Step-isotherm experiments

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. 

Step Transient Experiments at 800 Torr. Activation of pretreated silver was performed under isothermal conditions at 493, 523, and 543 K and 800 torr using a step transient format. A typical spectrum collected at 493 K, obtained by simultaneously pulsing ethylene-d4 and oxygen-18 from separate pulse valves into a continuous helium flow, is plotted in Figure 4. In this example, the oxygen to ethylene ratio was 2 1. As observed in the steady-flow TPSR experiments, the pretreated sample is readily activated, while the preoxidi samples remain inactive. 

Instead of changing the temperature it is also possible to determine lateral interactions from isothermal experiments/ or from multi-isotherm experiments in which the temperature is increased in steps. " ... 

Oxidative purification of ND powders was conducted under isothermal conditions using the heating stage and a tube furnace. Isothermal experiments included two steps (1) rapid heating at 50°C/min to the selected temperature and (2) isothermal oxidation for 5 h in ambient air at atmospheric pressure. ND powders used for crystal size characterization were oxidized for 2, 6, 17, 26, and 42 h at 430°C, in a closed tube furnace in static air at atmospheric pressure. 

After the adsorption isotherm experiments have been completed, an isotherm equation must be chosen. This equation should fit the experimental data. Often are the experimental data (the experimental adsorption data acquired by the FA method or the perturbation retention times acquired by the PP method) only compared with the ones calculated using the adsorption isotherm parameters acquired from some adsorption isotherm models [131], This is sometimes the only validation done in this field [131], However, the adsorption isotherm parameters should preferably be validated in two step (1) the different isotherm models should be compared using statistical calculations, e.g., an F-test, and (2) by using the parameters to computer simulate elution profiles and then compare them with experimental ones. 

Non-isothermal measurements (Chapter 2) have yielded valuable information about reaction temperatures and the successive steps in the removal of water from crystalline hydrates, e g. oxalates [14], sulfates [15-17]. DTA and DSC studies have sometimes provided additional information on the recrystallization of the dehydrated product [18]. The problems of relating kinetic parameters obtained by non-isothermal measurements to those from isothermal experiments are discussed in Chapter 5. The effects of heat transfer and diffusion of water vapour may be of even greater consequence in non-isothermal work. Rouquerol [19,20] has suggested that some of the above problems may be significantly decreased through the use of constant rate thermal analysis. 

In a next step the result obtained with the help of the first estimate on E/R is optimized by trial and error. The assessment of the degree of congruence achieved can either be performed with optical control or with the help of statistical methods vriiich are used to characterize variability. For this example, the highest degree of congruence of the curves obtained fi om the transformation to equivalent isothermal experiments was reached with a value of E/R = 7300 K (c.f. Figure 4-74). 

Figure 2.106. Reversible melting of poly(ethylene terephthalate) as recorded in a quasiisothermal step heating experiment. Comparisons can be made to a conventional DSC run performed at 10 °C/min. For the quasi-isothermal run, the conditions used were a period of 60s, an ampUtude of 1°C, and a sample mass of 5mg. [From Wunderlich et al. (1998) reprinted with permission from Elsevier Ltd.]...

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

Along the length of the tube, there was an about 600 mm isothermal region. After experiment finished, the most severe scaled region was at site 200-300 mm away from TiCU entrance where temperature was just lower than isothermal region. Then the scale became smooth step-by-step from front to rear of reactor. 

For instance, the time course of SPE demonstrates that the solvent phase surfactant concentration steadily decreases (Fig. 3) [58]. The w/o-ME solution s water content decreases at the same rate as the surfactant [58]. The protein concentration at first increases, presumably due to the occurrence of Steps 2 and 3 above, but then decreases due to the adsorption of filled w/o-MEs by the solid phase (Fig. 3) [58]. Additional evidence supporting the mechanism given above is the occurrence of a single Langmuir-type isotherm describing surfactant adsorption in the solid phase for several SPE experiments employing a given protein type (Fig. 4) [58]. Here, solid-phase protein molecules can be considered as surfactant adsorption sites. Similar adsorption isotherms occurred also for water adsorption [58]. 

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