Thermomechanical characteristics of calcium aluminate cement and sand tapes prepared by tape casting

The present paper concerns the mechanical characterization of calcium aluminate cement and sand tapes prepared by tape casting, including ultrasonic measurements of Young's modulus at high temperature and evaluation of four point flexural behavior after heat treatments in the range of 20–1400 °C. It is shown that dehydration strongly affects mechanical properties in the 400–900 °C range, but that treatments at temperatures higher than 1200 °C increase both Young's modulus and strength. By correlation with thermal and X-ray diffraction analyses, the evolutions of thermomechanical properties have been related to phase and microstructural changes when heating the material after hydration: conversion of hydrates and dehydration at low temperature, then, crystallization of C–A and C–A–S phases and finally sintering at the highest temperatures. In a last part, it is shown that the reinforcement by glass fibres enhances the mechanical properties, in particular in the temperature range of dehydration, and gives to the material a non-brittle behavior.     

INVESTIGATION OF IMPINGEMENT HEAT TRANSFER COEFFICIENT AT HIGH TEMPERATURES

Very little information exists for the impingement heat transfer coefficient at high temperatures. All available empirical correlations are mainly based on experiments conducted at relatively low impingement temperatures, and thus cannot describe the heat transfer characteristics of the impingement air at high temperatures with sufficient accuracy. A comprehensive study of the impingement heat transfer coefficient at high temperatures is carried out and presented in this paper. The aim of the study is to give a summary of the experimental results of the impingement heat transfer covering a large impingement air temperature range from 100 to 700°C. Heat transfer measurements were carried out on a laboratory-scale test rig. The main parts of the rig were a fan, a gas burner for air heating, a heavily insulated nozzle array with 300 × 500 mm impingement surface, a 40 mm thick and 300 × 500 mm sized aluminium plate for determination of heat transfer, and a data acquisition system. The heat transfer rate was determined from the heat-up rate of the aluminium plate due to the high temperature jet impingement.     

Circulation rates of boiling liquids in a vertical tube

An experimental study of heat transfer to boiling liquids under natural convective flow has been carried out in a single tube vertical thermosiphon reboiler to investigate the effect of heat flux and submergence on circulation rates. The test liquids used were distilled water, various concentrations of propan-2-ol in water and their azeotrope. The test section was an electrically heated stainless steel tube of 25.56 mm i.d., 28.85 mm o.d. and 1900 mm long with twenty-one spot welded copper constantan thermocouples to measure the variations in wall temperature along its axis. The uniform heat fluxes in the range of 3.5–42.4 kW/m² were employed while inlet liquid sub-cooling varied from 0.2 to 30.7 °C. The liquid submergence levels were maintained in the range 30–100%. The typical experimental data has been graphically presented and discussed. An empirical correlation has been developed from the experimental data of the present study through regression analysis.     

Vacuum pyrolysis of Prince Mine coal, Nova Scotia, Canada

Preliminary results are given on thermal decomposition characteristics of a high volatile A bituminous coal from Eastern Canada using vacuum pyrolysis experiments (pressure 2–200 mm Hg) over the temperature range 322–1000 °C. The objectives of the study were to determine the optimum temperature range for the formation of coal tar and to study the influence of reaction temperature on the nature of the solid residues. Significant decomposition reactions start at 300–400 °C and the optimum temperature range for the production of coal tar was 450–600 °C. The major gaseous products H₂S, CO₂ and CH₄ are formed up to 600 °C. Above 600 °C the coal decomposes mainly into CO and H₂. The solid residues are characterized by volatile matter content, calorific values and elemental analysis. The volatile matter content decreases rapidly from 322 °C and stabilizes at reaction temperatures > 750 °C. The 15% VM level, a minimum requirement in coal combustion processes, was reached at 500 °C. The changes in calorific values do not show any significant trend up to 600 °C, but decrease markedly above 600 °C. From the preliminary results vacuum pyrolysis is regarded as an effective process in which valuable coal tar by-products can be obtained from coal prior to its combustion.     

Direct oxidation of H2S into S. New catalysts and processes based on SiC support

Nickel sulphide, supported on SiC, exhibits a very high activity and selectivity for the direct oxidation of H₂S into S at medium temperatures (100–120°C) or at room temperature (20–40°C). Iron oxide, also supported on SiC, is highly reactive, and selective, for the same reaction at higher temperatures (210–240°C). This support is very stable, insensitive to steam and to any sulphur compounds in this range of temperature.     

