Search results for " Chemical Engineering"
showing 10 items of 2965 documents
Modelling and Simulation of Gas–liquid Hydrodynamics in a Rectangular Air-lift Reactor
2013
Abstract Computational Fluid Dynamics is a quite well established tool for carrying out realistic simulations of process apparatuses. However, as a difference from single phase systems, for multiphase systems the development of CFD models is still in progress. Among the two-phase systems, gas–liquid systems are characterised by an additional complexity level, related to the fact that bubble sizes are not known in advance, being rather the result of formation and breakage-coalescence dynamics and therefore of complex phenomena related to flow dynamics and interfacial effects. In the present work, Euler–Euler Reynolds-averaged flow simulations of an air-lift reactor are reported. All bubbles …
Improvement of boehmite nanoparticles' aqueous dispersability by controlling their size, shape and crystallinity
2014
In this work, results on the control of size, shape, crystallinity and aqueous dispersability of boehmite prepared by a hydrothermal process are reported. The two step synthetic procedure entailed the precipitation of a xerogel by adding NaOH until pH 10 to a solution of aluminum nitrate at 100 °C without or with additives such as tartaric acid and maltitol, and a subsequent hydrothermal treatment at 150 and 200 °C for different periods of time. The final materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and elemental analysis. The stability and other characteristics of the boehmite nanoparticle aqueous dispersions were determined by measuring t…
Dipole moments of 4′-aminoflavonols determined using electro-optical absorption measurements or molecular Stark-effect spectroscopy
2002
The electro-optical absorption measurements (EOAM) were used to measure the dipole moments of the normal form of 4 � -(methoxy)-3hydroxyflavone (FOM), 4 � -(dimethylamino)-3-hydroxyflavone (FME), and 4 � -N-(15-azacrown-5)-3-hydroxyflavone (FCR). For these probes the excited state intramolecular proton transfer (ESIPT) takes place. For comparison, the dipole moments of 4 � -(dimethylamino)-3methoxyflavone (FME3ME), for which ESIPT is lacking, were measured. In the case of FCR, FME, and FME3ME the equilibrated ground (µg) and excited Franck–Condon state (µ FC ) electrical dipole moments are parallel to each other and also parallel to the transition dipole moment. The electrical dipole moment…
Structural symmetry breaking in octupolar tetrastyrylpyrazines and their dipole moments in equilibrium ground and Franck-Condon excited state
2018
Abstract From electrooptical absorption measurements (EOAM) follows that the dipole moment of octupolar tetrastyrylpyrazines in the equilibrium ground state is large and that the change of dipole moments upon transition to the excited Franck–Condon state is significant. Obtained results unambiguously testify to structural symmetry breaking in the studied octupolar tetrastyrylpyrazines. Molecular mechanics and semi-empirical calculations evidence that tetrastyrylpyrazines have non-planar configurations. The non-planar geometry of the molecules causes large μg values and significant change of dipole moment Δaμ after excitation. Due to large μg and Δaμ values the maxima of the first absorption…
Dipole moments of aminophthalimides determined by modified electro-optical absorption and emission measurements
1995
Modified electro-optical absorption and emission methods were used to measure the electric dipole moments of five aminophthalimides in their ground and excited states. The equilibrated ground and excited state dipole moments measured by these methods are in reasonable agreement with those derived from other measurement techniques, confirming the general validity of these methods. A difference is observed between the properties of 3-aminophthalimide (3AP) and 4-amino-N-methylphthalimide (4ANMP) in different solvents. The equilibrated dipole moment of 3AP in its excited state is practically independent of the solvent polarity in contrast with 4ANMP. The possible mechanism of this effect is di…
Dipole Moments and the Direction of the Transition Dipole Moment of Some Intramolecular Exciplexes
1988
The dipole moments of some intramolecular exciplex systems have been determined from the effect of an electric field on their fluorescence and are compared to those estimated from solvent shift measurements. The charge transfer between the donor and acceptor subunits is between 0.6 and 0.9 of a full charge transfer. The face-to-face type exciplexes show considerable deviations from the general behaviour what might be due to some admixture of locally excited configurations to the exciplex wave function, in this way reducing the dipole moment values and changing the direction of the transition dipole moment.
Considerations on the dipole moment of molecules forming the twisted intramolecular charge transfer state
1992
Several derivatives of dual-fluorescing dimethylaminobenzonitrile (DMABN) are compared regarding their ground and excited-state dipole moments extracted from electro-optical emission and solvatochromic measurements. The comparison includes bridged model compounds as well as compounds with different degrees of steric hindrance to planarity, as well as ester and carbazolyl derivatives. The results are discussed on the basis of the twisted intramolecular charge transfer (TICT) model, the most widely accepted model. They show that (i) the dipole moment of the A (TICT) state (long-wavelength band) shows only little solvent sensitivity (polarizability), (ii) this observed small dependence of the …
Electronic Properties of Mixed-Valence Manganates: The Role of Mn Substitutional Defects
2002
Single-phase perovskites in the solid solution series La0.7+yA0.3-yMn1-xMxO3 (with 0.00 ≤ x ≤ 0.10; A = Sr2+, M = Cu2+, Zn2+, Sc3+, Cr3+, Co3+, and Ga3+; A = Ba2+, M = Cu2+, Zn2+, and Sc3+) have been prepared via the acetic acid solutions freeze-drying method. This soft procedure makes possible strict stoichiometric control, and the synthetic variables allow one to maintain a constant proportion of Mn4+ (ca. 32%) in the 47 compounds prepared. In this way, the concentration of cationic vacancies at A and B sites is practically negligible in all cases. X-ray powder diffraction patterns corresponding to the 47 compounds have been completely indexed with rhombohedral perovskite cells. The cryst…
Synthesis, crystal structure and magnetic properties of the complex [ReCl3(tppz)]·MeCN
2015
The reaction of the starting materials [ReIIICl3(MeCN)(PPh3)2] or [ReVOCl3(PPh3)2] with 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) in acetonitrile yielded the Re(III) complex [ReCl3(tppz)]·MeCN (1). This complex crystallizes in the monoclinic space group P21/n and its crystal structure consists of neutral mononuclear entities with meridional geometry of the chloride ligands, and the six-coordination of the Re(III) ion being completed by the tridentate tppz ligand. Each metal centre exhibits a highly distorted octahedral coordination with Re–Cl and Re–Ntppz bond lengths covering the ranges 2.3590(9)–2.3606(8) and 1.971(2)–2.096(2) A, respectively. The magnetic properties of 1 have been inves…
Surface water RO permeate remineralization through minerals recovery from brines
2022
Assisted-Reverse Electrodialysis (A-RED) technology was applied following reverse osmosis (RO) of a surface water resource in order to recover minerals from its brine and directly remineralize the RO unit's permeate. Four different sets of cation/anion exchange membranes were benchmarked using single- and mixed-salts synthetic solutions as well as real brine and permeate streams produced from three-stage reverse osmosis applied to Seine River water. The process, operating under equal permeate and brine channel flows (2 cm/s velocities) and applied voltage varying from 0 to 10 V, showed viable remineralization results. Optimal recovery at 10 V applied allowed increasing permeate mineral cont…