N.K. Kanellopoulos

Development of porous structure during air drying of agricultural plant products

Abstract The porous structure of dehydrated plant materials is a key parameter that affects the transport properties and a number of quality characteristics of these materials. The porous structure may be determined by the bulk porosity, pore sizes and pore size distribution parameters. Helium pycnometry and mercury porosimetry were used for porosity and pore size distribution measurements, respectively. The porous structure was measured in four key plant materials: apple; potato; cabbage; and, carrot. The dehydrated materials were either air- or freeze-dried or they were dried partially by air and partially by freeze drying, therefore a wide variety of porosities and pore size distributions were measured. It was found that the freeze drying resulted in a very high bulk porosity, while a gradual involvement of air drying resulted in a decreasing porosity. Two discrete peaks of the pore size distribution function were found for potatoes, cabbage and apples and three peaks for carrots. The three peaks for the above materials were found at around 20 μm, 1 μm and 0-2-0-04 μn, depending on the material. The pore size of air dried materials was much smaller than the size of freeze dried samples, due to the collapse of structure in air dried samples during dehydration. The surface areas of freeze dried samples were of the order of 1 m2/g, which is typical value for dehydrated foodstuffs.

Prediction of binary adsorption isotherms of Cu2+, Cd2+ and Pb2+ on calcium alginate beads from single adsorption data

The binary adsorption of Cu(2+)-Cd(2+), Pb(2+)-Cd(2+) and Pb(2+)-Cu(2+) mixtures onto Ca-Alginate beads, prepared from Laminaria digitata, was studied using batch experiments. Competitive sorption models including extended Sips, extended Langmuir, Jain and Snoeyink modified Langmuir (JS modified) as well as Ideal Adsorpted Solution Theory (IAST) models were applied to predict the binary adsorption using single component adsorption parameters. The extended and the JS modified Langmuir approaches provide excellent prediction of the binary adsorption, while the extended Sips fails to predict the experimental data, giving only fair results in the case on Pb(2+)-Cu(2+) mixtures. On the contrary, the IAST models, though they are more complicated, provide less accurate estimation of sorption in binary metal ion solutions. In general, single component adsorption parameters can be effectively used for the prediction of a materials adsorption performance in binary metal ion solutions.

High pressure gas permeability of microporous carbon membranes

Abstract Microporous carbon membranes are prepared, characterised structurally and tested in terms of high pressure CO 2 permeability at temperatures around the critical. A maximum in the permeance versus relative pressure curve is observed in close analogy to the case of mesoporous membranes. This weakens considerably as the temperature is increased above the critical. The results offer significant input for an improved understanding and theoretical modelling of the process and may be potentially useful for the identification of the optimal pressure and temperature conditions for efficient gas separations.

Double-side active TiO2-modified nanofiltration membranes in continuous flow photocatalytic reactors for effective water purification.

A chemical vapour deposition (CVD) based innovative approach was applied with the purpose to develop composite TiO(2) photocatalytic nanofiltration (NF) membranes. The method involved pyrolytic decomposition of titanium tetraisopropoxide (TTIP) vapor and formation of TiO(2) nanoparticles through homogeneous gas phase reactions and aggregation of the produced intermediate species. The grown nanoparticles diffused and deposited on the surface of γ-alumina NF membrane tubes. The CVD reactor allowed for online monitoring of the carrier gas permeability during the treatment, providing a first insight on the pore efficiency and thickness of the formed photocatalytic layers. In addition, the thin TiO(2) deposits were developed on both membrane sides without sacrificing the high yield rates. Important innovation was also introduced in what concerns the photocatalytic performance evaluation. The membrane efficiency to photo degrade typical water pollutants, was evaluated in a continuous flow water purification device, applying UV irradiation on both membrane sides. The developed composite NF membranes were highly efficient in the decomposition of methyl orange exhibiting low adsorption-fouling tendency and high water permeability.

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 network model for the permeability of condensable vapours through mesoporous media

The paper aims at introducing a discrete (network) approach to modelling transport of condensable vapours in mesoporous structures. Such models possess the potential for improving the understanding of the mechanisms responsible for the observed transport behaviour. The basic elements of a typical pore network representing a mesoporous medium are summarized and the current state concerning the simulation of the related phenomena is given. The main processes involved (adsorption, mass diffusion, surface flow, capillary condensation) are simulated over the entire range of relative pressure. Finally, the effects of material structural parameters (average pore radius, pore size distribution and standard deviation, pore connectivity) and other relevant factors (relative pressure, temperature, resistance to surface flow, total pressure drop) on vapour permeability are presented and the future research directions are pointed out.

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.

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.

On the structure of an asymmetric carbon membrane with a novolac resin precursor

An asymmetric tubular carbon membrane, appropriate for gas separation applications, was made through carbonization at 800°C of a precursor structure containing two phenol-formaldehyde resins, a partially cured novolac resin in 30–60 μm grains (bulk material), and a resole resin (membrane skin material). A replica of the skin material was deposited separately on a stainless steel substrate. The samples were analyzed by nitrogen adsorption, small-angle neutron scattering, and scanning electron microscopy (SEM). The basic structural entities of both skin and the bulk part were low-aspect-ratio carbon domains with a characteristic dimension in the 4.0–4.5 nm range. Further, the materials were characterized by microporosity in the 0.30–0.50 range with isotropic pores having a 1.3 nm diameter. The results are discussed with the help of a systematic survey of possible carbon structures with an intermediate level of microporosity.