STUDIES OF MAGNETITE DEPOSITION FROM A FLOWING SUSPENSION

The deposition of magnetite from flowing water suspensions is a problem particularly important in boiler plant. Measurements have been made of the rate of deposition of magnetite particles from suspension in demincraliscd water pumped through vertical aluminium tubes of 19 mm bore over a range of temperatures I2-75°C and Reynolds number 7900 to 88500 under isothermal conditions. Bulk concentrations of magnetite particles (of the order of 2 μm diameter) were in the range 200-600 ppm.

It would appear that deposition is mass transfer controlled. The initial deposilion rate for a given temperature and concentration varies as the suspension velocity to the power 0.73. The measured asymptotic deposition quantity varies as the suspension velocity to the power -0.66.

The conclusions imply a sticking probability for the magnetite on to the aluminium lube walls as 1 for the experimental conditions.     

Correlation crystallinity and physical properties of heat-treated cellulose triacetate fibres

Previous measurements of crystallinity in fibres of cellulose triacetate have been qualitative or semi-quantitative; in this work quantitative measurements of relative crystallinity have been made on cellulose triacetate yarn heat treated in the range 20–300°C following the x-ray diffraction method of correlation crystallinity index. The onset of crystallization is clearly marked by a transition in the crystallinity index at 172°C, and beyond this annealing temperature tenacity and crystallinity are inversely correlated. Orientation also improves with annealing temperature with a less well defined transition around 120–180°C. Initial Young's modulus increases with temperature whilst other physical properties have optimum values in the range 180–220°C. Cutting and grinding are found to have an adverse effect on the correlation crystallinity index which is in fact a measure of lattice perfection.     

Densification of nanocrystalline MgO ceramics by hot-pressing

Densification of pure nanocrystalline MgO powder with 10 nm particle size by hot-pressing was investigated in the temperature range 700–800 °C, applied pressure range 100–200 MPa, and for durations of up to 240 min. It was shown that significant densification under the pressure begins above 440 °C. Densities higher than 99.5% with grain size of 73 nm were achieved at 790 °C and 150 MPa for a 30 min duration. Remarkable densification from 90 to 99.5% was observed by temperature change from 700 to 790 °C, for which the grain size was doubled only. The final grain size decreased with increasing the applied pressure. Higher shrinkage rates and cumulative shrinkages were recorded by the application of pressure at 550 °C rather than from room temperature. The temperature at which the pressure was applied is crucial in determining the maximum shrinkage rate in the nanocrystalline compacts. This effect was related to the morphological changes of the particles caused by plastic deformation at lower temperatures. Analysis of the densification rate and its comparison to the literature data was in agreement with Coble creep, where self-diffusion of Mg²⁺ cations along the grain boundaries acts as a main densification mechanism.     

Viscosity measurements and empirical predictions for fluxed Australian bituminous coal ashes

Recent interest in the suitability of Australian bituminous coals for use in integrated gasification-combined cycle (IGCC) technologies has provided the opportunity to determine viscosity data for a range of coal ashes slags fluxed with limestone at the tapping temperatures of entrained flow gasifiers. Experimental viscosity measurements have been made over a range of slag compositions covering the anorthite region at the 0–2.5, 2.5–5, 5–7.5 and 7.5–10 wt% FeO levels of the quaternary SiO₂–Al₂O₃–CaO–FeO system. Contour plots of viscosities at 1450°C for the four FeO ranges are presented as an example for predicting slag behaviour in entrained flow gasifiers. The viscosity measurements have also been fitted empirically using a modified Urbain treatment to give separate models for the four FeO levels. Polynomial expressions are given for the evaluation of viscosities covering the temperature range 1400–1550°C for slags within the compositional range used in the derivation.     

Thermal conductivity of nanocomposites based on diamonds and nanodiamonds

The thermal conductivity of composites sintered from natural microdiamond (5–7 and 10–14 μm) and nanodiamond powders under pressure of  6.0 to 6.5 GPa at the temperature  1000 to 2000 °C for 6–20 s was measured in a steady heat flow in the temperature range of 50–200 °C. It was found that the thermal conductivity of nanodiamond composites produced in these conditions was less than 10 W/(mК) while that of natural microdiamonds was as high as 500 W/(mК).     

