Heating-curve determination

The adsorption of the gas should not be limited by diffusion, neither within the adsorbent layer (external diffusion) nor in the pores (internal diffusion). Should diffusion fimitations occur, then adsorption on active but less accessible sites may only occur after better exposed but less active sites have interacted. Diffusion may, thus, cause the smoothing out of significant details in the energy spectrum, and the differential heat curves determined under the influence of diffusion phenomena may indicate less surface heterogeneity than actually exists on the adsorbent surface [7]. 

Evaluation of catalyst for oxidation of pollutants usually involves an Ignition Curve determination. This is a slightly overused expression, because only heat generation is evaluated, not heat removal. For a true ignition curve representation, heat removal evaluation would also be required. 

For the experiments in type C catalysts, the pellets were overfilled with cyclohexane and initially cooled to 230 K. They were then reheated in steps of 1 K and allowed to equilibrate for 10 min before each measurement. The signal was determined from 32 accumulations with an echo sequence of 20 ms echo time to ensure that the signal from the plastically crystalline phase of cyclohexane had decayed fully. The typical heating curves of cyclohexane in the fresh and coked catalyst are displayed in Figure 3.3.3(a) As the temperature is increased, larger and... 

The diode laser is scanned up and down in frequency by a triangle wave, so that the scan should be linear in time and have the same rate in both directions. In the thermal accommodation coefficient experiments, the external beam heats the microsphere to a few K above room temperature and is then turned off. The diode laser is kept at fairly low power ( 7 pW) so that it does not appreciably heat the microsphere. Displacement of a WGM s throughput dip from one scan trace to the next is analyzed to find the relaxation time constant as the microsphere returns to room temperature. Results from the two scan directions are averaged to reduce error due to residual scan nonlinearity. This is done over a wide range of pressures (about four orders of magnitude). The time constant provides the measured thermal conductivity of the surrounding air, and fitting the thermal conductivity vs. pressure curve determines the thermal accommodation coefficient, as described in Sect. 5.5.2. 

More advanced techniques are now available and section 4.2.1.2 described differential scanning calorimetry (DSC) and differential thermal analysis (DTA). DTA, in particular, is widely used for determination of liquidus and solidus points and an excellent case of its application is in the In-Pb system studied by Evans and Prince (1978) who used a DTA technique after Smith (1940). In this method the rate of heat transfer between specimen and furnace is maintained at a constant value and cooling curves determined during solidification. During the solidification process itself cooling rates of the order of 1.25°C min" were used. This particular paper is of great interest in that it shows a very precise determination of the liquidus, but clearly demonstrates the problems associated widi determining solidus temperatures. 

The response is indicative of die electrothermal behavior of die bridgewire-explosive interface. Bridgewir.es which deviate from the characteristic heating curve have been dissected and examined to determine the cause of die abnormality. Deliberate faults have been fabricated into squibs. The relationship of the specific abnormality and the fault associated with.it have been demon strated (Ref 1, abstracted in Ref 3)... 

In a DSC analysis of a semi-crystalline polymer, a jump in the specific heat curve, as shown in Fig. 2.22, becomes visible. The glass transition temperature, Tg, is determined at the inflection point of the specific heat curve. The release of residual stresses as a material s temperature is raised above the glass transition temperature is often observed in a DSC analysis. 

MDSC, by varying the furnace temperature sinusoidally, has been used to determine the specific heat of PA6 materials (similar measurements have been performed for polypropylene nanocomposites). The materials were heated from -80°C to 250°C at 2°C/min. The reversible signal recorded during the experiment is related to the specific heat of the sample. The specific heat values versus temperature for the different PA6-based formulations are given in Figure 19.3, showing no significant differences between different formulations. The peaks noted on the specific heat curves correspond to the transition from the solid to the liquid states. 

The anhydrous salt exists as white crystals, of density 4-1833 at 18-5° C. and 4-1743 at 20-5° C., which melt at 698° On heating it undergoes two transformations, the first -with considerable development of heat, and finally boils. The transition temperatures between the polymorphic forms thus indicated have been determined from the cooling and heating curves as follows ... 

Effect of UFe on the MoFe Solid-State Transformation. The temperature of the solid-state transformation of pure MoFe obtained by thermal analysis was —10.8 0.1°C. from cooling curves and —9.8 0.05°C. from heating curves. The value of —9.8°C. is considered the better one since supercooling occurred while determining cooling curves. Literature values for this solid-state transformation are — 8.7°C., obtained from the intersection of vapor pressure curves (3), —9.6°C., obtained in calorimetric measurements (2), and —9.68°C., also obtained in calorimetric measurements (14). 

M. M. Dubinin (Academy of Sciences of the USSR, Moscow, USSR) In the work under discussion, the authors for the first time undertook an extensive and systematic investigation of heats of immersion into water of various zeolites in different cation-exchange forms containing varied amounts of preadsorbed water. On the basis of their experiments, they calculate the dependence of differential molar heats of adsorption on the adsorption values of water. In principle, assuming that equilibrium states are reached, the curves obtained should coincide with similar curves determined calorimetrically in adsorption of water vapors or with... 

Li per unit cell, isosteric heats were determined by R. M. Barrer and R. M. Gibbons for 68 Li" 6 Na", calorimetric measurements were made by N. N. Avgul, E. S. Dobrova, and A. V. Kiselev for 40 LL + 25 Na", calorimetric (points) and isosteric (filled curve) heats were obtained by N. N. Avgul, B. G. Aristov, A. V. Kiselev, L. Ya. Kurdyukova, and N. V. Frolova. In the case of GO2 adsorption, the calorimetric heat values coincide with the isosteric. These examples clearly show that the physicochemical constants calculated from experiments (Henry constant, second virial coeflBcient, corresponding heat of adsorption, etc.) are influenced by the zeolite structure and chemical composition. Therefore, it is quite necessary to indicate this composition in the representation and discussion of the thermodynamic results. Uncertain results were often obtained for zeolites having a binding material. 

Surface-cooled heat exchangers must be designed using large diameter tubes, low temperature drops, and low ATs between the cooling media and the slurry to obtain reasonable operating cycles. The steepness of the solubility curve determines these parameters. 

Numerous determinations of differential heats of adsorption have been made, both by direct calorimetric and by indirect thermodynamical evaluations. Generally speaking, these determinations cannot be regarded as highly accurate. For the calorimetric measurement, difflculties are encountered due to the frequently low heat conductivity of the adsorbent at a low pressure, leading to abnormal heat curves as shown frequently in the earlier literature. 

The maximal heat output can be determined based on the qualitative course of the heating curve. The pertinent time indicates how quickly the maximal heal output is reached. 

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