Modeling desorption of fluids from disordered mesoporous materials

The desorption mechanism of fluids in disordered mesoporous glasses is studied by Monte Carlo simulations of a coarse-grained lattice model with realistic matrix configurations representative of Vycor. Two methods of simulation are considered: grand canonical ensemble Monte Carlo simulations and dynamic Monte Carlo simulations which mimic the diffusion of the fluid in and out of the material using Kawasaki dynamics. In the grand canonical simulations, cavitation via nucleation of bubbles inside the pores plays the dominant role in determining the fluid configurations along the desorption isotherm. The Kawasaki dynamics simulations indicate that such configurations are achieved dynamically via the gradual advancement of macroscopic front interfaces toward the interior. This is made possible by the bubble nucleation mechanism operating on a length scale that is determined by both the typical pore size and the strength of the solid-fluid interaction.     

Dynamic aspects of mercury porosimetry: a lattice model study

Grand canonical Monte Carlo simulations using both Glauber dynamics and Kawasaki dynamics have been carried out for a recently developed lattice model of a nonwetting fluid confined in a porous material. The calculations are aimed at investigating the molecular scale mechanisms leading to mercury retention encountered during mercury porosimetry experiments. We first describe a set of simulations on slit and ink-bottle pores. We have studied the influence of the pore width parameter on the intrusion/extrusion curve shapes and investigated the corresponding mechanisms. Entrapment appears during Kawasaki dynamics simulations of extrusion performed on ink-bottle pores when the system is studied for short relaxation times. We then consider the more realistic and complex case of a Vycor glass building on recent work on the dynamics of adsorption of wetting fluids (Woo, H. J.; Monson, P. A. Phys. Rev. E 2003, 67, 041207). Our results suggest that mercury entrapment is caused by a decrease in the rate of mass transfer associated with the fragmentation of the liquid during extrusion.     

Collective dynamics near a phase transition in confined fluids

We performed molecular dynamics simulations in the microcanonical ensemble (MEMD) for a "simple" fluid confined between two solid substrates. From the calculation of the intermediate scattering function F(k( parallel ),t) and through the memory function formalism, we extract material ( i.e. transport and thermodynamics) coefficients in the vicinity of the liquid-gas phase transition. Our results show that approaching the limit of stability ( i.e. the spinodal), the dynamics of the system changes markedly.     

Mercury porosimetry in mesoporous glasses: a comparison of experiments with results from a molecular model

We present results from experiments and molecular modeling of mercury porosimetry into mesoporous Vycor and controlled pore glass (CPG) solid materials. The experimental intrusion/extrusion curves show a transition from a type H2 hysteresis for the Vycor glass to a type H1 hysteresis for the CPG. Mercury entrapment is observed in both materials, but we find that the amount of entrapped mercury depends on the chosen experimental relaxation time. No additional entrapment is found in a second intrusion/extrusion cycle, but hysteresis is still observed. This indicates that hysteresis and entrapment are of different origin. The experimental observations are qualitatively reproduced in theoretical calculations based on lattice models, which provide significant insights of the molecular mechanisms occurring during mercury porosimetry experiments in these porous glasses.     

Fast pyrolysis of coals under N2 and CO2 atmospheres

Oxyfuel combustion represents one way for cleaner energy production using coal as combustible. The comparison between the oxycombustion and the conventional air combustion process starts with the investigation of the pyrolysis step. The aim of this contribution is to evaluate the impact of N₂ (for conventional air combustion) and CO₂ (for oxy-fuel combustion) atmospheres during pyrolysis of three different coals. The experiments are conducted in a drop tube furnace over a wide temperature range 800–1400 °C and for residence time ranging between 0.2 and 1.2 s. Coal devolatilized in N₂ and CO₂ atmospheres at low temperatures (?1200 °C) and longer residence times (>?0.5 s), the char-CO₂ reaction is clearly observed, whose intensity depends on the nature of the coal. Furthermore, the volatile yields are simulated using Kobayashi’s scheme and kinetic parameters are predicted for each coal. The char gasification under CO₂ is also accounted for by the model.