Hydrogenation of anthracene over active carbon-supported nickel catalyst

Hydrogen transfer behavior over active carbon and carbon-supported Ni catalyst was examined in the hydrogenation of anthracene with three kinds of hydrogen sources: hydrogen gas, hydrogen-donor tetralin and the combination of both. In tetralin, active carbon itself provided higher conversions of anthracene in the temperature range of 350–400°C than Ni/C catalyst, while under the pressure of hydrogen gas, the addition of Ni metal onto active carbon remarkably promoted the hydrogenation of anthracene, providing a complete conversion at 300°C. When both tetralin and hydrogen gas were used together, an apparent improvement in both conversion and product distribution was observed with active carbon, whereas with Ni/C catalyst, the rate of hydrogen consumed in the hydrogenation was apparently low in the temperature range of 300–320°C, compared to that observed at the same temperatures using hydrogen gas alone.     

Nitridation of silicon powder studied by XRD, Si MAS NMR and surface analysis techniques

The nitridation of elemental silicon powder at 900–1475 °C was studied by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), XRD, thermal analysis and ²⁹Si MAS NMR. An initial mass gain of about 12% at 1250–1300 °C corresponds to the formation of a product layer about 0·2 μm thick (assuming spherical particles). XPS and XAES show that in this temperature range, the surface atomic ratio of N/Si increases and the ratio O/Si decreases as the surface layer is converted to Si₂N₂O. XRD shows that above 1300 °C the Si is rapidly converted to a mixture of - and β-Si₃N₄, the latter predominating >1400 °C. In this temperature range there are only slight changes in the composition of the surface material, which at the higher temperatures regains a small amount of an oxidised surface layer. By contrast, in the interval 1400–1475 °C, the ²⁹Si MAS NMR chemical shift of the elemental Si changes progressively from about −80 ppm to −70 ppm, in tandem with the growth of the Si₃N₄ resonance at about −48 ppm. Possible reasons for this previously unreported change in the Si chemical shift are discussed. ©     

Formation of textural and mechanical properties of extruded ceramic honeycomb monoliths: An H NMR imaging study

The effects of the nature of oxide component and binder, and thermal treatment temperature (100 °C and from 600 to 1300 °C) on textural and physicochemical properties of honeycomb monoliths based on alumina, titania and aluminosilicates have been studied.

The main regularities of the texture formation have been revealed using ¹H NMR imaging, SEM, XRD, adsorption technique and others. It has been shown that the textural changes of monolith samples at different preparation stages from extrusion to thermal treatment are mostly caused by removal of capillary water (20–100 °C), sintering of small pores and consolidation of oxide particles (100–700 °C), and phase transformation of the oxide component or the binder (900–1300 °C).     

Effect of the condensing cover''s slope on internal heat and mass transfer in distillation: an indoor simulation

The objective of the study was to determine a relation for predicting convective and evaporative heat transfer coefficients for all three condensing surfaces inclined at 15°, 30° and 45° under indoor simulation. The condensing covers were made of the same flat transparent glass as found in any solar distillation unit. The operating temperature range for the experiment was maintained at steady state from 40°C to 80°C by using a constant temperature bath. The temperatures and yields obtained for a 10-min interval were used to determine the values of constants C and n and consequently convective and evaporative heat transfer coefficients. It was found that a higher yield was obtained with an increase in temperature for a 30° slope compared to 15° and 45° slopes of the condensing cover.     

Experimental and microkinetic modeling of steady-state NO reduction by H2 on Pt/BaO/Al2O3 monolith catalysts

Experimental results describing the product distribution during the reduction of NO by H₂ on Pt/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts are presented in the temperature range 30–500 °C and H₂/NO feed ratio range of 0.9–2.5. A microkinetic model that describes the kinetics of NO reduction by H₂ on Pt/Al₂O₃ is proposed and most of the kinetic parameters are estimated from either literature data or from thermodynamic constraints. The microkinetic model is combined with the short monolith flow model to simulate the conversions and selectivities corresponding to the experimental conditions. The predicted trends are in excellent qualitative and reasonable quantitative agreement with the experimental results. Both the model and the experiments show that N₂O formation is favored at low temperatures and low H₂/NO feed ratios, N₂ selectivity increases monotonically with temperature for H₂/NO feed ratios of 1.2 or less but goes through a maximum at intermediate temperatures (around 100 °C) for H₂/NO feed ratios 1.5 or higher. Ammonia formation is favored for H₂/NO feed ratios of 1.5 or higher and intermediate temperatures (100–350 °C) buts starts to decompose at a temperature of 400 °C or higher. The microkinetic model is used to determine the surface coverages and explain the trends in the experimentally observed selectivities.     

