (>6mM) SDS solutions was also determined. The measured solubility of gris increased linearly with SDS concentrations above the cmc. Drug solubility in SDS concentrations below the cmc was also higher than that in water. Gris diffusion coefficients were measured using pulsed-field gradient NMR spectroscopy. To determine the controlling mechanism for surfactant-enhanced dissolution, a mathematical model was developed. The model solution, an equation for drug dissolution rate, was compared with experimental data to demonstrate that drug transport away from the solid surface is the slow step in the process. Measured gris diffusion coefficients and solubility values were used as constants in the mathematical model solution and were combined to calculate an effective gris diffusion coefficient. Using these experimentally determined properties, model-calculated dissolution rates were within 7% of the measured values. As hypothesized, dissolution rates were found to be directly proportional to the transport properties of the system (effective drug diffusion coefficient and fluid flow rate) as well as to the drug solubility. To further verify transport-limited dissolution, the measured dissolution rates were found to be proportional to the surrounding medium flow rate to the 1/3 power, as predicted by the model dissolution rate equation