Study of nonisothermal gas-oil catalytic cracking applying the microactivity test

Catalytic cracking of gas oils has been studied in a standard microactivity test (MAT) reactor. The cracking product distribution was measured as a function of temperature. Based on these experimental results, a four-lump kinetic model was developed. Kinetic constants were estimated using the sequential step-optimization method. A nonisothermal nonsteady-state model for a fixed-bed MAT reactor was proposed. The overall heat of the reactions were taken from the macroscopic differences in the enthalpies of the products and reactants. The influence of the feedstocks used and reactor temperature were discussed. The reactor and kinetic model were validated with results from MAT test data. The simulation results are in good agreement with the experimental data.     

Macroscopic modelling of prestressed microperiodic elastic media

Summary. The aim of this contribution is to formulate and apply a macroscopic model for the analysis of the dynamics and stability of prestressed microperiodic elastic media. This modelling problem has been solved by Kolpakov [1] using the asymptotic homogenization method. However, the method used in [1] neglects the effect of the period length on the macroscopic (overall) solid behavior. To eliminate this drawback we propose a nonasymptotic modelling approach which is a certain generalization of the tolerance averaging method recently presented in [2] and a series of papers. The presented general results are illustrated by the analysis of a certain special problem.     

Mechanical,thermal, and barrier properties of NBR/organosilicate nanocomposites

Nanocomposites of intercalated and exfoliated organosilicates in acrylonitrile butadiene rubber (NBR) were prepared by a solution‐blending method. The dispersion and intergallery spacings of organosilicates in these nanocomposites were examined by transmission electron microscopy and X‐ray diffraction. Dramatic enhancements in the mechanical and thermal properties of NBR are found by incorporating less than ten parts of organosilicate. In particular, the addition of ten parts of organosilicate provided a more than sixfold increase in tensile strength, a twofold increase in M500, and 168% and 39% enhancements in tear strength and elongation at break compared with pure NBR. The degradation temperature for NBR with ten‐parts' loading of organosilicate was 25°C higher than that of pure NBR. In addition, the relative vapor permeability of the NBR nanocomposites for water and methanol were 85% and 42% lower, respectively, than that of pure NBR. Polym. Eng. Sci. 44:2117–2124, 2004. © 2004 Society of Plastics Engineers.     

Process kinetics of UASB reactors treating non‐inhibitory substrate

The degradation of a non‐inhibitory substrate (sucrose) in upflow anaerobic sludge bed (UASB) reactors with different superficial flow velocites (u_s) was performed to generate experimental data. Additionally, a kinetic model accounting for the mass fraction of methanogens (f) and granule size distribution in UASB reactors is also proposed. At the volumetric loadings of 2.65–21.16 g COD dm^?3 day^?1, both the COD removal efficiency and granule size of the UASB reactors increase with increasing u_s. The f values determined experimentally increase from 0.13–0.24 to 0.27–0.43 if the volumetric loading is increased from 2.65 to 5.29 g COD dm^?3 day^?1. With a further increase in volumetric loading, the f values decline because of the accumulation of volatile fatty acids (VFAs). The predicted residual concentrations of VFAs and COD are in fairly good agreement with the experimental data. From the calculated effectiveness‐factor values, the influence of mass transfer resistance of the substrate sucrose on the overall substrate removal rate should not be neglected. From parametric sensitivity analyses together with the simulated concentration profiles, methanogenesis is the rate‐limiting step. Copyright © 2003 Society of Chemical Industry     

Improvement of the thermal stability of polyhydroxybutyrates by grafting with maleic anhydride by different methods: Differential scanning calorimetry,thermogravimetric analysis,and gel permeation chromatography

The crystallization and thermal degradation behaviors of polyhydroxybutyrate (PHB) grafted with maleic anhydride (MA) by different techniques were analyzed with differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and gel permeation chromatography (GPC). The results of DSC, TGA, and GPC analyses indicated that the grafting method could affect the crystallization rate, crystallinity, and thermal stability of PHB because of changes in the molecular weight of PHB and the amount of MA grafted during the reaction. The reduction of the molecular weight of PHB that reacted during the processing followed this order of methods: melt grafting > solvent grafting > mechanical grafting. However, the grafting ratio of MA followed this order of methods: melt grafting > mechanical grafting > solvent grafting. All three grafting methods significantly improved the thermal stability, therefore increasing the crystallization rate and melting temperature of the as‐received PHB. A grafting ratio of MA as low as 0.07 wt % could result in a significant improvement in the heat resistance of PHB. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The electrical model of polycrystalline CdSe Thin films

Studies of the electrical properties of polycrystalline CdSe thin films evaporated onto ruby substrates show that the mobility of charge carriers depends on the longitudinal electric field in some ranges of the field intensity. A structural model of a polycrystalline thin film has been developed by other authors. A diode model for the boundary regions has also been used.By analogy with other work we decided to construct an electrical model of the CdSe films that were studied. The proposed model contains two kinds of elements—diodes and resistors. This model was analysed numerically to verify its static electrical characteristics. The qualitative resemblance of the I-U, U_b-U and U_m-U characteristics of the model and the real sample (where U_b is the total voltage drop on all grain boundary regions and U_m the total voltage drop on all microcrystallites) is shown. It seems that after development the proposed electrical model can be used with advantage in the analysis of the electrical properties of semiconductor polycrystalline thin films.     

