Athanasios K. Stubos

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.

Diffusion and Flow in Porous Domains Constructed Using Process-Based and Stochastic Techniques

Two model mesoporous materials, namely Vycor glass and alumina membrane made by compaction of spherical monosize alumina particles, are reconstructed and the transport properties (Knudsen diffusivity, molecular diffusivity and permeability) of the resulting 3-dimensional binary domains are investigated through computer simulations. For the alumina membrane, two reconstruction alternatives are used, a ballistic deposition of spherical particles (process-based approach) and a stochastic procedure based on the porosity and the two-point correlation function of the porous matrix. For Vycor glass, only the stochastic reconstruction technique is employed. The obtained 3-D samples are structurally characterized in terms of their two-point correlation and chord length distribution functions. This type of information along with the comparisons between computed and reported transport coefficients indicate that the random sphere pack obtained from the ballistic-deposition procedure represents quite well the porous structure of the alumina membrane while the stochastically reconstructed domain fails to do so. In contrast, for the Vycor glass it is shown that the stochastic reconstruction technique is sufficient for a faithful representation of the porous matrix. It is furthermore argued that in the case of Vycor the stochastic reconstruction technique constitutes a kind of process-based approach.

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.

Modeling and Simulation for Absorption- Desorption Cyclic Process on a Three-Stage Metal Hydride Hydrogen Compressor

Abstract In the current work a mathematical and a simulation study on a three – stage metal hydride hydrogen compressor (MHHC) is presented. Multistage MHHC uses a combination of different materials acting as metal hydrides in order to increase the final compression ratio, while maximizing the absorption capacity of every single stage supply pressure. The performance of a MHHC can be predicted by solving simultaneously the heat, mass and momentum differential equations. The materials used for the current study are LaNi 5 , MmNi 4.6 Al 0.4 and Ti 0.99 Zr 0.01 V 0.43 Fe 0.99 Cr 0.05 Mn 1.5 . This three – stage compression system yields a pressure ratio of 25:1, for supply conditions 20 ° C and 5 bar. The delivery pressure achieved is 115 bar for 100 ° C desorption temperature.

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.

Characterisation of porous solids by simplified gas relative permeability measurements

A novel design for the application of a simplified experimental procedure, capable of providing satisfactory relative permeability data for porous media at low relative pressures, is presented. The technique avoids the two-component mixing complications as well as the problems arising from the sample macroscopic inhomogeneity. The latter has been reduced with the aid of a variable pressure compaction procedure. Thus, it becomes easier to utilise the powerful relative permeability technique for the characterisation of the structure of porous solids. Emphasis is given to the combination of the relative permeability data obtained with an analytical approach based on Effective Medium Theory for the calculation of pore connectivity.

Two-Stage Hydrogen Compression Using Zr-Based Metal Hydrides

Zr-based AB2-Laves phase type alloys containing the same type of A and B metals, have been prepared from pure elements by melting and subsequent re-melting under argon atmosphere by using a HF-induction levitation furnace. Characterization of the alloys has resulted from powder X-Ray Diffraction (XRD) measurements and SEM/EDX analyses. Systematic PCI (Pressure-Composition-Isotherms) measurements have been recorded at 20 and 90 °C, using a high-pressure Sieverts type apparatus. The purpose of this study is to find a series of alloys promptly forming metal hydrides (MH) with suitable properties in order to build a MH-based hydrogen compressor, working in the same way between 20 and ~100 °C.

Development and evaluation of materials for thermochemical heat storage based on the CaO/CaCO3 reaction couple

The current work relates to the development of synthetic calcium oxide (CaO) based compositions as candidate materials for energy storage under a cyclic carbonation/decarbonation reaction scheme. Although under such a cyclic scheme the energy density of natural lime based CaO is high (∼ 3MJ/kg), the particular materials suffer from notable cycle-to-cycle deactivation. To this direction, pure CaO and CaO/Al2O3 composites have been prepared and preliminarily evaluated under the suggested cyclic carbonation/decarbonation scheme in the temperature range of 600-800°C. For the composite materials, Ca/Al molar ratios were in the range between 95/5 and 52/48 and upon calcination the formation of mixed Ca/Al phases was verified. The preliminary evaluation of materials studied was conducted under 3 carbonation/decarbonation cycles and the loss of activity for the case of natural CaO was obvious. Synthetic materials with superior stability/capture c.f. natural CaO were further subjected to multi-cyclic carbonation/d...

A novel thermochemical energy storage and transportation concept based on concentrated solar irradiation-aided CaO-looping

To overcome the temporal and regional gap of surplus solar energy, the concept of thermochemical heat storage is discussed. In this particular case, the application of CaO and CaCO3 as energy carrying compounds for a trans-regional energy distribution concept is analyzed regarding the effective energetic and exergetic storage density. In a comprehensive sensitivity analysis, the influences of reaction temperature, conversion and heat recovery strategies are worked out. It can be seen that the effective storage density is strongly influenced by the preheating of reactants from ambient to reaction temperature. Thus, high conversion rates during forward and reverse reaction as well as improved heat recovery ratios are necessary to achieve a high energetic storage density. In case of effective exergetic storage density, carbonation temperature reaches an optimum. The method presented in this contribution can be applied to similar thermochemical heat storage systems and the results are of great importance for ...