Phase change in porous media

We review notable advances published in the year 1999 in the area of phase change and phase growth in porous media. Phase equilibria thermodynamics, particularly in micropores, and growth kinetics, emphasizing the pore-network structure, are highlighted. Advances reported include the effects of confinement in phase transitions in micropores, and of the pore microstructure in the growth and dissolution of gas and liquid phases with applications ranging from capillary condensation to drying to gas evolution to condensation.     

Permeability of complex porous media

Absract  Hydrodynamic permeability of membrane composed of a set of porous spherical particles with rigid impenetrable cores is calculated. The cell method proposed by Happel and Brenner is used in calculations. All known boundary conditions on the cell surface, such as the Happel, Kuwabara, Kvashnin, and Cunningham (Mehta-Morse) models, are considered. The flow of liquid is described by the Brinkman equations. The problem of flow around a single spherical particle covered with porous layer by the uniform flow of viscous incompressible liquid is solved. Theoretical and empirical results are compared. Different limiting cases for which the derived formulas lead to results known from published literature are considered. Original Russian Text ? S.I. Vasin, A.N. Filippov, 2009, published in Kolloidnyi Zhurnal, 2009, Vol. 71, No. 1, pp. 32–46.     

Foam flow through porous media

There are two parts to the interaction of foam with porous media. How the foam interacts with the surface and the flow within the substrate, which is the focus of this review. Flow-through porous media has been investigated experimentally with the main focus in literature being on enhanced oil recovery and remediation. Recently, investigation of the flow of foam through a deformable substrate for dishwashing application has led to the development of mathematical models. It has been proposed that foam flow through pore channels is similar to the behaviour observed within microchannels. Meaning that to investigate the effects these properties have on foam flow it is best to observe them within a model channel then build up to a 3D structure of interlinking channels to resemble porous media. In this review, it is highlighted that a large amount of work is needed in understanding the interaction of foam and/or liquid within porous networks. Methods that can be applied to better represent foam and liquid flow in porous media are discussed within this review, including both using microchannels to simulate individual pores and using these systems to build up to a 3D structure of interlinking pores. In addition, more advanced imaging techniques to observe the flow through porous materials are discussed, including computed tomography scanning nuclear magnetic resentence and confocal microscopy. There is still more work required to fully understand the flow within porous media, including observing the affect of dead-end pores, closed loops and rough channel walls have on the flow.     

New hierarchically porous titania monoliths for chromatographic separation media

Separation media based on hierarchically porous titania (TiO₂) monoliths for high‐performance liquid chromatography (HPLC) have been successfully fabricated by the sol–gel process of titanium alkoxide in a mild condition utilizing a chelating agent and mineral salt. The as‐gelled TiO₂ monoliths were subjected to a simple solvent exchange process from ethanol (EtOH) to H₂O followed by drying and calcination. The resultant monolithic TiO₂ columns consist of anatase crystallites with the typical specific surface area of more than 200 m²/g. The resultant monolithic TiO₂ column calcined at 200 and 400°C exhibited a good separation performance for organophosphates as well as for polar benzene derivatives in the normal‐phase mode.     

Ionic transport in porous media for high zeta potentials

Ionic transport coefficients are numerically calculated in various porous media for high zeta potentials zeta by solving the Poisson-Boltzmann, the Stokes, and the convection-diffusion equations on the pore scale. Theoretical formulae for the simple case of a plane Poiseuille flow are derived in this limit. These formulae, when expressed in terms of the characteristic length Lambda, provide an excellent estimate of the ionic transport coefficients whatever the porous medium and whatever the value of zeta.     

Electrokinetic coupling in unsaturated porous media

We consider a charged porous material that is saturated by two fluid phases that are immiscible and continuous on the scale of a representative elementary volume. The wetting phase for the grains is water and the nonwetting phase is assumed to be an electrically insulating viscous fluid. We use a volume-averaging approach to derive the linear constitutive equations for the electrical current density as well as the seepage velocities of the wetting and nonwetting phases on the scale of a representative elementary volume. These macroscopic constitutive equations are obtained by volume-averaging Ampère's law together with the Nernst-Planck equation and the Stokes equations. The material properties entering the macroscopic constitutive equations are explicitly described as functions of the saturation of the water phase, the electrical formation factor, and parameters that describe the capillary pressure function, the relative permeability functions, and the variation of electrical conductivity with saturation. New equations are derived for the streaming potential and electro-osmosis coupling coefficients. A primary drainage and imbibition experiment is simulated numerically to demonstrate that the relative streaming potential coupling coefficient depends not only on the water saturation, but also on the material properties of the sample, as well as the saturation history. We also compare the predicted streaming potential coupling coefficients with experimental data from four dolomite core samples. Measurements on these samples include electrical conductivity, capillary pressure, the streaming potential coupling coefficient at various levels of saturation, and the permeability at saturation of the rock samples. We found very good agreement between these experimental data and the model predictions.     

