A.Ch. Mitropoulos

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...

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.

Novel design for high pressure, integral, differential, absolute, and relative multicomponent permeability measurements

A high pressure apparatus of novel design, used for the performance evaluation of ceramic and polymeric membranes, is presented. The apparatus can test the performance of several membranes simultaneously and provide significant time saving by reducing the time required for the equilibration and mixing of gases. The apparatus is capable of providing measurements of single and multicomponent permeability for pressures up to 70 bar, under integral or differential modes of operation.

A novel experimental technique for the measurement of the single-phase gas relative permeability of porous solids

A novel design of a single-phase item of equipment, capable of providing satisfactory relative permeability data at low relative equilibrium pressures, is presented. Following certain experimental procedure, the complications of the two-phase flow and that of the macroscopic inhomogeneity have been reduced. Thus it becomes easier to utilize the powerful relative permeability technique for the characterization of porous solids.

Coal studies by small angle X-ray scattering

Two highly volatile bituminous coal samples were characterized by adsorption and small-angle X-ray scattering (SAXS). The SAXS measurements were repeated on samples that had been brought to equilibrium at various relative pressures of carbon tetrachloride which has a similar electron density as the coals. The validity of this combined technique was ascertained by reconstructing the adsorption isotherms from the SAXS data. A theoretical approach connecting the two methods was considered, and simple formulae were derived. The reconstruction of the adsorption isotherm is influenced by several parameters. Adsorption in conjunction with SAXS can provide information on the pore structure, the adsorption mechanism, and the swelling of the solid matrix upon adsorption.

Water adsorption and small angle X-ray scattering studies on the effect of coal thermal treatment

Abstract The effect of thermal treatment, at a relatively high temperature (∼- 340 °C), on two highvolatile bituminous coals is examined by water adsorption and small angle X-ray scattering (SAXS). The results suggest that the mesopore structure in both samples undergoes a partial collapse. Furthermore, a strong hydrophobic effect, at the very early stages of the adsorption process, is noted for only one of them. This observation is attributed to differences in the rigidity of each of the structures. Analysis of the spectra in terms of fractal geometry is also performed. It is proposed that thermal treatment changes the coal matrices from a dilute to a dense configuration. Heating causes a breakage of some oxygen and sulphur bridges giving rise to these structural alterations.

The combination of equilibrium and dynamic methods for the detailed structural characterisation of ceramic membranes

The combination of equilibrium and dynamic techniques provides powerful tools for the characterisation of the porous structure of ceramic membranes. We present the type of information obtainable by the permeability, adsorption and small angle scattering (SAS) methods and important combinations of them. Subsequently, we apply selected combinations of the above methods to three characteristic ceramic membrane materials: (a) a mesoporous alumina produced by symmetric compaction, (b) a Vycor®-type mesoporous silica, and (c) an asymmetric carbon membrane with a microporous gas-separating layer. Important conclusions are drawn regarding the pore and internal surface morphology of the studied membranes.

Neutron diffraction study of adsorbed CO2 on a carbon membrane

Abstract A novel high-pressure device allowing adsorption of gases in situ with neutron diffraction is constructed. By using a carbon membrane the diffraction pattern of the adsorbed CO2 is obtained.

Neutron scattering from water adsorbed on an alumina membrane

Abstract The pore structure of an alumina membrane was studied by adsorption in conjunction with small-angle neutron scattering (SANS). The effect of an incomplete contrast matching was investigated and simple formulae allowing the calculation of the amount of water adsorbed and the film areas, at various relative pressures, were derived for the ternary system. In the vicinity of the contrast matching point, however, incomplete contrast matching does not introduce a substantial complication to data analysis. The adsorption isotherm was reconstructed from the SANS data and agrees well with the experimental adsorption isotherm. For the alumina membrane, a coordination number of ≈10 was estimated from the SANS determined areas.

Small-angle X-ray scattering studies of adsorption in Vycor glass

Porous Vycor is examined by measuring CH(2)Br(2) adsorption in situ with small-angle X-ray scattering. When a class of pores fills with condensed vapors of this particular adsorbate it ceases to act as scatterer and only the remaining empty pores produce a measurable intensity. By determining a number of scattering curves at various relative pressure loadings details on the structure of the glass as well as on the adsorption/desorption mechanism are obtained. Pore chord length and specific surface area are estimated from Porod tangent analysis to 78 A and 108 m(2)/g. Comparison of the results with those reported for N(2) and Ar adsorption is also given. The role of network effects on the shape of the hysteresis loop is considered and the pore-blocking hypothesis is verified from the scattering spectra. The pore connectivity is calculated to 5.6. The X-ray data are further treated with the inverse Fourier transformation technique. Pore-size distributions are extracted and weighed against the prediction of the Kelvin equation.