Hydrogen production in solar reactors

The present work summarizes the recent activities of our laboratory in the field of solar-aided hydrogen production with structured monolithic solar reactors. This reactor concept, “transferred” from the well-known automobile exhaust catalytic after-treatment systems, employs ceramic supports optimized to absorb effectively solar radiation and develop sufficiently high temperatures, that are coated with active materials capable to perform/catalyze a variety of “solar-aided” reactions for the production of hydrogen such as water splitting or natural gas reforming. Our work evolves in an integrated approach starting from the synthesis of active powders tailored to particular hydrogen production reactions, their deposition upon porous absorbers, testing of relevant properties of merit such as thermomechanical stability and hydrogen yield and finally to the design, operation simulation and performance optimization of structured monolithic solar hydrogen production reactors. This approach, among other things, has culminated to the world's first closed, solar-thermochemical cycle in operation that is capable of continuous hydrogen production employing entirely renewable and abundant energy sources and raw materials – solar energy and water, respectively – without any CO₂ emissions and holds, thus, a significant potential for large-scale, emissions-free hydrogen production, particularly for regions of the world that lack indigenous resources but are endowed with ample solar energy.     

Toward a unified framework for the derivation of breakage functions based on the statistical theory of turbulence

The evolution of droplet or bubble size distribution in turbulent flow is of great significance in a variety of technological fields. Modeling this evolution by employing a population balance approach requires knowledge of the so-called breakage functions (rate and kernel). Over the years, a large number of phenomenological breakage functions with various degrees of sophistication have been proposed in the literature. Among them, those based on the statistical theory of turbulence are of particular interest, in that they attempt to take into account the structure of the flow field responsible for breakage. The purpose of the present work is to present a unified framework for developing this type of breakage functions and to show how existing models can be derived in a systematic and consistent way. The key parameters in this modeling approach are identified, which have to be determined by comparison with experimental data. It is also shown that the breakage functions, obtained within the framework presented here, lead to predictions of a droplet size evolution whose main features are consistent with experimental observations. It is suggested that this framework is an important step toward the development of a standard approach for modeling droplet size evolution in turbulent flow.     

Solvothermal synthesis,nanostructural characterization and gas cryo-adsorption studies in a metal–organic framework (IRMOF-1) material

A nanoporous metal–organic framework material, exhibiting an IRMOF-1 type crystalline structure, was prepared by following a direct solvothermal synthesis approach, using zinc nitrate and terephthalic acid as precursors and dimethylformamide as solvent, combined with supercritical CO₂ activation and vacuum outgassing procedures. A series of advanced characterization methods were employed, including scanning electron microscopy, Fourier-transform infrared radiation spectroscopy and X-ray diffraction, in order to study the morphology, surface chemistry and structure of the IRMOF-1 material directly upon its synthesis. Porosity properties, such as Brunauer–Emmet–Teller (BET) specific area (～520 m²/g) and micropore volume (～0.2 cm³/g), were calculated for the activated sample based on N₂ gas sorption data collected at 77 K. The H₂ storage performance was preliminary assessed by low-pressure (0–1 bar) H₂ gas adsorption and desorption measurements at 77 K. The activated IRMOF-1 material of this study demonstrated a fully reversible H₂ sorption behavior combined with an adequate gravimetric H₂ uptake relative to its BET specific area, thus achieving a value of ～1 wt.% under close-to-atmospheric pressure conditions.     

Influence of phytosterol and phytostanol food supplementation on plasma liposoluble vitamins and provitamin A carotenoid levels in humans: An updated review of the evidence

Phytosterols and phytostanols (PAP) compete with cholesterol absorption in the intestine, resulting in a 5–15%-reduction in plasma total and LDL cholesterol. An important issue is the PAP potential to reduce the plasma concentrations of fat-soluble vitamins and provitamin A carotenoids. Here, an update of the scientific evidence is reviewed to evaluate plant PAP-enriched foods impact on plasma fat-soluble vitamins and carotenoid levels, and to discuss potential implications in terms of cardiovascular risk. Based on 49 human interventional and 3 bioavailability studies, results showed that regular consumption, particularly over the long term, of foods fortified with PAP as recommended in labeling does not significantly impact plasma vitamins A, D, and K concentration. A 10% significant median reduction was observed for α-tocopherol. Concerning carotenoids, while 13 studies did not demonstrate statistically significant plasma β-carotene reduction, 20 studies showed significant reductions, with median effect size of ?24%. This decline can be mitigated or offset by increased fruits and vegetables consumption. Furthermore, higher cardiovascular risk was observed for differences in plasma β-carotene concentration of the same magnitude as the estimated average decrease by PAP consumption. These results are supported by the only study of β-carotene bioavailability showing decrease in absorption by phytosterols daily intake.     

