Thermokinetics

Hence, the main aim of the technological process in obtaining fibres from flexible-chain polymers is to extend flexible-chain molecules and to fix their oriented state by subsequent crystallization. The filaments obtained by this method exhibit a fibrillar structure and high tenacity, because the structure of the filament is similar to that of fibres prepared from rigid-chain polymers (for a detailed thermodynamic treatment of orientation processes in polymer solutions and the thermokinetic analysis of jet-fibre transition in longitudinal solution flow see monograph3. ... 

At present, it is known that the structures of the ECC type (Figs 3 and 21) can be obtained in principle for all linear crystallizable polymers. However, in practice, ECC does not occur although, as follows from the preceding considerations, the formation of linear single crystals of macroscopic size (100% ECC) is not forbidden for any fundamental thermodynamic or thermokinetic reasons60,65). It should be noted that the attained tenacities of rigid- and flexible-chain polymer fibers are almost identical. The reasons for a relatively low tenacity of fibers from rigid-chain polymers and for the adequacy of the model in Fig. 21 a have been analyzed in detail in Ref. 65. 

Reaction calorimetry is a technique which uses data on the rate of heat evolution or consumption to evaluate the thermokinetic reaction characteristics needed for reactor scale-up and/or optimization and safety. Since the late seventies, the application of this technique has been steadily growing and reaction calorimeters are now commercially available. Probably the first commercial reactor calorimeter was developed by CIBA-GEIGY (Bench Scale Calorimeter BSC) (see Beyrich et al, 1980 and Regenass et al., 1978, 1980, 1983, 1984, 1985, 1997))... 

Thermal methods in kinetic modelling. Methods for the estimation of thermokinetic parameters based on experiments in a reaction calorimeter will be discussed below. As mentioned in section 5.4.4.3, instantaneous heat evolved due to a single reaction is directly proportional to the reaction rate. Assume that the reaction is of first order. Then for isothermal operation ... 

If a reliable kinetic model and data on cooling capacity are at hand, runaway scenarios can be examined by computer simulations and only final findings have to be tested experimentally. Such an approach has been presented, e.g. by Zaldivar et al. (1992). However, the detailed reaction mechanism and reaction kinetics are rarely known. Therefore, thermokinetic methods with gross (macro-)kinetics dominate among methods for data... 

Results of regression are given in Table 7.4-4. The cycle time is size-dependent whereby this dependency is stronger for the semicontinuous process at these particular process conditions. The exponent p is almost the. same for both processes and is approximately 1/3. Clearly, the physical properties of the reaction mixture and thermokinetic data are needed to evaluate processing times and cycle times at a large scale. 

Conclusions concerning further process studies can be drawn from this simple analysis. (1) There is a need to try to reduce foaming at stage 2 of process A. If the requirement concerning the volume of vessel 2 in process A could be reduced by 40%, tank AE2500 with a capacity of 3.46 m might be sufficient, which would significantly decrease the capital costs. (2) Physical properties of all mixtures should be determined and thermokinetic data for the second steps of both processes provided to make calculations of the schedule more reliable. 

The development of the theory of heat-flow calorimetry (Section VI) has demonstrated that the response of a calorimeter of this type is, because of the thermal inertia of the instrument, a distorted representation of the time-dependence of the evolution of heat produced, in the calorimeter cell, by the phenomenon under investigation. This is evidently the basic feature of heat-flow calorimetry. It is therefore particularly important to profit from this characteristic and to correct the calorimetric data in order to gain information on the thermokinetics of the process taking place in a heat-flow calorimeter. 

The problem is apparently simple and may be expressed in the following way knowing g(t), the thermogram, and h(t), the calorimeter response to a unit impulse, solve Eqs. (20) or (35) and determine/(<), the thermokinetics of the phenomenon taking place in the calorimeter. However, the digital information which is used in the computer does not allow the continuous integration of Eq. (35). Both functions g(t) and h(t) are indeed stored and manipulated as series of discrete steps (samples). For a computer s convenience, Eq. (35) must therefore be written... 

FIGURE 2.8. Typical Results of Autocatalytic Thermokinetics as Obtained by Isothermal Analysis. 

The data required for the emergency vent design includes [191] (1) the thermokinetic and pressure history monitored under near adiabatic conditions, (2) the character of the type of vented system (vapor, gassy, or hybrid), (3) the phase of the vented material (vapor, liquid, or two-phase), and (4) the degree of two-phase disengagement (turbulent, bubbly, or homogeneous). To determine these characteristics, the VSP defines the system as viscous (100 cp) or nonviscous, and also whether or not it has a foaming tendency. 