Analysis of Possibilities of Application of Nanofabricated Thermal Probes to Quantitative Thermal Measurements

A steady-state thermal model of the nanofabricated thermal probe was proposed. The resistive type probe working in the active mode was considered. The model is based on finite element analysis of the temperature field in the probe-sample system. Determination of the temperature distribution in this system allows calculations of relative changes in the probe electrical resistance. It is shown that the modeled probe can be used for measurements of the local thermal conductivity with the spatial resolution determined by the probe apex dimensions. The probe exhibits the maximum sensitivity to the changes in the thermal conductivity of the sample between 2 W·m⁻¹ ·K⁻¹ and 200 W·m⁻¹ ·K⁻¹. The influence of the thermal conductivity of the probe substrate on metrological characteristics of the probe as well as the thermal resistance of the probe-sample contact on the determination of the sample thermal conductivity were also analyzed. The selected results of numerical analysis were compared with data of preliminary experiments.     

Electrochemical behavior of Ni3Al-based intermetallic alloys in NaOH

The corrosion behavior of Ni₃Al-based intermetallic alloys in a 0.5 M NaOH solution was studied at 25 °C. The open circuit potential, cathodic and anodic potentiodynamic polarization, Tafel plots and linear polarization resistance measurements were used to characterize the corrosion behavior. For the Ni₃Al(B, Zr) alloy, potentiodynamic polarization curves showed a wide passive region that can be found between about ?0.220 V_SCE and 0.520 V_SCE. On the other hand, a narrow passive region, in the range of potentials from about ?0.180 V_SCE to 0.180 V_SCE, was observed for the Ni₃Al(B, Zr, Cr, Mo) alloy. Chromium, as an alloying element in the Ni₃Al(B, Zr, Cr, Mo) alloy, contributes to transpassive dissolution of the passive film at much lower anodic potentials and remarkably reduces the passivation region. The experiments indicated also that damaged passive films on alloys repairs itself and pits do not initiate. The surface of both alloys and passive films possess extremely high corrosion resistance in a studied solution. However, Tafel and linear polarization tests revealed that freshly exposed surfaces of the Ni₃Al(B, Zr) alloy exhibited better corrosion resistances than the Ni₃Al(B, Zr, Cr, Mo) alloy. Both methods, used for the determination of corrosion rates gave very similar results. The calculated corrosion rates are about 2.8 ·10^?3 and 6.0·10^?3 mm year^?1 for the Ni₃Al(B, Zr) alloy and B, respectively.     

Characterization of Alumina Scales Grown on a 2nd Generation Single Crystal Ni Superalloy During Isothermal Oxidation at 1050, 1100 and 1150 °C

This paper concerns microstructural characterization of alumina scales formed on a 2nd generation single crystal Ni superalloy during isothermal oxidation in dry oxygen at 1050, 1100 and 1150 °C for 100 h. Samples for high resolution characterization of Al₂O₃ scales were prepared using a focused ion beam (FIB) method. High resolution TEM and S/TEM techniques were used for a detailed characterization and a direct comparison of the phase composition and chemistry of the oxide scales formed during high temperature oxidation. The growth of transient θ-Al₂O₃ and its transformation to α-alumina is addressed for each oxidation temperature along with the differences in the diffusion of reactive elements, such as Hf, Zr and Y, through grain boundaries of the α phase. The θ to α transformation front was proven to move from the metal-scale to the scale-gas interface. The results presented in this paper indicate that after 100 h of oxidation at 1050 and 1100 °C there are still some θ-alumina grains remaining and even in the regions where the transformation to α was finished the surface retained the whisker-like morphology.     

Performance of high‐resolution TVD schemes for 1D dam‐break simulations

Abstract

The performance of high‐resolution total variation diminishing (TVD) schemes for simulating dam‐break problems are presented and evaluated. Three robust and reliable first‐order upwind schemes, namely FVS, Roe and HLLE schemes, are extended to six second‐order TVD schemes using two different approaches, the Sweby flux limiter approach and the direct MUSCL‐Hancock slope limiter. For idealized dam‐break flows, comparisons of the simulated results with the exact solutions show that the flux vector splitting (FVS) scheme coupled with the direct MUSCL‐Hancock (DMH) slope limiter approach has the best numerical performance among the presented schemes. Application of the FVS‐DMH scheme to a dam‐break experiment with sloping dry bed shows that the simulated water depths agree well with the measured.