Alumina, specific heat

Instrumental. The Mettler TA2000B thermal analysis system is equipped with an interface system, and Hewlett-Packard 9815 desk top calculator and 7225 plotter. Samples, weighing 5-10 mg, sealed in aluminum pans and under a nitrogen blanket, were heated in the calorimeter at a rate of 10 deg/min from -35 or 10 deg C to 180 deg C respectively, for specific heat and kinetic scans. Specific heat measurements were calibrated with alumina to an accuracy of 3%. Temperatures and enthalpies were calibrated with an Indium sample. The accuracies were .02 deg C and 2% (Indium 28.5 J/g), respectively. 

Example 13-5 Using the one-dimensional method, compute curves for temperature and conversion vs catalyst-bed depth for comparison with the experimental data shown in Figs. 13-10 and 13-14 for the oxidation of sulfur dioxide. The reactor consisted of a cylindrical tube, 2.06 in. ID. The superficial gas mass velocity was 350 lb/(hr)(ft ), and its inlet composition was 6.5 mole % SO2 and 93.5 mole % dry air. The catalyst was prepared from -in. cylindrical pellets of alumina and contained a surface coating of platinum (0.2 wt % of the pellet). The measured global rates in this case were not fitted to a kinetic equation, but are shown as a function of temperature and conversion in Table 13-4 and Fig. 13-13. Since a fixed inlet gas composition was used, independent variations of the partial pressures of oxygen, sulfur dioxide, and sulfur trioxide were not possible. Instead these pressures are all related to one variable, the extent of conversion. Hence the rate data shown in Table 13-4 as a function of conversion are sufficient for the calculations. The total pressure was essentially constant at 790 mm Hg. The heat of reaction was nearly constant over a considerable temperature range and was equal to — 22,700 cal/g mole of sulfur dioxide reacted. The gas mixture was predominantly air, so that its specific heat may be taken equal to that of air. The bulk density of the catalyst as packed in the reactor was 64 Ib/ft. ... 

The specific heat, entropy, heat and Gibbs free energies of formation of alumina are given in Table 14, from [28], Above 2,790°K, the boiling point of aluminum, there is a discontinuous change in the heat of formation of alumina. 

Table 14 Specific heat and thermodynamic properties of a-alumina as a function of temperature...

The specific heat of a substance can be determined conveniently and rapidly using the techniques of DTA and DSC (173, 88). The method (173) is illustrated by the DuPont DSC curves for a-alumina, as given in Figure 7.62. A curve for the empty sample container is first run, as indicated by the upper curve. The sample is then placed in the sample container and its curve recorded, using the same instrument adjustments. The relationship between the blank (empty container) and the sample (empty container plus sample) then is... 

Figure 7.62. Specific-heat determination curves of a-alumina (173). (Cp) 32r — 0.279 mcal/mg.

How much energy is needed to raise the temperature of 2.5 moles of alumina from 0°C to 120 C, taking the specific heat capacity of alumina, 0.907 JK g , to be independent of temperature ... 

Size, weight, specific heat and composition of alumina balls (should not be less than 90% Al Oj). Balls of other materials or metals can also be tried after due study and trials. 

Specific Heat Capacity of Pure a-Alumina in the Temperature Region from 120 to 780 K... 

The thermal endurance factor is a function of temperature in that several of the variables, particularly the thermal conductivity and the specific heat, are functions of temperature. From Table 4.10, it is also noted that the thermal endurance factor may drop rapidly as the alumina-to-glass ratio drops. This is because of the differences in the thermal conductivity and TCE of the alumina and glass constituents, which increase the internal stresses. This is true of other materials as well. 

The specific heat of amorphous plastics increases with temperature in an approximately linear fashion below and above Tg, but a steplike change occurs near the Tg. No such stepping occurs widi crystalline types. The high degree of die molecular order for crystalline TPs makes their values tend to be twice those of the amorphous types. The TSs has the highest values. To increase TC the usual approach is to add metallic fillers, glass fibers, foamed structure, or electrically insulating fillers such as alumina. 

Fig. 85. - Schema of the high temperature instrument (Netzsch DSC 404 C Pegasus, version up to 1650 °C). Right, the calibration curve of specific heats obtained on a-alumina samples between -50 and 1600 °C using platinum crucibles with lids under an inert atmosphere.

Fig. 86. - Left, apparent specific heat of water below and above its melting point is shown using the alumina crucibles with lid and carried out under helium atmosphere. It includes the inset with a comparison of the measured specific heat with the data tabulated. Right, the apparent specific heat of glassy Ti6oCr4o samples arc publicized between the room and high temperatures (up to 1500 C) employing platinum crucibles with alumina liners and lids with the inner crucible surface protected by yltria coating (to avoid reaction between the titanium alloy and the alumina liner). Courtesy of Netzsch application laboratory, Selb, Germany.

Alternatively, the evaluation of volume thermal expansion and specific heat of alumina-magnesium systems with temperature have been evaluated using both molecular dynamics and experimentally derived interatomic potentials [20]. In spite of a well reproduced heat capacity curve, the predicted thermal expansion appears to be significantly lower at high temperature than experimentally observed. 

Parallel changes in specific heat and thermal expansion coefficient of alumina. 

Reference Material—Glass beads, alumina powder, silicon carbide, or any material known to be unaffected by repeated heating and cooling and free from interfering transitions. The specific heat capacity of the reference should be as close as possible to that of the test material. 

In conventional differential thermal analysis (DTA), the temperature at the center of the sample is compared with a reference material (often powdered alumina) as both are heated at a uniform rate. Any change in the sample s specific heat as at Tg, any structural change that is endothermic or exothermic as at T , or chemical reactions will show as changes in the temperature difference between sample and reference T. A primitive instrument consists of thermocouples inserted in the sample and the reference. The sample and the reference material are held in a heated metal block at a controlled temperature Tq. As Tq is raised (a rate of 10°C/min is common), Tj and follow perhaps by as little as 0.1°C. If an endothermic reaction takes place, Tj will lag behind temporarily. If an exothermic reaction takes place, will exceed Tj temporarily. Superficially, DTA resembles DSC in that temperatures of a sample and a reference material are increased at almost the same rate. However, in DTA the difference in temperature, AT, is the measured quantity rather than the difference in... 

Silica gel and activated alumina present few practical problems. They are easily reactivated after use by heating in a ventilated oven, to 130-300°C for silica gel, and 150-700°C for activated alumina. British standard specifications have been published for desiccants for packaging which regulate the contents of soluble chloride and sulphate, dust content and absorptive capacity. 

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