In order to determine the thermokinetics in experimental runs, the temperature is varied between certain limits depending on the thermal stability of the original components, the intermediates, final products, and by-products. For example, if the optimum temperature is Topt/ the temperature may be varied between Topt 25°C. 

Once the cooling liquid has been chosen (i.e., Cpm fixed), the variables of mass and temperature of the cold liquid remain and depend on the desired temperature decrease (T - Te). This value can be derived from the thermokinetics of the given reaction system. 

From the temperature in the system and thermokinetic data, the time available to the maximum rate of the runaway can be calculated or determined experimentally. This represents the maximum time available to discharge the contents of the reactor. The actual time needed for dumping should be less than the maximum to add a safety factor. The driving force for dumping can be gravity or vessel pressure. 

Steinbach, J., "Grenzen der Interpretierbarkeit Thermokinetischer Messungen" ("Limits to the Evaluation of Thermokinetic Measurements"), DECHEMA Tagung, Bad Soden, Germany (1990). 

This test can be used to give early detection of the initial exothermicity. It is possible to estimate thermokinetic parameters (e.g., the activation energy and the adiabatic self-heat rate) and to estimate how the initial temperature for self-sustaining reactions will vary with the quantity of material present. 

J. Nielsen, Metabolic control analysis of biochemical pathways based on a thermokinetic description of reaction rates. Biochem. J. 321(1), 133 138 (1997). 

In this case, although the reported enthalpy of reaction refers to the difference 7/yh (B) —//ja (A), where Ta and 7], represent the selected temperatures of the peak onset and offset, respectively, an approach based on the thermokinetic analysis of the measured curve was used to compute the peak baseline, and a very detailed description of the method used to derive the thermodynamic and kinetic data is given by the authors. Finally, a general and very important application of... 

The thermokinetic method [107,108] uses the measurement of the forward rate constant of the equilibrium... 

The thermokinetic method takes advantage of the correlation observed between... 

Fig. 2.23. Determination of the proton affinity of cyclohexanecarboxamide (a) by the kinetic method, and (b) by the thermokinetic method. The horizontal lines show the indicative values ln[AH]V[BrefH] = 0 in (a) and REs according to different authors in (b). [101] Adapted from Ref. [109] by permission. IM Publications, 2003.

Proton transfer is one of the prominent representatives of an ion-molecule reaction in the gas phase. It is employed for the determination of GBs and PAs (Chap. 2.11.2) by either method the kinetic method makes use of the dissociation of proton-bound heterodimers, and the thermokinetic method determines the equilibrium constant of the acid-base reaction of gaseous ions. In general, proton transfer plays a crucial role in the formation of protonated molecules, e.g., in positive-ion chemical ionization mass spectrometry (Chap. 7). 

Martinez, M. R., Estimation of Gas-Phase Thermokinetic Parameters Thermochemical Group Additivity Computer Program (TGAP). NTIS AD-762-614 and AD-769-631, 1973. 

FIGURE 13.5 Calorimetric and volumetric data obtained from adsorption calorimetry measurements Raw pressure and heat flow data obtained for each dose of probe molecule and Thermokinetic parameter (a), Volumetric isotherms (b), Calorimetric isotherms (c), Integral heats (d), Differential heats (e), Site Energy Distribution Spectrum (f). (From Damjanovic, Lj. and Auroux, A., Handbook of Thermal Analysis and Calorimetry, Further Advances, Techniques and Applications, Elsevier, Amsterdam, 387-438, 2007. With permission.)... 

Plot of the variation of the thermokinetic parameter as a fnnction of the adsorbed amount of probe. 

Measurement of the thermokinetic parameter can be used to provide a more detailed characterization of the acid properties of solid acid catalysts, for example, differentiate reversible and irreversible adsorption processes. For example, Auroux et al. [162] used volumetric, calorimetric, and thermokinetic data of ammonia adsorption to obtain a better definition of the acidity of decationated and boron-modified ZSM5 zeolites (Figure 13.7). 

FIGURE 13.7 Variation of the thermokinetic parameter (in minutes) versus the ammonia uptake for a H-ZSM5 sample and boron-modified H-ZSM5. (From Auroux, A., Sayed, M. B., and Vedrine, J. C., Thermochim. Acta, 93, 557-60, 1985. With permission.)... 

When the thermokinetic parameter was plotted versus the amount of NH3 adsorbed for samples of H-ZSM5 (Si/Al = 10.3) pretreated at 400 and 800°C it was found that the maximum time constant is higher for the sample pretreated at 800°C than for that pretreated at 400°C [103]. In fact, the increase of the pretreatment temperature caused dealumination extra-framework aluminum species were created that restricted the access to the channels and created diffusional limitations. 

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