Pore structure of plain cement pastes hydrated at different temperatures

Concrete outside the laboratory cures at temperatures other than 20°C. This paper describes an investigation of the pore structure of plain cement pastes hydrated at 5°, 20°, and 50°C to reflect a range of temperatures encountered in practice. Parallel specimens of 0.50 water/cement ratio pastes were examined using mercury intrusion porosimetry and backscattered electron image analysis. Increases in curing temperature resulted in increased porosity, particularly for pores of radius 200–1000 as measured by mercury intrusion, or 2500–12,500 as measured in the backscattered electron images. The difference between the two results indicates the magnitude of the “ink bottle effect” inherent in the mercury intrusion technique. However, both methods suggest that elevated curing temperatures could have a deleterious effect on the durability of plain cement concretes.     

HEAT TREATMENT OF CHOPPED ALFALFA IN ROTARY DRUM DRYERS

This paper deals with the heat treatment of alfalfa chops during the high temperature dehydration process. It outlines the dryer characteristics, difficulties and potential errors in measuring temperatures in the dryer, computation techniques, the relationships between moisture and temperature during drying, and the potential effect of dehydration on the destruction of the insect Hessian Fly due to elevated temperatures.

From the analysis based on an existing computer model for dehydration of alfalfa chops, and the available field data, it is shown that the dried chops will attain a temperature of 90°C or higher when the input temperatures are between 500°C and 800°C. These conditions apply to the drying of wet alfalfa (moisture content more than 55 percent wet basis). The plant material loses a large portion of its moisture in the first few seconds in the dryer. The rapid release of moisture may cause the rupture or detachment of particles such as eggs, larva, pupa, and insects from the plant material. These small particles are exposed to an intense heat and rapid dehydration.     

Influence of Impingement Temperature and Nozzle Geometry on Heat Transfer—Experimental and Theoretical Analysis

Based on the earlier studies the most common empirical correlations in the literature predict the impingement heat transfer coefficients rather well at low (close to 100°C) temperatures, but get inaccurate at higher temperatures. Impingement temperatures used in paper drying applications are typically 300 to 700°C, and there has been a need to improve the existing heat transfer correlations at high temperature area. This study presents experimental results of impingement heat transfer measurements with a laboratory-scale heat transfer test rig. Five nozzle configurations were measured. The parameters varied in the investigation were nozzle-to-plate distance, nozzle open area, nozzle diameter, impingement velocity and impingement air temperature. Regression model of heat transfer was developed based on the measurement data. A correction factor including the impingement and heat receiving surface temperatures is proposed. The correction factor can be used to improve the existing correlations at large jet to heat receiving surface temperature differences.     

Effect of temperature on the structure and density of ammonium chloride particles

The structure and density of individual ammonium chloride particles formed at 0 and −20°C by homogeneous nucleation were studied using electron microscopy and X-ray diffraction. The crystal size apparently increased at the lower temperature and many of the particles formed at −20°C were single crystals or had an oriented polycrystalline structure. These results differ from those reported previously for particles formed at room temperature (23–26°C), which showed an amorphous or randomly-oriented fine crystal structure. Coagulation was more frequently observed as the temperature decreased and the porosity present in the particles appeared to be much finer and more uniform. The density of these particles decreased from about 0.26 g cm⁻³ for particles of size 0.1–0.2 μm to approximately 0.1 g cm⁻³ for particles slightly smaller than 1 μ.     

Thermal phase sequences in gibbsite/kaolinite clay: electron microscopy studies

In Brazil, gibbsite/kaolinite clay is used extensively in the manufacture of refractory products. In this study, electron microscopy was used to characterize the phase sequences occurring during thermal transformation in this type of clay. The clay was powdered, heated on platinum foils to temperatures of 200–1550 °C and program cooled. The heated powders were characterized using transmission electron microscopy, selected area electron diffraction, elemental microanalysis and X-ray diffraction.

After heating the gibbsite/kaolinite clay to 200 °C or better at 300 °C, morphological characterization under a transmission electron microscope revealed that the chi-alumina from the gibbsite is identical in size and shape (hexagonal) to the undecomposed kaolinite crystals. The thermal transformations of gibbsite and well-crystallized kaolinite follow independent specific patterns up to 1100 or 1200 °C. Examining the organization of the internal granules formed from the dehydroxylation of gibbsite and kaolinite at increasing temperature (400–800 °C), a morphological difference was observed between the aluminas (chi and kappa) and the pseudomorphic metakaolin crystals. The two reaction sequences (a) metakaolin   spinel   mullite; (b) kappa-alumina   alpha-alumina occur in the 900–1100 °C range and characteristic morphological differences among the crystals are evident, especially in the spinel, which presents elongated particles. Mullite content increases between 1000 and 1550 °C, whereas alpha-alumina increases up to 1300 °C and decreases by 1550 °C, indicating interaction among the several high-temperature phases.