The effect of porosity and number of unit cell on applicability of volume average approach in closed cell porous media

Purpose The purpose of this study is to investigate the applicability of volume average which is extensively used for analyzing the heat and fluid flow (both for single-phase and solid/liquid-phase change) in a closed cell porous medium numerically. Design/methodology/approach Heat conduction equations for the solid frame and fluid (or phase change material) are solved for pore scale and volume average approaches. The study mainly focuses on the effect of porosity and the number of porous media unit cell on the agreement between the results of the pore scale and volume average approaches. Findings It is observed for the lowest porosity values such as 0.3 and the number of porous media unit cell as 4 in heat transfer direction, the results between two approaches may be questionable for the single-phase fluid. By increasing the number of porous media unit cell in heat transfer direction, the agreement between two approaches becomes better. In general, for high porosity values (such as 0.9) the agreement between the results of two approaches is in the acceptable range both for single-phase and solid/liquid-phase change. Two charts on the applicability of volume average method for single-phase and solid/liquid-phase change are presented. Originality/value The authors’ literature survey shows that it is the first time the applicability of volume average which is extensively used for analyzing the heat and fluid flow in a closed cell porous medium is investigated numerically.

Hydrogenation of benzoic acid derivatives over Pt/TiO2 under mild conditions

Hydrogenation of benzoic acid (BA) to cyclohexanecarboxylic acid (CCA) has important industrial and academic significance, however, the electron deficient aromatic ring and catalyst poisoning by carboxyl groups make BA hydrogenation a challenging transformation. Herein, we report that Pt/TiO2 is very effective for BA hydrogenation with, to our knowledge, a record TOF of 4490 h−1 at 80 °C and 50 bar H2, one order higher than previously reported results. Pt/TiO2 catalysts with electron-deficient and electron-enriched Pt sites are obtained by modifying the electron transfer direction between Pt and TiO2. Electron-deficient Pt sites interact with BA more strongly than electron-rich Pt sites, helping the dissociated H of the carboxyl group to participate in BA hydrogenation, thus enhancing its activity. The wide substrate scope, including bi- and tri-benzoic acids, further demonstrates the high efficiency of Pt/TiO2 for hydrogenation of BA derivatives.

Hydrodynamics and mass transfer in Kühni extraction columns

This work provides a review of hydrodynamic characteristics and mass transfer in the K?hni extraction columns. The experiments, as reported in the literature, were performed in the presence and absence of mass transfer. The results showed that the Sauter mean drop diameter was strongly affected by the rotor speed and interfacial tension, whereas the effects of the dispersed and continuous velocities were negligible. Empirical correlations for the Sauter mean drop diameter, taken from the literature, were discussed. It was experimentally determined that the dispersed-phase holdup depended to a great extent on the rotor speed, mass transfer direction between the phases, physical characteristics of fluids in the liquid-liquid system, and the dispersed-phase flowrate whereas it increased with the increase in mixing in the two-phase system and the ratio of phase flowrates. On the other hand, it has been shown that the mass transfer rate increases with increasing the level of back mixing. It was found that the mass transfer coefficient depends on the rotor speed and the direction of mass transfer between the phases. At the same time, it has been shown that the mass transfer coefficient depends relatively little on the phase flowrates. An empirical correlation was proposed for prediction of the overall mass transfer coefficient based on dimensionless numbers. Also, novel empirical correlations for prediction of the Sherwood number in the continuous phase were presented based on the dispersed-phase holdup, Reynolds number, and mass transfer direction between the phases. Empirical correlations based on dimensionless numbers can be considered as a useful tool for the design of the K?hni columns.

Pt/TiO2 Catalyzed Hydrogenation of Benzoic Acid with Unprecedented High Activity

The hydrogenation of benzoic acid (BA) to corresponding cyclohexanecarboxylic acid has important industry and academia significance, however, the electron deficient aromatic ring and the catalyst “poison” by carboxyl group make BA hydrogenation as one of the most challenging transformations. Herein, we found that Pt NPs deposited on TiO2 were very effective for BA hydrogenation with a record TOF of 4490 h− 1 under 80 oC and 50 bar H2 in hexane, one order higher than the reported results. DFT calculation showed that Pt NPs had a weaker interaction with BA than Ru and Pd NPs commonly used for BA hydrogenation, which improved the toxicity resistance of catalyst to BA. Pt/TiO2 catalysts with electron deficient and electron enriched Pt sites were successfully synthesized by modifying the electron transfer direction between Pt and TiO2. Isotopic experiments suggested the participation of dissociated H from carboxyl group in BA hydrogenation. Consequently, the electron deficient Pt sites with stronger adsorption of BA were more active than electron rich Pt sites in BA hydrogenation. In addition to BA, terephthalic acid, iso-phthalic acid, trimesic acid and other BA derivatives could also be efficiently converted to corresponding aromatic saturated products, demonstrating the wide substrate scope of Pt/TiO2.

Unusual Breathing Behavior of Optically Excited Barium Titanate Nanocrystals

Coherent X-ray diffraction patterns were recorded by using an X-ray free-electron laser to illuminate barium titanate nanocrystals as a function of time delay after laser excitation. Rather than seeing any significant thermal expansion effects, the diffraction peaks were found to move perpendicular to the momentum transfer direction. This suggests a laser driven rotation of the crystal lattice, which is delayed by the aggregated state of the crystals. Internal deformations associated with crystal contacts were also observed.