Non-isothermal mass transport in porous media

Propagation of acoustic waves in non-isothermal dense media produces radiation-pressures whose nature is analyzed here. The case of a flux of thermal — rather than acoustic — energy can be analogously treated, and a molecular theory of thermal diffusion is accordingly formulated. p]The special case of thermal diffusion occurring within the liquid-filled channels of a thin partition — “thermodialysis” — is quantitatively analyzed within the frame of reference of the radiation-pressure approach. p]Some apparatuses developed for the experimental study of thermodialysis are described. The results obtained are discussed and shown to be in accord with the theoretical expectations.     

Knowledge-based reconstruction of random porous media

An evolutionary optimization technique is used to reconstruct digitized material models of 300(3) nm3 size for mesoporous two-phase systems. The models are adapted to the two-point probability (TPP) and to a volume-based pore-size distribution (PSD) which were derived from SANS and adsorption experiments and which carry statistical information about morphology and topology of the pore system. To avoid extreme update-costs, the bulk of mutations are assessed by means of a suitable approximation of the PSD; it is demonstrated that a sporadic insertion of the PSD suffices to drive the algorithm towards satisfactory models in acceptable time. Our approach is knowledge-based in the sense that (i) the mutations are restricted to expedient exchanges of phase-voxels by a heuristic rule, and (ii) the sporadic calculation of the PSD from the current state of the model, in essence, provides an efficient self-control for the evolutionary process. We applied the method to reconstruct periodic models of the xerogel Gelsil 200. Such reconstructs of real mesoporous solids could be utilized, for instance, to verify theories of adsorption and capillary condensation.     

Modeling heating curve for gas hydrate dissociation in porous media

A method for modeling the heating curve for gas hydrate dissociation in porous media at isochoric conditions (constant cell volume) is presented. This method consists of using an equation of state of the gas, the cumulative volume distribution (CVD) of the porous medium, and a van der Waals-Platteeuw-type thermodynamic model that includes a capillary term. The proposed method was tested to predict the heating curves for methane hydrate dissociation in a mesoporous silica glass for saturated conditions (liquid volume = pore volume) and for a fractional conversion of water to hydrate of 1 (100% of the available water was converted to hydrate). The shape factor (F) of the hydrate-water interface was found equal to 1, supporting a cylindrical shape for the hydrate particles during hydrate dissociation. Using F = 1, it has been possible to predict the heating curve for different ranges of pressure and temperature. The excellent agreement between the calculated and experimental heating curves supports the validity of our approach.     

Universal aspects of polymer diffusion in random media

We discuss statics and dynamics of an excluded volume polymer moving in a Gaussian distributed random one body potential. Stressing the intimate connection of universality and renormalizability we point out which features can be expected to be universal. We in particular note that universality holds only as long as the excluded volume in some sense dominates over disorder. Whereas universal static properties up to a trivial redefinition of the excluded volume interaction coincide with those found for the standard excluded volume problem, we find highly nontrivial results for universal dynamic quantities. We in particular discuss our recent results on the center of mass motion.     

Energy-saving drying technology for porous media using liquefied DME gas

Process design and energy requirement for a practical plant are investigated for an energy-saving drying (dewatering) process invented by the authors in 2002 for high-moisture porous materials. The basic concept of the process involves the extraction of water from a high-moisture porous material by bringing it in physical contact with liquefied dimethyl ether (DME) at room temperature. Water content of DME asymptotically increases to the saturation value and the high-moisture porous material is dried almost perfectly. DME from the DME-water mixture is vaporized by decompression. DME and water are separated by flash distillation. DME vapor is compressed and cooled in a heat exchanger, and the latent heat of condensation is reused to vaporize the DME in the heat exchanger. Multistage compression and multistage flash distillation are employed. After compression, the temperature of DME is less than 50?°C. Because specific heat ratio of DME is only 1.11, the energy consumption of the compressor is reduced. Considering the adiabatic efficiency of the compressor and the net thermal efficiency, the total energy for dewatering is about 1100 kJ per 1-kg-water-content of the material being dewatered This process has significant potential and is compact than the existing dewatering processes.     

A volume averaging approach for asymmetric diffusion in porous media

Asymmetric diffusion has been observed in different contexts, from transport in stratified and fractured porous media to diffusion of ions and macromolecular solutes through channels in biological membranes. Experimental and numerical observations have shown that diffusion is facilitated in the direction of positive void fraction (i.e., porosity) gradients. This work uses the method of volume averaging in order to obtain effective medium equations for systems with void fraction gradients for passive and diffusive mass transport processes. The effective diffusivity is computed from the solution of an associated closure problem in representative unit cells that allow considering porosity gradients. In this way, the results in this work corroborate previous findings showing that the effective diffusivity exhibits important directional asymmetries for geometries with void fraction gradients. Numerical examples for simple geometries (a section with an obstacle and a channel with varying cross section) show that the diffusion asymmetry depends strongly on the system configuration. The magnitude of this dependence can be quantified from the results in this work.     

