Eustathios S. Kikkinides

Simulation of self-diffusion of point-like and finite-size tracers in stochastically reconstructed Vycor porous glasses

Aim of the present study is to simulate self-diffusion in three-dimensional images of reconstructed Vycor porous glass, which have the same statistical content as the actual material in terms of porosity and autocorrelation function. Effective diffusivities are determined from a step-by-step random walk process at different porosities and diffusion regimes. In all cases, the effective diffusivity curves show a sharp decrease below 20% porosity and drop to zero below a porosity of about 15%, a value suggested independently from the theory of spinodal decomposition. Comparison between the computed and experimental diffusivity values obtained in the Knudsen regime, shows a relative difference of less than 6%. Additional simulations in the molecular diffusion regime are performed using inert tracers of finite size. In these simulations, a transition is found in the value of diffusivity from a high value at small time scales, to a lower constant value achieved at large times. The time at which this crossover t...

A Study on Structural and Diffusion Properties of Porcine Stratum Corneum Based on Very Small Angle Neutron Scattering Data

AbstractPurpose. Generation of valuable information about the biphasic geometrical configuration of porcine stratum corneum from Very Small Angle Neutron Scattering (VSANS) data and investigation of its effect on the corresponding effective diffusivity. Methods. Spectra of porcine stratum corneum are mathematically transformed in order to obtain the corresponding auto-correlation function (ACF). Model stratum corneum structures, matching this experimentally determined ACF, are then produced based on the “brick-and-mortar” configuration. The effective diffusivity through these model domains is calculated using an appropriate numerical method. Results. The most appropriate geometry of porcine stratum corneums lipid and protein phases in a “brick-and-mortar” configuration is quantitatively determined and correlated with the barrier properties (diffusivity) of the stratum corneum model structures. Conclusions. The ACF analysis indicates the most appropriate values for the dimensions of the corneocyte thickness and the surrounding lipid gap, while the corneocyte length is estimated from the diffusion study.

Numerical and experimental investigation of the diffusional release of a dispersed solute from polymeric multilaminate matrices

In the present work the release behavior of special, multilaminate matrix-type polymer systems, is studied both theoretically and experimentally. Two different mathematical models have been employed to describe the release of a dispersed solute from both single- and multilayer matrices. A parameter sensitivity study shows that the incorporation of supersaturated matrices in the formation of multilaminate devices, with a nonuniform initial solute loading, can provide a delivery system with optimized performance compared to monolithic ones. Finally, the findings of this theoretical analysis show good agreement with measurements of the release rates of a model disperse dye from both single- and multilayer matrices.

Adsorption–desorption gas relative permeability through mesoporous media—network modelling and percolation theory

Gas relative permeability in mesoporous media is investigated during both adsorption and desorption of a condensable vapour which causes pore blocking in the structure of the porous network. Three-dimensional regular lattices of various pore connectivities are considered and compared to effective medium (EMA) models and Bethe trees of infinite size. During adsorption, the corresponding relative permeability curves exhibit a nearly quadratic behaviour with the volume of adsorbate, in full agreement with percolation theory, while EMA predicts a linear relation between these properties. Bethe trees give a very good approximation of the relative permeability curve of three-dimensional regular networks, provided that the connectivity of the Bethe tree is properly adjusted so that the same percolation threshold is obtained with corresponding regular network. During desorption, relative permeability is only a function of accessible pores. However, the trapped supercritical pores contribute to vapour occupancy during adsorption and not during desorption resulting in hysteresis loops in the corresponding volume of adsorbate. In the case of gas relative permeability experiments using different sets of mesoporous materials and adsorbates, when reduced relative permeability curves are considered in the vicinity of the percolation threshold, they all exhibit a universal behaviour regardless the topology of the porous medium and the nature of the adsorbate.

Effects of bed pressure drop on isothermal and adiabatic adsorber dynamics

Abstract A theoretical and experimental study on the effects of pressure drop on the dynamics of an isothermal fixed bed during adsorption is presented. The system of adsorbate-adsorbent used is oxygen in 13X-PSO 2 zeolite. Simple analytic solutions as well as detailed numerical solutions are compred with the experimental breakthrough curves. The theoretical study is further extended to the case of an industrial-size adiabatic fixed-bed adsorber. The complete numerical simulation involves the solution of heat, mass and momentum balances, using the Galerkin finite element method. An approximate analysis is also attempted using the theory of simple and shock waves. The first conclusion is that the presence of pressure drop leads to an early breakthrough for both concentration and thermal waves, as compared to the case with no pressure drop. Moreover, the constant pattern behavior of concentration and temperature that exists in the absence of pressure drop is distorted when sufficiently high pressure drop is present along the bed, because the concentration—temperature—pressure path in the hodograph space is no longer unique, but varies with time. Another significant conclusion is that the presence of pressure drop can either deflate or inflate the plateau zones of both concentration and temperature waves, depending on the values of certain bed parameters, and in all cases will facilitate the formation of a “pure” thermal wave.

