Quasi-isothermal conditions

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

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

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). 

In cases where only one temperature-dependent constant appears, it can be calculated in real-time from TS-BR data and, if the reaction is carried out at quasi-isothermal conditions, can also be calculated in real-time from TS-PFR data (see Chapter 11). In all other cases TS-PFR data processing must be done after the experiment is completed. 

A more general method of using integrated forms involves using the standard procedure where one waits until the TSR experiment is completed and then proceeds to smooth the (X, T, t) surface as described in previous discussion. On the smoothed surface one identifies the operating lines for the system and uses them in an integral method of data interpretation. This removes the restriction employed in the hydrolysis study, that the system be at quasi-isothermal conditions, and makes the method more general. 

The samples were saturated with liquid vapours in a vacuum desiccator at plpo = 1. The Q-TG mass loss and Q-DTG differential mass loss curves were measured under quasi-isothermal conditions in the temperature range 20-250 °C at a heating rate of 6 7min. 

Figure 7. Fractal dimension method of calculation on the basis of Q-TG thermodesorption of 1-butanol from La-montmorillonite surface under the quasi-isothermal conditions.

A new method for the calculation of diffusion coefficients from Q-TG data has been presented. The programmed thermodesorption of polar and nonpolar liquids from aluminium oxide and montmorillonite-Na and -La samples under quasi-isothermal conditions has been studied. The results from above methods were compared with literature data and good correlation was obtained. 

Figure 11 Schematic representation of stepwise heating to obtain melting temperatures under quasi-isothermal conditions...

Quasi-plug-flow mode in turbulent flows as a key to realization of fast chemical processes in quasi-isothermal conditions... 

Under quasi-isothermal conditions and at steady state, the solution of Equation (6) can be written with constants =Aa/N and A =Azj(RT )y where is the activation energy for the formation of the high-energy configurations, assumed to be describable by an Arrhenius e qiression t = B exp z/(RT) (P) ... 

Thus, even in an adiabatic mode of tubular turbulent chlorination reactor operation (without heat removal), the temperature growth in the reaction zone in the case of BR chlorination (12-15% solution) with molecular chlorine in a tubular reactor, operating in the optimum plug-flow mode in turbulent flows, does not exceed 2 1 °C. The process can be thought to proceed under quasi-isothermal conditions and does not require external or internal heat removal, or special stirring devices for heat and mass exchange intensification. 

Formation of the quasi-plug flow mode in the reaction zone in turbulent flows, due to the intensive convective heat exchange in proximity to the reactor walls and resulting quasi-isothermal conditions, allows efficient control of the thermal mode of the neutralisation process by external cooling. In order to decide whether it is reasonable to use external cooling for the neutralisation reaction in a tubular turbulent reactor of cylinder form, it is necessary to estimate the amount of heat which is released in the reaction zone under conditions of adiabatic heating. If the density and thermal capacity of the reaction mixture in the first approximation correspond to the values for water, then [15] ... 

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