Tetrahydrofuran hydrate decomposition characteristics in porous media

Many tetrahydrofuran (THF) hydrate properties are similar to those of gas hydrates. In the present work THF hydrate dissociation in four types of porous media is studied. THF solution was cooled to 275.15 K with formation of the hydrate under ambient pressure, and then it dissociated under ambient conditions. THF hydrate dissociation experiments in each porous medium were conducted three times. Magnetic resonance imaging (MRI) was used to obtain images. Decomposition time, THF hydrate saturation and MRI mean intensity (MI) were measured and analyzed. The experimental results showed that the hydrate decomposition time in BZ-4 and BZ-3 was similar and longer than that in BZ-02. In each dissociation process, the hydrate decomposition time of the second and third cycles was shorter than that of the first cycle in BZ-4, BZ-3, and BZ-02. The relationship between THF hydrate saturation and time is almost linear.     

Vapor and liquid equilibria in porous media

The alteration of the vapor–liquid equilibrium (VLE) of volatile organic mixtures by placing porous media at the liquid–vapor interface was studied. Kelvin, assuming ideal behavior of fluids, first introduced the vapor pressure of liquid over a meniscus as a function of its surface tension and the radius of the curvature. A thermodynamic model (SS_mod model) predicting the VLE of non-ideal organic mixtures in porous media was developed as a function of pore sizes. The model was used to predict the VLE of two aqueous alcohol solutions, ethanol–water and propanol–water, and two binary solutions, methanol–isopropanol and ethanol–n-octane. Experiments were conducted using sintered metal and fritted glass plates as porous media, and the results were compared with the model predictions. Using the actual diameter of the porous media, the model prediction showed good agreement with the experimental results.     

Concentration dependence of the distribution coefficient for macromolecules in porous media

Partitioning of macromolecules between pore and bulk solutions directly affects both equilibrium and transport processes such as exclusion chromatography and movement of solutes through porous media. Because of interactions between macromolecules and the pore wall, the variation of the macromolecule activity with concentration is different inside the pore than in bulk solution. This difference causes a concentration dependence of the distribution coefficient, as reported in experiments involving exclusion chromatography. In order to explain this effect, we develop a model for a concentration-dependent distribution which explicitly accounts for a coupling between pore–macromolecule and macromolecule–macromolecule interactions. Predictions using this model are reported for the case of rigid spherical macromolecules in both cylindrical and slit pores, including both steric (hard sphere–hard wall) and long-range (screened electrostatic) interactions. An important result is the existence of a general correlation between the first order concentration effect and measurable properties of the macromolecule and porous medium.     

Constrained equilibrium as a tool for characterization of deformable porous media

A new method for characterizing the deformable porous materials with noncritical adsorption probes is proposed. The mechanism is based on driving the adsorbate through a sequence of constrained equilibrium states with the insertion isotherms forming a pseudocritical point or a van der Waals-type loop. In the framework of a perturbation theory and Monte Carlo simulations we have found a link between the loop parameters and the host morphology. This allows one to characterize porous matrices through analyzing a shift of the pseudocritical point and a shape of the pseudospinodals.     

Evaluation of bacterial detachment rates in porous media

The ability of published biomass detachment rate expressions to describe experimental data obtained from porous media reactors usingPseudomonas aeruginosa grown aerobically on glucose was evaluated. A first-order rate expression on attached biomass concentration best reflected effluent substrate concentration for combined data sets. Detachment rate coefficientk _d1 was dependent on initial substrate concentration. Simulation of porous media reactor experiments indicated that responses using higher influent substrate concentrations possessed greater sensitivity to variations ink _d1. Simulations of field bioremediation systems suggest the use of accurate biofilm development kinetics is important in the prediction of well bore biofouling.     

Competitive positron and positronium trapping in porous media

Positron annihilation acquired acceptance for structural investigation of solids but results in porous media—where positron lifetime spectroscopy (LT) reveals substantial Ps formation—were ambiguous. Data on zeolites lead to the conclusion that Ps trapping in competing “extended free volume” sites, inhomogeneous regions and grain boundaries occurs. Furthermore, positron trapping must also be considered. Systematic errors due to incomplete time range selection are discussed, significance and importance of corrections for 3γ/2γ counting efficiency differences are shown in practice.     

Force-driven migration of particles in ordered porous media

Brownian dynamics simulation has been employed to study the behavior of force-driven particle migration in different ordered porous media comprised of periodically interconnected spherical cavities, representing inverted colloidal crystals. The effects of the imposed field strength and direction on the particle mobility and direction are investigated. The simulation results find that in a weak or intermediate field, the mobility normalized by the value in free solvent behaves in a similar way as the normalized diffusivity when the porosity is varied. Under a strong field, the normalized mobility can increase or decrease with the field strength, depending on the field direction relative to the cavity arrangement. If the imposed field is not aligned with any unobstructed pathway, the mobility tensor may become anisotropic and prolonged particle entrapment may also take place.