     

Modeling mercury porosimetry using statistical mechanics

We consider mercury porosimetry from the perspective of the statistical thermodynamics of penetration of a nonwetting liquid into a porous material under an external pressure. We apply density functional theory to a lattice gas model of the system and use this to compute intrusion/extrusion curves. We focus on the specific example of a Vycor glass and show that essential features of mercury porosimetry experiments can be modeled in this way. The lattice model exhibits a symmetry that provides a direct relationship between intrusion/extrusion curves for a nonwetting fluid and adsorption/desorption isotherms for a wetting fluid. This relationship clarifies the status of methods that are used for transforming mercury intrusion/extrusion curves into gas adsorption/desorption isotherms. We also use Monte Carlo simulations to investigate the nature of the intrusion and extrusion processes.     

Enhanced CO2 solubility in hybrid MCM-41: molecular simulations and experiments

Grand canonical Monte Carlo simulations are performed in a hybrid adsorbent model in order to interpret the CO(2) solubility behavior. The hybrid adsorbent is prepared by confining a physical solvent (OMCTS) into the pores of a mimetic MCM-41 solid support. As a result, simulated adsorption isotherms of CO(2) nicely match the experimental data for three distinctive systems: bulk solvent, raw MCM-41, and hybrid MCM-41. The microscopic mechanisms underlying the apparition of enhanced solubility are then clearly identified. In fact, the presence of solvent molecules favors the layering of CO(2) molecules within the pores; therefore, the CO(2) solubility in the hybrid adsorbent markedly increases in comparison to that found in the raw adsorbent as well as in the bulk solvent. In addition, a good understanding of confined solvents' properties and solid surface structures is essential to fully evaluate the efficiency of hybrid adsorbents in capturing CO(2). The sorbent-solid interactions along with the solvent molecular size's impact on CO(2) solubility are therefore investigated in this study. We found that an ideal hybrid system should possess a weak solvent-solid interaction but a strong solvent-CO(2) interaction. Besides, an optimal solvent size is obtained for the enhanced CO(2) solubility in the hybrid system. According to the simulation results, the solvent layer builds pseudomicropores inside the mesoporous MCM-41, enabling more CO(2) molecules to be absorbed under the greater influence of spatial confinement and surface interaction. In addition, the molecular sieving effect is clearly observed in the case of larger solvent molecular sizes.     

Mean-field theory of liquid droplets on roughened solid surfaces: application to superhydrophobicity

We present calculations of the density distributions and contact angles of liquid droplets on roughened solid surfaces for a lattice gas model solved in a mean-field approximation. For the case of a smooth surface, this approach yields contact angles that are well described by Young's equation. We consider rough surfaces created by placing an ordered array of pillars on a surface, modeling so-called superhydrophobic surfaces, and we have made calculations for a range of pillar heights. The apparent contact angle follows two regimes as the pillar height increases. In the first regime, the liquid penetrates the interpillar volume, and the contact angle increases with pillar height before reaching a constant value. This behavior is similar to that described by the Wenzel equation for contact angles on rough surfaces, although the contact angles are underestimated. In the second regime, the liquid does not penetrate the interpillar volume substantially, and the contact angle is independent of the pillar height. This situation is similar to that envisaged in the Cassie-Baxter equation for contact angles on heterogeneous surfaces, but the contact angles are overestimated by this equation. For larger pillar heights, two states of the droplet can be observed, one Wenzel-like and the other Cassie-like.     

Analysis of heat and mass transfers in two-phase flow by coupling optical diagnostic techniques

During the course of a hypothetical accident in a nuclear power plant, spraying might be actuated to reduce static pressure in the containment. To acquire a better understanding of the heat and mass transfers between a spray and the surrounding confined gas, non-intrusive optical measurements have to be carried out simultaneously on both phases. The coupling of global rainbow refractometry with out-of-focus imaging and spontaneous Raman scattering spectroscopy allows us to calculate the local Spalding parameter B _M, which is useful in describing heat transfer associated with two-phase flow.