Effective approach to cyclic steady state in the catalytic reverse-flow combustion of methane

Computer-based simulations of a reverse-flow reactor should be carried out till the attainment of the so-called cyclic steady state. Usually this state is achieved by the method of direct dynamic simulations. In the paper of Unger et al. (Comput. Chem. Eng. 21 (1997) 5167.) special approaches, making use of various minimization algorithms based most often on Newton algorithms, are proposed. In the present paper one deals with a comparison and an appraisal of these methods, applied to the reverse-flow catalytic combustion of methane that occurs in coal-mine ventilation air.     

The finite element method application to linear closed loop steady system vibration analysis

An original method for steady, linear, mechanical closed loop system vibration analysis is presented in the paper. The finite element method, the abilities of which are extended, including the coupling element term, is applied to modelling. The problem of excited periodic vibration has been formulated and resolved numerically using the Fast Fourier Transform (FFT) for this purpose. An illustrative example is presented using a computer model of a real milling machine to consider stationary cutting conditions. Some comparative calculations for various types of feedback interaction are performed and time plots for steady-state vibration are obtained.     

Melting model for intermeshing counter‐rotating twin‐screw extruders

Previous experimental studies of melting of pellets in an intermeshing counter‐rotating twin‐screw extruder have shown that melting is initiated both between the screws and at the barrel. Models are developed for melting in both those regions. The melting between the screws is initiated by frictional work on the pellets by the calendering stresses between the screws. The melting action at the barrel is induced by a barrel temperature higher than the melting point and propagated by viscous dissipation heating of the melt film produced.     

Step-scan alternating DSC study of melting and crystallisation in poly(ethylene oxide)

Step-scan alternating differential scanning calorimetry (SSA-DSC) method was applied to investigate the phase behaviour of well-characterised poly(ethylene oxide) (PEO). Influence of the three main measurement's parameters of SSA-DSC method: length of the isothermal segment (t_iso/s), temperature jump between two subsequent isothermal segments (step/deg) and linear heating rate in dynamic segments (b/K/min), on the shape of reversing and non-reversing components during the melting and crystallisation of PEO, has been evaluated. It was found that the reversing component during melting of PEO is increasing with an increase of the isothermal segment length. This effect is due to the existence of defected polymer crystal structures that form metastable regions between crystal phase and already melted polymer. Reversible recrystallisation in the presence of still existing polymer crystals is facilitated by longer isothermal segments. By increasing the step, the equilibrium of reversible processes is shifted towards products and activation of rate-controlled processes takes place; molecular nucleation is hampered and partial melting and/or recrystallisation proceed slower—this effect can be observed as a decrease of reversing signal with increasing step.     

Super soft elastomers as ionic conductors

Melts of linear brush polymers with PEO side chains attached at each repeat unit of the backbones have been doped with CF₃SO₃⁻Li⁺. Mechanical properties and ionic conductivity of such systems have been analyzed using mechanical and dielectric spectroscopies. Mechanical spectra indicated a presence of super soft states for samples with long backbones or for systems which have been slightly cross-linked (G′<10⁴ Pa). In the case of the polymer with longer crystallizing PEO side chains (MW_av=1100 g/mol), the ionic conductivity reaching the 10⁻³ S/cm level at the optimum CF₃SO₃⁻Li⁺ concentration (EO/Li⁺=10:1) have been detected at temperatures not far above the room temperature. The presence of lithium ions suppresses completely the crystallization of PEO side chains.     

