Stepped isotherm mode

In this case the initial 10°C/min heating rate switches automatically to an isothermal mode near 350 °C, when the first degradation step associated with acid loss exceeds a rate of 3 mass%/min, and after 5.8 min returns to the starting heating rate of 10°C/min when the degradation rate falls below 0.20 mass%/min. 

Table 5.3 lists the principal experimental methods used in dynamic mechanical testing. Of the experiments considered below, the thermal scan mode (method 1) is the technique most commonly used by thermal analysts. Here typical applications in quality control or processing look for differences in material batches, thermal history, different grades, reactivity, and other characteristics. The stepped isotherm (or step isothermal) experiment (method 2) is used mainly in studies involving detailed mechanical property determination for structural analysis, vibration damping applications, and for determining time-temperature superposition master curves. Method 3 (fast scan or single isotherm) is application specific. 

To determine the OIT, an isothermal mode was used where the isothermal temperature (140 °C) was reached at a heating rate of = 120 "C min . On the curve, the time between the beginning of the isothermal step and the onset of the oxidation peak corresponds to the OIT. 

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. 

Microcalorimeters are well suited for the determination of differential enthalpies of adsorption, as will be commented on in Sections 3.2.2 and 3.3.3. Nevertheless, one should appreciate that there is a big step between the measurement of a heat of adsorption and the determination of a meaningful energy or enthalpy of adsorption. The measured heat depends on the experimental conditions (e.g. on the extent of reversibility of the process, the dead volume of the calorimetric cell and the isothermal or adiabatic operation of the calorimeter). It is therefore essential to devise the calorimetric experiment in such a way that it is the change of state which is assessed and not the mode of operation of the calorimeter. 

A neutron diffraction study was undertaken by Madih et al. (1989) alongside methane adsorption measurements on the MgO(lOO) powder. The well-defined stepwise character of the CH4 isotherm at 87.4 K in Figure 10.26 is again indicative of the layer-by-layer mode of adsorption. A somewhat similar isotherm was given by C2H6 on the MgO(l 00) surface at 119.68 K, although the higher steps were not as distinctive as those for CH4. 

Polyvinylchloride (PVC) decomposes into two distinct steps, the first yielding hydrogen chloride and benzene, the second a mix of aromatics. The kinetic results vary with the amount of sample and the experimental modes (programmed heating or isothermal) are different for hydrogen chloride evolution, with activation energy 136 vs 120 kJ/mol, and reaction order 1.54 and 1.98. 

The catalytic tests were carried out in flow mode with the effluent gas analyzed by an on-line gas chromatograph which was provided with an Alltech CTR-1 column and a hot wire detector. The catalyst (ca.l20 mg) underwent the standard pretreatment in flowing 10% O2 in He at 773 K overnight. The reactant blend (0.46% NO + 0.46% CsHg + 5% O2 + He balance) was passed through the catalyst at the total flow of 100 mL (NTP)/min and total pressure of 1 atm in the range of temperature 423-873 K with a 0.5 h isothermal step at any selected temperature. GHSV = 25.000 h. ... 

In order to model the oscillatory waveform and to predict the p-T locus for the (Hopf) bifurcation from oscillatory ignition to steady flame accurately, it is in fact necessary to include more reaction steps. Johnson et al. [45] examined the 35 reaction Baldwin-Walker scheme and obtained a number of reduced mechanisms from this in order to identify a minimal model capable of semi-quantitative p-T limit prediction and also of producing the complex, mixed-mode waveforms observed experimentally. The minimal scheme depends on the rate coefficient data used, with an updated set beyond that used by Chinnick et al. allowing reduction to a 10-step scheme. It is of particular interest, however, that not even the 35 reaction mechanism can predict complex oscillations unless the non-isothermal character of the reaction is included explicitly. (In computer integrations it is easy to examine the isothermal system by setting the reaction enthalpies equal to zero this allows us, in effect, to examine the behaviour supported by the chemical feedback processes in this system in isolation... 