Effects of polyacrylamide‐co‐acrylic acid on cellulose production by Acetobacter xylinum

Polyacrylamide‐co‐acrylic acid (PA) added to shake flask cultures of Acetobacter xylinum at concentrations up to 3 g dm^?3 resulted in increased production of bacterial cellulose. For PA concentrations of 0–3 g dm^?3, 7‐day cellulose production rose monotonically from 2.7 ± 0.8 to 6.5 ± 0.5 g dm^?3 at a shaker speed of 175 rpm, and from 1.7 ± 0.01 to 3.7 ± 0.5 g dm^?3 at shaker speed of 375 rpm. Addition of PA also changed the morphology of the biomass from amorphous/stringy forms to spheroidal particles with diameters ≤2 mm. Similarly, bioreactor cultures grown in the absence of PA formed long fibrous masses which deposited on the internals, while those grown in the presence of 1–2 g dm^?3 PA formed small discrete particles with diameters ≤0.1 mm. Tests performed with 1 and 2 g dm^?3 PA, and stirrer speeds from 500 to 900 rpm, appeared to give the highest cellulose concentration of 5.3 ± 0.7 g dm^?3 in 64–68.5 h in the presence of 2 g dm^?3 PA at 700 rpm, although this value was statistically indistinguishable from that obtained at 1 g dm^?3 PA and 900 rpm. A qualitative model is proposed to describe the mechanisms by which PA affects biomass morphology, resulting in its advantageous formation as small, dispersed, spheroidal pellets. Quantitative analysis of the results gave inverse correlations between both the fraction of fructose carbon going to cellulose synthesis and the specific fructose consumption rate, and the maximum cellulose concentration and the fraction of fructose carbon going to by‐product formation. Since cellulose yield was almost universally improved by higher polyacrylamide concentration, it appears likely that increased viscosity reduces fructose uptake rate by limiting mass transfer. Copyright © 2003 Society of Chemical Industry     

Miscibility of some chlorinated poly(hydrocarbons) with ethylene-acrylic rubber

The compatibility of an ethylene-acrylic rubber (R) with poly (chloroprene) (CR) and two chlorinated poly-ethylenes, containing 48 (CPE 48) and 25 (CPE 25) wt.-% chlorine, was investigated. Blends with the latter polymer were studied in the complete composition range. The techniques used were phase-contrast microscopy, differential scanning calorimetry, dynamic mechanical and stress-strain testing. The single T_g relaxation of blends and its almost linear variation with composition, together with results obtained using the techniques mentioned, support the view that the systems are miscible at the segmental level. At low rubber contents an antiplasticisation effect was observed for the R/CPE 25 system. At high rubber compositions a small reduction of crystallinity and a melting point depression of the PE phase in CPE were observed. Various equations proposed to predict the T_g of blends and their modulus using pure component data were also tested at varying compositions and temperatures.     

Setting Up a Numerical Model of a DAF Tank: Turbulence, Geometry, and Bubble Size

This paper discusses the modeling framework and identifies a number of parameters relevant when setting up a computational fluid dynamics simulation of a dissolved air flotation (DAF) tank. The selection of a turbulence model, the choice between performing two-dimensional (2D) or three-dimensional (3D) simulations, the effects of the design of the flow geometry and the influence of the size of the air bubbles are addressed in the paper. The two-phase flow of air and water is solved in the Eulerian-Lagrangian frame of reference. The realizable k-ε model with nonequilibrium wall functions is suggested as a compromise between a need to effectively resolve the flow and the cost of the simulations. There is a discussion on the conditions for which the steady-state simulations are appropriate. We demonstrate that a steady 2D model can simulate a stratified flow pattern. Our results show that 2D models require adjustments in geometry (e.g., substitution of the outlet pipes to an outlet distributed over the total width of the tank) and in the parameters governing the flow in order to account for the true 3D nature of some of the flow patterns. In addition, we show that the bubble size has a larger influence on the flow in the separation zone than in the contact zone.