Lithium Type X Zeolite as a Superior Sorbent for Air Separation

Abstract Li-X, Ba-X, and Li-A zeolites are prepared by ion exchange with the commercial Na-X (13X) and Na-A (4A) zeolites. At the ambient temperature (298 K), only Li-X exhibits a significant enhancement of the N2/O2 adsorption selectivity over the commercial zeolites. At a pressure of 1 atm, the N2/O2 selectivity is increased from approximately 3 for 13X and 4A to approximately 7 for Li-X. Air separations by the two sorbents Li-X and Na-X are compared using a commercial 5-step air-separation pressure swing adsorption (PSA) cycle. The comparison is made by using a mathematical model of proven predictive ability. A parametric comparison shows superior separation results for the Li-X zeolite under all PSA conditions. For example, the Li-X sorbent gives >90% O2 purity and an O2 recovery of over 70% at a feed rate of >5.65 s× 105 L STP/h. In addition, for the same O2 purity and feed rate, the Na-X sorbent gives an O2 recovery of less than 30%. Hence the replacement of the commercially used Na-X sorbent by the...

Combination of small angle scattering and three-dimensional stochastic reconstruction for the study of adsorption-desorption processes in Vycor porous glass

We study sorption and transport processes in dry and wet (preadsorbed with CH2Br2) Vycor glass by combining small angle scattering and three-dimensional (3D) stochastic reconstruction methods. Three-phase systems of solid, condensate, and void space, are generated for the first time, by the combination of the above methods. The resulting 3D images can visualize the evolution of the adsorption process and show how sorption alters the pore space characteristics of the material. Desorption is modeled in this system with the additional employment of an invasion percolation algorithm to account for the hysteresis effect caused by the inaccessible regions of the porous matrix. It is found that desorption is simulated very well provided that the main mechanism for hysteresis depends only on the topology of the pore space and not on thermodynamic effects. Based on a random-walk procedure, Knudsen transport properties of the reconstructed images are also determined for different degrees of saturation, providing ve...

Innovative methods for preparation and testing of Al2O3 supported silicalite-1 membranes

Abstract The aim of the present study is to develop a novel type of zeolite membrane and exploit some of the inherent and unique advantages of this membrane in gas separations. The development of inorganic membranes formed from a coherently grown layer of zeolite crystals is a particularly promising approach, since such membranes offer substantially higher permeabilities and selectivities compared with polymeric materials and can perform under extreme conditions, e.g. at elevated temperatures and in aggressive chemical environment. Such zeolite membranes are prepared by in-situ hydrothermal synthesis of zeolite layer within the macropores of alumina ceramic supports (tubes/discs). In this way more stable zeolite membranes are produced, avoiding outer separating layers which are prone to mechanical damage. These membranes have also been shown to have good thermal stability up to 500°C, with no evidence of crack formation. The synthesized materials have been characterized by several techniques such as Hg and N2 porosimetry, scanning electron microscopy (SEM), and X-ray diffraction (XRD). From these measurements it appears that the structure of the zeolite membrane is influenced by several factors, such as structure and size of the pores of the support, chemical composition of the precursor solution from which the crystals will be harvested, reaction conditions, etc. Finally, gas permeabilities and/or selectivities have been measured for several important gas systems. The results of this study indicate the important role of influencing the outcome of the crystal formation by controlling the conditions of the hydrothermal treatment in obtaining optimum gas permeabilities and/or selectivities of these systems.

Experimental investigation on separations of condensable from non-condensable vapors using mesoporous membranes

Abstract He/H 2 O and He/n-C 5 H 12 permeabilities as well as single-component permeabilities were measured on two mesoporous alumina membranes, in order to determine the optimum conditions for separating condensable from non-condensable vapors. The membranes were produced by compaction of non-porous alumina particles and exhibited similar pore structures but different porosities. The results show that, if the relative pressure of the condensable vapor in the condensable/non-condensable mixture is properly selected, very high selectivity and permeability values can be achieved simultaneously. The optimum relative pressure of the condensable vapor corresponds, in all cases studied, to the maximum condensable vapor permeability and to a relative gas permeability of zero. The pressure gradient of the condensable vapor across the membrane should be kept small, to avoid the formation of low permeability regions at either end of the membrane.

Application of effective medium approximation for the determination of the permeability of condensable vapours through mesoporous media

Single-bond effective medium approximation (EMA) is considered as an alternative method of modelling transport of condensable vapours in mesoporous structures. EMA is simpler and requires minimal computational time and effort compared to the network model. The effects of material structural parameters (pore size distribution, average pore radius, pore connectivity), fluid properties and operating conditions (relative pressure, total pressure drop) on vapour permeability are presented and compared with the corresponding results of the network model. In all cases, for a small pressure drop along the porous medium, there is an excellent agreement between EMA and network model. In general, EMA performs better for porous lattices with narrow pore size distributions, large fractions of small pores and high coordination numbers. The difference between the two models is detectable only in the transition to liquefaction regime, where the presence of the neighbouring pores becomes significant (neighbouring effect). When a high pressure drop is applied across the porous medium, single-bond EMA fails to be in close agreement with the network model. This is because a high pressure drop creates anisotropy in the network giving rise to a pressure distribution in the transverse direction. Consequently, a transverse flux is generated, moving along a direction which is perpendicular to the main longitudinal flux, resulting in considerably lower values of effective permeability. Finally, for a two-dimensional square lattice with a binary pore size distribution EMA was found to be in excellent agreement with the network model even near the percolation threshold. This is not surprising and relates to the well known fact that EMA predicts exactly the percolation threshold of a two-dimensional square lattice.