Luminescence study of europium（Ⅲ） tris（β-diketonato）/phosphonate complexes in chloroform

The extraction of Eu（Ⅲ） with β-diketone, HA, and monodentate or bidentate Lewis bases, B, into chloroform and the luminescence properties of the extracted species were studied. Pivaloyltrifluoroacetone, Hpta, and 2-thenoyltfifluoroacetone, Htta, were used as the β-diketones. The Lewis bases, B, were tetraethyl methylene diphosphonate, POPO, which was bidentate, and diethyl benzylphosphonate, PhPO which was monodentate. Based on the extraction data, the stability constants, log β, of the first complexes between tfis（β-diketonato）Eu（Ⅲ） and the phosphonate, EuA3B, were determined to be 6.0 for the POPO complex and 3.40 for the PhPO complex. The Eu（Ⅲ） luminescence intensity in the EuA3POPO was larger than EuA3 where A was either pta or tta at similar concentrations of Eu（Ⅲ）, while that in Eu（pta）3PhPO was stronger than EuA3; however, in Eu（tta）3PhPO, it was weaker than Eu（tta）3. The POPO functions as a sensitizer, and the PhPO functions as a quencher for the tta chelate and as a sensitizer for the pta chelate. From the lifetime and quantum yield, φ, of the Eu（Ⅲ） luminescence in the complexes as well as the observation of the extractability of Eu（Ⅲ） with the Hpta and the phosphonates and of the luminescence spectra of the complexes, it was confirmed that the extraction of Eu（Ⅲ） was remarkably enhanced with a β-diketonate and a strong Lewis base, and also the ternary complex that was formed as the extracted species, showed luminescence enhancement. This phenomenon may be due the formation of a strong bond between the Eu（Ⅲ） and the strong Lewis base leading to more hydrophobicity in the extracted species and also to more effective energy transfer from the Lewis base to the Eu（Ⅲ）. It was not significant whether the donor atoms were N or O.     

A simple and rapid HPLC technique for vanillin determination in alcohol extract

This paper describes a simple and rapid HPLC technique for vanillin determination in alcohol vanilla extract. Vanillin was separated on a Nucleosil C18 column by using water and methanol (40:60) as the mobile phase and retention time was only 2.2 min. The measurements were made by using a photodiode array detector of the most adequate maximum wavelength absorbance at 231 nm. This method has been successfully applied for the determination of vanillin in some commercial extracts.     

Unusual Quadrupedal Locomotion in Rat during Recovery from Lumbar Spinal Blockade of 5-HT7 Receptors

Coordination of four-limb movements during quadrupedal locomotion is controlled by supraspinal monoaminergic descending pathways, among which serotoninergic ones play a crucial role. Here we investigated the locomotor pattern during recovery from blockade of 5-HT₇ or 5-HT_2A receptors after intrathecal application of SB269970 or cyproheptadine in adult rats with chronic intrathecal cannula implanted in the lumbar spinal cord. The interlimb coordination was investigated based on electromyographic activity recorded from selected fore- and hindlimb muscles during rat locomotion on a treadmill. In the time of recovery after hindlimb transient paralysis, we noticed a presence of an unusual pattern of quadrupedal locomotion characterized by a doubling of forelimb stepping in relation to unaffected hindlimb stepping (2FL-1HL) after blockade of 5-HT₇ receptors but not after blockade of 5-HT_2A receptors. The 2FL-1HL pattern, although transient, was observed as a stable form of fore-hindlimb coupling during quadrupedal locomotion. We suggest that modulation of the 5-HT₇ receptors on interneurons located in lamina VII with ascending projections to the forelimb spinal network can be responsible for the 2FL-1HL locomotor pattern. In support, our immunohistochemical analysis of the lumbar spinal cord demonstrated the presence of the 5-HT₇ immunoreactive cells in the lamina VII, which were rarely 5-HT_2A immunoreactive.     

Hydrothermal Crystallization of Zirconia and Zirconia Solid Solutions

Zirconia as well as yttria-zirconia and calcia-zirconia solid-solution powders were crystallized under hydrothermal conditions from (co)precipitated hydroxides. The morphology of the powder particles is strongly dependent on the crystallization conditions. The powders crystallized in a water solution of Na, K, and Li hydroxides show elongated particles of much larger sizes than those which result from the process carried out in pure water or a water solution of Na, K, or Li chlorides. The shapes of the latter particles are isometric. The growth mechanism of the elongated particles is suggested.