Equilibration with water vapor Substantially less work was done in this area in the early phase of research on PFSAs because of previous emphasis on membranes which would be in contact with liquid water or aqueous solutions (e.g., for chlor-alkali technology). However, water supplied from the vapor phase could be a principal mode of external hydration of the membrane in a PEFC, particularly hydration of the anode side, and thus it is an important focus of study in fuel cell R D. The shape of the sorption isotherms shown in Fig. 29 (a) and (b) is generic for ion-exchange polymers. With increasing PhsO. water is sorbed in two steps as evidenced by the sorption isotherm ... 

There are several procedures to perform pyrolysis flash pyrolysis (pulse mode), slow gradient heating pyrolysis (continuous mode), step pyrolysis, etc. Commonly, the pyrolysis for analytical purposes is done in pulse mode. This consists of a very rapid heating of the sample from ambient temperature, targeting isothermal conditions at a temperature where the sample is completely pyrolysed. Controlled slow temperature gradients are also possible in pyrolysis, but their use in analytical pyrolysis is limited. Step pyrolysis heats the sample rapidly but in steps, each step following a plateau of constant temperature kept for a limited time period. 

There are two possibilities for performing a frontal chromatography experiment for the purpose of the determination of equilibrium isotherms. The step-series method uses a series of steps starting from C = 0 to C +i. After each experiment, the column has to be reequilibrated and a new step injection with a different end concentration C +i can be performed. In the staircase method, a series of steps is performed in a single run with concentration steps from 0 to Q, Q to C2,.. ., C to C +i. The column does not have to be reequilibrated after each step and, therefore, the staircase method is faster than the step-series method. Both modes of frontal analysis give very accurate isotherm results. 

The method is advantageously combined with the frontal analysis method, which also requires a concentration plateau and thus shares the disadvantage of high sample consumption if operated in open mode. As indicated in Fig. 6.24, the measurement procedure starts at maximum concentration. This concentration plateau is reduced step-by-step by diluting the solution. To reduce the amount of samples needed for the isotherm determination the experiments can be done in a closed loop arrangement (Fig. 6.17). It is also possible to automate this procedure. 

Although the TS mode of operation does not require isothermal or steady-state conditions, it is assumed that the reaction is at all times in steady state with respect to certain steps in the reaction. For example, in catalytic TS-PFR operation, it is taken that the adsorption/desorption steady state is achieved much mare rapidly than the time scale involved in the temperature scanning procedure. In the TS-CSTR we assume this, as well as the fact that complete mixing of reactor contents takes place on a time scale much shorter than the temperature ramping. Moreover, although there may be temperature differences and heat flows between various components of the reactor, of the catalyst, and of the reactants, these should not be flow-velocity-dependent, nor should there be any flow-velocity-dependent diffusion effects. 

The first steps towards an explanation of these extra modes of hydrocarbon oxidation were taken by Frank-Kamenetskii who proposed that the multiple cool flames observed were oscillatory and that the period of oscillation reflected the underlying chemistry. Experimental investigations continued to concentrate upon traditional measurements of pressure versus time, of induction period, and o establishing the identity of stable intermediates and reaction pathways. Interpretations of these results continued to be made on isothermal degenerate branched-chain reactions direct measurements of temperature were not made. These interpretations were very incomplete, and much better understanding has emerged from application of thermokinetic theory. 

For commercial applications, an adsorbent must be chosen carefully to give the required selectivity, capacity, stability, strength, and regenerability. The most commonly used adsorbents are activated carbon, molecular-sieve carbon, molecular-sieve zeolites, silica gel, and activated alumina. Of particular importance in the selection process is the adsorption isotherm for competing solutes when using a particular adsorbent. Most adsorption operations are conducted in a semicontinuous cyclic mode that includes a regeneration step. Batch slurry systems are favored for small-scale separations, whereas fixed-bed operations are preferred for large-scale separations. Quite elaborate cycles have been developed for the latter. 

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