Quasi-isothermal

B1.27.4.1 CLASSIFICATION BY PRINCIPLE OF OPERATION ISOTHERMAL CALORIMETERS (MORE PRECISELY, QUASI-ISOTHERMAL)... 

In order to compare the finite element model with the one-dimensional Chiao model, an extremely simple mesh of only five elements extending in a column from the laminate centerline to the outer surface was used to model the gradients in the laminate through-thickness direction. Figure 7 shows the reaction history (fractional concentration of reactive species, C, versus time) obtained from this run, selected at the location nearest the heated surface. This figure also shows the comparison with the quasi-isothermal and Chiao models. 

Hexcei/Chiao Quasi-Isothermal Finite Element... 

Thermolysis was carried out under quasi-isothermal conditions (in other cases under dynamic conditions with a heating rate of 10°C/min). 

Some types of isoperibolic (quasi-isothermal) equipment using relatively large quantities of sample—see Section 2.3.I.2. 

Fig. 56. Dissociation of BaC>2 during stepwise heating, approaching quasi-isothermal conditions...

If, as shown in figure 12.9a, the whole transformation strictly occurs in the interval (Te—(), IK —> 7 c, then the corresponding enthalpy change// can be calculated from equation 12.37 by using the areas of two peaks only (i) the large peak recorded in the main experiment between Te—0.1K and Te, and (ii) the corresponding peak for the zero line. In this case, the process is quasi-isothermic because the initial and final temperatures differ by only 0.1 K. Therefore, unless... 

Hub, L. Medium-small-scale safety calorimeter, usable under isothermal, quasi-isothermal or adiabatic conditions. 

Figure 17.21. Distribution of adsorbate along a column operating quasi-isothermally with a Type IV isotherm. The dotted line is the temperature distribution at the breakpoint of a column of length 36 units. Air flow 8.8 x 10 6 m3/s. Temperature of jacket 303 K. Relative humidity of feed 95 per cent. The volumes to breakpoint of each bed are indicated on the curves.

The thermal decomposition of a number of double complexes containing the [Cr(CN)6]3-anion has been investigated and the results obtained under quasi-isothermal and -isobaric conditions are summarized in Scheme 31.336 The CN-flipping reactions involved in the scheme resemble similar reactions of Prussian blue analogues such as Co3[Cr(CN)6]2 337 and Fe3[Cr(CN)6]2-338 The unusually low magnetic moment of Cr(CN)6Cr indicates considerable metal-metal interaction via the CN bridges. 

Figure 6.5 Substitution reaction under quasi isothermal conditions The jacket temperature (Tc) must follow drastic dynamic changes in order to maintain the reaction medium temperature (T,) only roughly constant.

The next important step in the study of the regularities of the autowave modes of cryochemical conversion was to perform a series of experiments with thin-film samples of reactants. The changeover to such objects, characterized by the most intense heat absorption, allowed the realization of quasi-isothermal conditions of the process development and thus favored the manifestation of the abovementioned isothermal mechanism of wave excitation, which involves autodispersing the sample layer by layer due to the density difference between the initial and final reaction products. The new conditions not only not suppressed the phenomenon, but made it possible to reveal some details of the traveling-wave-front structure, which will be discussed here and also in Section X. 

More complex temperature programmes are sometimes useful. These might combine periods of variable heating and cooling rates with isothermal periods. For example, stepwise heating can be used to detect the onset of melting under quasi-isothermal conditions (Laye, 2002). Modulated temperature DSC (MTDSC), in which the linear temperature scan is perturbed by a sinusoidal, square or saw-tooth wave, or other modulation of temperature, has a number of potential advantages over the conventional linear scan. These include increased sensitivity and resolution, and the ability to separate multiple thermal events (Laye, 2002). 

To keep the temperature low the heat of reaction must be removed in an appropriate way, and to achieve a sufficient reaction rate effective catalysts have to be applied. The process is therefore performed in steps, with intermediate heat removal between the individual catalyst beds, in which the reaction runs adiabatically. Recently, quasi-isothermal reactors have been developed in which cooling tubes run though the catalyst layers. 

Irrespective of the catalyst type used, the high initial carbon monoxide concentrations mean that the reaction must generally be performed in steps, with intermediate cooling. But it has also been reported that the CO content can be reduced from 50 to 0.8 % in a single step in a large hydrogen plant by using a quasi-isothermal reactor (e.g., the Linde spiral-wound reactor). 

P. Staszczuk, D. Stemik, G. W. Ch dzyhski Application of quasi-isothermal thermogravimetry and sorptometry for estimation of heterostructure and fractal dimension of high temperature superconductors. J. Therm. Anal. Cal. (2003) in print. 

Manduva R, Keti VL. Banks SR. et al. Calorimetric and spatial characterisation of polymorphic transitions in caffeine using quasi-isothermal MTDSC and localised thermo-mechanical analysis. J Wiarm Sci 2007 97(2008) 1285-1300,... 

Figure 7 Vertical profiles of formaldehyde, HCHO, in the snowpacks in central Alaska described in Figure 3 and subjected to different thermal regimes a temperature gradient (HGM) regime, and a quasi-isothermal (QIM) regime.

Sorenson, O.T. Quasi-isothermal methods in thermal analysis. Thermochim. Acta 1981, 50,163-175. 

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